Fine-tuning equilibrium water content and mechanical properties of acrylic-based copolymers for intraocular lens applications.

We report on the development of copolymers of 2-Hydroxy Ethyl Acrylate (HEA) with 2-(2-Ethoxy Ethoxy) Ethyl Acrylate (EEEA) grafted with Ethylene Glycol Di-Methacrylate (EGDMA), designed for use as an intraocular lens (IOL) material. Various HEA/EEEA monomer ratios were synthesized via thermal copolymerization, with the HEA concentration pregressively increasing from 3.5% to 28%, while the EEEA concentration decreased proportionately. The physical-chemical, optical, and mechanical properties of the newly developed materials, fabricated as discs (~3.2 mm thick, 11 mm in diameter) and strips (~3.2 mm thick, 80×15 mm2), were comprehensively analyzed. Fourier-Transform Infrared Spectroscopy confirmed the successful copolymerization, as characteristic peaks corresponding to the monomers were observed. Since the development of IOL materials hinges on understanding their physical-chemical, optical, and mechanical characteristics - particularly the equilibrium water content (EWC) - our initial focus was on identifying EWC as a key factor in the development of IOLs. The results showed that the EWC value increased with higher HEA concentrations. Contact angle measurements indicated that the polymers exhibited hydrophilic behavior, with values ranging from 68 to 76°. X-ray diffraction analysis demonstrated that the HEA concentration influenced the crystalline structure, which, in turn, affected the mechanical properties. The results indicated that higher HEA concentrations, corresponding to increased EWC values (i.e., ~8%), led to enhanced flexibility, as evidenced by a decrease in tensile strength from 1.71 to 1.13 MPa, and reduced hardness, which declined from 57.5 to 47.5 Shore A. Additionally, refractive index analyses revealed a gradual decrease with increasing HEA concentrations, ranging from 1.565 to 1.543 when measured at 480 nm and from 1.547 to 1.528 when measured at 660 nm. The evaluation of the coefficient of variation and Pearson's correlation coefficient demonstrated strong material consistency and clear trends across formulations, reinforcing the reliability of the observed properties. These findings emphasize the significance of EWC and the ratio of hydrophilic monomers in acrylic-based copolymers, suggesting that future research could benefit from designing copolymers with tailored physical-chemical, optical, and mechanical properties for IOL applications.