Dynamic Study of Flexible Sensors to Reduce Motion Artifacts
Rodeheaver, Nathan A.
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The field of wearable electronics is changing healthcare and increasing possibilities for human-machine interfaces. Soft electronics stretch with the skin to monitor long-term heart rate trends or direct the motion of smart prosthetics. The capabilities are only as good as the signal quality. A significant challenge for these devices is that by their very definition – wearable – these flexible sensors suffer from motion artifacts not previously found when measured in a stationary setting. This thesis investigates three significant sources of motion artifacts for flexible sensors: relative motion between sensor and signal source, the unique challenges of skin strain, and change in contact impedance. Relative motion is not a unique problem for wearable electronics. Still, human tissue's elastic nature means that most body-mounted sensors undergo more relative motion than on a comparable rigid machine. Device design and placement are analyzed to reduce the movement between the sensor and signal source. Dynamic effects of jogging are numerically simulated for a chest-mounted device showing a small form factor, and lightweight designs reduce device motion. Human skin is an unstable platform to mount devices. Skin strain causes device movement and changes the biopotential during measurement. Experimental examples show material and design solutions to increase adhesion, reduce strain within the device, and maintain breathability for long-term recordings. Flexible sensors measuring biopotential are susceptible to changes in contact impedance. Skin strain and vibrations create motion artifacts that can mimic or disrupt many biosignals, making them hard to filter out. A prototype device is presented that uses a strain isolating layer to reduce skin strain at the electrode, which stabilizes contact impedance and reduces motion artifacts. Experimental data from the device compensating for these three sources of motion artifacts is presented for quantitative comparison.