An Anti‐Fracture and Super Deformable Soft Hydrogel Network Insensitive to Extremely Harsh Environments

Abstract Design of hydrogels with superior flexible deformability, anti‐fracture toughness, and reliable environment adaption is fundamentally and practically important for diverse hydrogel‐based flexible devices. However, these features can hardly be compatible even in elaborately designed hydrogels. Herein soft hydrogel networks with superior anti‐fracture and deformability are proposed, which show good adaption to extremely harsh saline or alkaline environments. The hydrogel network is one‐step constructed via hydrophobic homogenous cross‐linking of poly (sodium acrylate), which is expected to provide hydrophobic associations and homogeneous cross‐linking for energy dissipation. The obtained hydrogels are quite soft and deformable (tensile modulus: ≈20 kPa, stretchability: 3700%), but show excellent anti‐fracture toughness (10.6 kJ m−2). The energy dissipation mechanism can be further intensified under saline or alkaline environments. The mechanical performance of the hydrophobic cross‐linking topology is inspired rather than weakened by extremely saline or alkaline environments (stretchability: 3900% and 5100%, toughness: 16.1 and 17.1 kJ m−2 under saturated NaCl and 6 mol L−1 NaOH environments, respectively). The hydrogel network also shows good performance in reversible deformations, ion conductivity, sensing strain, monitoring human motions, and freezing resistance under high‐saline environments. The hydrogel network show unique mechanical performance and robust environment adaption, which is quite promising for diverse applications.

The compression and the tension tests were performed using an Instron 5569 electronic universal test machine. The hydrogels were under the as-prepared state. All the tests for the mechanical performance of the hydrogels were performed at room conditions (15~25 ℃, 70~85% relative humidity) without special measures to avoid dehydration of hydrogels. Cylindrical specimens with a size of ø16×10 mm were prepared to perform uniaxial compression tests with a displacement rate of 3 mm min -1 . Compressive modulus was calculated from the slope of the linear region of the stress-strain curve during the compression tests. The tension tests and pure shear tests of the samples were performed with a displacement rate of 100 mm min -1 without specific description.
Dumb-bell-shaped specimens of PANaD n hydrogels were prepared standardized as ISO 37-2017 (overall length: 50 mm, width of ends: 8.5 mm, length of narrow portion: 16 mm, width of narrow portion: 4 mm, thickness of the sample: 2 mm) for tension tests. Tensile modulus of each stage was calculated from the slope of the linear region of the stress-strain curve during the tension tests. The work (W) during the tension process of PANaD n hydrogels was calculated by integrating the area under the stress-strain curve. In the cyclic tension tests, the crosshead returned to its original position immediately with a displacement rate of 100 mm min −1 after the sample was stretched to a 1500% tensile strain with no recovery interval. In the stress relaxation tests, the hydrogels were stretched to a certain strain, then the displacement was held constant. Meanwhile, the force sensor recorded the stress as a function of time. Two identical pieces of hydrogels were prepared with 1.5 mm in thickness and 60 mm in width for the pure shear test. The samples were clamped along the long edges with an initial test length of H=10 mm. A notch with a length of 30 mm was introduced into one sample using a razor blade. The fracture energy or toughness (G) of PANaD n hydrogels was calculated from the stress-strain curve of the unnotched sample according to the equation: in which λ c stands for the critical stretch that the crack propagates for the notched sample. In the tear tests, the samples were prepared with two inextensible backing layers and an initial crack. The two arms of the samples were clamped to perform tear tests and the force sensor recorded the stress at various displacement rates (2-300 mm min -1 ).
The ionic conductivities of the hydrogels were measured by electrochemical impedance spectroscopy (EIS) with an EG&G Princeton Applied Research P4000+ workstation (Frequency range: 100 kHz~0.01 Hz, applied voltage: 10 mV) using two aluminum foils as electrodes. The resistance changes of the hydrogels during deformations were recorded by a KEITHLEY DMM6500 6 1/2 digit multimeter. In the section of human motion detections, informed written consent from all participants was obtained prior to the research. During the tests, the hydrogel sensors were only temporarily attached to the skin surface of the participants, rather than implanted in human body.
The aqueous solutions were ultrasonically oscillated for 1 h before polymerization for homogenous dispersion of DVB. DLS measurements revealed that the hydrophobic DVB crosslinker was uniformly dispersed and formed nanodroplets with an average diameter of ~100 nm in AANa solutions ( Figure S2a). The diameter of DVB nanodroplets was increased with the increase of DVB content at n < 0.3. However, the diameter was almost constant by further adding DVB into solutions, which suggested that the further added DVB was separated from solutions. The maximum content of DVB introduced by ultrasonic oscillation was 0.3 mol L -1 . The solution was injected into PTFE molds with specific shapes, and the polymerization was initiated by ammonium persulphate at 60 ℃. After 4h, transparent hydrogels PANaD n with about 70 wt % water were obtained. The water contents of the hydrogels were determined by a gravimetric method. The as-prepared hydrogels were stored at 80 o C under vacuum, and weighed at appropriate intervals until the weight was constant. The water content was determined by the weight loss divided by the initial weight. The water contents determined by gravimetric methods for PANaD 0.1 , PANaD 0.2 , and PANaD 0.3 hydrogels were 68.9%, 68.5%, and 67.9, respectively, which were quite close to the calculated water contents (69.4% for PANaD 0.1 , 69.0% for PANaD 0.2 , 68.4% for PANaD 0.3 hydrogels).
DLS measurements of the as-prepared hydrogels are shown in Figure S2b. The PANaD 0.2 hydrogels were freeze dried to constant weight, then the freeze-dried hydrogel networks were immersed into saturated NaCl solutions or 6 mol L -1 NaOH solutions at room temperature until the

FT-IR spectrum of the PANaD 0.2 hydrogel network
The FT-IR spectrum of freeze-dried PANaD 0.2 hydrogel is shown in Figure S4. The absorption peaks at 3320 cm -1 and 1550 cm -1 were assigned to the vibration of -OH and COO-groups, respectively. [1] The C=C stretching at 1640 cm -1 was not observed in the FT-IR spectrum of freezedried PANaD 0.2 hydrogel, which confirmed the successful polymerization of the hydrogels [2] .  Figure S17. DSC traces of PANaD n and PANaD n -NaCl hydrogels during cooling

Skin allergy tests of the hydrogels
Skin allergy tests were performed to test the skin sensitization of the hydrogels using the Buehler test in guinea pigs, according to ISO 10993-10:2021. The test article and control article were cut into suitable size. The test samples were patched to ten guinea pigs. Five control animals were treated accordingly but with the negative control. The topical challenge with the test article excited no skin reaction in the test and in the control animals. The skin sensitization rate determined from the tests was 0%. Figure S18. Test report of Skin allergy tests for the hydrogels.

Stable in Saturated
NaCl, 6 mol L -1 NaOH Yes a) from pure shear tests b) from tear test, c) -: not given, d) under saturated NaCl environment, e) under 6mol L -1 NaOH environment