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

## TL;DR

Researchers improved the pH stability of a hydrogel used for artificial cartilage by adding acrylamide, allowing it to maintain its structure and properties in more extreme pH conditions.

## Contribution

The study introduces a modified hydrogel with enhanced pH stability up to pH 9 by incorporating acrylamide into the secondary network.

## Key findings

- A PMOx/P(AA90-co-AAm10) hydrogel remains stable and retains cartilage-like properties at pH 9.
- Adding 10 wt.% acrylamide to the secondary network prevents deprotonation and dimensional changes in the hydrogel.
- The modified hydrogel's stability is suitable for biomedical applications with varying pH environments.

## Abstract

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.

A double network hydrogel (DNH) based on poly(2‐methyl‐2‐oxazoline) PMOx as primary network and poly (acrylic acid) as secondary network has cartilage‐like properties, which are not affected up to a pH of 7.4. By copolymerizing acrylic acid and acrylamide as a secondary network, a PMOx‐based DNH that is stable up to a pH of 9.0 was obtained.