# Whole Organ Engineering: Approaches, Challenges, and Future Directions
## 5 Recent Developments and Accomplishments
### Skin  
Engineered full-thickness skin grafts should ideally be developed with scaffolding representative of the diverse structure in native epidermis, dermis, and subcutaneous fat, which, similar to other organ engineering methods, has been accomplished using decellularization methods [66]. Using perfusion decellularization of porcine full-thickness skin flaps, [PERSON] et al., created a novel method for generating an acellular skin flap scaffold in 2017 [66]. In vitro tests revealed that the decellularized skin tissue maintained native mechanical properties and could be successfully reendothelialized after perfusion with HUVECs [66]. The most notable results from [PERSON] and colleagues were from their in vivo studies, which found that their engineered skin underwent neovascularization and complete tissue integration at greater rates than native xenogeneic skin or commercially available dermal substitutes when implanted onto full-thickness skin defects [66]. Analysis of the immune cells that migrated to the implant site revealed no evidence of rejection and only showed signs of inflammation associated with the wound healing process, with the acellular scaffold promoting tissue regeneration without signs of scarring [66]. The ability of [PERSON] and colleagues' graft to promote regeneration without scar tissue forming is vital, as scar formation is a significant concern with full-thickness dermal injuries as it can result in severe limitations in function and mobility of an individual.  
Other strategies for skin tissue engineering involve the use of de novo ECM bioscaffolds on which cells are seeded so that they can proliferate and generate ECM and tissue [140; 141].  
[PERSON] et al., utilized a biodegradable composite scaffold comprising collagen I and chondro-sulfate, a component of cartilage [140]. The scaffolds were seeded with human dermal fibroblasts and then epidermal keratinocytes, evaluating the engineered skin substitute with both in vitro and in vivo studies with athymic mice [140]. The in vitro results demonstrated that mechanical properties of [PERSON] and colleagues' skin substitute were dependent on the combination of materials and cell types used [140]. When comparing the mechanical properties of engineered skin before and after implantation, it was found that implantation was beneficial for the graft to develop more stable mechanical properties that resembled native tissue, as the graft was remodeled by the host [140]. These results were indicative of a viable skin substitute, as the ability of an engineered graft to be remodeled is important for implant integration.  
Bottom-up 3D bioprinting approaches have also been utilized in generating full-thickness tissue constructs, particularly to control the placement of structures such as pores and vasculature for the engineered tissue [142; 143]. [PERSON] et al. showed that 3D bioprinting could be used to generate a construct with vasculature, in which several cell types could be cultured [142]. Similarly, [PERSON] et al. used the resolution and control with which bioprinted scaffolds can be generated to create a gelatin scaffold loaded with ECM-components with precise placement of pores [143]. This allowed for the growth of sweat glands within the construct in vitro [143]. The ability to precisely place and grow glandular components of engineered skin is important in skin regeneration, as natural repair mechanisms fail to do so when repairing full-thickness skin injuries.