Control of the Reversible Shear-Induced Gelation of Amphiphilic Polymers through Their Chemical Structure

All-Natural, Robust, and pH-Responsive Glycyrrhizic Acid-Based Double Network Hydrogels for Controlled Nutrient Release

Supramolecular hydrogels self-assembled from naturally occurring small molecules (e.g., glycyrrhizic acid, GA) are promising materials for controlled bioactive delivery due to their facile fabrication processes, excellent biocompatibility, and versatile stimuli-responsive behaviors. However, most of these natural hydrogels suffer from poor mechanical strength and processability for practical applications. In this work, through adopting a multicomponent gel approach, we developed a novel mechanically robust GA-based hydrogel with an interpenetrating double network (DN) that is composed of a Ca2+-enhanced hydrogen-bond supramolecular GA nanofibril (GN) network and a Ca2+cross-linked natural polysaccharide sodium alginate (ALG) network. Compared to the single GN network (SN) hydrogel, the GN-ALG hybrid hydrogels (GN-ALG-DN) with the hierarchical double-network structure possess excellent mechanical properties and shaping adaptation, encouraging small and large amplitude oscillatory shear (SAOS and LAOS) rheological performances, better thermal stability, higher resistance to large compression deformations, and lower swelling behaviors. Furthermore, the GN-ALG-DN hydrogels exhibit a pH-responsive and sustained release behavior of nutrients (i.e., vitamin B12, VB12), showing a faster VB12 release rate with a higher swelling ratio in an alkaline condition (pH 7.5) than in an acidic condition (pH 2.5). This is ascribed to the fact that the higher dissociation degree of carboxylic groups in GA and ALG molecules in an alkaline environment induces the erosion and looseness of the self-assembled GN network and the ionic-cross-linked ALG network, which can lead to the decomposition of the hybrid hydrogels and thereby increases the release of nutrients. Cytotoxicity tests further demonstrate the excellent biocompatibility of the GN-ALG-DN hydrogels. This study highlights the design of robust shaped and structured supramolecular hydrogels from natural herb small molecules, which can serve as solid, edible, and stimuli-responsive active cargo delivery platforms for food, biomedical, and sustainable applications.

Biocatalytic Feedback-Controlled Non-Newtonian Fluids

Non-Newtonian fluids are ubiquitous in daily life and industrial applications. Herein, we report an intelligent fluidic system integrating two distinct non-Newtonian rheological properties mediated by an autocatalytic enzyme reaction. Associative polyelectrolytes bearing a small amount of ionic and alkyl groups are engineered: by carefully balancing the charge density and the hydrophobic effect, the polymer solutions demonstrate a unique shear thickening property at low pH while shear thinning at high pH. The urea-urease clock reaction is utilized to program a feedback-induced pH change, leading to a strong upturn of the nonlinear viscoelastic properties. As long as the chemical fuel is supplied, two distinct non-Newtonian states can be achieved with a tunable lifetime span. As a proof of concept, we demonstrate how the physical energy-driven nonequilibrium properties can be manipulated by a chemical-fueled process.

Thermo-reversible sol–gel transition of aqueous solutions of patchy polymers

Aqueous solutions of an amphiphilic thermoreversible patchy polymer show abrupt gelation upon cooling by the combined effect of percolation and transition from intra to intermolecular hydrophobic associations.

Vortex-induced injectable silk fibroin hydrogels

A novel, to our knowledge, technique was developed to control the rate of beta-sheet formation and resulting hydrogelation kinetics of aqueous, native silk solutions. Circular dichroism spectroscopy indicated that vortexing aqueous solutions of silkworm silk lead to a transition from an overall protein structure that is initially rich in random coil to one that is rich in beta-sheet content. Dynamic oscillatory rheology experiments collected under the same assembly conditions as the circular dichroism experiments indicated that the increase in beta-sheet content due to intramolecular conformational changes and intermolecular self-assembly of the silk fibroin was directly correlated with the subsequent changes in viscoelastic properties due to hydrogelation. Vortexing low-viscosity silk solutions lead to orders-of-magnitude increase in the complex shear modulus, G*, and formation of rigid hydrogels (G* approximately 70 kPa for 5.2 wt % protein concentration). Vortex-induced, beta-sheet-rich silk hydrogels consisted of permanent, physical, intermolecular crosslinks. The hydrogelation kinetics could be controlled easily (from minutes to hours) by changing the vortex time, assembly temperature and/or protein concentration, providing a useful timeframe for cell encapsulation. The stiffness of preformed hydrogels recovered quickly, immediately after injection through a needle, enabling the potential use of these systems for injectable cell delivery scaffolds.