Hydrogel is a soft material similar to biological tissue composed of a three-dimensional polymer network and water. It is widely used in many fields such as tissue engineering, biomedical equipment, actuators, drug delivery, stretchable electronic devices, and soft robotics. In recent years, people have devoted themselves to preparing hydrogel materials with high mechanical stiffness, strong ductility, good toughness and good self-healing properties. However, these methods tend to make the hydrogel have greater hysteresis, resulting in fatigue damage. In addition, the complex preparation process, cumbersome synthesis route, single function, uncontrollable modulus, etc. greatly limit its wider application. Therefore, it is still a challenge to prepare a multi-purpose hydrogel with adjustable elastic modulus, high elasticity, and a simple method.

The team of Professor Li Jianshu and Professor Chu Hetao of Sichuan University prepared a new type of hydrogel material derived from a solvent-induced dynamic micellar sea-island structure through the copolymerization of acrylamide (AM) and PF127-DA in an aqueous solution. This hydrogel has super stretch (435-2716%, 10-40 wt %), variable modulus (0.36-112.79 MPa, 30 wt %), toughness (1.13-7.17 MJ m−3, 10-40 wt %), low hysteresis (about 10%, 30 wt %), transparency (27.98 -83.78%, 10-40 wt %), shape memory (fixed ratio up to 97%, recovery rate up to 100%, 10 wt %) . This new type of hydrogel material can be used in stress sensors, stretchable electronics and soft robots. The work is titled "Ultra-Stretchable, Variable Modulus, Shape Memory MultiPurpose Low Hysteresis Hydrogel Derived from Solvent Induced Dynamic Micelle Sea-Island Structure" published in "Advanced Functional Materials".

Figure 1 Synthetic principle diagram and performance characterization of PAM/PF127 hydrogel. (A) Synthesis of PF127-DA; (b) Synthesis of PAM/PF127 hydrogel and its microstructure in water or ethanol; (c) Performance test of PAM/PF127 hydrogel with a solid content of 10 wt% , Including knotting, bending, ductility, stretching, shape memory, self-healing, variable modulus, etc.
Article highlights:
1. The author achieves variable modulus and shape memory through the solvent-induced crystallization and matrix volume phase change of the polyethylene oxide (PEO) segment of the PF127-DA molecular chain. This is a purely cyclic physical process.
2. The author uses the PF127-DA micellar sea-island structure to optimize the mechanical properties, instead of introducing sacrificial bonds, which reflects the high toughness and low hysteresis of the hydrogel material. In addition, PAM/PF127 hydrogel has high sensitivity to deformability, elasticity, solvent-induced shape memory and variable modulus characteristics.
3. The author characterizes the microstructure and properties of the PAM/PF127 hydrogel, proves the existence of the sea-island structure of the hydrogel, and discusses that the crystallization in ethanol is caused by the dissociation of hydrogen bonds.

Figure 2 Mechanical test and structure analysis of PAM/PF127 hydrogel in water. (A) Under different solid content, the tensile stress-strain curve of PAM/PF127 hydrogel; (b) Under different solid content, the toughness of PAM/PF127 hydrogel; (c) The solid content is 30 wt% PAM/PF127 hydrogel under different strains for 10 consecutive loading-unloading tensile cycle tests; d) Toughness-hysteresis diagram; (e) Compressive stress-strain curve of PAM/PF127 hydrogel with different solid content; ( f) PAM/PF127 hydrogel with a solid content of 30 wt% was tested under different strains for 10 consecutive loading-unloading compression cycles; (g) polyacrylamide, PF127-DA and 30 wt% solid content PAM/PF127 water FTIR spectrum of the gel; (h) TEM image of 30 wt% solid content PAM/PF127 hydrogel; (i) SAXS curve of different solid content PAM/PF127 hydrogel.