Multi-color dual wavelength vat photopolymerization 3D printing via spatially controlled acidity

Additive manufacturing (AM) enables precise control over object geometry but typically prints single materials. Multimaterial AM expands design freedom by spatially varying properties within printed parts. In vat photopolymerization (VP), achieving multimateriality is challenging because a single homogeneous resin vat is used. Prior strategies often exchange entire resins between layers, which introduces complexity, contamination risks, and mainly allows variation only along the build (z) axis. Chemistry-centered approaches instead modulate properties in situ using light intensity and/or wavelength to control local conversion or orthogonal reactions, but demonstrations have largely focused on stiffness changes and often require post-processing or yield limited contrast. This study asks whether dual-wavelength VP can decouple object formation from color modulation to produce multi-color objects from a single resin vat. The authors propose using UV-triggered photoacid generation to control local acidity and protonate pH-responsive dyes while visible light cures the network, enabling spatially programmed color independent of the curing chemistry.

- Deposition-based multimaterial AM (material extrusion, inkjet, DIW) achieves property variation via material delivery but differs from VP approaches. - Multimaterial VP commonly swaps resins (vat exchange or fluidics), which is slower, complex, and mainly permits z-axis variation. - Intensity-tuned VP in a single vat can modulate properties via degree of conversion but often needs post-cure or yields limited disparity. - Wavelength-selective, orthogonal photochemistries in VP afford additional control; prior dual-wavelength work mainly targeted stiffness using combinations such as radical/cationic polymerizations or thiol-ene/coumarin photocycloadditions. - Prior color demonstrations include grayscale intensity-controlled radical generation with anthraquinone dyes, coupling printing and color changes, or two-step color modulation post-print. A need remains to decouple curing from color control and to implement simultaneous multi-wavelength control for on-the-fly coloration.

- Resin design: Employed photoacid generators (PAGs; triarylsulfonium hexafluorophosphate salts, TAS) with pH-responsive dyes to modulate color via UV-induced acidity while curing networks with visible light. - Dyes: Bromocresol green (BG; blue→green→yellow upon protonation) and methyl red (MR; yellow→orange→red). Dye mixtures (BG/MR ratios 3:1, 1:1, 1:3) expanded color palettes. - Representative formulations (wt% relative to monomer): • F1: PEGDA-250 100; BAPO 0.4; TAS 3; BG 0.05; 3 M NaOH 1. • F2: As F1 but no TAS (control). • F3: As F1 but with MR (0.05 wt%) instead of BG. • F4: As F1 but with BG+MR mixtures. • F5: PEGDA-250 100; TAS 6; BG 0.01; 3 M NaOH 1; hydroquinone (HQ) 0.2; camphorquinone (CQ) 0.4; EDMAB 0.4; no BAPO (type II system to reduce 365 nm reactivity and mitigate outgrowth). - Solution and solid-state color tests: UV (365 nm, 10 mW/cm²) exposure to TAS+dye+NaOH solutions and to cured PEGDA-based disks assessed UV dosage vs. color shift. Control (F2) probed non-PAG effects. - Commercial VP 3D printing (two-step workflow): Elegoo Mars 3 (405 nm LCD). Typical layer thickness 50 µm; example layer time 12 s (optimized per resin). Post-print color modulation via UV (365 nm) with masks or projections; multiwavelength tests at 365/405/455 nm (10 mW/cm²). Post-cure: 405 nm, 1 min. - Environmental stability: Ambient light aging (RGB tracking up to 6 weeks); mitigation via adding 0.2 wt% avobenzone (UVA absorber). Leaching tests by water immersion with UV–Vis of supernatant up to 4 weeks. - Grayscale patterning (two-step): Generated a calibration swatch by projecting grayscale (0–255) with a 365 nm projector (≈2 mW/cm²) for 3 h onto a uniform printed substrate, photographed to extract an available color map (MATLAB). Converted target images (e.g., camouflage) to reduced palettes via minimum variance quantization, then mapped to grayscale projections to reproduce patterns. - Dual-wavelength VP 3DP (single-process workflow): Custom printer with a 365 nm UV projector (max ≈2 mW/cm²) and a visible-light projector (max ≈200 klux). MATLAB with Psychtoolbox controlled both channels; geometric alignment via keystone correction and transformation. Determined visible light cure parameters per resin; explored UV layer times (e.g., 60–180 s). To mitigate outgrowth in DW mode, adopted F5 with increased TAS, added HQ, and used CQ/EDMAB; protonated BG served as an opaquing agent to CQ. - Characterization: SEM of features; penetration-depth assessment (top-side UV exposures; color vs. depth via image analysis); RGB tracking; UV–Vis; tensile testing (ASTM D638 Type V; 10 mm/min; video-based strain in MATLAB).

- UV-triggered acidity enables robust, dosage-dependent color modulation in both solution and solid networks: TAS + BG systems shift blue→green→yellow with 365 nm exposure; control without PAG shows muted blue shift consistent with dye photolysis rather than acid formation. - Successful two-step VP printing from a single vat with post-print UV patterning produced multi-color objects with fine features: 50 µm layers, line slits as small as 100 µm, walls as thin as 200 µm, sharp overhangs; complex shapes (lattices, frogs, gyroids) demonstrated. - Wavelength dependence: 365 nm induces the strongest color change; 405 nm yields slower, moderate change; 455 nm shows no effect, consistent with TAS absorption (negligible beyond ~390 nm). This decouples color modulation from 405 nm curing and allows conventional post-curing. - Penetration-depth limitation benefits and tradeoffs: For >1 mm thickness, color changes initiate at the surface with limited depth until outer layers fully shift, reducing unintended color modification in already-printed layers but constraining bulk recoloring. - Environmental stability: Ambient light gradually drives color toward the most acidic form; parts stored in the dark retain color. Adding 0.2 wt% avobenzone suppresses UV-driven color change (blocks UVA), improving stability but preventing rapid recoloring on demand. Minimal dye leaching into water detected over 4 weeks, attributed to highly crosslinked networks. - Expanded color palettes: MR-based resin (F3) provides orange→red hues; mixing BG and MR (F4) yields distinct palettes tunable by dye ratio (e.g., 3:1, 1:1, 1:3 BG:MR) with vibrant, programmable colors. - Grayscale workflow reproduces target multi-color patterns: Established calibration maps from grayscale intensity (0–255) to obtainable colors and used MATLAB to convert reduced-color target images (e.g., camouflage) into grayscale projections to pattern parts with good visual fidelity. - Dual-wavelength single-process 3DP demonstrated multi-color parts within single layers: Initial BAPO-containing resins showed outgrowth at long UV exposures (>3 min/layer). Reformulated F5 (TAS 6 wt%, HQ 0.2 wt%, CQ/EDMAB) reduced outgrowth, accelerated color change (<3 min UV layer time), and leveraged protonated BG as an opaquing agent to CQ. - Mechanics unaffected by color state: ASTM D638-V dogbones (F5) printed as blue vs. yellow exhibited no statistical differences in ultimate tensile strength, modulus, or elongation to break. - Demonstrations: Multi-color octet truss with four grayscale levels; cube with mid-height color rearrangement; gradient-colored pyramid; two-toned (countershaded) shark; four simultaneous Benchy prints (yellow, green, blue, multi-color) from a single resin vat.

The study demonstrates that spatially controlled acidity using UV-activated photoacid generators can decouple structural curing (via visible light/radical polymerization) from color modulation, directly addressing the challenge of creating multi-color, multimaterial-like VP prints from a single homogeneous vat. UV dosage and patterning control local protonation state of pH-responsive dyes, enabling pixel-level color control independent of the chemistry setting network structure. This approach generalizes across multiple dyes and dye mixtures, supports programmable palettes via grayscale calibration, and integrates with both sequential (post-print) and simultaneous dual-wavelength workflows. The results underscore the utility of photochemical orthogonality in VP to expand beyond stiffness control to aesthetic/functional coloration, while maintaining mechanical performance across color states. The grayscale mapping pipeline provides a practical route to replicate complex patterns and, in the future, could be paired with grayscale-visible curing to co-program mechanical and optical properties within the same build. Although limited by UV penetration in thick parts and ambient-light sensitivity, the method offers a versatile platform for single-vat multi-color VP with potential applications in data storage, camouflage, education, and custom patterning.

This work introduces a versatile, single-vat strategy for multi-color vat photopolymerization by combining visible-light curing with UV-controlled color modulation via photoacid generators and pH-responsive dyes. The method functions in two modes: (1) two-step printing followed by UV patterning and (2) single-process dual-wavelength printing with synchronized visible and UV projections. It achieves vibrant, spatially programmable colors, is compatible with multiple dyes and mixtures, preserves mechanical properties across color states, and enables complex multi-color geometries within single layers. Future directions include exploring additional resin chemistries (including nonpolar systems with suitable PAGs), employing more efficient PAGs and higher UV intensities to shorten layer times, integrating UV-blocking coatings for long-term color stability, and leveraging grayscale control of both visible and UV channels to co-program multiple properties (e.g., stiffness and color) within a print.

- Limited UV penetration restricts rapid bulk color changes in thick (>1 mm) parts; color propagates from the surface inward. - Ambient light can slowly shift colors toward the acidic state; UV absorbers improve stability but hinder re-coloring responsiveness. - Initial dual-wavelength attempts with BAPO showed outgrowth due to 365 nm absorption; mitigation required resin reformulation (increased TAS, HQ inhibitor, CQ/EDMAB) and still involved relatively long UV layer times (≤3 min/layer). - TAS showed solubility limitations in nonpolar monomers (e.g., isobornyl acrylate, butyl acrylate), potentially constraining resin scope without alternative PAGs. - Dye degradation under UV (e.g., BG photolysis) can mute colors; dye concentration must balance visibility and penetration. - Color palette and fidelity depend on grayscale calibration and projector intensity uniformity; environmental conditions and optics may introduce variability.