CENTER FOR BIOMEDICAL ENGINEERING

BE 306 Bio-Instrumentation (Lecture)

Sample Problem Set 5

 

 

  1. Please draw the equivalent circuit of the electrode-electrolyte interface. Label all components. Indicate the physical meaning of each component.

    2.  

  2. Two electrodes were implanted into the body and their combined electrical characteristics were measured. When no current passed through the circuit, 180 mV was measured between the electrodes. When 10 m A DC current was applied, voltage seen between the two electrodes increased until it reached to 290 mV and remained constant. What are the lumped sum value of the Rs and Rd for the two electrodes and the tissue? What can you tell about these electrodes? Are they made from the same material?

  3. If the the lumped sum value of the Rs and Rd for the two stimulation electrodes mentioned above were known to be 6 KW, what would be the voltage seen across the two inner measuring electrodes when the stim current is 1 mA?

    4.  

     

  4. Two electrodes are implanted into the body. When 10ľA was passed through the electrodes, a potential difference of 300 mV was observed. When 20 ľA was applied, a potential difference of 400 mV was observed. What is the lumped value of Rs, Rd and Rbody? Show your work!

    5.   

  5. An electrical stimulation pulse is being sent to a sphincter muscle to prevent incontinence. If the cathodic portion of the pulse has an amplitude of 8 volts and pulse width of 1 milli-second, what should be the amplitude of the 4 milli-second anodic portion of the pulse to minimize the polarization on the stim electrode?

    6.  

  6. Below is a diagram of an electrochemical sensor:

 

 

6.a ) What are the symbols and represent?

: Antigen (AGN). In this case, it represents the drug which concentration is being studied.

: Patient's own antibodies (ABY) against the drug .

 

6.b) Explain how this sensor works

On the absence of antibodies, the potassium ions are able diffuse through the PVC membrane and maintain an ionic current between the two electrolytes. If the patient develops an immune reaction against the drug, then the antibodies will attach to the drug immobilized on the PVC membrane, and reduce the ionic current. In the worst case, the combined AGN-ABY complex would completely block the surface of the membrane and isolate two electrolytic chambers à open circuit. This will be detected by the voltmeter.

6.c) Is this a potentiometric or an amperometric sensor? Why?

The particular implementation shown for problem 6 is a potentiometric type sensor since a voltmeter is measuring the potential difference between the two electrodes. If this was an amperometric sensor, then there would have been a power supply (e.g. a battery) and a current meter on the circuit.