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Dissertation Defense – Mehdi Kohani

Title: Electrostatic Discharge (ESD) Risks in Wearable Medical Devices: Evaluating the Standard Test Methods and Developing a Current Prediction Model

Day: Tuesday, August 14th
Time: 11am – 1pm
Location:  EGR-0159 (ENGR)

Committee:
Professor Michael Pecht, Chair/Advisor
Professor Patrick McCluskey
Professor Aristos Christou
Associate Professor Jin-Oh Hahn
Professor Christopher Davis (Dean’s representative)
Abstract:
Electrostatic discharge (ESD) is a critical reliability concern for wearable medical devices. In recent years, numerous reports of device malfunction resulting in patient adverse events, and medical device recalls have been attributed to ESD. To mitigate the risk of device malfunction, sufficient ESD immunity standards and accurate ESD prediction models that represent reasonably worst-case discharges during usage are necessary. Thus, ESD test configurations that represent realistic discharges of wearable devices in healthcare applications need to be developed, and the severity of the ESD events need to be compared with the existing ESD immunity standards. The U.S. Food and Drug Administration recognizes the IEC 60601-1-2 collateral standards, within which the IEC 61000-4-2 standard is the recommended ESD test method.
The severity of the discharges depends on the electrical impedance of the body and the discharging structure. Thus, to identify the worst-case realistic discharge scenarios for wearable medical devices, the proper body posture, device location, and the realistic discharge setup need to be determined. The ESD literature on wearable devices has only focused on standing person discharging to a vertical plane, as the worst-case ESD. Moreover, previous current prediction models did not consider the impedance parameters of the human body and the test setup on the waveform parameters.
Through conducting laboratory experiments in a climate chamber, the worst-case charging activities performed routinely by patients and hospital personnel were identified and the peak voltage levels during the activities were measured. ESD measurements for these scenarios showed that the IEC 61000-4-2 is not sufficient for these devices since the waveform parameters (peak currents and maximum current derivatives) of realistic discharges were up to 1.9 and 2.4 times larger than the standard test method, respectively.
A physics-based model for current waveform prediction with reasonable accuracy (<10% error) was developed using the impedance of the discharging structures and the human body for three body postures (standing on the floor, sitting and laying down on a hospital bed) and two device locations (hand and waist). The trends in the waveform parameters of the realistic discharge configurations were explained in terms of the body resistance and capacitance.