The study evaluates the photopolymerization and cytocompatibility of a water-soluble lithium acylphosphinate salt (LAP) as a photoinitiator for polymerizing diacrylated poly(ethylene glycol) (PEGDA) monomers into hydrogels. The photoinitiator, lithium phenyl-2,4,6-trimethylbenzoylphosphinate, was synthesized and characterized for its ability to polymerize PEGDA monomers rapidly while maintaining high cell viability during encapsulation. Compared to I2959, a commonly used photoinitiator, LAP exhibits a higher molar extinction coefficient at 365 nm, leading to faster polymerization rates and greater water solubility. The study demonstrates that LAP-initiated polymerization reaches gelation in one-tenth the time of I2959 under similar conditions. Additionally, LAP allows for polymerization at lower initiator concentrations and longer wavelengths, such as 405 nm, which is not feasible with I2959 due to its low absorbance at these wavelengths. When tested with human neonatal fibroblasts, survival rates in hydrogels polymerized with LAP exceeded 95% after 24 hours, indicating high cytocompatibility. The study also addresses light attenuation in thick hydrogel samples, showing that LAP's bleaching characteristics mitigate this issue. Overall, LAP offers significant advantages over I2959 in terms of polymerization efficiency, water solubility, and cytocompatibility, making it a promising photoinitiator for cell encapsulation and biomedical applications.The study evaluates the photopolymerization and cytocompatibility of a water-soluble lithium acylphosphinate salt (LAP) as a photoinitiator for polymerizing diacrylated poly(ethylene glycol) (PEGDA) monomers into hydrogels. The photoinitiator, lithium phenyl-2,4,6-trimethylbenzoylphosphinate, was synthesized and characterized for its ability to polymerize PEGDA monomers rapidly while maintaining high cell viability during encapsulation. Compared to I2959, a commonly used photoinitiator, LAP exhibits a higher molar extinction coefficient at 365 nm, leading to faster polymerization rates and greater water solubility. The study demonstrates that LAP-initiated polymerization reaches gelation in one-tenth the time of I2959 under similar conditions. Additionally, LAP allows for polymerization at lower initiator concentrations and longer wavelengths, such as 405 nm, which is not feasible with I2959 due to its low absorbance at these wavelengths. When tested with human neonatal fibroblasts, survival rates in hydrogels polymerized with LAP exceeded 95% after 24 hours, indicating high cytocompatibility. The study also addresses light attenuation in thick hydrogel samples, showing that LAP's bleaching characteristics mitigate this issue. Overall, LAP offers significant advantages over I2959 in terms of polymerization efficiency, water solubility, and cytocompatibility, making it a promising photoinitiator for cell encapsulation and biomedical applications.