Jessie Qian and Lacey Cirinelli

Spring 2005 Senior Design Project

Dept. of Bioengineering-The Pennsylvania State University, University Park, PA 16802

Sponsor: Dr. Ryan S. Clement, Neurotechnology Lab

Implantable electrodes (Fig. 1) record neural activity in the brain which is generally used for any type of neurophysiology research done today; however, it's been shown that over time, implanted electrodes tend to lose their ability to record brain activity.  The main reason that this is believed to happen is due to gliosis or glial scarring which is caused from electrode micromotion in the brain [1].  Thus far, little research has been done to determine both the type and amount of electrode motion and the affects of this motion on tissue scarring.  Therefore, there is a need for a data acquisition system which will be able to correlate heart beats (through the use of an electrocardiogram (EKG)), respiration, and electrode movement (through use of a linear variable differential transformer (LVDT)). 

Figure 1. Electrode used for neurophysiology research
 

The final report for this project may be downloaded in Microsoft Word format: Final Paper

Design Criteria

• Mobile, basic breadboard circuit protected in project box for sturdiness and reusability

• •Operates over normal rat cardiac rate (225-495 heart beats/min) and body temperature (90^o-110^oF)

• •Viewable on a multi-channel oscilloscope

• ¡Compatible with DataWave software (DataWave Tech., Wausau, WI) 

Original EKG Circuit

The first circuit that was tried may be seen below in Fig 2, 3 and was composed of two main components.

Fig. 2: EKG schematic [2]                                                                                                   Fig. 3:  Original combined EKG circuit (from lab)

                                                     
Tests could then be conducted by testing the first component using Agilent Technologies' (Agilent Technologies, Palo Alto, CA) Signal Generator Cardiac Signal.  The first component was also tested on humans.

Fig. 4: Component 1 output                                                                                               Fig. 5: Lab setup

From Fig. 4, it is possible to see the results for just the first component of the circuit.  The input wave (top waveform) is first generated by the Signal Generator.  The second waveform is the output that is made by the circuit.  As we can see from the signal above, it is very close to the input being generated which is what we were looking for.

After adding the second component of the original EKG design, the waveform had a lot of noise and as seen in Fig. 6, was not able to give any information with regard to where the PQRST data were located.

Fig.  6: Combined EKG output for the original circuit

Revised EKG Circuit

As seen from the waveform from Fig. 6, it is not possible to get an accurate reading of the heartbeat from the images generated from the first circuit.  It is important then, to find alternative designs for the EKG in order for it to be feasible for testing on a rat.  In order to do this, the circuit must gain extra amplification (through an additional op amp component and a voltage divider to prevent amp saturation) while at the same time be reduced for noise (through less wiring, additional filters, and extra capacitors to ground noise artifacts).  Appropriate electrodes must also be made for testing purposes on the rat.

Fig. 7: Revised EKG circuit schematic [3]                                                                          Fig. 8: Revised combined EKG setup (from lab)

Using the circuit schematic from Fig. 7, it is possible to retest the circuit using the signal generator:

Fig. 9: Combined output for revised EKG circuit                                                                     Fig. 10: Printout of combined output

From both the circuit waveform output and the printout, it is possible to see that the PQRST waves are clearly defined.  This combined circuit was also tested on humans and on the rat during acute surgery.

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