Devices that monitor the human body's processes are being researched and tested for biological sensing or for prosthetics but materials scientists at the University of Washington have taken that a step farther. They have built a transistor that uses protons and could communicate directly with living things. But the current prototype has a silicon base and could not be used in a human body, so don't get prepared to cyberpunk yourself just yet.
Currently sensing technology typically uses electrons, negatively charged particles, rather than protons, which are positively charged hydrogen atoms, or ions, which are atoms with positive or negative charge.
Why does that make a difference? Human devices like light bulbs and the device you're reading Science 2.0 on send information using electrons but living things, like our bodies, send signals and perform work using ions or protons.
In the body, protons activate "on" and "off" switches and are key players in biological energy transfer. Ions open and close channels in the cell membrane to pump things in and out of the cell. Animals including humans use ions to flex their muscles and transmit brain signals. A machine that was compatible with a living system in this way could, in the short term, monitor such processes. Someday it could generate proton currents to control certain functions directly.
A first step toward this type of control is a transistor that can send pulses of proton current. The prototype device is a field-effect transistor, a basic type of transistor that includes a gate, a drain and a source terminal for the current. The UW prototype is the first such device to use protons. It measures about 5 microns wide, roughly a twentieth the width of a human hair.
The device uses a modified form of the compound chitosan originally extracted from squid pen, a structure that survives from when squids had shells. The material is compatible with living things, is easily manufactured, and can be recycled from crab shells and squid pen discarded by the food industry.
First author Chao Zhong, a UW postdoctoral researcher, and second author Yingxin Deng, a UW graduate student, discovered that this form of chitosan works remarkably well at moving protons. The chitosan absorbs water and forms many hydrogen bonds; protons are then able to hop from one hydrogen bond to the next.
"So there's always this issue, a challenge, at the interface – how does an electronic signal translate into an ionic signal, or vice versa?" said lead author Marco Rolandi, a UW assistant professor of materials science and engineering. "We found a biomaterial that is very good at conducting protons, and allows the potential to interface with living systems. In our device large bioinspired molecules can move protons, and a proton current can be switched on and off, in a way that's completely analogous to an electronic current in any other field effect transistor."
Computer models of charge transport developed by co-authors M.P. Anantram, a UW professor of electrical engineering, and Anita Fadavi Roudsari at Canada's University of Waterloo, were a good match for the experimental results.
"So we now have a protonic parallel to electronic circuitry that we actually start to understand rather well," Rolandi said.
Applications in the next decade or so, Rolandi said, would likely be for direct sensing of cells in a laboratory. Longer term, however, a biocompatible version could be implanted directly in living things to monitor, or even control, certain biological processes directly.
The study is published online this week in Nature Communications.