Recent Advances in Tissue Engineering and Regenerative Medicine

The research in the field of tissue engineering and regenerative medicine is exponentially growing to meet the demands for organ transplantation. The advantage of tissue engineering over conventional organ transplantation is the personalized development of whole organ or a particular part of the organ. To meet these organ demands, there are various approaches of tissue engineering such as traditional approach of using scaffold to grow cells and advanced 3D printing technology. The inkjet bioprinters are used along with bio-ink for bio-fabrication of different organs. The other bio-printing techniques such as extrusion-based and laser-assisted bio-printing can also be employed based on the requirement. The extracellular matrix (ECM) materials are used as a bio-ink but are limited largely to non-vascularized organs. The decellularized extracellular matrix (dECM) bio-inks are the recent advancement in the field which can be employed to generate the vascular organs like lungs and blood vessels. Even though tissue engineering shows a promising future there are various issues to be dealt with including ethics, approval from regulatory bodies and high cost of the technology.


Introduction
Organ transplantation has been the only feasible option for millions of people around the world with organ failure, and this problem is further challenged with increasing wait list for the organ transplantation and increase in mortality rate due to organ failure [1]. The issues associated with organ transplantation are complicated by finding a suitable donor for organ transplantation and storing it for a longer period of time [2]. According to the U.S government on organ donation and transplantation report, more than 100,000 patients are in the 2019 waiting list for organ transplantation and the organ donor shortage is at its peak than ever [3].
Tissue engineering is considered as the "holy grail" in the medical field and is growing at the fast pace allowing the tailormade organs to be an alternative and a viable solution to replace failed organs [4,5]. Tissue engineering has made the dream come to reality of having a fully functional artificially produced organ. Every year, there is exponential increase in the number of publications in the field of tissue engineering and regenerative medicines [6,7].
Although the non-vascularized organs such as skin [4,8], urinary bladder [9], urinary tract [10], bone [11] and blood vessels [12] are commercially available, thick vascularized organs such as liver, kidney and heart are still far from reality [1,13]. Commercial applications of tissue engineering are high and versatile [14]. It will help the world not just to solve the problem of organ donor scarcity for transplantation but can also be used for research purposes, for example, to study the effect of drug toxicity on different organs [7], and to study pharmacokinetics of a drug [13,[15][16][17]. Research to create artificially engineered organs is carried out in different research laboratories around the world and successful treatments by bio fabricated organs has been reported [18]. According to Lee et al (2013), scaffold market was 4.75 million US dollars year which is expected cross more than 10 million US dollars by 2020; whereas market for stem cell will cross 11 billion US dollars by 2020 [19].
Apart from medical applications, many researchers around the world are exploring possibilities to produce leather and meat artificially through bio fabrication techniques [20,21]. The idea of bio fabrications is also being tested to produce microelectronics and biosensor [22].

Traditional Tissue Engineering
The idea of grafting or organ transplantation is not new. First skin grafting can be traced back to 3000 BC in India [23]. Although, the foundation of tissue engineering was laid by Dr. Ross in 1907 [24]. Dr. Ross studied nerve fiber development from embryonic tissue [25]. After four decades, in the year 1948, the first artificial kidney was made. Although it was a failure but conceived the idea of tissue engineering to produce artificial organs [17]. From early 1950s to 1960s, numerous articles were published on tissue assembly on which the present regenerative medicine and tissue engineering is established [23]. The tissue engineering can be defined as the methodology to replace the damaged tissues or organs with new tissues or working organ [26,27]. There are three ways by which it can be achieved: Damaged cells can be replaced with new cells,
The first two methods are useful when damage is minimum, but to third method can be employed when there is need to replace larger part of organ or whole organ. The organ can be grown either as tissue scaffolds using 3D bioprinter or recolonization of decolorized organ. The major challenge in tissue engineering is to mimic the microenvironment of extracellular matrix and to arrange different cell types correctly when multiple cell types are present [29].
Growing cells in a scaffold is a traditional method of tissue engineering [30,31]. In this method, cells are seeded in the scaffold, which are usually porous and allowed to mature in the bioreactor [32]. The scaffold mimics the extracellular matrix of the cells.
Extracellular matrix (ECM) is very important for the cell as it allows the cells to interact, provides nutrients and supplies oxygen [33].
Scaffold is typically made of either synthetic or natural polymers to provide the structural design and property of the organs [14,26,34]. Though this methodology is successful, but it has some shortcomings as well. This is not suitable for vascularized organs, as the cells in these organs are situated over the 200μm of vascular structure [35,36]. Blood vessels help in transfer of nutrient and oxygen which pose a major threat to the cell differentiation and maturation [37]. The second problem is when the cells are seeded in the scaffold, at times they do not adhere to the scaffold. Finally, synthetic grafts are prone to bacterial infections and thrombin formations [35]. been fulfilled by organ donations and therefore need to regenerate organs through tissue engineering is gaining popularity. Tissue engineering has the ability to revolutionize the medical field with the bio fabricated organs and tissues [28,40]. In 1995, 3D printing technology and regenerative medicine converged together and gave rise to new era of 3D organ printing. Bio fabrication is defined as the process of using living cells, biomaterials, extracellular matrices and molecules to generate complex living as well as non-living biological products [24]. 3D printing provides more mechanical stability and nutrition diffusion than that of scaffold and applying In the year 2008. This was achieved by completely removing cells from the organ. Decellularization is done by three processes.  1948

DECM Bio ink
In order to overcome the shortcomings of the decellularized organs and bio inkjet printing organ new technology is emerged by merging both. In this method, organs are decellularized first [82,83] and then ECM from decellularized organs is used as the bioink for the bioprinter [79]. Decellularized matrix is considered as next generation bioink [78,80]. Pati et al (2014) developed this technology and year later in 2015 they developed soft tissue using decellularized adipose tissue [83]. By this method, a layer of 400-300μm thickness is made and stacked up to 10 layers which is twice the size of the traditional printing method [76]. This technology is so far found to be useful in producing the whole organ and the mechanical property of decolorized organs was improved by hybrid technology, which uses re-absorbable polymer scaffold [84].

Future Aspect
Advances in tissue engineering research and its methodology lead to the formation of scaffold to bioprinter and then to the decellularized organs. More research in methodologies are overcoming the shortcomings of previous ones. Improving knowledge in the biology of regeneration, development in microelectronics and 3D printing technology is helping in further overcoming the hurdles [85]. Production of commercially available organs is not the distant future anymore. FDA regulations [26], associated cost [86] and ethical issues [87,88] may delay the technology, however research trend suggests death due to organ scarcity will be reduced in foreseeable future [89].