Improved Comonomer Reactivity Enhances Optical Clarity and Mechanical Properties of pH-Independent Synthetic and Mucin-Cross-Linked Boronic Acid Hydrogels

• Published in: Macromolecules 2024-02, Vol.57 (3), p.887-893
• Main Authors: Davidson, Cullen L. G., Deng, Christopher, Trachsel, Lucca, Brooks, William L. A., Sumerlin, Brent S.
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.3c01471

Boronate esters are formed from the complexation between boronic acids and cis-diols and are widely used as cross-links in self-healing hydrogels. Boronate esters are typically formed in a pH range of 8–9, although a variety of strategies allows for their formation at lower pH values. One such strategy is the use of an intramolecularly coordinating boronic acid monomer, 2-acrylamidophenylboronic acid (2APBA), where the internal coordination between the carbonyl oxygen and the boronic acid boron stabilizes cross-links formed at acidic and neutral pH. However, poly­(vinyl alcohol) (PVA)/boronic acid hydrogels created from 2APBA become opaque over time, likely due to the development of regions of high cross-link density, which potentially precludes use in applications where optical transparency is needed. We attribute this phenomenon to the 2APBA copolymer precursors containing long runs of boronic acid units as a result of the disparate reactivity ratios with acrylamido comonomers. In this report, we synthesized a new boronic acid monomer, (2-(3-acrylamidopropanamido)­phenyl) boronic acid (APAPBA), which exhibited improved reactivity with N,N–dimethylacrylamide and enables the synthesis of copolymers with more uniform distribution of boronic acids. The resulting hydrogels with PVA demonstrate higher moduli and optical transparency. Finally, these APAPBA-based copolymers were used to synthesize biohybrid hydrogels with natural mucin, highlighting the potential of this monomer for use in biological applications where optical clarity is important.