# Bioengineered Skin Substitutes: Advances and Future Trends
## 3 Cell-Based Skin Substitutes
### Bioengineered Dermo-Epidermal Skin Substitutes (Under Development)
#### 3.2.1 Cell-Laden Hydrogels as Wound Dressings  
More recently, attention has been attracted to engineering hydrogels as skin templates to encapsulate cells and bio-macromolecules to support cell-cell and cell-microenvironment interactions. Due to their attractive properties, hydrogels are the most common materials used as a scaffold to culture cells for skin repair applications. Hydrogels can provide an appropriate scaffold for cell encapsulation due to their 3D matrix, which is replete with water and their biodegradability properties. Additionally, the majority of them are biocompatible, which means the hydrogel does not provoke any adverse reaction, rejection, or immune response upon implantation [80; 81; 82; 83; 84]. Furthermore, mechanical properties of hydrogels including stiffness, viscoelastic behavior, and initial state recovery (self-healing) are tunable by copolymerization, nanoparticle incorporation, and changing the polymer concentrations  
Figure 2: (**a**) A collagen type I hydrogel containing human adipose-derived microvascular networks cultured for 21 days in vitro, (**b**) A pre-vascularized bioengineered human dermo-epidermal skin substitute 7 days after transplantation onto the back of nu/nu rat, (**c**) Human engineered capillaries are visualized by the human-specific CD31-antibody (red) present in the CD90-positive (green) human neo-dermis. (**d**) Human CD31-capillaries (red) co-stained by human/rat alpha smooth muscle actin (aSMA) (pericyc marker; green). [PERSON] stains the nuclei in blue. White dotted line indicates the dermo-epidermal junction. Scale bar = 100 \(\upmu\)m (**a–d**) and 1 cm (**b**) [12; 20]. and ratios [3; 83; 84]. Moreover, hydrogels are biomimetic structures, mimicking the natural microenvironment enabling cell-cell interactions and interactions with the surrounding tissue [85; 86]. There are some commercial cell-laden hydrogels available on the market. TransCyte (Advanced BioHealing, Inc., New York, NY, USA and La Jolla, CA, USA) is a bio-engineered skin wound dressing, formerly marketed as Dermagraft-Transitional Covering [87]. This dermal substitute is composed of human newborn fibroblasts, which are then seeded on the nylon mesh of Biobrane with a thin silicone layer regulating moisture vapor from the wound. TransCyte can be applied to the wound site and is protected by adhesive strips; in some cases, surgical staples are used [88]. In general, Figure 3 represents a concept of a cell-laden hydrogel for skin tissue engineering. First, cells are isolated from a patient and cultured in a hydrogel matrix. The hydrogel supports the growth of cells and skin formation, which can be grafted back to treat the skin defect of the patient. Cell-laden hydrogels are usually prepared by mixing the isolated cells in a pre-polymer solution followed by a crosslinking using an ionic or chemical crosslinking such as a thermal or photo-crosslinking mechanism. However, the type of photoinitiator, light, and temperature need to be controlled to protect the cells and increase their viability after crosslinking [3; 89; 90; 91]. There are different kinds of cell-laden hydrogels, which can be divided based on their scaffold structure such as porous and stimuli-responsive hydrogels. In porous hydrogels, a porous bioscaffold containing cells forms a foam or crosslinked hydrogel, which can be applied as a skin substitute onto a wound site [92; 91]. In particular, porosity plays a critical role to allow host cell infiltration into the 3D network and to improve protein transport and diffusion to mimic native tissue structure and function. For instance, if the pores are too small, they might be blocked by cellular penetration, ECM formation, and vascularization of the inner areas of the scaffold [81].