Preparation And Performance Of Knitted Fabric Strain Sensors For Flexible Joint Rehabilitation Training

During the past few years,emerging flexible smart wearable electronic devices have been developing rapidly human-computer interaction,activity monitoring,medical rehabilitation,and telecommunication,providing a healthy and convenient lifestyle for humans.Flexible strain sensors provide a reliable treatment basis and early warning method for patients with chronic diseases by monitoring human breathing,pulse beat and joint movement in real-time.Fabric-based flexible sensors,prepared by physicochemical methods with fabric as a flexible base,are widely appreciated because of their softness,comfort,fit,warmth,and breathability.Sensitive materials are key to the preparation of fabric-based strain sensors,which can convert strains in sensors induced by human body activity into accessible electrical signals.Carbon materials have been used in the preparation of wearable electronic devices due to their excellent electrical conductivity,mechanical properties,thermal stability,and ease of chemical modification.However,there are some problems such as poor bonding fastness,uneven loading,and easy shedding of the carbon material to the fabric.Such problems hindered companies from selecting the right carbon material for large-scale and low-cost production and application.In addition,fabric sensors may fail when disturbed by complex environments in real life,such as water washing,sweat,grease and wet and cold environments.In addressing the above challenges,this topic used a close-fitting,highly elastic and breathable knitted fabric as a flexible substrate and compared the performance of different kinds of carbon materials loading knitted fabric sensors,aiming to select a carbon material-loaded knitted fabric sensor suitable for company scale production.Then,functionalized modification of fabric sensors to provide sensor durability to complex environments and improve the wear comfort of fabric-based sensors.Finally,a double-layer conductive network was constructed to further improve the sensor’s ability to discriminate between strain degrees.The prepared fabric-based sensors were applied to human activity monitoring to confirm their enormous potential for activity management,medical rehabilitation and human-machine interaction.The details and conclusions of the study are as follows:（1）The cotton knitted fabrics（CKF）with structures of plain,1×1 rib and 2×2 rib were selected as a flexible substrate and reduced graphene oxide（r GO）,carboxylic multi-wall carbon nanotubes（c-MWCNTs）and carbon black（CB）were chosen as the conductive materials.The chitosan-modified cotton fabrics were loaded with three carbon materials to prepare r GO/CKF,c-MWCNTs/CKF,and CB/CKF sensors respectively.The electro-mechanical characteristics of the three-carbon material decorated fabric sensors were investigated and compared.The resistance of the fabric sensor increased and then decreased at high strain（40%）,which is significantly different from the thin-film type strain sensors.Therefore,knitted fabric resistance models were developed and structural changes under strain in conductive fabrics were observed to explain the resistance phenomenon.Under small strains（5%）,the fabric length resistance was the main influencing factor resulting in an increase in the overall resistance of the fabric sensor,while under large strains the fabric contact resistance was the main affecting factor making the overall resistance of the sensor decrease.Among three carbon materials,the fabric sensor made by graphene as a conductive material has the best electro-mechanical property and washing durability.Therefore,the r GO/CKF sensor was applied to joint activity monitoring and showed a high stretchability and stability in finger,wrist,arm,ankle and other joint activities,which could be used for intelligent clothing in medical rehabilitation training and sports management.（2）Still using graphene as the conductive material,the same modification and combination method was applied to polyamide/spandex knitted fabrics to prepare graphite conductive fabrics with excellent conductive properties.Then,graphene-loaded multifunctional fabric sensors were prepared by functionalizing the fabric sensors with adsorbed nanoscale silica and polydimethylsiloxane（PDMS）through a simple and efficient dip-coating process.Graphene-loaded superhydrophobic fabric sensors were prepared using a Si O₂ concentration of 12 mg/m L and a PDMS treatment time of 10 min.A broad working range of 60%,a fast response time（22ms）and stable cycling durability over 4000 cycles were simultaneously achieved using the prepared sensor.Furthermore,the sensor showed excellent superhydrophobicity（156°）,self-cleaning,air permeability,photothermal effects and UV protection,due to the synergistic effect of graphene,silica and PDMS.The superhydrophobic properties were retained even after repeated washing and chemical attack over a long period.This multifunctional sensor could be mounted on human joints to perform tasks,including activity monitoring,medical rehabilitation evaluation and gesture recognition,due to its superior electromechanical capabilities.Based on its multiple superior properties,this sensor could be used as winter sportswear for athletes to track their actions without being impacted by water and as a warmer to ensure the wearer’s comfort.（3）Graphene conductive fabric was used as the first conductive network layer and silver conductive ink was sprayed onto the surface of the graphene fabric by a spraying process to build a silver conductive network layer.A multifunctional fabric sensor with a two-layer conductive network structure was then prepared by using PDMS treatment to prevent the shedding of the conductive material.The resistance of the sensor dropped from 4.8 x 104Ωto 5.2Ωafter spraying with silver conductive ink.The sensor simultaneously achieved ultra-high sensitivity（gauge factor of 3.6×10⁵）,fast response（80 ms）and stable cycling durability（>5000 times）related to differences in conductivity and deformation of the graphene fabric and the Ag layer.The breakage of the silver conductive layer under strain achieved a significant change in resistance and the graphene fabric safeguarded the completeness of the conductive pathway.Moreover,with the help of PDMS,the fabric achieves hydrophobic properties,effectively preventing interference from external contamination and allowing underwater activity monitoring.The thermal conductivity network of graphene and silver provided photothermal and electrothermal functions for thermal therapy and body temperature regulation.Based on these sensing properties and multifunctionality,the sensor detected not only subtle muscle movements（such as facial expressions,throat movements and abdominal breathing）but also large joint movements at different angles,showing enormous potential in the evaluation of joint rehabilitation treatments.