Continuous Spinning of High‐Tough Hydrogel Fibers for Flexible Electronics by Using Regional Heterogeneous Polymerization

Abstract Hydrogel fibers have attracted substantial interest for application in flexible electronics due to their ionic conductivity, high specific surface area, and ease of constructing multidimensional structures. However, universal continuous spinning methods for hydrogel fibers are yet lacking. Based on the hydrophobic mold induced regional heterogeneous polymerization, a universal self‐lubricating spinning (SLS) strategy for the continuous fabrication of hydrogel fibers from monomers is developed. The universality of the SLS strategy is demonstrated by the successful spinning of 10 vinyl monomer‐based hydrogel fibers. Benefiting from the universality of the SLS strategy, the SLS strategy can be combined with pre‐gel design and post‐treatment toughening to prepare highly entangled polyacrylamide (PAM) and ionic crosslinked poly(acrylamide‐co‐acrylic acid)/Fe3+ (W‐PAMAA/Fe3+) hydrogel fibers, respectively. In particular, the W‐PAMAA/Fe3+ hydrogel fiber exhibited excellent mechanical properties (tensile stress > 4 MPa, tensile strain > 400%) even after 120 days of swelling in the pH of 3–9. Furthermore, owing to the excellent multi‐faceted performance and one‐dimensionality of W‐PAMAA/Fe3+ hydrogel fibers, flexible sensors with different dimensions and functions can be constructed bottom‐up, including the one‐dimensional (1D) strain sensor, two‐dimensional (2D) direction sensor, three‐dimensional (3D) pressure sensor, and underwater communication sensor to present the great potential of hydrogel fibers in flexible electronics.


Experiment and Method
The universality of the SLS strategy: The spinning solution of PAM, PAA, HEA, HEAA, HEMA, and DMAA hydrogel fibers consisted of 3 mol/L monomers, I1173 (1 mol% of monomer), MBA (0.01 mol% of monomer), and 15 mL H2O.The spinning solution of APMS and SBMA hydrogel fibers consisted of 1 mol/L monomer, I1173 (1 mol% of monomer), MBA (2 mol% of monomer), and 15 mL H2O.The spinning solution of META hydrogel fiber consisted of 3 mol/L META, I1173 (1 mol% of monomer), MBA (2 mol% of monomer), and 15 mL H2O.The spinning solution of hydrogel fibers copolymerized with AM and other monomers consisted of 3 mol/L total monomers (85% AM and 15% other monomers), I1173 (1 mol% of total monomer), MBA (1 mol% of total monomer), and 15 mL H2O.The feed speeds of all hydrogel fibers mentioned above were presented in Fig. 3f, and other parameters were consistent with the fabrication of the highly entangled hydrogel.
FTIR tests: Fourier transform infrared (FTIR) tests of the samples were acquired using a Nicolet IS50-Nicolet Continuum produced by Thermo Fisher Scientific, Ltd., where transmission mode was employed for sample detection.
XPS tests: X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos Axis Ultra-DLD X-ray photoelectron spectrometer.

Mechanical properties tests:
The mechanical properties of hydrogel fibers were tested by using a universal tester (AGS-X).The stress (σ) was calculated as σ = F/A0, where F was the load force, and A0 was the original cross-sectional area.The strain (ε) was estimated as ε = ∆l/l0, where ∆l was the length of the stretch and l0 was the original length.The elastic modulus (E) was obtained by calculating the slope of the stressstrain curve in the region between 0-5% strain.The bulk toughness (W) was obtained , where εb and σload were the corresponding breaking strains and stresses in the loading process, respectively.Energy dissipation (Uhys) was obtained as , where εx was the preset strain, and σload and σunload were the corresponding stresses in the loading and unloading processes, respectively.

Conductivity test:
The electrical resistance of hydrogel fiber was measured by using a SourceMeter (Keithley DMM7510).The conductivity (σ) was calculated as σ = Δl/(R × S), where Δl, R, and S were the length, electrical resistance, and cross-sectional area of hydrogel fiber, respectively.

Sensing tests of hydrogel fiber-based sensors:
The sensing tests of the hydrogel fiber-based sensor were carried out by the combination of a universal tensile testing machine (AGS-X) and a CHI660D electrochemical workstation at a constant voltage of 1 V.The gauge factor (GF) was estimated as GF = ∆R/R0/ε, where ∆R was the resistance changes with strain, R0 was the resistance of the hydrogel fiber at the original length, and ε was the applied strain.The sensitivity (S) of the pressure-sensing unit was estimated as S = ∆R/R0/σ, where ∆R/R0 was the relative resistance changes, σ was the applied stress.
Fig. S1 The drying weighing method to measure the water content of the lubricating solution.Where  819  and  819  represented the absorption at 819 cm -1 of PAM hydrogel fiber and monomer, respectively.
Fig. S6 The effect of the feed speed of the spinning solution on the mechanical properties of PAM hydrogel fibers.

Fig. S3
Fig. S3 The hydrophilicity of the (a) polytetrafluoroethylene (PTFE) and (b) silica gel mold.The regional heterogeneity of AM solutions polymerized in the (c) PTFE and (d) silica gel mold.

Fig. S7
Fig. S7 Effect of varying the (a) monomer concentration, (b) crosslinker concentration, (c) initiator concentration, and (d) UV distance on the starting and ending points of Region II.

Fig
Fig. S14 (a) Schematic diagram of the 2D direction sensor based on the W-PAMAA/Fe 3+ hydrogel fiber.The red line was the positive pole and the black line was the negative pole.The ∆R/R0 of two fibers when the 2D direction sensor was stretched at (b) 0°, (c) 30°, and (d) 45°.(e) The ∆R/R0 during the interaction of tensile angle and strain.(f) The directional selectivity factor of the 2D direction sensor.

Fig
Fig. S15 (a) Schematic diagram of the pressure-sensing unit based on the W-PAMAA/Fe 3+ hydrogel fiber.The red line was the positive pole and the black line was the negative pole.(b) the ∆R/R0 of the pressure-sensing unit during pressuring.(d) The response and recovery delay of the pressuresensing unit.(e) the ∆R/R0 of the pressure-sensing unit at 10-90 KPa pressure.(f) The long-cycle stability of the pressure-sensing unit.