1. INTRODUCTION:

The 3D bioprinting procedure includes creating functional tissue structures and organs from 3D digital models using cell-laden bioinks¹. Compared to traditional tissue engineering techniques, 3D bioprinting has a number of benefits^2,3. Due to the limitations of traditional tissue engineering techniques, which make it impossible to create sophisticated biomimetic structures, the manufactured tissue is imprecise and has erroneous cell microenvironments.⁴, whereas 3D There is potential for bioprinting to fabricate complex, sophisticated, biomimetic tissue constructs. Automation, high accuracy, control and freedom in geometry (pore size, porosity, and connection, customizability, and printability of a wide range of materials, ability to include, and precision spatiotemporal placement of proteins, growth factors, drugs, DNA, and other biochemical cues along with the cells⁵, wide range of cell density and possibility of cell density gradient, reproducibility, and repeatability are some of the many advantages of this technology. 3D bioprinting has numerous applications in the healthcare industry, including disease modelling, drug development and testing, high-throughput screening, and regenerative medicine, due to its capacity to manufacture three-dimensional biomimetic functional tissue structures. Since the terms "3D printing" and "3D bioprinting" are frequently used interchangeably in the scientific community, It is crucial to understand the differences between the two concepts⁶. Both the processes build a 3D object from a 3D model, layer by layer.

While Unlike 3D printing technologies, which do not use cells or biologics, 3D bioprinting uses cell- and biologic-rich bioinks to construct living tissues. It's critical to recognise the differences between bioprinting using cell-filled bioinks and 3D printing of porous polymeric scaffolds for cell seeding. 3D printing also has numerous biomedical applications including medical devices, surgical instruments, prostheses, customized implants (inert materials such as metals, ceramics, or polymers without cells), and surgical planning and training using anatomical models^7,8.

This review solely focuses on the use of cell-laden bioinks and other processes to print organs and tissues for use in regenerative medicine applications. hence, general 3D printing technologies and applications do not fall within the scope of this work. The demand for bioprinting technology supports the quick development seen in the area over the previous ten years.

One of the biggest issues in healthcare is the ongoing organ shortage. In the USA, there were 122, 071 patients anticipating an organ transplant in 2016, with 48% of them waiting for more than 2 years⁹. Despite a significant growth inhow popular organ transplants are, the supply has been essentially unchanged for ten years.

This growing organ shortage dilemma could be resolved in large part by bioprinting. Although There is still more work to be done before fully functional organs can be bioprinted¹⁰, a lot of progress has been made toward achieving the bigger objective of printing whole organs.

In order to reduce the high cost and expedite the drug discovery process, bioprinting could be used in pharmaceutical research as in vitro models for assessing medication efficacy, toxicity, chemotherapy, or chemo-resistance.^{11, 12}. Over the past ten years, research on bioprinting has increased significantly. The quantity of articles on 3D The number of people using bioprinting surged by 330% between 2000 and 2015 (from 24 to 792) illustrating how quickly the field is developing¹³. With a CAGR of 26.5%, the market for The 3D bioprinting is anticipated to grow from USD 411.4 million in 2016 to USD 1, 332.6 million in 2021¹⁴.

2. STEPS IN 3D- BIOPRINTING PROCESS:

The three key steps in the traditional bioprinting process are pre-processing, processing, and post-processing.Pre-processing comprises imaging the tissue or organ using computed tomography moreover, magnetic resonance imaging (MRI)(CT). The processing stage begins with collecting primary cells from patients, growing, and expanding them ex vivo for the bioprinting procedure.

The resulting 3D models are then transformed into STL file format. The cells are suspended in appropriate bioinks that mirror the properties of the tissue that will be printed.. Post-processing comprises maintaining before being transplanted into patients or utilised as in vitro models for disease simulation or drug testing, the bioprinted tissue or organ must undergo tissue maturation in the bioprinter.¹⁵

3. BIOPRINTING TECHNIQUES:

3.1 Laser-based Bioprinting:

laser-based bioprinting With the use of Computer-aided Design and Manufacturing (CAD/CAM), laser-based bioprinting technologies use laser energy to shape cell-laden bioinks in a three-dimensional spatial arrangement. Laser radiation, It has been used to accurately shape metals like Ag, BaTiO3, and NiCr into active and passive mesoscopic circuit components such as conductors, capacitors, and resistors. It has a high spatial resolution of 1-3 m and is very monochromatic, coherent, and focussed¹⁶.

This method is a promising choice for usage in biological applications like cell printing due to its high resolution and reproducibility¹⁷. Approach using laser-induced forward transfer (LIFT)(Figure 5A) is extensively used for tissue bioprinting and cell patterning^18–20. The usage of NIH3T3 fibroblasts and HaCaT keratinocytes has been documented in the literature in a number of successful attempts to print cells in a 3D spatial layout using laser-based bioprinting.

human mesenchymal stem cells^21,22 human dermal fibroblasts (hMSCs)²¹, human umbilical-vein endothelial cells (HUVEC) and human umbilical-vein smooth muscle cells (Even while laser-based bioprinting has several advantages over traditional bioprinting methods, there are still a number of challenges. The issue of metallic nanoparticles induced cytotoxicity is brought up by the employment of metals as a laser-energy absorption layer in procedures like LIFT and AFA-LIFT²⁴. Laser-based bioprinting has more drawbacks than benefits.

The method's nature prevents it from being used to fabricate full-scale tissue constructions that are appropriate for regenerative medicine. However, the method can be utilised to pattern multicellular microenvironments at the cell level with great resolution, such as tumour microenvironments (TME), for the study of drugs or the modelling of diseases⁹³.

Since optical forces and principles underlie laser-based bioprinting, optically selective cell targeting and transfer may one day be made achievable¹⁷.

3.2 Droplet-based bioprinting with inkjet technology:

3.2.1 Inkjet printing:

Inkjet printing is a non-contact technique that prints on a surface with exceedingly tiny ink droplets (Xu et al., 2013; Abeyewickreme et al., 2009; Fang et al., 2012). Inkjet bioprinting is an adaptation of inkjet printing that makes use of cell-filled bioink cartridges rather than conventional printing ink cartridges. Inkjet printing techniques like drop-on-demand (DOD) and continuous inkjet (CIJ) can be widely categorised. Rayleigh-Plateau instability happens when CIJ (Figure 5F) bioprinting is forced via a nozzle, ejecting a stream of droplets containing cells as bioink.²⁵

Since the droplet could not be precisely controlled in CIJ due to the nature of the process, DOD is the best option for bioprinting.²⁵ Bone^26,27, cartilage^28,29, skin³⁰, heart³¹, and neural tissue³² have all been printed using inkjet bioprinting. Its advantages include inkjet bioprinting, fast printing speeds (up to 10, 000 droplets per second), and quick resolution (50m). There are a few negatives, though the only bioinks that can be printed are those with low viscosities (between 3 and 12mPa s), which necessitate an additional cross-linking process to give the build structural integrity. Another issue with inkjet bioprinting technologies is nozzle blockage, which restricts the cell density in the bioink to less than 106 cells per millilitre³³.

3.2.2 Electro-hydrodynamic jetting (EHD-jetting)-based Bioprinting:

A high voltage (0.5-20 kV) is applied between the nozzle and the substrate as a back-pressure supply delivers the bioink to the nozzle tip^{34, 35}. When the electric field force supplied by the source surpasses the viscoelastic force and surface tension force of the bioink at the nozzle tip, bioink droplets are expelled from the nozzle by the formation of a Taylor cone^36–37.

EHD-jetting can be used in a variety of ways, including dripping, spindle, oscillating jet, and cone-jet mode³⁸, the voltage applied and the separation between the nozzle and the substrate. Using EHD-based bioprinting, living cells like Jurkat cells³⁹, mouse neuronal cells (CAD)⁴⁰. The patterning of human embryonic kidney cells has shown that the cells can withstand the strong electric fields and stresses produced during the jetting procedure (HEK 293T)⁴¹ and 3T3 murine fibroblasts⁴².

DNA Oligonucleotides⁴⁴ and patterned protein arrays⁴³ had both been printed using the method with success. The high resolution that EHD-jetting-based Bioprinting provides in comparison to other bioprinting techniques is its greatest benefit. Using this technique, a nanoscale resolution of about 100 nm had been attained.⁴⁴

3.2.3 Sound-based bioprinting:

Acoustic Bioprinting is relatively a new method of bioprinting that can eject droplets of cell-laden bioinks on demand. Cell-laden An acoustic actuator sits in the middle of the open pool where the bioink is kept. Surface tension forces keep the bioink static at the tiny nozzle tip. When the piezoelectric substrate and interdigitated gold rings in the acoustic actuator are triggered, circular acoustic waves are produced with their focus point at the air-bioink interface at the nozzle tip. These waves are able to break surface tension forces and eject bioink droplets onto the substrate.⁴⁵

There is no chance of introducing harmful stressors like heat, high pressure, or high voltage to the cells because the bioink is in an open pool rather than a nozzle as with other DOD techniques²⁵.

3.2.4 Microvalve Bioprinting:

Typically, a laser beam is focused to create a light-trap that is utilised to direct the cells on the substrate (Malda et al., 2013).

For printing cells, It is more common to use a different variation of this approach termed laser-induced forward transfer (LIFT) (Bohandy et al., 1986; Odde and Renn, 2000; Schiele et al., 2010). In a microvalve bioprinting system, the ejection of droplets of cell-laden bioink is regulated by electromechanical or solenoid valves.

Pneumatic pressure is used to deliver the bioink to the nozzle tip. Application of voltage pulse to the valve causes the valve to open due to solenoid action and when the pneumatic pressure overcomes the viscoelastic force of the bioink⁴⁶.

3.3 Extrusion-based Bioprinting:

An orifice made of syringes, a motor that moves the syringes in a three-axis motion, and a stage on which the item is built make up the typical printer configuration (Fang et al., 2012).

The usual printer configuration consists of a syringe-based orifice, a motor that moves the syringes in a three-axis motion, and a stage on which the item is constructed (Fang et al., 2012). UExtrusion printing of cells typically uses a highly viscous hydrogel with cell content that can flow through the nozzle without the need for high temperatures (Fedorovich et al., 2007). Extrusion-based bioprinting is the method that is employed the most frequently⁴⁶. Bone^{47, 48}, cartilage⁴⁹, skeletal muscle⁵⁰, skin⁵¹, cardiac tissue⁵², mental tissue⁵³, and liver⁵⁴ are only a few examples of the different tissue types.

3.4 Stereolithography Bioprinting:

Bioprinting Despite being created in 1996 by 3D Systems, stereolithography is still a relatively new technology when it comes to bioprinting.

Stereolithography's operating system is covered elsewhere⁵⁵. In a nutshell, when the build stage is shifted vertically, producing layer of the object by layer, a layer of photopolymer resin is cured (or polymerized) by light (typically UV) irradiation. The light movement is controlled by computer code, pictures, or CAD files. In terms of modes, stereolithography has two.

The light source in the first one (Figure 5N) is computer-controlled and travels in accordance with each structure's requirements. The second method uses a Digital Micromirror Device (DMD), which is an array of thousands of tiny mirrors that can individually be programmed to reflect light in a specific pattern (such as an image or CAD/STL file), polymerizing an entire layer at once⁵⁶. Human dermal fibroblasts (HDFs)⁵⁷, spinal root ganglia (DRG)⁵⁸. Stereolithography bioprinting offers the highest resolution (~6µm) of all the bioprinting methods⁵⁶. With the advent of two-photon polymerization based stereolithography⁵⁹, very high resolution in nanometre scale (~200nm) could be obtained.

Reduced printing time is a benefit of DMD-PP. Because there are no nozzles involved in the process, nozzle clogging is not an issue, allowing for the use of Reduced printing time is a benefit of DMD-PP.>106 cells per millilitre). Despite having the finest resolution available, this method has significant drawbacks that prevent the bioprinting of cells and tissues.

First, only bioinks that can be photopolymerized or bioinks that have a UV-activated photoinitiator(Irgacure2959(2-hydroxy-1-4-(2-hydroxyethoxy)phenyl-2-methyl-1-propanone is commonly used) can be used. A few studies have been done on the development of biodegradable photopolymerizable bioinks, such as Poly(ethylene glycol) dimethacrylate (PEGDMA), Poly(propylene fumarate) (PPF), Trimethylene Carbonate (TMC), and E-Caprolactone (CL)^{57, 60,61}.

However, the photoinitiator (Irgacure 2959, which is the least cytotoxic among the photoiniti ⁵⁷. Secondly, the cells are exposed to UV radiation and are prone to cell lysis and DNA damage⁸³. The UV radiation could also damage the DMD system. A visible-light DMD-PP has been disclosed that employs an eosin-Y-based photoinitiator that is activated by visible light⁶². A few works have reported this pattern.

Kim et al.⁶³ A hybrid bioprinting technology capable of printing simultaneously with both extrusion- and inkjet-based dispensing methods. They gave evidence. a single-step bioprinting of a 3D replica of human skin employing this hybrid system's three printing techniques: melt extrusionwas used to print PCL mesh, Extrusion-based bioprinting for the skin's dermal layer, which acts as a scaffold or support structure, inkjet DOD bioprinting for keratinocyte patterning over the dermal layer which forms the epidermal layer.

4. D BIOPRINTED TISSUES AND ORGANS:

4.1 Skeletal System:

Skeletal System the primary skeletal system elements are bones and cartilages, as well as bands of fibrous connective tissues, such as tendons and ligaments. The human body's structural framework is provided by this system, which also gives the body stability and shape. Numerous factors, including as trauma, disease, injury, and ageing, contribute to the loss or deterioration of skeletal tissue, which has a considerable morbidity and socioeconomic impact⁶⁴.

More than 500, 000 individuals need bone defect repairs annually in the US alone, demonstrating the extreme demand for functional bone transplants.⁶⁵

4.2 Bone Gao et al.⁶⁶ employed inkjet bioprinting to create human MSCs (extracted from a 22-year-old man) suspended in a PEG-GelMA hydrogel (10% w/v of PEG and 1.5% w/v of GelMA, supplemented with 0.05% w/v of Irgacure 2959) at a concentration of 6X106 cells per millilitre, with PEG hydrogel as a control. RUNX2, SP7, DLX5, ALPL, Col1A1, IBSP, BGLAP, SPP1, Col10A1, MMP13, SOX9, Col2A1, ACAN, among others) gene and protein expression study, the addition of GelMA accelerated early differentiation of hMSCs to osteogenic lineage in contrast to the control, 21 days after printing.. In another study by Campos et al.⁶⁷, 5.1.2. Cartilage A degenerative joint disease called osteoarthritis causes the synovial joints' hyaline cartilage to gradually disappear.⁶⁸

Having a global impact on millions of individuals. Osteoarthritis affects 37% of those over 65 in the USA, and it has an annual economic cost of $3.4 to $13.2 billion⁶⁹. The jo technique is the current gold standard for repairing damaged cartilage and underlying bone.int arthroplasty, however it has significant drawbacks and post-operative problems ⁶⁸. Tissue-engineered cartilages are therefore considered to show promising alternative treatment. Due to its low cell density (10–15%) and avascular nature, cartilage has a restricted capacity for regeneration^49,70. Alginate (3.5% w/v), agarose (2% w/v), PEGMA (a commercial product called BioINKTM), and GelMA (10% w/v with 0.05% Irgacure) were the four bioinks that Daly et al.⁷¹ tested for their effectiveness in the bioprinting of fibrocartilage and hyaline cartilage. The outcomes demonstrated that PEGMA and GelMA favoured the development of fibrocartilage-like tissue 40 percent of BioCartilage (cartilage extracellular matrix particles), 3 percent of gellan, and 2 percent of alginate were used to generate a special bioink.

Extrusion-based bioprinting for the skin's dermal layer, which acts as a scaffold or support structure, inkjet DOD bioprinting for keratinocyte patterning over the dermal layer. Kristi et al.⁷² created a unique bioink with 40% of BioCartilage (cartilage extracellular matrix particles), 3% of gellan, and 2% of alginate. After being taken out of the medial and lateral femoral condyles of four 6-month-old calves' full-thickness articular cartilage, bovine chondrocytes were suspended in the hydrogel at a density of 6 x 106 cells per millilitre. The post-printing cell viability was more than 95% after 7 days.

Auricular, nasal, meniscal, and vertebral cartilage were printed as a proof of concept (shown in Figure 7 G-J, L-N) and the in vitro study proved that the bioink supported the proliferation of chondrocytes after 8 weeks, with deposition of cartilage matrix proteins.

4.3 Challenges and future outlook In vitro cultivation of bioprinted bone in therapeutically relevant sizes is the main obstacle to bioprinting of bone⁷³.

There are still issues to be solved, such as how to maintain cell viability across the full thickness and provide uniform perfusion of nutrients and growth hormones throughout the three-dimensional structure. examination of additional methods, like microcarrier-based bioprinting⁷⁴ and the obstacle might be overcome by bioprinting a developmental precursor that would serve as a model for organogenesis in vivo. Second, a porous scaffold structure is required in order to create biomimetic bone tissue.

4.4 Muscular system:

Muscular structure By using the inkjet bioprinting approach, Phillippi et al.⁷⁵ muscle-derived stem cells (MDSCs) that have been printed isolated from adult mice) under culture conditions. The objective of the project was to produce spatially defined patterns of immobilised growth factors for regulated stem cell development, however this was the first effort to bioprint muscle cells that had been documented. Mouse C2C12 myoblasts were also imprinted on micro cantilevers using Cui et al⁷⁶ inkjet bioprinting technique for use in biosensor and Bio-MEMS applications.

4.5 Nervous System:

Nervous System Using an extrusion-based bioprinting approach, Hsieh et al.⁷⁷ created a polyurethane (PU) nanoparticles-based bioink (25–30% w/v) for bioprinting muscle-derived stem cells (MDSCs) that were isolated from adult mouse brain at a density of (4 106 cells/ml).. More than 65% of the PU's chemical makeup was created using up of a soft segment. PCL diol/poly(L-lactide) (PLLA) diol or PCL diol/poly(D, L-lactide) (PDLLA) diol (4:1 M ratio) served as the soft segment (4:1 M ratio).

In comparison to the former, PU bioink with the later soft section preferred good cell proliferation and differentiation. In vivo study in a zebrafish embryo, neural injury model also demonstrated nervous regeneration. In another study⁷⁸, Schwann cells (isolated in primary culture from the sciatic nerve of Sprague-Dawley rats) suspended in a fibrin-factor XIII-hyaluronic acid hydrogel (50mg/ml, 1U/ml, and 4mg/ml respectively) was printed into a thrombin/PVA solution (50U/ml, and 1.4% w/v respectively) with post-printing cell viability of greater than 95% after 7 days.

4.6 Lymphatic system:

Lymphatic System Nakamura et al.⁷⁹ attempted to engineer an artificial lymph node using an extrusion-based printing method using micro-fabricated spinneret nozzles.

This study demonstrated that lymph node-like structure could be fabricated using their method, however, no cell-laden hydrogels was printed nor characterized.

4.5 Endocrine System:

Endocrine System Hormone replacement therapy is now used to address hormonal or gland malfunction. However, because it prevents the delicate modulation of hormones in response to varying physiological conditions, this therapy strategy is not ideal.⁸⁰

A working mouse thyroid gland with vascularization was generated utilising bioprinting, according to a recent publication by Bulanova et al.⁸⁰. In mouse embryos, they eliminated thyroid spheroids (TS), while in e8.5 mouse embryos, they removed allantoic spheroids (AS). Researchers employed mouse embryos to obtain thyrocytes and endothelial cells (EC), which "self-assembled" to produce a thyroid tissue.

After 4 days, EC (from AS) invaded and vascularized the TS, while TS epithelial cells finally developed into follicles. A vascularized tissue construct was produced as a result. By implanting the construct under the kidney capsule in a hypothyroidism mouse model, the construct's performance was assessed.

Five weeks after kidney transplantation, blood samples for plasma T4 levels and histological analysis of the kidneys showed effective host tissue integration and maturation, assisting in they reported to have bioprinted a functional endocrine gland. The thyroid gland, on the other hand, has a very simple structure, therefore the same approach might not be successful when printing more complex glands, like the pancreas.

Beta cells, one of the five main cell types found in the pancreas (as mentioned in the preceding sentence), are of particular significance since they are in charge of producing insulin. Diabetes is a disease that affects millions of individuals worldwide due to dysfunctional insulin production. Hence, research on bioprinting of beta cells is of a particular interest to the bioprinting community

4.6 Reproductive System

Reproductive System Atala et al^81,82 implanted a collagen matrix containing autologous cavernosal smooth muscle and endothelial cells in a rabbit model. Results showed the development of penile tissue phenotypes, and cavernosography, cavernosometry, and mating investigations showed structural and functional biomimicry.

Al.⁸³ explored the possibility of developing a bioartificial testis as a viable option for fertility restoration of prepubertal boys who underwent chemo/radiotherapy. When these individuals reach the appropriate age, prosthetic testicles can be created using their cryopreserved immature testicular tissue (ITT) to restore their fertility.

4.7 Integumentary System:

In a rabbit model, Atala et al.^84,85 implanted a collagen matrix containing autologous cavernosal smooth muscle and endothelial cells. The formation of penile tissue phenotypes was demonstrated by the results, and structural and functional biomimicry was revealed by cavernosography, cavernosometry, and mating studies. In a two-cell layer model, the epithelial and endothelial layers are represented by A549 (alveolar epithelial type II cells; AT-II) (4.5 x 106cells/ml) and EA.hy926 (endothelial A549 cells and HUV-EC were combined to create a hybrid human cell line, and they were separated from one another by a thin layer of MatrigelTM (which symbolises the basement membrane).^86.87 Rimann and co. A multi-layered skin construct was produced using microvalve bioprinting.

In a PEG-based photo-polymerizable bioink for bioprinting, human primary dermal fibroblasts (9 x 106 cells/ml) and epidermal keratinocytes (1 x 107cells/ml) were suspended. One of the longest time periods evaluated among the other bioprinted skin experiments, the printed structures were described for up to 42 days. After culturing the printed dermal construct for varied lengths of time up to 6 weeks, epidermal layer printing was done.

Formation of stratum corneum was observed only on those constructs that were cultured for 6 weeks prior to epidermal layer printing and not seen on those constructs that were cultured for less than 6 weeks. This observation emphasized the importance of the quality of dermal layer for fabricating a biomimetic skin tissue.

4.8 Respiratory System:

Respiratory System Shan et al.⁸⁶ BMSCs (derived from the rabbit tibial plateau) were planted after creating a biomimetic tracheal graft utilising 3D printing (Fused Deposition Modelling) to test the graft's biocompatibility. Such works have been the subject of in-depth assessments elsewhere^87,88. To the best of our knowledge, just one study has been published so far on bioprinting an analogous lung tissue.

Using a bioprinting technique, a lung tissue analogue that accurately replicates the structure of the human air-blood barrier may be created. A549 (alveolar epithelial type II cells; AT-II) (4.5 x 106 cells/ml) and EA.hy926 (endothelial hybrid human cell line derived by fusing A549 cells with HUV-EC) cells represent the epithelial and endothelial layers, respectively, of a two-cell layer model, with a thin layer of Matrige lTM(representing the basement membrane) separating them.

The functionality of the printed construct is not assessed, despite the fact that this study was effective in fabricating a lung tissue mimic with an air-blood barrier topology.

4.9 Digestive System:

4.9.1 Liver:

Arai et al.⁸⁹ suspended primary hepatocytes (isolated from the liver tissue of male 6- to 8-week- old ICR 12 mice) in a galactosylated alginate hydrogel (12mg/ml)and used inkjet bioprinting method to fabricate a two-layered liver tissue.

Better cell adherence was facilitated by an interaction between the galactose and the hepatocyte-specific asialoglycoprotein receptor (ASGPR) chain of the galactosylated alginate hydrogel. The printed construct had morphological characteristics with native liver tissue and expressed proteins and receptors unique to the liver, including albumin, MPR2, and ASGPR, demonstrating the usefulness of the printed liver tissue. Using an extrusion-based bioprinting system, Kim et al.⁹⁰ printed mouse primary hepatocytes (isolated from the livers of mice aged 6–8 weeks) suspended in 3% w/v alginate hydrogel (4 x 107 cells/ml) into 3D liver tissue constructions. The liver-specific genes albumin, hepatocyte nuclear factor 4 alpha (HNF-4a), fork head box protein A3 (Foxa3), and asialoglycoprotein receptor 1 (ASGR1) steadily increased up to day 14 during the cells' 14-day viability.⁹¹

4.10 Urinary System:

4.10.1 The Kidneys:

Extrusion of urothelial cells (UCs) (isolated from New Zealand white rabbits) and bladder SMCs (isolated from rabbit bladder) suspended in a gelatin/fibrin/hyaluronic acid hydrogel (35, 30, and 3mg/ml respectively) were used to create scaffolds in a bioprinted urethra using a PCL/PLCL combination.

The printed construct had mechanical and morphological properties that were on par with those of the native tissue after 7 days, and post-printing cell viability was above 80%. Although the urinary system has been produced via bioengineering before, bioprinting has not yet been investigated in this setting⁹².

4.11 Circulatory/Cardiovascular System:

Circulatory/Cardiovascular System Duan et al.⁹³ bioprinted an aortic valve conduit from porcine aortic VICs and human aortic root SMCs (isolated from the aortic root of a 12-year old young patient) (2 x 106cells/ml) suspended in a gelatin/alginate hydrogel (0.06, and 0.05g/ml respectively). After 7 days, the post-printing cell viability was greater than 80%. This study only assessed the vitality of valve cells after printing. Other cardiac tissues save heart valves such as myocardium has also been bioprinted.

Gaebel et al. ⁹⁴ patterned HUVECs (4 x 106 cells/ml) and hMSCs (2 x 106 cells/ml) using laser-based bioprinting (LIFT method) on a polyester urethane urea (PEUU) cardiac patch. PEUU patch with randomly seeded cells without bioprinting was used as the control.

In vivo studies on Hearts of rats with infarcts (after left anterior descending (LAD)-ligation) demonstrated that the bioprinted patch with spatial cell arrangement had more blood vessel formation, enhanced capillary density, and resulted in significant improvement in the functionality of the infarcted hearts in rats, 8 weeks after transplantation, compared to the control.

5. CONCLUSIONS:

A wide range of diverse tissues related to various organ systems have been created using bioprinting, which has been the subject of substantial research. There are still many unexplored areas when it comes to bioprinting, such as lymphatic tissues and endocrine glands, even if some tissue types, such as bone and skin, have attracted a lot of attention. There are a number of common issues, which are addressed in the relevant parts, with applying it to each organ system.

The main difficulties include the scarcity of primary cells, bioprinting in therapeutically useful sizes, bioinks with suitable rheological properties, vascularization, and innervation. The issue of the scarcity of primary cells can be solved by employing stem cells to control differentiation to obtain the appropriate cell lineage. sues in clinically relevant sizes is another challenge. Extrusion The only technique that can now produce human-scale tissues is bioprinting.

However, it is a monumental joto ensure cell viability across the full thickness and to guarantee uniform perfusion of nutrients and growth hormones throughout the 3D structure. Alternative methods include bioprinting a developmental precursor or the self-assembly of several tiny tissue constructs into the entire organ.

The final obstacle and intriguing future approach is vascularizing and innervating the bioprinted constructions. Future study will focus on incorporating relevant growth factors and cells to encourage the growth of vasculature and nerves, as well as combining bioprinting and microfluidics to produce vascularized and innervated bioprinted tissues.

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Received on 08.05.2023 Accepted on 05.07.2023

Int. J. Tech. 2023; 13(1):57-67.

DOI: 10.52711/2231-3915.2023.00007