Skin is an excellent candidate for the forefront of tissue engineering: producing tissue in thin sheets reduces the scope of many of the hurdles involved, such as blood vessel formation to support cells in the tissue interior, or the need for innovation in the growth environment and equipment used to host and encourage growing tissue. Nonetheless, it's still a work in progress. Skin is still a complicated item, for all that it might be simple in comparison to a lung or a kidney - getting the skin layers right, with the correct cell types doing the correct things in the correct places, is one challenge only recently solved, for example. That's not even to talk about the difficulties of adding hair to the mix.
One of the things we might expect to see in the years ahead is an evolution of specialized tissue printing machinery, each focused on one particular type of organ or tissue. Different shapes and forms of tissue have very different requirements for growth, so why not specialize the equipment? Here is a popular science article looking at the early development of a skin fabricator:
Leng's modest prototype looks like a small open box of clear hard rubber, the layered floor of which contains a delta of microfabricated pathways. These lead from seven reservoir stations to a single output stream. Just like the colour cartridges in my printer, Leng's reservoirs of live cells are computer controlled to dispense precise amounts exactly where needed.
She switches on a variable pressure device that drives the material into a base stream of alginate - a medically approved derivative of algae. The cells that would be placed in the reservoirs to fabricate human tissue would ideally be drawn from the patient, but could also be compatible donor or stem cells. "The stream flows into a liquid-filled reservoir that contains calcium chloride," Leng says, as a milky-white ribbon appears in a tank at the output end. "The calcium ions bind to the alginate chain and cause it to become a gel."
The ribbon of skin-like tissue winds around a small turning spindle or drum, the speed of which can determine the tissue's thickness and even its texture. More pull from the drum makes the material tougher, like muscle tissue.
The drum could also be used to make a tissue cylinder - a vein. "Think of something like duct tape," says Guenther, who also directs the underground lab that bears his name. "You make the tissue adhesive on one side, it sticks to itself, and you create a physical tube." The alginate, which degrades over time, provides only a temporary matrix for the cells, Guenther says. The cells attach to each other, and replace the disintegrating matrix with their own.
Guenther says the prototype printer cost a few hundred dollars to make. Apart from the micro-fabricated elements in the base, the whole thing looks like it could be assembled in a handyman's basement. One early version of the receiving chamber was a fish tank purchased in Chinatown. Guenther tells me, with amused pride, that it cost $20 - then shows me a pouch as big as his hand that contains a $4,000 sheet of dermal regeneration template - the stuff used in more laborious forms of tissue engineering.
The living cells in the tissue printer were taken from neonatal rats. The first clinical trials will be undertaken with mice, then pigs. Another hurdle to be cleared is the matter of cell supply. Ideally, says Guenther, cells for a human recipient should be made from the patient's own tissue. How that or donated material could be harvested quickly enough to feed a high-capacity tissue printer still has to be worked out. Jeschke says he hopes that a trial with five to 10 human patients could be possible within two to three years.
You might also look at an article from early last year on an automated artificial skin factory in Germany that produces small skin sections for research or grafting - an early model for what will come later when tissue engineering is more advanced.