Synthesis, Characterization, and Modeling of Thermo-Responsive Hydrogels with Improved Mechanical Strength and Swelling Kinetics

Hydrogels have been extensively studied for use in applications such as tissue engineering, drug delivery systems, wound dressing, actuators, valves, and sensors in Micromechanical Systems (MEMS), among others. Thermo-responsive hydrogels in particular pose various advantages such as their capability to respond to an external stimulus and reverse the response when the stimulus is removed and the ability to imbibe a significant amount of solvent and increase their volume by over 1000%. Extensive research has been conducted to tune and improve hydrogel’s rate and degree of swelling and mechanical properties. Previous work showed that when synthetic clay (Laponite) is cross-linked with a well-known thermo-responsive hydrogel composed of N- Isopropylacrylamide (NIPA) the mechanical properties of the hydrogel network improved. It was demonstrated that increasing the cross-linker concentration will increase the stiffness of the hydrogel network, but the swelling ratio will decrease. This imposes a tradeoff between two desirable properties, swelling ratio, and mechanical strength. In this work, a unique thermo-responsive nanocomposite hydrogel was synthesized with two types of synthetic clays, Laponite XLG and its modified version (with ionic dispersant) Laponite XLS, at different concentrations to explore the interrelation of the two clays synthesized with the NIPA gel with respect to the gel’s microstructures, mechanical properties, swelling degree, and kinetics. It was found that the N-Isopropylacrylamide (NIPA) gel cross-linked with the combined synthetic clays XLS and XLG exhibited improved mechanical properties and swelling ratios at equilibrium. The Young Modulus improved by 247% (from 24.6 kPa to 85.3 kPa) and the swelling ratio at equilibrium exhibited an improvement of 35% (from 605% to 814%) when compared to N-Isopropylacrylamide (NIPA) cross-linked with either of the clays separately. The short and extended time swelling was characterized and compared with mathematical models to understand the swelling kinetics of the combined gels