Abstract
Conductive hydrogels are widely used in tissue engineering, drug delivery carriers, soft robotics, flexible electronics, bioelectrodes, electronic skin, and sensors due to their excellent biocompatibility, high water content, adjustable modulus, and conductivity. These applications typically require conductive hydrogels to have good mechanical and electrical properties and maintain outstanding water stability. Traditional hydrogels often suffer from poor conductivity and weak mechanical properties. A common approach to increase hydrogel conductivity is introducing conductive fillers; however, this usually implies a partial sacrifice of stretchability, biocompatibility, and water content. In addition, the electrical properties of hydrogels tend to be unstable due to rehydration in aqueous environments. In this thesis, hydrogel materials with different structures were constructed by combining different processes using poly(vinyl alcohol) (PVA) and silver nanowires (AgNWs) as raw materials. Meanwhile, a series of conductive hydrogels with high conductivity and mechanical properties were developed by systematically investigating the effects of different structures on the hydrogel properties. Finally, based on the excellent properties of the prepared hydrogels, we explored their potential applications in soft electronics.First, we fabricated a conductive hydrogel composite from AgNWs and PVA by employing a synergistic method of freeze-thawing and salting‐out treatments. This combined method constructs a hierarchical hydrogel structure and increases the local concentration of AgNWs by inducing continuous phase separation. The resultant conductive hydrogel composites exhibited ultra‐high electrical conductivity (~1739 S/cm) and electrical stability in aqueous environments while maintaining high water content (~87%), stretchability (~480%), and excellent biocompatibility. The high conductivity and stretchability can be attributed to the synergistic effect of freezing and salting-out treatments. This combined approach induced continuous phase separation, establishing hierarchical hydrogel structures at different length scales. The resulting hierarchical structure introduced multiple strengthening and toughening mechanisms, significantly increasing stretchability. Meanwhile, the local concentration of AgNWs increased as the hydrogel structure evolved during successive phase separations. This process induced percolation, thereby creating more conductive paths. In addition, the hydrogel composites demonstrated excellent water stability, retaining their electrical properties even after four months of immersion in water. Furthermore, these hydrogel composites exhibited remarkable in-vitro and in-vivo biocompatibilities.
Among different conductive hydrogels, compared to two-dimensional films and three-dimensional bulk conductive hydrogels, one-dimensional conductive hydrogel fibers exhibit the inherent structural advantages, such as small diameter, lightweight, and good flexibility, easy to weave, and can be restructured into various multidimensional structures. Second, we propose a facile strategy to construct hydrogel fibers with ultrahigh conductivity and toughness by exploiting the synergistic effects of freezing-thawing, salting-out, and drying-annealing. The continuous phase separation induced by the combined processes results in hierarchical structures, promoting the formation of interconnected conductive networks and increasing the fiber's crystallinity and crystal domain size. The prepared conductive hydrogel fibers exhibited ultrahigh conductivity (958 S/cm), excellent mechanical properties (strength (~6.2 MPa), stretchability (>300%), and toughness (~10 MJ/m3)), high water content (~75%), outstanding water stability, and fatigue resistance properties. More importantly, this fiber preparation strategy is versatile and highly flexible, allowing precise tuning of the various properties of hydrogel fibers, including size, electrical conductivity, and mechanical properties, to meet practical applications. In addition, these performance-tunable conductive hydrogel fibers can be reconfigured into various multidimensional structural materials, such as hydrogel yarns and hydrogel fabrics, due to the intrinsic structure advantages of light weight, flexibility, and ease of twisting.
To further improve the mechanical properties of hydrogels, we introduced a polymer entanglement enhancement mechanism based on our previous work. Conductive hydrogel fibers with more tangles than cross-links were fabricated by freeze cross-linking. Subsequently, high crystallinity was obtained by salting-out and drying-annealing treatment. The fabricated conductive hydrogel fibers achieved both high crystallinity and high entanglement. Owing to the synergistic enhancement mechanism of entanglement and crystallization, we further improved the mechanical properties of the conductive hydrogel fibers, resulting in high modulus (~29.8 MPa), high toughness (~36.92 MJ/m³), extensibility (~634%), strength (~11.33 MPa), and high conductivity (~242.56 S/cm).
Finally, based on the excellent electrical and mechanical properties, we demonstrated the potential applications of the prepared conductive hydrogel materials in soft electronics. We showcased their use as underwater conductors and bioelectrodes by integrating conductive hydrogel films into flexible shark surfaces, underwater electric vehicle circuit systems, and commercial ECG measurement device electrodes. Additionally, we obtained hydrogel materials with multidimensional structures by weaving conductive hydrogel fibers into yarns and fabrics. We also demonstrated their applications as soft bioelectronics, such as artificial nerves, artificial ligaments, and biofiber electrodes.
| Date of Award | 20 Dec 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jinlian HU (Supervisor) & Lixin DONG (Co-supervisor) |