A Review of Soft Robotic Actuators and Their Applications in Bioengineering, with an Emphasis on HASEL Actuators’ Future Potential

Abstract

This review will examine the rapidly growing field of soft robotics, with a special emphasis on soft robotic actuators and their applications in bioengineering. Bioengineering has increasingly utilized soft robotics due to their mechanical adaptability and flexibility, with applications including drug delivery, assistive and wearable devices, artificial organs, and prosthetics. Soft robotic applications, as well as the responsive mechanisms employed in soft robotics, include electrical, magnetic, thermal, photo-responsive, and pressure-driven actuators. Special attention is given to hydraulically amplified self-healing electrostatic (HASEL) actuators due to their biomimetic properties and innovative combination of dielectric elastomer actuators (DEAs) and hydraulic actuators, which eliminates the limitations of each actuator while introducing capabilities such as self-healing. HASEL actuators combine the fast response and self-sensing features of DEAs, as well as the force generation and adaptability of hydraulic systems. Their self-healing ability from electrical damage not only makes HASELs a unique technology among others but also makes them promising for long-term bioengineering applications. A key contribution of this study is the comparative analysis of the soft actuators, presented in detailed tables. The performance of soft actuators is assessed against a common set of critical parameters, including specific power, strain, maximum actuation stress, energy efficiency, cycle life, and self-healing capabilities. This study has also identified some important research gaps and potential areas where soft robotics may still be developed in the future. Future research should focus on improvements in power supply design, long-term material durability, and enhanced energy efficiency. This review will serve as an intermediate reference for researchers and system designers, guiding the next generation of advancements in soft robotics within bioengineering.

1. Introduction

The role of actuators is to convert input energy into motion. Traditional actuators like electric motors are used very widely for propulsion, rotation, lifting, and positioning tasks because of their high specific power. Actuators have emerged as a critical component in robotics, with significant advancements in their technology in recent decades. Even though conventional actuators like electric motors have very high power-to-weight ratios up to 300 W/kg compared with hydraulic actuators that have 2 W/kg, there are many limitations due to the rigidity of actuation materials and the support of the materials. To ensure a safe interaction between these rigid robots and humans, they should be closely observed and controlled. Soft robotics technologies address these shortcomings by taking inspiration from the compliance and adaptability of biological muscles in animals and humans. Soft actuators are made from flexible viscoelastic materials to implement the capability of even load distribution, stiffness modulation, and energy dissipation for safe and efficient interactions in a range of environments [1].

There are tremendous advancements, and a vast amount of research has been performed in the area of soft actuators. A comprehensive literature review is essential to find out the current state of the knowledge and explore the most impactful innovations. This review is specifically aimed at researchers and practitioners who are already familiar with soft robotics and actuator technologies. It delves into various soft actuators responsive to different stimuli, such as electrically responsive, magnetically responsive, thermally responsive, photo responsive, and pressure-driven actuators. Dielectric elastomer actuators (DEAs) and piezoelectric actuators are discussed within the electrically responsive actuators. DEAs operate using the Coulombic attraction of two flexible electrodes. They are used for their high energy density and large strain potential, though they have challenges such as a risk of electrical breakdown and high voltage requirements. Piezoelectric actuators, which leverage the piezoelectric effect, are known for their precision, quick response, and light weight. Magnetically responsive actuators use magnetic fields to generate mechanical deformation. This type of actuation is used in applications that require wireless control, such as hazardous environments and medical procedures inside the human body. Under thermally responsive actuators, shape memory alloys (SMAs), shape memory polymers (SMPs), and liquid crystal elastomers (LCEs) are discussed. They use temperature changes to induce mechanical shape changes. This class of actuators is used in applications that require reversible shape changers due to their ability to remember specific shapes and alter them in response to temperature variation. Within photo-responsive actuators, visible light-driven actuators (VLDA) and near-infrared (NIR)-driven actuators are reviewed. These two actuators use light energy to create mechanical motion. They are used in medical applications such as drug delivery due to their advantages such as biocompatibility and remote-control capability. Finally, pneumatic and hydraulic actuators are discussed under pressure-driven actuators. They use compressed air or pressurized fluids to generate large deformations and force. Both these actuators are used in prosthetics and artificial muscles. Pneumatic actuators are used for their lightweight and flexible design, and hydraulic actuators are used for their high force generation and precision. The requirement of an external fluid reservoir or air compressor is a key drawback of pressure-driven actuators when it comes to portable applications.

Among all the actuators reviewed, hydraulically amplified self-healing electrostatic (HASEL) actuators are particularly exceptional due to their unique features and potential in future applications. The HASEL actuators combine the properties of DEAs and fluid-driven actuators, which are two prominent soft robotic technologies. They inherit the high-speed and self-sensing abilities of DEAs and the adaptability of fluid-driven actuators. Most importantly HASEL actuators have overcome many limitations of the DEAs and fluid-driven actuators. HASEL actuators overcome the disadvantages of the DEAs, such as the risk of dielectric breakdown and the requirement for pre-stretchable materials for dielectric layers. These actuators also overcome the main drawbacks of fluid-driven actuators, such as bulk and complexity, leakage issues, slow response times, and limited precision. HASEL actuators not only overcome the challenges of the fluid-driven and DEAs but also introduce self-healing from electrical damage, a feature that is not available in any other soft actuator. However, taking inspiration from HASEL actuators, the recently developed shape memory polymers (SMPs), liquid crystal elastomers (LCEs), and pneumatic actuators have self-healing properties but will require external sources like heat or sunlight for healing [2,3,4]. HASEL actuators are capable of a wide range of motions, including linear contraction, bending, and twisting, while remaining as soft and adaptable as needed for a safe human–robot interaction. Furthermore, their self-healing capability makes them more reliable for applications where they would be used in the long term, such as bioengineering and medical devices. Interestingly, there are many types of HASEL actuators, such as Peano, planar, and donut, that can be used according to the required application. Out of them, Peano HASEL actuators closely mimic the linear contraction of human muscles, making them a particularly promising candidate for use in prosthetics and wearable devices [5,6].

Each type of actuator that is discussed in this review is detailed with a brief introduction, working mechanism, materials used, fabrication methods, new findings, advantages, limitations, and applications. Proper decisions can be made given the characteristics and requirements of different types of actuators by researchers and practitioners to properly apply suitable actuators in a certain bioengineering application or even to further design and modify technologies to adapt to their projects. Following this, the review focuses on the bioengineering applications of soft actuators, such as prosthetics, assistive devices and wearables, drug delivery, and artificial organs. Analyzing and reviewing the innovations of these biomedical applications will help to find the gaps in current research, identify the unmet needs, and guide future research and development on soft actuators, focusing on superior performance, efficiency, and applicability in real-world scenarios. Thereafter, we compare the mechanical properties of soft actuators that have applications in bioengineering. Finally, the current limitations of the soft actuators and the areas that need further development are discussed.

2. Classification of Soft Robotic Actuators

The classification of soft actuators is fundamental to understanding the ever-growing field of soft robotics. This section will classify soft actuators according to the type of stimulus that triggers their actuation as shown in Figure 1. Categories include electrical, thermal, magnetic, photo-responsive, and pressure-driven mechanisms. Each of these categories has its own characteristics, materials, fabrication techniques, and actuation methods, presenting a wide array of functionalities for their applications. Classifying these actuators will provide the foundation to understand the applications and potential applications of soft actuators in bioengineering, which will be addressed later in this article.

2.1. Electrically Responsive Actuators

Soft actuators that are capable of converting electrical energy to mechanical motion are classified as electrically responsive soft actuators. DEAs and piezoelectric actuators are discussed in this section. These actuators use mechanisms such as electrostatic force and the piezoelectric effect to convert the electrical energy to mechanical energy. HASEL actuators also fall within this category, which are discussed in Section 3 of this paper.

2.1.1. Dielectric Elastomer Actuators

Dielectric elastomer actuators (DEAs) operate via Coulombic attraction, utilizing two flexible electrodes on a compressible membrane. DEAs are known for a high energy density, self-sensing capabilities, and significant strain, making them suitable for biological applications such as artificial muscles. However, they require high voltages, which can lead to electrical breakdown and leakage currents [7,8,9,10].

The performance of DEAs is influenced by factors like the membrane material, dielectric constant, and voltage stability. Common materials include silicones, acrylics (e.g., VHB 4910), and polyurethanes. VHB 4910, a frequently used acrylic elastomer, demonstrates the highest strain performance but is highly viscoelastic, affecting stiffness under varying conditions [11].

DEAs could be stacked, folded, framed, inflated, or rolled in different ways and bent, stretched, or buckled differently. Examples include annelid-like robots, electro-adhesive grippers, and soft crawling robots [9,10]. Using multilayer fabrication techniques, newer DEA-based systems exhibit rapid actuation, a high energy density, and low voltage requirements compared to earlier unimorph designs [12].

Recent advancements, like roll-to-roll processing, have improved the manufacturing efficiency and enabled rolled DEA designs with high flexibility, a high electro-elastomeric-to-weight ratio, compactness, and cost-effectiveness. Despite their benefits, these designs face limitations in bandwidth and pre-stress dependency [13].

Low-voltage stacked DEAs (LVSDEAs) are a significant development [14], as exemplified by robots like the DEAnsect, a lightweight, wireless, and autonomous robot, showcasing the growing potential and adaptability of DEA technologies in robotics.

2.1.2. Piezoelectric-Based Soft Actuators

Piezoelectricity is the property of certain materials to generate an electric charge or voltage when mechanical forces are applied and, conversely, to produce mechanical deformation when exposed to an electric field [15]. These materials are widely used as sensors for their ability to convert mechanical forces into electrical signals, and as actuators, though their use in the latter is limited by low strain, low force, and the need for high electric fields. Despite these drawbacks, piezoelectric actuators are valued for their light weight, precision, fast response, and high actuation frequency, making them ideal for applications like soft assistive gloves and sensors in rehabilitation devices [16,17].

Electrospinning and inkjet printing are common methods for creating piezoelectric actuators [18]. For example, polyvinylidene fluoride (PVDF) serves as a base material, with electrodes printed using silver ink and a piezoelectric layer of P(VDF–TrFE) built through precise deposition [19]. These films are then thermally treated to enhance their properties, enabling high-performance piezoelectric devices.

A study by Wu et al. showcased a miniaturized robot (3 cm × 1.5 cm) inspired by biological motion [20]. The robot featured a piezoelectric PVDT layer, thin palladium–gold electrodes, and a silicone adhesive layer, all supported by a PET base. By applying alternating current, the robot achieved a wave-like motion similar to running animals. Operating near its resonant frequency (~850 Hz) enhanced its speed, achieving up to 20 body lengths per second. This work highlights the promise of piezoelectric materials in soft robotics, particularly for imitating natural movement patterns.

2.1.3. HASEL Actuators

HASEL (hydraulically amplified self-healing electrostatic) actuators are a revolutionary development in the field of electrically responsive actuators, whose innovative design is currently in patent pending status [21]. These actuators resolve some of the shortcomings of more conventional technologies like DEAs, which are reviewed in Section 2.1.1, by incorporating aspects of hydraulic actuators that are presented in Section 2.5.2. These novel HASEL actuators address the limitations of DEAs, including the requirement of pre-stretching, risk of failure after dielectric breakdown, limited flexibility, and insufficient output force. They also overcome many disadvantages of hydraulic actuators, such as the slow response time and the exclusion of bulky and complex components. Their characteristics make them suitable for applications requiring precision, adaptability, and bioinspired motions, like soft grippers and prosthetics. The inherent flexibility and scalability in this technology provide an opportunity to integrate into a complex, lifelike system, whereas traditional actuators are less effective [6].

Section 3 provides further details on HASEL actuators based on the working principles, self-healing and self-sensing mechanisms, types of materials used, fabrication methods, advantages, and disadvantages, with a discussion of their application potential.

2.2. Magnetically Responsive Actuators

Magnetically responsive actuators generate mechanical deformation in response to an external magnetic field. One of the standout features of magnetically responsive actuators is the ability to be controlled remotely. Magnetic fields can penetrate through various media and initiate motions or shape changers without physical contact. This characteristic is important in applications where direct contact is either challenging or undesirable, such as inside the human body for medical procedures or in hazardous environments where human intervention is risky [22]. The magnetically responsive actuators are made by integrating soft mediums with magnetic fillers. For the soft mediums, polymers, gels, paper, and fluids are typically used. When exposed to magnetic fields, these particles align in a way that changes the shape or causes motion in the actuator. This mechanism allows for a wide range of movements and deformations, making these actuators versatile and adaptable to various tasks and environments [23]. Magnetic fillers convert electromagnetic energy into mechanical deformation or thermal energy. There are different types of magnetic fillers, like ferromagnetic particles, superparamagnetic particles, and permanent magnets. Each type of magnetic filler reacts differently when a magnetic field is applied. Examples of soft magnetic fillers are soft ferrite, iron, and iron-based alloys. The special properties of these materials are that they have high permeability and low coercivity. This means they can be easily magnetized and have a low resistance to demagnetizing. Therefore, the materials can be remagnetized and demagnetized easily to the required directions. When a magnetic field is applied, these magnetic fillers that are within the polymer matrix align quickly with the magnetic field, leading to a deformation in the polymer matrix. This process is known as magnetostriction [24].

One of the most conventional methods of fabrication for soft magnetic actuators is molding and casting. When molding these materials, applying a uniform magnetic field during the process aligns the magnetic particles within the material along a specific direction, forming chained microstructures. Molding can be used not only for simple shapes but also for complex shapes like lattice structures using 3D-printed molds. Then, the mold can be filled with magnetorheological fluids or elastomers containing soft magnetic particles, adding magnetic responsiveness to these metamaterials. The advantage of molding is that it does not require significant material modifications and it is applicable for a wide range of elastomers and hydrogels [25,26]. The other way of fabricating soft magnetic actuators is 3D printing. Three types of 3D printing techniques are used for fabrication, and they are fused deposition modeling (FDM), digital light processing (DLP), and direct ink writing (DIW) [27].

A unique technique for creating magnetic soft actuators using multi-material DLP 3D printing was introduced by Zhongying Ji et al. [28]. The magnetic resin used in these actuators is made from acryloyl-modified polyethylene glycol, reactive diluents, and a photoinitiator, designed for both magnetic responsiveness and stability. Using DLP 3D printing, actuators are fabricated layer by layer with UV curing, based on a digital model prepared using slicing software. The printer alternates between magnetic and nonmagnetic resins to create actuators with integrated complex structures. These actuators, composed of segments with varying diameters and magnetic particle concentrations, exhibit diverse mechanical responses and bending properties under magnetic fields. Designs such as grippers and other complex structures are achieved, with potential applications in biology, medicine, and robotics due to their flexibility and noiseless operation.

An article by Xufeng Cao et al. describes the use of FDM 3D printing for the fabrication of biomimetic magnetic actuators [27]. The fabrication process involves creating a composite material from thermoplastic rubber (TPR) and carbonyl iron particles (CIPs). TPR serves as a flexible matrix due to its thermoplastic and rubber-like properties. The process begins with softening TPR by heating, mixing it with CIPs for uniform dispersion, and processing the composite in a twin-screw extruder at 190 °C. The extruded material is cooled, shaped into filaments suitable for 3D printing, and used in a deposition-based printer to form structures layer by layer. Key parameters, such as the print speed, layer height, and temperature, are optimized for precision. The magnetic and mechanical properties are characterized using SEM microscopy, tensile testing, and deformation analysis. Actuators fabricated from this material exhibit biomimetic movements under magnetic fields, with FEM analysis used to model and predict the deformation behavior under varying magnetic field strengths, aiding in the design of advanced structures.

Magnetically responsive actuators face challenges regarding fabrication and implementation. Challenges in fabrication include the uniform embedding of magnetic materials into soft matrices, maintenance of mechanical stability during repeated actuation, and precision during microfabrication. During actuation, challenges arise in creating sufficient force and torque on a small scale, precision in localization and navigation, and the control of multiple actuators in space without interference. Due to the micro-scale design, integrating power sources and sensors remains a key challenge. Ensuring the biocompatibility and development of biodegradable material are the limitations related to medical applications. Innovations in material science, fabrication techniques, and control systems are required to address these challenges in the future [29].

2.3. Thermally Responsive Actuators

Soft actuators that use temperature differentials to drive mechanical motion are classified as thermally responsive soft actuators. This section discusses shape memory alloys (SMAs), shape memory polymers (SMPs), and liquid crystal elastomers (LCEs) as the actuators. These actuators can switch between predefined shapes using temperature differences, making them commonly used in applications that require reversible shape changes. SMA actuators are frequently used in prosthetic hands because they can produce high stress. Even though these actuators have impressive performance, all thermally responsive actuators have a common limitation, which is a slow response mainly due to the thermal conductivity of the materials. Overcoming this limitation is essential to have day-to-day applications of this set of actuators in the bioengineering field.

2.3.1. Shape Memory Alloys (SMAs)

Shape memory alloy soft actuators are a transformative technology in soft robotics. These actuators use the unique properties of shape memory alloys like nickel–titanium alloys to produce mechanical deformation in response to changes in temperature. The fundamental mechanism behind these actuators is the shape memory effect. When SMA materials (mostly metallic) are heated above a specific transformation temperature, they can return to a precise predefined shape that is programmed while manufacturing. Compared to other soft actuators, SMAs can generate high stress due to their energy absorption capacity [30,31,32]. Nickel–titanium (NiTi) is the most commonly used SMA due to its high density and strain recovery properties. SMA wires are embedded in soft materials such as polydimethylsiloxane (PDMS) to create flexible and biocompatible soft actuators [33]. Mersch et al. combined SMA wires with textile reinforcement to overcome the limited maximum stroke in soft actuators [34]. It was achieved previously by extending the SMA wire length, which increased the weight of the actuator.

There are three main methods for fabricating SMA actuators: straight, curved, and complex shapes. Embedding SMA wires in a flexible matrix is one of the methods that is used to fabricate SMA soft actuators. In this method, SMA wires that are pre-strained are placed inside a mold, and an elastomer called polydimethylsiloxane (PDMS) is poured around them. Once the elastomer is cured, it will solidify and bond with the SMA wires, resulting in a composite material that is capable of substantial deformation [32,35]. For creating curved actuators, a double-casting method is used. First, a straight actuator is formed by embedding SMA wires in an elastomer matrix. Then, the formed straight actuator is placed inside a second mold with a desired curvature to generate the curved actuator. This process of recasting will make the actuator perform more complex shape changes utilizing the predefined shape change [36]. Additionally, multi-material 3D printing techniques are employed to fabricate SMA actuators. This method is used to create complex structures with precise control over the geometry and material properties [33,37].

A significant advantage of using SMAs, particularly NiTi alloys, is their high energy density, which allows them to produce a large force-to-weight ratio. This makes them suitable for applications such as prosthetics that require powerful and lightweight actuation. SMAs can also recover to their original shape after large deformations, which is beneficial for applications such as grippers and locomotives that require a wide range of motion [35,38,39]. Temperature changes greatly influence SMA performance. In some applications, achieving precise temperature control can be challenging, especially in varying environmental conditions. SMAs’ relatively slow heating and cooling process, along with a slow response time compared to other soft actuation types, make them less suitable for scenarios requiring rapid actuation [31].

2.3.2. Shape Memory Polymers (SMPs)

Similar to SMA actuators, shape memory polymer (SMP) actuators use materials designed to return to a preprogrammed shape. SMPs use lightweight and flexible materials such as acylate and polyurethane to make the base material of the actuators, unlike the metallic materials used in SMA actuators. Due to this, SMP actuators have a lower weight and potentially lower manufacturing costs compared to SMA actuators. SMP actuators can be classified into three types: one-way, two-way, and multiple-way. One-way actuators can only remember the permanent shape, and they cannot go back to the temporary shape; therefore, they can be used for single-use applications only. Two-way SMP actuators can remember two shapes, which are the original and programmed shapes, and repeatedly switch between them with temperature cycling. Multiple-way actuators can remember multiple shapes based on the number of transition temperatures; they can also switch between shapes [2].

A two-way SMP actuator can be used as an example to explain the mechanism of SMP actuators. The polymer is heated above the transition temperature (T_trans) to soften and deform it into the required shape when an external force is applied. While the external force is applied and maintained in the desired position, the SMP is cooled to a temperature that is below the glass transition temperature. The polymer’s molecular chains become immobilized during the cooling process, converting from a rubbery state back to a glassy state. The temperature of the SMP is maintained at the ambient temperature until further thermal activation. When the SMP is reheated above the glass transition temperature, again the SMP is returned to its original shape by releasing its strain energy. In one-way SMP actuators, a polymer is heated above T_trans to soften and deform it into the temporary shape while above T_trans, and then cooled below T_trans to fix it. When the polymer is reheated above T_trans, it can return to the original shape, but it will not go back to the temporary shape like the two-way actuators once it is cooled again. Multiple-way SMP actuators are sequentially heated above multiple temperatures, cooled, and reheated to achieve different shapes [40].

SMP actuators are typically manufactured using conventional methods like injection molding, shape extrusion, laser cutting, and soft lithography. However, due to their high costs and lengthy processes, researchers have discovered 3D printing as a cost-effective and efficient fabrication method. This allows for the creation of complex structures that are difficult to create using conventional methods. Because the SMP structures that are 3D printed can change their shape with external stimuli, they are called 4D printing. Normal 3D printing methods such as fused deposition modeling (FDM), stereolithography (SLA), and direct ink writing are used in 4D printing as well [41].

One of the key benefits of SMP actuators is that they are lightweight and can be used as flexible structures in wearable and implantable devices. Additionally, SMP actuators can be designed to respond to stimuli other than heat, such as light and magnetic fields [42]. SMP actuators can be built with self-healing properties to reduce maintenance costs and increase durability. This is performed by embedding capsules that are filled with healing agents inside the SMP material. When the material cracks, the capsules break open and release the healing agent that is capable of filling and fixing the cracks. The slow response time and temperature sensitivity are the main disadvantages of SMP actuators, where the actuation speed can be slower compared to other actuators due to the thermal conductivity of the material. SMPs are used for biomimetic applications in wearable technology and soft robotic grippers [2].

2.3.3. Liquid Crystal Elastomers (LCEs)

Liquid crystal elastomers (LCEs) are materials that combine the elastic properties of polymers and the anisotropic properties of liquid crystals. These materials can respond not only to thermal stimuli but also to light and electric fields. LCEs consist of polymer networks filled with liquid crystalline mesogens that align in a certain direction, resulting in a macroscopically anisotropic material. When the mesogens are stimulated with heat, they lose their ordered alignment and thereby cause the material to contract along the alignment direction and expand perpendicularly. This reversible transition enables LCEs to undergo large deformations [43].

Researchers have developed various materials and composites to enhance the performance and functionality of LCE actuators [43]. As mentioned before, they are primarily synthesized using mesogens, which are polymerized into an elastomer network in which the molecular order is coupled to the elastic properties of the polymer. Popular mesogens include acrylates and methacrylates due to their general compatibility with most polymerization techniques. Crosslinking agents play a very significant role in the architecture of the LCE polymer network [44,45]. Nanomaterials are often integrated into the LCE matrix to improve the actuation performance and include multifunctional properties. These components synergistically enhance the actuating performance, responsivity, and multifunctionality of LCEs [43,45].

LCE actuators are fabricated using two-stage polymerization, direct ink writing (DIW), and 3D printing. The two-stage polymerization technique involves a diacrylate mesogen, a dithiol spacer, and a photoinitiator to create a lightly cross-linked network and achieve a monodomain structure. Finally, a second stage of polymerization is initiated in UV light so as to fix the aligned structure permanently [43]. DIW is combined with 3D printing to fabricate complex structures of LCEs. The process is initiated with the preparation of a computer-aided design model of the actuator. In this model, the main frame of an actuator is printed using an inkjet 3D printer. Following printing, conductive silver ink is printed onto the frame using a DIW printer, followed by thermal curing. Finally, pre-programmed monodomain LCE strips are placed and bonded onto the frame, realizing the actuator [46,47].

LCE actuators have many advantages, including large, reversible deformations in response to heat, light, and electric fields. In addition, their multifunctionality enables them to respond to multiple stimuli, which makes possible tasks such as shape morphing and self-healing. Their molecular alignment could be programmed during fabrication, enabling complex motion patterns and high-precision control. Self-sensing LCE actuators, made of materials like liquid metal channels, enable closed-loop control [43,45]. LCE actuators have disadvantages such as a low actuation speed and force, limiting their applications in fast or high-force actuation. Additionally, they are sensitive to environmental conditions, such as temperature variation; these affect the performance, causing inconsistent behavior. Moreover, some nanomaterials incorporated into LCEs may introduce mechanical constraints, thus affecting the flexibility and ultimately the lifespan of the actuators [44,45]. LCE actuators have excellent potential to work as artificial muscles due to their ability to mimic the contraction and relaxation behaviors of natural muscles. Additionally, LCEs are beneficial in microactuators and microfluidic systems, where their responsive actuation enables the creation of precise, controllable components such as valves and pumps, which are crucial for lab-on-a-chip devices and other miniaturized applications [43].

2.4. Photoresponsive Actuators

Photoresponsive actuators are a class of soft actuators that, under the stimulus of light, would convert light energy to mechanical motion. The ability to control the actuators remotely is one of the main advantages of these actuators; therefore, they are used in applications that require wireless or remote-control actuation, such as underwater actuation. There are a few types of photoresponsive actuators that are designed to be triggered by various wavelengths of light. The most common types are discussed in this section, which are visible light-driven actuators and near-infrared-driven actuators.

2.4.1. Visible Light-Driven Actuators (VLDAs)

VLDAs can produce mechanical deformation from sunlight. These actuators draw inspiration from the natural movements of plants, such as the hygroscopic seed dispersal mechanism, the opening of pinecones, and the twisting of Bauhinia variegata pods [48]. VLDAs are operated based on four primary mechanisms: photothermal, photochemical, photoelectronic, and photomechanical effects. The photothermal mechanism actuators are made from carbon nanotubes (CNTs) or graphene oxide (GO) embedded into a polymer or hydrogel matrix. When the actuator is exposed to light, the materials embedded in the actuator convert the light energy into heat. Materials with exceptional light absorption qualities and thermal conversion properties are used to integrate with the polymer or the hydrogel matrix. When the heat is transferred from the heat-absorbing materials to the polymers or hydrogel mix, it causes thermal expansion, resulting in mechanical deformation. This is the primary actuation type among all the photoresponsive actuation schemes. For example, the article by Deng et al. presents a VLDA fabricated using CNTs embedded in paraffin wax [49]. This actuator can bend and twist in response to light, and the tunability of the actuator is controlled by the alignment and the concentration of the carbon-based material.

The photochemical mechanism uses photoactive molecules that undergo a reversible isomerization when exposed to light. Liquid crystal polymer networks (LCPNs) incorporate these photoactive molecules within a polymer matrix, allowing complex and reversible deformations; similar to the photothermal mechanism, the actuator’s response to light can be precisely controlled by aligning the liquid crystal molecules in specific orientations [50]. Photoelectronic actuation uses materials that can generate electronic responses when exposed to light. They use materials like single-walled CNT (SWCNT) bundles and graphene sponges prepared using Hummer’s method to show movement and propulsion [51]. Without intermediate chemical or thermal processes, photomechanical actuators can convert the light energy to mechanical motion. Materials that can undergo structural changes, such as contraction and expansion, in response to light are used in these actuators. Single-walled and multi-walled CNT films are used to prepare the photomechanical actuators by vacuum filtration [50].

The fabrication methods for visible light-driven actuators depend on the materials used in the actuators. The primary fabrication techniques are spin coating and embedding, layer-by-layer assembly, and copolymerization. The article by Yang et al. explains the method of GO layered with polymers to create photoresponsive actuators. Each layer is designed to serve a specific function, such as light absorption, mechanical support, or actuation [50]. GO is used for its excellent photothermal properties, while the polymer layers are used for their structural integrity and flexibility. The repeated layers of GO and the polymer create a material that can convert light into mechanical motion. Deng et al. describe the fabrication of photoresponsive actuators by spin coating [49]. It is a process that involves applying liquid paraffin wax into a polyamide substrate and then rapidly spinning it to ensure that the solution spreads evenly. The spinning speeds range from 400 to 2000 rpm. The films are reheated to 80°C and then cooled to room temperature before cutting them in different directions. This method is used to create thin and uniform films.

The article by Xiang et al. uses copolymerization to fabricate VLDAs. In this process, simple molecules that can join together to form a structure known as monomers are used [52]. Monomers like polyethylene glycol (PEG1000), hexamethylene diisocyanate (HDI), pentaerythritol, and tetrahydroxy-functionalized hexaarylbiimidazole (HABI) are mixed in a liquid solvent called anhydrous dimethylformamide (DMF) so they can react together. To increase the speed of chemical reaction, di-n-butyltin dilaurate is used. Then, the mixture is left to polymerize. Two types of bonds are made from this polymerization, one is dual cross-linked polyurethane (PU) to make the material strong, and the second is dynamic covalent HABI cross-linked points that can break under light to make the material change its shape. The final step is to dry the resulting gel under a vacuum at 70 °C to remove the DMF solvent, creating a solid form of the hydrogel. The actuators that are made from this method are used as underwater actuators. The advantages of visible light-driven actuators are remote-control accessibility, precise control of the actuators, and biocompatibility, making them suitable for biomedical applications. While it is safer to use visible light, it has a limited penetration depth compared to UV light, which can be a disadvantage for certain applications. The materials are also susceptible to fatigue from repeated actuation. The applications of visible light-driven actuators are smart walkers, drug delivery, microswitches, biomedical devices, artificial muscles, and photo origami applications [50].

2.4.2. Near-Infrared Light-Driven Actuators

Near-infrared (NIR) light has longer wavelengths than visible light, and due to this, NIR light has a lower loss than visible light while penetrating through biomaterial. This makes them highly suitable for biomedical devices that need wireless actuation. Even though the light-driven actuators use many methodologies, most of the NIR light-driven actuators are operated on a photothermal mechanism [53]. Even though all of them use photothermal mechanisms, the materials used in the actuators are changed according to the actuation strategies. Hydrogels, shape memory polymers (SMPs), and liquid crystal elastomers are some strategies used to make the NIR light-driven actuators.

In their article, Zhang X. et al. discuss the integration of metal–organic frameworks (MOFs) into poly(N-isopropylacrylamide) (PNIPAM) hydrogels to serve as photothermal agents [54]. MOFs can absorb NIR light and convert it to heat efficiently. PNIPAM is a hydrogel temperature-sensitive material that can expel water and contract upon heating. Thin films of poly(dimethylsiloxane) (PDMS) are used as the backing material on which the hydrogel is polymerized. To fabricate the actuator, a process called in-situ photopolymerization is used. It is a process where the polymerization will take place on the actual substrate, in this case, the PDMS thin films. This is achieved through exposure to light that activates the polymerization process. The use of MOFs accelerates the response time of the actuator, enabling applications like aquatic soft robotics. The study by Zhan X. et al. also used the same methodology but, additionally, they added Fe3O4 nanoparticles for better photothermal conversation and a shape memory polymer (SMP) for a better actuation response and temperature range [55].

The research conducted by Fang et al. demonstrated the creation of the NIR light-driven actuator using SMPs with carbon nanotubes (CNTs) and reduced graphene oxide (RGO) integrated as photothermal fillers [56]. These materials are mixed and then hardened to make the actuators. Once heat is applied and the temperature reaches a certain transition point, they can return to their original shape. These materials can provide robust and reliable actuation, even in challenging aquatic environments. This method has few requirements to operate, such as the material must be stable underwater and to keep the actuator temperature under 100 °C. Yu et al. have shown through their research a mechanism of a two-stage NIR light-driven actuator using a highly elastic form-stable phase-change polymer (HEPCP), LCEs, and MWCNTs. HEPCP is a polymer that can transition between liquid and solid states with a temperature change [57]. When NIR light is applied to the actuator, the MWCNTs within the actuator absorb the light and convert it into heat. When the temperature reaches the phase transition temperature of the HEPCP, it shifts from a solid state to a liquid state. This transformation changes the mechanical properties of the material, softening the actuator. The phase change of HEPCP and the temperature change in the composite affect the surrounding LCEs to realign their molecular chains and extend or contract. The configuration of the LCEs is responsible for the actuator’s deformation, such as bending, twisting, or contracting. These actuators are capable of reversible shape changes, making them suitable for tasks requiring precision and a fast response. With similarities to this work, the research conducted by Qin et al. focuses on the development of NIR light-driven actuators using liquid crystal polymers (LCPs) [58]. LCPs have the mechanical properties of the polymers and the fluidic properties of liquid crystals. LCPs can be engineered to not only absorb the NIR light but also different wavelengths of light. The precise control offered by LCPs is ideal for the fluids of micro-scale devices. They can act as pumps or guides for fluid streams.

2.5. Pressure-Driven Actuators

Pressure-driven actuators produce motion through the expansion and contraction of flexible chambers that are filled with gases or liquids. Therefore, these actuators are biocompatible to be applied in bioengineering applications, including prosthetics, rehabilitation devices, and wearables. Pressure-driven actuators have many applications in bioengineering due to their high power-to-weight ratio compared to other soft actuators. Even though these actuators have many advantages, there are also several drawbacks. Due to their dependence on air compressors or fluid reservoirs coming from external sources, it is challenging to make portable devices using them.

2.5.1. Pneumatic Actuators

Soft pneumatic actuators (SPAs) are valued for their high force and torque combined with lightweight and flexible structures, making them prominent in real-life applications. Unlike traditional McKibben actuators, modern SPAs are fully constructed from soft materials like silicon rubber, providing enhanced flexibility and range of motion [59]. SPAs operate through volumetric expansion or contraction and support movements such as bending, twisting, and extending. There are two types of SPA actuators: ones that operate with positive pressure and ones that use negative pressure. Positive pressure actuators use compressed air to activate, while negative pressure actuators use a vacuum to be activated. Between these two types, negative pressure actuators are preferred over compressed actuators mainly due to the fail-safe feature. This implies that in the event of system failure, minor leaks or structural damage are less likely to lead to a catastrophic failure. Negative pressure actuators also have more durability than positive pressure actuators. The negative pressure actuators have a limited force output compared with positive pressure actuators; therefore, in applications that require a high force and lightweight and flexible structures, positive pressure actuators are preferred [60].

Conventional SPAs are created via molding and casting, while modern SPAs use advanced 3D printing methods such as SLA, FDM, selective laser sintering, and material jetting. Materials include silicon, TPU, and hydrogels. A notable innovation is self-healing SPAs, which can repair small damages using sunlight [61].

SPAs are utilized in soft grippers for diverse object handling, artificial muscles in humanoid robots, prosthetic devices, exoskeletons, and rehabilitation equipment [3,62,63]. Despite their advantages, they face challenges like a dependence on external compressors, noise, and vibration, which restrict their use in quiet environments [3].

2.5.2. Hydraulic Actuators

Hydraulic actuators excel among soft actuators due to their high specific power and power density. They operate by generating pressure in a liquid-filled flexible chamber (typically using water or oil), which deforms the chamber to produce movement. Depending on their design, these actuators can perform diverse motions, including stretching, bending, twisting, contracting, or spiraling, making them suitable for a wide range of tasks [64].

Soft hydraulic actuators are made from materials like elastomers, fibers, hydrogels, and anisotropic fabric composites to ensure flexibility, durability, and structural integrity. Silicone rubbers (e.g., Ecoflex 0050 and Dragon Skin 30) provide elasticity and allow significant deformation. Kevlar fibers reinforce structures by restricting radial growth while permitting axial extension. Hydrogels add compliance and flexibility, while anisotropic fabrics provide directional strength for complex movements. Water or other incompressible, non-toxic fluids are typically used as the hydraulic medium due to their safety and availability [65].

Hydraulic actuators are primarily fabricated using molding and casting, where materials are poured into CAD-designed molds and cured to achieve the desired shapes [64]. Advanced designs can also utilize 3D printing for molds or complete actuator structures. Key components, such as reinforcement fibers, hydraulic chambers, and silicone hoses, are manually assembled and bonded with silicone adhesive for structural integrity. The use of anisotropic fabric composites enhances the durability, flexibility, and performance, enabling the creation of highly sophisticated hydraulic actuators [65,66,67].

Soft hydraulic actuators offer significant advantages, including high flexibility, a safe interaction with humans and delicate objects, versatility in motion, eco-friendliness, and efficiency in underwater operations. They enable precision control, modular design, and a good strength-to-weight ratio, making them suitable for applications like industrial robots, prosthetics, underwater vehicles, and wearable technology [64,66,67]. However, challenges include hysteresis, nonlinearity requiring advanced control strategies, fluid leaks leading to performance issues and environmental risks, and bulky components that hinder miniaturization. Addressing these limitations is crucial for their broader adoption in applications like bionic robots, rehabilitation devices, and educational research [64].

3. HASEL Actuators

Biomimicry is a scientific field that is used to solve human problems by studying patterns, functions, and processes in nature. Soft robotics is a branch of study that applies biomimicry. In soft robotics, actuators have been developed to replicate the natural skeletal muscle functionality. They are also referred to as “muscle-mimicking actuators”. The “Hydraulically amplified self-healing electrostatic (HASEL) actuator” is one of the most recent developments in the soft robotics sector [68].

3.1. Working Principle

The fundamental working principle of a HASEL actuator is shown in Figure 2. This is based on the donut HASEL actuator described by Acome et al. [69]. This actuator has an elastomeric shell that is partially covered in the middle by a pair of opposing electrodes and filled with liquid dielectric. The application of voltage (V1, V2, and V3) creates an electric field that produces an electrostatic Maxwell stress, which displaces the liquid dielectric from between the electrodes, resulting in actuation strain. When the voltage increases past the threshold voltage, the electrostatic force becomes greater than the mechanical restoring force, which makes the electrodes pull together abruptly.

3.2. Materials and Fabrication

Elastomers and thermoplastic polymers are the materials used to fabricate the HASEL actuators. The actuators that are made from elastomers include donut and planar HASEL actuators. On the other hand, Peano, quadrant donut, curling, and high-speed Peano HASEL actuators are made using thermoplastic polymers. Each of these actuators comes with unique functions and can be used in different applications. Elastomeric donut HASEL actuators are fabricated using a circular shell elastomer that is filled with liquid dielectric and electrodes are located at the middle of the actuator. Upon the application of a voltage, the actuator deforms in the middle and the dielectric liquid flows to the outer ring of the actuator, transforming it into a shape of a donut as illustrated in Figure 3B. These actuators have widely been known for their large actuation strains and have been applied as soft grippers for delicate handling. The planar HASEL actuators are designed to linearly expand. When a voltage is applied, the dielectric liquid redistributes over the actuator, making the actuator thinner while increasing the area. Planar HASEL actuators are capable of high linear strains up to 124%, and six planar HASEL actuators have been tested to lift a gallon of water [69]. Peano HASEL actuators are designed for linear contraction. Unlike the planar HASEL actuators, these actuators do not require rigid frames or pre-stretching. Peano HASEL actuators, as shown in Figure 3A, are made of dielectric liquid-filled rectangular shells with electrodes inside the shells. Contraction of the actuator happens due to the electrostatic zipping of the electrodes, which incrementally compresses the dielectric liquid within the shell. The linear contraction of these actuators closely mimics the contraction of human muscles. A significant advantage of Peano HASEL actuators is their scalability. To increase the overall force output, multiple actuators can be combined in parallel, and to improve the stroke length, the number of actuators in the series must be increased depending on the specific application requirements. Cost-effective production techniques, such as heat sealing, are used to fabricate these actuators due to the use of thermoplastic as the material [70].

Quadrant donut HASEL actuators are an advanced version of the classic donut HASEL actuator. In these actuators, the structure segmented into four quadrants brings an exactly unique shape to the actuator alone. It is performed in strategic ways such that the improvement of the actuator’s control over deformation would be linear in expansion. Hence, each quadrant uniformly deforms upon the application of a voltage to present smoother and more predictable actuation with respect to the standard donut design. One of the key advantages of quadrant donut HASEL actuators is modularity. They can be stacked or otherwise arranged in various configurations to create many different mechanical outputs. This stacking of actuators greatly increases stroke length [71]. Finally, high-strain Peano HASEL actuators are designed where electrodes are relocated to achieve larger deformation strains. They have a maximum theoretical train up to 36% compared to the 10% in normal Peano HASEL actuators, making them suitable for applications that require higher deformation [5]. In all, each type of HASEL actuator offers unique advantages in flexibility and adaptability for a wide variety of applications in soft robotics and bioengineering.

HASEL actuators are fabricated in a multi-step process, which has been specially tailored to fabricate actuators that are flexible and have high performance with various design configurations. They consist of deforming shells, which could be made from either elastomeric material or thermoplastic material. Elastomeric actuators usually consist of silicone elastomers, characterized by excellent stretchability and good dielectric properties. The thermoplastic actuators are consequently fabricated from biaxially oriented polypropylene, a material with excellent mechanical and dielectric properties. In the fabrication process, two layers of the material are bonded, where the shell has the shape of a pouch filled with a liquid dielectric, such as silicone oil or vegetable-based Envirotemp FR3. Normally, thermoplastic actuators are bonded into shape through heat sealing or CNC-based thermal bonding. Heat sealing for rapid prototyping normally includes precision tools, such as CNC machines or 3D printers that have been converted into heat sealers. Such machines can make complex geometries in an actuator. Once the shell is formed and filled, the next step involves the application of the electrodes. Depending on the type of HASEL actuator, electrodes can be made of flexible conductive materials such as thin metal films, conductive inks, or even hydrogels. Screen printing and vapor deposition are used to apply the electrodes to the surface of the shells over the region where deformation will take place. In applications that involve a high force output or an extended working stroke, multiple actuators are stacked or connected in parallel. For durability of operation, after being assembled, the actuators are sealed to prevent the leakage of the liquid dielectric. In addition, the process also allows customization to specific application requirements, such as the integration of a self-healing dielectric material or the integration of other sensing elements that incorporate a capacitive sensor for self-sensing capabilities [20,71].

3.3. Advantages and Limitations

HASEL actuators are demonstrated to possess an impressive self-healing capability, which is primarily against the damage caused by dielectric breakdown. Dielectric breakdown happens due to excessive electric fields destroying the insulating properties of a dielectric material. Upon the occurrence of a breakdown, the liquid dielectric redistributes into the affected region to re-establish electrical insulation. It effectively heals the actuator to continue functioning without repair or other external intervention. This novel mechanism ensures much higher reliability and a longer lifetime for HASEL actuators, making them more appropriate for challenging and repetitive usage. The HASEL actuators have the ability to self-heal because they use the liquid dielectric. Solid dielectrics permanently get damaged from breakdown, but liquid dielectrics have the capability to return to an insulating state. The second important element in the self-healing process is the elastomeric shell that encloses the liquid dielectric. These shells are made to have a sealing property to avoid any probable leakage of the liquid dielectric in case of physical punctures. While the shell does not mechanically repair punctures, its flexibility and elasticity keep the liquid dielectric inside such that the actuator maintains structural and functional integrity. This containment is crucial for preserving the performance of actuators after damage and speaks to the robustness of the designs of HASEL actuators [69].

The active self-healing mechanism is inspired by the principles of liquid dielectric systems in high-voltage transformers. Using similar principles may reinstate insulating properties after electrical damage. By employing such principles in soft robotics, HASEL actuators overcome a significant challenge faced by DEA actuators [72]. Donut HASEL actuators are able to self-heal from 50 instances of dielectric breakdowns. Because of this property, gas bubbles, which have a low breakdown strength, were created as the liquid broke down, but they had little of an impact on the self-healing process because they were pushed out of the area with the strongest electric field among the electrodes. Dielectric breakdown voltage is not a fixed value, but it is a distribution of values that depends on various factors, such as the presence of defects and the insulation quality. Since HASEL actuators can self-heal from electrical damage, a larger actuation stroke can be produced by assembling multiple actuators together. A linear strain of 37% can be achieved by stacking five actuators. This is close to the linear strain achieved by muscles in our body [69].

HASEL actuators can use their structures as deformable capacitors, enabling self-sensing capabilities. They will be able to monitor the state of their deformation through changes in capacitance. The activation of the actuator or external forces applied to the actuator would change the size and spacing of electrodes, thus producing a measurable capacitance change, which directly indicates deformation. This immediate feedback allows very precise control without external sensors. The additional self-sensing capability for compactness and reliability in HASEL actuators makes them very suitable for robotics and wearable devices. HASEL actuators create an avenue for life-like motion and adaptability through the emulation of proprioceptive feedback from biological muscles, hence extending the functionality in dynamic applications that require high precision and responsiveness [69]. However, HASEL actuators exhibit poor accuracy in self-sensing in high frequencies of actuation, typically above 1 Hz, due to a phase lag, where there is a delay between the actual motion of the actuator and the sensed signal. The high frequency of the actuator also introduces electrical noise. As a solution for this limitation, Vogt et al. introduced a design of an actuator called Frog-HASEL (F-HASEL) with a different set of electrodes dedicated only to sensing. By segregating sensing from actuation, the interference is reduced drastically, and the accuracy increases [73]. Two methods of sensing are discussed: the voltage method and impedance method. In the voltage method, changes in voltage are measured to estimate movements directly, and the impedance method utilizes voltage and current to calculate changes in electrical impedance. These complementary approaches create a performance gain of high accuracy at frequencies as high as 10 Hz and functionality at frequencies as high as 20 Hz with advanced signal processing. The paper further demonstrates the practical utility of F-HASELs for wearable devices by tracking joint rotation in real time for virtual reality systems. HASEL actuators have a few other limitations, including the need for high voltages, and even though the HASEL actuators have self-healing capabilities, repeated breakdowns can cause the formation of gas bubbles in the liquid dielectric, which can result in performance degradation [6].

3.4. Applications

Due to the novelty of the technology and current limitations, the practical implementation of HASEL actuators remains limited. However, because of the immense potential of technology, there is a wide range of future applications across various fields. HASEL actuators have the potential to make life-like robotics due to the ability to replicate the adaptability and responsiveness of biological organisms [6]. Configurations of HASEL actuators, such as donut HASEL actuators have been used to make soft grippers that are capable of handling raspberries without crushing them. Therefore, they can be used to make soft grippers that can handle delicate objects [69,74]. HASEL actuators also have potential applications in prosthetics, assistive, and wearable devices due to their light weight and muscle-like performance [75]. Due to their dynamic actuation capabilities, they are suitable for legged locomotion and bio-inspired flight. In their article, Buchner et al. applied HASEL actuators to a musculoskeletal robotic leg to illustrate HASEL’s potential in agile, adaptive, and energy-efficient systems [76]. The robotic leg is actuated by antagonistic pairs of HASEL actuators, biomimicking the function of biological muscles through tunable stiffness and high agility sphere motions beyond 5 Hz. This system is capable of detecting and adapting to obstacles using the self-sensing capability of HASEL actuators without using external sensors. Furthermore, energy efficiency results in only 1.2% of the energy consumption required from conventional DC motors during static tasks. Lastly, they can be used in biomimetic systems such as robotic tails and limbs that are inspired by natural mechanisms [71].

4. Applications of Soft Robotics in Bioengineering

Incorporating soft robotics into bioengineering has opened new frontiers in medical technology, offering new adaptive and safe solutions for human interaction. Due to the flexibility and adaptability of the soft actuators, they are more suitable for bioengineering applications than traditional rigid robotics, which can be hazardous. Soft robotic applications in bioengineering, such as prosthetics, assistive and wearable devices, drug delivery, and artificial organs are discussed here. The working mechanisms of the applications are outlined. We begin by investigating some of the early inventions in bioengineering and trace their development to the way they have handled their limitations over time. The discussion includes the improvements that are required in the new inventions for widespread real-life applications.

Most of the applications have seen different types of soft actuators applied to the same bioengineering application, and each of them has their relative merits and demerits. While multiple actuators were employed across different applications, pneumatic actuators stand out as the most commonly used due to their high force-to-weight ratio. This section not only underlines the current impacts of soft robotics on bioengineering but also unravels the enormous potential for future innovations. Figure 4 illustrates examples of applications of soft actuators in bioengineering.

4.1. Prosthetics

Soft robotic technologies are increasingly influencing the development of prosthetic devices, especially in the area of upper limb prosthetics. Soft robotic actuators are integrated into prosthetics in various forms, including grippers, hands, and electric muscles. A wide range of these grippers and hands employ soft robotic actuators, such as pneumatic actuators and shape memory alloy actuators. The primary goal of incorporating actuators in upper limb prosthetics is to reduce the device’s weight. According to the users, the main dissatisfaction with conventional electric upper limb prosthetic devices is the weight distribution [84]. Rigid actuators require a complex mechanical structure to replicate human-like movements.

Low et al. have made a prosthetic hand that includes three independently controlled pneumatic finger actuators [85]. These actuators are designed to mimic the thumb, index, and middle fingers of a human hand. These fingers are the most crucial for gripping and handling tasks. The hand is also designed to be lightweight, and it is attached to a robotic arm, LBR iiwa 14 R820 (Kuka, Germany), for testing. The actuators are designed with pneumatic channels to replicate the bending of the natural finger joints called metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints. The length of each actuator is 146 mm, which is equal to the length of the index finger. The actuators are fabricated with a material called Dragon Skin 10-Medium (DS10-M) due to their hyperelastic properties. When air is pumped into the channels, the actuators inflate, causing the fingers to bend at the joints. The amount of air pressure can be controlled to change the degree of the flexion, allowing for precise control over the finger movements. This prosthetic hand is tested and capable of grasping and holding up to 600 g. This is essential for everyday activities, such as picking up objects and opening doors. This makes the force generated by each finger 2 N, which is significantly lower than the force a healthy individual can produce—typically around 300 N for females and 450 N for males.

An article by De Pascali et al. introduced a prosthetic hand that is integrated with pneumatic artificial muscles [86]. They have named the artificial muscle actuators GeometRy-based Actuators that Contract and Elongate (GRACE). GRACE actuators are designed to mimic the contraction and elongation of the human muscles. They are made from a pleated, single-material membrane that expands and contracts pneumatically. The actuators are fabricated using 3D printing technology, employing a flexible and durable material known as flexible 80A resin. The prosthetic hand is also fabricated using 3D printing, integrating all the components, including palms, fingers, flexible joints, and tendons. Each finger in the prosthetic hand is equipped with GRACE actuators arranged in series. Pneumatic lines are connected to inflate and deflate the actuators. The hand is designed in a way that each finger can move independently. In this prosthetic hand, the wrist can also be moved using the GRACE actuators. The actuators in the wrist are incorporated in the antagonistic setup, which means some actuators contract while others extend. This design allows the prosthetic to mimic the movements of a biological wrist.

There are a few types of GRACE actuators, and GRACE-C is the type of actuator that is designed to maximize contraction. In isometric tests, GRACE-C actuators recorded a maximum pulling force of 18 N at a 4 mm stretch and also underwent 1000 cycles of contraction and extension tests, showing good durability. Similar to the prosthetic hand presented by Low et al., when the performance of the pneumatic actuators is compared with natural muscles, the force output of the actuators is significantly lower than that of natural muscles. The study conducted by Kandasamy et al. introduces body-powered, portable soft hydraulic actuators [87]. These actuators are designed to mimic the human fingers. A syringe filled with fluid is connected to soft actuators in the hand. When the user shrugs their hand, which pushes the plunger of the syringe, increasing the pressure of the syringe drives the hydraulic fluid into soft actuators, causing them to bend. Currently, this hand can only grip cylindrical objects, and they have tested that it takes 5.8 N to dislodge an item from the gripper.

Shape memory alloys (SMAs) are used in prosthetic hands to develop devices that are lighter, quieter, and capable of more natural movements compared to motor-driven systems as shown in Figure 4A. The prosthetic hand developed in the study by Lee et al. operates in the principle of SMA actuation, where SMA wires get contracted when heated and return to their original form when cooled [77]. Each finger of this hand is equipped with multiple SMA wires to increase the force output. This setup amplifies the actuation force and increases the range of motion. The SMA wires work as an artificial muscle in this hand. The hand is tested for its capability to grab different shapes and objects, such as cups, balls, bottles, and pens. It is measured that the prosthetic hand can handle small objects that weigh around 500 g. A similar prosthetic hand designed by Kim et al. has five tendon-driven fingers attached to a 3D-printed palm [88]. The artificial fingers are made from smart soft composite (SSC) with SMA wires embedded within them. This allows the actuators to bend like a natural finger, achieving a bend of 305 degrees with a weight of 20 g and 61 degrees with a weight of 60 g. While the main purpose of using SMA actuators in prosthetic devices is to reduce their weight, there are a few limitations to overcome. These include the susceptibility of SMA wires to wear and tear, the configuration of the wires can be complex, slow reaction speed, and maintaining optimal temperatures for SMA heating and cooling can be challenging [89,90].

The are many artificial muscles that are made from technologies like pneumatics, hydraulics, and DEAs that are tested in prosthetics and assistive devices. All these devices have significant limitations, including a lack of force, lower speed, controllability, and reliability, which restrict their everyday use [84]. In the article by Yoda et al., they introduced a high-speed finger driven by Peano HASEL actuators, functioning similarly to an electric muscle [75]. They used a finger from the Bebionic v2 Prosthetic Hand and measured the force generated from the electric muscle in the study. By comparing the test results with an electric hand that is actuated by DC motors, the Peano HASEL custom finger is 10.6 times faster and consumes 8.7 times less electricity and has an 11.1 times higher bandwidth. The only limitation is that the fingertip force produced by the DC motor-driven finger is 10 times higher. This technology presents new opportunities in prosthetic limb design when it comes to weight distribution.

4.2. Assistive and Wearable Devices

Different types of wearable soft robotic devices are introduced in rehabilitation and disability engineering as shown in Figure 4C–G, to address various health-related issues. These include rehabilitation and assistive devices, compression therapy devices, and performance augmentation devices [91]. These devices aim to restore the ability to perform physical tasks impaired by injury, disease, or congenital conditions. Research on rehabilitation devices has produced several innovative devices. For instance, Polygerinos and colleagues developed a robotic glove for chronic stroke survivors who suffer from hemiparesis or related conditions utilizing soft fiber-reinforced silicone actuators that function under fluid pressurization. This glove is designed to solve issues in traditional rehabilitation, such as the need for repetitive tasks with occupational therapists, which can often be expensive and labor-intensive [92]. Many other rehabilitation gloves are made using hydraulic and pneumatic soft actuators. For instance, Lee et al. have optimized the design of the glove to increase the degree of freedom [93]. The article by Jiang et al. introduces a fabrication method for a rehabilitation glove that can bear 2.5 times more pressure than a regular glove, presented visually in Figure 4E [81]. Using the same pneumatic actuation method, Yap et al. and Hu et al. introduced soft rehabilitation gloves using different fabrication methods and designs [80,94]. Figure 4D shows the glove designed by Yap et al.

Kang et al. designed tendon-driven gloves that help restore hand movements for daily tasks using pneumatic actuation [95]. Unlike other gloves, this one uses a polymer base material for the fabrication of the glove instead of fabric. This makes the glove more adaptable for different hand sizes, and it can also be cleaned easily using an alcohol swab, unlike fabric-based gloves. Zhang and colleagues explored a different approach with wearable pneumatic rings that act like a bubble on the finger that can help the user grasp objects without ending the hand are demonstrated in Figure 4F,G. The glove is controlled by forearm muscle signals and it is tested in grasping a 645 g water bottle [82] Park et al. have worked on lower limb supports using McKibben pneumatic artificial muscles for ankle–foot rehabilitation [96]. This device is designed to be used for patients with abnormal gait caused by stroke, cerebral palsy, amyotrophic lateral sclerosis, and multiple sclerosis. There are four pneumatic artificial muscles used in this device and they can assist with movements such as dorsiflexion, plantarflexion, inversion, and eversion. Pressure and strain sensors are embedded to analyze the gait patterns. The device is tested and can make an ankle range of motion of 27°. Thalman and Artemiadis further summarize the variety and applications of soft wearable robots for movement assistance in their review [97].

4.3. Drug Delivery

In soft robotic actuation, magnetic fields are used to control soft robots inside an enclosed space. This technology is used in drug delivery inside the human body. The soft actuators are made from elastomers and gels embedded with magnetic materials. Recent advances in drug delivery use 3D printing, origami techniques, elastomers filled with liquid metal, tough hydrogels, and mechanical metamaterials [98]. In the article by Fusco et al., they have designed a microrobot that can be used to carry drugs inside the body and a mechanism that can release them at the required location [99]. The microrobot is constructed with a core of magnetic alginate microbeads, which are encapsulated within a hydrogel bilayer structure. The hydrogel bilayer is sensitive to near-infrared (NIR) light and temperature changes. This structure allows the microrobot to be loaded with the desired drug and released by opening the hydrogel bilayer at the target location. The unloading can be achieved by raising the temperature to 40 °C or by exposing it to near-infrared (NIR) light. The base layer of the hydrogel bilayer is made of poly(ethylene glycol) diacrylate (PEGDA) and the top layer is made of graphene oxide nanocomposite. The responsiveness to the NIR light is enabled by the inclusion of graphene oxide. During the fabrication process, the drug or therapeutic agent can be embedded into the beads. The navigation of the microbots through the body is performed by a magnetic manipulation system known as “OctoMag”. This system can precisely control the movements of the microbots throughout the body.

The article by Breger et al. also describes the development of self-folding soft microgrippers, as shown in Figure 4B that are responsive to both temperature and magnetic fields [78]. They have used a hydrogel called poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAM-AAc) to make the microgrippers and mix it with a harder, non-expandable polymer called polypropylene fumarate (PPF) to increase the stiffness and the gripping ability. They have also added iron oxide nanoparticles into the hydrogel, making the microgrippers responsive to magnetic fields and allowing remote control. The grippers are stored at a temperature of 4 °C and they open when the temperature approaches 37 °C. These microgrippers can be directed to specific tissue locations to deliver the drugs.

Exceeding limitations such as unchangeable magnetization and limited movement types, the article by Liu et al. introduces a millirobot that can change its morphology while navigating by reprogramming the magnetization profile [100]. The robot can also switch motion modes, such as from rolling to gliding or flipping to rotating, without needing to stop for adjustments. The adaptation in the movement allows the robot to navigate in narrow passages. The millirobot’s body includes a photothermal hydrogel containing magnetic particles. The NIR (near-infrared) laser can target sections in the millirobot and heat them. When exposed to NIR laser light, the hydrogel transitions from a solid state to a gel state, temporarily freeing the magnetic particles from their fixed positions. While the magnetic particles are mobile, an external magnetic field is applied. This field realigns the magnetic particles into new orientations. The magnetic particles lock into their new configurations when the laser is turned off and the hydrogel is cooled. This introduction of morphology-reconfiguration technology holds promising potential for improving drug delivery using millirobots.

Another application of soft actuators in drug delivery involves bio-hybrid soft actuators. This drug delivery system represents a transformative approach in modern medicine. Recent advancements in bio-hybrid actuators have been highlighted in various studies, each contributing to the potential and application of these systems. The article by Shin et al. discusses the development of nanomaterial-based biohybrid hydrogels for bioelectronics [101]. These materials are used to create responsive drug delivery systems. Nanomaterials such as carbon nanotubes and metallic nanoparticles are embedded within hydrogels. These actuators can be engineered to use physiological signals as inputs to release therapeutic agents. This capability is crucial, as it ensures that drugs are delivered at the optimal time and location. In addition, they have developed a system that can deliver drugs in real time in response to a patient’s condition by integrating bio-responsive elements that can dynamically interact with the biological environment. These types of dynamic systems are beneficial for managing diseases that require continuous monitoring and adjusting the therapeutic control according to the condition of the patient. In their study, Lingyu Sun et al. explore the use of microorganism-actuated biohybrid robots for precise drug delivery [102]. They use genetically modified bacteria that are capable of navigating to the tumor sites, which are then used to transport and release chemotherapeutic agents directly where needed. This targeted approach enhances the impact on the tumor cells and significantly reduces the adverse effects associated with the distribution of toxic drugs in unwanted locations.

4.4. Artificial Organs

One of the key benefits of soft robotics actuators rather than hard ones is that they can be applied to intimate human contact. This will make them suitable for applications such as artificial organs, where the special material features of soft robotics are most useful. Out of all the artificial organ applications, the creation of a soft robotic heart has gained more attention than the rest due to its critical nature of functionality. McKibben actuators and pneumatic artificial muscles are used in this application as shown in Figure 4H [83]. McKibben actuators have an inflatable bladder that can contract linearly and expand radially when pressure is applied. This mechanism is used for the functionality of soft robotic sleeves designed to wrap around the heart, assisting in both the systolic and diastolic phases of the cardiac cycle. These McKibben actuators are synchronized with the heart’s natural rhythms while improving mechanical coupling and cardiac output. Besides the above-highlighted use of soft robotics in ventricular heart devices, other soft robotic applications include implantable linear pneumatic muscles to assist the right ventricle in cases of right heart failure [103].

The soft total artificial heart (sTAH) is a breakthrough in soft robotics used on a fully implanted heart. Through 3D printing and lost-wax casting processes, the device is entirely fabricated from silicone elastomers, enabling the unit to mimic the natural movements shown by the human heart. The design includes two ventricles and an internal expansion chamber that is pneumatically actuated to form pulsatile blood flow. This biomimetic approach by the sTAH ensures the replication of the physiological motion of a real heart to achieve adequate blood flow, meeting the demands of the body. Despite its potential, the sTAH faces challenges in material durability and operational longevity, which are crucial for it to be applied effectively over time as a solution for the long-term replacement of the heart. This early prototype has been shown to have very low durability and will need more research to improve the longevity of these devices through better material properties [104].

Soft robotic ventricular assist devices (VADs) provide another example of the application of soft robotics to support cardiac function. These devices use soft pneumatic artificial muscles, which provide direct compression of the cardiac muscles to assist in the systolic phase and aid in diastolic refilling. VADs have been especially important for temporary support as a bridge to transplantation for patients awaiting heart transplants. In some cases, VADs serve as a permanent solution for those not eligible for transplantation. The design of these devices is aimed at minimizing blood clotting and stroke complications that are quite common in conventional mechanical pumps. Soft robotic technology has been incorporated for the safety and better efficacy of these assistive devices [105]. One of the most interesting inventions in this field is the soft robotic ventricular assist device with septal bracing for heart failure therapy. It is designed for either isolated left or right heart failure by applying rhythmic loading to either ventricle. The device applies forces to the free wall of the ventricle through the interventricular septal attachment such that in systole, there is an approximation of the septum and free wall; in diastole, it assists with recoil. Physiologic sensing of native hemodynamics enables the autonomous augmentation of the heart function. This device has demonstrated effectiveness in restoring blood flow and pressure in a variety of heart failure models through in vivo studies in a porcine model. Large, significant reductions in diastolic ventricle pressure were also obtained, indicative of evidence for diastolic unloading or an improvement in ventricle filling with consequent support of cardiac output over the long term [103].

4.5. Applications of Soft Robotic Actuators in Bioengineering

Table 1 summarizes the types of soft actuators in each bioengineering application and gathers all the references of the papers discussed. For each bioengineering application, such as prosthetics, wearable devices, drug delivery, and artificial organs, multiple inventions have been developed using different actuators or using the same type of actuator with varieties and improvements. The table organizes these applications by actuator types, such as DEAs, SMAs, LCEs, magnetically responsive actuators, pneumatic actuators, and HASEL actuators. This structuring also helps us to find out which actuators are most used in different applications and which type of soft actuator has the most inventions in each application. Pneumatic actuators turn up repeatedly in various applications due to their high specific power and adaptability. This table is intended to guide researchers and practitioners toward the identification of pertinent studies and technologies that have been applied to specific bioengineering challenges, leading toward a better understanding of the actual state and future possibilities of soft robotics for bioengineering applications.

Table 2 summarizes the role of each actuator in bioengineering applications and their limitations. The analysis reveals that while these actuators are well-suited for certain engineering applications, challenges related to power requirements, portability, and mechanical constraints persist. This summary therefore provides insights into the choice of an appropriate actuator based on the application needs and challenges.

5. Comparison of Soft Robotic Actuators in Bioengineering

This section is divided into two sections. In the first section, the performance of soft actuators that are used in bioengineering applications are compared. Then, the impact of the fabrication technique on the performance of the actuators is summarized in a table. In the second section, the limitations of the soft actuators are discussed and a table outlining future research directions for each actuator type is presented.

5.1. Comparison of the Performance of Soft Robotic Actuators

Table 3 reviews the actuation parameters of soft actuators to compare their performances. Out of all the soft actuators, Peano HASEL actuators, SMAs, SMPs, LCEs, HAMs, and PAMs are selected for comparison because they had the most applications in bioengineering compared to the others. In addition to that, the actuation parameters of biological muscles are also included in the table to compare them with artificial muscles. These actuators can be categorized as actuators designed for bending and actuators designed for linear actuation (expansion and contraction). HASEL, HAM, and PAM are actuators that can perform linear actuation and can be compared with natural muscles. SMAs, SMPs, and LCEs are designed for bending actuation and can be compared with each other based on their performance. The actuation parameters that will be used to compare the actuator’s performance are the specific power, strain, maximum actuation stress, energy efficiency, number of cycles, and self-healing capabilities.

Specific power is the ratio of the power that is generated by the actuator to the mass of the actuator [123]. This is an important parameter in bioengineering applications, specifically when it comes to prosthetics, wearable devices, and artificial organs. Pneumatic actuators are also mentioned, as pneumatic artificial muscles have the highest specific power out of all the actuators, which is 10 kW/kg. SMAs have the second highest specific power after PAMs, which explains why many inventions use SMAs in prosthetic applications.

Strain in the soft actuators is calculated as the ratio of how much a material deforms compared to its original size in the direction of the applied force [123]. SMPs have the highest strain out of all the actuators. More than 100% strain can only be achieved by the bending actuators. However, as linear actuators, HAMs and PAMs have the next highest strain values, which are up to 90% and 70%, respectively.

Maximum actuation stress in soft actuators refers to the maximum amount of force per unit area that the soft actuator can apply when it is activated [123]. SMAs have the highest maximum actuation stress, which is 700 MPa. This is due to the strength of the alloy materials used, such as NiTi, in the SMAs. Out of the actuators that are designed to perform linear actuation, PAMs have the highest maximum actuation stress, which is up to 1 MPa.

The energy efficiency of the soft actuators is calculated by the ratio of the output mechanical energy to the input energy [123]. PAMs have the highest energy efficiency, followed by the HAMs. All three thermally responsive actuators, SMAs, SMPs, and LCEs, have low efficiency due to heat dissipation.

The number of cycles refers to the number of times the actuator can be activated before it stops functioning properly [123]. The biological muscles have 10⁹ cycles, which is way higher than all the soft actuators that are discussed here. However, Peano HASEL actuators have 10⁴–10⁵ cycles. HASEL actuators have other variants of actuators, such as, donut HASEL actuators that are capable of 10⁶ life cycles.

The self-healing capability refers to the ability of the actuators to fix themselves after damage. Out of all the soft actuators discussed here, only HASEL actuators are capable of self-healing, and the other soft actuators that are mentioned in the self-healing category require external support, such as heating or sunlight, for healing. The healing capabilities of the SMPs, LCEs, and PAMs are recent findings and are not available in all their actuators.

By analyzing Table 3, it can be seen why PAMs have so many applications compared to other actuators. PAMs have the best parameters among all the actuators, other than the number of cycles and the maximum actuation stress. Even though the SMAs have the highest actuation stress, they are not used in many applications due to their slow actuation speed. The Peano HASEL actuators have many parameters that are close to the biological muscles, with a high number of cycles, self-healing, and self-sensing capabilities. They also have fast actuation speeds that are not compared in Table 3 but are an important parameter when it comes to applications.

Even though Table 3 highlights specific power and maximum actuation stress, a more practical way to compare the power generation from actuators is to compare the force measured in real-world applications. In this case, we are using the force measured from the prosthetic hands and fingers powered by different actuators, which is discussed in the bioengineering applications section. The main ways the force is measured in the prosthetic hands include direct force measurements using sensors and the measure of the maximum weight that the prosthetic hand can hold without failure. The prosthetic hand actuated from pneumatic actuators can lift objects up to 600 g. which is equal to 5.884 N [85]. The maximum force is calculated as 1.372 N in the prosthetic hand actuated by SMAs [77]. The prosthetic hand actuated from LCE actuators can lift a bag that weighs 252 g, which is equal to 2.471 N [141]. The maximum fingertip force measured from the hydraulic actuator-powered prosthetic finger is measured as 2.9 N [142]. The pinch force measured from the Peano HASEL actuator-powered finger is calculated as 0.91 N [75].

Key fabrication techniques in the development of soft actuators are summarized in Table 4, showing their impacts on performances and associated limitations. Each of these methods is assessed according to whether it improves precision, scalability, integration of materials, or customization. Techniques such as 3D printing and DIW are optimized for customization and functionality and are hence best for prototyping and complex designs. In contrast, fabrication methods such as roll-to-roll processing and injection molding are easily scalable to large volumes. While high-precision methods for nanoscale features, such as electrospinning and micro/nano-patterning, are required, they do have issues with cost and scalability. A comparison of these trade-offs is shown below, which can be used to select fabrication techniques based on the application of soft actuators.

5.2. Challenges and Limitations of Soft Robotic Actuators in Bioengineering Applications

Soft robotics provides a revolutionary solution in bioengineering that would enable virtually limitless opportunities for building medical devices for interactions with humans softly and effectively. These new systems will be promising for prosthetics, assistive and wearable devices, drug delivery, and artificial organs. However, identifying the challenges and limitations of soft robotic actuators in bioengineering is essential to fully harness their potential in real-world applications. Despite their potential, current soft robotic actuators have several limitations, including biocompatibility, durability, actuation power sources, complex manufacturing processes, and low specific power generation from the actuators. In this section, we discuss some of the critical challenges and look at barriers that need to be overcome to help make the way forward for soft robotics in bioengineering.

Soft robots should be made durable to withstand repetitive use in medical applications. The materials are inherently flexible, and, hence, will easily wear out due to their nature, thereby restricting their lifetimes. Consistent performance over time should be a top priority, since any malfunction or failure during use can cause harm to the user’s health. The most challenging points for actuation technologies in soft robotic fluidic actuators and shape memory alloys are the high power consumption, relatively slow response time, and complex control requirements, limiting practical applications in fields requiring precision and a fast response, like medicine. Furthermore, actuator miniaturization for in-body operation presents continuous challenges. The development of small but strong actuators for safe operation inside a human body is required to further develop the capability of soft robots for medical applications.

Another major challenge is to develop suitable power sources for soft robotic actuators. Traditional power sources are often bulky or generate heat, which is unsuitable for many biomedical applications. That would make it mandatory to work out advanced battery technologies to be compact, portable, efficient, and able to deliver power without changing the safety and comfort of the user. As highlighted in the applications section, pneumatic soft actuators are used in more applications than any other actuator due to their high specific power. Most of the other actuators have a low power-to-weight ratio to use in real-life situations. So, an increase in specific power is one of the main factors that needs to be developed to expand the real-life applications of soft robotics. Ethical concerns are also quite pertinent, and for the public at large, acceptance and trust will be gained, especially with human augmentation and enhancement. Early engagement with regulatory bodies and ethics committees during the development process could help to streamline the path to clinical adoption. These challenges emphasize the interdisciplinary character of soft robotics, as materials science, engineering, biology, and medicine are all involved. Overcoming them is, however, key to fully realizing the potential of soft robotic actuators for bioengineering applications.

Table 5 summarizes the future research directions for each type of soft actuator from all the articles that are reviewed in this article. Each cell details the key developments required to overcome existing limitations and improve performance. Material innovations, improvements in actuation mechanisms, design and geometry innovations, control approaches, scalability, and application-specific developments are the main categories considered in the future research section.

For sustainable development, biodegradable and renewable materials will be essential for the manufacture of HASEL actuators. As the use of these actuators increases, it becomes increasingly important to address the sustainability challenges associated with e-waste. The future of biodegradable and renewable actuators and prosthetic devices presents a promising environment for researchers to innovate, achieve sustainability, and introduce composite materials. Factors to consider are (a) the recyclability and lifecycle, (b) durability and environmental impact, and (c) integration with prostheses and implantable devices.

(a) Recyclability and lifecycle: Maintaining the performance of actuators over a long lifespan, even under harsh environmental conditions, is crucial to ensure their economical and practical application. Biodegradable actuators should be developed with recyclability in mind, ensuring their safe decomposition or recycling at the end of their lifecycle. Research into an optimal balance between durability and biodegradability is essential, as the materials must retain functionality (e.g., biomimetic and self-healing properties) for the necessary duration.

(b) Durability and environmental impact: The development of biodegradable and renewable actuator technology requires research on durability and the environmental impact. A key challenge is ensuring that these devices retain their performance under adverse environmental conditions, such as exposure to moisture, temperature fluctuations, and mechanical stress. Encapsulation is critical as it allows sensors to withstand various environmental conditions while maintaining optimal performance. The use of bio-based materials in the form of coatings and encapsulation can significantly enhance the durability of biodegradable and renewable actuators in harsher environments.

(c) Integration with prostheses and implantable devices: The integration of biodegradable and renewable actuators with prosthetic devices is another key area of focus for sustainable technology advancement. These devices must work seamlessly with other components, such as sensors, processors, and power sources, without sacrificing performance. The challenge lies in ensuring that the materials are compatible with existing soft robotic devices while maintaining eco-friendly properties. Research into optimal system designs for prosthetic devices that can adapt to compact and diverse requirements, such as implantation, will be essential for effective integration.

Trade-offs between sustainability and technical performance also require careful consideration. For example, while biodegradable and renewable actuators can contribute to environmental benefits, their performance needs to be examined to compete with that of traditional materials, potentially impacting specific forces and self-healing. The complexity of integrating these materials into existing systems may require more robust designs, which could counteract some of the sustainability goals by increasing resource consumption during manufacturing. Balancing these factors will be crucial in advancing the field.

Future studies could explore innovative solutions that enhance power efficiency while minimizing the ecological footprint, ensuring that the transition to biodegradable technologies does not compromise the effectiveness of bioengineering systems. In the following subsections, we will review several prospects that highlight emerging trends in the field.

6. Conclusions

In conclusion, soft robotics has seen remarkable growth over the last decade, with the development and application of soft actuators in bioengineering. A range of actuation mechanisms, such as electrically actuated, magnetically actuated, and thermal and pressure-driven actuators, further underscores this, each offering unique advantages for specific applications. More specifically, HASEL actuators are emerging as a promising technology for their high-speed response, self-healing capabilities, high actuation strain, self-sensing, high efficiency, low energy consumption, and functions that mimic the movement of biological muscles. These would be appropriate and useful in bioengineering applications like prosthetics or wearables.

Advanced materials and fabrication technologies, including 3D printing and soft lithography, have enabled the creation of complex, multicomponent actuators with superior levels of performance. Nevertheless, various challenges still need solutions, which include enhanced durability, energy efficiency, an increase in power generation, and biocompatibility. These issues should be addressed toward the wider adoption of soft robotics in medical applications where reliability and patient safety are considered most important. Future research must overcome these limitations by developing robust materials, power sources with high efficiency, and advanced control mechanisms. Like public trust and acceptance, ethical and regulatory concerns also render the issues imperative with the deployment of soft robotic devices in health care.

In summary, while significant progress has been made, further interdisciplinary research is required for the full realization of the potential of these devices in transforming bioengineering and improving patient outcomes. This review, therefore, serves as a comprehensive reference for the current state of the art and future directions in soft robotics, with a particular emphasis on exciting possibilities and ongoing challenges in this dynamic field.

Author Contributions

writing—original draft preparation, O.P.; writing—review and editing, O.P., G.G., R.L. and U.G.; visualization, O.P.; supervision, G.G., R.L. and U.G.; funding acquisition, R.L.; All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

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Figure 1. Classification of soft robotic actuators.

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Figure 2. The working principle of HASEL actuators is represented using a donut HASEL actuator. The green arrows show the Maxwell stress acting on the dielectric liquid and the red arrows show the hydraulic pressure acting on the shell of the actuator.

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Figure 3. (A) Peano HASEL actuator with 6 pouches. (B) Stack of ten elastomeric donut HASEL actuators.

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Figure 4. (A) Prosthetic hand actuated from SMA actuators developed by Lee et al. [77]. (B) Thermally responsive reversible self-folding microgrippers [78]. (C) Soft wearable elbow exoskeleton actuated from SMAs developed by Copaci et al. [79]. (D) Soft robotic glove designed to assist stroke survivors, developed by Yap et al. [80]. (E) Soft robotic glove developed by Jiang et al. for hand rehabilitation [81]. (F) Pneumatically actuated assistive grasping device [82]. (G) Application scenario of an assistive grasping device holding a box. (H) Pneumatically actuated soft robotic device for a failing heart developed by Payne et al. [83].

Table 1

Applications of soft robotic actuators in bioengineering.

Applications	Actuator Type	References
Prosthetic devices	DEA	[13,106,107,108]
	HASEL actuator	[75]
	Pneumatic actuator	[85,86,87,109]
	SMA	[77,88,89,90]
Artificial muscle	DEA	[110]
	Magnetically responsive actuator	[111,112]
	SMP	[113]
	LCE	[114,115,116,117]
	Pneumatic actuator	[60]
Assistive gloves	Pneumatic actuator	[16,80,81,82,91,92,93,94,95,96]
Assistive and wearable devices (excluding assistive gloves)	DEA	[106,118,119,120]
	SMA	[121,122]
	Pneumatic actuator	[3]
Drug delivery	Magnetically responsive actuator	[22,78,98,99,100,101,102]
Artificial organs	Pneumatic actuator	[83,103,104,105]

Table 2

Roles of soft actuators in bioengineering applications and their limitations.

Actuator Type	Role in Bioengineering	Limitations
Pneumatic Actuators	For prosthetic hands, assistive and rehabilitation devices require a high specific power, light weight, and flexible designs.	It requires external compressors, reducing portability and generating noise.
Hydraulic Actuators	For prosthetic joints and assistive devices needing high precision and force generation.	Leakage risks and bulky components hinder their use in compact devices.
Shape Memory Alloys (SMAs)	For lightweight prosthetic hands, high stress is generated.	A slow response time, wear and tear susceptibility, and complex configurations.
Dielectric Elastomer Actuators	For artificial muscles in assistive and wearable devices.	High voltage requirements and risks of electrical breakdown in interactive applications.
Magnetically Responsive Actuators	For drug delivery systems and remote-controlled biomedical devices.	Limited force output and challenges with mechanical constraints in high-strength tasks.
Photoresponsive Actuators (VLDA/NIR)	For drug delivery systems and biomedical grippers requiring wireless, biocompatible actuation.	Fatigue from repeated actuation and limited light penetration for deep tissue applications.
HASEL Actuators	For prosthetic hands, offering human-like motion, self-healing, and self-sensing.	High voltage requirements.

Table 3

Comparison of the performance of soft robotic actuators.

Criteria	Peano HASEL	SMA	SMP	LCE	HAM	PAM	Biological Muscle
Actuation mechanism	Electrostatic forces in fluid-filled pouches	Thermally activated to change shape based on heat	Thermally activated to change shape based on heat	Thermally activated liquid crystal phases	Hydraulic pressure inflates or deflates a flexible membrane	An inflatable bladder contracts radially and expands axially	-
Specific power (kW/kg)	0.18 (average)-0.36 (peak) [6]	1–6.6 [123]	-	0.001 [123]	2 [123]	10 [123]	0.28 [1]
Strain	15% [6]	<8.5% [124]	Up to 300% [123]	19–45% [123]	45–90% [123]	10–70% [123]	40% [1]
Maximum actuation stress (MPa)	0.21 [6]	700 [124]	130 [123]	0.45 [123]	0.6 [123]	Up to 1 [123]	0.35 [1]
Energy efficiency	20% [6]	<15% [124]	<10% [123]	5% [123]	23–59% [123]	70% [123]	40% [1]
Number of cycles	104–105 [6]	300–107 [123]	20,000 [125]	100 [116]	-	30,000 [123]	109 [6]
Self-healing capability	Yes, can self-heal from electrical damage [5]	No	Yes, able to self-heal cracks and damage [9]	Yes, can heal from heating to 80 °C [4]	No	Can self-heal via sunlight [10]	Yes
References	[5,6,70,71,75,126,127,128,129,130,131]	[30,31,32,33,34,35,37,38,39,121,122,123,124,132]	[2,40,41,42,113,123,125,133,134]	[4,45,46,114,115,116,117,123,135,136]	[64,65,66,67,123,130,137]	[3,59,60,123,138,139,140]	[1,6]

Table 4

Impact of the fabrication techniques on the performance of the actuators and their limitations.

Fabrication Technique	Impact on Performance	Actuator Types	Limitations
3D Printing (FDM, SLA)	Enables the customization of complex geometries, improves repeatability, and integrates multiple materials for multifunctionality.	Pneumatic [61], hydraulic [65], SMP [41], and magnetically responsive actuators [27]	Limited material compatibility, and potential resolution issues for small-scale features.
Electrospinning	Produces nanoscale fibers for lightweight and highly responsive actuators.	Piezoelectric-based soft actuators [18], LCE [143], and photoresponsive actuators [144]	It requires precise control and specialized equipment and has limited scalability for large-scale production.
Roll-to-Roll Processing	Enhances scalability and cost-efficiency for thin-film actuators.	Dielectric elastomer actuators [13]	Limited to thin-film actuators, requires a high initial setup cost.
Layer-by-Layer Assembly	Provides nanometer-level control over the film thickness and composition, enabling precise material customization.	Photoresponsive actuators and magnetically responsive actuators [145]	It is time-consuming for multilayer designs and challenging for large-scale production.
Injection Molding	Provides the rapid production of actuators with consistent quality and durability.	Pneumatic [146] and SMP [147] actuators	Limited customization, requires costly molds and setup.
Direct Ink Writing (DIW)	Integrates diverse materials, including conductive or magnetic fillers, for actuators with multifunctional capabilities.	Magnetically responsive actuators [27] and LCEs [46]	The fabrication process is slower than other methods, and material compatibility can be challenging.

Table 5

Future research directions of each soft actuator type.

Actuator Type	Future Research Directions
Dielectric Elastomer Actuators (DEAs)	-Development of elastomers with higher dielectric constants to reduce the operational voltage-Self-healing materials to improve reliability and durability; multilayer fabrication to reduce the actuation voltage-Miniaturization for microscale actuators-Advanced control strategies for precise actuation-Enhanced integration with wearable and soft robotic systems
Piezoelectric Soft Actuators	-Improved piezoelectric particulate composites (e.g., PZT/SMPU) for nanoscale applications-Development of biocompatible materials for biomedical use-Enhanced precision in nanoscale displacement and actuation-Advanced 3D printing techniques for fabricating piezoelectric actuators-Integration of real-time feedback systems-Applications in nanofabrication, cell manipulation, and minimally invasive medical devices
HASEL Actuators	-High-performance materials with better dielectric properties-Encapsulation techniques to prevent liquid dielectric leakage-Advanced fabrication methods like additive manufacturing for complex geometries-Development of multifunctional designs integrating sensing and actuation-Lower voltage actuation using thinner elastomer layers-Applications in robotics, prosthetics, and wearable devices
Magnetically Responsive Soft Actuators	-Biocompatible and biodegradable magnetic materials for biomedical applications-Optimization of magnetic field systems for efficient actuation-Miniaturization for microrobots-Real-time control strategies for dynamic environments-Exploration of hybrid systems integrating magnetic actuation with other mechanisms-Applications in minimally invasive surgeries and diagnostics
Shape Memory Alloy (SMA) Actuators	-Hybridization of SMA materials to improve performance and durability-Faster thermal responses using advanced heat management methods-Miniaturization for micro/nano-scale applications-Improved 3D printing methods for diverse SMA geometries-Use of biocompatible coatings for biomedical applications-Development of untethered systems using alternative energy sources
Shape Memory Polymer (SMP) Actuators	-Multifunctional SMPs with self-healing and recyclability-Development of SMPs responsive to non-thermal stimuli like light and magnetism-Integration with 4D printing for time-dependent deformations-Biocompatibility improvements for biomedical applications-Research on energy-efficient recovery mechanisms-Use of hybrid materials combining SMPs with elastomers for enhanced performance
Liquid Crystal Elastomer (LCE) Actuators	-Exploration of covalent adaptable networks for dynamic programmability-Development of 4D printing for precise and scalable fabrication-Enhanced actuation force and speed; self-healing and recyclable LCEs for sustainability-Applications in biomimetic robotics and soft medical devices-Control systems for localized programming and real-time feedback
Pneumatic Soft Actuators	-Biodegradable and environmentally friendly materials-Integration of flexible sensors for real-time monitoring-Advanced control algorithms like iterative learning and machine learning-Portable power supplies for untethered systems-Modular designs for scalability and diverse applications-Improved manufacturing methods for better strength and durability
Hydraulic Soft Actuators	-Development of anisotropic composites for enhanced stiffness and control-Integration of compact electro-hydraulic systems for miniaturization-Advanced mathematical models for precise actuation control-Real-time monitoring through self-sensing capabilities-Optimization for extreme environmental conditions-Applications in medical robotics and industrial automation
Visible Light-Driven Soft Actuators	-Development of new photoresponsive materials for improved actuation efficiency-Exploration of biocompatible materials for medical uses-Advanced 4D printing for complex and customizable geometries-Real-time feedback systems for adaptive control-Improved light penetration and actuation under natural light conditions

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