BME 552 - Electrodes and packaging

BME 552 - Electrodes and packaging - 1/30/2009...

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1/30/2009 1 Device/Tissue Interface II BME 552 January 30, 2009 Today’s Business • HW1 due today at 5PM to Tim Nayar • HW2 available on blackboard • Reading – Merrill section 2 –Rose Najaf (packaging o er ie – Najafi (packaging overview)
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1/30/2009 2 Overview of next 5 classes We will examine the signal path from current generator to neurons (not in that order) Class 2-3 – Electrodes/Packaging Class 4-5 – Stimulating neural activity: short, medium, long-term effects Class 5 – Circuits for neural stimulation Class 6 – applications of neural stim: retina, deep brain stimulation, peripheral nerve For now, assume circuit has power and data source Overview of this class • Review of electrochemical methods Response of electrode to constant current • Response of electrode to constant current pulse • Charge storage capacity vs. injectable charge • Pt pseudo capacity • Non-linearity of electrodes • Non-uniformity of current at disk electrodes • Definition of Electrochemical Safety • Packaging Microelectronics
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1/30/2009 3 Electrochemical Methods The electrochemical measurement methods can be used to define safe operation limits for stimulating electrodes, predict stimulating electrode behavior, and evaluate electrodes implanted in nerve tissue What makes a good neural prosthesis electrode material • Stability under conditions of use which include – Saline environment – High current density – If chemical reactions occur, it does not produce harmful reactants • For stimulation, high frequency signals used, so low frequency electrode behavior of little concern as long a charge balanced stimulus is used • For recording, spike bandwidth centered at 1 kHz, evoked potential or EEG are lower down to 1 Hz but evoked potential or EEG are lower, down to 1 Hz, but signals are small. • Thus for both stimulus and recording, many of the undesirable non-linear properties of electrodes are avoided, within certain restrictions.
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1/30/2009 4 CV determines operating range for stimulation • CV delineates “water window”. At negative potential, H 2 evolves, at positive potential O 2 evolves. • In between these potentials, reactions should be reversible. During stimulation During stimulation, electrode potential should stay within water window • Rose Paper to cover this CV determines reversibility of reactions • For perfectly reversible reaction – Position of peaks doesn’t vary – Ratio of current peaks equal to one – Peak current proportional to (volt/sec)^0.5
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1/30/2009 5 Impedance vs. freq • Frequency content of stimulus pulse stimulus pulse, recorded signal, will determine impedance that the signal “sees” • Impedance data uses small signa small signal Constant current stimulation CV and EIS waves not what is used for neural stim Constant current used because it….
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BME 552 - Electrodes and packaging - 1/30/2009...

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