Nanowires That Behave Like Cells
Researchers at the Lawrence Livermore National Laboratory have sealed silicon-nanowire transistors in a membrane similar to those that surround biological cells. These hybrid devices, which operate similarly to nerve cells, might be used to make better interfaces for prosthetic limbs and cochlear implants. They might also work well as biosensors for medical diagnostics.
Biological communication is sophisticated and remains unmatched in today’s electronics, which rely on electrical fields and currents. Cells in the human body use many additional means of communication including hormones, neurotransmitters, and ions such as calcium. The nexus of biological communication is the cell membrane, a double layer of fatty molecules studded with proteins that act as gatekeepers and perform the first steps in biological signal processing.
Aleksandr Noy, a chemist at the national lab, gave silicon nanowires a cell membrane in the hopes of making better bioelectronics. “If you can make modern microelectronics talk to living organisms, you can make more-efficient prosthetics or new types of biosensors for medical diagnostics,” says Noy. For example, if the electrodes connecting a prosthetic device with the nervous system could read chemical signals instead of just electrical ones, the person wearing it might have better control over the prosthetic.
Noy started by making arrays of silicon-nanowire transistors–rows of 30 nanometer-diameter wires bounded at either end by electrical contacts–using methods developed by other researchers. The arrays were placed in a microfluidic device. Noy’s group used the microfluidics to deliver hollow spheres of fatty membrane molecules. The spheres are attracted to the negatively charged surfaces of the nanowires, where they accumulate and fuse together to form a continuous membrane that completely seals each nanowire just as a biological membrane seals the contents of a cell. Bare nanowire transistors exhibit a measurable change in their electrical properties when exposed to acidic or basic solutions; the membrane-protected nanowires do not, because the fatty layer seals out the harsh solution–just like a biological cell membrane.
To give the coated nanowires electrical gates–essentially, a means of making them responsive to the surrounding chemical environment–Noy added proteins to form ion channels, which control the flow of charged atoms and molecules across cell membranes. When put into solution with the nanowires, these proteins insert themselves into the membrane. Noy’s group tested the devices with two types of ion channels: one that always allows small, positively charged ions to pass through and one that does so only in response to a voltage change that can be produced by the nanowire. This voltage-responsive protein is often used to mimic nerve-cell electrical signals. The nanowires with ion channels were able to sense the presence of ions in the solution. By using the nanowire to create a voltage difference across the membrane, the voltage-responsive protein can be opened and closed, effectively allowing the nanowire to turn its chemical-sensing ability on or off. “The neuron is a good analog in some ways,” Noy says of these devices.
Noy’s work, described this week in the Proceedings of the National Academy of Sciences, opens new avenues because it makes the nanowires more like cells, says Yi Cui, assistant professor of materials science and engineering at Stanford University. With Charles Lieber, a chemist at Harvard University, Cui has made silicon nanowires into very sensitive sensors by coating the nanowires with antibodies. The sensors could, for example, detect blood proteins characteristic of cancer. Noy’s work, Cui says, “is a really creative way to integrate a transistor with a cell membrane.” By coating the nanowires, Noy can take advantage of everything that biological cell membranes have to offer, including the ability to sense and respond to voltage changes, as well as ions, proteins, and other biomolecules. This range of functionality can’t be achieved with antibodies, says Cui.
Next, Noy plans to develop more-sophisticated nanowire-hybrid devices. So far, each device has been equipped with only one type of ion channel, which limits the complexity of the functions they can carry out. (Biological cells are coated with many different membrane proteins.)
The researchers will also begin testing the devices’ interactions with living cells. Other researchers, including Peidong Yang at the University of California, Berkeley, and Harvard’s Lieber, have used bare silicon nanowires to interface with neurons, stem cells, heart cells, and other tissues. They’ve shown that the nanowires can send and receive electrical signals with very high spatial resolution, even within single cells. Noy’s initial work remains a proof of concept.
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