| Sensor is the key component of information acquisition.With the rapid growth of market demand for flexible wearable electronic products,there is an urgent need to develop flexible sensors with various detection functions.In recent years,the paper material has shown broad application prospects in the field of flexible sensors because of its flexible,low-cost,hydrophilic,renewable,degradable,green and environmental protection properties;However,how to organically integrate paper material,electrode material and sensing functional material to develop flexible paper-based(PB)sensors with different detection functions still faces many challenges.In view of this research status,a series of multifunctional flexible PB sensors including humidity,pressure and bending strain detections were developed around the ordinary printing paper material,and their potential applications on human body were demonstrated.The main research contents and results are as follows:1.The resistance flexible PB humidity sensor was designed and fabricated using the hydrophilicity of paper and the excellent flexibility of polyester fiber conductive tape.The morphology characterization shows that the paper is composed of rough fiber network structure;The photoelectron spectroscopy shows that there are hydrophilic functional groups(–OH)in the paper;Further,the contact angle confirms that the paper has a good hydrophilicity,indicating that the paper can be used as the humidity sensing material.With the increase of the relative humidity(RH),the conductivity of the PB humidity sensor gradually increases,showing good linear response of more than three orders of magnitude in the range of 41.1%–91.5%RH.The response and recovery times of the PB humidity sensor are about 472 s and 19 s,respectively.Benefitting from the good flexibility and humidity sensing response of the PB humidity sensor,it has potential applications in respiratory rate,diaper monitoring,non-contact switch,skin humidity and so on.In addition,the PB humidity sensor can also be used to monitor the humidity distribution in horizontal space by array integration of unit device.2.The voltage type flexible PB humidity sensor was designed and fabricated by integrating the humidity sensor and primary battery.Specifically,the paper is used as the substrate and hydrophilic material,sodium chloride(NaCl)is used as electrolyte,Cu and Al foil tape are used as positive and negative electrodes of the voltage type flexible PB humidity sensor,respectively.The characterization results show that NaCl is uniformly distributed in the paper,and the NaCl paper has a good hydrophilicity.The humidity sensing test results show that the as-fabricated voltage type flexible PB humidity sensor can spontaneously generate voltage and distinguish different humidity(41.1%-91.5%RH).A single voltage type PB humidity sensor can output 0.58 V with a power of 1.33?W at 91.5%RH.The response and recovery times of the sensor are about 109 s and 113s,respectively.Through the series integration of eight voltage type PB humidity sensors,it can realize an output voltage of 4.21 V and charge the capacitor,proving the potential power supply characteristic of the voltage type flexible PB humidity sensor and providing a new strategy for the development of self-powered humidity sensor.Similarly,it is confirmed that the voltage type flexible PB humidity sensor can be used in the fields of respiratory rate,diaper monitoring and non-contact switch.3.The paper/paper single-layer microstructure(SLM)flexible piezoresistive sensor was designed and fabricated using the rough surface structure of the paper and the conductivity of the carbon ink.The characterization results show that the carbon ink contains well dispersed and conductive carbon nanoparticles;The surface of the carbon ink paper still maintains the inherent rough surface structure of the paper.The pressure sensing test results show that the SLM piezoresistive sensor has fast response time and wide pressure detection range;The sensitivities of the SLM piezoresistive sensor are0.614 k Pa-1 and 0.064 k Pa-1 in the pressure ranges of 0.1-40 k Pa and 6-40 k Pa,respectively.The pressure sensing mechanism of the SLM piezoresistive sensor is explained by combining the morphology characterizations and finite element simulation analysis.In addition,the potential applications of the SLM piezoresistive sensor in wrist pulse,finger joint movement,abdominal breathing rate and pressure distribution detection are demonstrated.4.The paper/electrodes double-layer microstructure(DLM)flexible piezoresistive sensor was designed and fabricated using the rough surface structures of the paper and polyester fiber conductive tape electrode,and the conductivity of the carbon ink.The morphology characterization shows that the polyester fiber conductive tape electrode has rough fabric structure.The pressure sensing test results show that the sensitivities of the DLM piezoresistive sensor are 5.54 k Pa-1 and 1.61 k Pa-1 in the pressure ranges of 0.5-5k Pa and 5-60 k Pa,respectively,which are much higher than that of the SLM piezoresistive sensor.Combined with morphology characterizations and finite element simulation analysis,the reason why the DLM interface is conducive to improve the sensitivity of the sensor is analyzed.Furthermore,the DLM flexible capacitive pressure sensor was designed and fabricated using the functional structure of the DLM piezoresistive sensor.The paper is directly used as dielectric layer,which reduces the carbon ink material and greatly simplifies the fabrication process of the flexible PB pressure sensor.The pressure sensing test results show that the DLM flexible capacitive pressure sensor has a wide pressure detection range of 0.1-80 k Pa and very low detection limit of 3 Pa.Similarly,the multifunctional applications of the DLM flexible piezoresistive and capacitive pressure sensors in pressure related fields are demonstrated.In particular,the DLM flexible capacitive pressure sensor can be used as a non-contact switch.5.The flexible PB bending strain sensor was designed and fabricated by using the bending flexibility of paper,the rough structure in the paper cross-section and the conductivity of the carbon ink.The characterization results show that the cross section of the carbon ink paper has a rough structure,which is conducive to the bending strain response.The bending strain test results show that the sensitivities of the flexible PB bending strain sensor are 1.59/°and 0.3/°in the bending angle ranges of 14.4-41.4°and41.4-68°,respectively.The response mechanism of the flexible PB bending strain sensor is analyzed by combining the characterization of cross-section morphology and the optical images of the sensor under different bending angles.Benefitting from the cross-section interface sensing mechanism,the surface of the PB bending strain sensor can be directly encapsulated using adhesive tape,so as to achieve good water resistance.In addition,the potential applications of the flexible PB bending strain sensor in human body and safety door are demonstrated. |
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