Elucidating rheological property enhancements in supramolecular hydrogels of short poly[(R,S)-3-hydroxybutyrate]-based amphiphilic triblock copolymer and α-cyclodextrin for injectable hydrogel applications

Abstract

Injectable hydrogels are an increasingly important materials in biomedical applications for drug delivery as well as injectable scaffolds. Although injectable supramolecular hydrogels based on the host–guest interaction of α-cyclodextrin (α-CD) and hydrophilic polymers have been reported, systematic studies on their dynamic rheology properties in relation to the supramolecular architecture of the hydrogel are limited. We recently found that low molecular weight (Mn < 5000) poly[(R,S)-3-hydroxybutyrate]-poly(ethylene glycol)-poly[(R,S)-3-hydroxybutyrate] (PHB-PEG-PHB) triblock copolymers formed unexpectedly strong hydrogels with α-CD. Herein, we elucidate the unexpected rheological behavior of the supramolecular hydrogel using dynamic rheology and X-ray diffraction measurements, in comparison with model systems that use low or high molecular weight PEGs as alternative guest polymers. Based on fluorescence probe spectroscopy, dynamic light scattering measurements and transmission electron microscopy, the self-assembly property of PHB-PEG-PHB was confirmed, and it is this hydrophobically associative nature of PHB-PEG-PHB that confers a high G′ value in the order of 104–105 Pa to the supramolecular hydrogels, along with high yield stress value of over 102 Pa and fast structural recovery after yielding. Hence, this study highlighted the usefulness of the hydrophobic interaction of a guest polymer, in addition to polymer molecular weight, polymer/α-CD ratio and overall solid content, to modulate the hydrogel stiffness, strength, and a very important parameter for injectable hydrogels: structural recovery rate.