Jeong, Dahee

Department of Mechanical EngineeringThis thesis presents improvement reliability in a conductive liquid metal, Eutectic gallium-indium (eGaIn), based soft sensors and verification of the sensor lifetime. The soft sensor is applied to measure human motion as wearable systems which have repeated over thousands of cycles. Hence, the soft sensor should have high reliability and predictable lifetime.

To improve reliability we suggested direct wiring of eGaIn to connect electrodes which is the most fragile in the sensor system. The liquid metal was used instead of rigid metal wire, which is usually inserted to microchannel to connect the sensor and the electrode. Also, Sil-poxy which is one of the silicone bonds was selected to reinforce the interface without cure inhibition of the elastomer. Therefore, the reinforced electrode part can endure 100% strain over 1,000 cycles. In addition, different 4 elastomers were suggested to acquire enough stretchability which is larger than the strain range required by applications. Then, the performance of soft sensors which were made of suggested elastomers was verified in terms of stretchability, the relationship of strain-normalized resistance, linearity, hysteresis, and the response at the different cyclic speed by a pull-to-failure and cyclic loading test. It is found that no significant variation or degradation of sensor signal even softer and stretchable elastomers.

To predict the soft sensor lifetime we conducted an accelerated life test, which was a cyclic loading test until failure of the sensor. Samples were made of Dragon Skin 30 and KE-1190 respectively. From the test, the life distribution was expressed by Weibull distribution with similar shape parameter. It showed the sensor lifetime was accelerated by strain range under the same failure mode. Then, the relationship of the number of cycles and applied strain was represented by inverse power mode. Two failure modes were observed during the test, crack propagation and increase in resistance. The sensor which had low hardness had bur in knife cutting process and the bur cause crack propagation in large strain range. In case of small strain range, eGaIn was oxidized during a large number of cycles and it caused the increase of resistance of the eGaIn sensor. As a result, the lifetime of the soft strain sensor was divided into 3 stages related failure modesclos

Improvement and Verification of Reliability in Conductive Liquid Metal based Soft Sensors

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