Research On Liquid Type Flexible Strain Sensor Based On Low-Temperature-Molded Gallium-Indium Alloy

Normally,flexible strain sensors which consist of conductive materials and the elastomer can detect the deformation by the change in electrical properties of conductive materials(e.g.electrical resistance).Flexible strain sensors have shown broad application potentials in motion monitoring,human-machine interaction and so on.At present,most flexible strain sensors adopt solid conductive materials,which results in the greater friction between solid conductive materials and the elastomer,so it is difficult for solid conductive materials to return to initial position after being stretched.If solid conductive materials are substituted by liquid conductive materials,we can alleviate or even solve above problems by liquid conductive materials with good fluidity and ductility.At this situation,flexible strain sensors are called liquid type flexible strain sensors.However,the fabrication process of most liquid type flexible strain sensors is complicated.Generally,the liquid channel mold with a certain shape and length is encapsulated into the elastomer.After the elastomer fully cures,the channel mold is removed by direct extraction and so on.Thus,the elastomer substrate with a liquid channel is formed.Therefore,in this study,we fabricated two kinds of liquid type flexible strain sensors using tin-bismuth alloy and low-temperature-molded gallium-indium alloy respectively to alleviate or even solve the problem related to the cumbersome fabrication process.We encapsulated the cuboid tin-bismuth alloy into the elastomer and then removed the alloy by heating and extruding to form a liquid channel.We also encapsulated the pre-molded gallium-indium alloy into the elastomer and then gallium-indium alloy automatically turned back into liquid metal at room temperature,which eliminated two steps of removing the mold and injecting liquid conductive materials.Finally,we tested these two kinds of sensors.Specific work and research results are as follows:(1)Initially,the cuboid tin-bismuth alloy(melting point of 80?,length of 40 mm,width of 5 mm,thickness of 1.5 mm)was encapsulated into the elastomer(Ecoflex 0050)which could tolerate high temperature above 100?.Then Ecoflex 0050 was heated to 100? in a water bath so that the alloy could be removed thoroughly by extruding after it turned back into liquid.Finally,the first liquid type flexible strain sensor was fabricated.The test results show that the detectable tensile strain range is 0.5?200%,the tensile strain gauge factor is 5.2 and the degree of hysteresis is 6.9%.However,the sensor only completes 500 times tensile strain fatigue tests and the fabrication process is still so complicated that the step of removing the mold is inevitable.(2)To further simplify the fabrication process and improve the durability,liquid metal whose melting point of 16? was cooled down to 0? by cooling-spray so that liquid metal became solid and cuboid liquid channel mold(length of 40 mm,width of 5 mm,thickness of 1 mm)was formed.Then silver-plated copper wires and the mold which had been connected were placed on the surface of Ecoflex 0050 which had cured for an hour,ensuring that silver-plated copper wires and the mold would not sink.Finally,we poured Ecoflex 0050 on the mold to complete the encapsulation so that the second liquid type flexible strain sensor was fabricated.The test results show that the minimum detectable limit is 1%tensile strain or pressure of 7 kPa and the maximum detectable limit is 300%tensile strain or pressure of 211 kPa.The gauge factor of tensile strain and compressive stress are 2.15 and 0.62 kPa-1 respectively.The degree of hysteresis is 10.2%and the sensor completes 5000 times tensile strain fatigue tests and 4000 times compression stress fatigue tests.When temperature of the sensor rises from 20 to 80?,the temperature coefficient of resistance is only 6.4×10-4?-1 which shows good thermal stability.(3)We applied the liquid type flexible sensor based on low-temperature-molded gallium-indium alloy to human motion monitoring successfully,not only for detecting the bending of elbows,fingers,knees and rotation of the nape,but also for detecting the undulation of the biceps and vocal band vibrations.