1. Introduction
Human organs are highly specialized tissue structures performing particular distinctive functions. In the case of dysfunctional organs, clinical treatments are often limited by a scarcity of available donors and by immune rejection of donated tissue [1]. To overcome the lack of available transplantable organs, tissue-engineering approaches are used, which face some challenges [2]. Borrowing the concept of three-dimensional (3D) printing from additive manufacturing technologies, whereby a digital design for a 3D structure is fabricated layer by layer following the bottom-up approach, 3D bioprinting is now being pursued as a potential solution to some of the challenges faced in tissue-engineering methods [3,4,5,6,7,8]. Layer-by-layer precise positioning of biological materials, and biochemical and living cells, with spatial control of the placement of functional components, is used to fabricate 3D tissue structures [9].
A typical bioprinting process consists of three major steps, namely pre-processing, processing, and post-processing (Figure 1). Pre-processing involves imaging of the tissue or organ and the reconstruction of 3D models from the imaging (Figure 1a–d). Multidetector computed tomography (MDCT) is widely used for rapid prototyping because of its simpler image-processing requirements. Cone-beam computed tomography (CBCT), positron emission tomography (PET), single-photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), and ultrasonography (US) are other non-invasive imaging modalities [10]. The processing step involves the bioprinting process using an appropriate bioink [11,12,13,14] (Figure 1e–g). The bioprinting process can be classified into four different categories, including laser-based bioprinting [15,16,17,18,19,20,21,22,23], droplet-based bioprinting [24,25,26,27,28,29,30,31], extrusion-based bioprinting [28,29,32,33,34,35,36,37,38,39,40,41,42], and stereolithography-based bioprinting [13,43,44,45,46,47,48,49,50]. Post-processing involves maturation of the bioprinted tissue before its intended use [9,51] (Figure 1h–i).
2. Laser-Based Bioprinting
The main components of a laser-based bioprinter are the laser source, a laser transparent print ribbon coated with a layer of cell-laden bioink, and a substrate or collector slide on a motorized stage. The energy from the laser is utilized to pattern cell-laden bioinks in a three-dimensional spatial arrangement with the aid of computer-aided design and manufacturing (CAD/CAM). The high resolution and reproducibility of this process makes it a viable option for use in biomedical applications [53]. Some of the variations of this method based on the type of laser source and laser transparent print ribbon are shown in Table 1. Stem-cell grafts, skin tissue, multicellular arrays, and biopapers were reported to be printed using this method [23,54]. The major advantage of laser printing is the non-contact process. This eliminates nozzle clogging and also results in high cell viabilities [55]. However, there are several disadvantages of laser-based bioprinting, which outweigh the advantages. Laser exposure on the cells is not without risk, and the use of metal to absorb the laser energy can induce cytotoxicity [53].
3. Droplet-Based Bioprinting
Such a process ejects cell-laden bioink out of the nozzle onto a substrate in the form of droplets [51]. Inkjet printers are one of the most commonly used type for both non-biological and biological applications [9]. Inkjet printers can use thermal [27] or acoustic [56] forces, among others, to eject drops of liquid onto a substrate [9], as seen in Table 2. One of the major advantages of droplet-based bioprinting is its compatibility with a wide variety of biological materials. Furthermore, such bioprinters provide high resolution (20–100 µm) and speed (1–10,000 droplets/s) while being a low-cost alternative [29]. For example, a high-throughput cell printing system was demonstrated for drug screening [30,57,58,59,60,61]. At the same time, a major drawback of this technique is the requirement for the biological material to be in a liquid and less viscous form [29], which may not always be the case.
4. Extrusion-Based Bioprinting
In extrusion-based bioprinting, the bioink is extruded out of the nozzle using pneumatic pressure or mechanical force. The biggest advantage of extrusion-based bioprinting is the scalability due to the continuous flow of bioink and large deposition rate (Table 3). At the same time, the resolution of this method is lower than other methods [62,63]. While the printability of high-viscosity bioinks and high cell concentrations is an advantage [37], the inherent nozzle-clogging problem is a disadvantage [64]. Due to their low cost and simple-to-use nature, extrusion-based bioprinters are the most widely used of all bioprinters [51] (Table 4). Cell-laden constructs with tunable 3D microenvironments were constructed by bioprinting gelatin methacryloyl (GelMA)/alginate core/sheath microfibers using extrusion-based bioprinting and subsequent ultraviolet (UV) cross-linking [40]. Further stabilization strategies in extrusion-based bioprinting were also reported, in order to successfully complete the printing of intact, accurate, and biologically relevant constructs with desirable properties [65].
5. Stereolithography-Based Bioprinting
In stereolithography-based bioprinters, UV light is used to cure layers of photopolymer, stacks of which form the 3D object (Table 3). The biggest advantage of stereolithography in general and stereolithography-based bioprinting in particular is its very high resolution. Other advantages include high cell concentrations and no problem of nozzle clogging. The preparation of three-dimensional biodegradable poly(ethylene glycol)/poly(d,l-lactide) hydrogel structures using stereolithography at high resolutions was shown [46]. Cell-encapsulated hydrogels were also shown to be 3D-printed using stereolithography [48]. Cell-attachable and visible-light cross-linkable bioinks, based on gelatin methacryloyl (GelMA) with eosin Y (EY) photoinitiation, for stereolithography three-dimensional (3D) bioprinting were developed and used to print cell-laden hydrogels [13]. However, there are many disadvantages of this method. The biggest disadvantage is that only photocurable bioinks can be used. Another disadvantage is that the cells will get exposed to harmful UV light, which affects the cell viability [44].
6. 3D-Printed Tissues and Organs
Bioprinting was used to generate two-dimensional (2D) and 3D structures for various purposes, including fabrication of scaffolds and tissue constructs for tissue regeneration (Table 5). Some examples of printed tissues are shown in Figure 2. Markstedt et al. [66] printed shapes resembling human ear and sheep mensci using a bioink containing alginate and nanofibrillated cellulose and an inkjet-based 3D bioprinter which is largely applied for fibrous nanomaterials packaging [67]. Duan et al. [62] printed aortic valve conduits using hydrogel-based bioinks laden with aortic root sinus smooth-muscle cells and aortic valve leaflet interstitial cells and an extrusion-based 3D bioprinter. Table 5 summarizes the applications of 3D bioprinting in tissue engineering.
Therefore, it can be said that bioprinting holds tremendous potential and is fast moving toward fully functional 3D-printed organs. For example, in the future, chronic toxicological diseases that are majorly due to industrial particulate pollutants such as pulmonary fibrosis could be cured by transplanting 3D-printed lungs from patients’ own programmed cells. Life expectancy can be increased because patients will not be left waiting until a suitable organ is available from an organ donor. Body cells taken from patient blood or from a skin biopsy will be transported to a laboratory [87]. Here, cells will be programmed into routine culture to be transformed into diseased organ cells (e.g., lung cells) and will be expanded in volume/number for the 3D bioprinting to resemble a lung after a few weeks. After maturation into sterile culture conditions, the artificial lung will be ready to be implanted inside the patient (Figure 3) to replace the dysfunctional organ [88]. This whole process will take just a few months and will also produce personalized organs for the patient from their own cells. This will reduce the possibility of rejection by the body, and the patient will not have to spend the rest of his/her life on anti-rejection drugs and having to deal with all of the associated side effects. Petersen et al. [89] (Figure 3) proposed using scaffolds of extracellular matrix from lungs of adult rat that retain the hierarchical branching with cellular components removed. A bioreactor was used to culture pulmonary epithelium and vascular endothelium on the acellular lung matrix, resulting in hierarchical organization within the matrix and efficient repopulation of the vascular compartment. When implanted into rats in vivo for short time intervals (45 to 120 minutes) the engineered lungs participated in gas exchange, although the inflation of engineered lung was found to be less than that of the native lung, and some bleeding and clotting was observed. While this represents a step toward developing a viable strategy for generating fully functional lungs in vitro, there remains the issue of extracting scaffolds from lungs, among others. This is where 3D bioprinting can help by developing patient-specific, on-demand biological scaffolds.
The entire process involves nano- and micro-to macroscale bottom-up engineering [90,91] using a simple desktop 3D printer. This opens up the possibility that, one day, we will be able to bioprint amputated sub-organs, missing organs, and digitally designed cosmetic body parts. Instead of using plastics to print structures, researchers will use living cells mixed with biocompatible scaffolds to build living tissue inside a sterile safety cabinet to keep the cells protected from harmful foreign substances. In this context, it is also important to discuss the relevance of four-dimensional (4D) bioprinting, the fourth dimension being time [92]. While 3D bioprinting is set to make our lives easier by printing required living tissue on demand, in some cases, it may lose relevance if it is too time-consuming. Therefore, time taken to create the end-product is an important parameter to consider when judging the effectiveness of bioprinting processes. Furthermore, it was suggested that a universal bioink would be a significant technological advancement that could standardize the bioprinting field and accelerate the realization of human tissue product biomanufacturing [93]. With advancement in 3D printing, parallel advancements in 3D bioprinting can also be expected in the future. For example, dip-pen nanolithography is being developed, combining advantages of electron beam lithography, inkjet printing, and microcontact printing [94]. Such methods also allow the parallel application of different inks, which may be useful for printing complex tissue structures when integrated with in silico modeling [95,96].
7. Future Outlook: 3D Bioprinting Air–Liquid Interface (ALI) as an Artificial Material for Nanotoxicity Assessment of Particulate Matter
With the huge potential that 3D bioprinting holds, other applications apart from tissue/organ regeneration can be realized [97], for example, printing a lattice-like membrane, which can act as a biological tape. Such a membrane when placed in a culture microincubator could be used to recreate the microenvironment of the human body. To mimic the air–liquid interface in vivo, 3D bioprinting can build living lung-like tissues via printing the inside of the incredibly intricate branching network of tubes [98,99]. Each of these tubes ends in a tiny structure of air sacs/pouches where oxygen and carbon dioxide are exchanged, which gives an idea of just how complex this structure is. There are 300 million of these tiny air sacs in each lung [100], which makes it a very challenging structure to bioprint. However, a part of it can be printed to be used as a very useful model for toxicological research [101]. Another example is an asthma attack, where patients breathe in certain particulate allergens (micro- and nano- to macroscale particles/airborne spores) which aggravate muscle contraction of reduced-diameter airways [102]. In the future, starting from those airway muscle cells, one can recreate them in the lab and print them into tubular structures [103]. Further incubating these constructs in a suitable microenvironment to mature into similar functional airway muscles is possible via adding a stimulatory compound like histamine. It is released in asthmatic patient airways during an asthma attack, causing muscle contraction. We will be able to 3D print an airway muscle tissue mimicking the biological lifelike contraction and, thus, test advance therapeutics to reverse the contraction, relaxing the air tube as an anti-asthma drug does in patients. The fact that drug tests can be performed in these tissues is a very important point, because the drug development industry faces a big challenge of human trials after testing in vitro cells grown in petri dishes and preclinical tests in in vivo rodents. Rodents such as mice and rats respond very differently to test therapeutic compounds than humans do [104]. There is a huge chasm between the preclinical tools that we test the drugs on and the humans for whom the drugs are designed to help. Therefore, 90% of drugs that show promise in animals actually fail to work in humans, usually because they are just not effective at fighting disease or, sometimes, they are downright toxic [105]. These 3D-printed tissues can help the drug development process by enabling pharmaceutical companies to test these compounds in tissues that reproduce the complexity of the human body [106]. This will save lives by providing better drugs to patients faster and for less expense. It also has an ethical and moral impact, because research can drastically reduce the number of animals that are used for drug development. By the year 2050, it is estimated that the meat and leather industry combined will need around one hundred billion farm animals to supply us with our animal-based needs such as meat, leather, milk, etc. [107]. To supplement those kinds of needs, animal cells can now be grown in the laboratory in just the same way as human cells; thus, there is potential here to replace a large proportion of these animals using bioprinted cells. We can differentiate them into muscle-like cells and then print those cells into meat products. The first bioprinted beef burger was revealed back in 2013, although incurring high costs (approximately $300,000) [108]. As technology moves forward rapidly, bioprinted leather is also a potential use for this technology. Skin cells can be grown, and the industry could generate customized leather products with specific thicknesses or textures or colors, making it feasible for the potential replacement of animal products by even better bioprinted animal products [109].
Laser-Based Bioprinting | ||||||
---|---|---|---|---|---|---|
Category | Laser-Induced Forward Transfer (LFT) | Absorbing Film-Assisted Laser-Induced Forward Transfer (AFA-LIFT) | Biological Laser Processing (BioLP) | Matrix-Assisted Pulsed Laser Evaporation Direct Writing (MAPLE-DW) | Laser-Guided Direct Writing (LG DW) | |
Difference | Laser transparent print ribbon | With thin metal layer | With thick metal layer | With biopolymer layer | / | |
Laser pulses | High power | High power | High power | Low power | / | |
CCD camera | / | / | Included | / | / | |
Optical fiber | / | / | / | / | Included or not included | |
Advantages | Overall |
1. High cell viability 2. High resolution 3. High cell densities 4. Low-viscosity cell suspensions | ||||
Individual | / | Thick metal layer reducing the risk of laser energy on cells damage | Biopolymer facilitating initial cell attachment | / | ||
Disadvantages |
1. A risk of photonic cell damage 2. Scalability limitation 3. Fabrication of the laser print ribbon 4. High cost of laser system 5. Complexity of controlling the laser pulses |
Droplet-based Bioprinting | |||||
---|---|---|---|---|---|
Category | Inkjet Bioprinting | Electro-Hydrodynamic Jetting-Based Bioprinting | Acoustic Bioprinting | Microvalve Bioprinting | |
Continuous Inkjet | Drop-on-Demand | ||||
Trigger Difference | Pneumatic actuator | Thermal, piezo-electric, electrostatic actuator | Electric field | Acoustic actuator | Pneumatic actuator |
Advantages |
1. High resolution 2. High printing speed 3. Affordability 4. Cell concentration gradient |
1. High resolution 2. High-viscosity bioink |
1. Without detrimental stressors 2. High resolution 3. High printing speed |
1. Synchronized ejection from different print heads | |
Disadvantages |
1. Low-viscosity bioink 2. Nozzle clogging 3. Droplets cannot be controlled precisely |
1. Low-viscosity bioink 2. Nozzle clogging |
1. Electric field might affect the long-term cell viability 2. Precise spatial placement of cells is onerous |
1. Not too high a viscosity of bioink 2. Not too high a cell concentration |
1. Nozzle clogging 2. Low resolution 3. Damage of cells |
Category | Extrusion-Based Bioprinting | Stereolithography-Based Bioprinting |
---|---|---|
Trigger difference | Pneumatic pressure or mechanical force | Light (usually UV) irradiation |
Advantages |
1. Scalability 2. High-viscosity bioink 3. High cell concentration |
1. Highest resolution 2. Reduced printing time |
Disadvantages |
1. Lowest resolution 2. Nozzle clogging 3. Shear-thinning bioink |
1. Nozzle clogging 2. Photopolymerizable bioinks or bioinks containing UV-activated photo initiated damage of cells 3. UV irradiation damage of DNA and promotion of cell lysis |
Properties | Laser-Based Bioprinting | Inkjet Bioprinting | EHD Jetting-Based Bioprinting | Acoustic Bioprinting | Microvalve Bioprinting | Extrusion-Based Bioprinting | Stereolithography-Based Bioprinting |
---|---|---|---|---|---|---|---|
Bioink viscosity | 1–300 mPa·s | 3–12 mPa·s | 1–1000 mPa·s | NA | 1–200 mPa·s | ~600 kPa·s | ~5 Pa·s |
Cell density | 108 cells/mL | 106 cells/mL | 106 cells/mL | 106 cells/mL | 106 cells/mL | 108 cells/mL | >106 cells/mL |
Speed | 200–1600 mm/s | 10,000 droplets per second | 10–500 mm/s | 10,000 droplets per second | 1000 droplets per second | 10–50 μm/s | High |
Resolution | 50 μm | 50 μm | 100 nm | 37 μm | – | 100 μm | 200 nm–6 μm |
Accuracy | High | Medium | Low | Medium | Medium | Low | High |
Cell viability | >95% | >80% | >80% | >90% | >80% | 40–95% | 25–85% |
Structural integrity | Low | Low | High | Low | Low–medium | High | Medium–high |
Scalability | Low | High | High | Medium | High | High | Medium–high |
Cost | High | Low | High | Medium–high | Medium | Low–medium | Medium |
Tissue | Techniques | Cell Types | Growth Factors | Materials | References |
---|---|---|---|---|---|
Heart valve | Extrusion-based bioprinting | Aortic valve interstitial cell
Aortic root sinus smooth-muscle cell | – | Hyaluronic acid
Gelatin Alginate | [62,68] |
Myocardial tissue | Extrusion-based bioprinting | Cardiomyocyte progenitor cell | – | Alginate | [69,70] |
Blood vessel | Jetting-based bioprinting | Endothelial cell
Smooth-muscle cell Mesenchymal stem cell | – | Fibrin | [71,72] |
Extrusion-based bioprinting | Endothelial cell
Cardiac cell Smooth-muscle cell Fibroblast | – | Collagen
Agarose Alginate | [52,73,74] | |
Musculo-skeletal tissue | Jetting-based bioprinting | Muscle-derived stem cells
Myoblast Mesenchymal fibroblast | BMP-2
FGF-2 | Fibrin | [75,76,77] |
Extrusion-based bioprinting | Bone marrow stromal cell
Endothelial progenitor cell Endogenous stem cell | TGF-β | Agarose
Alginate Hydroxyapatite Polycaprolactone | [78,79] | |
Nerve | Jetting-based bioprinting | Embryonic motor neuron cell
Hippocampal cell Cortical cell Neuronal precursor cell Neural stem cells | CNTF
VEGF | Soy agar
Collagen Fibrin | [80,81] |
Extrusion-based bioprinting | Bone-marrow stem cell
Schwann cells | – | Agarose | [82,83] | |
Skin | Jetting-based bioprinting | Dermal fibroblast
Epidermal keratinocyte | – | Collagen | [84] |
Extrusion-based bioprinting | Epitheleal progenitors | EGF BMP-4 | Gelatin | [85] | |
Bone | Extrusion-based bioprinting | Human mesenchymal stem cells | – | GelMA | [86] |
Funding
This research received no external funding.
Acknowledgments
A.V.S. thanks the Max Planck Society for the grassroot project grant 2017 (M10335) and 2018 (M10338).
Conflicts of Interest
The authors declare no conflicts of interest.
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Abstract
Borrowing the concept of three-dimensional (3D) printing from additive manufacturing technologies, whereby a digital design for a 3D structure is fabricated layer by layer following the bottom-up approach, 3D bioprinting is now being pursued as a potential solution to some of the challenges faced in tissue-engineering methods [3,4,5,6,7,8]. Cone-beam computed tomography (CBCT), positron emission tomography (PET), single-photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), and ultrasonography (US) are other non-invasive imaging modalities [10]. Table 5 summarizes the applications of 3D bioprinting in tissue engineering. [...]it can be said that bioprinting holds tremendous potential and is fast moving toward fully functional 3D-printed organs. While 3D bioprinting is set to make our lives easier by printing required living tissue on demand, in some cases, it may lose relevance if it is too time-consuming. [...]time taken to create the end-product is an important parameter to consider when judging the effectiveness of bioprinting processes.
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Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer