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Shape Memory Polymer Micromachining for Wearable Sensors and Health Monitoring


Despite many advances in mobile communications and data science remote, continuous health monitoring remains a challenge due to a dearth of appropriate wearable sensors. These sensors must bend and stretch along with the human body while maintaining consistent performance. As most standard electronics component are made of rigid, brittle materials, there is an inherent mechanical mismatch that limits the usefulness of current sensor technology for wearable health monitoring.

This work introduces pre-stressed thermoplastic shape memory polymers as a fabrication tool to produce complex, hierarchically wrinkled thin films for use as wearable sensing electrodes and wearable strain sensors. Additionally, a series of scalable, polymer compatible micromachining techniques are discussed for fabricating these sensors.

Hierarchical wrinkled structures significantly enhance the surface area (>600%) as compared to planar thin films. Integrated with a flexible polymer carrier, wrinkled electrodes can provide increased efficiency with low sample volumes for electrochemical sensing on human skin. This enables potential application of bioanalyte detection in human sweat.

Out of plane wrinkling in thin films can also serve as a strain relief. By transferring wrinkled metal thin films into an elastic carrier, highly elastic strain sensors can be fabricated and used to detect and monitor human motion. These sensors show very high gauge factors (as high as 42) over a wide dynamic range (>150%). Worn on the chest, these sensors can monitor breath rate and approximate lung volume based on chest wall displacement as is demonstrated in this work.

Finally, future work with possibility of incorporating a water soluble sacrificial layer are discussed. The addition of hydrophilic polymers as a lift off layer increases flexibility and ease of transferring wrinkled thin films. Importantly water-based processing would convert the current solvent-base lift off process to a more green manufacturing process.

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