Current advancements in the development of bionic organs using regenerative medicine and 3D tissue engineering

ABSTRACT

Bio-artificial organs, commonly referred to as bionic organs, are artificial structures that perform the same functions as natural organs. In regenerative medicine, damaged organs are repaired using biological components including growth factors and stem cells. The advancement of tissue engineering, which aims to harness the inherent regenerative potential of human body organs to rebuild normal biological function, has benefited greatly from the use of biomaterials. The rapidly expanding field of regenerative medicine has brought the usage of biomaterials and their functions in the production of new tissue. Organs-on-chips are seen as a concept performer in tissue engineering with significant potential for future ‘clinical trials on a chip’ and a step towards developing customized medicine. With the advancement of 3D printing technology, the manufacturing constraints of biomedical devices have been solved using cutting-edge biomimetic structures. This review covers the areas on bionic organs such as organ on a chip with tissue engineering, bio grafting, 3D bio-printing and the stem cell techniques that are employed in regenerative medicine.

KEYWORDS:

• Bionic organs
• 3D tissue engineering
• regenerative medicine
• biomaterials
• biomimetic

Introduction

The human body is made up of intricate structures and material systems, current manufacturing techniques, which are constrained in their ability to choose and fabricate materials, are unable to produce biomedical devices that have intricate bio-inspired architectures and structures, which hinders the development of biomedical devices for a variety of applications. Due to their exceptional fabrication abilities, 3D printing techniques are widely used in many industries, including biomedical engineering, automotive, aviation, telecommunications, civil engineering, and sustainable energy [1,2]. By layer-by-layer depositing materials, such as metals, polymers, and ceramics, three-dimensional (3D) printing, sometimes referred to as additive manufacturing, is a technique that can create items with complicated architecture. Technology for 3D printing in the medical areas is a promising method for individualised care [3]. One of the most alluring applications of bioprinting and bio fabrication is the development of biological 3D tissue models, such as those for drug screening, disease models, tissues or organ on a microchip, healthcare sensors, and biological actuators. To aid surgeons in their work and to better educate patients about their illnesses, 3D copies of a patient’s damaged organ could be created. Scaffolds and implants are created using 3D printing processes for regenerative medicine. Stem cells for example, are routinely expanded ex vivo as part of the current tissue engineering approach before being transplanted into injured sites. Stem cells have considerable promise in tissue engineering and reconstructive therapeutics because of their distinct regeneration capacity and immune modulatory qualities. Tissue engineering depends heavily on biomaterials, but this does not mean that conventional, artificial biomaterials must be employed [4]. By using stem cells sown on artificial structures to replace damaged tissues, tissue engineering aims to regain the normal regulation of cell activity. The 3D surroundings inside the stem cell niche have an impact on stem cell therapy self-renewal and differentiation [5]. The scaffold approach has been abandoned, and attempts to create 3D tissue using cells and growth regulators are now regularly documented. Bio fabrication is the term used to describe this approach, which includes organ printing and bioprinting [1].

The creation of novel biomaterials-bound techniques that better imitate tissue and organ architecture is largely responsible for the recent successes in tissue engineering [6]. Tissue engineering can be approached in a number of ways, including scaffold- and biomaterial-based procedures, the use of decellularized organic substances, scaffold-free techniques, and the inclusion of cellular components. The factors around a cell or a group of cells that have a direct or indirect impact on cell activity via physicochemical, biochemical, or other processes make up the cell microenvironment. Given that stem cells are presently the most technically possible source that can supply the significant number of cells required for engineering clinically relevant portions of tissue, the microenvironment of stem cells is an especially significant subject matter in regenerative medicine and tissue engineering [7]. The potential and capacity to rejuvenate and rebuild injured organs and tissues is the foundation of the potential of regenerative medicine. With the potential to even treat some congenital defects, regenerative medicine has demonstrated encouraging outcomes for the restoration and regeneration of a number of tissues and organs, such skin, heart, kidneys, and liver. The components used in regenerative medicine strategies must be capable of replacing the damaged tissue/organ and operate as the original tissue/organ or be able to promote the restoration of the original tissue tissue in order for them to be effective. These components are typically mixtures of scaffolds, biomaterials, growth factors, and stem cells. In the past ten years, the Food and Drug Administration as well as the European Medicines Agency have approved a number of 3D-bioprinted structures and stem cell treatments. These treatments and goods include biologics, medical equipment, and biopharmaceuticals [8]. In this review, we will focus on different aspects of tissue engineering applications with particular focus on bionic organs, 3D bioprinting, tissue grating, stem cells and regenerative medicine.

Bionic organs

An organ is a unique part of the animal or plant that carries out special functions. These organs can become damaged and should be replaced with a prosthetic device or artificial organ [9]. These replacement devices are often made of natural-based or synthetic-based materials and should be compatible with the blood and tissues. The bionic organs resemble the functioning of the original defective organ. These bionic organs are broadly divided into four types as depicted in Figure 1 [10].

Figure 1. Classification of bionic organs.

The bionic or artificial organ serves as a substitute for a biological organ in the human body. An organ is described as a specialised structure such as a brain, heart, and lungs in a human or animal which is made up of distinct tissues and cells and is specialised for its particular functions [11]. The bionic organ development technology originated as a curative therapy for many difficult chronic illnesses with the use of stem cell technology along with growth factors and biomaterials. Although it is still in the development phase, regenerative medicine and tissue engineering can reduce the challenges associated with bionic organ development and implantations (Figure 2) [12].

Figure 2. Development of bionic organ.

Classification of bionic organs

Sensory organ

For the replacement of the outer portions of the ear, polymeric biomaterials like silicons can be used. Additionally, some study has included inserting electrodes into the cochlea and connecting them to an outside microphone [13]. People who are deaf have been given considerable hearing restoration because of using these gadgets. Generally, plastic is used for coating the cables and electrical components of the device. Several people wear contact lenses, which are the most useful application of polymeric materials. Hydrogel polymeric materials are used to make these contact lenses [14,15]. Additionally, some research is being done to directly stimulate the visual part of the brain using electrodes coupled to a camera, giving the illusion of vision to the blind.

Internal organs

The heart, liver, kidney, pancreas, gastrointestinal tract and lungs are among the artificial internal organs. These prosthetic replacements frequently perform specific tasks that are hard to replicate with artificial materials [16]. Over one million pacemaker devices are implanted every year to control heart rate and these devices have biomaterial coatings to shield the electrical components from the bloodstream. To cure Stokes-Adam’s illness, pacemaker devices have been used on animals for experimental studies since 1932 and people since 1952 for treating heart blocks. Even though the heart has four chambers, the arterial or ventricular valves are always replaced [17]. These valve replacements come in a wide range of designs that make use of both synthetic-based and natural-based biomaterials.

Tissue replacement

The skin is the greatest organ in the body. Although several people are seriously injured each year from skin damage that requires rapid treatment to avoid microbial contamination, there is currently no substance that can replace the functioning of skin [18]. The effective solution to this issue has made use of a composite material with an outside layer made of silicone rubber and an interior membrane made of collagen-glycosaminoglycan. Other composite materials include collagen, polypeptides, dextran and hydrogels. These materials effectively mimic the skin’s barrier functions, but none of the materials have been able to mimic the other functions of the skin [19]. The human body is mainly composed of connective tissues, soft tissues and fatty tissues. This kind of tissue is used extensively in the field of reconstructive or plastic surgery. At least 30,000 hernia repairs, 100,000 breast augmentation and 100,000 face plastic operations are performed annually [20]. An appropriate soft tissue material must be physically suitable over the long term and should not have any negative effects on the patient and only a small range of materials can satisfy the required needs. Although polymers and fabrics may contain the right amount of soft texture, the ingrowth of fibrous tissue rapidly makes these materials unsuitable since the implantation is becoming more stiff or hard.

Joint/bone replacement

To treat an injury or chronic disease-related condition, it is essential to replace a damaged joint with a prosthetic device. Several joint replacement implants are performed each year with a poly-dimethylsiloxane polymer [21]. Silicone polymer has also taken the place of several applications, including hand bone replacement. A metal ball comes into contact with a plastic surface consisting of high-molecular-weight polyethylene biomaterial in other types of joint replacement such as the knee and hip. Most of the time, a cement-like material made of polymethyl methacrylate that is polymerised in place is used to secure these plastic and metal components to the body [22]. Designing an effective prosthesis arm is more challenging than designing a hand. It is more challenging to develop lower limb implants than upper limb implants. This is because the main use of the lower limbs is walking, in contrast to numerous upper limb usage which cannot be accomplished with one limb [23]. Although there is still more work to be done in this field, advanced electrical stimulation-actuated devices offer great potential for mimicking the original arm functions [24].

Organs-On-Chip (OOC) development

The Organs-on-chip (OOC) that emerged in 2010 is regarded as an extremely promising prospect for studying the mechanisms behind drug screening and organ functioning platforms with future developments to be utilised for targeted therapy and clinical diagnostics (Figure 3) [25]. Donald Ingber was the first to establish the idea of OOC. A microfluidic culture known as an OOC has chambers that are continually stimulated with live cells inside of them to mimic the motions, physiological reactions and biomechanics of tissue or organ. The first lung-on-a-chip model was created by Dr. Ingber and his colleagues at the Wyss Institute. It is a microfluidic chip that simulates breathing, highly resistant lungs made of human passages, vasculature, and inflammatory responses [26].

Figure 3. Organ-On-Chip development.

Other early instances of OOC, such as lymph node-on-a-chip, blood vessels-on-a-chip and intestine-on-a-chip have been introduced. Then more advanced devices that link many organ systems were introduced. OOCs are models produced by integrating advanced chip technology, which may offer these new models. In comparison to other methods mentioned, this can modify human reactions such that they can be effectively mimicked in the laboratory [27]. In the chip tissue models, microfluidic technologies provide unique and improved controllability over cultural and cellular conditions. The idea of OOC refers to the emerging autonomous and active tissue and organs system with a structure that aims to mimic in vivo organ conditions [28]. Another critical step for the modification and analysis of cellular activity is the integration of simulation and sensor technology on-chip and their interface with cell cultures. The various organ development using the chip and its function has been mentioned in Table 1 [29].

Table 1. Various Organs-On-Chip and its functions.

Different Organs-On-Chip	Function	Reference
Brain-on-chip	The brain is among the most complicated organs. Medical researchers with interests in neuroscience, neurodegenerative illnesses and pharmacological investigations on the blood-brain barrier have been particularly interested in this organ for decades. To study human cell development and chemotoxicity several attempts have been made to construct in vitro studies.	[30,31]
Lung-on-chip	For many years, the proportion of lung illnesses has been rising. Lung transplantation is the best medication for these illnesses. An attempt to develop an in vitro heart-on-a-chip device for pharmaceutical analysis and molecular diagnostics uses 3D bioprinting using scaffold biomaterials for constructing myocardium tissue.	[32,33]
Heart-on-chip	Creating the cardiac medicine and toxicity of the drug and there have been problems with medication interactions. Before clinical trials, microscale on-chip systems provide an effective platform for conducting economical and quick drug testing.	[34,35]
Liver-on-chip	The design of a suitable in vitro model for assessing hepatotoxicity and evaluating drug processing is in high demand. Recent developments in 3D bioprinting and microfabrication methods have led to the development of liver-on-chip systems.	[36–39]
Bladder-on-chip	Finding a means to detect bladder-related cancer at an early stage is essential for improving patient chances of survival. A study team has been working on the quantitative detection of the Galectin-1 (Gal-1) protein which is a marker for the development of several cancer-related illnesses including bladder cancer. Entrapped Gal-1 antibodies on the microelectrode surface were employed to increase selectivity and immobilisation.	[40]
Uterus-on-chip	The female reproductive system is very complex. A whole uterine system reproduction on a microfluidic chip might have significant effects on in vitro fertilisation-embryo transplantation (IVF-ET) and medication analysis of toxicology and effectiveness.	[41,42]

Challenges in bionic organ development

Bioartificial tissue design is not simple despite the fact it is created using a variety of component tools, including scaffolds, bioreactors, growth and differentiation agents, cells and so on [43]. None of these parts make up a device with separate functions, hence neither does their combination in an in vitro method for bionic organ synthesis result in a product with a modular structure. Biofilm development is one of the challenges of bionic organ development which is detailed in Table 2.

Table 2. Challenges in the development of bionic organs.

Sl.No	Challenges	References
1.	Flexibility is low and highly limited	[44]
2.	Limited for long-period storage	[45]
3.	Biofilm development	[46]
4.	Biomaterial preservation	[47]
5.	Developing and building devices with desired features	[51]
6.	Transparency in implants and the associated features	[48]
7.	Combining biomaterial to a defined structure	[49]
8.	ECM progress is slow.	[45]
9.	Specific effects that are related to cells and tissues response	[50]
10.	Structural loss due to contaminations and loss of bioavailability	[35,51]

Biofilm development in implants

A bionic organ or a prosthetic device develops bacteria growth and becomes the cause of device-related infections known as biofilm development [52,53]. The definition of biofilm was suggested by Donlan and Costerton as a microbially formed motile organism containing cells that are physically attached to surfaces or their interfaces, incorporated in matrix substances they have developed and displaying irregular growth patterns and expression of genes [54]. This new generation of environmental diseases is especially dangerous to vulnerable people who might not have recovered previously. They make use of the biofilm approach that has worked so effectively for them in their natural surroundings. Some human illnesses are not linked to devices but are driven by bacteria that form a biofilm [55]. It is debatable if biofilms are the true aetiological agents in these situations because the conventional Koch’s postulate cannot be used. Heart infections are caused by staphylococci 25%, fungi and gram-negative bacteria caused 30% and Streptococci produced 56% of infections [56]. They enter the bloodstream through the gastrointestinal tract.

Biofilm in medical devices

Over the past 20 years, a great deal of study has been done on the biofilms that develop on various medical implants. According to Donlan and Costerton, a characterisation of the biofilm’s attachment on certain devices, including artificial heart valves, urinary bladder, central venous catheters, intrauterine devices and contact lenses [57]. For some devices like contact lenses research has also clarified the sensitivity of different materials to microbial attachment and biofilm development. The implants that are mainly harmed by infections are dental implants, neurological implants, ophthalmological implants and orthopaedic fracture fixation implants [58]. Along with adhering to hygiene standards while implanting medical devices, another emerging approach is the development of novel materials that could be resistant to microbial adhesion and colonisation [59]. The best options to prevent colonisation and contamination would be medical devices constructed of a material that is generally colonisation-resistant or antiadhesive.

Dawn of tissue engineering in bionic organs

The tissue engineering approach is evolving rapidly to meet the challenges and main issues, taking into consideration the advancements in tissue engineering using animal modelling, scaffold preparation techniques, and 3D bioprinting [60]. The tissue engineering field has great potential for overcoming a variety of obstacles with inventive advancements, and will soon bring numerous advancements that can effectively be applied to humans [61]. There are new opportunities for research in several scientific fields, such as genetics, human biology, pharmacokinetics, and others by involving stem cells and tissue engineering for the regeneration of bionic organs [62]. The fundamental of tissue engineering is to transform the cells that can begin and maintain the regeneration process, maybe using growth hormones or genetic material, so that they can produce new, functioning tissue of the requirement [63]. This can be done with scaffold biomaterials to direct the structural form of the new tissue, and it can happen on an individual basis at the location of the tissue or organ damage in a single patient or on a larger scale in a fermentation broth by which the newly generated tissue is implanted into the patient body [64].

Biomaterials for bionic organ development

The primary function of biomaterials in tissue engineering is to regulate the activity of the regenerated tissue also providing temporary mechanical support and mass transfer to promote cell attachment, multiplication, and division. Considering that extracellular matrix (ECM) biomaterials contain the intrinsic signals vital for interacting with and controlling niche cells [65,66]. Biomaterials which are frequently referred to as scaffolds may present biological and chemical signals with spatio-temporal accuracy (Figure 4). These signals are of high significance to the regulation of cell effectiveness and function and tissue regeneration. To help in the restoration of a tissue’s entire function and structure, tissue-engineered structures must resemble some of the natural complexity of a tissue [67]. A scaffold should act as a temporary construction that over time, will be gradually decayed or recycled back in line with the rate at which new tissue is developing. The potential of a biomaterial to regulate human host reactions to externally transplanted cells may alter cellular mechanisms and function and in turn, have a significant impact on the formation of the desired tissue [68]. The developments in the field of biomaterials, along with recent advances in the understanding of the biology of the (ECM) and the significance of environmental factors in tissue development, have resulted in the new design of material templates that are altered to provide suitable structural support and in some cases, to promote the safe and successful regeneration of tissue [69]. Using scaffolding materials to build artificial organs and tissue or as material devices or implants to cure or replace original organs or tissues are the main application of biomaterials [70]. Tissue-engineered scaffolds aim to at least partially resemble the biological ECM and produce a supportive environment to promote tissue development. The development of many medical implants for clinical rejuvenation treatments and the bioengineering design paradigm both owe much to the biomaterials that support and encourage restorative cell growth. Natural and synthetic biomaterials made of two or more materials are only a few of the many alternatives available for constructing a specific biomaterial to be utilised as a matrix pattern. It is necessary to assess the benefits and drawbacks of using these biomaterials as well as their compatibility for use. These biomaterials are classified into natural and synthetic-based biomaterials.

Figure 4. Biomaterials for the development of bionic organs.

Naturally derived biomaterial

Due to their inherent structural similarity to native tissue ECM, biocompatibility, mechanical dynamics and degradation, natural biomaterials make up an essential subgroup of biomaterials for use as bioengineering templates. These natural biomaterials are synthesised using eco-friendly methods [72]. Environmentally friendly aqueous-based processing techniques are frequently used to handle natural biopolymers. They are not toxic when applied to biological systems and the beginning formulation and processing conditions may be changed to alter the rate of degradation [73]. Natural biomaterials have the property of naturally stimulating cellular recognition, which may help to increase cell attachment and functioning. A broad range of hierarchically organised materials in nature depend on polymers including chitin, collagen and cellulose for their growth and activity [72]. Protein-based biomaterials include gelatin, collagen, fibrin, keratin, eggshell membrane and silk fibroin [74]. Polysaccharide-based biomaterials are cellulose, hyaluronan, alginate, glucose, chondroitin and chitin. While polysaccharide-based biomaterials are mostly derived from microbial or algal sources, such as in the case of dextran and its derivative, protein-based biomaterials are often derived from human and animal sources and contain bioactive compounds that resemble the extracellular environment [72]. Decellularized tissue-derived biomaterials are a category of natural biomaterials that are produced by removing all nuclear and cellular components from native organs and tissues. Examples of these materials include decellularized skin, blood vessels, heart valves and the liver [75]. Natural polymers have been investigated in the development of tissue engineering templates due to their key advantage in supporting cell attachment, propagation, and differentiation [76]. Frequently combined with molecular and mechanical signals, these templates are used for applications ranging from tissue regeneration to functional organ replacement. These polymers are often prepared for transplantation as porous scaffolds, thin membranes or hydrogel particles and are mainly enzymatically biodegradable into non-toxic end products for medical or therapeutic uses [77]. These biomaterials are still effective if local, immediate action is adequate, despite the kinetics of the destruction being difficult to manage. Injectable hydrogel, for example, is a unique type of natural polymer that may be given non-invasively to a target region of tissue injury.

Synthetically derived biomaterial

In comparison to naturally derived biomaterials, the use of synthetically derived biomaterials such as matrix and templates in tissue engineering offers several significant advantages, including appealing options for the architecture and control of shape to produce acceptable substitutes for limitations of ECM systems-based biomaterial on a human origin that mimic biomaterial functions [78]. Polyglycolic acid (PGA), (Poly-hydroxy acids) which include polylactic acid (PLA) and their copolymer, poly(lactic-co-glycolic acid) (PLGA), are the most often employed synthetic polymers for tissue regeneration [78]. Glycolic acid and lactic acid, the harmless breakdown products of these polymers, are produced via chemical hydrolysis of the polymeric materials and eliminated through regular metabolic processes. Chemical hydrolysis may be simpler to predict and regulate than enzymatic decay in vivo due to its independence from local enzyme concentrations [79]. By adjusting the lactide ratio and polymerisation conditions, synthetic polymers’ characteristics, such as mechanical modulus, degradation rate and tensile strength may be easily adjusted for specified applications [80]. These materials have been effectively used in the clinic to replace or create urethral tissue in patients. The wide range of biomedical applications of several synthetic polymers, such as PEG, PCL, PLGA, PLA and PVA are due to the tissue regeneration of ECM-like biomaterial because of its biocompatibility [70]. While scaffolds made of synthetically derived biomaterials can have fully interlinked holes, some types, such as polyhydroxy esters have the potential to release acidic breakdown products that can change the pH of the tissues around them. Consequently, this pH fluctuation may have a severe effect on cell activity and viability and may result in tissue damage and inflammation. However, due to a lack of physiologically functioning features, synthetic biomaterials normally do not pose a threat of eliciting an immune reaction [81]. Integrating biological domains into synthetic biomaterial templates using a variety of synthesis processes that have been developed and perfected makes it possible to create scaffold biomaterials with a known and adjustable structure. The most effective method for inducing desirable cell-material interactions is to place bioactive substances on synthetic biomaterial templates.

ECM (Extracellular Matrix) based biomaterial

The ECM or extracellular macromolecule matrix is a multi-component structural element that is created and put together by the effector cells. It consists of a variety of multidomain biopolymers as well as common structural biomolecules [82]. Its protein network controls the fibre width, structure, and arrangement, maintaining stability with the cells around it. In most organs and tissues in the human body, the ECM complex, biomaterial 3D network of proteins and carbohydrates, cell composition and set of physical characteristics. The extracellular matrix (ECM) of living things can be distributed as an undifferentiated ‘organic matrix’ or organised into interconnecting fibre structures that are ordered in a cell or tissue-specific way [83]. It is well recognised that the living-tissue ECM has several roles in tissue formation and restoration, including determining the stability and structure of tissues, acting as a reservoir for morphogens, and supporting resident cells physically [83]. ECMs are the subject of extensive research work carried out in various parts of the world. These projects aim to demonstrate the nature and special characteristics of ECMs in physiology and composites science as well as for tissue engineering and applications in regenerative medicine [84–86]. Understanding the biological data and elements of ECM for biomaterials design has received a lot of interest over the past few years. These tissues have ECM biomaterial arrangement and composition which reflect adaptation to pressurised conditions and each ECM substance interacts with tissue-forming cells in a distinctive way that has been researched for application in various regeneration approaches [87]. The development of medical devices for tissue regeneration depends on the design and structure of the biomaterials used. Recently, there has been a rise in interest in the ECM-based biomaterials of a certain tissue composition as well as in each matrix component’s physiological and developmental responsibilities. The chemical, physical and structural characteristics of numerous ECM components and naturally occurring proteins and substrates that could facilitate the application of studies on scaffold biomaterials have been and the overall results are successful in tissue regeneration [88]. The highly organised pattern of any ECM-embedded and cell surface-associated constituents suggests essential structural and regulatory functions in tissue function as well as translational effects for the diagnosis and treatment of disease [89].

Challenges in developing biomaterial

The injury due to illness or infection of each tissue frequently exhibits a distinct sequence of tissue regeneration mechanisms. However, comparable cellular and molecular activities during tissue regeneration do occur. The tissue regeneration stages comprise several signalling elements while maintaining strict temporal and spatial control, resulting in the optimum potential tissue regeneration [90]. The cellular scaffold is a biomaterial with mechanical and physical properties that match with the target tissue and that contain a variety of growth factors and adhesion molecules from cells that can stimulate a regenerative extracellular matrix for the right cell populations and alters their behaviour in addition to biocompatibility [91]. A template can not fulfill the criteria for a regenerative extracellular matrix because it lacks the required bioavailability and mechanical characteristics, and more critically, it lacks the target cell interactions to regulate the expression of genes, structure, and kinetics [70].

Tissue grafting

Currently, a wide range of substances can be considered biomaterials, including man-made structures, therapeutic cells, and even a variety of living tissues or organs utilised in transplantation [71]. Consequently, tissue grafts can be viewed as the standard ‘biomaterials’ for reconstructive therapies [92]. Tissue grafting is a transplant of tissues or cells from one part of a person’s body to another. Soft and hard tissue grafts for tissue reconstruction in clinics have increased in popularity during the past two decades [93]. It is evolved in a desire to develop and improve bionic organs and to recover damaged or impaired organs. The use of cells and biomaterials in tissue grafting to create lab-grown tissue/organ replacements that mimic the functionality of native tissue has been driven by efforts in the field of organ transplants, as well as in 3D bioprinting [94]. The previous research was carried out over several years, and the scientists working in the modern bioengineering fields, whether it be tissue (or organ) auto/allo grafting or regenerative medicine or tissue engineering, still adhere to some of the ideas set by those pioneering researchers, the primary objectives are still the same. Here, we have a general understanding of the benefits and drawbacks of various tissue resources and graft types that can give surgeons verified guidelines for graft selection in medical research [95]. Additionally, knowing the composition and structure of native tissue and reviewing the clinical advantages of tissue grafts can provide practical tissue engineers with crucial knowledge on the development of cutting-edge biomaterials and new design innovations (Figure 5).

Figure 5. Different types of tissue grafting.

Autologous tissue grafts

Tissue grafts are the first application of human-origin biomaterials for the improvement of bionic organs and therapeutic purposes. Even now, a lot of biomedical professionals still claim that autologous tissue is the ideal tissue for repairing the majority of tissue damage [96]. Autologous tissue grafts (autografts) retain significant quantities of living cells and have all the characteristics necessary for new tissue development and structural restoration, they are the standard to which all other implanted biomaterials are compared. The main objective of tissue engineering techniques is to create a tissue composite that, when implanted, performs biologically in a manner identical to or comparable to that of an autologous tissue graft [97]. The application of autologous tissue grafts remains the clinical gold standard for tissue regeneration and reconstruction; a primary vasculature in the implants or bionic organ is frequently grafted to the graft as a resource of blood perfusion [98]. There’s been a lot of research on soft-tissue grafts, especially the use of autologous grafts for the treatment of cutaneous injuries (skin injury), the treatment of soft-tissue volume deficits, and the rebuilding of artificial human body parts [99]. However, the method’s application has been severely constrained due to worries regarding graft survival after in vivo transplantation. To overcome these challenges, recent research has concentrated on the fabrication of cellularized artificial grafts. A healthy endothelium membrane is the best anti-thrombogenic component, as is widely recognised. To assess the theory, autologous endothelial cells (EC) obtained from the saphenous vein were inoculated into the non-heparinised blood and employed to pre-clot the graft before implantation [100]. This technique raised the patency rate of dacron prostheses. Dacron is a polyester graft that is usually employed in the fabrication of artificial organs.

In past years, the target of dentistry research has been to enhance the quality of tooth and dental implant grafts [101]. Gingival recession treatment is a common demand from patients because of its major impact on dentin hypersensitivity and aesthetics. In this regard, gingival recession can be effectively addressed with a free gingival graft (FGG), which is most frequently combined with a coronally positioned graft [102]. The free gingival graft (FGG) is a soft tissue transplant obtained from the palate that has the epithelium still attached [103]. As [104] previously described, an FGG can be used directly to fill the root and restore the gingival margin to its appropriate configuration. In the attempt to increase root coverage capacity and mucogingival junction alignments, customised procedures based on FGGs be effective in the treatment of isolated and numerous gingival recessions along both the upper and lower incisors and premolars. This effective implementation is especially applicable in the fields of implant surgery and periodontics [105].

Allogenic tissue grafts

Even though an autograft is the greatest option for managing artificial organs, there are drawbacks to autologous grafts, including the lack of graft supply and the toxicity of the grafts. Numerous alternative grafting components that fall under the following classifications can be employed to overcome the restrictions associated with the acquisition of autologous tissue grafts:

• (i) Xenografts are components obtained from another species and are commonly referred to as xenogenic grafts or heterografts (Namekawa et al., 2019).

• (ii) Synthetic grafts, also known as alloplastic grafts, are artificial materials that are classified according to their origin of production and chemical configuration (Popov et al., 2020).

• (iii) Allografts are made of components obtained from another member of the same species and are also referred to as homografts, homologous or allogenic grafts (Prat-Vidal et al., 2020).

Effective allogenic material transplantation into individuals relies on minimising risks while maximising benefits. An important key factor influencing the rejection of grafts in individuals is the immunological response. However, with optimal enhanced immunosuppressive medication and improved human leukocyte antigen (HLA), the allograft prognosis can be improved [106].

3D bioprinting

Three-dimensional tissue engineering

A significant number of tissue engineering research has been carried out on the three main components which are growth factors, cells and scaffolds. However, there are several drawbacks to employing scaffolds. The idea of the scaffold has fallen out of favour in recent years, and attempts to synthesise 3D tissue using cells and growth factors are now regularly documented [107]. This technology is known as biofabrication, and it includes the techniques of organ printing and bioprinting [108,109]. Furthermore, while 3D structures employing inkjet printer technology have already emerged as the quick prototype, a 3D printer with an inkjet nozzle out of which droplets with a volume identical to that of cells are embossed and can be handled in a sterile atmosphere has been designed [110]. This could pave the way for the development of 3D tissues [111].

3D printing of artificial organs

3D bioprinting is a revolutionary approach capable of designing and manufacturing tissue-specific constructs by constructing complex, heterocellular formations with micro-scale precision. With the use of 3D bioprinting, a variety of biologics, such as genes, cells, hydrogels, growth factors and neo-tissues with ECM protein modifications, can be deposited [96]. Even though these discoveries have resulted in significant advancement over time, bioprinting has drawn criticism for its limited survival durations, capacity to only produce thin tissues, and difficulty to replicate complex composite tissues [112]. However, bioprinting is gaining popularity in the fields of regenerative medicine and pharmaceuticals. This technique especially serves as the foundation for designing and enhancing bionic organs, tissues, organoids, and organs-on-chip models [114]. Bionic organs currently play a significant part in several areas, offering a viable method to create biological substitutes for natural human tissues or organs for a variety of applications, including tissue synthesis, drug screening and organ transplantation in the field of biomedical engineering [113]. Artificial organs and tissues help to address the issue of limited organ donation for organ transplantation. Numerous ways have been employed to create artificial organs and tissues using palliative techniques [115]. Nevertheless, the morphologies and material systems of organs and tissues in humans are complex. Most existing manufacturing techniques can only accurately reproduce the early stages of the disease, and there are numerous barriers to achieving the living organism’s normal tissue functions [116]. Even though some techniques can simulate several types of tissues, they weren’t ideal for high-throughput validation because of their significant expense, several intricate fabricating steps, and time-consuming trials [117].

Advanced bioprinting has been extensively researched and employed to attempt to build artificial tissues and organs, including the heart, skin, liver and muscle. These organs and tissues can be utilised for disease modelling, drug testing, organ transplantation and toxicity testing [118,119]. For instance, airflow-assisted 3D bioprinting is a cutting-edge technique to fabricate tissues on spiral-based microspheres with great resolution and complex microarchitectures. By embedding multiple cells into the spiral-based spheroids, which were geometrically multiple scalars and cell-orientational, it was possible to create viable organoids in vitro and create new, asymmetrical biomimetic models for regenerative medicine and basic biomedical research [106]. Additionally, heterogeneous spheroids provide benefits like multi-components, adjustable morphology, and usability [120].

In recent years, 3D bioprinting has been utilised to deposit layers of bio-inks made of biocompatible polymer matrix and living cells to create sophisticated artificial tissues and organs [121,122]. However, it’s not always possible to replicate the organs. Overall, most of the stated problems of bioprinting technologies must be addressed to mass-produce artificial tissues and organs, and printable biomaterials that are expected to imitate the material constituent of target tissues and organs.

Methods of 3D bioprinting

Bioprinting allows for the controllable and automatic fabrication of live biological models [123]. However, it depends on the selected carrier. The critical factors of this entail:

1. Production of a bio-ink for bioprinting that accurately resembles the environment of native cells.

2. Enhancing the atmosphere for the bioprinting procedure.

3. Appraisal of the produced bioconstruct.

4. Assessing the status of bioprinted cells.

As shown in Figure 6, Inkjet bioprinting, Laser assisted bioprinting, Stereolithography bioprinting and Microextrusion bioprinting were all widely employed in the construction of artificial organs and tissue. Table 3 provides a brief comparison of various approaches in 3D bioprinting techniques.

Figure 6. Various bioprinting techniques.

Table 3. Comparison of various 3D bioprinting techniques.

Feature	Inkjet	Stereolitho -graphy	Microextrusion	Laser	Reference
Dosing approach	Drop	Tank	Fibre	Drop	[44,124]
Bioprinted cells viability	More than 85%	More than 90%	40–95%	More than 95%	[124,125]
Bioinks used	Hydrogels, agar, alginate, collagen, fibrin	Methacrylates of gelatin, hyaluronic acid, collagen; polyethylene glycol diacrylate, dimethacrylate	Hyaluronic acid, gelatin, alginate, collagen, fibrin	Hydrogels, nano- hydroxyapatite	[126]
Bioink’s viscosity	Low	Medium-high	High	Medium	[44,124]
Bioprinting speed	Medium	High	Low	High	[126–128]
Crosslinking agent	Chemical, light, thermal, enzymatic, ion	Light	Chemical, light, thermal, enzymatic, ion, shear, pH	Chemical, light ion	[126–129]
The integrity of the final structure	Low	High	High	Low	[126]
Cost	Low	Low	Medium	High	[44,130]
Used in	Skin, blood vessels, cartilage	Pulmonary alveolus, blood vessel, cartilage, organ-on-a-chip	Trachea, heart valve	Skin	[126–131]
Advantages	During bioprinting, shear forces are maintained at a minimum; high resolution; low costs; fast print; printing using low-viscosity liquids is possible	High accuracy; nozzle-free technique; cell viability.	Enables printing with high cell density material; simple technique; suitable for various biomaterials including ECM mimetic hydrogels.	Biomaterial distribution in the liquid or solid phase; high resolution.	[130–132]
Disadvantages	Low cell density;may lead to cellular mechanical deformation; problems with vertical structures; It is impossible to print continuously.	Lack of printing multicells; cellular toxicity by near-UV blue light and UV light source; cells are damaged during photocuring.	Produces significant shear stresses that may have an impact on the phenotype and viability of cells; Only appropriate for viscous liquids.	High temperatures during the laser pulse cause damage; high costs.	[130,132–134]

Functionalization of bionic organs by regenerative medicine

Regenerative medicine is the next phase in the progression of bionic organs. It is generally prompted by the same medical demands as for the development of artificial organs, transplants and replacement therapy, but has much more ambitious goals than traditional methods [135]. It seeks not only to replace what is defective, but also to offer the materials essential for in-vivo restoration, to design replacements that integrate perfectly with the live body, and to stimulate and assist the body’s inherent abilities to regenerate and heal oneself [136]. The essence of regenerative medicine is recognising and utilising the latent potential of cells, and it leads to a huge amount of research in the area [137–139]. It goes beyond conventional transplantation and replacement therapies by combining a number of convergent techniques, in both traditional and developing approaches [140]. One of the main challenges in the development of the field of tissue engineering is the requirement for cell sources. The phrase ‘regenerative medicine’ was coined as a result of the increased need for renewable cells, such as stem cells (Figure 7) [8].

Figure 7. Fields currently contributing to regenerative medicines.

Stem cells in regenerative medicines

In the discipline of regenerative medicine, stem cells are employed to regenerate the body’s damaged parts, their capacity to self-renew and specialise into every type of cell in the body demonstrates their vital significance [141]. Stem cells offer the ability to create human disease models, which would improve understanding of the pathogenetic mechanism of human diseases and allow for advancements in cell-based treatment for degenerative disorders [142]. According to a study published in 2010 [143], human-derived stem cells grown on polymer scaffolds weren’t any longer noticeable after a few days of implantation in an immune-deficient mice recipient. Instead, murine monocytes rapidly overpopulated the scaffolds, followed by smooth muscle and endothelial cells. The pre-seeded and transplanted stem cells are believed to have produced a large number of monocyte chemoattractant protein-1, which has increased early monocyte recruitment in the recipient, according to the authors’ original hypothesis. These observations imply that tissue regeneration is not solely a cell-restoration mechanism, but also involves an inflammatory mechanism. However, it remains completely unclear what biological function and regulatory mechanisms are engaged in this secretion-induced inflammatory process. Various sub-types of stem cells are employed in the tissue regeneration mechanism, embryonic stem cells are one of them. The blastocyst is where embryonic stem cells are produced following fertilisation. They are naturally pluripotent and also can produce practically every organ. Induced pluripotent stem cells (iPSC) and Somatic stem cells are widely used cell sources in regenerative medicines (Figures 8 and 9).

Figure 8. Differentiation of unipotent and pluripotent stem cells.

Figure 9. Functionalization of stem cells in bionic organs.

Stem cells involved in regenerative medicine for the development of bionic organ

Induced pluripotent stem cells (iPSC)

Since the term ‘Regenerative medicine’ was introduced, clinical applications have grown significantly, beginning with the studies on human embryonic stem cells (ESC). Amid disappointment that human ESC could neither be employed in biological research or medicine. To overcome it, the phenomenon of nuclear transplantation was accomplished in an extensively organised experiment involving four gene transfers [1], These cells are generally referred to as induced pluripotent stem cells (iPSC). iPSC seems to be the main focus of study in regenerative medicine. Studies into the clinical use of iPSC showed that mice models for haemophilia [144], sickle cell anaemia [145], and parkinson’s disease [146] were successfully treated using iPSC. These studies suggest that human diseases might be treated using the same approach.

Xenogeneic materials are currently utilised in several procedures in standard ESC/iPSC culture. Both ESC and iPSC require feeder cells to preserve their undifferentiated phase; typically, these cells are mouse embryonic fibroblasts (MEF) that have had their development arrested by mitomycin C (Figure 10). It was suspected that xenogeneic cell contamination could result in infection if these xenogeneic cells were employed in therapeutic conditions [147]. Multiple researchers have reported coating cell culture dishes with completely synthetic materials and chemically defined-culture media to prevent xenogeneic contamination. And also chitosan and alginate combined in a 3D porous biopolymer – based scaffold assist human ESC self-renewal. On the other hand, human iPSCs could be cultured employing autologous fibroblasts as feeder cells.

Figure 10. Xenogenic materials in induced pluripotent stem cells (iPSC) culture.

However, drawbacks to the usage of iPSCs involve the creation of teratomas from undifferentiated cells, carcinomas by gene transfer and diseases with xenogeneic materials employed in cell cultures. Nearly the whole reprogramming procedure in iPSC is yet unclear.

Somatic stem cells

For a very prolonged time, it was considered that adult cardiomyocytes were in a condition of terminal differentiation and that the heart was incapable of self-healing or regaining its homoeostatic functions. These characteristics sparked a rise in interest in somatic stem cells, such as mesenchymal stem cells, bone marrow-derived stem/progenitor cells and adipose-derived stem cells, among researchers studying heart regeneration. Because foetal tissue-related cells (e.g. amniocytes) have a multipotent capacity. Amniocyte transplantation resulted in heart regeneration in rats with myocardial infarction [148]. Meanwhile, other scientists have consistently claimed that, in the adult heart, cardiomyocytes can be born again. This assertion was based on estimating the age of cardiomyocytes by measuring carbon-14. Following that, several researchers reported that the heart contains stem cells and progenitor cells. The discovery of a method that appeared to be based on embryonic bodies provided the fuel for the extension of this field of study: by constructing a sphere with cardiac-tissue-derived cells, a cluster of almost undifferentiated cells could be enriched. The procedure is injecting cardiac stem cells intramuscularly while performing coronary artery bypass grafting. The cardiac tissue removed during a prior biopsy from the right ventricular septum is where the injected stem cells are extracted from. Recombinant basic fibroblast growth factor (bFGF) is employed during cell culture in place of xenogeneic components, and blood serum is obtained from autologous blood. A simple prolonged gelatin sheet of bFGF is applied to the injection sites before the cells are injected through the epicardium. After this experiment, it is anticipated that more cases will be collected for multi-facility clinical trials and that this technique will grow into a highly advanced biomedical technique.

Conclusion

A rising body of research indicates that biomaterials derived from humans are producing an increasing number of goods and useful clinical techniques to preserve, improve, or regenerate tissues and organs. The future era of biomaterials for healthcare are being created by the integration of sophisticated manufacturing and biomimetic design of structure and material systems. To create biomedical functional equipment with biomimetic architecture and bionic material systems, researchers and scientists have widely developed a variety of advanced additive manufacturing technologies. An enhanced library of biomaterials will open up more opportunities for future biomimetic medical application. Novel biomaterials are a crucial component enabling the formation of natural extracellular matrix. By treating serious, incurable illnesses like diabetes, stroke, and paralysis, regenerative medicine has the potential to save millions of lives. Additionally, it can be used to replace amputated limbs or treat congenital malformations. In a few years, it’s anticipated that some traditional therapy lines will be replaced by techniques including gene editing, 3D bioprinting, living robots, soft nanorobotics, and blends of these techniques based on artificial intelligence. With the use of biofabrication technology, stem cells may be differentiated and given microenvironmental cues that will allow them to develop into adult tissue. The requirement for individual organ donation would therefore be reduced or eliminated as it would be possible to create organs on needed in vitro. Given the potential uses of stem – cell organs, this manufacturing technique has the potential to completely alter how current medicine and healthcare are provided.

Author contributions

Conceptualisation: D.K.S., Y.M., S.M. and D.C.V.; literature search: Y.M., D.K.S., S.C.A. and V.S.; data extraction: D.K.S. and S.C.A.; formal analysis: Y.M. and V.S.; original draft preparation: Y.M., D.K.S., S.S. and D.C.V.; manuscript review and editing: S.M. and D.C.V.; supervision: D.C.V. and S.M. All authors have read and agreed to the published version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data Availability Statement

Data sharing does not apply to this article.

Additional information

Funding

The authors acknowledge the ICMR-NCD ad-hoc oral health grant (Ref. No. 2022-18903) for financial support.