Shear Thickening Behavior in Injectable Tetra-PEG Hydrogels Cross-Linked via Dynamic Thia-Michael Addition Bonds

Area of Science:

• Materials Science
• Polymer Chemistry
• Biomedical Engineering

Background:

• Injectable hydrogels are crucial for minimally invasive biomedical applications.
• Understanding the rheological properties of hydrogels under shear is essential for predicting their performance during injection and in vivo.
• Dynamic covalent chemistry offers a route to tunable and responsive hydrogel networks.

Purpose of the Study:

• To investigate the shear thickening behavior of injectable poly(ethylene glycol) (PEG)-based hydrogels cross-linked via thia-conjugate addition.
• To correlate hydrogel properties, such as plateau moduli and relaxation times, with shear thickening response and injectability.
• To elucidate the underlying mechanism of shear thickening in these dynamic covalent hydrogels.

Main Methods:

• Synthesis of tetra-PEG hydrogels using thia-conjugate addition chemistry.
• Rheological characterization using continuous flow shear rheometry to measure viscosity, plateau moduli, and relaxation times.
• Assessment of injectability by measuring injection forces at varying polymer concentrations and bond exchange kinetics.

Main Results:

• The PEG-based hydrogels exhibited shear thickening at low shear rates, with tunable moduli (60–650 Pa) and relaxation times (2–8 s).
• Higher polymer concentrations and slower bond exchange kinetics led to increased injection forces.
• Reversible viscosity changes with varying shear rates suggest chain stretching as the primary mechanism for shear thickening.

Conclusions:

• Dilute, dynamic covalent tetra-PEG hydrogels display tunable nonlinear rheology and shear thickening behavior.
• The findings provide insights into the shear-induced dynamics of these hydrogels, crucial for optimizing their injectability.
• These dynamic PEG hydrogels show promise for biomedical applications requiring shear-thinning or shear-thickening properties during delivery.