| The development of technology and the emergence of flexible electronic devices have driven the demand for strain sensors.The advent of flexible strain sensors has addressed the limitations of traditional rigid materials,enabling strain sensors to adapt to different shapes and surfaces,providing better wearability and comfort,especially for sensing tasks in curved or dynamic environments.Multifunctional flexible strain sensors,characterized by their low cost,portability,and ease of fabrication,have gradually gained popularity among the general public.Hydrogel,as one of the flexible materials,possesses advantages such as simple preparation and good flexibility,meeting the material requirements of flexible strain sensors.Through printing technology,stretchable hydrogel devices with complex patterns can be fabricated to fulfill personalized needs.To address the limitations of hydrogels,including high water loss,inadequate frost resistance,and low sensitivity,this study prepared multifunctional P/M/L hydrogels by introducing superabsorbent salt Li TFSI and MXene nanosheets with high conductivity and high photothermal conversion efficiency,thereby improving their long-term stability,sensing,and photothermal performance.Additionally,printed hydrogels can construct complex multilevel internal structures at the millimeter or even micrometer scale,enhancing the performance and functionality of the devices.This study primarily focused on the application of P/M/L hydrogels in flexible strain sensors and the field of photothermal therapy,exploring the influence of structural design on the performance of strain sensors.The specific contents are as follows:In response to the issues of low mechanical strength,poor environmental stability,and limited functionality of traditional polyvinyl alcohol(PVA)hydrogels,we have developed multifunctional P/M/L hydrogels.The abundant functional groups on MXene nanosheets interact with PVA molecules through hydrogen bonding,enhancing the mechanical strength of the hydrogels.The maximum fracture strain of P/M/L hydrogels can reach 957%,surpassing the maximum strain experienced during human movement(approximately 55%),thereby meeting the mechanical requirements of wearable electronic devices.By adding the highly water-absorbent salt Li TFSI as an anti-freezing and moisturizing agent,the environmental stability of the hydrogels has been improved.Compared to the traditional glycerol displacement method,the intrinsic charge properties of lithium ion salts result in minimal conductivity loss in hydrogels.The strong hydrogen bonding between the cations,anions,and water molecules in the hydrogels disrupts the hydrogen bonds between water molecules,endowing the hydrogels with excellent dehydration resistance and freeze tolerance.Compared to existing hydrogels,P/M/L hydrogels exhibit superior environmental stability and retain their flexibility even after 200 days of storage at room temperature.Furthermore,these hydrogels demonstrate excellent freeze resistance,mechanical flexibility,and extensibility under extremely low temperature conditions(-80 ~oC,24 h).Additionally,by introducing MXene nanosheets with excellent conductivity and remarkable photothermal properties as fillers,the hydrogels fulfill multifunctional applications in sensing and photothermal therapy.Thanks to the outstanding conductivity of MXene nanosheets,P/M/L hydrogels,as conductive hydrogel-based flexible strain sensors,exhibit high sensitivity(GF≈3.74),a wide detection range(1%-100%strain),and good sensing stability(stable signal output over 50 cycles).Based on these properties,we fabricated P/M/L hydrogel sensors for human motion monitoring.Moreover,owing to the excellent photothermal effect of MXene nanosheets,P/M/L hydrogels demonstrate rapid photothermal responsiveness and remarkable photothermal stability under 808 nm near-infrared light irradiation.They also exhibit controllable surface temperature adjustment capability(23 ~oC-130 ~oC).Based on this,we applied the hydrogels to perform photothermal therapy by attaching them to the human body.Addressing the difficulties in preparing high-resolution complex micro/nano-structured hydrogels,the inability of traditional template methods to achieve complex shape construction,high template preparation costs,and the challenge of removing hydrogels from templates,we employed printing as a method for fabricating hydrogel sensor components,enabling the establishment of layered and multi-level internal structures in hydrogel sensors at the micrometer scale.First,we tested the rheological properties of the printing ink(shear thinning,thixotropy)to confirm its printability.We also explored the optimal printing parameters for hydrogel inks and successfully printed sensors with micrometer-scale features.The printed sensors exhibit real-time responsiveness,stable sensing performance(maximum tensile stress maintained at 96.7%of the initial value over15 cycles),and a wide sensing range(capable of real-time monitoring strain from 1%to 50%).Addressing the limited methods of improving hydrogel sensing performance through macro-level structural design,we designed and printed sensors with different fine structures(gridless film,rectangular grid,diagonal grid,honeycomb grid,circular grid)and performed sensing performance testing and simulation on the different structures.The experimental results demonstrate that the circular grid structure generates a larger longitudinal load under the same tension,resulting in enhanced local strain distribution and a faster change in the cross-sectional area of the conductive pathway,thereby exhibiting higher sensing sensitivity.This further confirms the feasibility of changing sensor performance theoretically through structural design.Finally,inspired by the traditional Chinese art character"Huluwa"(calabash gourd),we printed flexible electronic tattoo electrodes using the hydrogel and applied them to strain sensors for monitoring wrist bending in human subjects. |
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