Abstract
SummaryHydrogels have gained tremendous attention due to their versatility in soft electronics, actuators, biomedical sensors, etc. Due to the high water content, hydrogels are usually soft, weak, and freeze below 0°C, which brings severe limitations to applications such as soft robotics and flexible electronics in harsh environments. Most existing anti-freezing gels suffer from poor mechanical properties and urgently need further improvements. Here, we took inspirations from tendon and coniferous trees and provided an effective method to strengthen polyvinyl alcohol (PVA) hydrogel while making it freeze resistant. The salting-out effect was utilized to create a hierarchically structured polymer network, which induced superior mechanical properties (Young's modulus: 10.1 MPa, tensile strength: 13.5 MPa, and toughness: 127.9 MJ/m3). Meanwhile, the cononsolvency effect was employed to preserve the structure and suppress the freezing point to −60°C. Moreover, we have demonstrated the broad applicability of our material by fabricating PVA hydrogel-based hydraulic actuators and ionic conductors.
Highlights
Hydrogels are three-dimensional (3D) networks of hydrophilic polymers holding a large amount of water (Ahmed, 2015)
The frozen gel was placed into a 1.5 M sodium citrate (SC) solution, which has a strong salting-out effect where the high ionic strength of salt ions reduces the solubility of polymers (Baldwin, 1996)
In summary, we provide an effective way of achieving high toughness and freeze resistance at the same time in polyvinyl alcohol (PVA) gel
Summary
Hydrogels are three-dimensional (3D) networks of hydrophilic polymers holding a large amount of water (Ahmed, 2015). Because of the low polymer density and high water content, hydrogels fail to meet mechanical property requirements in many important fields such as artificial tissue, actuator, and soft robotics. In order to toughen the typically soft and weak hydrogels, researchers developed various methods such as introducing anisotropic structures (Zhang et al, 2014), compositing (Chen et al, 2020), double network (Sun et al, 2012), mechanical training (Lin et al, 2019), thermal annealing (Owusu-Nkwantabisah et al, 2018), and ice templating (Zhang et al, 2005). Fibrils group together to become fibers, which constitute fiber bundles and come together to form fascicles (Benjamin et al, 2008). This anisotropic hierarchical structure endows tendon with superior mechanical properties. Inspired by the sophisticated structures of tendon, tough hydrogels have been successfully fabricated through the synergistic effect of ice templating and salting out (Hua et al, 2021)
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