| The acceleration of population aging processes has brought a great pressure to society,especially in the medical system.In recent years,scientists are committed to develop self-monitoring wearable medical equipments to solve the potential health problems of unmanned elderly.At present,the 24-hour ECG portable device is widely used in the medical system and is favored by many heart patients.It can enable patients to carry out ECG tests at home in a more comfortable way and reduce the hospitalization rate of the hospital,which preliminarily reflects the convenience brought by wearable medical devices for people’s livelihood.The development of wearable medical devices is in the preliminary stage.In addition to heart diseases,lung and brain diseases also account for a large proportion of medical treatments.Electrical impedance tomography（EIT）can be used in vitro non-injury monitoring because it uses the impedance difference of the internal body to reconstruct the image without radiation to the body.With the development of the computer field in recent years,this technology has been continuously optimized and applied in clinical practice.Its application in medical wearable devices will be soon.To ensure a long-term stable monitoring of wearable medical devices,it is necessary to consider both the comfort of medical electrodes and the stability/accuracy of bioelectrical signal.Currently,disposable Ag/Ag Cl wet electrodes are commonly used.The conductivity of its conductive gel will decrease in long-term monitoring due to the evaporation of water,and its long-term contact with the skin will destroy the metabolic balance of the skin surface.Firstly,A resistance network model was proposed to accurately predict electrodes’contact resistance,and the influencing factors of contact resistance were analyzed in this paper.Secondly,a new convex electrode was obtained by optimizing the material and the woven structure.In order to further understand the electrode-skin interface system,its dynamic equilibrium process was analyzed.Based on the dynamic system,one kind of medical double-layer woven electrode,which could not only monitor for a long time but also keep the dynamic balance of the electrode-skin interface system,was developed.Finally,the electrode was applied to a simple electrical impedance tomography system to verify its practical value.The specific research work is summarized as follows:（1）The resistance of woven electrodes is affected by many factors,including the warp and weft density,the material of conductive yarn,and the number of conductive yarns in the warp and weft.Different applications have different resistance requirements of fabric materials.A model that can accurately predict the resistance of conductive fabrics is very useful and convenient for the design and application of conductive fabrics.In this paper,a racetrack model was proposed to simulate the high warp/weft density fabric structure and an elliptical model was proposed to simulate the low warp/weft density fabric structure.At the same time,Wheatstone resistance network was used to simplify the fabric resistance network model.In order to verify the accuracy of the resistance network model,138 conductive fabric samples（combinations of 6 types of warp yarns and 23 types of weft yarns）were weaved to verify the accuracy of the warp resistance network model based on the racetrack model and the accuracy of the weft resistance network model based on the elliptical model.By comparing the theoretical resistance and the experimental resistance of the conductive fabric,we found that the resistance network model could accurately predict the actual resistance of the conductive fabric.Compared with an existing conductive fabric resistance model,the model established in this paper has higher accuracy for the resistance prediction.（2）The passive change of contact resistance in textile electrode will reduce the acquisition accuracy of bioelectrical signal.The key to solve this problem is to identify the key influencing factors and understand how these factors affects the contact resistance.First,we built a contact resistance model based on some conductive yarns interwoven and explored the effect of the contact pressure and the contact angle on the contact resistance.We found that the contact resistance of conductive yarn decreased with an increase in the contact pressure,but the rate of change decreased as the contact pressure reached a certain point,i.e.,further increase in contract pressure did not cause much decrease of the contact resistance.In addition,the contact resistance of copper wire and silver-plated filament decreased with an increase in contact angle.The study of the above laws can provide a reference for the future application of electronic textiles.（3）The optimized design of electrode structure and the improvement of electrode material were carried out respectively.The woven structure was chosen as the basic structure of the textile electrode.Two kinds of single-layer medical electrodes were developed with either the woven plain structure or the plain jacquard（convex）structure,and both were fabricated with increased tightness.The woven structure was resisted to deformation,which could reduce the signal noise caused by deformation-induced resistance change during dynamic measurement.The high tightness could reduce the contact resistance value caused by the interweaving point of warp and weft yarns and its changes in dynamic measurements.The convex electrode design was to increase the contact area of the electrode-skin interface and reduce the interface impedance.In addition,the convex structure could increase the friction between the electrode and the skin and reduce the motion artifacts.Silver-plated filament was selected as the electrode material,and the electrode was chlorinated to obtain silver/silver chloride woven electrode.Silver is of good conductivity,cheaper than gold and other precious metals,softer than stainless steel filament.Additionally,Silver has higher durability and stability than coating electrode with conductive materials,including carbon（carbon nanotubes,graphene,etc.）and conductive polymer（polyaniline,polypyrrole and polythiophene）.Therefore,it is the primary consideration of medical electrode material.Medical electrodes need good potential stability in biological electrical signals acquisition.Based on this,we introduced a chloride deposition method for woven silver-plated electrodes,and we designed 32 kinds of deposition scheme to study the effects of influencing factors of electrochemical deposition.Specifically,we investigated how the uniformity of electrode deposition was affects by deposition method（constant potential method,constant current method,multi-step potential method and multi-step current method）,voltage/current size,deposition time,and electrolyte concentration.The results show that the optimal deposition scheme was"small step"as multi-step potential method in 1-1.5-2 V mode,the deposition rate as 20-40 s/cm~2,and the electrolyte concentration as 0.1 M.Firstly,the frequency response and impedance curve with time were recorded using the developed woven plain electrode and convex electrode and compared with that recorded using the disposable Ag/Ag Cl wet electrode.The results shown that the biological impedance logarithm has a negative linear relationship with frequency.The slope and intercept of convex electrode had the lowest standard error,followed by plain electrode and disposable wet electrode.In addition,the impedance stability range of the two woven electrodes was larger than that of the disposable wet electrode.The bioimpedance of the wet electrode was stable for the first 300 seconds,whereas the impedance of the woven electrode decreased with time.On the contrary,the impedance of the woven electrode stabilized after3 hours,while the impedance of the wet electrode increased after 3 hours.At the same time,we analyzed the impedance-time changes of human body in the process of small swing of arm,large swing and wrist rotation within 500s.The results shown that,compared with the disposable wet electrode,both plain Ag/Ag Cl electrode and convex Ag/Ag Cl electrode had relatively small motion interference,while plain Ag/Ag Cl electrode only showed relatively stable impedance change in small amplitude motion.Additionally,the convex Ag/Ag Cl electrode had better resistance to motion interference,which could maintain good monitoring effect in large motion.In addition,the impedance-time curves of woven electrode before and after chlorination were analyzed.It was concluded that plain-woven electrode with chlorination deposition had more stable and smaller impedance output.The adaptation process was shorter,and stable human impedance monitoring could be carried out after 3 hours.Compared with the woven electrodes,the wet electrode was not suitable for long-term monitoring of human impedance.Compared with the plain electrode,the convex electrode had more stable impedance acquisition and smaller impedance value.Therefore,the design of chlorinated deposition and convex structure effectively improved the impedance acquisition stability of the electrode system under dynamic monitoring.（4）In order to keep the microenvironment stable between the electrode and skin during long-term monitoring,an electrode-skin interface equilibrium system model was established,and the effects of skin metabolites（vapor sweat and liquid sweat）on the microenvironment were analyzed through this model.On this basis,six kinds of double-layer wearable medical woven electrodes were developed independently.Sweat from skin surface was exported to the outer layer of the electrode through superconducting junction warp/weft yarn.Then,the sweat was absorbed and evaporated by the cotton yarn with excellent moisture absorption.The air permeability,moisture permeability,moisture absorption and quick drying,contact angle,and impedance of the double-layer electrode were tested respectively to explore the comfort and human body signal acquisition performance.（5）To study the application value of the developed electrode in the field of electrical impedance tomography,the equivalent circuit model of bioimpedance system was established.Secondly,we designed the samples for EIT system and experimental schemes of human forearm test and simulation test.Through the analysis of the imaging results,the generated impedance image could clearly show the bone distribution of the forearm. |
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