You reach out to pick up an apple. You can do it thanks to brain-based robotics. But since you have a prosthetic hand, you can't feel it. You can't feel it when you hug someone, either. (That's arguably even worse.)
That's the reality for many of the millions of amputees fortunate enough to have access to artificial limbs (many more millions worldwide don't). But Zhenan Bao is out to change all that. The Stanford chemical engineer is working with a team of researchers to develop a new "skin" that could stand in for the real thing and give amputees back sensation. Previously, researchers have been able to offer some sense of touch, but not with flexibility at the same time. Bao's skin, by comparison, would allow amputees to feel even with bendable prosthetics.
The science of how you feel
To get a sense of just how incredible Bao's feat is, consider how your regular sense of touch works: Inside your skin are millions of nerve receptors. These receptors gather information about force, pain and temperature. The receptors then send electrical impulses to your neurons (nerve cells). The impulses pass rapidly from neuron to neuron, to the spinal cord, and finally to your brain. The brain then has the job of translating the incoming signals. To work properly, Bao's skin had to replicate that entire sequence, all with an adaptable, flexible material that still would be durable enough to avoid interruption of the process.
Bao came up with a two-layer design of ultra-thin plastic, with the first layer standing in for your nerve receptors and the second layer containing circuits that get the electrical signals to your brain.
- The researchers investigated and described how to use plastics and rubbers as pressure sensors, measuring how much natural rebound or spring the molecular structures in those materials provide.
- Bao and her team indented a waffle pattern into the top layer of thin plastic to compress the "springs" in the material even more, thereby increasing pressure sensitivity.
- The team inserted carbon nanotubes through the waffled plastic. When you increase pressure on the plastic, the nanotubes are brought closer together. Subsequently, they conduct electricity to sensors better. When you decrease pressure on the plastic, the nanotubes move further apart, and electricity doesn't travel to the sensors as easily.
- Bao connected the first layer of the "skin" to the second circuit layer.
- The researchers adapted an optogenetics technique developed by Karl Deisseroth. The technique allows scientists to bioengineer cells to turn "on" and "off" in response to specific light frequencies. Bao was able to translate the electronic signals from the "skin" into light, which then theoretically would activate neurons that would carry the messages to the brain.
One sensing mechanism down, five to go
Bao stresses that the skin is still only in the proof of concept phase, and that much more work is required to get the full touch capability you naturally have. She still has to develop and incorporate systems that will mimic the remaining five types of biological sensing mechanisms regular skin holds. But already, the basic, two-layer foundation Bao has makes such additions theoretically feasible, and her team is partnering with PARC (of Xerox) to adapt inkjet printing technology that would make the skin practical over a large area. Not only that, but Bao's plastic fabric also should be able to "heal" and power itself. It might take time, but molecule by molecule, it's coming.