Supramolecular and Covalent Polymer Hybrids

Cotey, Thomas James

doi:https://doi.org/10.21985/n2-ww0e-wa14

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Supramolecular and Covalent Polymer Hybrids

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The development of functional materials with rationally designed hierarchical structure is an interdisciplinary challenge. Looking to nature for inspiration, we use small molecules that engage in directed self-assembly through carefully tuned intermolecular interactions to construct materials that have structure at multiple length scales. In this work, supramolecular structures formed using peptide amphiphiles (PA) are used to create novel biomaterials. First, this work examines the formation of bulk gels through the interfacial complexation of PA nanofibers with an oppositely charged covalent polymer. These gels were formed by the rapid mixing of solutions, one containing negatively charged PA nanofibers and the other the positively charged biopolymer chitosan. During mixing, complexation occurs at the interface of the two solutions, leading to the formation of a contact layer that locks in the fluid structure formed during mixing, yielding a hydrogel with a lamellar microstructure and many internal interfaces between the supramolecular and covalent components. The nanofiber morphology of the PA is essential to this process because gels do not form when solutions of supramolecular assemblies form spherical micelles. We found that rheological properties of the gels can be tuned by changing the relative amounts of each component. Furthermore, both positively and negatively charged proteins are easily encapsulated within the contact layer of the gel. Building off these findings, we sought to gel peptide amphiphiles during flow using controlled laminar flow in flow focusing microfluidic devices. PA fibers align in the flow direction, and a solution of inorganic multivalent ions is used to gel the PA stream within the microfluidic device, leading to the continuous formation of a highly aligned microgel that we termed a “superbundle.” We explored the processing parameter space of this flow-focusing microfluidic system and developed design rules for producing superbundles with a variety of supramolecular nanofibers and gelators. We found that high concentrations of PA nanofibers as well as high volumetric flow rate ratios between the gelator flow and the PA flow were necessary to form superbundles. We noted a remarkable similarity between the superbundles structure and the structure of the extracellular matrix, the biological framework that provides an environment which supports cellular migration, proliferation, and differentiation. In addition to mimicking the structure of the extracellular matrix, we demonstrated the superbundles’ ability to encapsulate a range of proteins. Using lessons learned from the preceding studies, we investigated the ability to form superbundles using complexation with covalent polyelectrolytes as well as other peptide amphiphiles as gelators. We confirmed the general design principles that we developed for microfluidic production of superbundles using inorganic multivalent ions were relevant for a broad range of gelators. The structure of the supramolecular polymer, high supramolecular polymer concentration, and the confinement of the supramolecular polymer solution by an impinging gelator flow were all crucial to the formation of superbundles regardless of the gelation mechanism used. We also demonstrated that both negatively and positively charged proteins can be encapsulated in superbundles made by complexing covalent and supramolecular polymers as well as those formed by complexing oppositely charged supramolecular polymers. Collectively, the systems investigated in this work demonstrate that rationally designed processing methods can be used to create a diverse set of supramolecular materials with tunable chemistry and hierarchical order.

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