Load-Relaxation Characteristics of Chemical and Physical Hydrogels as Soft Tissue Mimics

Background Load-relaxation under a constant state of deformation is a common characteristic of hydrated materials, including hydrogels and biological tissues. Overall, mechanical response in such materials is a strong function of underlying structure, which in hydrogels depends on whether the gel is formed through physical or chemical cross-linking. In order to use hydrogels in biomedical applications where their properties are matched to those of native tissues, it is critical to understand these underlying structure-properties relationships. Objective The objective of current work is to quantitatively characterize the load-relaxation behavior of physical and chemical gels and perform a comparative analysis with several biological tissues. Methods Microindentation-based load-relaxation experiments were performed on three physical (agar, alginate, and gelatin) gels and one chemical (polyacrylamide) gel with a range of experimental time frames. Results All three physical gels exhibit strong time-dependent load-relaxation behavior where faster indentation leads to pronounced load-relaxation over short time-scales. The polyacrylamide gel is largely time-independent and exhibits negligible relaxation within short time-scales. The material property intrinsic permeability, which relates to underlying pore structure, was time-independent for both physical and chemical gels. Conclusions A comparative analysis reveals that different aspects of the time-dependent properties of biological tissues are captured by physical and chemical hydrogels, with implications for tissue engineering applications.