Responsiveness of Charged Double Network Hydrogels to Ionic Environment

Charged double network (DN) hydrogels are widely studied for their desirable mechanical strength and tunable properties. In this work, the influence of polymer concentration on microstructure and properties of agarose/polyacrylic acid DN hydrogels is studied. Agarose, the first network, is a brittle biopolymer, while polyacrylic acid (PAAc) is a weak polyelectrolyte. The microstructure, visualized in liquid environment, displays an agarose scaffold coated and interconnected by PAAc, deviating from the common assumption of an entangled double network. Importantly, the charging of PAAc in the hydrogel is regulated not only by the pH and weak polyelectrolyte effects, but also by the restricted swelling of the double network, and hence, it is an inherent regulation mechanism of charged hydrogels. The interactions between the hydrogel and the ionic environment induce microstructural changes and charging of the double network, impacting surface properties such as topography, stiffness, and adhesion, which are spatially resolved by liquid-environment atomic force microscopy. The responsiveness of the DN hydrogels significantly depends on both polymer concentrations and ion concentrations. These findings provide insights into the responsive behavior of double network hydrogels and reveal universal mechanisms for charged hydrogels, which can guide the future development of functional soft materials.