Skin is one of the easier starting points for 3D bioprinting, the application of rapid prototyping technologies to the construction of living tissue. Since skin is a thin tissue, the challenging issue of producing the intricate blood vessel networks needed to supply inner cells with oxygen and nutrients can be skipped. Thin tissue sections can be supported in a suitable nutrient bath, and after transplant, patient blood vessels will grow into the new skin. Further, there is a fairly large and long-established research and development industry involved in various forms of skin regeneration. Numerous forms of prototype skin-like tissues have been created over the years, lacking many of the features of the real thing, but still useful in the treatment of, for example, burn victims. Further, skin structure is by now well understood, and considerable progress has been made in deciphering the signals and environment needed for suitable cells to self-assemble into the correct arrangements. All told, it should not be a complete surprise to see significant progress emerge in this part of the field.

Significant progress has been made over the past 25 years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin, or for the establishment of in vitro human skin models. In this sense, laboratory-grown skin substitutes containing dermal and epidermal components offer a promising approach to skin engineering. In particular, a human plasma-based bilayered skin generated by our group, has been applied successfully to treat burns as well as traumatic and surgical wounds in a large number of patients in Spain. There are some aspects requiring improvements in the production process of this skin; for example, the relatively long time (three weeks) needed to produce the surface required to cover an extensive burn or a large wound, and the necessity to automatize and standardize a process currently performed manually. 3D bioprinting has emerged as a flexible tool in regenerative medicine and it provides a platform to address these challenges.

In the present study, we have used this technique to print a human bilayered skin using bioinks containing human plasma as well as primary human fibroblasts and keratinocytes that were obtained from skin biopsies. We were able to generate 100 cm2, a standard P100 tissue culture plate, of printed skin in less than 35 minutes (including the 30 minutes required for fibrin gelation). We have analysed the structure and function of the printed skin using histological and immunohistochemical methods, both in 3D in vitro cultures and after long-term transplantation to immunodeficient mice. In both cases, the generated skin was very similar to human skin and, furthermore, it was indistinguishable from bilayered dermo-epidermal equivalents, handmade in our laboratories. These results demonstrate that 3D bioprinting is a suitable technology to generate bioengineered skin for therapeutical and industrial applications in an automatized manner.