Flexible Sensors For Bio-signals Detection

Advances in non-invasive,convenient and real-time physiological signal detection technology have played a key role in the development of health assessment systems and human-machine interfaces.At present,commonly used physiological signal sensors are primarily fabricated using silicon-based materials.However,their efficiency to extract physiological signals on the human body is restricted by their stiff,brittle,and nonstretchable characteristics.Therefore,utilizing flexible electronics to further develop physiological signal sensors can break through the limitations of traditional devices and extract physiological signals with improved efficiency and accuracy.Currently,flexible electronic skin can be mainly divided into three categories: multifunctional flexible sensor systems,self-powered flexible sensors,and self-healing material-based flexible sensors.The development of self-healing material-based flexible sensors can avoid the structural damages of the multi-functional and selfpowered flexible sensors,which is generally inevitable especially after long-term application or repeated measurement.Hydrogel is one of the promising candidates for conductive self-healing materials due to its softness,high stretchability,and tunable mechanical and electrical properties.However,currently used conductive hydrogels based sensors can only detect large-scale body movements,which are unable to detect weak physiological signals(breathing,pulse,etc.)with high precision.To solve this,we designed a novel injectable conductive self-healing carbon nanotube-Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)-polyvinyl alcohol-polyacrylamide(CNT-PEDOT-PAM-PVA)hydrogel.This material owes ultralow Young's modulus(~ 1 kPa),ultra-high stretchability(> 2000%),favorable injectability,and high mechanical and electrical self-healing efficiency.Besides,the impedance of the material responds rapidly to deformation.Owning to these properties,a miniature flexible strain sensor via material injection technique can not only achieve an accurate quantitative analysis of tiny limb movements but also successfully distinguish breathe frequency and intensity of a mouse under different activities.A pressure micro-sensor fabricated through bottom-up printing–injection technique can successfully monitor precise sphygmus signals from the human radial and carotid arteries,and has shown great potential in the diagnosis of cardiovascular diseases.Taken together,the feasibility and advantages of ICSH CNT-PEDOT-PAM-PVA hydrogels in the detection of physiological signals in vivo are demonstrated,suggesting that those miniature sensors can contribute to healthcare monitoring and the understanding of the biological basis of a particular behavior or neuropsychiatric disorder.Flexible non-contact proximity sensors can sense the approaching of objects,providing versatile sensing capabilities and richer operating experiences compared with traditional contact sensors,thus having a profound impact on human-machine interfaces such as security protection systems and intelligent electronics.However,the current extensive adoption of low-dimensional materials,together with expensive and timeconsuming manufacturing processes,may limit the development of flexible proximity sensors.Paper-based electronics have the merits of low cost and large-area manufacturing.Therefore,the development of paper-based flexible proximity sensors has extraordinary significance for the future development of the human-machine interfaces.To this end,we have developed a flexible paper-based PEDOT/PSS-PVA proximity/touch sensor(3P sensor).We first deposited PEDOT/PSS-PVA on the surface of the hierarchical fiber network inside the filter paper,which is then wrapped by a layer of conducting polymer-hydrogel film,and further fabricated the 3P sensor.We found that the component PVA can increase the sensitivity and the stability of the PEDOT/PSS fiber network.When a finger touches,its impedance level at high frequency is about 3 times larger than the PVA-free sensors.The electrochemical impedance spectrum(EIS)of the 3P sensor showed that during finger-touch,the capacitance of the sensor is 2.26 times higher than that tested without finger-touching.This may be caused by the induced inductance during finger-touching,which results in the re-arrangement of charges inside the sensor.This subsequently increases the capacitance of the sensors and the high-frequency impedance.We then systematically investigated the response characteristics of the 3P sensors and found that the electrical response of the sensor exhibited an exponential relationship with the proximity distance.The maximum detection distance of the 3P sensor reaches 20 cm,and it showed a fast and stable response in the cycling test.Besides,the electrical response level of the sensor when the finger is touching is three times larger than when the finger is at 0.1 cm distance,indicating that the sensor can significantly differentiate the finger-touching and fingerapproaching.Besides,the 3P sensor only responds to human approaching and touching,demonstrating human specificity.Based on the above 3P sensor's excellent performance,we have developed a security protection system with multi-level response capabilities,which showed different levels of response to human approaching and touching.In the end,we have fabricated a 3P sensor array via multi-channel detection technology and further demonstrated a contactless keyboard to achieve the contactless performance of a simple melody.The low-cost 3P sensor's excellent ability to detect human proximity and to distinguish between approaching and touching make it have broad prospects in human-machine interfaces such as security protection and smart electronics.