3D printing of hollow geometries using blocking liquid substitution stereolithography

Abstract

Micrometer scale arbitrary hollow geometries within a solid are needed for a variety of applications including microfluidics, thermal management and metamaterials. A major challenge to 3D printing hollow geometries using stereolithography is the ability to retain empty spaces in between the solidified regions. In order to prevent unwanted polymerization of the trapped resin in the hollow spaces-known as print-through-significant constraints are generally imposed on the primary process parameters such as resin formulation, exposure conditions and layer thickness. Here, we report on a stereolithography process which substitutes the trapped resin with a UV blocking liquid to mitigate print-through. We investigate the mechanism of the developed process and determine guidelines for the formulation of the blocking liquid. The reported method decouples the relationship between the primary process parameters and their effect on print-through. Without having to optimize the primary process parameters to reduce print-through, hollow heights that exceed the limits of conventional stereolithography can be realized. We demonstrate fabrication of a variety of complex hollow geometries with cross-sectional features ranging from tens of micrometer to hundreds of micrometers in size. With the framework presented, this method may be employed for 3D printing functional hollow geometries for a variety of applications, and with improved freedom over the printing process (e.g. material choices, speed and resulting properties of the printed parts).

Figure 1

Fabrication of hollow geometries and mitigation of print-through using the blocking liquid substitution process. (A) Schematic of the bottom-up stereolithography system and the fabrication steps used to fabricate hollow geometries. The sponge, resin (light green) and the blocking liquid (orange) are localized on separate regions of a vat which is translatable with respect to the substrate and the part being printed (dark green). The blocking liquid substitution is selectively utilized whenever hollow geometries that are susceptible to print-through are encountered (see Supplementary Note 1), otherwise, the printing process operates in conventional mode. (B) Printed device (transparent) containing microfluidic channels that are filled with the substituted blocking liquid (orange). (C) Reconstructed 3D rendering of the hollow microfluidic channels (blue) of a printed device which was characterized using Nano-CT imaging after the substituted blocking liquid was drained out of the channels. (D) Comparison of the cross-sections of the hollow channels (blue) fabricated using the conventional process to those fabricated using the blocking liquid substitution process. The cross-sections were obtained using Nano-CT imaging and corresponds to the y-y’ line in (C). The channels had a designed height of 100 μm, and the solid capping thickness above the trenches was 250 μm. (E) Quantitative comparison of the hollow channels fabricated using the conventional method to the blocking liquid substitution method. The schematic of the channel cross-section in (D) defines geometrical parameters shown in the plot. The horizontal dashed line represents the designed channel height of 100 μm. The error bars represent standard deviation of process repeatability for n = 3 trials.

Figure 2

Interaction between the blocking liquid and the resin in the bottom-up stereolithography system. Schematic showing regions of interaction between the resin (light green) and the blocking liquid (orange) after substitution of the blocking liquid and during the polymerization of the principal layer. Mixing between the two liquids occurs when the part containing the blocking liquid filled trenches is brought into contact with the resin pool. The effect of liquid mixing within Regions I, II and III are discussed throughout the main text.

Figure 3

The effect of blocking liquid formulation on the effective channel height. (A) Comparisons of the two different blocking liquids to the conventional process. (B) The effect of Sudan I absorber concentration, [Ab], in HDDA blocking liquid. In (A) and (B), the designed channel height is shown as horizontal dashed lines, the principal exposure dose was four times higher than regular exposure dose, and the error bars represent standard deviation of process repeatability for n = 3 trials.

Figure 4

Characterization of the resin, blocking liquid and principal exposure dose, and their effects on print-through. (A) Photopolymerization characteristics of the resin (Primary Resin) and the blocking liquid (Blocking Resin) used for the results shown in (B) and (C). This plot shows the maximum polymerization thickness (cured depth, C_d) that is expected for a given incident exposure dose, E. The trendline is fitted to the fundamental “working curve” equation of stereolithography, where D_p is the characteristic penetration depth of light and E_c is the critical energy dose required for the onset of polymerization. The dashed horizontal line at 25 μm is the layer thickness and the dashed vertical line at E_reg is the exposure dose of the regular layers for the results shown in (B) and (C). (B) The effect of principal exposure dose on the effective channel height, where the exposure dose is normalized to E_reg. (C) Fidelity and comparison of fabricated channels with varying designed height, where the principal exposure dose used was 4.0E_reg. In (B) and (C) the horizontal dashed lines represent the designed channel height, and the error bars represent standard deviation of process repeatability for n = 3 trials.

Figure 5

3D printed microdevices comprising of complex hollow geometries. (A) Picture of devices fabricated using a HDDA based resin. The hollow regions of these devices were characterized using Nano-CT imaging, for which the renderings are shown in (B) and (C). (B) Device comprised of channels (blue) with connected paths that form hexagonal geometric patterns (designed height of 100 μm and width of 75 μm). (C) Device comprised of multiple channels (designed height of 150 μm and width of 100 μm) spanning across all three-dimension of the device (multilevel channels). Each continuous channel path is colorized (red, green, blue) to illustrate that the intricate channels are separate but are interlocked. The computer-aided design (CAD) model of this device is shown in Supplementary Fig. 4. (D) Microscope picture of an optically transparent device comprising of multilevel channels (designed height of 250 μm and width of 125 μm) fabricated using a PEGDA based resin. (E) The channels of the device shown in (D) are filled with colored food dyes to illustrate functionality for microfluidic applications. The CAD model of the device in (D) and (E) is shown in Supplementary Fig. 5.