Shape memory hydrogels offer new opportunities for the development of smart wearables due to their intelligent responsiveness. In this study, a self-healing hydrogel with multiple shape memory properties was prepared via one-step polymerization. The synergy of dynamic electrostatic interactions and imine bonds afforded a physically and chemically dual-crosslinked hydrogel with high stretchability (maximum strain of 556%), tensile fracture strength (63 kPa), and self-healing properties (99% healing efficiency at 60 °C within 4 h). Through the reversible construction of physical crosslinking points, the temporary shape of the hydrogel was fixed by two different types of stimuli, pH and metal ions (Ag+, Zn2+, etc.), with the shortest shape fixation time of 15 s. Furthermore, the hydrogel was used to fabricate resistive strain sensors and capacitive pressure sensors, and both types displayed high sensitivity, stability, and cyclability. The sensors could detect various types of human body movement, such as finger bending, swallowing, and pulse, indicating a promising future in smart wearables and healthcare monitoring.
Hydrogels with multiple shape-memory ability have aroused great interest due to their promising applications in various fields. Nevertheless, the weak mechanical performance of most shape-memory hydrogels seriously impedes the practical application in more complex environments. Herein, we reported a novel hydrogel with both high mechanical and multi-shape memory properties composed of stearyl methacrylate (SMA), acrylic acid (AA) and quaternary chitosan (QCH). The electrostatic interactions between AA and QCH together with the hydrophobic interactions of alkyl chains in SMA endowed the hydrogel with great strain–stress and fatigue resistance. Furthermore, due to the reversible destruction and construction of physical cross-links, the prepared hydrogels also exhibited the shape-memory ability in response to different stimuli, such as temperature, pH and NaCl solution. Additionally, the multiple shape-memory effect could be accomplished via programmable combination because of the relatively independent physical interactions in the hydrogels.
Shaojun Chen,Zhankui Mei,Huanhuan Ren,Haitao Zhuo,Jianhong Liu,Zaochuan Ge
This paper reports a new type of zwitterionic polyurethane containing pyridine type zwitterionic segments for shape memory materials. The pyridine type zwitterionic shape memory polyurethanes (SMPUs) were synthesized using polyurethane polymerization from N,N-bis(2-hydroxylethethyl)isonicotinamide (BINA), hexamethylene diisocyanate (HDI), diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO), followed by a ring-opening reaction with 1,3-propanesultone (PS). The structure and properties of the pyridine-type zwitterionic SMPUs (PyZPUs) were investigated systematically. The results demonstrate that the PyZPUs form phase-separation structures composed of a hard phase and an amorphous soft phase with a broad glass transition. The shape memory tests show that the PyZPUs have thermal-induced dual-shape memory effects (SMEs), triple-SMEs and quadruple-SMEs and also moisture-sensitive SMEs. Comparatively, the samples with PS/BINA molar ratios of 0.6–0.8 have more than 86% shape recovery and more than 95% shape fixity with a heat stimulus. Under moisture conditions, these samples also show the quickest moisture-sensitive strain recovery, up to 86%. Moreover, the PyZPUs show good biocompatibility when the content of pyridine type zwitterionic segments is beyond 21 wt%. Thus, the PyZPUs are expected to have many promising applications in smart biomedical fields.
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