Stimuli-Responsive Phosphate Hydrogel: A Study on Swelling Behavior, Mechanical Properties, and Application in Expansion Microscopy.

Phosphorus-based stimuli-responsive hydrogels have potential in a wide range of applications due to their ionizable phosphorus groups, biocompatibility, and tunable swelling capacity utilizing hydrogel design parameters and external stimuli. In this study, poly(2-methacryloyloxyethyl phosphate) (PMOEP) hydrogels were synthesized via aqueous activators regenerated by electron transfer atomic transfer radical polymerization using ascorbic acid as the reducing agent. Swelling and deswelling behaviors of PMOEP hydrogels were examined in different salt solutions, pH conditions, and temperatures. The degree of swelling in salt solutions followed CaCl < MgCl < KCl < NaCl with a decrease in swelling rate at higher concentrations until reaching a saturation point. In water, the degree of swelling increased significantly around neutral pH and remained constant at basic pH values. The effects of polymerization conditions, including pH, temperature (30, 40, 50 °C), and MOEP concentration (40, 50, 60% v/v MOEP/HO), on the hydrogel swelling behavior in various salt solutions were also investigated. PMOEP hydrogels showed a decrease in the degree of swelling as the pH was increased above the native pH of the monomer solution. Scanning electron microscopy and energy-dispersive spectroscopy were utilized to examine the microstructure and chemical composition of the dried hydrogel after salt solution swelling. Cytotoxicity testing using rat bone marrow stem cells confirmed the biocompatibility of the PMOEP hydrogels. A unique feature of this effort was evaluation of these phosphate hydrogels for use in expansion microscopy where a significant twofold enhancement in cellular expansion capacity was showcased utilizing 4T1 mouse breast cancer cells. This comprehensive study provides valuable insights into the stimuli-responsive behavior and expansion characteristics of phosphate hydrogels, highlighting their potential in diverse biomedical applications.