Bioengineers who hope to help paralyzed patients by melding electronics with nerve or brain tissue face a materials challenge: living tissue and microelectronics could hardly be more different. Most tissues are supple, while the semiconductors and metals used in electronics are brittle and stiff. As a result, the implanted electronics can irritate and damage surrounding tissue. It is precisely this material difference that Stéphanie Lacour is trying to bridge.
As a postdoctoral researcher at Princeton University, Lacour fabricated thin gold strips on elastic rubber substrates that could be stretched like a rubber band without losing electrical conductivity. The Princeton group, led by electrical-engineering professor Sigurd Wagner, then used these strips as the foundation of the first stretchable integrated circuit. Connecting small, rigid islands of conventional semiconductors with the gold strips, the researchers built simple electronic devices that still worked after repeated stretchings. While these circuits consisted of just a few transistors, they demonstrated a way in which engineers might make everything from electronic “skin” for robots to extremely flexible displays.
But it’s the potential applications in biology and medicine that are, Lacour says, “really thrilling.” Now a research project manager at the University of Cambridge in England, she is heading an effort to create implants that surgeons could use to repair nerves severed in an injury.
At the back of her mind, says Lacour, is the goal of creating electronic skin that could cover prosthetic limbs. Eventually, the electronics could be directly connected to a person’s nerves, providing mental control over the prosthetic and, through a network of sensors, “feelings” in the limb. Any application that requires an electronic interface with the nervous system could use stretchable electrodes, says Barclay Morrison, a professor of biomedical engineering at Columbia University. For example, neuroengineers are developing micro- electrode arrays that neurosurgeons have begun implanting in quadriplegic patients to allow them to control computer cursors or robotic arms with their minds (see “Implanting Hope”). But conventional metal electrodes are 100 million times stiffer than the brain tissue. “You’re implanting really rigid needles into the brain,” Morrison says. Lacour’s electrodes much more closely match the elasticity of brain tissue, potentially reducing the chance of damage.
Morrison has begun using Lacour’s stretchable metal electrodes in experiments to study brain injuries. The stretching of brain tissue during an accident can set off a chain of cellular events leading to the death of neurons days after the accident.
Morrison is re-creating the injuries by violently stretching thin slices of brain tissue. Lacour’s elastic electrodes can stretch with the tissue, recording in real time the changes in the electrical activity of the neurons.
Still, says Princeton’s Wagner, the field of stretchable microelectronics is very much in its infancy. It will be at least a decade, he predicts, before the technology is ready for use in consumer products like flexible displays.
But for now, bioengineers are just happy to have a way to bridge the material gap between tissue and electronics. A material that can stretch to twice its size and still be conductive is “unheard of,” Morrison says. “It’s incredible.”