Wavelength-Matched Process Control as a Prerequisite for the Post-TPO Photopolymer Transition

The 2023 European Union regulatory transition of the photoinitiator TPO — SVHC listing under REACH followed by harmonised Repr. Cat. 1B classification under Delegated Regulation (EU) 2024/197 — is driving substitution to TPO-free formulations across the European dental and biomedical photopolymer additive manufacturing segment. Substitution at the photoinitiator chemistry level is necessary but not sufficient: conventional TPO formulations absorb broadly across the 350–420 nm range and cure adequately under both 365 nm Hg-lamp and 385/405 nm LED post-cure cabinets that constitute the dental laboratory installed base; many TPO-free chemistries absorb primarily in the 350–410 nm window matched to current LED hardware. Operating an unaudited 365 nm Hg-lamp cabinet on a TPO-free formulation produces a latent process deviation — sub-threshold degree of cure not visible at the dimensional level — that propagates into elevated extractables and into cytotoxicity concerns at device-level biocompatibility evaluation. This third perspective develops a four-step wavelength-matched audit methodology for the post-TPO chemistry transition: cabinet emission spectrum measurement, photoinitiator absorption verification, spectral overlap-based dose calibration, and FTIR or DSC degree-of-cure validation. A simplified spectral overlap integral (effective dose D_eff) is introduced as a laboratory-floor screening metric for gross wavelength mismatch; provisional engineering bands are proposed (D_eff,local / D_eff,reference > 0.9 acceptable; 0.7–0.9 conditional; < 0.7 high mismatch). The overlap metric and the engineering bands are intended for first-pass triage of cabinet-chemistry pairings prior to release-grade cure validation; they are not regulatory acceptance thresholds and do not replace full photochemical quantum-yield characterisation. The audit output is intended for integration into the ISO 13485 quality management documentation as a chemistry-cabinet pairing record.