Unformatted text preview: BME 552 Homework Set 4 Assigned Feb 20, 2009 Due Feb 27, 2009 1. Neural system you are interested in stimulating has chronaxie of 0.6 ms and rheobase of 40 uA. You are working on an implantable device that will have limited voltage. The limit on the voltage for the proposed device is +/‐5V for stimulation (i.e. the circuit has +/‐ 6V to operate), but in order to obtain the data you use a stimulator with +/‐ 100V. Your electrode diameter is 100 microns. Assume tissue resistivity of 100 Ohm‐cm, capacitance of 20 uF/cm, and Rp of 10 MOhm. At what pulse width do you achieve the lowest voltage? Is this voltage within the limits of the stimulator? Use the strength duration data to generate a set of (pw, pa) pairs, then plug these into the equation v(t) = IRs + IRp(1 – e(‐t/RpC)), where I = pa and t = pw. Find the minimum V. My plot is below. Voltage has a minimum value at pw = 0.15 ms. The voltage is within the limits of the high voltage stimulator but not within the limits of the implantable stimulator being designed. 2. You have a constant voltage stimulator in a DBS system. Initially, resistance seen by the output is 5KOhm (assume it can be modeled as a single resistor for this problem). Over time, this resistance increases to 15KOhm. Your initial settings for stimulation are 3V, 0.1 ms. Your stimulator has a 5V output limit. How do you need to adjust your output to have equivalent charge output at the higher resistance? Why may you need to increase the charge with the higher resistance? Initially, 3V for 0.1 ms will result in current of 0.6 mA and charge of 0.06 uC. If the load is 15 kOhm, then increasing voltage to 5V will result in 0.333 mA. You will need to increase the pulse width to 0.18 ms to apply equivalent charge. Since longer pulses are less efficient at stimulating neural tissue, then a longer pulse with equivalent charge may not result in the same neural response. 3. Dr. Wills Problem, see following pages. Solution below. ...
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 Spring '09
 Berger,Weiland
 Interest, 100 ohm, 100 microns, Dr. Wills Problem, constant voltage stimulator, high voltage stimulator

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