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This project involves designing electronics for in-vitro neural recording systems.
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 tran?sistors are significantly smaller than individual neurons and could in principle provide information on how subsections of a neuron behave.
Nerve cells are attached to open, metal-free gates of field-effect transistors submersed in electrolyte. The intracellular voltage is modulated by small ac signals from 0.1 Hz to 5000 Hz using a patch-clamp technique. The source-drain current is affected in amplitude and phase through a modulation of the extracellular voltage in the cleft between transistor and cell. The ac-signal transfer is evaluated on the basis of linear response theory. We use the model of a planar two-dimensional cable which consists of the core of an electrolyte sandwiched between the coats of a cell membrane and silicon dioxide of the transistor surface. Comparing experiment and model we obtain the resistances of core and coat, i.e., of the seal of cell and surface and of the attached membrane. The resistance of the membrane varies in different junctions. It may be lowered by two orders of magnitude as compared with the free membrane. This drop of the membrane resistance correlates with an enhancement of the seal resistance, i.e., with closer adhesion. The linear ac-transfer functions are used to compute the signal transfer of an action potential. The computed response is in good agreement with the observations of excited nerve cells on transistors.
These circuits are potentially useful for invariant pattern recognition.
An analytical technique to be used in MOVPE must be applicable at pressures from 20 hPa up to atmospheric pressure. This rules out the standard methods of electron diffraction (LEED, RHEED) used in MBE, since they only function in ultrahigh vacuum (approx. 10-10hPa).
500 nm mesa of a resonant tunneling transistor for digital applications and single electron tunneling
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The brain is a collection of about 10 billion interconnected neurons. Each neuron is a cell [right] that uses biochemical reactions to receive, process and transmit information.