(355g) Designing Gels That Mimic the Rheology of Animal Tissues

Soft animal tissues, such as the lungs, display a highly compliant and dissipative response, facilitating repeated deformation with minimal energy loss, whereas stiffer tissues, like the trachea muscles and tendons can withstand high impact and considerable deformation while maintaining structural integrity. What exactly is the rheology of these tissues? Are they elastic or viscoelastic? How does tissue rheology relate to function, especially to their protective ability? Can we create synthetic materials, i.e., hydrogels, with rheology comparable to those of tissues? Motivated by these questions, we have embarked on a systematic study on tissue rheology and on the design of tissue-mimetic gels.

Our focus for this talk will be on different tissues ranging from soft tissues like lungs to stiff tissues like cartilage, known for its ‘cushioning’ or ‘shock absorbing’ ability, extracted from different animals. We find that while all tissues show the expected gel-like response in dynamic rheology in the linear regime (i.e., its moduli are independent of frequency), the elastic modulus (G’) of bovine trachea is greater than 10⁵ Pa and that of lung is ～1000 Pa. Curiously, the viscous moduli (G’’) of these tissues are also high, resulting in high loss tangent (tan δ = G”/G’). Thus, tissues is indeed viscoelastic and their tan δ exceeds those of typical hydrogels. Turning to non-linear rheology, these tissues exhibit a large ‘hysteresis loop’ in cyclic compression. The area enclosed by this loop correlates with energy dissipation.

For gels to mimic the rheology of tissues, they have to be rendered more viscoelastic. We have explored several strategies towards this end. Gels of gelatin and acrylamide have been prepared with different additives that can enhance their viscous dissipation, notably starch granules. We have been able to design a class of gels that exhibit the combination of rheological properties seen in different tissues, i.e., (1) high stiffness (G’); (2) high viscoelasticity (tan δ; and (3) high ability to dissipate energy at large deformations. The unique properties of these tissue-mimicking gels will be revealed through a series of demonstrations.