Abstract:

Objective Conventional ionogel sensors are critically limited by dense film formats, suffering from poor moisture/air permeability and wearing comfort. This study combines a self-healing polyurethane (SHPU) matrix, based on dynamic imine-urea bonds, with the ionic liquid EMIM:DCA to create a material engineered in both film and fiber forms. The primary objective is to establish a scalable material fabrication route (solution casting, wet spinning), optimizing conductivity, robust mechanical properties, efficient intrinsic self-healing, and reliable sensing capabilities essential for practical, long-term wearable health monitoring.

Method SHPU was synthesized via catalytic reaction using poly(tetramethylene ether) glycol (PTMEG), isophorone diisocyanate (IPDI), and dynamic chain extenders (2-amino-4-methyl-6-hydroxypyrimidine (UPy), dimethylglyoxime (DMG) and glycerol). SHPU dissolved in tetrahydrofuran (THF)/ethanol was blended with 10%-40% ionic liquid EMIM:DCA. Films were prepared by solution-casting, and fibers were wet-spun into a water coagulation bath, using solvent ratio and drawing speed to control the fiber morphology. The chemical structure was confirmed by Fourier transform intrared spectroscopy (FT-IR), and surface wettability, thermal stability, mechanical/self-healing properties, and morphology were characterized via contact angle, thermogravimtric analysis (TGA), tensile tests, and scanning electron microscopy (SEM), respectively. Fiber sensing performance was assessed by recording resistance changes during cyclic stretching using a coupled universal tester and source meter.

Results The experimental results revealed that the prepared films possess excellent thermal stability, with an initial thermal decomposition temperature exceeding 150 ℃. As EMIM:DCA content increased, the films became noticeably more hydrophilic, with water contact angle decreasing from 109.6° for pure SHPU to 47.91° for films containing 40% EMIM:DCA.

In terms of mechanical properties, the SHPU film containing 10% EMIM:DCA exhibited a tensile strength of 10.2 MPa and an elongation at break of 685%. Higher ionic liquid content caused reduction in strength but improved material compliance. All formulations showed outstanding self-healing ability. After being cut and healed at 80 ℃ for 12 h, stress healing efficiency exceeded 88% and strain healing efficiency exceeded 89% across the series. Notably, the 40% EMIM:DCA film achieved full (100%) stress self-healing. Optical microscopy confirmed that the cut interfaces closed effectively after healing.

SEM images confirmed that wet spinning produced continuous, uniform EMIM:DCA/SHPU fibers with smooth surfaces without obvious defects. The sensing performance of the fibers depended strongly on the EMIM:DCA content. Fibers with 30% EMIM:DCA offered an optimal balance, acting as effective strain sensors with a gauge factor of 3.58 within the 50%-120% strain range. These sensors responded rapidly, exhibiting both response and recovery times within 703 ms during hand-motion detection. In practical tests, the fiber sensors reliably monitored and distinguished complex human movements. Real-time resistance signals clearly captured variations corresponding to flexion and extension of the wrist, elbow, index finger, and middle finger. For example, elbow bending at different angles (0°, 30°, 60°, 90°) produced a distinct stepwise increase in relative resistance. Moreover, the sensors maintained stable signal output over 500 stretch-release cycles at 50% strain, demonstrating good durability for dynamic motion tracking.

Conclusion EMIM:DCA/SHPU iongel films and fibers with different EMIM:DCA contents were successfully prepared by solution casting and wet spinning techniques. By regulating the content of EMIM:DCA, the sensing performance of the composite materials was enhanced. EMIM:DCA exhibited excellent compatibility with the SHPU matrix and could be uniformly dispersed within the SHPU matrix. The EMIM:DCA/SHPU films showed good mechanical self-healing properties and outstanding thermal stability. After cutting, the EMIM:DCA/SHPU conductive composite fibers could be self-healed. The EMIM:DCA/SHPU conductive fibers featured excellent sensitivity, with both response time and self-healing time for hand movements reaching the millisecond level. The sensors could accurately capture the movement states of the wrist, elbow, fingers and other parts, and output stable resistance response signals, demonstrating broad application potential in the field of flexible sensing.

Key words: functional polymer material, self-healing polyurethane, wet spinning, ionogel fiber, flexible sensor