Poster N7 - Designing Gels that Mimic the Rheology of Cartilage at Low and High Deformations

Introduction: Tissues in our body like articular cartilage, muscles, and tendons are able to withstand high impact and considerable deformation. Can we create synthetic materials, i.e., hydrogels, with mechanical properties comparable to those of the above tissues? In particular, cartilage has been a tissue that has inspired numerous studies by scientists aiming to mimic its properties. Cartilage is known to have a high elastic (storage) modulus G’ as well as toughness. While the modulus is obtained from linear rheology (i.e., at low deformations), toughness correlates with the rheology at large deformations. Moreover, cartilage is well-known for its ability to absorb impact and serve as a ‘cushioning’ or ‘shock absorbing’ material. Impact-absorption is again reflected in the non-linear rheology of cartilage.

The goal of this study is to develop hydrogels that mimic the rheology of cartilage in all the above respects, i.e., both at low deformations (linear rheology) as well as large deformations (non-linear rheology). Such hydrogels could function as effective replacements for cartilage for various biomedical applications. We will show that one route to synthesizing such gels is to embed starch granules in a gelatin matrix.

Materials and

Methods: Gelatin-starch gels were prepared by adding gelatin powder and starch granules into deionized water and mixing with a stir bar at 60°C. The resulting sample was poured into a Petri dish and cooled to room temperature to form the gel.

Rheological experiments under oscillatory shear as well as compression were conducted on an AR2000 rheometer at 25°C using parallel plates (20 mm in diameter). Tissue and gel discs (4-10 mm in thickness) were used for the experiments. For the oscillatory shear, frequency sweep was performed at 1% strain. Compression tests were performed in the squeeze-test mode, at a rate of 10% strain per minute, determined based on the initial thickness. The normal-stress transducer was used to collect the normal force during compression, which was further converted to stress based on the initial surface area of the gel.

Results, Conclusions, and Discussions: Data from dynamic rheology on bovine cartilage are shown in Figure 1A. The frequency sweep shows that cartilage has an elastic modulus G’ > 105 Pa, as expected. However, the viscous (or loss) modulus G” is also very high for cartilage and in turn, the loss tangent tan  is quite high (0.39 at a frequency of 10 rad/s). This means that cartilage is a viscoelastic tissue. As a comparison, dynamic rheological data on a gelatin-starch gel are shown in Figure 1B. The elastic modulus G’ again exceeds 105 Pa and the value of tan  is 0.38 at 10 rad/s. Thus, the gel again has is both stiff (high G’) and viscoelastic.
Turning to the non-linear rheology, we study cartilage in cyclic compression, and find that it exhibits a large ‘hysteresis loop’ (Figure 1C). The area enclosed by this loop is 5.7 kJ/m3 and is a measure of how well the tissue dissipates energy. For comparison, the same data are shown for a gel of gelatin alone and one of gelatin-starch.

As shown in the figure, incorporating starch in the hydrogel matrix dramatically increased the elastic modulus of the gel and the hysteresis area. The gelatin-starch gel had a hysteresis of 2.8 kJ/m3 which is comparable to the hysteresis of cartilage. These results indicate that gels containing starch are capable of high elastic energy dissipation through internal friction.

The overarching similarity in mechanical properties of starch-bearing gels to shock-absorbing tissues and the gels’ outstanding energy absorption could be the key to mimicking tissue-like behavior. Based on these findings, it is expected that these gels have potential applications in the field of tissue engineering like articular cartilage replacement and repair.

Ganesh, S., Subraveti, S. N., & Raghavan, S. R. "How a gel can protect an egg: a flexible hydrogel with embedded starch particles shields fragile objects against impact." ACS Appl. Mater. Interfaces 2022, 14, 17, 20014–20022

Acknowledgements (Optional):