3D Printing PDMS Elastomer in a Hydrophilic Support Bath via Freeform Reversible Embedding

Thomas J Hinton, Andrew Hudson, Kira Pusch, Andrew Lee, Adam W Feinberg, Thomas J Hinton, Andrew Hudson, Kira Pusch, Andrew Lee, Adam W Feinberg

Abstract

Polydimethylsiloxane (PDMS) elastomer is used in a wide range of biomaterial applications including microfluidics, cell culture substrates, flexible electronics, and medical devices. However, it has proved challenging to 3D print PDMS in complex structures due to its low elastic modulus and need for support during the printing process. Here we demonstrate the 3D printing of hydrophobic PDMS prepolymer resins within a hydrophilic Carbopol gel support via freeform reversible embedding (FRE). In the FRE printing process, the Carbopol support acts as a Bingham plastic that yields and fluidizes when the syringe tip of the 3D printer moves through it, but acts as a solid for the PDMS extruded within it. This, in combination with the immiscibility of hydrophobic PDMS in the hydrophilic Carbopol, confines the PDMS prepolymer within the support for curing times up to 72 h while maintaining dimensional stability. After printing and curing, the Carbopol support gel releases the embedded PDMS prints by using phosphate buffered saline solution to reduce the Carbopol yield stress. As proof-of-concept, we used Sylgard 184 PDMS to 3D print linear and helical filaments via continuous extrusion and cylindrical and helical tubes via layer-by-layer fabrication. Importantly, we show that the 3D printed tubes were manifold and perfusable. The results demonstrate that hydrophobic polymers with low viscosity and long cure times can be 3D printed using a hydrophilic support, expanding the range of biomaterials that can be used in additive manufacturing. Further, by implementing the technology using low cost open-source hardware and software tools, the FRE printing technique can be rapidly implemented for research applications.

Keywords: 3D printing; Carbopol; FRE printing; PDMS; freeform fabrication.

Conflict of interest statement

The authors declare the following competing financial interest(s): Carnegie Mellon University has filed for patent protection on the technology described herein, and T.J.H. and A.W.F.are named as inventors on the patent..

Figures

Figure 1
Figure 1
FRE printing is performed by extruding PDMS prepolymer in a support bath consisting of Carbopol gel. (A) A 3D file of a vase is imported and processed into G-code before being 3D printed. (B) The 3D file is replicated, layer-by-layer, from PDMS embedded within the Carbopol support bath by a syringe-based 3D printer. (top) A schematic of the printing process showing the vase in (A) printed within a 50 mL centrifuge tube filled with Carbopol. (bottom) A photograph of the actual vase being 3D printed from PDMS in the Carbopol, due to similar index of refraction it is difficult to see the vase. After printing the PDMS is cured for 72 h at room temperature or 2 h at 65 °C. (C) Following curing of the embedded PDMS, the Carbopol bath is liquefied by addition of monovalent cations, in this case a PBS solution, combined with mechanical agitation. (D) After the support bath is liquefied, the print can be removed. Scale bar is 1 cm.
Figure 2
Figure 2
PDMS prepolymer filaments extruder and cured at different temperatures and in different Carbopols are dimensionally stable. (A) Representative phase-contrast images of PDMS filaments extruded into Carbopol 940, ETD 2020 and Ultrez 30 and cured at 65 °C for 2 h or 20 °C for 72 h showed small morphological differences due to the type of Carbopol, but not due to cure temperature (scale bar is 200 μm). (B) Quantification of PDMS filament diameter for target extrusion diameters of 140, 280, and 400 μm (green lines) showed the ability to generally achieve diameters within 10%. The cure temperatures of 65 °C for 2 h (red) and 20 °C for 72 h (blue) did not have a statistically significant effect on diameter (as determined by t tests between cure temperatures for each target diameter and Carbopol type, P < 0.05). (C) Surface and cross-sectional renderings of PDMS filaments imaged using laser scanning confocal microscopy verified the smooth surface and circular cross-section of PDMS extruded in Carbopol 940 ETD 2020 and the rough surface of PDMS extruded in Ultrez 30 (units in 3D rendering are in micrometers, scale bar is 100 μm).
Figure 3
Figure 3
Release of cured PDMS prints by using NaCl solution to decrease yield stress and viscosity. (A) Rheometry of Carbopol gels diluted in water and in PBS buffer demonstrated a thinning behavior in the presence of ionic buffer solutions, whereby they transition from a yield-stress fluid to a shear-thinning fluid. (B) An example of a FRE printed PDMS cylinder (dyed black) in the Carbopol support submerged in a larger beaker consisting of PBS and a magnetically driven stir bar for mechanical agitation. (C) A time-lapse showing the print being released from the Carbopol support by taking advantage of this thinning behavior. Apparent image blurriness is an effect of the Carbopol suspension.
Figure 4
Figure 4
Representative FRE printed PDMS structures using the Carbopol support. (A) The Carbopol gel is capable of supporting freeform extrusion such as this helical path rendered in G-code. (B) The helical extrusion appears identical to the G-code when embedded in the Carbopol (dyed black for visualization). (C) After curing and release, the PDMS helix print retains its geometry when floating in water. (D) G-code for a cylindrical tube created using a helical extrusion. (E) The layers of PDMS filaments fuse into a monolithic surface. (F) After curing and release, the printed tube remains fused between layers and is stiff enough to maintain its geometry while being handled. (G) The G-code for more complex helical tube. (H) As with the tube, the layers of the helical tube are supported within the Carbopol. (I) Release of the helical tube from the Carbopol gel shows the maintenance of geometrical features, supported in water because it cannot support its own weight, even when cured. (J) A PDMS tube to demonstrate the manifold nature of the print’s outer surfaces (scale bar is 4 mm). (K) A time-lapse heat map of dye perfused through the tube (scale bar is 4 mm.) (L) G-code of a bifurcation with a webbed fork for stability. (M) The FRE printed PDMS bifurcation embedded in the Carbopol. (N) Perfusion of dye through the bifurcation, splitting fluid flow.

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