auto accidents, Atala says. “The [AFIRM] trials were open to any patient, whether a civilian or a member of the military.”
The encouraging results of AFIRM’s early work prompted military officials to begin planning for a second phase that would continue to support the work of research centers and biomedical companies. In September, the Army chose the Wake Forest Institute for Regenerative Medicine to lead AFIRM-II. Atala is the lead investigator and director of the initiative. Its goals are to further improve technologies for the regeneration of limbs, skin, structures of the face and skull, and organs of the lower abdomen that aren’t protected by the body armor that shields the torso.
Among the AFIRM projects, perhaps the furthest along on the path to approval as treatments in humans are skin healing therapies—which may also hold out the greatest potential revenues as mainstream medical treatments.
John Jackson, an associate professor at the Wake Forest Institute for Regenerative Medicine, says AFIRM has been supporting a number of different skin healing technologies in the hope that one or more of them will work well to reduce scarring, infections, and other complications. Current products on the market, such as synthetic dressings or a collagen matrix derived from cows, can be used as temporary measures to cover large burns and other wounds, Jackson (pictured) says. But the bovine tissue is eventually rejected by the human body and needs to be removed, he says.
In current medical practice, once the wound is stabilized by temporary measures, the standard treatment to permanently close large wounds is a skin graft, using skin removed from an uninjured part of the patient’s body.
“But then, you produce another wound when you harvest that good skin, and that has to heal as well,” Jackson says.
So what could improve temporary coverings or even replace skin grafts? One idea is a spray-on wound covering called ReCell, which is made by Avita Medical of Cambridge, U.K., and Northridge, CA. ReCell is a suspension of skin cells taken from the patient’s body, and is being tested in a late stage clinical trial at Wake Forest under the AFIRM initiative.
Wake Forest is also among the medical research centers that are testing Madison, WI-based Stratatech’s skin substitute StrataGraft in a mid-stage clinical trial. StrataGraft is a sheet of living tissue made from a line of human cells called NIKS cells, which were discovered at the University of Wisconsin-Madison.
In mid-2013, Stratatech was awarded a five-year contract for as much as $47.2 million to develop StrataGraft by the U.S. Department of Health and Human Service’s Biomedical Advanced Research and Development Authority (BARDA). The agency is part of a government defense network preparing for the possibility of mass casualties due to terrorist attacks. But Stratatech points out that about 45,000 patients in the United States are hospitalized every year for burn injuries, creating a peacetime need for improved burn treatments.
Replacing skin—a relatively thin sheet of tissue—might seem like much less complex problem than AFIRM’s other challenges, such as replacing whole organs or limbs. But the skin is actually a complex structure of dozens of different cell types performing a large array of different functions. The upper layers of the skin—the epidermis—retain moisture and serve as the body’s physical barrier against germs. Hair follicles not only shield the body from injury, but also contain immune cells that make up the first line of biological defense against infections. Cells called melanocytes make the pigment melanin, which protects skin cell DNA from damage caused by light. In the lower levels of the skin, the elastic structural fibers of the dermis support the epidermis and hold nerves and blood vessels in place.
While tests of ReCell and Stratagraft have been encouraging, AFIRM scientists are digging even deeper into the toolbox of regenerative medicine for alternatives that may be even better at restoring the complex structure of natural skin. Ideally, therapies would improve not only the functions of healing skin, but also its appearance. For example, the pigment of the new skin would match the host site.
The larger the wound, the more important it is to duplicate full functioning, says Jackson, a leader of the Wake Forest researchers who have been working on 3-D skin printing in animal models.
The idea is to “print” the wound with several types of cells, along with key proteins. The Wake Forest bioprinter is equipped with a scanner that creates a three-dimensional computerized map of the wound bed, taking note of all the variations in thickness and the shape of its outer contours. Instead of using different colored inks like the inkjet printers we’re familiar with, or the plastic “ink” that 3-D printers use to create product prototypes, the bioprinter’s ink is made of living cells suspended in a gel. The gel consists of the proteins fibrinogen, collagen, and thrombin, which form a network of chemical crosslinks. The cells are taken from a skin biopsy of the patient, and then coaxed to multiply their numbers in a nutrient bath before they’re mixed into the gel.
So far, the Wake Forest researchers have used two types of cells to create their ink—fibroblasts, which give rise to collagen and fibers found in the dermis, and keratinocytes, the cell type that forms the epidermis. In animal studies, the printer deposited layers of these two cell types at varying thicknesses, guided by the computer’s 3-D map of the wound bed’s contours.
In mice and pigs, the results have been encouraging. Jackson says the printed skin cells promoted wound healing within two or three weeks in pigs. The printed cells organized themselves into differentiated skin cell layers—apparently guided by the body’s own natural cell signaling molecules. By comparison, wounds in pigs usually take about six weeks to heal without treatment, he says. And the natural process of healing creates scars, because it contracts the wound area by drawing the raw edges of skin together. The hope is that bioprinted skin in humans will minimize scarring, Jackson says.
Wake Forest has filed patents on its printer technology, and the researchers hope to advance the bioprinter into clinical trials in four or five years, Jackson says. Wake Forest has set up its own GMP manufacturing unit for the cell preparations, and could conduct early stage clinical trials until it attracts investments from a commercial partner, he says.
There are still many improvements the researchers could make to the 3-D skin printer, Jackson says. So far, they’ve used only two cell types, which are loaded into two different syringe pumps that deliver them to the printer heads. But the bioprinter can accommodate eight syringe pumps—and thus eight different cell types. Jackson foresees that they’ll try including melanocytes and follicle cells.
“That’s the next generation,” he says.
Jackson’s group has also been experimenting with stem cells derived from human amniotic fluid. These cells produce growth factors that could promote wound healing. They also might suppress immune system rejection if the Wake Forest researchers load a second bioprinter syringe with off-the-shelf human skin cells instead of culturing the patient’s own skin cells—a process that can take as much as 10 days.
The ultimate promise of the printing technology goes far beyond flat structures like skin. In AFIRM’s second phase, researchers will be working with 3-D printers to try to produce bone, cartilage, and muscle, as well as combinations of these tissues for facial and skull reconstruction.
Whether through bioprinting or other regenerative medicine technologies, the AFIRM-II goal is to master the reconstruction of lost body structures at increasing levels of intricacy, size, and function. Atala says the aim is to progress from skin to tubular body structures such as blood vessels, hollow organs such as the bladder, and eventually to solid organs such as the liver.
“We continue to increase the complexity of the technology,” Atala says. By funding these constant scientific tweaks and improvements, the military may someday be able to honor the sacrifices of its seriously wounded combat veterans by lessening those injuries.
