Flexible And Highly Sensitive Inductive Pressure Sensors And Their Applications In Wearable Electronics

Flexible sensors are made by integrating functional components into flexible substrates.Comparing with the traditional rigid sensors,flexible sensors have advantages of malleability,bendability,miniaturization,and probability.Flexible sensors lead to the rapid development of wearable devices.Future wearable devices will focus on the optimization on safety,comfort,cost,functional diversification,accuracy and speed of data acquisition,which increases the demand for flexible sensing devices with high sensitivity,high environmental adaptability and low cost.Inductive sensor is based on electromagnetic induction and converts the measured signal into the change of coil self-inductance or mutual inductance to realize non-electrical measurement.Here,we report a novel flexible inductive pressure sensor array with a simple construction of three layers:a planar spiral inductor layer and ferrite film units attached on a polyethylene terephthalate(PET)membrane,which are separated by an elastic pillar layer.The device is suitable for large-area contact pressure measurements.Importantly,by introducing the ferrite film with an excellent magnetic permeability,the effective permeability of the inductor is greatly influenced by the separation distance between the inductor and the ferrite film.As a result,when an external load is applied to the sensor,the values of the inductance change largely as the separation distance varies.The ferrite film effectively increases the inductance of the inductor and greatly increases the sensitivity of the sensor.Our device has achieved a high sensitivity of 1.60 kPa-1 with a resolution of 13.61 Pa in the pressure range of 0-0.18 kPa,which is very competitive.In addition,our device shows an outstanding stability(within 0.076%and 1.07%variations,respectively)when exposed to environmental interferences(e.g.,human bioelectricity and electro magnetic interference,EMI),in comparison with its capacitive counterpart.In addition,the device exhibits a fast response time of 111 ms and a good stability of 0.23%variations under a constant load of 62.48 Pa for 32 hours.To demonstrate the potential of the sensor array in the field of flexible wearable electronics,we have successfully developed a 4 × 4 inductive pressure sensor array into a wearable keyboard for a smart electronie calendar application,and built a measurement system for the data acquisition of the sensor array.The device is combined with the software part applied to the intelligent electronic calendar.Afterwards,the sensor is further applied to human respiratory signal detection,demonstrating the potential of the sensor in the field of human physiological signal detection.