3-D printing is a tool that has blossomed given the cheap computing resources to control it. It has long been possible to print three-dimensional structures in a variety of mediums, but efficient automation makes it cheaper to do this reliably and repeatedly, and also allows for the accurate manufacture of objects with very small scale features. Since cheap computing resources also drive progress in biotechnology, it is only natural that advances in tissue engineering go hand in hand with 3-D printing. These possibilities had to occur at the same time, as they depend on the same underlying technological capabilities. Tissue engineers want the ability to produce structures that mimic the collagen scaffold of the extracellular matrix, webbed with blood vessels and all sorts of other structural features on scales varying from millimeters to micrometers. As a goal that is yet to be achieved completely, but so far good enough attempts have been produced to create several less complex forms of tissue: a scaffold is printed and in the process of its construction is seeded with cells and proteins that encourage growth.
Researchers have been working with 3-D printers for some years now. Some of the formative research programs and first companies in the space are on their way to being a decade old, such as Organovo, whose founders count the Methuselah Foundation among their investors. The focus today is still largely on the production of products for research groups, producing small tissue structures such as printed blood vessels that can speed up the research process. Later, we will see more in the way of larger organs and tissue sections printed for transplant, not research. That is not too many years ahead.
Central to the lab's work are three customized 3-D printers, each worth a quarter of a million dollars. Lewis led me through a warren of corridors and offices to a room where one of the printers sat on supports. It was immense. The base of the printer was a granite block five feet long, four feet deep, and a foot high, weighing a ton and a half. The printer does such fine-scale work that a stable base is essential, Lewis said. Resting on the block was a flat stage or platform, above which, in a vertical row, stood four rectangular steel containers, each a foot or so tall - the ink dispensers. A tangle of colored wires connected the dispensers to some machinery behind them, and each dispenser was controlled at the top by a robotic arm. To the side sat a large monitor and a computer, which controlled the printer.
Each dispenser contained a different biological material, Lewis explained. One held an aqueous suspension of chemically treated collagen, which serves as the matrix on which many of the body's tissues take shape. Two others held suspensions of fibroblasts, the gristly cells that form the body's connective tissue. The last dispenser contained the fugitive ink that Lewis had developed to create channels within materials. On the computer, Kolesky called up a software program and found an image representing the block of tissue that he would be printing. It looked like a rectangle of semi-clear gelatin, within which was a vascular network: a channel entered at one end and branched into smaller vessels, which looped around and ultimately joined back into a single vessel that exited at the other end. It was a simple network, approximating the way that an artery divides into smaller capillaries that eventually recombine into a vein.
The dispenser with the fugitive ink moved quickly and almost imperceptibly, releasing an exceptionally thin stream of what looked like agar onto the glass slide. The printer clacked and clattered like a busy riveting machine. In a minute or so, the job was done; the printer had left a trail of gelatinous ink that exactly matched the pattern on the computer. The stream of ink was about a tenth of a millimetre in diameter, and the entire pattern covered an area a little larger than a matchbook. The printer wasn't rigged to finish the job, but Kolesky explained what would typically happen next. The other ink dispensers would take their turn, laying down a lattice of collagen and fibroblasts that would solidify around the network of fugitive ink, encasing it in tan-colored living tissue. To drain the fugitive ink, Kolesky would place the tissue on a chilled stone cube; this would cause the ink to change from a gel to a liquid, after which he could then extract it with a small suction device. The end result would be a block of living tissue suffused with intricate vessels capable of carrying nutrients to the cells within.
The last step was to me the most remarkable. Once the vessels were empty, Kolesky would take a suspension of endothelial cells - the cells that line the insides of blood vessels - and inject it into the vessel network. The cells would settle in and multiply to line the insides of the channels, effectively turning the channels into blood vessels. And then the cells would spread - they would begin to branch off the existing vessels and form new ones. In effect, Lewis and her team have created an environment that the cells consider home - it is far more natural to them than a petri dish or the inorganic scaffolds that had previously played host to cultured tissues.
"I like to say that we design the highway and then get out of the way and let the endothelial cells create their own driveways," Lewis said. "It's better to rely on the intelligence of the cells themselves in terms of how they like to sprout."