Creating a durable, natural-looking skin substitute to cover burn injuries or other wounds has been a bioengineer’s holy grail for decades. Now, we may be much closer, thanks to a new technique for 3-D printing skin complete with working blood vessels.
The research, done at the Rensselaer Polytechnic Institute (RPI) and Yale University, uses living human skin cells turned into a liquid “bio ink.” The bio ink is used to print artificial skin, which then grows its own blood vessel system.
“The vasculature is very important because that’s how the host and the graft talk to each other,” says Pankaj Karande, a professor of chemical and biological engineering at RPI, who led the research. "Communication between host and graft is critical if the skin substitute is not to be rejected by the body."
Currently, patients in need of skin grafts have two options. First, there are autologous skin grafts, where doctors shave off a piece of healthy skin to cover the damaged area. Second are artificial skin products made from materials ranging from bovine collagen to polymer foam. Both have disadvantages. Autologous skin grafts are painful and create a new wound. Artificial skin products have a range of limitations—they’re often temporary, don’t cover deep wounds or don’t resemble human skin.
“They are just dressings or Band-Aids,” says Karande, of the artificial skin products on the market.
The RPI and Yale team’s new grafts are made with bio ink containing cells from infant foreskin, human endothelial cells from umbilical cord blood, human endothelial colony forming cells, and human placental pericytes from placenta tissue, all suspended in collagen from rat tails. This forms the inner layer of the skin, the dermis. A second bio ink, made from another type of human foreskin cells, keratinocytes, is printed on top to form the outer layer of the skin, the epidermis. Then, in the petri dish, endothelial cells and the placental pericytes begin to assemble themselves into tiny vascular networks.
The team implanted the grafts on mice and found that the blood vessels connected with the mice’s own vascular networks within four weeks. That meant blood was flowing between the mouse and the skin graft.
“We see that the graft stays there longer, and that the skin matures and becomes closer to what we would see in native human tissue,” Karande says.
Translating this early research, recently published in the journal Tissue Engineering Part A, into a product for human use will involve many steps. The team needs to find a way of making a graft without rat collagen or other animal products, to make it less likely to be rejected by the human body. The mice used in the experiment had their immune systems “turned down” to prevent rejection; ordinary humans will need a product that’s as close to their own tissue as possible. One way of lessening the likelihood of rejection is by editing donor cells with CRISPR technology to make them more universally acceptable. Then there’s the question of skin structures like sweat glands and hair follicles, which can be damaged or destroyed by burns and injuries.
“Some of these are important for function and aesthetics,” Karande says. “You want the skin at the graft site to look as much like the surrounding skin.”
Another aesthetic question is color. A graft for a person of Indian origin should be darker than one for a pale Northern European, for example. Skin color is determined by concentration of cells called melanocytes. Melanocytes can be added to the bio ink in greater or lesser numbers depending on the color desired.
“Bioprinting could revolutionize the field of burn care,” by replacing currently available graft options, write Mathew Varkey, Dafydd O. Visscher, Paul P. M. van Zuijlen, Anthony Atala, and James J. Yoo, researchers at Wake Forest University and Amsterdam University Medical Center, in a paper summarizing current research on the subject.
While several investigations of bioprinting skin with blood vessel systems have shown success, 3-D printing sweat glands and controlling skin pigmentation has had “varying results,” they write. They also point out that new skin products will face regulatory hurdles, and that hospitals will need to develop new infrastructures and processes for creating and using the grafts.
“We’re still at the basic research stage,” Karande says. “We’re still figuring out the basic problems and what the right answers might be.”