Spatial selectivity mechanisms in volumetric photopolymerisation

Volumetric photopolymerisation has emerged as a family of light-based additive manufacturing methods that solidify a three-dimensional resin volume without forming the part through sequential layers, with implementations spanning tomographic and dual-wavelength architectures. This Perspective examines volumetric photopolymerisation through the spatial selectivity mechanism rather than through hardware architecture, distinguishing integrated dose contrast in tomographic methods, chemically gated coincidence in dual-wavelength methods and nonlinear absorption in two-photon polymerisation as an earlier historical precedent. Each mechanism encounters distinct dominant limits: integrated-dose systems are constrained by background dose accumulation and by the continuous chemical response of the resin to cumulative exposure; chemically gated systems are constrained by photoswitch kinetics and by the spectral separation of the activation and projection wavelengths; nonlinear absorption systems are constrained by small focal volume and by volumetric throughput. Material development is therefore not architecture-agnostic: in integrateddose systems the chemistry must accommodate cumulative exposure, while in chemically gated systems the photoinitiator participates directly in spatial selectivity, with reported progress including improved dualcolour photochemical contrast, water-soluble photoinitiator systems for hydrogel printing and applicationspecific optimisation of matrix and co-initiator chemistry. The Perspective synthesises documented mechanism-specific constraints and does not propose a quantitative ranking across architectures, a standardised material-design framework or a roadmap for volumetric photopolymerisation development.