論文の公開元へStudy on facile fabrication of super strong cellulose hydrogel with anisotropic hierarchical fibrous structure
概要
1.1 Overview
Natural structural materials are built at ambient temperature from a fairly limited selection of components. They usually comprise hard and soft phases arranged in complex hierarchical architectures, with characteristic dimensions spanning from the nanoscale to the macroscale.[1] The resulting materials are lightweight and often display unique combinations of strength and toughness. With the unique mechanical performance, natural structural materials like tendon, ligaments and bones can provide essential support for all kinds of functions. The early technological development of humanity was supported in its early stages by natural materials such as bones, wood and shells.[2] As history advanced, these materials were slowly replaced by synthetic compounds that offered improved performance. Today, scientists and engineers continue to be fascinated by the distinctive qualities of the elegant and complex architectures of natural structures, which can be lightweight and offer combinations of mechanical properties that often surpass those of their components by orders of magnitude. Contemporary characterization and modelling tools now allow us to begin deciphering the intricate interplay of mechanisms acting at different scales — from the atomic to the macroscopic — and that endow natural structures with their unique properties. At present, there is a pressing need for new lightweight structural materials that are able to support more efficient technologies that serve a variety of strategic fields, such as transportation, buildings, and energy storage and conversion. [ref] To address this challenge, yet-to-be-developed materials that would offer unprecedented combinations of stiffness, strength and toughness at low density, would need to be fashioned into bulk complex shapes and manufactured at high volume and low cost.
Hydrogels are formed through the cross-linking of hydrophilic polymer chains within an aqueous microenvironment, the water rich nature of hydrogels makes them broadly applicable to many areas, including tissue engineering, drug delivery, soft electronics, and actuators. [3] Conventional hydrogels usually possess limited mechanical strength and are prone to permanent breakage. The lack of desired dynamic cues and structural complexity within the hydrogels has further limited their functions. Broadened applications of hydrogels, however, require advanced engineering of parameters such as mechanics and spatiotemporal presentation of active or bioactive moieties, as well as manipulation of multiscale shape, structure, and architecture.
Inspired by nature biological materials, many methods have been developed to make polymer fibers in dry or hydrogel form including wet/melt spinning, electrospinning, and microfluidic techniques, where polymer solutions or melts are drawn as fibers with diameters on the order of micrometers to sub-micrometers.[4] But constructing an anisotropic bulk hydrogel containing biomimetic hierarchical fibrous architecture is still a challenge. In the previous work, we developed a facile method called Drying in Confined Condition method, simple as DCC method, [5] through which we successfully fabricated anisotropic alginate hydrogels with hierarchical fibrous architectures. Besides the biomimetic structure, the alginate hydrogels also exhibit mechanical properties similar to those of load bearing natural tissues such as ligaments and tendons.
1.2 Outline of this dissertation
It’s been widely studied that the extreme mechanical property of connective tissues like tendon and ligaments result from the highly aligned and hierarchical fibrous structure. Besides the extreme strength, the toughness of those connective tissues are also excellent. Preparation of finely organized fibrous structure which can optimize the strength and toughness is really challenging for hydrogel scientists.
Previously, our lab developed a facile method called Drying in Confined Condition method, simple as DCC method, which can effectively produce highly aligned and hierarchical fibrous structure on alginate hydrogels, the resulting strength can reach the human ligaments level, but the extreme strength of achilles tendon (~80MPa) is still far from reaching. And since alginate hydrogel is not stable in saline solution due ionic cross-linking, which would limit the application prospect.
Hence in this study, we choose cellulose as raw material, and focus on preparation of super strong cellulose hydrogel through DCC method. We successfully improved the mechanical strength of purely cellulose based hydrogel to human achilles tendon level, and meanwhile there is no sacrificing in toughness. Through multiple characterization method, we confirmed the high orientation degree through different scales and the way of fibrils aggregation play important role in the improved mechanical performance.
In chapter 1, general introduction and outline of the dissertation are discussed.
In chapter 2, a brief review on the strong and tough hydrogels, strengthen and toughening strategies are introduced. Meanwhile, brief introduction on the DCC method and the validity of this method are discussed. The effect of DCC method on different polymers are compared.
In chapter 3, the ideal environmental condition on preparation of DCC-Cellulose hydrogels are discussed.
In chapter 4, extremely strong and purely cellulose-based cellulose gels are prepared, The high strength and toughness of the DCC-E gels were realized by optimizing the cellulose fibril arrangement from nanoscale to macroscale, which was done by selection of an appropriate solvent used for cellulose regeneration. Parallel aggregated fibrous structures observed in the DCC-E gels are thought to play a central role in the enhancement of both toughness and strength.
In chapter 5, the toughening mechanism of DCC-Cellulose hydrogel have been explored through SEM and cycling tests. It’s been clarified that untwisting of hierarchical fibrous structure under loading is the main mechanism for energy dissipation. And abundant hydrophobic interaction between fibrous structure have been confirmed through immersing the samples in urea solution to observation the change of fibrous structure and compare the hysteresis change in cycling tests.
In chapter 6, by using DCC process as training method, we successfully further improved the mechanical property of cellulose hydrogel. By various characterization, we confirmed that during repetitive DCC process, cellulose fibrils become more aggregated together. And by giving a extreme pretrain in the second DCC process, we find the fracture stress is hard to breakthrough 90 MPa.
In chapter 7, conclusions of the dissertation are summarized.
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