Silicon is cheap — a feature that has made it a darling of the electronics industry. But it has a major problem: It doesn’t play well with people.
“People are the worst enemy of silicon,” said Paul Berger, an electrical and computer engineering professor at Ohio State University. “When we sweat, we sweat sodium and potassium out of our pores. That is the bane of a silicon device.” The electrolytes are common inside the body too, where they perform tasks like regulating fluid content and controlling muscles. Silicon sensors inserted under the skin quickly attract and absorb sodium and potassium, rendering them unreliable.
Berger and his colleagues decided to search for a way to protect silicon from electrolytes. They settled on aluminum oxide, which is used in everything from toothpaste to sandpaper. When a silicon sensor is coated with aluminum oxide, it can last up to 24 hours inside the human body.
The coating’s first application could be a quick 5-to-10 minute diagnostic test that can determine if a transplant recipient is in danger of rejecting an organ. Medical professionals would insert a needle covered in silicon sensors into a patient’s body to sense if proteins that indicate organ stress are present. Then they would do something Berger said is key: throw the needle away. Needles need to be disposable, and silicon can make that affordable.
Eventually, Berger would like to see electronics that can be implanted directly in contact with human tissue for long periods of time. That doesn’t exist right now. Pacemakers, for instance, are built in a stainless steel housing for protection. Small wire electrodes are the only electronic component outside the housing.
“The smart electronics are sequestered from the body,” Berger said. “What I’m trying to advocate is to put electronics in intimate contact with the body.”
While the aluminum oxide coating could be improved to protect silicon for a longer period of time, it’s not a permanent solution. The immune system still sees the sensor as a foreign body. And like a sliver, the body will eventually push it out.
The next revolution will be organic semiconductors, which Ohio State University will soon test in mice, Berger said. These could pave the way for more permanent electronic implants. His colleagues are already working on an artificial neuron built from transistors, but there are many more applications.
“You could do a heart stent,” Berger said. “Wouldn’t it be nice if you had some intelligence built in that could wirelessly transmit the health of that artery?”