Mechanical properties of hydrogels are crucial to emerging devices and machines for wearables, robotics and energy harvesters. However, traditional strategies for preparing mechanical property desirable hydrogel are usually of low throughput, limited in shape control and difficult in upscaling, therefore limiting their applications in sophisticated soft devices. We propose macromolecule conformational shaping of a single-composition hydrogel via a pH-dependent antisolvent phase separation process, which enables extreme mechanical programming of hydrogel microfibers and various functional all-soft hydrogel fiber electronic devices.

Fig. 1 Conformational shaping of a sodium polyacrylate (PANa) polyelectrolyte macromolecule. a, (i) Schematic illustration for the macromolecule conformation change and (ii) the antisolvent phase separation process. b, Atomic force microscopy images of the single-composition hydrogel microfibers of different pH. c, Polarized optical microscopy images of hydrogel microfibers of different pH, and pH 9.14 microfiber at 800% strain.

Our method shows that the conformation of a poly(acrylic acid) based polyelectrolyte macromolecule can be synthetically engineered from compactly coiled to extended, aligned states based on a pH dependent antisolvent phase separation process (Fig. 1a). The pH determines the original conformations of the polyelectrolyte macromolecule, while the phase separation drives aggregation of the macromolecule and generates densely entangled macromolecule networks. The resulting hydrogel microfibers represent a new type of unconventional polymer networks based on the single-composition entangled polyelectrolyte macromolecule with controllable conformations. The macromolecule conformations evolve from tightly coiled to extended, aligned states with increasing pH, corresponding to the various nanostructured hydrogel networks in atomic force microscopy images. Those include aggregated globules, aggregated coils, extended interpenetrated coils, partially aligned chains, and compactly aligned chains (Fig. 1b). And we observe birefringence colors in the polarized optical microscopy images fibers of pH ranging from 13 to 14, confirming the gradual alignment of extended macromolecule chains. And the appearance of brilliant birefringence colors in the stretched pH 9.14 hydrogel fiber indicates that the randomly entangled macromolecule coils can be straightened and aligned under tensile strains (Fig. 1c).

Fig. 2 Mechanical characterization for the single-composition polyelectrolyte hydrogel microfibers. a, Schematic illustration of the response of different macromolecule network architectures to the mechanical loading. b, Strain-stress curves of the hydrogel microfibers with pH from 3.95 to 13.97. c, Mechanical parameters scope of the hydrogel microfibers. d, Mechanical resilience and strain recovery of the microfibers under a 200% strain cycle. e, Mechanical gradation of the microfibers with increased pH.

Diverse macromolecule conformational shaping realize continuous and wide-range mechanical programming of the single-composition hydrogel microfibers, in terms of both stretchability and resilience properties (Fig. 2a). The successive molecular tuning comprises of i) aggregated macromolecule coils with strong intermolecular interactions of low stretchability but high strength, and the resulting fiber is anelastic with slow tensile strain and strength recovery; ii) extended, interpenetrated coils that are highly stretchable and elastic with reversible random entanglement to orientated state, and deformation of the fiber can be instantly recovered; iii) aggregated, aligned chains network of low stretchability and plastic deformation due to the tight bonding of extended macromolecule chains.

As measured, hydrogel microfiber of the lowest pH is stiff and not stretchable and the strength continues to decrease alongside stretchability enhancement until the pH increases to 13.34 (Fig. 2b), when extended, linear macromolecules start to align. The pH 13.34 fiber is soft and ultrastretchable with a breaking strain reaching 2693%, yielding the highest toughness of 20.3 MJ m^–3. The pH-dependent mechanical parameters correspond to the scope of elongation ratio of 105% ± 2% to 2630% ± 120%, tensile strength of 1210 ± 120 kPa to 47 ± 5 MPa, modulus of 240 ± 30 kPa to 2050 ± 370 MPa, and toughness of 1.7 ± 1.1 MJ m^-3 to 17.8 ± 1.6 MJ m^-3 (Fig. 2c). Mechanical resilience properties were studied via loading-unloading tests. The hydrogel microfibers at around pH 5 show anelastic property, hydrogel microfibers in the moderate pH range (6.35 to 12.38) are fast resilient, and plastic deformation are produced in the microfibers of pH above 13 under stretching (Fig. 2d,e).

Fig. 3 Ultrastretchable polymorphic hydrogel fiber electronic devices. a, Fast resilient ionic hydrogel fiber sensors wirelessly monitoring high-speed wing-flapping motions of a robotic bird. b, (i) Elongation and spring index versus the spring diameter of the programmed helical hydrogel fibers. (ii) Ultrastretchable and low temperatures tolerable LED lighting. c, (i) A Janus hydrogel spring composed of >100 thermoelectric coils electrically connected in series. (ii) Wearable thermoelectric bracelet harvesting human body heat.

The heterogeneous mechanical properties resulting from the complex macromolecule architectures can be arbitrarily layered in one-step via our pH-dependent phase separation process, thereby generating hydrogel fiber devices with readily customizable shapes. Polymorphic hydrogel fibers of fibers/ribbons, Janus fibers, multilayered fibers, core-shell fibers, helical Janus fibers, Janus springs and beyond, can be fabricated. We demonstrate the programming of ultra-large strain capable, all-soft hydrogel electronic devices that operate durably in ambient air: 1000% strain and fast response (~30 ms) fiber sensors wirelessly monitoring robotic bird dynamics, extremely large deformations (3800 to 6000%) and antifreezing helical electrical conductors, and wearable, stretchable (700%) thermoelectric Janus springs.

For more details of this work, please see our recent publication in Nature Communications:

https://www.nature.com/articles/s41467-022-31047-3