There are currently hundreds of thousands  of people on transplant lists, waiting for critical organs like kidneys,  hearts, and livers that could save their lives. Unfortunately, there aren’t nearly enough donor organs  available to fill that demand. What if instead of waiting, we could create brand-new, customized  organs from scratch? That’s the idea behind bioprinting, a branch of regenerative medicine  currently under development. We’re not able to print complex  organs just yet, but simpler tissues including blood vessels and tubes responsible for nutrient  and waste exchange are already in our grasp. Bioprinting is a biological  cousin of 3-D printing, a technique that deposits layers of  material on top of each other to construct a three-dimensional object  one slice at a time. Instead of starting with metal, plastic,  or ceramic, a 3-D printer for organs and  tissues uses bioink: a printable material that  contains living cells. The bulk of many bioinks are water-rich  molecules called hydrogels. Mixed into those are  millions of living cells as well as various chemicals that  encourage cells to communicate and grow. Some bioinks include a  single type of cell, while others combine several different  kinds to produce more complex structures. Let’s say you want to print a meniscus, which is a piece of cartilage in the knee that keeps the shinbone and thighbone  from grinding against each other. It’s made up of cells called chondrocytes, and you’ll need a healthy supply of  them for your bioink. These cells can come from donors whose cell lines are replicated in a lab. Or they might originate from a  patient’s own tissue to create a personalized meniscus less likely to be rejected by their body. There are several printing techniques, and the most popular is extrusion-based  bioprinting. In this, bioink gets loaded into a  printing chamber and pushed through a round nozzle  attached to a printhead. It emerges from a nozzle that’s rarely  wider than 400 microns in diameter, and can produce a continuous filament roughly the thickness  of a human fingernail. A computerized image or file guides the  placement of the strands, either onto a flat surface or into a  liquid bath that’ll help hold the structure in place  until it stabilizes. These printers are fast, producing the  meniscus in about half an hour, one thin strand at a time. After printing, some bioinks  will stiffen immediately; others need UV light or an additional  chemical or physical process to stabilize the structure. If the printing process is successful, the cells in the synthetic tissue will begin to behave the same way cells do in real tissue: signaling to each other, exchanging  nutrients, and multiplying. We can already print relatively simple  structures like this meniscus. Bioprinted bladders have also been  successfully implanted, and printed tissue has promoted facial  nerve regeneration in rats. Researchers have created lung tissue, skin, and cartilage, as well as miniature, semi-functional  versions of kidneys, livers, and hearts. However, replicating the complex  biochemical environment of a major organ  is a steep challenge. Extrusion-based bioprinting may destroy a significant percentage of cells in the  ink if the nozzle is too small, or if the printing pressure is too high. One of the most formidable challenges is how to supply oxygen and nutrients  to all the cells in a full-size organ. That’s why the greatest successes so far have been with structures  that are flat or hollow— and why researchers are busy  developing ways to incorporate blood vessels  into bioprinted tissue. There’s tremendous potential to use  bioprinting to save lives and advance our  understanding of how our organs function  in the first place. And the technology opens up a dizzying  array of possibilities, such as printing tissues with  embedded electronics. Could we one day engineer organs that  exceed current human capability, or give ourselves features like  unburnable skin? How long might we extend human life  by printing and replacing our organs? And exactly who—and what— will have access to this technology  and its incredible output?