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EE 4BD4 Lecture 28 Electrical Muscle Stimulation 1 Purpose • Provide muscle control where path from brain is interrupted • Provide feedback control during muscle retraining • Keep muscle healthier when it has been dennervated (no alpha motor unit connection) • Strengthen or maintain muscle when joints have been immobilized 2 Muscle Stimulation to Maintain Health • What happens to muscle after the nerve supply is lost due to an accident 3 Muscle Structure after 1 Month • Muscle Cross-sections A – denervated, B – denervated and stimulated, C – innervated; bar = 100 µm 4 After 3 months + 3 months Nerve Attached 5 General Instrumentation Layout Control Source Pulse Generator Pulse Shaping Pulse Output Electrodes 6 Control Source • Generate the pulse timing information and the pulse amplitude and duration • Can generate single pulse timings or the start and stop of trains of pulses at a set frequency (i.e. the start and number of pulses) • Can provide constant pulse amplitudes or shaped trains • Usually computer or microcontroller 7 What Pulse Shapes Have Been Used? 8 Final Forces Produced by Muscle 9 What Pulse Train Frequency? 10 Pulse Generator • Can use constant voltage (i.e. current determined by patient electrode/skin impedances) or constant current (voltage across electrodes determined by patient skin/electrode impedances) • Stimulation is determined by current flow so constant current is more modern design • For surface stimulation and motor nerve close to skin with 50 μsec pulses get sensation around 10 ma and nerve stimulation around 15 ma 11 Constant Current Designs • Patient not grounded 12 Howland Current Pump • Patient grounded 13 Limitations of These Designs • Voltage compliance (maximum voltage resulting from impedance) is limited to power supply voltages) • With typical load impedances of 1 kΩ and 30 ma stimulating current you already require 30 V, requiring high voltage op amps (and power supplies) 14 Simpler Designs Transistor Based Op amp Added 15 Safety Lockout • Transistor switch added to stop inadvertent stimulations 16 Specs for Last Circuit • Transistor is TIP50 • Op amp is LM358M • Setting resistor determined by current required at that input voltage • VHV Can be high voltage source 17 Overall System 18 +15v Rat Muscle Stimulator 5V power Power 5V Isolator isolator PC USB Microcontroller CNTRL 1 High CNTRL 2 High CNTRL 2 Low CNTRL 1 Low To constant current source and H-bridge components To CCS Optocoupler Optocoupler H-bridge Inside the H-bridge Constant current source Inside the constant current source (CCS) From H-Bridge From H-bridge Vin DAC + - From optocoupler Rsense Current through Vin ______ rat = Rsense 19 20 Safety Lockout • Transistor switch added to stop inadvertent stimulations 21 High Voltage Application • What do we do if our compliance voltage exceeds the supply voltage? • (1) Use a DC-DC converter to give high voltage source to drive circuit Fig 3.8 slide 21 • (2) Replace load in Fig 3.8 by the input coil of a pulse transformer • If output current to electrodes is to be e.g. 40 ma, the input current must be 480 ma if transformer ratio is 1:12 • May then replace transistor in Fig 3.8 by a Darlington pair for current amplification 22 Darlington Pair • Used as a current amplifier for higher current loads • Can be constructed from two bipolar transistors or implemented with an IC • If very high currents req’d (amps) use power transistors • Disadvantage is there is considerable voltage drops across the junctions so that the voltage seen at the input coil is reduced 23 Darlington Pair for Current Amplification 24