Decoupling the dependence of rheological/mechanical properties of hydrogels from solids concentration

Hydrogels are being increasingly used in medicine due to their potential to be delivered into the body in a minimally invasive manner (e.g. injection in solution form) for subsequent gelation at the site of introduction. Both the flow properties of the solution and mechanical properties of the gels are critical to their applications. However, both properties depend on the concentration of polymer, and simply increasing the concentration to improve the gel properties often leads to unacceptably high fluid viscosities. Thus, we hypothesized that hydrogels with a bimodal molecular weight distribution (MWD) (i.e. a mixture of high MW polymer and polymer tailored to have a lower MW but still able to participate in gel formation) would allow one to readily decouple the dependence of the two properties from the overall concentration. This hypothesis was investigated using alginate hydrogels, and we found that increasing the concentration of alginate (Calginate) with high MW alginate from 2 to 5% raised the viscosity (η) of solution from 0.7 to 20 Pa s, while enhancing the shear modulus (G) from 25 to 50 kPa. In contrast, increasing Calginate of binary solutions at a weight fraction of high MW alginate of 0.50 raised η from 0.2 to only 3.6 Pa s, while enhancing G from 15 to 52 kPa. Strikingly, the low η of binary solutions can be attributed to a significant decrease in physical interactions between the chains, while strong gel strength could be attributed to an increased fraction of intermolecular cross-links and stiffened molecules as compared to gels comprised of high MW alginates. This approach of adjusting the MWD of gel forming solutions to control the fluid and solid properties in an independent manner may be broadly utilized in designing other hydrogels and materials for a variety of applications.