Macromolecule conformational shaping for extreme mechanical programming of polymorphic hydrogel fibers

The study addresses the challenge of achieving extreme and programmable mechanical properties in single-composition hydrogels and translating them into integrated, robust devices. Conventional covalently cross-linked hydrogel networks are often fragile and difficult to scale into high-performance devices. Although unconventional network architectures (e.g., double networks, slide-ring cross-linkers, crystalline or supramolecular cross-links, nanoparticle reinforcement) have improved toughness and resilience, they typically require complex chemistries, multi-step processing, molds with low throughput, and face interfacial issues when integrating with elastomeric encapsulants. The authors propose a materials strategy using macromolecule conformational shaping of a polyelectrolyte (sodium polyacrylate) via pH-dependent antisolvent phase separation, enabling continuous tuning from coiled to extended/aligned states. This approach aims to program mechanical responses—modulus, stretchability, resilience—within a single composition, facilitate scalable fiber fabrication (including layered/Janus architectures), and realize durable, all-soft electronic devices operable in ambient and low-temperature environments.

The paper reviews prior hydrogel mechanical enhancement strategies, including interpenetrating polymer networks with covalent/physical hybrid cross-linking, slide-ring cross-linkers, and networks reinforced by crystalline domains, hydrogen/ionic bonds, and nano-/microparticles. These have yielded remarkable properties (high toughness, strength, resilience, interfacial toughness) but often require intricate compositions, sequential or prolonged polymerization, and specialized molds that limit throughput and shape control. Few strategies enable printing or spinning of mechanically robust hydrogels. Existing devices typically require elastomer encapsulation to prevent dehydration, introducing interfacial bonding/adhesion issues. Thus, there is a gap for scalable manufacturing and integration methods that program mechanical heterogeneity within a monolithic hydrogel system.

Materials and dope preparation: Sodium polyacrylate (PANa) polyelectrolyte was used due to high molecular weight (reported ~3×10^7 Da in Results; Methods specify Mw ~3×10^6 Da), hygroscopicity, and abundant carboxylate groups. Typical dope: 0.73 g PANa in 20 ml deionized water, stirred/heated at 80 °C for 5 h to obtain a uniform hydrogel, then centrifuged at 400×g for 30–60 min to remove bubbles. pH adjustment: pH 3.00–9.14 via HCl (1.00–0.01 mol/l); pH 12.38–13.97 via NaOH (0.2–1.0 mol/l). Example: pH 9.14 from 0.01 mol/l HCl, pH 3.95 from 0.25 mol/l HCl. Composite dopes prepared by dispersing SWCNTs (10–40 wt% relative to PANa) with 30 wt% SDS in 0.5 mol/l NaOH, ultrasonicated (200 W, 20 kHz, 10 s on/off, 30 min, ice bath), then dissolving PANa to form SWCNT–PANa dopes. Process: pH-dependent antisolvent phase separation filamentation. Dope extrusion into methanol bath (antisolvent) at 0.15 ml/min through a 900 µm inner-diameter nozzle. Methanol extracts water due to higher water affinity, causing rapid dehydration (150–250 s), phase separation, macromolecule aggregation, and formation of densely entangled networks. Low pH dopes solidify fastest. Solidified microfibers are collected without tension on a roller, then exposed to ambient air (65% RH) where porous networks collapse/self-assemble and re-equilibrate water (12–40 wt% depending on conformation). Cross-sectional areas ~0.08–0.2 mm^2. Structural characterization: AFM to visualize nanostructures (aggregated globules to aligned chains with increasing pH); polarized optical microscopy to monitor birefringence and strain-induced orientation; FTIR to assess protonation states (diminishing 1700 cm^-1 C=O, increasing 1546/1395 cm^-1 COO^- bands with pH); SAXS/WAXS to determine nanostructural evolution and orientation factors under strain (orientation factor f increases from 0.42 at 0% to 0.91 at 800% strain). Rheology of dopes (viscoelastic, shear-thinning) and viscosity/moduli dependence on pH and shear rate. Mechanical testing: Tensile tests across pH 3.95–13.97 to obtain strength, modulus, elongation, toughness; cyclic loading for resilience and strain recovery; strain-rate and humidity studies; long-term stability (5 months ambient storage). Ribbons fabricated via flat nozzle (10 mm×0.7 mm) show similar behavior. Fabrication of polymorphic fibers: One-step, layered monolithic structures via synchronized filamentation of varied-conformation microfibers to form conformal anti-fracture interfaces. Janus fibers via parallel-axial dual-spinneret (two 3 mm×0.5 mm nozzles, each 0.15 ml/min), combining a fast-resilient pH 12.38 elastic phase with an ultrastretchable plastic pH 13.34 phase doped with SWCNTs (20 wt%) to introduce conductivity. Helical Janus springs produced by prestraining Janus fibers (200–900%) at 40 mm/min, then releasing to induce helices due to resilience mismatch. Core–shell fibers via coaxial spinneret (inner diameters 330 µm and 1.8 mm). Multilayered fibers possible with multi-channel parallel-axial spinnerets; compatibility with direct ink writing demonstrated. Device demonstrations: Ionic strain sensors using pH 12.38 fibers; characterization of resistance change vs strain (0–1000%), frequency response (up to 8.6 Hz at 100% strain; up to 5.7 Hz at 600%), durability (>1000 cycles at 600% and 4.2 Hz), and wireless integration on a robotic bird (ESP32, Bluetooth). Helical conductors (Janus with SWCNT-doped phase) tested for stretchability (up to 6000%), resistance evolution during uncoiling/straightening, cyclic durability, antifreezing (DSC freezing points ~−28.6 °C and −33.3 °C), LED lighting at room and sub-zero temperatures, recovery after liquid nitrogen exposure. Thermoelectric (TE) Janus springs with 40 wt% SWCNTs in pH 13.34 phase, ~3 mm diameter coils, series-connected coils (e.g., 11 coils, >100 coils) characterized for Voc/Isc under varying strain (0–1000%) and temperature differences (5–40 K), long-term stability on hot tube and wearable tests.

• Macromolecule conformation control: PANa chains transition from coiled/aggregated to extended/aligned with increasing pH via antisolvent phase separation, forming densely entangled single-composition networks with tunable water content (~12–40 wt%). Orientation factor (WAXS) increases from 0.42 (0% strain) to 0.91 (800% strain).
• Mechanical tunability (pH 3.95–13.97):
  • Elongation at break: 105% ± 2% to 2630% ± 120% (pH 13.34 microfiber reached 2693% with reported highest toughness 20.3 MJ m^−3).
  • Tensile strength: 1210 ± 120 kPa to 47 ± 5 MPa.
  • Elastic modulus: 240 ± 30 kPa to 2050 ± 370 MPa.
  • Toughness: 1.7 ± 1.1 to 17.8 ± 1.6 MJ m^−3 (also single sample reported 20.3 MJ m^−3 at pH 13.34).
  • Resilience: Fast-resilient in moderate pH (6.35–12.38); for pH 9.14, strain recovery 97.7% and resilience 90.1% at 200% strain; low and high pH regimes show large hysteresis; pH ~5 exhibits anelastic recovery over 1–2 h.
  • Long-term stability: Mechanical performance stable after 5 months ambient storage.
• Processability: One-step monolithic layered fibers with conformal, anti-fracture interfaces. Janus fibers formed via dual-spinneret; well-bonded interfaces confirmed across flow rates.
• Helical Janus springs: Prestrain-programmable diameter (1.8–6.7 mm) and turns/cm; elongation limit increases from ~3800% to 6000% with diameter. Negligible residual strain at 4000–5000% cycling for 2.1–2.9 mm coils. Resistance vs strain shows distinct regimes (uncoiling 0–310% with faster ΔR/R0 increase; coil straightening 310–1000% slower increase). Durable at 1000% cyclic strain.
• Antifreezing/low-temperature tolerance: DSC freezing points ~−28.6 °C (pH 12.38) and ~−33.3 °C (pH 13.34 + 20 wt% SWCNTs). Conductors operate at −30 °C; after immersion in liquid nitrogen (−196 °C), mechanical stretchability recovers within ~5 s in ambient. LEDs remain lit up to 1200% strain at ambient and at −30 °C. Functionality retained after 5 months ambient storage.
• Strain sensors (pH 12.38 fibers): Gauge factors GF ≈ 2.29 (0–400%) and 4.41 (400–1000%); minimal creep at fixed strain; frequency response up to 8.6 Hz at 100% strain and up to 5.7 Hz at 600% strain; stable over >1000 cycles at 600% and 4.2 Hz. Wireless robotic bird demonstration shows dynamic signals up to ~16.5 Hz with ~30 ms response/recovery.
• Thermoelectric Janus springs: With 40 wt% SWCNTs (conductivity ~400.6 S m^−1), Voc and Seebeck coefficient remain stable from 0–1000% strain. An 11-coil spring at ΔT ≈ 40 K shows Voc tracking ΔT; sustained Voc ~5.7 mV and Isc ~2.6 µA over 5 h on a ~60 °C tube. At 700% strain, peak power increases from ~0.1 to 5.7 nW as ΔT increases (5–40 K). Wearable/coilable bands yield Voc ~11.5 mV, Isc ~0.5 µA (on body) and Voc ~65.7 mV, Isc ~2.7 µA (on hot tube).

The work demonstrates that programming the conformation of a single polyelectrolyte (PANa) via pH-controlled antisolvent phase separation enables continuous and extreme tuning of hydrogel fiber mechanics, addressing the need for robust, scalable, and integrable hydrogel materials. By shifting macromolecular architectures from coiled aggregates to extended/aligned states, the authors control modulus, strength, stretchability, and resilience within one composition, avoiding complex chemistries and interfacial issues common in heterogeneous systems. The ability to form monolithic layered/Janus architectures with conformal interfaces allows combining disparate mechanical and electrical properties in a single fiber platform. This material paradigm directly translates into durable, all-soft devices operable in ambient and sub-zero environments: ultrafast, large-strain sensors; ultrastretchable conductors with antifreezing performance; and highly deformable thermoelectric springs maintaining performance under strain. Collectively, the findings validate that macromolecule conformation shaping is an effective, scalable route to mechanically programmable, multifunctional hydrogel fibers suitable for realistic wearable/robotic applications.

The study introduces a macromolecule conformational shaping strategy for single-composition PANa hydrogels, enabling unprecedented mechanical programmability (modulus spanning ~4 orders of magnitude; elongation from ~100% to >2600%; tunable plasticity/anelasticity/elasticity). The antisolvent phase separation filamentation produces polymorphic fibers and layered/Janus constructs with robust interfaces, facilitating scalable fabrication of all-soft devices. Demonstrations include: (i) 1000% strain, ~30 ms response fiber sensors for robotic birds; (ii) helical conductors tolerating 3800–6000% strains and sub-zero temperatures; (iii) wearable/stretchable (up to 700%) TE Janus springs with stable outputs. Future directions include extending the approach to other polyelectrolytes and fillers to broaden functional modalities, optimizing processing to refine mechanical gradients, scaling multi-channel architectures for higher integration density, and enhancing power outputs for energy harvesting via coil design and materials optimization.

• Material specificity: The approach is demonstrated with PANa; generalization to other polymers requires validation.
• Environmental dependence: Although water content and mechanics were stable over months at 65% RH, hygroscopicity implies potential sensitivity to humidity and temperature extremes outside tested ranges.
• Processing considerations: Use of methanol as antisolvent and high/low pH dopes may constrain certain biocompatible or in situ manufacturing settings.
• Performance bounds: While mechanical and sensing/TE performance is impressive, TE power outputs are in the nW range, limiting immediate powering of higher-load devices without further scaling or optimization.
• Parameter variability: Reported toughness values vary across sections (e.g., up to 20.3 MJ m^−3 vs summary 17.8 ± 1.6 MJ m^−3), indicating sample-to-sample/process sensitivity requiring tight control for reproducibility.