DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

326 STIMULATION OF EXCITABLE TISSUES(a)RfRSlave-VSupply-VSupplyVinRin42 -3 +7+VSupply6A1Master____ RfGain =RinLOADA2SlaveVLoad Max = +/- 2VSupply647- 2+ 3+VSupplyGain = 1RSlave(b)RfRSlave-VSupply-VSupplyRinRin42 -3 +76RSenseVin+VSupply+VSupplyRfSlaveMasterVCompliance = +/- 2VSupplyFigure 7.14 The bridge connection of two op-amps provides output voltage swings twice that of one op-amp, making it possible to reachhigher stimulation or compliance voltages in applications that operate amplifiers near their maximum voltage ratings. (a) Generic voltageoutputbridge connection of two op-amps. (b) The master amplifier can be set up as any op-amp circuit: for example, as a Howland currentpump.LOAD647- 2+ 3RSlaveelectrode pair is equal to the amount of current desired. Current leak between electrodepairs is minimized and channel isolation is improved significantly. Figure 7.15c showshow the circuit could be built with Howland current pumps.Charge Injection through Implanted ElectrodesElectrodes used for stimulation behave radically different than electrodes that are usedonly for sensing biopotentials. This is because much stronger currents need to be producedthrough the tissue by converting electron flow in the leads to ion movement in theaqueous solution. Safe electrical stimulation of the nerves and muscle through directcontactelectrodes requires this conversion to happen only through reversible chargeinjectionprocesses. Irreversible reactions such as electrolysis result in the evolution ofgases and acid or alkali solutions that can destroy the tissues. Other harmful irreversiblefaradic reactions include saline oxidation, metal dissolution, and oxidation of organicmolecules.