In the past decade, neuroscientists and clinicians have begun to use implantable MEMS multielectrode arrays (e.g., [1]) to observe the simultaneous activity of many neurons in the brain. By observing the action potentials, or "spikes," of many neurons in a localized region of the brain it is possible to gather enough information to predict hand trajectories in real time during reaching tasks [2]. Recent experiments have shown that it is possible to develop neuroprosthetic devices -machines controlled directly by thoughts -if the activity of multiple neurons can be observed.Currently, data is recorded from implanted multielectrode arrays using bundles of fine wires and head-mounted connectors; all electronics for amplification and recording is external to the body. This presents three major barriers to the development of practical neuroprosthetic devices: (1) the transcutaneous connector provides a path for infection, (2) external noise and interfering signals easily couple to the wires conveying weak neural signals (<500µV) from high-impedance electrodes (>100kΩ), and (3) the connector and external electronics are typically large and bulky compared to the ~5mm electrode arrays. To eliminate these problems, data from the implanted electrodes should be transmitted out of the body wirelessly. This requires electronics at the recording site to amplify, condition, and digitize the neural signals from each electrode. These circuits must be powered wirelessly since rechargeable batteries are relatively large and have limited lifetimes. Low power operation (<100mW) is essential for any implanted electronics as elevated temperatures can easily kill neurons.A wireless, fully-implantable neural recording system is being developed to facilitate neuroscience research and neuroprosthetic applications (see Fig. 30.2.
In a recent paper on deviations from Matthiessen's rule for platinum Klemens and Lowenthal (1961) classified the deviation patterns, calculated for a number of different platinum resistance thermometers, into three groups, and reported that only one of these groups followed the pattern predicted by Sondheimer and Wilson's (1947) two-band conduction theory. They suggested that if resistors belonging to one particular group (though no matter which group) were selected for use in low temperature platinum resistance thermometry then the resistance-temperature relationship could be expressed accurately by a relatively simple formula. We believe that Klemens and Lowenthal's method of classifying the resistors into groups is open to serious objection and that consequently some of their important conclusions are not necessarily valid.According to Sondheimer and Wilson (1947) the resistivity p(T) for two-band conduction can be expressed in the following mann-er :(1) where p(T) and p(O) are the resistivities at temperature T oK and 0 oK respectively and the subscript i refers to ideally pure metal. The last term on the right-hand side represents the deviation from Matthiessen's rule and should always be positive. According to Wilson (1953) the quantities a and b should be of the order of unity and may be temperature-dependent.To evaluate the parameter a (in fact they preferred to use the reciprocal) Klemens and Lowenthal used the relation given below which was derived from equation (1) for the case where p;>p (O) :~~t:~:;:(~) (l-cu;(T))(1 +1/a), (2) and where for any given resistor cu(T) and cu(O) are the ratios of the resistivity at T oK and 0 oK respectively to the resistivity at 273 oK; the subscript T4 refers to their reference resistor T4. Their calculated values of 1/0, for 17 platinum resistance thermometers were found to vary among the different thermometers from -0·4 to +8·6 and in addition were usually temperature dependent for any particular thermometer. They then classified the resistors into three groups with group 1 having l/a both small and constant with temperature; group 2 having l/a small and increasing with temperature; and group 3 having l/a large and increasing with temperature. In conclusion, they stated that the two-
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