THE power to perform a wide range of tasks by using thought to control a computer or robotic arm has come a big step closer, thanks to 鈥渁rtificial synapses鈥�. These are rather like the links between brain cells but formed by connecting silicon nanowires to individual neurons.
鈥淭he neurons grew on the chip and made connections with the nanowires鈥�
Electrodes have already been implanted into the brains of some paralysed people, and have been used to control a computer cursor and robotic arm (快猫短视频, 15 July, p 28). But these neural prostheses measure signals from groups of neurons, and can only sample a fraction of brain activity. 鈥淭hey鈥檙e amazing advances, but it鈥檚 still pretty crude,鈥� says Charles Lieber of Harvard University. 鈥淭he electrodes are just poking in there, at the scale of the whole cell.鈥� Lieber鈥檚 aim now is to communicate with neurons in the way the brain itself does. This would enable researchers to convert a wider range of thought processes into electrical signals, helping paralysed people make more movements and giving neuroscientists more information on how the brain works, he says. With finer implants 鈥測ou鈥檙e going to be able to do so much more鈥�, Lieber says.
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Lieber and colleagues built a chip with 20-nanometre-thick silicon wires running across its surface (Science, vol 313, p 1100). On top of the chip the researchers grew rat neurons, which stretched out their axons (the long projections that transmit signals to other cells) and the shorter extensions called dendrites, which receive signals. The axons and dendrites formed more than 50 connections per neuron with the nanowires, each about the size of a natural synapse (see Image).
The researchers were able to watch signals as they passed down the axon and through the nanowires. They were also able to enhance the signals by stimulating the axon with electrical pulses from the nanowires. This would enable neural prosthetic devices to transmit signals to each neuron containing feedback on movements being carried out by the robotic limb or computer cursor, allowing the person to make any necessary adjustments. The researchers are now building larger arrays of nanowires, on which they hope to grow a network of neurons.
The work will dramatically improve our understanding of how neurons process information, says bioengineer Jose Carmena of the University of California, Berkeley. However, he says the team has yet to make the leap from developing cell cultures to producing devices that can be implanted into live animals. So far, Lieber鈥檚 group has been able to keep the cells alive and firing on the nanowires for around 10 days.
Lieber is also interested in connecting neurons to electronic devices to create hybrid systems that could lead to new types of computers. He and his colleagues connected a neuron to their silicon nanowires to create a simple circuit that acted as a logic gate, the basic building block of computers.
