Granular hydrogels as brittle yield stress fluids.

Area of Science:

• Materials Science
• Rheology
• Biomedical Engineering

Background:

• Granular hydrogels are widely used in biomedical applications.
• Current rheological characterization methods often focus on shear-thinning, self-healing, or ensemble metrics.
• A need exists for detailed analysis of transient rheological behaviors during yielding and unyielding processes.

Purpose of the Study:

• To develop and apply an analytical framework to comprehensively characterize the rheological behavior of granular hydrogels.
• To investigate the influence of microgel and granular assembly properties on steady and transient rheology.
• To establish a quantitative model for the rational design of granular hydrogels.

Main Methods:

• Utilized oscillatory shear testing combined with the Kamani-Donley-Rogers (KDR) model, incorporating Brittility (Bt).
• Quantified steady and transient rheology by varying microgel composition and diameter, and granular packing and droplet heterogeneity.
• Analyzed mixtures of polyethylene glycol and gelatin microgels.

Main Results:

• Granular hydrogels were characterized as brittle yield stress fluids with complex transient rheology.
• The KDR model with Bt effectively captured granular hydrogel behavior across various design parameters.
• Rheological behavior and model parameters were correlated with microgel composition in monolithic and mixed hydrogels.
• Self-healing behavior was robustly captured, and granular relaxation time was found to depend on strain amplitude.

Conclusions:

• The KDR model with Bt provides a quantitative framework for understanding and predicting granular hydrogel rheology.
• This approach reduces complex transient rheology to manageable model parameters.
• The findings facilitate the rational design of granular hydrogels for diverse applications, including injection, *in situ* stabilization, and 3D bioprinting.