Design And Preparation Of Flexible Non-contact Capacitive Sensor

For many years,human beings have been fighting against health problems,and the emergence of diseases often causes people great anxiety and pain.Long-term monitoring of physiological signals such as body temperature,pulse,and respiration is of great help in the diagnosis,treatment and postoperative rehabilitation of the disease.At present,contact sensors are widely used in hospitals and daily life,which are closely attached to the skin surface to monitor physiological signals.There are limitations such as uncomfortable wearing,waste of resources,and interference with normal human activities.Therefore,the method of non-contact monitoring physiological signals came into being,which can detect physiological signals such as respiration,pulse and heart rate without interfering with normal human activities.At present,the commonly used non-contact monitoring methods include thermal infrared imaging,Doppler radar detection and capacitive coupling.Among them,the capacitive coupling method is an ideal method for non-contact health monitoring due to its simple structure,light weight,and low consumption.The capacitive coupling sensor includes two types: self-capacitance and mutual capacitance.Capacitive sensors based on the principle of mutual capacitance are ideal monitoring devices because they are not easy to be affected by environmental common-mode noise and effects of changes in air circulation.At present,non-contact capacitive sensors based on the mutual capacitance principle mainly realize the detection of the proximity of fingers and the shape of conductive objects.Due to their low sensitivity and weak detection ability,it is difficult to detect weak physiological signals such as breathing and pulse.In order to improve the sensitivity of the capacitive sensor in non-contact mode and successfully monitor the weak physiological signals of the human body,this thesis mainly carried out the following work:(1)Liquid metals(LMs)and PDMS are mixed at different mass ratios to obtain a paste with good conductivity and rheological properties.Then the LMs @ PDMS electrode is prepared by template printing method,and the electrode template with the required shape is directly etched by laser engraving machine.At the same time,a porous PDMS with a low dielectric constant is prepared by utilizing the feature that water will leave holes after the water and PDMS are mixed and solidified.(2)By analyzing the working principle of the capacitive sensor based on the mutual capacitance principle in non-contact mode,it is found that the determining factor affecting the non-contact detection rate of the capacitive sensor is the dielectric constant of the electrode structure and the dielectric layer;The simulation verification finally found that the capacitive sensor with the upper electrode in the shape of a circular ring,the lower electrode in the shape of a disk,and a low dielectric constant of the dielectric layer has the highest capacitance change rate in the non-contact mode.(3)According to the simulation results,the circular and circular LMS @ PDMS electrodes are prepared by the template printing method,and porous PDMS is prepared after water and PDMS are mixed and solidified.Then the capacitance sensors were prepared by using them as the upper electrode,the lower electrode and the dielectric layer respectively.The capacitive sensor has excellent non-contact detection performance in a non-contact state,and is insensitive to tensile and compressive strains,and has good anti-deformation and interference ability;Meanwhile,the sensor also has excellent anti-fatigue performance.Finally,after the capacitive sensor was implanted into clothing and furniture,non-contact detection of human breathing and pulse signals was successfully achieved,and it showed great application potential in the fields of human-computer interaction and software robots.