Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel.
Materials Research Laboratory, University of California, Santa Barbara, CA 93106, USA; Institute for Polymer Chemistry, University of Stuttgart, D-70569 Stuttgart, Germany.
Acta Biomater. 2019 Sep 15;96:303-309. doi: 10.1016/j.actbio.2019.07.005. Epub 2019 Jul 15.
The penetration of water into rubber-like protein networks such as cross-linked resilin, which is found in insects, can lead to changes in stiffness that range over several orders of magnitude. This softening effect cannot be explained by the volumetric changes associated with pure swelling/deswelling used to describe networks with covalent bonds. Rather, this property stems from the reversible swelling-induced breaking of hydrogen cross-linking bonds that connect the chains in the network. This work presents a model for the swelling and the mechanical response of hydrogen-bond dominated biopolymer networks. It is shown that the penetration of water molecules into the network leads to the breaking of non-covalent cross-linking sites. In turn, the network experiences a reduction in the effective chain-density, an increase in entropy, and a consequent decrease in free energy, thus explaining the dramatic softening. Additionally, the breaking of hydrogen bonds alters the micro-structure and changes the quantitative elastic behavior of the network. The proposed model is found to be in excellent agreement with several experimental findings. The merit of the work is twofold in that it (1) accounts for the number and the strength of non-covalent cross-linking bonds, thus explaining the drastic reduction in stiffness upon water uptake, and (2) provides a method to characterize the micro-structural evolution of hydrogen-bond dominated networks. Consequently, the model can be used as a micro-structural design-guide to program the response of synthetic polymers. STATEMENT OF SIGNIFICANCE: Hydrogen-bond dominated biopolymer networks are found in insects and have a unique structure that allows a dramatic reduction of several orders of magnitude in stiffness upon hydration. Understanding the micro-structure of such networks is key in the fabrication of new biomimetic polymers with tunable mechanical properties. This work introduces a microscopically motivated model that explains the dramatic reduction in stiffness and quantifies the influence of key micro-structural quantities on the overall response. The model is validated through several experimental findings. The insights from this work motivate further attempts at the fabrication of new biomimetic polymers and serve as a micro-structural design guide that enables the programming of the elastic swelling-induced response.
水进入橡胶状蛋白质网络（如昆虫中发现的交联弹性蛋白）的渗透会导致刚度发生几个数量级的变化。这种软化效果不能用与共价键网络相关的纯溶胀/去溶胀所引起的体积变化来解释。相反，这种特性源于可逆溶胀诱导的氢键交联键的断裂，这些交联键将网络中的链连接起来。本文提出了一个用于描述氢键主导的生物聚合物网络溶胀和力学响应的模型。结果表明，水分子进入网络会导致非共价交联点的断裂。反过来，网络的有效链密度降低，熵增加，自由能降低，从而解释了剧烈的软化。此外，氢键的断裂改变了网络的微观结构并改变了其定量弹性行为。所提出的模型与几个实验结果非常吻合。这项工作的优点有两个：（1）考虑了非共价交联键的数量和强度，从而解释了水合作用导致刚度急剧降低的原因；（2）提供了一种方法来描述氢键主导的网络的微结构演化。因此，该模型可用于作为微结构设计指南，以编程合成聚合物的响应。
氢键主导的生物聚合物网络存在于昆虫中，其独特的结构允许在水合作用下刚度急剧降低几个数量级。了解这种网络的微观结构对于制造具有可调机械性能的新型仿生聚合物至关重要。这项工作介绍了一个基于微观动机的模型，该模型解释了刚度的急剧降低，并量化了关键微观结构量对整体响应的影响。该模型通过几个实验结果得到验证。这项工作的见解激发了对新型仿生聚合物的进一步制造尝试，并作为微结构设计指南，可实现弹性溶胀诱导响应的编程。
