Wearable Sensing Interface And Method For Leaf Capacitance Monitoring

It is of great significance to monitor plant health status in-time.While the traditional approaches for plant health status monitoring,such as direct observation of plant appearance and detection of the characteristic products cannot meet the need for the early,real-time and in-situ detection,many scholars try to monitor the changes in leaf capacitance signal to reflect the changes in plant health status.The traditional types of leaf capacitance sensors consist of rigid functional materials,which cannot ensure the conformal contact between the sensor and the irragular surface of plants.Additionally,to realize persistent monitoring of leaf capacitance,the devices must be fixed on the leaf surface with the help of other tools.As the plant tissues are fragile,the rigid devices or the tools may cause damage to them,which will make the detection results unreliable and inaccurate.To solve the problems,we propose a wearable sensing approach for leaf capacitance monitoring.It is carried out via the fabrication of the wearable sensing interface,test of the sensing performance,real-time monitoring of leaf capacitance signal,and the evaluation of the electrodes’influence on plant health status.In this study,Epipremnum aureum L.are taken as the model,the feasibility of the sensor to realize in-situ monitoring of leaf capacitance is verified,and its influence on plant health status is evaluated.On that basis,the sensing system that can realize real-time sensing and wireless transmission of capacitance signal is developed and its feasibility to persistently monitor leaf capacitance is explored,which lays the foundation for the in-situ and non-invasive monitoring of plant physiological information.The main results are as follows.（1）To explore the wearable sensing approaches of leaf capacitance,a layer of Au is magnetron sputtered on the PET substrate to form the wearable electrode（Au@PET electrode）,which can ensure the conformal contact between the electrode and the leaf.The relative resistance change of the electrode under cyclic bending test is less than 2%,which demonstrates its good flexiblility and stable conductivity.The capacitance sensor formed by Au@PET electrodes and dielectric layer（silicon rubber etc.in this study）shows good capacitance sensing stability,with the relative capacitance change less than 5%under various environment.As leaf is a poor conductor of electricity,it can be treated as the dielectric layer and sandwiched between two Au@PET electrodes to form parallel plate capacitance sensor.Leaf capacitance measured by this sensor shows same results compared with the traditional rigid sensor.The evaluation of plant health status during leaf capacitance monitoring indicates that the wearable electrodes do not affect the growth and development of the lateral buds and roots,as well as the water status of the plant.However,the net photosynthesis rate of the leaf decreases with the presence of Au@PET electrodes due to their opacity,with the degree of 18.77%.Additionally,it is found that the nitrogen and chlorophyll content in the leaf under the electrode is lower than the other part,with the extent of 57.77%and 62.69%,respectively.（2）To mitigate the adverse effect of the wearable electrodes on plant health status during leaf capacitance monitoring,electrospinning is employed to fabricate PLA NFM,and the wearable and porous electrode（CNT/PLA electrode）is obtained after the vaccum filtering of CNT on PLA NFM.The relative resistance change of the electrode under cyclic bending test is less than 6%,which demonstrates its stable conductivity.The capacitance sensor formed by CNT/PLA electrodes and dielectric layer（silicon rubber etc.in this study）shows good capacitance sensing stability,with the relative capacitance change less than 5%under various environment.The electrode shows a water vapor transmission rate of 959.55±20.81 g·m^-2·d^-1.The electrodes can be conformally attached to leaf surface to form parallel plate capacitance sensor.Leaf capacitance obtained from this sensor is the same with the results obtained from traditional rigid sensor.The evaluation of plant health status during leaf capacitance monitoring indicates that the CNT/PLA electrodes do not affect the growth and development of the lateral buds and roots,as well as the water status of the plant.Similarly,the net photosynthesis rate of the leaf decreases with the presence of CNT/PLA electrodes,with the degree of 17.85%.It is found that the nitrogen and chlorophyll content in the leaf under the electrode is lower than the other part,with the extent of 23.79%and 20.49%,respectively.Due to the gas-permeability of the electrodes,the adverse effect of Au@PET electrode on plant health status is mitigated.（3）To lower the influence of the electrodes on plant health status during leaf capacitance monitoring furtherly,the flexible,self-adhesive electrode with sandwich structure（PLA@CNT@PLA electrode）is fabricated by the in-situ electrospinning of PLA NFM and spraying of CNT on PLA NFM.The electrode shows stable conductivity,with the relative resistance change less than 2%under cyclic bending test.The capacitance sensor formed by PLA@CNT@PLA electrodes and dielectric layer（silicon rubber etc.in this study）shows good capacitance sensing stability,with the relative capacitance change less than 3%under various environment.The water vapor transmission rate of CNT/PLA electrode is 963.06±31.91g·m^-2·d^-1.The electrode can be self-adhered to the leaves to form parallel plate capacitance sensor.Leaf capacitance obtained from this sensor is the same with the results obtained from traditional rigid sensor.The evaluation of plant health status during leaf capacitance monitoring indicates that the wearable electrodes do not affect the growth and development of the lateral buds and roots,as well as the water status of the plant,but they can decrease the net photosynthesis rate of the leaf,with the content of 17.81%.It is found that the nitrogen and chlorophyll content in the leaf under the electrode is lower than the other part,with the extent of 4.95%and 4.97%,respectively.Due to the absence of the adhesive,the influence in plant health status during leaf capacitance monitoring is mitigated furtherly.（4）To monitor the plant health status in time and realize persistent monitoring of leaf capacitance signal,a real-time sensing system for leaf capacitance is developed by the integration of the self-adhesive and sandwich-structured electrode,flexible circuit board,and power supply module.The system can be further used in the in-situ and persistent monitoring of leaf capacitance.The results show that leaf capacitance decreases since the first day of drought stress,and the decreasing trend continues during the drought stress process,during which time the capacitance difference between daytime and night becomes smaller as the physiological activities rate became lower.The evaluation of plant health status under drought stress indicate that leaf color becomes yellow and plants become wilted at the third day of drought stress.Compared with the traditional method of direct observation of plant appearance,this system can detect the change of leaf health status 1–2 days earlier and it shows potential in the process of the informationization and intellectualization in agricultural production.