Ed Yong takes us through the reason recent breakthroughs in transplants (e.g. a new trachea) doesn’t mean we’ll be printing more complex anytime soon:

“A good way to think about it is that there are four levels of complexity,” says Anthony Atala from the Wake Forest Institute for Regenerative Medicine, one of the leaders of the field. The first level includes flat organs like skin, which comprise just a few types of cells. Next up are tubes, like windpipes or blood vessels, with slightly more complex shapes and more varied collections of cells. The third level includes hollow sac-like organs, like the bladder or stomach. Unlike the tubes, which just act as pipes for fluid, these organs have to perform on demand – secreting, expanding or filtering as the situation arises.

Scientists have fashioned lab-grown organs from all three of these categories. Surgeons have implanted artificial skin and cartilage into thousands of patients. Synthetic windpipes are now a reality. Artificial blood vessels are going through clinical trials for patients on dialysis and children with congenital heart problems. Atala himself has transplanted lab-grown bladders into several patients, the first of whom has now been living with her new organ for over a decade.

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The cells also need to grow along the right shapes, so getting the right scaffold is essential. For simple organs, like Beyene’s windpipe, it is possible to fabricate the whole scaffold from scratch. But solid organs have more complex shapes, so some teams start with existing organs, taken either from cadavers or from animals. They use detergents to strip away the cells, leaving behind a natural scaffold of connective tissues and blood vessels, which can then be seeded with a patient’s stem cells. It is the equivalent of stripping a building down to its frame and filling the walls back in. Scientists have made liverslungs and even beating hearts in this way, and some have started to transplant their organs into animals.

Some researchers are excited by the potential organ-building capabilities of three-dimensional (3-D) printers. These devices are modified versions of everyday inkjet printers that squirt living cells rather than drops of ink. Layer by layer, they can make three-dimensional structures such as organs and, as of September last year, the blood vessels they contain. Atala is developing this technique – he wowed the audience at a TED conference last year by printing a kidney on stage (although not a functional one).  He says, “For the level four organs, it’s just a matter of time,” says Atala. “We’re still a long way from full replacement, but I do believe that these technologies are achievable.”