Designing Highly Stretchable Resilient Hydrogels

第1部分：非技术概述水凝胶被用于许多应用，包括生物医学领域，药物配方和食品材料。开发高度可拉伸和有弹性的水凝胶可以开辟这些材料的其他应用，例如在人造肌肉中，在假肢装置中帮助行动不便的个人，能量收集，以及在软体机器人中实现功率放大过程。生物弹性蛋白（如弹性蛋白）表现出高拉伸性和弹性，但由合成单体制备的含水聚合物网络通常不表现出这种特性。生物弹性体蛋白的高拉伸性和高回弹性归因于亲水性和疏水性片段的存在所产生的熵弹性和焓弹性的平衡组合。在本研究中，以这些生物聚合物为灵感，采用简单的化学合成路线合成具有亲水性和疏水性的水凝胶。本项目将开发和实施的合成策略和表征技术将适用于其他软质材料。由于该项目的跨学科性质，拟议的研究将影响许多学科，即聚合物科学，物理学，机器人和生物学。除了本科生和研究生直接参与拟议的研究活动外，教育和推广部分还将包括实验室演示和K-12学生积极参与研究活动，目的是向他们介绍聚合物科学和工程，并鼓励他们对STEM的兴趣。在这个项目中，将研究由亲水性和疏水性组分组成的水凝胶弹性的起源和性质。特别是，本项目考虑的水凝胶将通过丙烯酸（AAc）、甲基丙烯酰胺（MAAm）和聚丙烯二丙烯酸乙二醇酯（PPGDA）的自由基聚合得到。PPGDA在室温下稀溶于水，可作为疏水块。这些块的倍数可以在这些凝胶中结合形成疏水区域。所得的水凝胶具有高度的可拉伸性和弹性，当从拉伸状态释放时可以达到非常高的收缩速度。据推测，在拉伸过程中，亲水块表现出熵弹性，而疏水区域的解离、折叠的聚丙烯乙二醇块在水介质中的解绕和拉伸将产生显著的焓损失。与在蛋白质弹性体中观察到的情况类似，在焓弹性和熵弹性方面的平衡将使水凝胶变得坚硬、可拉伸和有弹性。然而，对这些水凝胶的结构和性质的基本认识还不清楚。在计划的研究中，这将通过系统地改变单体浓度、疏水含量、疏水块分子量和溶剂质量或温度来解决。这些条件的变化可能会导致凝胶结构的变化，这将与凝胶性能有关，特别是拉伸性和回弹性。系统的理解将导致建立从合成聚合物合成可拉伸和弹性水凝胶的设计原则。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

PART 1: NON-TECHNICAL SUMMARYHydrogels are being used in many applications including in the biomedical field, pharmaceutical formulations, and food materials. Developing highly stretchable and resilient hydrogels could open up additional applications of these materials, for example in artificial muscles, in prosthetic devices towards assisting individuals with restricted mobility, energy harvesting, and enabling power amplification process in soft-robots. Biological elastomeric proteins such as resilin display high stretchability and resilience, but water-containing polymer networks prepared from synthetic monomers typically do not display such properties. High stretchability and resilience in biological elastomeric proteins have been attributed to the balanced combination of entropic and enthalpic elasticity originating from the presence of hydrophilic and hydrophobic segments. In the proposed research, taking inspiration from these biopolymers, hydrogels with hydrophilic and hydrophobic contents will be synthesized using a facile chemical synthesis route. The synthesis strategy and characterization techniques that would be developed and implemented in this project will be applicable to other soft materials. Because of the cross-disciplinary nature of the project, the proposed research will impact many disciplines viz., polymer science, physics, robotics, and biology. In addition to the direct involvement of undergraduate and graduate students in the proposed research activities, the educational and outreach component will also include laboratory demonstrations and active involvement of K-12 students in research activities with the goal of introducing them to polymer science and engineering and encouraging interest in STEM. PART 2: TECHNICAL SUMMARYIn this project, the origin and nature of elasticity in hydrogels consisting of both hydrophilic and hydrophobic components will be investigated. Particularly, the hydrogel considered for this project will be obtained by using free-radical polymerization of acrylic acid (AAc), methacrylamide (MAAm), and polypropylene glycol diacrylate (PPGDA). PPGDA is sparsely soluble in water at room temperature and will act as a hydrophobic block. Multiples of those blocks can associate to form hydrophobic domains in these gels. The resultant hydrogel is highly stretchable, resilient, and can achieve very high retraction velocity when released from a stretched state. It has been hypothesized that during stretching, the hydrophilic blocks display entropic elasticity, whereas dissociation of the hydrophobic domains, unwinding and stretching of the collapsed polypropylene glycol blocks in the aqueous media will have a significant enthalpic penalty. Similar to that observed in protein elastomers, a balance in enthalpic and entropic elasticity will render a hydrogel stiff, stretchable, and resilient. However, a fundamental understanding linking the structure and properties of these hydrogels is not clearly available. In the planned research, such will be addressed by systematically varying the monomer concentrations, hydrophobic content, hydrophobic block molecular weight, and solvent quality or temperature. Variation of these conditions will likely lead to the change in gel structure, which will then be linked to gel properties, particularly stretchability and resilience. A systematic understanding will lead to establishing design principles for synthesizing stretchable and resilient hydrogels from synthetic polymers..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.