Abstract
In this work, biodegradable macroporous stimuli-responsive polypeptide hydrogels based on l-glutamic acid (Glu) and its copolymers with equimolar amounts of l-phenylalanine (Phe) or l-lysine (Lys) were prepared by deprotection of the corresponding organogels under acidic conditions. The organogels were synthesized by ring-opening polymerization of N-carboxyanhydride (NCA) derivatives of the corresponding α-amino acids in oil-in-oil high-internal phase emulsions (HIPEs) using the di-NCA derivative of l-cystine as a cross-linker. The organogels exhibit the typical interconnected porous polyHIPE morphology, which is completely preserved in the hydrogels after removal of the protecting groups of the Glu and Lys repeating units. The pH-dependent behavior and mechanical properties of the obtained hydrogels were studied in buffer solutions with different pH values. At pH 7.5, P(Glu) and P(Glu-co-Phe) can be compressed to half their original height and both return to their initial state after unloading. By lowering the pH to 5.5, P(Glu) remains soft, while P(Glu-co-Phe) already becomes much stiffer. In contrast, for the P(Glu-co-Lys) hydrogel, high buffer uptake was observed only at high or low pH values, whereas at intermediate pH values, the low buffer uptake and the impaired ability to return to the original height are attributed to the attractive ionic interaction between the oppositely charged side groups. We have shown that by tuning the chemical composition of the polypeptides, the uptake, in vitro enzymatic degradation, and compression behavior of the hydrogels can be modulated.
Highlights
Hydrogels are water-swollen three-dimensional viscoelastic macromolecular networks cross-linked by either covalent bonds or non-covalent interactions
The obtained polyHIPE organogels were purified by Soxhlet extraction with dioxane to remove the solvents, surfactant, and any soluble polymer chains that were not part of the polyHIPE network
Polypeptide−polyHIPE hydrogels were prepared via an organogel-to-hydrogel route
Summary
Hydrogels are water-swollen three-dimensional viscoelastic macromolecular networks cross-linked by either covalent bonds or non-covalent interactions. There has been a great deal of interest in the development of stimuli-responsive hydrogels that can rapidly change their structure and volume in a controlled manner depending on environmental temperature, pH, ionic strength, or redox potential.[32,33] A key element in the pH sensitivity of a polymer is the presence of ionizable pendant groups. A solution of triphosgene (1.38 g, 4.7 mmol) in anhydrous THF (10 mL) was added dropwise to the suspension under argon. Enzymatic degradation assays were performed using protease XIV from S. griseus (specific activity: 3.5 U/mg protein) according to previously reported procedures.[48] First, purified and freeze-dried hydrogel samples (10 mg dry weight) were soaked in pH 7.5 buffer After the hydrogels reached uptake equilibrium, they were immersed in distilled water, which was replaced three times to remove the buffer salts.
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