This popular science piece argues that the future of bioartificial organ development will be a process of top down and bottom up tissue engineering attempts eventually meeting somewhere in the middle, and there providing the ability to generate complex replacement organs to order:
Although a body, no less than a car, may eventually need replacement parts, surgeons cannot simply place an order at the Organ Zone. One alternative to waiting for a donor organ is to handcraft one. Doing so is as inconvenient at it sounds. First, you need a scaffold. This, too, can come from a donor, a cadaver even, provided all the cells are removed, leaving only extracellular matrix consisting of collagen or cartilage. To date, this approach has been used to accomplish relatively simple repairs in human patients. For example, scaffolds fashioned from cadaver materials and plastics have been seeded with stem cells to create new windpipes.
While optimism does seem to be in order, this approach to tissue engineering, with its arduous and time-consuming shaping of scaffolds and pipetting of stem cells, has something of a preindustrial, handicraft feel to it. Moreover, it seems inherently resistant to the sort of top-down optimization you might see in an industrial setting. Mass production would seem to be out of the question. But what about mass customization? Mass customization refers to a kind of bottom-up process that relies on computer-aided design (CAD) tools and rapid prototyping platforms such as 3D printers. Since it was learned that more than 90% of bioprinted cells manage to survive the rigors of bottom-up processing, researchers interested in generating 3D replacement organs have been pressing the "print" button. If they succeed, they will create an industry that has the distinction of skipping the mass production phase of industrial development.
That may seem too large a chasm to cross. The other side may be reached, however, partly thanks to insights gleaned from top-down systems. For example, it is already well understood that printing an organ won't be as simple as dropping cells and supporting materials in the right place and calling it a finished product. An organ grows and develops over time amidst a storm of signals and signal responses, and these depend on environmental cues and cellular sensitivities and propensities for self-organization. Exactly how all these interactions can be orchestrated is unclear, but researchers, undaunted, are pressing forward.