Dual-wavelength volumetric photopolymerization confinement

A unique aspect of our system is the use of a multicolor system to achieve volumetric patterning by the photochemical generation of both polymerization initiation and polymerization inhibition species. Common among all contemporary SLA devices is the use of a single wavelength of light to initiate polymerization patterned in a plane. In contrast, we use one wavelength to photochemically activate polymerization and a second wavelength to inhibit that reaction. Here, photopolymerizable resins are formulated with camphorquinone (CQ; Fig. 1B) and ethyl 4-(dimethylamino)benzoate (EDAB; Fig. 1C) as a visible light photoinitiator and co-initiator, respectively (18), and bis[2-(o-chlorophenyl)-4,5-diphenylimidazole] (o-Cl-HABI; Fig. 1D) as a photoinhibitor. Whereas HABIs are well known as effective photoinitiators in the presence of complementary, hydrogen-donating co-initiators (19), in the absence of co-initiators, the lophyl radicals transiently generated upon HABI photolysis efficiently inhibit radical-mediated, chain-growth polymerization (fig. S1) by rapidly recombining with propagating, carbon-centered radicals and thus can be used to prevent polymerization adjacent to the illumination window.

Independently controlling initiation and inhibition necessitates that photoinitiating and photoinhibiting species have complementary absorbance spectra. As shown in Fig. 1E, o-Cl-HABI exhibits very weak absorbance in the blue region of the spectrum and moderate absorbance in the near ultraviolet (UV), complementing the absorbance spectrum of CQ that absorbs blue light (λ max = 470 nm) but absorbs poorly in the near UV. This minimal overlap in the absorbance spectra of CQ and o-Cl-HABI in the near UV to blue region of the spectrum enables polymerization to be selectively initiated with blue light and inhibited with UV light.

The thickness of the polymerization inhibition volume can be controlled by varying the ratio of the intensities of the two illuminating light sources. When both UV and blue light are supplied to the resin, an inhibition volume with no polymerization is generated adjacent to the window. Above this region, polymerization occurs, allowing the continuous printing of objects, such as those shown in Fig. 1F, without deleterious window adhesion. The inhibition volume thickness (i.e., the vertical distance into the resin from the transparent window in which no polymerization occurs) is dependent on the incident initiating and inhibiting light intensities (I blue,0 and I UV,0 , respectively) such that (1)

Here, the inhibition coefficient (β) is a constant for a given resin composition and incorporates the ratio of inhibitor to initiator absorbance cross sections, quantum yield, and reaction rate constants (20). The absorption height of a material, h UV and h blue , is defined as the inverse of the sum of the concentrations of all absorbing species (c i ) multiplied by their wavelength-specific absorptivity (ε i ) (i.e., ) and is equal to the depth into an absorbing medium at which the light is 90% attenuated.

Figure 1G shows that inhibition volume thickness, calculated using a subtractive technique (15), is controlled by varying both the ratios of the incident radiation and the concentration of the UV absorber. Adjustment of I UV,0 /I blue,0 changes the relative rates of initiating and inhibiting radical generation within the resin [trimethylolpropane triacrylate (TMPTA)] and can be used to control the inhibition thickness. Alternatively, the UV absorber concentration (Tinuvin 328; see fig. S3) can be changed to achieve a similar control over the inhibition volume thickness. Increasing the UV absorber concentration to decrease h UV selectively confines UV light, and hence generation of inhibiting radicals, to progressively thinner regions above the projection window. It is important to note that a minimum intensity ratio at which initiation and inhibition rates are balanced is required to generate an inhibition volume and can be shown to equal (I UV /I blue ) crit = 1/β (see supplement 1). In this TMPTA-based system, 1/β is found to be approximately 1; nevertheless, this value is dependent on resin composition, necessitating experimental determination for specific resin formulations.

The thickness of this polymerization inhibition volume adjacent to the projection window is a critical parameter for continuous stereolithographic fabrication. Previously reported inhibition layers resulting from oxygen inhibition are typically only tens of micrometers thick (15, 16). Although this inhibition layer eliminates adhesion to the window, its small thickness curtails resin reflow underneath the emergent object, especially in objects with large cross-sectional areas (21), and necessitates the use of low-viscosity resins or fabrication of objects with small cross sections. Here, the inhibition volume thickness can be modulated by altering the UV absorbance of the resin or varying the intensities of the initiating and inhibiting light sources such that inhibition volume thicknesses in the hundreds of micrometers are readily obtained. These thick inhibition volumes are particularly desirable when using viscous resin formulations, further expanding the monomer palette, or to allow resin reflow into the print area for objects with large cross-sectional areas. Nevertheless, increases in the inhibition volume thickness are typically accompanied by decreased polymerization rates, and hence slower print speeds, owing to attenuation of the initiation wavelength intensity within the resin bath. Notably, the system described here can negate this limitation and achieve equivalent polymerization rates for different inhibition volume thicknesses by accompanying any variation in the inhibition wavelength intensity with a corresponding initiation wavelength intensity change (see fig. S2).