Disappearing chip: A transistor array made of biodegradable materials dissolves over time. The image at top left shows the chip before it’s placed in solution; the gray strips are electrical contacts. Subsequent photos were taken on days 10, 30, 40, 50, and 70. The red line in the bottom right image is a scale bar representing five millimeters.
While the silicon devices might be better suited for longer-term implants such as brain interfaces, where high performance is crucial, fully biodegradable devices might be better suited to applications where it’s important for the device to disappear over time, such as tissue engineering or drug delivery, says Bao.
The Stanford researchers next plan to bring down the operating voltage of the devices. Right now it’s high enough to split water, which is too high to be safe inside the body. The source of the problem is the insulating layer, or dielectric. In the demonstration devices, the dielectric is an 800-nanometer-thick film of polyvinyl alcohol, which the researchers chose for its biodegradability. But the polyvinyl alcohol layers are thick and tangled, which means the voltage has to be relatively high for electrons to travel through it. The Stanford researchers are currently testing thinner dielectrics, including lipid membranes, that are just tens of atoms thick.
The Stanford group is also testing different materials as substrates for the electronics. The organic electronics are flexible, but the device is built on a brittle plastic. The group will test substrates made of rubbery, stretchy polymers that conform well to biological tissues such as the heart. They’re also testing different coatings for the devices. Once exposed to pH levels that mimic those inside the body, the current devices immediately begin to degrade. Bao would like to coat them with materials tuned to dissolve after a desired amount of time.