Photoinitiator Migration and Substitution in Vat Photopolymerisation: Spectral Compatibility, Conversion, Residual Species, Photoproducts and Qualification

Photoinitiators control the conversion of optical dose into reactive species, but unreacted initiator and photolysis products can remain in cured materials and may migrate under use or extraction conditions. This creates a substitution problem with at least four coupled axes: spectral compatibility, initiation efficiency and conversion, residual parent-photoinitiator and photoproduct inventory, and the intended-use interpretation of chemical release. Direct DLP evidence comparing multiple photoinitiator systems shows that changing the photoinitiator can simultaneously alter cytotoxicity, colour, dimensional accuracy, conversion and mechanical properties. Broader UV-curing literature shows that low-molecular-weight photoinitiators or their photoproducts can migrate, while polymeric, multifunctional and self-anchoring designs can reduce migration in particular coating or ink systems. Those adjacent results are useful mechanism and qualification evidence but are not quantitative VPP transfer rules or a selection guide for alternative photoinitiators. ER-95 therefore proposes a substitution qualification matrix that requires optical-source matching, controlled formulation basis, conversion and print-performance checks, targeted parent-photoinitiator analysis, screening for relevant photoproducts, defined extraction or leaching conditions, and application-specific toxicological interpretation where required. TPO is used only as a bounded regulatory case study: its constituent-level status can motivate substitution, but it does not establish that any replacement is safer or that a cured VPP device is qualified.