Rheological aspects of hydrogel processing

To screen and design hydrogels for specific applications, the ability to characterize and tailor their rheological properties is essential. During the processing and application of hydrogels, rheology plays a threefold role to determine the processability (= injectability or printability) of the materials, to determine their macroscopic mechanical performance in the end-use application together with the evolution from the liquid-like state under processing conditions toward the more solid-like state in the end-use application and finally to act as an indirect structure probing technique. In the present chapter, an overview is given of the different rheological characterization techniques and the material properties resulting from these characterizations. The material properties are subdivided into linear viscoelastic properties, which are characteristic for the small deformation behavior, and non-linear viscoelastic properties that characterize the response to large deformations. In the latter category, both non-linear deformations experienced during use as well as the flow behavior relevant for processing are considered. In addition, a critical overview is given of the main experimental challenges that complicate the rheological characterization of hydrogels. Finally, a brief introduction is given to microrheology, which provides the possibility for non-contact, high-throughput, local characterization of the rheological properties on minute sample quantities.