Flexible Piezoresistive Composites Based On Low-cost Carbon Materials And Their Applications In Strain Sensors

Sensors are being widely used in aerospace weapon,traffic,building and medical care areas.Nowadays,with the rapid development of various intelligent wearable electronic devices,the sensors that are able to monitor the ability of human touching,smelling,tasting,viewing and feeling have gained increasing attention in the human fit,hospitality and motion areas.However,conventional sensors are rigid and have low-sensing ability,they cannot meet the high requirements for the good flexibility and high sensitivities of the wearable features.Recently,the flexible sensors reported are based on expensive materials and complex fabricating methods which are not suitable for large-scale production.Besides,sensors that could only identify stimulus in a certain direction could not work effectively in complex environment.It is of high importance to fabricate large-scale flexible sensors which have high sensing abilities with low-cost raw materials and simple techniques.In this work,the fabrication of a wearable strain sensor for human motion detection is first demonstrated with a commercially available nano-sponge as a starting material.The key resistance sensitive material in the wearable strain sensor is obtained by the encapsulation of a carbonized nano-sponge(CNS)with silicone resin,the CNS is obtained by high pyrolysis without air.The as-fabricated CNS/silicone sensor is highly sensitive to strain with a maximum gauge factor of 18.42.In addition,the CNS/silicone sensor exhibits a fast and reliable response to various cyclic loading within a strain range of 0–15% and a loading frequency range of 0.01–1 Hz.Finally,the CNS/silicone sensor as a wearable device for human motion detection including joint motion,eye blinking,blood pulse and breathing is demonstrated by attaching the sensor to the corresponding parts of the human body.Then,to gain a flexible strain sensor,which can simultaneously differentiate normal and lateral strain,cotton thread is used as raw material.And through a similar method,the cotton thread is carbonized into carbonization cotton thread(CCT).The strain sensor is developed based on the combination of a quasi-rectangular wave shaped carbonized cotton thread(Re-CCT)and a straight-shaped carbonized cotton thread(St-CCT)encapsulated by silicone resin.The unique structure design of the strain sensor enables the resistance of Re-CCT is highly stable under stretching,whereas the resistance of St-CCT is highly sensitive to lateral strain with a gauge factor(GF)of 24.17.At the same time,the Re-CCT and St-CCT in the strain sensor exhibits almost identical response to compression with a GF of 0.35 at the strain of 0-20%,and 2.06 with strain range in 28-40%.The combination of Re-CCT and St-CCT as strain sensing materials endows the sensor fabricated not only effective to monitor the strain magnitude,but also capable to differentiate normal and lateral strain,which is demonstrated by using it to detect the grip and lift gesture of human hand.Those flexible piezoresistive sensing materials based on low-cost materials and simple fabricating techniques response quickly and have high sensitivity in terms of strain and stress.Moreover,with novel structure of CCT,the composite is able to differentiate normal and lateral strain.Thus,the composite could used in some complex environment.In conclusion,the flexible piezoresistive strain sensors based on low-cost carbon materials and simple methods have high potential in the future wearable electronics.