Recent Advances in 3D Bioprinting of Human Organs

On the current basis organ shortage has been a medical challenge due to the scarcity of donors and patient immune rejections, furthermore, is as been hard to mimic or know the condition of diseases in animal models during preclinical studies, due to disease phenotype which differs between animal and humans. Numerous biological applications have made use of the quickly developing field of 3D bioprinting. It is different from the traditional 3D printing, that utilizes bioinks comprising of cells and various biomaterials allowing for the generation of functional complex tissues. Bioprinting deals with computational modelling, preparation of bioink, bioink deposition, and lastly subsequent printed product maturation; it is a complex procedure where the type of bioprinter, bioprinting technique, and composition of the bioink must all be taken into account while developing the build. This amazing technology has fully established success in human studies, where by varieties of functional complex tissues have been generated for both vivo and vitro applications. However, the major driving force behind this innovation has been utility in human organs and tissues.

On recent outlook the transplantation of organs has been an optimal treatment approach for patients with end stage organ failure, which also out here there has been shortage of organ donors which has been a posed challenge. It is challenging to fully replicate the intricate cellular milieu for drug testing in two-dimensional cell culture and animal trials. 3D bioprinting has been a rapid growing manufacturing technology to forge constructs, which includes; extrusion-based, jetting-based, vat photopolymerization-based methods and also other rising three-dimensional (3D) bioprinting organs and tissues in screening and transplantation summarized. Lastly, we will discuss the prospect and challenges of three-dimensional (3D) bioprinting of organs and tissues. The goal of this review is to make it easier to overcome the challenges encountered during the difficult process of producing and utilizing tissues and organs.

Introduction

Tissue damage and degeneration is a rather common phenomenon among humans; however, the regenerating capabilities of human body are rather insufficient to deal with this trauma. The traditional methods for treating these conditions are dependent upon tissue or organ transplantation which is again dependent upon the availability of a donor which can be rather scarce and comes with the risk of graft rejection due to immune response. 3D bioprinting is a rapidly evolving industry that has the potential to reshape regenerative medicine said by Tasnim et al., (2018). Tissue engineering and regenerative medicine are rapidly evolving fields that work toward solving these issues, statement suggested by Bose, Roy, and Bandyopadhyay (2012). Bose and Bandyopadhyay (2019) had clarified that additive manufacturing is one of the most advanced techniques that has been utilized in this area of tissue engineering. It encompasses the principles of material science with biology for the fabrication of organ and tissue framework (Bose & Bandyopadhyay, 2019;  Bandyopadhyay, Zhang, & Bose, 2020). Its primary objective is the restoration of damaged tissues or organs, with its fundamental goal being to emulate the native complexity of biological tissue (cellular niche) that will aid in the cell differentiation and tissue regeneration. Traditionally, this process requires the formation of an interphase between cell, scaffolds and growth factors. Scaffolds can provide the base on which cells can grow under the influence of growth factors (Satpathy et al., 2019) However, this process is rather random in nature and does not allow for a specific customized 3D distribution of cells or matrix (Bose, Vahabzadeh and Bandyopadhyay, 2013), in addition to being time consuming and less efficient. This leads to their non-feasibility for clinical applications from a logistical and economical viewpoint said by Singh and Williams (2008). With this regard, Melchels et al., (2012) made a statement, additive manufacturing is now being explored for tissue engineering as it involves the top-down approach of building the complex tissue in a layer-by-layer fashion, thereby producing precise geometries due to controlled nature of matter deposition with the help of anatomically accurate 3D models of the tissue generated by computer graphics. However, there are still several challenges that need to be overcome to unlock the full potential of 3D bioprinted tissues and organs.

Figure 1. Typical 3d bioprinting workflow. Tissues or organs are first imaged using a variety of techniques as to generate a guide that can be used while printing the desired constructs. The design approach is then selected, which includes biomimicry, self-assembly, or mini tissues. Materials and cells sources are then selected based on the desired tissue type, form, and function. Once these components have been selected, they are integrated into a bioprinting system and the 3D construct is generated. Following printing, the constructs can either be used immediately for in vitro purposes, in vivo transplantation, or can be matured further in a bioreactor before being utilized for other applications.

Regardless of the rapid advancements in additive manufacturing technologies, obtaining suitable seed cells persists in the pursuit of creating fully functional organs through 3D bioprinting (Bian, 2020), (Brezulier, Chaigneau, Jeanne and Lebullenger, 2021), and (van Daal et al., 2020) had similar statements. Isik et al., (2023) and Wang et al., (2021) had both given out several key statements on seed cells which they clarified play an essential role in 3D bioprinting technology since cells are the fundamental units of life. High quality cells are indispensable for generating applicable 3D bioprinted tissues and organs. An ideal cell source possesses several key characteristics.

1. Printability: The cells should demonstrate the ability to withstand the rigorous printing process, including shear stress, pressure, and temperature variations, without compromising their viability or functionality. Although suitable bioink materials could help to increase the printability of cells, it is still important for cells utilized for the generation of organs and tissues.

2. Proliferation: For organ and tissue fabrication, cells need to possess a proliferation capacity in order to expand to the required cell number and adequately populate the 3D bioprinted tissue. However, it is also important to have control over the proliferation of the seed cells. Excessive proliferation could result in hyperplasia which can disrupt the structure and function of 3D bioprinted tissues.

3. Functionality: The cells should either possess functional attributes or have the capability to differentiate into mature functional cells, thereby establishing the desired functionality of the 3D bioprinted organs.

4. Safety: When constructing transplantable normal tissues for therapeutic purposes, it is crucial to use cells with a normal karyotype, non-tumorigenic properties, and devoid of phycological toxicity. Immunological rejection induced by allogeneic or heterogeneous cells is also a serious problem, which should be carefully considered.

4. Economy: The construction of large-scale organs necessitates a substantial number of seed cells. Therefore, the cost-effective large-scale expansion of seed cells is crucial for the applications of 3D bioprinted organs.

5. Self-assembly ability: The microstructure of 3D bioprinted tissues plays a crucial role in achieving full functionalization. The microstructure is primarily formed through the natural self-assembly and organization of cells, which cannot be precisely designed using the current resolution capabilities of 3D bioprinting techniques. The vessel networks of 3D bioprinting vascular tissues mainly relied on the cells [Isik et al., (2023) and Wang et al., (2021)]

More Information

3D bioprinting is an extended application of AM that involves building a tissue or organ layer-by-layer using bottoms-up approach. The aim of 3D bioprinting is to somehow mimic the natural cellular architecture by depositing materials and cells in a particular fashion which can restore the normal structure and functionality of complex tissues. In 3D bioprinting, Xiongfa, Hao, Liming and Jun (2018) also said that cells or biomolecules are printed directly onto a substrate in a specific pattern such that the cells can hold together to form the required 3D construct. Bioprinting deals with the living entities such as cells, tissues, etc., hence the modalities associated with the living tissues has to be observed in it, such as biocompatibility of the material being used, cell sensitivity to the printing methods, growth factor delivery and perfusion explained by Murphy and Atala, (2014).  Knowlton, Anand, Shah and Tasoglu (2018) also brought out an understanding statement, Since the whole process is automated, it can give precise patterning of cells with controlled ECM organization. Because of the layer-by-layer construction of the bio printed tissues, they possess interconnected pores which are ideal for perfusion of gas and nutrients, as well as inter- and intra-cellular communications.

These bio-printed tissues with improved intercellular communications can give a decent reference to in vivo physiology, such a result can contribute toward the data obtained during pre-clinical trials, since animal model is not sufficiently equipped to predict human pathophysiological responses (Shanks, Greek, & Greek, 2009). One of the foremost requirements for 3D bioprinting is bioink. It is composite made up of biomaterials, cells, and other required components on Ozbolat (2015) view point. The technology can be used for fabrication of functional human tissue or organ such as heart, liver, skin, bones etc., along with generating microfluidic models of organs-on-a-chip in the near future (Guillemot et al., 2011).

However, Xu, Chai, Huang and Markwald (2012) still clarified, despite these advantages and convenience offered by the 3D bioprinting, the state-of-the-art technology involves several challenges such as vascularization of the tissue, gas and nutrient exchange, biocompatibility and biodegradability of the material that is used as substrate, shape-fidelity and preservation of functionality of the printed tissue. To this effect, synthetic and natural polymers such as alginate, gelatin, collagen, Polyethylene glycol (PEG), Hydroxyapatite etc., because of their biocompatible nature and controllable physio-chemical properties that can be modified to suit the ECM structure and formation [Tevlek & Aydin, (2017); Bodhak, Bose, and Bandyopadhyay, (2010)]

Literature review:

3D bioprinting

Agarwal et al., (2021) explained Three-dimensional (3D) bioprinting enables the fabrication of 3D architecture of complex spatial patterns through the layer-by-layer deposition of a range of biomaterials. 3D bioprinting allows for control over construct fabrication and cell distribution, with a printing resolution close to the finest features of tissue microarchitecture from ten to a few hundred micrometers (μm) [Daly et al., (2016), Kesti et al., (2015), Miri et al., (2019)]. With substantial repeatability, reproducibility, controllability, and printing throughput, 3D bioprinting can produce customized devices with continuous and stable biological patterns. This technology offers potential for tissues, organs, prosthetics, drug delivery systems, and, ultimately, high-resolution simulations of the heart [Agarwal et al., (2021); Liu et al., (2020); Serpooshan et al., (2016)].

The bioprinter is encapsulated by a set of consecutive manufacturing operations guided by integrated computer numerical control machinery. Basic industry references are indicated by fundamental operating parameters, crosslinking, and print rheology measurements [Kelly et al., (2019); Cui, Nowicki, Fisher, & Zhang (2017); Engler & Cooper-White (2020)]. During the printing process, Derakhshanfar et al., (2017) noticed the platform’s movement is governed by coordinates saved in file format, such as a g-code, that can be easily followed by the printer.

Print conditions, such as printing nozzle aperture, printing speed, printing temperature, number of printed layers, and layer thickness, can vary widely. Each variable can greatly impact cell survival and construct fidelity [Liu et al., (2020); Derakhshanfar et al., (2017); Blaeser et al., (2016), (Sun et al., (2020)]. Derakhshanfar et al., (2017) also said, printability should be optimized to improve the fabrication process and construct properties.

Cellular Component of Bioink

Bioinks to be frank are made of cells and various biomaterials and are characterized by their cytocompatibility and printability. While cytocompatibility determines cell viability, migration, proliferation, differentiation, and subsequent tissue creation, its printability affects shape fidelity and mechanical stability. The properties of bioink are picked to compliment the type of bioprinter as well as bioprinting approaches, according to the desired organ or tissue. Further, printer type and bioprinting approach must also be considered when choosing the appropriate bioink. Although somatic cells such as chondrocytes, fibroblasts, and cardiac myocytes have been used in 3D bioprinting, most applications rely on the inclusion of stem cells to facilitate de novo tissue development [Martínez, Ávila Schwarz, Rotter & Gatenholm, (2016); Albanna et al., (2019); Koti et al., (2019)]. The self-renewal capabilities combined with directed differentiation power of these cells allows control of tissue development during bioprinting processes for produced organs and tissues.

3D bioprinters use bioinks comprised of living cells and biomaterials to generate 3D printed tissues. This process follows a workflow comprised of computational modeling, bioink preparation, bioink deposition, and subsequent maturation of printed products explained by Murphy and Atala (2014). Bioprinting is a versatile tool able to produce a wide range of tissues and organs. Access to bioprinted organs could help resolve the current human organ shortage crisis.

3D bioprinting techniques

3D bioprinting is the most up-to-date technology based on digital model files to build biologically active tissues and organs through layer-by-layer printing. This layered manufacturing technique based on the principle of discrete stacking allows the printing of tissues that mimic the complex anatomy and physiological functions of the human body. Currently, various 3D bioprinting techniques, including jetting-based, extrusion-based, and vat photopolymerization-based methods as shown in Table 1 which was done by Jain, Kathuria, and Dubey (2022), and Rahmani, et al., (2023). This were employed to create highly functional 3D biostructures. Table 1 presents an overview and comparison of these 3D bioprinting techniques.

Bioprinting technologies	Printable viscosity	Cell density	Resolution	Print speed	Cell viability	Cost	Advantages	Disadvantages
Inkjet-based bioprinting	3–12 mPa ⋅ s	106 cells/ml	>10 μm	High	>85%	Low	Affordability, small droplet volume, high resolution	Clogging, shearstress, limited printedmaterials
Extrusion-based bioprinting	30-6 × 107 mPa ⋅ s	108 cells/ml	>100 μm	High	>90%	Low	Affordability, high deposition rate	High shear stress, low resolution
Light-assisted bioprinting	1-1,000 mPa ⋅ s	108 cells/ml	10−100 μm	Low to high	>85%	Medium	Nozzle-free, noncontact, high resolution, good cell viability	Timeconsuming, high cost, photonic cell damage

Figure 2:

Figure 2. Scheme of various 3D bioprinting technologies: (A) Extrusion-based 3D bioprinting. (B) Jetting-based 3D bioprinting. (C) Vat photopolymerization-based 3D bioprinting (Xuming et al., (2024).

Cell requirements in different 3D bioprinting techniques

In recent years, significant advancements have been made in 3D-bioprinting techniques, revolutionizing the field of bioartificial organ construction. 3D bioprinting involves the precise deposition of bioinks composed of living cells and biomaterials, enabling the fabrication of complex tissue structures [Joshi, Kaur, & Singh., (2022); Do, Khorsand, Geary, & Salem (2015); Erben et al., (2020)]. However, it is important to note that different bioprinting technologies possess unique characteristics, resulting in distinct requirements for seed cells (Table 2).

Table 2. Requirements of cell sources of different 3D bioprinting techniques.

Technique	Requirements	Ref.
Extrusion-based bioprinting	Suitable size shape, viscosity Stable viability and functionality	[Zhuang et al., (2019); Lee et al., 2016); Moncal et al., (2019)]
Inkjet bioprinting	Compatibility with bioink materials Jetting process resilience	[Bedell et al., (2022), Dufour et al., (2022)]
Laser-assisted bioprinting	Stable viability and functionality Laser resilience	[Kerouredan et al., (2019), Hakobyan et al., (2020)]
Stereolithography bioprinting	Compatibility with light-sensitive materials Light exposure resilience	Mahdavi et al., (2020)

Table 3: A summarization of four main bioprinting techniques.

	Droplet-Based	Laser-Assisted	Stereolithography & Digital Light Processing	Extrusion-Based
Advantages	Precise deposition, high cell viability, biomaterial compatibility, variable biomaterial concentrations, controllable growth factors	Non-contact printing, biomaterial compatibility, high cell viability, high cell densities	Non-contact printing, high resolution, high printing speed, cell viability, high cell densities	Biodegradability properties, simultaneous usage of multiple biomaterials, multiple nozzles, high cell densities, high viscosity bio-ink
Limitations	Inability to extrude continuous flow of bio-ink, low cell densities, low viscosity bio-ink	Time-consuming, high cost, limited construct size	Damage from UV exposure, cytotoxic effects, limited range of bio-inks	Low resolution, low precision

3D Bioprinted Tissue and Organ Constructs

3D bioprinted constructs have been utilized to investigate novel drug therapies, develop patient-specific treatment plans, and study complex physiological processes findings by Hong, Yang, Lee and Kim (2018). However, Eswaramoorthy, Ramakrishna and Rath (2019) recently stated for some, the ultimate goal of this technology is to fabricate fully functional organs for in vivo application. 3D bioprinted structures could replace diseased or damaged organs, alleviate strains associated with finding appropriate donor organs, and minimize immune complications and/or anatomical incompatibilities that can arise from allogenic transplant.

Recent advances in human research have led to the production of 3D bioprinted cardiac patches used to treat myocardial infarctions in rat models, bioprinted corneal constructs shown to successfully integrate with host porcine tissue, and many other promising preliminary studies, as outlined in Table 3. (Yeung  et al., 2019)

TABLE 4: Supplemental Table 3. Examples of in vitro and in vivo studies that utilize 3D bioprinting techniques to generate tissue constructs (Yeung et al., 2019)

Kidney	Human	iPSCs	Differentiation protocol was developed that allowed for the simultaneous differentiation of all four renal progenitors from iPSCs. Kidney organoids contained all the components of the native kidney and expressed appropriate specialised cell types	(92)Takasato et al. 2016
		iPSCs	Using a modified feeder-free protocol from Takasato et al. 2015, generated kidney organoids that were transplanted under the renal capsule of immunocompromised mice for up to 28 days. Resulted in progressive maturation of nephron structures and organoid vascularisation.	(93)van den Berg et al. 2018
Liver	Human	iPSC-HPCs	Two-step bioprinting approach that embedded iPSC-HPCs onto a scaffold that mimicked the anatomical structure of hepatocytes. 3D printed hepatic model demonstrated increased phenotypic and functional enhancements over several weeks of in vitro culture	(94)Ma et al. 2016
		iPSC-HLCs	3D bioprinted iPSC-HLC spheroids showed increased cell survival, and hepatic and metabolic function compared to single cell constructs.	(95)Goulart et al. 2019
Muscle	Human	hMPCs, hNSCs	3D bioprinted human neural-skeletal muscle constructs showed improved myofiber formation, long-term survival, neuromuscular junction formation in vitro. Constructs were implanted into rats and facilitated rapid innervation and matured into organised muscle tissue	(96)Kim et al. 2020
		hUCB-MSCs	3D bioprinted scaffolds filled with hUCB-MSCs improved regenerative processes in rabbits with full thickness rotator cuff tears	(97)Rak Kwon et al. 2020
Pancreatic         Cardiovascular	Human	HUVECs	Generated a protocol for preparing a pancreatic tissue-derived decellularized extracellular matrix (pdECM) bioink. Pancreatic tissue constructs were then created via micro extrusion-based 3D bioprinting methods	(102)Kim et al. 2019
		iPSCs	Generated islet organoids using a multilayer microfluidic chip device by initial embryoid body (EB) formation followed by pancreatic induction differentiation. Exhibited appropriate morphology, multicellular complexity, and enhanced expression of B-cell associated genes, insulin secretion levels, and the ability to appropriately respond to glucose	(103)Tao et al. 2019
	Human	iPSCs, CMs, FBs, ECs	Cardiac patches were produced from mixed cell spheroids via 3D extrusion bioprinting. Patches were implanted into rats to repair myocardial infarctions (MIs). Rats treated with the cardiac patch had a 100% survival rate, where control rats had an 83.3% survival. Rats treated with the cardiac patch had significantly less scarring, a significantly increased number of blood vessels, and better cardiac function compared to that of the control rats.	(84)Yeung et al. 2019

NOTE: 

iPSCs; induced pluripotent stem cells, MSCs; mesenchymal stem cells, hMSCs; human mesenchymal stem cells, HUVECs; human umbilical vein endothelial cells, CMs; cardiomyocytes, FBs; fibroblasts, ECs; endothelial cells, iPSC-HPCs; iPSC-derived hepatic progenitor cells, iPSC-HLCs; iPSC-derived hepatic-like cells, hMPCs; human muscle progenitor cells, hNSC; human neural stem cells, iPSC-NPCs; iPSC-derived neural progenitor cells, NSCs; neural stem cells. hUCB-MSCs; human umbilical cord blood-MSCs,

3D Bioprinted Tissue and Organ

Heart bioprinting

The heart is a highly segmented organ responsible for maintaining the body blood circulation. Unlike the lungs, liver, skeletal muscle, and skin, the mammalian heart is nearly non-regenerative, as it lacks sufficient stores of progenitor cells to generate new cardiomyocytes. Since it cannot produce new functional myocardium after acute or chronic injury, implantation of 3D bioprinted tissues is a potential effective method. Cardiac patching has been a significant area of research in cardiac tissue engineering, where cardiomyocytes are formed on specific scaffolds and then placed on the epicardial surface of the infarcted area for therapy. Yeung, et al. applied human iPSC-derived cardiomyocytes co-cultured with fibroblasts and endothelial cells to form cell sheets without a biomaterial scaffold using the 3D bioprinting technique (Yeung et al., (2019)

Liver bioprinting

Kang et al., (2017) explained, the liver is the biggest organ in the human body and performs vital functions such as metabolism, bile secretion, blood coagulation, and immunity. Liver diseases from various causes have become a global health concern. Liver transplantation is an effective treatment for various end-stage liver diseases. However, due to a growing organ shortage, the 3D bioprinted livers has the potential to be a promising alternative. A 3D liver model was created using mouse-induced hepatocyte-like cells (miHeps) that were generated by transforming mouse embryonic fibroblasts with pMX retroviruses expressing hepatic transcription factors Hnf4a and Foxa3.

Lung bioprinting

Chronic lung diseases, such as chronic pneumonia and chronic obstructive pulmonary disease, result in the irreversible loss of lung function, impaired gas exchange, and a reduction in the quality of life. Given the lack of regenerative capacity in the human lung, the most effective treatment for severe lung diseases is lung transplantation. The lungs facilitate gas exchange through a thin membrane in the alveoli, known as the air-blood barrier. The alveoli and air-blood barrier are both small in size and complex in structure, which presents a significant challenge in the bioprinting process. Consequently, there are few studies on the use of lung bioprinting technologies to mimic alveolar function (Wu, Qin & Yang, 2023)

The human lung is a primary target organ for numerous viral and bacterial pathogens, making 3D bioprinted lung a valuable tool for infectious disease studies and drug testing. Berg et al. bioprinted a lung model using primary human lung fibroblasts and monocytic THP-1 cells with the top layer of alveolar epithelial A549 cells for the study of viral inhibitors (Berg et al., 2021)

Current Challenges and Future Aspects of 3D Bioprinting Techniques

In its current challenges Balla et al., (2020) stated on how FDA has issued a guidance document for production of medical devices, “Technical considerations for Additive Manufactured Devices” that provides guidelines for the additive manufacturing including 3D printing. With the technological advancement in the printing technique and development of efficient and cost-effective printing methods, it becomes necessary to regulate the quality control standard before transplantation in each step during the process, such as while designing a model, selection of bioink, printing validation, maturation of post-printing and assessment of product quality.

Further, Starly and Shirwaiker (2015) clarified the number of components involved in printing process is one of the big issues with 3D bioprinting. Lack of software that can define the placements of cells, biomaterials and biological molecules virtually following the robust designing and translation that drive downstream manufacturing operations cause bioprinting to be hampered.

 Another challenge Hollinger, Brekke, Gruskin and Lee (1996) say is, it is necessary to manufacture an adequately stable as well as mechanically inflexible 3D construct during transplantation. During hard tissue repairing, porosity and structure designed by 3D bioprinting should maintain a high elastic modulus so that they can support the natural cell growth during implantation.

Due to scaffold deformation newly, formed tissues will probably fail if proper structural maintenance and mechanical support are not given by the scaffold (Hollister, 2005). Proper vascularization in vivo is another important need for a bio printed construct in TE that provides the cells’ growth factors, oxygen, nutrients and removes waste. In vivo capillaries, which are present within 100 mm from maximum cells, exhibit sufficient diffusion, that is needed for survival of the cell (Kaully, Kaufman-Francis, Lesman and Levenberg, 2009). Some of the issues that arise with the scalability and wide-spread adoption of bioprinting techniques have been highlighted here in Figure 3.

FIGURE 3. Current challenges in 3D bioprinting techniques.

Today organ transplantation could be a lifesaving treatment choice but few people are available as donors. According to Organdonor.gov, 18 people die in the US everyday due to appropriate organ transplant. Therefore, this emerging 3D bioprinting technology could be an option for organ transplantations around the world and could end the heavy demand on organs. Future developments in bioprinting are expected to witness rapid developments in bioprinters which can be readily deployed in hospitals. The bioprinters will be expected to perform bioprinting with high resolution, mechanical strengths and cell viability. In addition, to obtain bioprinted constructs for clinical translation, it is necessary to integrate functional vasculature in the grafts to ensure long term cell survival (Datta et al., 2018).

 Different tissues of the body have differing requirements of cell densities, number of different types of cells, spati-temporal distribution of cells inside the constructs. Moreover, when using stem cells, different matrix properties may modulate the differentiation and trans-differentiation of cells into specific lineages [Even-Ram, Artym & Yamada, 2006; Barui, Chowdhury, Pandit & Datta, 2018].

Bioprinting is multi-step process and each step should be well-coordinated with other steps in the process. Perfusion bioreactors are expected to be another key area where bioprinting technologies will witness increased integration. However, apart from all the above future directions, most important would-be rapid envelopment of bioinks with optimized bioprintability and biofunctional properties. At present, most biopolymers used in bioprinting are borrowed from polymers generally used for tissue engineering and seldom possess optimal rheological and crosslinking properties ideal for a bioprinting process. Therefore, there exists a significant challenge of developing ideal bioinks (Skardal, 2018).

Finally, since the ultimate aim of bioprinting is to provide functional tissue constructs there is also need to develop better assays, which can analyze cell functionality in 3D constructs. given the rapid pace at which bioprinting is emerging and the tremendous interest in this technology cutting across different scientific disciplines, it is expected that the above challenges could be overcome and bioprinted constructs will become available for translational studies as well as speed up the drug development process especially considering the fact that Pharmaceutical companies will spend over $50 billion dollars on research and development to get drug approval from the FDA for animal, preclinical, and clinical testing. It is expected that in the future, 3D Bio bioprinters can cheapen this expense and quicken testing time with better prediction of drug reaction and without waste money or time. Figure 4 provides a comprehensive picture of the current applications in bioprinting with some of the approved research going on in this field.

FIGURE 4. Different bioprinted organs.

 1) Skin construct includes 20 layers of keratinocytes and fibroblasts implanted into the wound Day 0 (at left) and Day 11 (at right) (He et al., 2018)

2) Cross-section of 3D bioprinted cardiac patches (at left), anterior aspect (at middle), In vivo transplantation of cardiac patches (at right) (Ong et al., 2017)

3) 3D bioprinted human ear (at left) and sheep meniscus (at right) (Mori, Fernandez, Blunn, Tozzi & Roldo, 2018)

4) Treatment of femur defect using polymeric hydrogel and growth factor (Mori et al., 2018)

5) Fixing of bone defects using in situ 3D bioprinting with alginate hydrogel, transparent before photopolymerization (at left), becomes milky white after photopolymerization (at right) (Li et al., 2017) (used from open access journals).

Ethical Considerations

The emergence of 3D bioprinting technologies has stimulated ethical discussions pertaining to their use. 3D bioprinting generally requires the use of stem cells in bioink formulations including embryonic, adult stem cells, and/or induced pluripotent stem cells. Stem cell acquisition has a history of ethical and political controversy as discussed above (Vijayavenkataraman, Lu, & Fuh (2016).

Higher adoption of 3D bioprinting technology likely will intensify stem cell debate because it requires more stem cells during production. Advancements in 3D bioprinting may provide opportunities for tissue and organ transplantation in veterinary medicine. Organ transplantation is a major procedure that has drastic, long-lasting effects on the recipient. Thus, it is important to consider the ethical responsibility of manufacturers and veterinary professionals in terms of donor and recipient selection. To ensure the safety of the recipient, quality assurance measures must be employed at every stage of the bioprinting process. Clinical trials will likely have to evolve from the transplantation of tissue and smaller organ structures before transplantations of larger organ structures can be attempted. Educational programs must be developed to ensure that veterinarians can provide owners with an accurate understanding of these technologies explained out by Liguori, Jeronimus, Aquinas, Moreira and Harmsen (2017).

 Although, Mason, Visintini, and Quay (2019) states it can still be argued that the safety of the recipients cannot be adequately ensured with any current processes involved in clinical translation. Therefore, it will be important to involve doctors, medical researchers, public health officials, regulatory authorities, and other community stakeholders in the development of new regulatory measures. Increased accessibility to 3D bioprinted tissues could also affect public behavior and disturb human-animal relations. For example, if 3D bioprinted human organs become easily accessible, individuals may be more likely to perform more harmful activities, such as smoking, excessive drinking, and/or drug use, with less fear of the repercussions. If so, it may be reasonable to assume that this lapse in judgment may influence their behavior toward animals (Pavlovich, 2016).

3D bioprinting could lead to conflicts between animal welfare and veterinary medical technology as in the case of double-muscled Belgium Blue Cattle. These cattle were produced through targeted breeding programs resulting in removal of the bovine myostatin gene (MSTN), a gene involved in the regulation of skeletal muscle development. The goal was to produce cattle with a greater meat yield and a higher percentage of high-value cuts. However, it inadvertently led to health issues such as dystocia, reduced calf fertility, and reduced calf survival, thereby negatively affecting the welfare of the cattle (Eriksson, Jonas, Rydhmer & Röcklinsberg, 2017). Similarly, 3D bioprinting could incentivize owners to perform augmentative treatments and/or numerous tissue replacements, particularly on performance animals such as racehorses, to further the longevity of their animal’s performance. This could threaten the quality of life of these animals and prolong suffering prior to compassionate euthanasia. Thus, education programs will need to be developed to inform stakeholders on the risks and limitations associated with bioprinting modalities. Though 3D bioprinting may help bolster the capacities of both veterinary and human medicine, if misused, it could drastically affect patient well-being.

Conclusions

Organ fabrication stands as one of the major applications enabled by 3D bioprinting technology. The main issue in conducting successful 3D bioprinting operations rests on obtaining sufficient cell numbers. The tissue and organ regeneration field has enormous potential through additive manufacturing strategies within bioprinting processes. The technology facilitates the creation of tissue that functions better and more predictably for patient applications. 3D bioprinting stands out as a preferable approach to autologous and allogeneic grafting because it avoids patient-related stress while addressing the scarcity of donor tissue and it constructs tissues through layer-by-layer building from the base up. Through personalized 3D bioprinting technology patients can get highly effective medical treatments with attractive outcomes. All platform developments have not eliminated the various barriers associated with connecting printed body constructs to biological tissue systems. Standardized printing methods alongside careful quality-control processes are needed to maintain printed construct quality after cells survive in the bio-ink formulation. The field of tissue engineering needs these technologies to develop multiple approaches for tissue fabrication that include inkjet printing, laser assisted bioprinting, extrusion bioprinting, and stereolithography and others. When compared to manual tissue culture methods, 3D bioprinting offers precise cell patterning with better spatial control and high-throughput tissue printing.

REFERRENCE

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