Autonomous self‐healing 3D micro‐suction adhesives for multi‐layered amphibious soft skin electronics

Abstract

Autonomously self‐healing, reversible, and soft adhesive microarchitectures and structured electric elements could be important features in stable and versatile bioelectronic devices adhere to complex surfaces of the human body (rough, dry, wet, and vulnerable). In this study, we propose an autonomous self‐healing multi‐layered adhesive patch inspired by the octopus, which possess self‐healing and robust adhesion properties in dry/underwater conditions. To implement autonomously self‐healing octopus‐inspired architectures, a dynamic polymer reflow model based on structural and material design suggests criteria for three‐dimensional patterning self‐healing elastomers. In addition, self‐healing multi‐layered microstructures with different moduli endows efficient self‐healing ability, human‐friendly reversible bio‐adhesion, and stable mechanical deformability. Through programmed molecular behavior of microlevel hybrid multiscale architectures, the bioinspired adhesive patch exhibited robust adhesion against rough skin surface under both dry and underwater conditions while enabling autonomous adhesion restoring performance after damaged (over 95% healing efficiency under both conditions for 24 h at 30°C). Finally, we developed a self‐healing skin‐mountable adhesive electronics with repeated attachment and minimal skin irritation by laminating thin gold electrodes on octopus‐like structures. Based on the robust adhesion and intimate contact with skin, we successfully obtained reliable measurements during dynamic motion under dry, wet, and damaged conditions.

INTRODUCTION

Owing to advancements in soft conductive polymers and hybrid materials, stretchable electronics such as organ-attachable bioelectronics or artificial electronic skin have recently received significant attention for next-generation biomedical and robotic applications.^1–3 In addition to various mechanical and electrical properties such as elasticity, adhesion, and conductivity, the importance of the application of devices capable of natural healing, such as living organisms, from wear and physical damage is also increasing.^4–7 Self-healing materials have been developed to address mechanical damage and restore sensing capabilities, with the aim of enhancing the longevity and durability of electronic devices.⁸ This innovative approach not only improves the overall lifespan of electronics but also contributes to a reduction in electronic waste.⁹ To develop durable skin electronics, it is important to use stretchable, self-healing materials that possess both softness and resilience to fracture energy.¹⁰ These materials should exhibit exceptional resistance to damage and prevent the propagation of small cracks. The capability to heal and recover when cracks occur is an important prerequisite for building robust skin electronic systems.¹¹ Most reported intrinsically self-healable materials contain soft rubber-like polymer components. Simply enhancing the strength or concentration of dynamic bonds within a polymer improves the tensile strength but may compromise the extensibility and dynamic behavior. Recently, various approaches, including controlling the molecular structure,^12–14 mixing strong/weak dynamic bonds,^15–17 and embedding nano–microstructures,¹⁸ have been reported to toughen self-healing polymers.

Despite the development of self-healing polymer (SHP) with improved mechanical properties, challenges persist in achieving reversible adhesion and conformal contact with dry and wet rough surfaces for their use in body-attachable bioelectronics interfaces.^19,20 To address this critical issue, self-healing adhesives based on chitosan, polyallylamine, and polyacrylic acid ester-based bridging polymers have demonstrated remarkable attachment to various skin electronic surfaces.²¹ In specific, a self-healing viscoelastic gum with excellent conformability to various surfaces and three-dimensional (3D) printability was reported,²² wherein a robust bionic glove was engineered to detect hand gestures and translate sign language exploiting these unique properties. In addition, a durable design for a proton-conductive ionic skin through the incorporation of a dual-network structure was demonstrated. This involves introducing an entropy-driven supramolecular zwitterionic competing network to the H-bonded polycarboxylic acid network, mimicking the functions of the flexible elastin network and rigid collagen fibers found in natural skin.^23,24 Although these materials exhibit excellent adherence and conformability to skin and tissues, their sustainability requires additional investigation regarding potential chemical contamination and repeatability.^25–27 Compared with the aforementioned approaches, physically based adhesives with bioinspired nano–microstructures have shown reversible attachment on various surfaces without chemical interactions.^28,29 Among these, dry adhesive patches inspired by the protuberance or infundibulum of octopus suckers exhibit strong adhesion to human skin under wet and dry conditions without chemical residue or skin irritation.^30,31 Due to these advantages, octopus-like structures have recently been studied in various fields such as drug delivery,³² bioelectronics,^1,33–35 and soft robotics.^36,37 However, because they are conventional cross-linked elastomers (PDMS, EcoFlex, etc.) they have limitations in that they are not resistant to unexpected damage.

In addition to their remarkable attachment performance and biocompatibility, the practical applications of SHP have encountered numerous challenges. Self-healing materials are primarily composed of dynamic bonds and have a low glass-transition temperature, making their performance inferior to that of non-self-healing materials.³⁸ This leads to poor mechanical properties, which make fabricating microstructures with SHP challenging because of fractures, defects, and elongation of the patterns. Furthermore, because of the increased chain dynamics above the glass transition temperature, maintaining the pristine structure over time has been a challenge for previous self-healable polymers.³⁹ Here, the development of functional small structured materials and interfaces with reversible adhesion, conductivity, and stretchability that can autonomously self-heal can pave the new way for versatile applications. To deal with the challenging issues, a new structure of the self-healing adhesive is essential for skin adhesive electronics.

In this work we propose an autonomous self-healing multi-layered adhesive patch with amphibious 3D micro-suctions inspired by octopus. Utilizing tough and water-resistive SHP based on polydimethylsiloxane (PDMS),^11,40 the thermoplastic SHP is patterned and maintained its structure by controlling the molar ratio of 4,4′-methylenebis(phenyl urea) (MPU) and isophorone bisurea (IU) moieties in the SHP. By placing an elastic layer between the upper and lower layers with excellent self-healing performance, mechanical properties and self-healing performance can be achieved simultaneously. The MPU rich middle layer (PDMS–MPU_0.8–IU_0.2) has excellent structural stability, which can be structured into an octopus inspired architecture and maintain initial state with good elastic recovery, as keeping the damaged area in close contact.⁴¹ Owing to excellent self-healing property of the optimized triple multi-layers, it can accelerate the contact and wetting between the two-dimensional interfaces of the bifurcated patterning layer, thereby improving the multi-layer self-healing property. In addition, the top layer enables stable fixation on rough skin owing to high flowability and adaptability. To understand the reflow phenomenon of self-healing 3D microstructures, we experimentally optimized the maintenance of the pillar pattern (diameter 100 μm, aspect ratio 1.3) structure, leading to the fabrication of an octopus-inspired hierarchical structure for conformal, reversible, contamination-free adhesion of deformable device. The octopus-inspired structures with a layer of very soft SHP can maintain high adhesion to diverse surfaces and rapid self-healing. With stable interfacial adhesion between the self-healable octopus-inspired adhesive (SOIA) and the deposited Au electrode, the multi-layered SOIA with electrode can measure electrocardiogram (ECG) signals during body movement under dry, wet, and even damaged conditions.

RESULTS AND DISCUSSION

Self-healable octopus-inspired adhesive

Figure 1A shows the tentacles of an Octopus vulgaris. In wildlife, skin and limb injuries are caused by capture, transportation, autodissection, mating, and competition. Cephalopods, including octopus, activate regenerative processes after wound healing in various organs, such as the cornea, peripheral nerves, and tentacles. In O. vulgaris, healing begins with muscular contraction and epidermal in-folding immediately after injury, with suckers near the lesion moving toward the wound. These can reduce the lesion size and lead to rapid healing (Figure 1A, (i)).⁴² To elucidate the mechanical and structural properties of octopus, the sucker structure consisted of a soft infundibulum and an elastic acetabulum. The suction chamber of the sucker can induce negative pressure through structural deformation to improve dry and wet adhesion (Figure 1A, (ii)). The soft infundibulum is vital for sustained attachment because of its soft tissue nature, which allows it to adapt to diverse surface roughness. This adaptability enables effective sealing and maintenance of negative pressure within the chambers.

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Inspired by wound healing and the mechanical and structural properties of the octopus, we devised a self-healing octopus-inspired architecture (Figure 1C). However, traditional dynamic polymers easily flow owing to their rapid relaxation dynamics, indicating a short time for rearrangement or flow to dissipate stress. This hinders the printing or molding of dynamic polymers into diverse shapes owing to their poor mechanical properties and structural stability. The relaxation timescale is material-dependent and can be tuned by modifying the bond chemistry. Therefore, we engineered relaxation dynamics by controlling the molecular design. We synthesized a self-healing elastomer with PDMS oligomers linked by an MPU unit to impart elasticity to better hold the elastomer together and an IU unit for effective energy dissipation (Figures 1B and S1). The microstructures of the polymers were verified using small-angle x-ray scattering (SAXS) (Figure 1D). All the polymers displayed specific peaks representing domain sizes (d) in the range of 6–7 nm. Increasing the number of MPU units increased the domain size. We also observed nano-scale morphologies of SHP as a function of MPU and IU unit content ratio using atomic force microscopy (AFM). In the AFM images, the clear phase-separation was shown and the width of fiber-like aggregation was increased from 2.78 nm (M_0.4I_0.6) to 8.75 nm (M_0.8I_0.2), which supports SAXS data, indicating an increase in domain size due to increased intermolecular urea bonding (Figures S2 and S3). Nanophase separation can improve the mechanical properties of materials.⁴³ In addition, increasing the number of MPU units may limit the terminal flow owing to reduced chain mobility through cooperative H-bonding and π–π stacking.⁴⁴ Therefore, the mechanical and rheological properties of the SHP can be controlled by tuning the molar ratio of the MPU and IU units.

To simultaneously obtain excellent self-healing and mechanical properties, we designed three self-healing elastomers with adjusted molar ratios of MPU and IU units (hereafter denoted as M_xI_1−x). (Figure 1E). Inspired by the adhesion and mechanical properties of the octopus tentacle, the thin top layer (M_0.2I_0.8) enabled stable fixation on rough skin and rapid surface self-healing owing to its fast self-healing properties and high adaptability. The elastic middle layer (M_0.8I_0.2) has high structural stability and is patterned with an octopus-inspired structure, which can induce a high negative pressure for dry/underwater adhesion (Figure 1E, (i)). The bottom layer (M_0.6I_0.4) can enhance the total patch self-healing property owing to its high self-healing property and the accelerating contact of the bifurcated patterning layer. Our tri-layered self-healing adhesive (t-SOIA) patch was resistant to unexpected damage owing to autonomous healing under ambient and underwater conditions (Figure 1E, (ii)). Thus, even if the adhesion value decreases owing to damage to the structure, the damaged area can be restored on its own to induce high adhesion (Figure 1F). The t-SOIA electrode was fabricated by depositing Au on the top layer via vapor deposition (Figure 1G). Electrical performance can also be restored by reforming the electrical pathway as the damaged area recovers. The t-SOIA electrode can be attached to sweaty and rough skin surfaces with high conformity to human skin and can detect the ECG signal under sweaty and underwater conditions. (Figure 1H).

Structural stability against flowing

PDMS–MPU–IU elastomers have excellent mechanical and self-healing properties, allowing them to be processed in a variety of ways, such as solution processing, molding, and bonding, using their self-healability at high and even room temperatures (~20°C).^11,40 However, above the glass transition temperature of the polymer, the viscosity decreases, flowability improves, and the structure reorganizes with minimal surface energy. Recent autonomous SHP with dynamic bonds have a very low glass transition temperature (T_g) and fail to maintain their structure owing to reflow when applied at room temperature or body temperature. Considering that the mechanical and rheological properties of our PDMS-based elastomer are highly dependent on dynamic bonds, we attempted to control the physical and rheological properties of the material by adjusting the sticker ratio to solve this problem.

Within same polymer backbone and dynamic bond concentration, by controlling the sticker ratio can adjust material morphology and intermolecular interaction. To investigate the mechanical and rheological properties of our samples, SHP with MPU-to-IU molar ratios of 8:2, 6:4, and 4:6 was synthesized. The Young's moduli of the films were calculated to be 0.827, 0.462, and 0.317 MPa (Figure S4). Polymer chain flow and segmental vibration play a pivotal role in self-healing. However, the high mobility of entangled chains degrades the mechanical properties of materials, whereas localized segmental motion can minimize mechanical performance weakening.⁴⁵ These polymer dynamics can be quantified by using a characteristic relaxation time, demonstrating polymer chain rearrangement and stress relaxation. For this reason, the rheological properties of the PDMS-based films were characterized by performing frequency sweeps at 20–100°C and performing time–temperature superposition (TTS) (Figure S5). In Figure 2A, master curves at 30°C depict the rheological behavior of each SHP. The crossover point between the loss modulus (G″) and storage modulus (G′) signifies the transition from liquid-like to solid-like behavior. The flow transition relaxation time (τ_f) was determined by observing the reciprocal frequency at the G′/G″ crossover point between the terminal and rubbery regions. The determined values were 2462 s for M_0.8I_0.2, 415 s and 198 s for M_0.6I_0.4 and M_0.4I_0.6, suggesting that M_0.8I_0.2 behaves more like a glassy solid than M_0.6I_0.4 and M_0.4I_0.6 (Figure 2B). As the proportion of MPU units in the polymer increased, the relaxation time of the polymer decreased. This is because of enhanced urea-urea bonding and π–π stacking within the MPU–MPU bonds, hindering chain rearrangement. This suggests that at short time scales, M_0.4I_0.6 exhibits greater relaxation and “flow” compared to M_0.8I_0.2. We also derive segmental relaxation time (τ_s) using the following equation: $J^{\prime }=G^{\prime }/{\left({\eta }^{*}\omega \right)}^2=\lambda /\left[{\eta }^{*}\right]$. where J′, G′, and [η*] represent the storage compliance, storage modulus, and complex viscosity, respectively. The τ_s was extracted from the λ value at 0.05 rad s⁻¹ (Figure S6).⁴⁶ We observed that calculated τ_s was similar within all ratios. Moreover, DSC analysis shows a constant T_g at around −124°C regardless of the MPU-to-IU ratio (Figure S7). These demonstrate that the MPU and IU segment relaxation is independent from the mechanical properties of this system. In addition, the Arrhenius temperature dependence was demonstrated by plotting the shift factors at multiple temperatures, which led to the estimation of the flow activation energy (E_a,flow) for each SHP (Figures 2C and S8). Owing to the strong hydrogen bonds between the MPU units, it was confirmed that the higher the MPU ratio, the higher the E_a,flow, resulting in resistance to flow. A similar tendency was observed when analyzing the degree of reflow of the actual pillar pattern over time (Figures 2D and S9). At temperatures above the T_g of the polymer, the pattern deforms until it reaches a thermodynamically stable state.

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The three patterns with the same AR value of 1.3 with diameter 100 μm showed different reflow conditions depending on MPU:IU ratio. Figure 2E depicts a schematic of the thermal reflow process of the pillar pattern transition between the initial shape (gray) and the intermediate shape (light gray). The confinement effect on the polymer reflow phenomenon is mainly attributed to the inherent rheological properties of the polymer. The reflow process includes the rounding of corners driven by Laplace pressure and the movement of the triple line along the substrate, which is influenced by interfacial tension and wetting forces.⁴⁷ Figure 2F is the data showing the experimentally obtained shape of the pattern after 7 days at 32°C. The lower the MPU ratio, the faster the pattern reflow occurs and the pattern becomes similar to the final shape. To quantitatively evaluate structural stability, we investigated the ability to maintain the structure over time using the rate of change in the pattern height compared to the initial height. Based on the reflow dynamics of the patterns, the experimental results for each MPU:IU ratio were analyzed with respect to the height of the pattern each time. Notably, both the speed and extent of the pattern reflow were significantly greater in terms of height than width, which is consistent with previous observations.^48,49 The reflow dynamics were determined by fitting the height over time to the following equation, together with the respective results for the pattern: $\ln \left(h_t\right)=\mathit{ln}{h}_{\infty }+\ln \left(h_0-h_{\infty }\right)-b/\mathit{De}$. Here, h_t is pattern height over time t, b is a compensation factor according to material properties, and De (= τ_f/t) is a Deborah number, which indicates a critical parameter for dynamic deformation of polymer materials. When process time of the pattern is shorter than τ_f (De >1), owing to material behaves like an elastic solid, no stress relaxation occurs. When observing time of the pattern is longer than τ_f (De <1), it behaves as a viscous liquid, and the dynamics of interchain interactions dominate.⁵⁰ For this reason, in Figure 2D, it can be seen that as the ratio of MPU of the self-healing polymer increases, its De increases, allowing that the aspect ratio of the structure is well maintained. As shown in Figure 2G, the rate of change of this phenomenon initially slows considerably over time. The dependent reflow properties of different SHP molar ratios, M_0.8I_0.2, showed good structural stability compared with the other ratios. On the other hand, in the cases of M_0.4I_0.6 and M_0.6I_0.4, it was difficult to maintain the structure because of the fast reflow rate and relatively low structural stability. This indicates that M_0.8I_0.2 is resistant to structural deformation and is suitable for various types of structuring.

Adhesion characteristics of the SOIA

Optical microscopy images of the SOIA at the three ratios are shown. The three SOIA with the same AR 1.3 value fabricated by the imprinting process also showed similar reflow tendencies depending on the MPU:IU molar ratio (Figures 3A and S10). In the cases of M_0.4I_0.6 and M_0.6I_0.4, dome structure deformation occurred rapidly after 48 h; however, in the case of M_0.8I_0.2, structural deformation hardly occurred until 48 h. This is closely related to the trend in adhesive performance. To understand the adhesion behavior in dry and underwater conditions, the adhesion strengths of the different samples (M_0.4I_0.6, M_0.6I_0.4, and M_0.8I_0.2) were measured on a silicon substrate with a preload of 3.5 N cm^–2 applied, as shown in Figure 3B.

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Assuming an axisymmetric peeling of the elastic film,⁵¹ the overall dry adhesion force on the dry surface (σ_o,dry) can be defined as the combined effect of the suction stress (σ_s) in ambient conditions, and the van der Waals force (σ_vdw) follows: (σ_o,dry) = (σ_s) + (σ_vdw).⁵² The suction stress under ambient conditions is determined by a simple volumetric change resulting from deformation behaviors, whereas the van der Waals force is calculated based on the actual contact area between the plane surface and the substrate.⁵² Under dry conditions, the SOIA film exhibited a higher adhesive strength (7.82 N cm⁻²) compared to the flat film (4.485 N cm⁻²), due to the impact of suction stress (Figures 3B and S11).

Unlike the dry surface caused by suction stress and van der Waals force, in case of underwater conditions, the overall adhesion force on the underwater condition (σ_o,underwater) combined with the suction stress (σ_s) and capillary stress (σ_c) play a dominant role (σ_o,underwater = σ_s + σ_c).³⁰ On the underwater conditions, the SOIA film exhibited a higher adhesive strength (4.43 N cm⁻²) compared to the flat film (1.69 N cm⁻²) (Figures 3B and S11). Based on the geometric factor (e.g., size and shape) and material properties, the suction adhesion (${\sigma }_s$) on the flat substrates is as follows⁵³: 1 $\begin{array}{rcl}{\sigma }_s& =& P_0\left(1-\frac{V_1}{V_2}\right)\\ \hspace{0.17em}& \cong & P_0\left[1-\left(\frac{{\left(r-v\bullet D_a\right)}^2\bullet \left(t-D_a\right)-V_{\mathit{\text{dome}}}}{r^{2}t-V_{\mathit{\text{dome}}}}\right)\right]\pi r^{2}n,\end{array}$ where P₀ is the initial pressure of the air sealed in an octopus-inspired pattern, V₁ is the deformed volume, V₂ is the volume of the final state, r is the radius of the octopus inspired architecture (OIA), t is the depth of the octopus-inspired cavities, v is Poisson's ratio of the polymeric materials, F_p is the applied preload, D_a is the vertical displacement of the OIAs, V_dome $(V_{\mathit{\text{dome}}}=\pi h_p\left(\frac{h_p^2+3r^2}{6}\right))$ is volume of the dome structure, h_p is height of the dome, and n is the number of populated octopus-inspired patterns (~5 × 10³ suckers cm^–2) as shown Figure 3C. The study identified that significant suction forces under diverse conditions were a result of numerous small contacts facilitated by the octopus-inspired adhesive present in both dry and underwater conditions. A finite element method (FEM) simulation was employed to theoretically analyze the attachment and restoring process of the SOIA (Figure S12). At the attachment stage, the volume of the SOIA chambers was easily reduced by applying normal force (3.5 N cm^–2) because the stress is concentrated on the wall of the SOIA. In the restoring stage, the volume of the SOIA chambers increased owing to the pulling deformation. This phenomenon is due to a specific elastic behavior caused by structural deformation due to preload control, which effectively removes air and water inside the chamber, increasing the possibility of suction occurring and improving the suction effect.^30,31,53,54 In Figure 3B, M_0.8I_0.2, which has good structural stability, did not undergo deformation of the dome structure, and its initial adhesive strength was maintained on the both dry and underwater condition even after 48 h. However, in the cases of M_0.6I_0.4 and M_0.4I_0.6, the adhesive strength decreased over time. This can be explained by the result of the volume change being reduced at the same preload, as the dome structure (V_dome) disappeared owing to the reflow. As Equation (1), decreasing the V_dome contributes to the decreased suction stress because of the decreased volume term (1 − V₁/V₂). This showed a similar trend in the adhesion performance of SOIA and hole pattern adhesive, and also showed significantly higher normal adhesion compared to various structures (Figure S11). Based on our experiments, we used M_0.8I_0.2 as the adhesive layer to achieve high adhesion performance and durability. To function effectively as a skin-attachable device, it is crucial for the interfacial adhesion of the device to maintain stable and conformal contact with the curved and rough skin. To increase the contact efficiency of a rough surface under dry and underwater conditions, the flexibility of the SOIA surface layer was modified by applying a soft, adaptable M_0.2I_0.8 layer (d-SOIA). Cross-sectional optical microscopy images of adhesion on a pigskin substrate demonstrated that the conformal contact provided by the soft layer on d-SOIA was superior to that achieved by a flat layer (Figures 3D and S13). Additionally, with the soft layer on top, the d-SOIA patch achieved superior conformal contact compared to the case without the soft layer, which is very important for improving the suction effect without leakage. (Figure 3E). As shown in Figure 3F, the adhesion of the d-SOIA patch in the normal direction was higher than those of the other three patches under both dry and underwater conditions in the pigskin replica (Figure S14). In addition, the d-SOIA patch maintained highly under repeated mechanical attachment–detachment cycles (>100) on the pigskin replica substrate under both dry and underwater conditions, as shown in Figure 3G. To investigate the autonomous adhesion healing ability of our adhesive patch, we monitored its adhesion strength in response to mechanical damage (Figure 3H). First, we induced a surface-level cut using a scalpel to damage the pattern layer (40 applications). Interestingly, we observed the autonomous self-healing of the pattern and adhesion strength after 24 h without any force to push into contact. This is thought to be the result of the rapid self-healing properties of the M_0.2I_0.8 layer. In addition, the d-SOIA patch (area: 4 cm × 4 cm) maintained good attachment on rough and hairy skin (weighing ~400 g on rough sweaty skin and ~500 g on rough dry skin) (Figures 3I, S15 and Videos S1–S3). Even if the pattern is damaged and a certain level of adhesion is lost, subsequently regained over time.

To date, most self-healing materials are limited to applying a dry adhesive layer, owing to the trade-off relationship between the self-healing properties and mechanical strength.⁵⁵ To deal with the trade-offs in fabricating SOIA, we designed a tri-layered self-healing film that consists of (1) a bottom layer (M_0.6I_0.4) for high self-healing efficiency, (2) a middle layer (M_0.8I_0.2) for patternability, and a top layer (M_0.2I_0.8) for conformal contact with human skin, which achieves controlled mechanical properties with high self-healing ability. Figure 4A shows a schematic of the self-healing process of the damaged tri-layer film. Strong and weak reversible hydrogen bonds were randomly formed between the MPU and IU moieties of the self-healing polymer, respectively. The tri-layered film was optimized by controlling the thickness ratio between M_0.6I_0.4 and M_0.8I_0.2 films. The thickness of the soft M_0.2I_0.8 layer is reduced to 20 μm, which is the minimum thickness necessary to accommodate the curved surface of the skin. As shown in Figure 4B, the optimal thickness ratio of the tri-layer film was determined by considering healing efficiency and Young's modulus. The Young's modulus of the tri-layered film was decreased by reducing the thickness ratio of the M_0.8I_0.2 layer to maintain the overall bulk film elasticity and micropatterning (see gray line in Figure 4B). In contrast, the healing efficiency of the polymer was determined by increasing the thickness ratio of the M_0.6I_0.4 layer because the soft SHP promotes the re-contact of the damaged regions of the patterned hard self-healing elastomer surface (see red gradient bar of Figures 4B and S16–S18). Consequently, considering the self-healing and mechanical properties, the optimal thickness ratio of M_0.8I_0.2:M_0.6I_0.4 in the tri-layered film was chosen to be 1:1, which has over 90% healing efficiency and proper mechanical properties for bioinspired self-healing adhesive architectures. The controlled tri-layered film had a good mechanical durability on multiple stretching cycles (Figure S19). In addition, it was confirmed that t-SOIA showed almost the same adhesion performance when compared to d-SOIA (Figure S20). As a result, t-SOIA exhibited higher adhesion performance with self-healing performance that previously reported octopus-inspired structures could not achieve (Figure S21).

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The difference in the modulus of the optimized tri-layered film was verified by peak-force QNM analysis using AFM, as shown in Figure 4C. We observed the clear interfaces of the cross-sectional tri-layered film in peak-force QNM image. To calculate the adhesion force of each interface of the multi-layered films, we measured peeling forces of the laminated films (1.M_0.8I_0.2/M_0.8I_0.2, 2.M_0.6I_0.4/M_0.8I_0.2, and 3.M_0.6I_0.4/M_0.6I_0.4) using a 180-degree peeling method. The interfacial toughness (adhesion force) values were over 3000 J m^–2 for all films (Figure S22). We also observed that the scar on the cut t-SOIA had almost disappeared after healing at 30°C for 48 h (Figure 4D). Furthermore, the healing efficiency of the patterned film was similar to that of the flat (non-patterned) film (Figure S23). To apply the tri-layered film to a self-healing dry adhesive patch for in situ biosignal monitoring, we deposited an Au electrode on top of the tri-layered film using vapor deposition (Figure S24). Before deposition of the Au electrode, oxygen plasma was treated on the tri-layered film surface to improve the adhesive properties of the metal-polymer interface. The surface energy of all the film was calculated using the Owens–Wendt method with two fluids: DI water and diiodomethane. A longer plasma treatment time led to a lower contact angle of the DI water, and the surface energy simultaneously increased (Figures 4E and S25). After 10 s of oxygen plasma treatment, the surface energy of the tri-layered film was saturated, which forms a good interface with the Au electrode. Furthermore, the electrode thickness was optimized to 50 nm by comparing the electrical conductivity with the gold deposition thickness (Figure 4F). To investigate the electrical healing capabilities of the t-SOIA electrode, it was completely cut using a scalpel, as shown in Figure 4G. The two fractured surfaces were re-contacted for 48 h (Figure 4H, top OM images). The disconnected conducting path of the Au electrode on the tri-layered film autonomously self-healed, and the conductivity of the electrode almost reached that of the pristine state (>10⁴ S cm^–1) after 18 h (Figure 4H, bottom graph). Furthermore, the conductivity of self-healing electrode was almost maintained up to 30% without mechanical damage and it was durable to multiple stretching up to 10 000 times at 30% strain as shown in Figures 4I and S26, respectively.

Biosignal monitoring in dynamic, harsh environments

Figure 5 shows the practical application of the t-SOIA electrode in ECG monitoring systems. ECG signals, a crucial vital sign, were measured using a t-SOIA patch deposited with an Au electrode and attached to the skin, thus demonstrating the application of the t-SOIA electrode for biosignal monitoring (Figure 5A). One of the distinctive features of the t-SOIA electrode is that, unlike conventional ECG electrodes, it can be attached to both dry and underwater environments without causing skin irritation while possessing self-healability, reusability, and durability. To evaluate the epidermal damage caused by our adhesive and commercial electrodes, changes in the skin surface were examined when both patches were attached and removed after the same amount of time (Figure 5B). Although the t-SOIA electrode achieved clean adhesion, the application of commercial electrodes resulted in allergic reactions such as itching, stickiness, and redness on the affected skin area. In addition, with high adhesion strength under wet and rough skin, our t-SOIA electrode then resists delamination from the wet skin of our volunteers against flowing water (Figure 5C and Video S4). Figure 5D illustrates the in-situ electrocardiogram signals of the PDMS–OIA and t-SOIA electrodes. To demonstrate real-time self-healing during operation, a scalpel was used to completely dissect the deposited electrode and wire of the PDMS–OIA and t-SOIA. PDMS–OIA did not recover to its initial state even after a sufficient amount of time after damage and did not show a signal; however, in the case of t-SOIA electrode, a slight signal was observed after 12 hours, it recovered to its original signal after 18 h. During the healing process, the ECG signal reflects the disconnected conductive pathway and rapidly stabilizes to show a certain signal value, indicating completion of the self-healing process. Figure 5D shows an enlarged image of the self-healing ability of the damaged t-SOIA electrode at room temperature for 18 h. After 18 h, the scar on the cut film almost disappeared; that is, it self-healed. Notably, optical microscopy images of pristine (Figure 5D, (i)) and self-healed Au electrode (Figure 5D, (iii)) nano-networks validated the reconstruction of the conductive networks after 18 h at room temperature. The conformality of the t-SOIA electrode was also maintained during light and extremely dynamic human motions while playing beach volleyball (e.g., walking, running, receiving, and diving), facilitating real-time tracing of the heart rate and heart rate variability data (Figures 5E, S27 and Video S5).^56,57 This allows continuous, non-invasive, time-efficient, and low-cost monitoring of changes in the autonomic nervous system and cardiovascular health.^58,59 Owing to the hydrophobicity of the t-SOIA patch, it could be reused by simply washing out the sea sand and dust on its surface (Figure 5F). Moreover, shallow scratches caused by sand on the outer surface were well recovered at room temperature owing to the excellent self-healing performance.

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CONCLUSION

We proposed a self-healable, stretchable, reusable, and skin-attachable patch inspired by an octopus. We established the conditions under which the SHP could be patterned and maintained its structure based on the MPU:IU molar ratio. To understand its pattern reflow phenomenon, we experimentally optimized the maintenance of the pillar pattern (diameter 100 μm, aspect ratio 1.3) structure, leading to the fabrication of an octopus-inspired hierarchical structure. The octopus-inspired structures with a layer of highly soft SHP maintained high adhesion against diverse nonflat and soft surfaces and rapid surface self-healing. With stable interfacial adhesion between the t-SOIA and the deposited Au electrode, the t-SOIA could measure ECG signals during body movement under dry, wet, and even damaged conditions.

EXPERIMENTAL SECTION

Synthesis of self-healing elastomers

The aminopropyl terminated polydimethylsiloxane (H2N-PDMS-NH2, MW = 5000, 50 g) was dissolved in chloroform (CF, 300 mL) solvent and stirred for 1 h to decrease temperature to 0°C in N₂ atmosphere. Triethylamine (Et3N, 5 mL) was added to solution and stirred for 1 h. After stirring process, 4,4′-methylenebis(phenyl isocyanate) (0.5–2 g, 0.2–0.8 eq) and Isophorone diisocyanate (1.8–0.45 g, 0.8–0.2 eq) were mixed in CF and added to solution with droplet phase. The solution with all components added was stirred for 1 h at 0°C at an appropriate rate to ensure mixing throughout the solution. The temperature was gradually decreased to room temperature and kept stirring for 3 days. A final product has to be purified through evaporation—precipitation—dissolution steps. Before purifying process, methanol (10 mL) was added to mixture solution and stirred for 30 min for removing non-reacted isocyanate. Then, the solvent is evaporated using a rotary vacuum evaporator until only half the volume of the total solution remains. The concentrated solution is aged until the upper precipitates settle to the bottom. The upper solution excluding the precipitate was removed and CF (50 mL) was added to dissolve the residuals. After repeating the above processes three times, the final product was poured into a Teflon mold and dried in vacuum overnight to remove residual solvent.

Fabrication of PDMS–MPU_x–IU_1−x (x = 0.2, 0.4, 0.6, and 0.8) films

The typical procedure for the preparation of PDMS–MPU_x–IU_1−x polymer films is as follows: 3–5 g of PDMS–MPU_x–IU_1−x was dissolved in 15–20 mL of CHCl3 and stirred at 50°C. The resulting viscous solution was stirred for more than 3 h and then gradually cooled to room temperature. The resulting solution was poured onto an OTS-treated Si substrate (4 in.) and dried at room temperature for 6 h, followed by drying at 80°C under reduced pressure (approximately 100 Torr) for 3 h. Polymer films were then peeled off after being cut into certain dimensions and ready for mechanical testing.

Fabrication of bi- and tri-layered films

Self-healing monolayered films were prepared with optimized thickness ratios of 2:1, 1:1, and 1:2 (PDMS–MPU_0.6–IU_0.4:PDMS–MPU_0.8–IU_0.2). A press machine was used for 1 h to create conformal contact between the two films and remove the bubble traps. After pressing, the bilayered film was placed on a hot plate and heated for 1 min to induce stress relaxation. For the tri-layered film, PDMS–MPU_0.2–IU_0.8 film was spin-coated onto OTS-treated silicon substrates and transferred to top of the bilayered film (PDMS–MPU_0.8–IU_0.2 side).

Fabrication of the PDMS–OIA and SOIA patch

To fabricate the octopus-inspired pattern master, a silicon mold with microhole patterns (diameter of 100 μm and width-to-depth ratio of 1) was prepared using photolithography and subsequent reactive ion etching. The mold was treated with a fluorinated self-assembled monolayer (SAM) solution (FOTCS) diluted to 0.03 M with anhydrous heptane (Samchun Chemical Co.) under an argon atmosphere. S-PUA liquid droplets were dispensed onto a mold with a polyethylene terephthalate (PET) film as the backplane, and a partial filling technique controlled the geometry, surface properties, and pressure to produce a polymeric master with octopus-inspired architectures, as previously studied.⁵¹ Air bubbles were trapped in the microhole chambers owing to the viscous properties and interfacial tension of the prepolymers. Consequently, the as-prepared polymeric master with an octopus-inspired architecture can be obtained by simple replication. The elastic PDMS precursor was poured into the prepared mold. After thermal curing at 80°C for 2 h, a PDMS patch with an octopus-inspired architecture was obtained. The SOIA patch was fabricated by exploiting the moldable features and bonding properties of the polymer at high temperatures. For the bi- and tri-layer films, the bonding process involved annealing at room temperature for 6 h after applying gentle pressure to keep the two pieces in good contact and obtain a stable interface. The polymer film on the substrate was pressed using a positive PDMS–OIA mold at 80°C and allowed to rest for 2 h. Then, after removing the PDMS mold, a successful pattern with an OIA pattern was confirmed, in which the thickness of the film was ~400 μm.

ACKNOWLEDGMENTS

The authors gratefully acknowledge support from the National Research Foundation of Korea (NRF-2021R1C1C1009925, 2020R1A6A1A03048004, and RS-2023-00214236). This work was supported by the Market-led K-sensor technology program (RS-2022-00154781, Development of large-area wafer-level flexible/stretchable hybrid sensor platform technology for form factor-free highly integrated convergence sensor), funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). This work was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIT) (No. CRC230231-000).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

The data that support the plots within this paper and other finding of this study are available from the corresponding author upon reasonable request.

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