Poly(vinyl alcohol)-Agar Double Network Hydrogels: Linking Formulation to Mechanical and Rheological Properties.

Hydrogels are used widely in healthcare disciplines due to factors such as their high water content and safety profile. However, the materials are typically soft and may not be suitable for applications under stress, such as implantation into load-bearing sites. It has been shown that tough hydrogels may be formed by combining brittle chemically-cross-linked polymers with a physically-entangled system to give “double-network” hydrogels. However, the process for chemically cross-linking polymers typically requires reactive species, which are unsafe to use outside of specialised facilities. Furthermore, once the chemical network is formed, the material cannot be remolded. In this study, double-network hydrogels have been formed from two physical networks, namely agar and PVA hydrogels. Agar forms a helical polymer network supported by non-covalent interactions, whereas PVA can form a so-called “cryogel” by freeze-thaw cycling to induce crystallites, which cross-link the network. It has been shown that this approach to producing double-network hydrogels gives tough materials without harsh cross-linking agents. Relationships between PVA molecular weight and gel mechanical properties are probed by approaches including needle-injection, tensile testing, and shear rheometric methods. Formulation factors such as concentration, freeze time, and storage time are also explored.