Extending the pH Stability of Poly(2‐Oxazoline)/Poly(Acrylic Acid) Double‐Network Hydrogels by Including Acrylamide as Comonomer

Double-network hydrogels (DNHs) have outstanding mechanical properties, making them suitable for biomedical applications, such as artificial cartilage. One example of a biocompatible DNH with a mechanical behavior that exactly matches that of natural cartilage is based on poly(2-methyl-2-oxazoline) (PMOx) as primary and poly(acrylic acid) (PAA) as a secondary network, which is stabilized by hydrogen bonds formed between the protonated PAA and the PMOx carbonyl groups. This DNH is shifting the deprotonation of PAA to a pH of 7.6, which makes it applicable in natural tissue. However, implanted hydrogels often encounter varying pH conditions within different tissues or in response to inflammation and infection. Thus, a further shift of the pKa toward more basic conditions is necessary. In this study, the secondary network in the PMOx-based DNHs was systematically varied by copolymerizing acrylic acid (AA) and acryl amide (AAm) and the mechanical properties were investigated in PBS buffer with varying pH. A ratio of 10 wt.% AAm in the secondary network resulted in a PMOx/P(AA₉₀-co-AAm₁₀) DNH that is not deprotonated even at pH 9 and does not change its dimensions or its cartilage-like mechanical properties up to this pH.