One week before the 2016 Games, the sensational Afghani gymnast suffers a bad dismount, destroying the ligaments in her left knee. The coach runs up to her, aghast. "What size are you?" he demands. "Four," she groans. A few hours later the coach enters the refrigerated vaults of the Kabul Tendon Bank. Artificial knees, elbows, and shoulders line the walls, each embraced by glistening webs of bioengineered ligaments. By the next morning, the custom-fitted replacement tissue has been installed into the gymnast's knee. Five days later—as she stands to receive her gold medal—a tiny scar is the only sign of her recent injury.

Tissue engineers trained in the newlywed fields of engineering and biology are striving for the Holy Grail of sports surgery: a mechanical soft-tissue replacement that can be attached directly to an athlete's musculoskeletal frame. "We're already making tissues that can be integrated over time," says Tony Keaveny, associate professor of bioengineering at UC Berkeley. "One problem is, the body sometimes rejects them. Another is that they don't have good mechanical properties at the outset. How do you manage it so they're functional right away?"

One solution will be to cultivate whole tissue systems using cells taken from the athlete's body. In several separate studies, rabbit knees have been injected with a mixture of cells and potency factors that grow into a cartilage-like replacement. The process, however, takes about a month, and the tissue may not be as good as the real thing. When the science is perfected, an athlete's tissue cells will be harvested in advance and grown over precise, computer-modeled replicas of his or her bones. Eventually, such components won't even have to be made from one's own cells. "We could have off-the-shelf components, genetically engineered to be compatible with your body," says Keaveny. "You'd remove them from their scaffoldings, and pop them right in."

This sci-fi scenario may be a reality within 20 years. Further afield are cybernetic implants like carbon nanotubes. When perfected, these microscopic filaments will be the strongest materials ever synthesized. Laced through an athlete's muscles, such fibers could allow athletes to raise the bar in many sports. And here's an even wilder possibility: Take that nanotube-reinforced arm and program the muscle cells with the genetic trigger of a common flea's jumping legs. Shot put, anyone?