Although surgeons dream of replacing severed limbs with mechanical prostheses controlled by the living nervous system, the creation of ‘bionic men’ remains purely imaginary. But now a team of German biophysicists has taken a small step toward merging man and machine by opening a two-way communications link between a silicon chip and a leech’s nerve cell, reports Malcom Browne of the New York Times. Dr Peter Fromherz and his team at the Max Planck Institute of Biochemistry, Munich, Germany, have reported that that they have built a junction between a spot on a silicon chip and a corresponding spot on a leech neuron. A novel feature is that no electric current passes from the chip to the neuron: the chip stimulates the neuron to fire, or respond, by using capacitive stimulation, in which electric charges are rearranged without any flow of current. Since the late 18th century, when it was shown that electrical stimuli cause muscle tissue to contract, scientists have explored the electrochemical pathways by which nerves are made to fire. In Fromherz’s work, an electric voltage applied to the interior of the chip produces an electric field that induces a charge inside the cell; when it reaches a threshold level, the cell fires, initiating the electrochemical sequence by which nerve cells communicate with their neighbours. This success complements an achievement by Fromherz’s team four years ago, when it found a way for a chip to translate the impulse of a leech nerve cell into a voltage change in a chip containing a Field effect transistor, also without any flow of electrical current. And the latest achievement has established a signalling channel between a nerve cell and a silicon chip that works in both directions – an essential requirement for any future prosthetic limb controlled by the brain through a living nervous system. The chips used by the group contain a charge-carrying silicon layer covered by a thin insulating layer of silicon oxide. This layer, in contact with the leech nerve membrane, blocks any flow of electricity, but is permeable to the electric field induced by the silicon layer beneath it. But the finding in itself open the way to building bionic limbs, Fromherz said, noting that capacitive coupling is not the mechanism by which living cells communicate. Neurons transmit their messages by chemical changes at their synapses. One of the obstacles to integrating a silicon chip with a living nervous system is maintaining intimate contact between the chip and the neurons. Various scientific teams in Europe, Asia and the US are experimenting with adhesive peptide chains and proteins, to maintain tight bonds between biological tissue and silicon-based contact points. The State University of New York at Buffalo has reported that it has bonded protein to etched patterns on a Teflon film and then used the patterns as templates for the directed growth of nerve cells. In principle, a Teflon template could act as a scaffold for growing a neural network, although the researchers’ interest lies in the healing of wounds. To etch a pattern on a Teflon film, the Buffalo group has used a technique that borrows ideas developed by the semiconductor industry, which uses lithographic technology to manufacture the intricate microscopic circuits that make up a chip. Teflon is masked with nickel, through which the pattern of an intended neural pathway has been cut out. It is then exposed to a blast of hot plasma which penetrates the gaps in the nickel mask and replaces fluorine atoms in the non-reactive Teflon with highly reactive hydroxyl groups along the etched lines. Then peptides derived from a cell-attachment promoter called laminin are brought into contact with the etched pattern and chemically bonded to the reactive hydroxyl groups along the etched lines. Finally, live neurons placed on the template bond to the peptides and use peptide channels as guides for their own growth. But the team hasn’t tried using primary human nerve cells, and does not know yet whether something like this could be implanted in a live person. Back
at the Max Planck Institute, Fromherz’s group has begun a study of the behaviour of neurons from the hippocampus of rat brains. The hippocampus is interesting as it is believed to be a seat of memory. Does this mean that scientists hope someday to create a direct interface between a silicon chip and one of the main engines of thought? Formherz explained We’ll never see a day when the Encyclopaedia Britannica could be transmitted by a chip into the brain, but what we do hope to create is a neural network of rat cells that would survive for long enough that we could study its development in detail, using thousands of attached transistors. A little at a time, we progress.
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