Hydrogels are three-dimensional porous structures that can absorb large amounts of water. They can be made up of polymers, protein, peptides, colloids, surfactants, or lipids.¹ Hydrogels’ ability to uptake large amounts of water is useful for many biological applications, including drug delivery and tissue engineering. Since a hydrogels’ properties change as it absorbs water, scientists must accurately characterize its behavior at different saturation amounts and in varying conditions.

An example of a naturally occurring hydrogel is collagen, the main structural protein in the human body. Collagen is an ideal material for tissue structures due to its fibrous nature – its molecules are tightly packed together in long fibrils and, depending on the exact structural arrangement, can produce fibrils with similar tensile strengths to steel but with some flexibility.

Collagen’s remarkable structural properties and biocompatibility lead to it becoming a widely used material, particularly as a scaffold in tissue engineering.² Since collagen is a natural material, its qualities vary from sample to sample. Therefore it can be challenging to manufacture collagen-based products with reproducible properties. However, the inherent variability of natural starting materials is not an insurmountable challenge – it simply requires the correct material property testing to be performed to predict the material’s behavior for the final application.

Throughout this blog we will use collagen as our primary example of a hydrogel to characterize. However, these same techniques can be applied to any hydrogel of any source material.

Rheology – the measure of flow and deformation behavior of liquids and solids – is an ideal methodology for studying hydrogels such as collagen. Hydrogels are inherently viscoelastic, meaning they exhibit both viscous and elastic behavior. Dynamic (oscillatory) rheology characterizes these complex materials and reports quantitative measurements of viscoelasticity:

• Storage modulus: how much energy must be applied to distort the sample
• Loss modulus: the energy lost as the material returns to its original shape after deformation
• Tan delta: the ratio of the loss modulus to the storage modulus and the measure of the damping abilities of the hydrogel

These viscoelasticity measurements are critical when trying to understand the differences in the degree of cross-linking, glass transition, heterogeneity, and molecular weight between hydrogel samples and how it impacts the end-use performance.¹ Rheometers easily measure viscoelasticity and are the ideal tool for optimizing these qualities.

Another important measurement that is critical for hydrogels is yield stress. A lot of hydrogels are injectable materials so yield stress is very important as it determines how the hydrogel is retained at the injection point. The shear response also determines how easy the hydrogel solution is to inject.

Since different collagen starting materials can have very different physical and thermal properties, researchers must carefully characterize collagen under realistic application conditions to select the best materials for specific uses.³ Rheology test equipment’s rapid screening capabilities makes industrial collagen processing both efficient and cost-effective.