Construction And Application Of Flexible Piezoresistive Sensor Based On Electrospun Nanofiber Mats

With the improvement of living standards,people pay more attention to their health.Wearable device has been regarded as a very promising instrument for everyday monitoring of human breathing,pulse,heartbeat,blood pressure and body temperature due to its high sensitivity,great portability and easy-operation.Flexible piezoresistive sensor is one of the most attractive wearable devices for human health monitoring.The sensor could sensitively detect slight skin deformation,which is caused by heart rate,respiration and wrist pulse.Electrospinning technique has been employed to prepare conductive nanofiber mats as sensing layers for the fabrication of flexible piezoresistive sensors.However,a packaging step is normally needed to encapsulate the electrospun sensing layer.Moreover,the use of polymer or fabric packaging substrates may influence the flexibility,air/humid permeability and comfort of the devices.To solve the above problems,a sandwich-structured flexible piezoresistive sensor was prepared,in which electrospun nanofiber mats function as supporting,sensing and packaging layers.Three works were conducted in this project as the followings:1.Preparation of electrospun nanofiber mats with biocompatible polymers as raw materialsIn this study,polylactide(PLA),silk fibroin(silk fibroin,SF)and collagen(collagen,COL)were utilized as raw materials to prepare biocompatible nanofiber mats(PLA-SF-COL mats).Scanning electron microscopy(SEM)shows that the prepared electrospun mats were composed of oriented nanofibers with the average diameter of122 ± 28 nm.The biocompatible nanofiber mats exhibit good skin affinity and flexibility,which can fit with the skin surface directly in practical application and can be used as flexible materials for the development of pressure sensors.2.Sandwich-structured flexible piezoresistive sensor with polypyrrole-coated nanofiber mats as sensing layersTo avoid the usage of air-impermeable substrates,electrospun nanofiber mats were utilized as supporting,sensing and packaging layers to fabricate a sandwich-structured flexible piezoresistive sensor.Firstly,polypyrrole(PPY)were coated at the PLA-SF-COL mats to form conductive nanofiber mats via in-situ synthesis.Field emission scanning electron microscope(FESEM)shows that PPY was successfully modified on the flexible nanofiber mats.By changing PPY concentration,the surface morphologies of the PPY layers could be different.After optimization,PPY-modified mats with the highest conductivity could be obtained.Secondly,two layers of conductive nanofiber mats were stacked face-to-face on a PLA-SF-COL mat.The multilayered structure was fixed on the electrospinning drum for the collection of another layer of PLA-SF-COL mat.Finally,a sandwich-structured flexible piezoresistive device with electrospun nanofiber mats as substrate,sensing and packaging layers was assembled.The sensor could distinguish finger tapping and pressing accurately with good stability(200 times loading/unloading cycles)and sensitivity.It can also be used to monitor human vital signs such as wrist pulse,breathing and speaking.3.Flexible piezoresistive sensor constructed via electrospinning technique with polyaniline nanofiber mats as sensing layersTo further simplify the preparation process,electrospun conductive polymer composite mats were directly adopted to serve as sensing layer for the fabrication of sandwich-structured piezoresistive sensor.The electrospinning parameters were optimized to obtain polyaniline(PANI)-PLA nanofiber mats with high conductivity.FESEM shows that the average diameter of PANI-PLA nanofiber was 100±30 nm.Then,PLA-SF-COL mat,PANI-PLA conductive mat and another PLA-SF-COL mat were electrospun on the collection drum successively to obtain a multilayered structure,in which the PANI-PLA mat works as sensing layer.The multilayers could be cut to form flexible piezoresistive sensors with various sizes.The devices could sense human pulse and the pressure change caused by different number of coins accurately.Due to their high sensitivity and excellent stability(2000 loading/unloading cycles),the as-prepared sensors are able to monitor human physical signs in real time.In this project,electrospinning technique was applied to prepare a biocompatible nanofiber mat with good air/humid permeability and great skin affinity.The biocompatible mats were utilized to package sandwich-structured flexible piezoresistive sensors with PPY-modified mats and PANI-PLA electrospun mats as sensing layer,respectively.In comparison with previous works,the as-prepared piezoresistive sensor were constructed with electrospun nanofiber mats without the use of air-impermeable substrates.The preparation,assembly and package of each components could be accomplished with only electrospinning method.The as-prepared devices possess high sensitivity,great flexibility,good skin affinity and excellent stability and may have great potentials in the daily monitoring of human vital signs.