Introduction

The conventional therapies for diseased tissue substitutes are organ transplantation (allografts, autografts and xenografts) and medical implants. The ultimate goal of tissue engineering is to fabricate functional tissue for the replacement of damaged tissue. Tissue engineering has been used in the clinical applications to overcome the limitation of organ shortage. Tissue-engineering approach for regeneration of tissue can be broadly divided into two approach: top-down approach and bottom-up approach. In top-down approach, scaffolds are prepared; cells are seeded within it, matured inside bioreactor to regenerate desired tissue. However, this approach is unable to mimic the microarchitecture of tissue. Traditional 3D printing is among one of the top-down approach of tissue engineering. Limitations of these techniquesare non-uniform distribution of cells, low vascularization and limited diffusion. Whereas bottom-up approach overcomes the limitations of top-down approach. In this method, cellularized scaffold is prepared which mimics extracellular matrix. This approach has advantages such as high cell density and superior diffusion.

3D printing technique evolved from the additive manufacturing technique works on the basic principle of layer by layer assembly of biomaterial to prepare the require geometry. Many traditional 3D printing techniques like selective laser sintering (SLS), Stereolithography (SLA) and fused deposition modelling (FDM) have been in the field of tissue engineering and regenerative medicine. Using this classical 3D printer, solid scaffolds are fabricated using thermoplastics or resins as biomaterial (Gao et al. 2017). This type of system either polymerize liquid resin, het filament while passing through nozzle or sinter material of powder form. The principle of conventional tissue engineering is to seed isolated cells on the pre-formed solid scaffolds, placing the construct inside bioreactor and then implanting it into patients. The 3D printing technique are capable to meet requirements of hollow organs, but still there are some limitations. Non-uniform seeding of cells into constructs, placement of different types of cells in defined position, not able to reconstruct complex 3D organs. If the cell seeding is non-uniform through scaffold, then incomplete healing may occur. Moreover, high temperature and use of toxic solvent can decrease cell survival. Also increased time span for cell expansion may delay the treatment.

Hence, improvement in pre-existing 3D technology to tissue engineering leads to evolution of 3D bioprinting. Harsh processing conditions such as exposure to UV light, high temperature and organic solvents make 3D printing incompatible for the incorporation of cells while printing. Bioprinting allows printing of tissue constructs using hydrogel material at ambient conditions allowing incorporation of cells with biomaterials as ink. Current 3D Bioprinting technique overcomes the shortcomings of 3D printing technique by organizing cells, macromolecules, extracellular components and biomaterials in a defined pattern in three or two dimensions to mimic functional tissue. The biomaterials available for 3D bioprinting are polymers (PCL, PLGA), extracellular matrix material gelatin, collagen), proteins (silk) and hydrogels. The unique advantage of designing scaffold with 3D bioprinting alleviates the problem occurring due to limited donor by using cell-infused bioink to print tissue constructs. Bioink is the most important component of 3D bioprinting. It consists of biomaterials, cells, growth factors and bioactive components. The advantages of bioprinting technology are positioning of cells at accurate position, in situ printing of cells, can print high cell density tissues and vascularized tissues.

Till date, many tissue constructs have been prepared using in vitro bioprinting. However, this printing approach is nowadays replaced by in-situ and in-vivo bioprinting. In this approach, denovo organ are fabricated directly at the damaged site of the patient. The applicability of in vitro tissue printed outside to design functional tissue. The 3D Bioprinting technique helps to create tissue engineered constructs to heal the morbidity of damaged site leading to development of complex organ. 3D model of organ can be fabricated from DICOM data obtained from imaging techniques. Suitable image recognition algorithm used to extract desired structure and converted into STL format. From this STL file, a 3D model can be created using 3D bioprinter. 3D bioprinting technology is been used in wide range of application areas as depicted in Fig. 1.

Fig. 1
figure 1

Schematic representation of application of 3D Bioprinting in Biomedical field

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The fabrication of functional organ of clinical dimensions is very challenging. Some of the difficulties are addition of different cell types to prepare complex organ geometry, integration of vascular network, structure and functionality. Bioprinting is the simultaneous printing of proteins, living cells, DNA, drug particles, biomaterials, growth factor in a prescribed layer-by-layer fashion for the fabrication of organs, tissues, organ-on-chip model or tissue construct. The bioink solution deposited on the support material can be polymerized through various crosslinking mechanisms (ionic, photopolymerization or enzymatic). Based on the working principle, bioprinting technology can be dividedas: inkjet based bioprinting, laser based bioprinting and extrusion based bioprinting.

Types of 3D bioprinting

On the basis of material delivery system, the 3D bioprinting technique can be classified as Contact bioprinting and non-contact bioprinting (Mironov et al. 2006).

  1. a.

    Contact bioprinting There is a contact between substrate and biomaterial delivery tool and substrate. Ex- Extrusion based bioprinting

  2. b.

    Non-contact bioprinting There is no contact between substrate and biomaterial delivery tool and substrate. Ex- Inkjet based bioprinting

Three main type of 3D Bioprinting techniques are used widely nowadays which are described as:

Extrusion based bioprinting

In this 3D bioprinting technology, materials dispensed by force (either pneumatic or mechanical) through a nozzle. It is one of the least expensive methods and even relatively slow as compared to laser-based and inkjet-based technologies. In this bioprinting method, temperature of nozzle and working platform can be controlled, allowing printing of large variety of cell encapsulated biomaterials with high cell densities and viabilities. As resolution of extrusion, bioprinting is directly proportional to the diameter of the needle, material viscosity and effect of shear stress during dispensing taken into consideration with this type of bioprinting. The biomaterial is melted through deposition and fused after extrusion (Gupta et al. 2018).

Inkjet bioprinting

This bioprinting technology offers high-resolution patterning of small amount of biomaterial to form a substrate. The basic printing process of inkjet printer is: Firstly, formation of drop directed to specific location. Secondly, interaction of drop with the substrate. Based on formation of drops of ink, inkjet printing can be classified as: (1) Continuous inkjet printing (CIJ) (2) Drop-on demand printing (DOD). CIJ printing mechanism operates at higher frequency whereas DOD works at lower frequency. The droplet size formed by CIJ is twice of the orifice size. The small volume droplets formed by DOD has greater resolution (Gurkan et al. 2014).

In Continuous inkjet printing approach with application of external pressure, a force is applied on the ink allowing streams of droplets to come out of orifice continuously. As the ink in this approach is electrically conductive, the drops when formed are charged and is deposited on the desired place by magnetic or electric field. The extra droplets are captured in the reservoir and recirculated as shown in the Fig. 2.

Fig. 2
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Diagrammatic representation of continuous inkjet 3D bioprinting technique

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In contrast, in Drop-on demand printing, pressure pulse applied into ink and the drop is generated only when needed as illustrated in Fig. 3.

Fig. 3
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Figure showing drop on demand 3D bioprinting technique using a resistive element b piezoelement

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During printing process, the biomaterial to be printed in liquid form is jetted continuously onto the printer platform through the actuating units, forming ink droplets on the platform. Before the next layer of droplets added in the platform, ink droplet solidifies into the pre-defined geometry. Nevertheless, this technology has some disadvantages, as material to be printed must have low viscosity; stress generated during bioprinting can affect cell viability; due to printer head, clogging only limited biomaterials used. (Gupta et al. 2018)

Laser based bioprinting

The main components of laser based bioprinting is

  1. a.

    Pulsed laser source This pulse actuate metal film vaporization and hence forming droplet, which deposits on the receiving substrate.

  2. b.

    Focusing system

  3. c.

    Target to act as support for printing material (Ribbon) thin layer of bioink spreadson thin laser absorbing layer. For different types of bioinks and cells , different types of ribbon are needed

  4. d.

    Receiving substrate

The laser from pulsed laser source passes through focusing system (lens) falls on ribbon. This technique has additional advantage such as prevent cell clogging (as nozzle independent), have control on cell density, high deposition speed, control on cell organization and non-contact printing. A detailed description of the working principle of bioprinter explained elsewhere (Gupta et al. 2018).

These bioprinters have the ability to print viable cells of multiple types into spatially defined 3D arrays, which mimics the structure of tissue or organ. Following the development of bioprinters, it became popular to print patient specific tissue constructs to supplement the activity of the natural tissue. Some of them are discussed in the following sections.

Hard tissue engineering

3D bioprinting technique has the potential of regeneration of hard tissue such as bone and cartilage. A mineralized extracellular matrix is required for hard tissue engineering to mimic tissue environment (Sun et al. 2018).Hard tissues contain unique cell types such as bone consists of osteoblasts cell with calcified ECM and cartilage consists of chondrocytes cells. The biological functions performed by hard tissues are metabolism and hematopoiesis. The hard tissues require string mechanical support hence the biomaterial selected should provide strength to the construct developed.

Bone

Bone is a type of connective tissue with calcified ECM. There are three main type of cell: osteoblasts, osteocytes and osteoclasts as illustrated in 3D model of bone in Fig. 4. The main challenges associated with the bone bioprinting is selection of biomaterial (must be biocompatible, promote uniform distribution of cells, printable) and functional vasculature. All these challenges can be addressed by employing suitable bioprinting technique.

Fig. 4
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Schematic diagram of 3-D model of trabecular bone

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Keriquel and colleagues have demonstrated in-situ laser-assisted bioprinting of mesenchymal stromal cells for in-vivo regeneration of bone tissue with high resolution and precision (Keriquel et al. 2017).

Byambaa and co-authors fabricated large bone tissue construct containing perfusable vascular lumen with functional vasculature by utilizing extrusion-based direct-writing bioprinting method by utilizing gelatin methacryloyl (GelMA) hydrogel at low methacryloyl substitution as bioink (Byambaa et al. 2017).

Titanium due to its low antigenicity and good durability can be used as biological implant material. However, when it encounters tissues, load transmits into the implants and it tends to loosen with time. Hence a functional connection i.e. osseointegration of titanium implant is required. McBeth et al improved this osseointegration of titanium implant by printing a gelatin methacryloyl (GelMA) scaffold via extrusion based 3D bioprinter. This scaffold can be either grafted or printed directly on titanium implant. This scaffold formed promotes calcium mineral deposition by osteoblasts and normal human primary osteoblasts without any exogenous osteogenic factors (McBeth et al. 2017).

Hydroxyapatite is the main component of natural bone and poses osteoconductivity, which helps to create bone like microenvironment. The addition of hydroxyapatite in hydrogel improves osteogenic differentiation and cell proliferation (Demirtaş et al. 2017). Wang et al reported about impairment of bone regeneration due to high levels of tumor necrosis factor (TNF)-a. They utilized the antagonist effect of Atsttrin and combined it with alginate (Alg)/hydroxyapatite (nHAp) to repair complex calvarial bone defects and improved tumor necrosis factor (TNF/TNFR) signals (Wang et al. 2015). Bendtsen and co-author prepared a novel hydrogel using alginate, polyvinyl alcohol (PVA) and hydroxyapatite (HA) hydrogel into which mouse calvaria 3T3-E1 (MC3T3) cells were added. The porous scaffold formed promote enhanced, uniform healing of bone defect (Bendtsen et al. 2017). Even complex craniofacial bone defects can be reconstructed by bioprinting of stromal vascular fraction (SVF) derived cells within polycaprolactone/hydroxyapatite (PCL/HAp) hydrogel under short-term hypoxia condition. The autogenous vascularized bone printed proves the ability of 3D bioprinting in tissue regeneration applications (Kuss et al. 2017).

Hyaluronic acid (HA) is a high molecular weight glycosaminoglycan, one of the polymer present in the extracellular matrix to provide mechanical support to ECM. Poldervaart et al. made hyaluronic acid (HA) to be a 3D printable scaffold by modification with methacrylate groups and further addition of photo initiators. The resultant methacrylated hyaluronic acid (MeHA) gels formed is resistant to degradation, good biocompatibility, mechanical stiffness, allow primary cell survival, supports osteogenic differentiation and have low cytocompatibility. Hence fulfilling the required characteristics of bone tissue engineering (Poldervaart et al. 2017).

Comparison between alginate and chitosan as bioink when printed through extrusion based bioprinter in terms of mechanical support, mineralization and osteogenic differentiation were studied and it was shown that chitosan proved to be superior when compared to alginate (Demirtaş et al. 2017).

Cui et al. also prepared vascularized bone biphasic construct using FDM and SLA 3D bioprinter (dual 3D bioprinting approach). The polylactide (PLA) fibers and cell-laden gelatin methacrylate (GelMA) hydrogels and addition of hMSCs and human umbilical vein endothelial cells (HUVECs) to obtain biomimetic vascularized construct (Cui et al. 2016).

Bioactive glasses has the potential to provide angiogenesis, increase mineralisation of bone related cells and oesteogenesis. Wang et al studied the effect of bioglass on biomineralization of bone related SaOS-2 cells. For this alginate/gelatin, hydrogel is taken and supplemented with bioglass. In addition, founded that incorporation of bioglass increases the SaOS-2 cells potency to mineralize and proliferate (Wang et al. 2014). Gao et al also conducted research to evaluate the osteogenesis property of bioactive glass. For this they co-printed hMSCs suspended in poly(ethylene glycol) dimethacrylate (PEDGMA) with bioactive glass and Hydroxyapatite. However, founded that hydroxyapatite promotes oestogenesis as compared to bioactive glass (Gao et al. 2014). Murphy and co-author used two syringe based extrusion bioprinting system in which one syringe consists of PCL polymer, bioactive glass and chloroform and the other containing human adipose stem cells (ASCs) suspended in Matrigel. Firstly, syringe containing bioactive glass is allowed to print, on the top of which another layer of bio-ink is printed. This 3D bioprinted polycaprolactone/bioactive borate glass scaffold has potential to be used for bone tissue engineering with enhanced angiogenesis (Murphy et al. 2017).

Gao et al. fabricated hMSCS incorporated PEG-GelMA hydrogel scaffold using inkjet bioprinter having simultaneous photopolymerisation capacity. This scaffold shows improved mechanical strength required for bone and cartilage tissue engineering (Gao et al. 2015a, b). In further studies, Gao and coauthor combined acrylated peptides and acrylated poly(ethylene glycol) (PEG) to form hydrogel into which human mesenchymal stem cells (hMSCs) were added to prepare bioink for inkjet bioprinting to print bone and cartilage. The scaffold exhibited good biocompatibility with enhanced chondrogenic and osteogenic differentiation (Gao et al. 2015a, b).

Cartilage

Cartilage is a connective tissue, which lacks vasculature system and nervous system. It contains special types of cell called chondrocytes as shown in Fig. 5. Due to absence of blood vessels, diffusion of nutrition takes place through matrix hence has slow self-healing capability. Cartilage is a type of hard tissue but is elastic and flexible in nature. Cartilage is broadly classified into three types: Fibrous, hyaline (or articular) and elastic cartilage (Lee et al. 2014). For the fabrication of artificial cartilage, bioink containing chondrogenic cells are preferred.

Fig. 5
figure 5

Diagram showing 3-D model of cartilage

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In cartilage based tissue engineering, materials such as PCL, alginate and Methacrylate containing materials are blended with suitable hydrogel and cells to be used as bioink compatible with cartilage cells to promote chondrogenesis. Bioink selection is one of the challenge of bioprinting. To support chondrocytes phenotype, many researches on alginate-blended bioink is carried out. To regenerate cartilage by the use of multi-head deposition system (MHDS), Kundu et al. prepared polycaprolactone (PCL) framework into which chondrocyte cell-encapsulated alginate hydrogel were dispensed to prepare a hybrid scaffold to prepare cartilaginous ECM for cartilage regeneration (Kundu et al. 2015). Markstedt et al. combined the shear thinning properties of nanofibrillated cellulose (NFC) with alginate to bioprint this hydrogel after addition of chondrocytes cells to fabricate cartilage tissue (Markstedt et al. 2015). Muller et al introduced sulfated version of alginate to bind growth factors (fibroblast, transforming and hepatocyte growth factor). The gel disc made of alginate sulfate-nanocellulose material allows cell spreading and proliferation (Müller et al. 2017). Apelgren and team fabricate in vivo human cartilage by combining nanocellulose and alginate with human chondrocytes and hMSCs using extrusion printer (Apelgren et al. 2017). Abbadess and co-author prepared polyethylene glycol and partially methacrylatedpoly(N-(2-hydroxypropyl) methacrylamide mono/dilactate) triblock copolymers as hydrogel to fabricate cartilage repair construct. This combination of polymer supported cartilage formation and provide good mechanical property (Abbadessa et al. 2016).

Mouser and co-author bioprinter chondrocytes laden Gelatin-methacryloyl (GelMA) blended with gellan gum to evaluate its suitability for use as bioink for cartilage fabrication. In addition, founded that this hydrogel supports cartilage matrix production and at high yield stress, viscosity is decreased resulting in high printability of hydrogel (Mouser et al. 2016).

In this study, Cui and coauthor suspended human chondrocytes into poly(ethylene glycol) diacrylate (PEGDA) to print neo-cartilage by delivering the bioink at precise location using thermal inkjet printing method. This neo-cartilage formed exhibits outstanding collagen (typeII) and glycosaminoglycan (GAG) production (Cui et al. 2016).

Soft tissue engineering

Soft tissue engineering is challenging as it aims to treat defect with high volume to surface ratio.

The prevailing therapeutic treatment by surgery or autologous tissue transfer introduces further complication of morbidity at donor site and even scar formation at injury site.

Vascular tissue

Any tissue construct either large or small tissues or organ requires vasculature to allow blood flow for its functioning. The incorporation of vessel-like structure is of critical importance to develop functional tissue suitable for tissue regeneration. Extracellular matrix based biomaterials (ex-gelatin, collagen) have limited mechanical strength to withstand the weight of hollow tubes but are biocompatible and allows cell proliferation. Hence blended with suitable biomaterial having mechanical stability to be used as bioinks to fabricate highly organized and intact vessels.The vascular tissue is composed of three main layers: Tunica interna, tunica media and tunica external as shown in Fig. 6. The innermost layer (tunica intima) consists of endothelial cells and provides resistance to infections and non-thrombogenicity. The next layer is the basement layer containing collagen type IV, laminin. The middle tunica media layer contains collagen type I and III and smooth muscle cells. These smooth muscle cells is responsible for the dilation and contraction of vessels (Yurie et al. 2017). The composition of the outermost layer named tunica adventitia is fibroblasts. There is a coordination between all the layers to perform their function such as perfusion. Depending on the size of vessels, vasculature can be classified as macrovasculature and microvasculature. Macrovasculature contains arteries and veins whereas microvasculature consists of arterioles, capillaries and venules (Yuan and Rigor 2010).

Fig. 6
figure 6

Schematic diagram of 3-D model of blood vessels

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The interconnected vascular networks within tissue constructs supports transportation of nutrients, oxygen and waste products has many applications in organ repair and transplantation. Conventional microfabrication techniques are unable to create such perfusable hollow vascular tubes leading to formation of necrotic region in bulk tissues. Jia et al biofabricated the vascular network through patterned cell and biomaterial deposition using an advanced extrusion system with cell responsive bioink containing gelatin methacryloyl (GelMA), sodium alginate, and 4-arm poly(ethylene glycol)-tetra-acrylate (PEGTA) (Jia et al. 2016).Kolesky and co-author constructed thick vascularized tissue (≥1 cm) using fugitive ink (pluronic and thrombin), cell-laden ink (gelatin, fibrinogen), silicone ink and cells as bioink to print construct within 3D perfusion chips (Kolesky et al. 2016) (Fig. 7).

Fig. 7
figure 7

Diagrammatic representation of 3-D model of skin

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Construction of small sized vessels of diameter <6 mm (periphery artery, coronary artery) is very challenging. Lee et al bioprinted small diameter vessel by adding bone marrow- derived mesenchymal stem cells (bMSCs) in sodium alginate as second layer on the top of diagonal cross- stripped PCL scaffold layer. Then a third layer of helical form PCL scaffold is placed on the top of second layer. This study is conducted to study the effect of pulsatile flow on bMSCs and it is observed that cells differentiated into endothelial-like cell (Lee et al. 2018).

Skin

Bioprinting technique is feasible to print artificial skin by combining two or more types of skin cells. In case of extensive burn injury, it is difficult to opt conventional treatment techniques such as autografts, allografts. This treatment options has many disadvantages such as limited size, very long preparation time and contracture.

Kyle et al proposed on-site in situ skin repair by printing keratinocytes and fibroblasts directly on the wound model using inkjet bioprinting. These constructs leads to production of skin identical to normal murine skin (Binder et al. 2010). Skardal et al. printed collagen-fibrin gel containing Amniotic fluid-derived stem (AFS) cells over the wound of mice for skin regeneration. The incorporation of AFS cells modulate inflammatory response and the prepared construct exhibit increased neovascularization and maturation of blood vessel (Skardal et al. 2012).

Tissue engineering for application in specific organs

The only treatment for end-stage organ injury/ failure is replacement of organ. There is an unmet demand of organ donors. This gap of donor organ can be covered by fabricating artificial organ using 3D bioprinting. This section explains about the status of organ bioprinting. The potential of organ bioprinting is to convert bioprinted cell laden hydrogel into vascularized organ like structure.

Liver

Liver is the largest and unique organ responsible for blood detoxification and plays an important role in metabolism of drugs. Some of its important functions are: bile secretion, detoxification and homeostasis. Anatomically the hepatocytes are polyhedral cells constituting 80% liver volume as depicted in Fig. 8. The liver function decreases with damage of hepatocytes leading to liver failure. The end stage liver disease, chronic liver injury or tumors of liver origin leads to liver malfunction. The advancement of stem cell based research allowed differentiation of stem cells into hepatocytes leading to development of artificial liver either by liver support systems, 3D bioprinting or by seeding cells in decellularized scaffold. Jones and others addressed for the first time the bioprinting of human induced pluoripotent stem cells (hiPSCs) using valve-based printing technique and established that post-printing hiPSCs differentiates into hepatocyte cell types (HLCS) (Faulkner-Jones et al. 2015). Ma et al. employed DLP based 3D bioprinter to print in vitro 3D liver model mimicking several features of liver by patterning hiPSC-derived hepatic progenitor cells (hiPSC-HPCs) in GelMA as first layer. And subsequently printing supporting cells such as human umbilical vein endothelial cells and adipose derived stem cells in GelMAand GMHA as second layer (Ma et al. 2016).

Fig. 8
figure 8

Schematic diagram of 3-D model of liver

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Bioprinting approach for liver model fabrication includes fabrication of 3D architecture combined with bioreactor to prepare organ-on-chip platform. During clinical trial of new drugs, the liver is most susceptible to toxic chemicals present in drugs. As liver is responsible for clearance of those toxic agents and can cause dame of liver i.e. hepatotoxicity. Hence, nowadays prediction of the toxicity of new drugs on tissues can be done quickly using liver-on-chip or mini liver (microfluidic chip containing liver cells) instead of performing in-vivo studies. Bhise and co-authors fabricated and demonstrated the use of a liver-on-chip platform, as a means to assess the drug toxicity. They developed perfusable bioreactor with printer to bioprint human hepatocarcinoma cells (HepG2/C3A) spheroids. The printed spheroid mixed with GelMA and 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone solution to again bioprint this solution into glass slide to prepare hepatic construct (Bhise et al. 2016).

To assess the drug induced liver injury (DILI) many researchers have created liver tissue model to perform drug diffusion and test toxicity. Nguyen and co-author bioprinted human hepatic stellate cells (HSC), human umbilical vein endothelial cells (HUVEC), and cryopreserved primary human hepatocytes containing bioink into plates to develop 3D liver tissue (Nguyen et al. 2016). Zhong et al prepared bioink by combining L02 cells (human hepatocytes cell line HL-7702) with collagen and chitosan solution buffered with morpholinoethanesulfonic acid solution (MES) and NaOH hydrogel to print 3D scaffold for liver tissue engineering (Zhong et al. 2016). In related studies, Massa et al. utilized sacrificial bioprinting technique by printing agarose fiber into HepG2/C3A cell laden GelMa hydrogel poured in PMMA mold. The mold then exposed to UV light for crosslinking polymer solution and agarose fiber then removed manually to form hollow microchannel in the construct. In addition, human umbilical vein endothelial cells (HUVEC) is addedto prepare vascularized liver tissue model (Massa et al. 2017) (Figs. 9, 10).

Fig. 9
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Kidney-on-a-chip in vitro model

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Fig. 10
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Diagrammatic representation of 3-D model of bladder

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Kidney

The kidney derived from mesoderm is the main organ of the body responsible for excretion of waste, xenobiotic and water homeostasis. The nephron is the functional unit of kidney containing glomerulus (for filtration of blood) and renal tubule (for flow of glomerular filtrate). The proximal renal tubule is near glomerulus and is responsible for renal filtrate transport to blood. Hence can be affected by accumulation of high quantity of circulating drugs or xenobiotic. The nephrons are connected to ureter via collecting duct.

Renal damage can be done at many locations like glomerulus, collecting duct or proximal tube. Renal injury such as acute kidney injury (AKI), end stage renal disease (ESRD) or chronic kidney disease (CKD) demands kidney regeneration treatment such as dialysis, transplantation and bioprinting (Zhang et al. 2018). The number of kidney donors are not sufficient to fulfill the requirement of the increased patients. The ability of differentiation of stem cells into renal cell types helps to prepare bioengineered kidney using bioprinting technique.

As stated earlier proximal tubule are more prone towards reel injury. Hence, many researchers have worked on the same. King et al bioprinted 3D proximal tubule containing interstitial cell types containing renal proximal tubule epithelial cells (RPTEC) model to study epithelial-interstitial interaction and human nephrotoxicity (King et al. 2017). Homan et al developed 3D convoluted renal proximal tubule on kidney-on chip device by printing firstly the fugitive ink on gelatin-fibrinogen ECM layer deposited at gasket. The 3D proximal tubule is formed by extruding fugitive ink from the nozzle of 3D bioprinter. Then additional ECM is casted on the top of printed fugitive ink. Model placed inside perfusable chip and liquefied to 4°C to remove the fugitive ink resulting into a tubular channel. Within this tubule, PTEC cells are introduced which after a period of time becomes polarized epithelium (Homan et al. 2016).

Bladder

Total substitution of bladder is required for patients undergone cystectomy. Many problems like bladder cancer, trauma, infection, inflammation, iatrogenic injury, interstitial cystitis can lead to bladder injury (Gürpinar and Griffith 1996).Problems associated with the artificial bladder are deposition of calcareous material, urine leakage resulting in peritonitis, encrustation and infection. Hence, artificial bladder should be made up of inert material, must be non-irritating and non-allergic. Bladder reconstruction can be done using prosthetic bladder. Bioprinting technique has also been able to develop artificial bladder to treat bladder damage. There is still limited number of literature available on printing of bladder, but still this 3D bioprinting technique is of great significance for bladder regeneration. Xu and co-authors developed a cell-encapsulating droplet generation system to print building blocks within a sterile hood to control humidity and temperature. This building blocks assembles together to create an SMC patch. This patch was cultured for 51 days to form 3D tissue construct similar to native rat bladder (Xu et al. 2010). Recently Zhang et al. used integrated bioprinting technology to fabricate urethra with poly(ε-caprolactone) (PCL) and Poly(lactide-co-caprolactone) (PLCL) thermoplastic polymers along with bioink made of fibrin, gelatin, hyaluronic acid as hydrogel with urothelial cells (UCs) and smooth muscle cells (SMCs) as cellular component to mimic the natural urethra of rabbits. With printing of tubular scaffold, simultaneously the bioink were delivered into scaffold layers formed (Zhang et al. 2017). Imamura et al. reconstructed urinary bladder using 3D bioprinting robot system. The biofabricated structure contains bone marrow-derived cells, which differentiate into smooth muscle cells to fabricate urinary bladders (Imamura et al. 2018). More development is still required to meet the ideal requirements of fabrication of artificial bladder. This bioprinting approach using inert biomaterial would find out the problem of calcareous deposits.

Retina

Retina responsible for the perception of sensory information is a nervous tissue that contains rods and cone photoreceptors. Human retina has immense structural complexity as shown in Fig. 11, it is highly vascularized tissue containing different cell types like horizontal cells, amacrine cells, retinal pigment epithelial cells, retina ganglion cells, bipolar cells, glial cells as well as muller cells. There are 55 different cell types and 105 per square millimeterphotoreceptor cells in the retina (Peterman et al. 2003). All these retinal cells must act in harmony with each other to transmit visual information to brain. Retinal pigment epithelium (RPE) is the outermost layer of retina containing single layer of hexagonal pigmented polarized epithelial cells. This RPE cells are responsible for the proper functioning of visual process and are difficult to duplicate. Any malfunction of RPE cells may cause retinal disease. The choroid is layer providing vascular structure for the supply of nutrients and oxygen to other layers of retina. In between RPE and choroid lies the Bruch’s membrane (Komez et al. 2016). The ganglion cells and the photosensors lies in the innermost and outermost portion of retina respectively. Light travels from the retina, strikes the photoreceptors and activates it. The photons gets absorbed by the visual pigments of photoreceptor and is translated into electrical signals. The electrical signals simulates neurons of retina and are transmitted to the brain from spike discharge pattern of ganglion cells. For vascularization of retinal layers, blood vessels passes through the retina. In some retinal disease, only particular cells need to be replaced as retina ganglion cells in glaucoma, but in others retinal area need to replaced. The retinal cells deteriorates with age or due to diabetes or occlusion and fails to regenerate which promotes the need of cell transplantation and 3D bioprinting techniques. Cell transplantation have limitation of axion orientation at in vivo condition. Bioprinting of complex retina containing many cell types too is very challenging. The scaffold for retinal tissue engineering must be porous, biocompatible, not induce foreign body response, mechanically stable and must be thin.

Fig. 11
figure 11

Schematic diagram of 3-D model of retina

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With advancement in stem cell biology, retinal equivalent construct can be bioprinted by using stem cells responsible for retinal tissue engineering such as embryonic stem cells (ES) and induced pluripotent stem cells (iPSC). Reconstruction of destroyed retina can be done by using the differentiation capability of induced pluripotent stem cells into different types of retinal cells (retinal pigment epithelial cells, retina ganglion cells etc.) (Yang et al. 2017).

The retinal tissue engineering may include development of Bruch’s membrane, printing of retinal pigment epithelium, retinal glial cells and retinal ganglion cells. The bioprinted scaffold act as a substitute to attach different types of retinal cells.

Among different types of retinal cells, retinal ganglion cells (RCGs) placed at ganglion cell layers transmits visual message from photoreceptors to brain, but it fails to regenerate. Important consideration for the RGCs development is to maintain the axon orientation as compared to existing axon orientation, RGC positioning and cell survival rate. These challenges can be meet by the providing physical support to the cells by means of scaffold. The scaffold must have potential to guide axon orientation, cell adhesion, cell differentiation, cell proliferation and cell migration. Kador and coauthors combined thermal inkjet 3D bioprinting technique to place RGCs precisely on electrospun scaffold surface to guide RGC neurite outgrowth (Kador et al. 2016). Lorber et al studied the effect of inkjet printing methods on retinal ganglion cells (RGC) and glia cells. And founded that even though there is reduction in cell population but there is no effect on the survival rate of cells during jetting (Lorber et al. 2014)

Bruch membrane is a specialized membrane which acts as a thin (2–4 µm) barrier between choroid and retina. It is an important component of retina tissue engineering as it acts as scaffold for cell growth. Its key characteristics are: ultrathin, porous (to have controlled diffusion rate) and permeable (to allow nutrient exchange). To address the pore-size distribution limitation of Bruch’s membrane, Tan et al. developed an ultrathin PCL membrane with regular and interconnected pores instead of larger pore and thick film reported earlier. The low cytotoxic membrane developed showed improved ARPE-19 cell arrangement forming a well-developed barrier to allow homeostatic functions (Tan et al. 2017). To extend the application of this ultrathin membrane Shi and coauthor combined human retinal pigmented epithelial cell line (ARPE-19) and the human retinoblastoma cell line (Y79) containing alginate/pluronic bioink to prepare retina model using microvalve based bioprinting method. Here the bioprinted retina model prepared using ARPE-19 cell seeded on ultrathin membrane represents RPE monolayer and Bruch's membrane into which Y79 cell containing bioink is printed (Shi et al. 2017).

Conclusion and future trends

The printing of functional human organ to overcome the issue of organ shortage is still being required to revolutionize the healthcare industry. There remains a need to develop 3D bioprinted functional organs of relevant dimensions. The combination of imaging techniques, cell-laden inks and 3D bioprinting technologies will bring a new concept in tissue engineering and regenerative medicine. The 3D printed tissue or organ based on imaging data are prone to imaging errors. Hence, imaging must be done accurately to create patient specific construct. It is important to check the response of cells post printing as cells are subjected to shear stress either at nozzle or at orifice and also experiences pressure, high frequency or heat to start the process of bioprinting. In organ-derived scaffold, no cells should remain at the time of transplantation to avoid immunological rejection.

Despite of so many researches, so far no ideal material is available for 3D bioprinting of bladder. The biomaterial for bladder reconstruction must be able to contract and expand as walls of bladder undergoes mechanical deformation during the normal function of bladder.

Also it is important to perform a detailed research on printing of different types of retinal cells and even photoreceptors using multi-head bioprinter to allow fabrication of functional retina. The main challenge of retinal bioprinting is to maintain cell density to develop the layers of retina, whichcan be addressed by the use of multiple passes of stem cells. Different retinal cells must show synaptic simulation among each other allowing proper transmission of visual signal.

Fabrication of stem cell based organoids has the ability to represent complex architecture, multilayer and multicellular organ model to resemble different types of organs similar to in-vivo counterparts. With this organoids, it is easy for researchers to develop new disease treatment.