Silicon on the Brain
A chip that reads neurons
Context: The neurons of the mammal brain are hard to study, even when they’re isolated in the lab. For more than a decade, scientists have analyzed the large neurons of leeches and snails by linking them directly to silicon chips that record their electrical activity. But mammalian neurons are smaller, and though they can be grown on silicon, the resulting signals are typically too weak to yield useful data. The electrical activity of mammalian brain cells can be read with electrodes, but that can be imprecise and requires careful preparation steps.
Moritz Voelker and Peter Fromherz at the Max Planck Institute for Biochemistry have now designed the first computer chip that can record the firing of mammalian neurons, though so far only in a petri dish.
Methods and Results: As a neuron fires, the voltage across it changes, so a neuron on a chip affects how transistors underneath it conduct electricity. But in chips with conventional transistor designs, there’s so much naturally occurring noise that it swamps neural signals. So Voelker and Fromherz changed the geometry of the transistors to suit the electrical properties of living neurons. They buried the conducting channels of their transistors a few nanometers deeper than usual, making the transistor more sensitive to the low voltages and firing speeds of neurons. The transistors could detect the signal of an individual rat neuron in a group, without the elaborate sample preparation that conventional electrodes require. What’s more, the transistors are significantly smaller than individual neurons and could in principle provide information on how subsections of a neuron behave.
Why it Matters: Electrodes implanted in human brains have allowed paralyzed patients to move computer cursors and prosthetic limbs (see “Implanting Hope,” March 2005, p. 48). While increased computing power helped enable that breakthrough, so too did the development of hardware suitable for detecting neural signals. A silicon interface could process data more nimbly and is the logical candidate for next-generation devices. Those are still years away; in the nearer term, neuron-silicon interfaces will help explain how groups of neurons communicate with each other and could be particularly helpful for understanding how neuroactive drugs such as antidepressants work.
Source: Voelker, M., and P. Fromherz. 2005. Signal transmission from individual mammalian nerve cell to field-effect transistor, Small 1:206–210.