Desktop-Stereolithography 3D-Printing of a Poly(dimethylsiloxane)-Based Material with Sylgard-184 Properties

Abstract

The advantageous physiochemical properties of poly(dimethylsiloxane) (PDMS) have made it an extremely useful material for prototyping in various technological, scientific, and clinical areas. However, PDMS molding is a manual procedure and requires tedious assembly steps, especially for 3D designs, thereby limiting its access and usability. On the other hand, automated digital manufacturing processes such as stereolithography (SL) enable true 3D design and fabrication. Here the formulation, characterization, and SL application of a 3D-printable PDMS resin (3DP-PDMS) based on commercially available PDMS-methacrylate macromers, a high-efficiency photoinitiator and a high-absorbance photosensitizer, is reported. Using a desktop SL-printer, optically transparent submillimeter structures and microfluidic channels are demonstrated. An optimized blend of PDMS-methacrylate macromers is also used to SL-print structures with mechanical properties similar to conventional thermally cured PDMS (Sylgard-184). Furthermore, it is shown that SL-printed 3DP-PDMS substrates can be rendered suitable for mammalian cell culture. The 3DP-PDMS resin enables assembly-free, automated, digital manufacturing of PDMS, which should facilitate the prototyping of devices for microfluidics, organ-on-chip platforms, soft robotics, flexible electronics, and sensors, among others.

Keywords: 3D-printing; elastomers; microfluidics; poly(dimethylsiloxane); stereolithography.

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figures

Figure 1.. Transparent SL prints with 3DP-PDMS-S resin

(using 0.6% TPO-L). CAD model, top view and isometric view of (a) solid cubes, (b) hollow cubes and (c) hollow tetrahedrons. (d) Transmittance of samples of molded Sylgard-184 PDMS, freshly printed 3DP-PDMS-S, isopropyl alcohol (IPA) extracted 3DP-PDMS-S print, and Dynasolve M10 extracted 3DP-PDMS-S print. (e) Comparison of the transparency of 4 mm-thick rectangular blocks of PDMS placed over printed text (Calibri 10-pt) and color images – one 3D-printed with 3DP-PDMS-S resin (top) and the other molded with Sylgard-184 (bottom).

Figure 2.. Resin absorbance and Z-Resolution of SL-printing.

(a) Absorbance measurements of the photoinitiator TPO-L (0.6%) alone, the photosensitizer ITX (0.1%) alone and the 0.6% TPO-L + 0.1% ITX mixture, compared with the power spectrum (green) of the UV-LED source used in the DLP SL-printer. (b) Cure depth determination: 2 mm wide lines of 3DP-PDMS (with 0.6% TPO-L and 0.3% ITX) formed after being exposed with 385 nm UV for different periods of time. (c) Log-linear plot of the cure-depth versus exposure time for different concentrations of TPO-L and ITX (n ≥ 3). The solid lines denote the logarithmic fits of the data points (R2 ≥ 98.5% for all the fits). The slopes of the lines determine the characteristic penetration depth of the resins. Error bars represent SEM. Of the three resins that have the smallest slopes (boxed), we chose the mixture with 0.6% TPO-L and 0.3% ITX because it produced the most transparent prints (see text).

Figure 3.. Microfluidic devices with 3DP-PDMS SL-printing.

(a) Bridge structures printed with 3DP-PDMS: Characterization of exposure times required for creating roof structures on top of voids. The walls (2 mm wide, 2 mm high) are 3D-printed and the intervening areas are exposed with UV for different times. The uncured resin from the voids was later cleared with isopropyl alcohol. (b) A microfluidic device with 500 μm wide channels SL-printed with 3DP-PDMS. (c) A central stream of yellow dye (9 mL/hr) flanked by two streams of blue dye (9 mL/hr each) produce a heterogeneous laminar flow (9 mL/hr) in the 3DP-PDMS microfluidic device.

Figure 4.. Mechanical Characterization of 3DP-PDMS.

(a) 3D printed flexible dog-bone structure made with 3DP-PDMS. (b) Representative stress-strain curves of dog-bone specimens printed with 3DP-PDMS prepared with different ratios of end group and side-chain macromers. (c) Young’s modulus of 3DP-PDMS prepared with different ratios of end group and side-chain macromers. Error bars are SEM. (d) Elongation at break values of 3DP-PDMS prepared with different ratios of end group and side-chain macromers and different photoinitiator concentrations. Error bars are standard deviations. The elongation-at-break of Sylgard-184 PDMS is shown as a dotted line.

Figure 5.. Biocompatibility of SL-printed 3DP-PDMS-S.

(a) CAD design of a 30 mm diameter petri dish. (b) SL-printed PDMS petri dish. (c) Phase-contrast micrograph of CHO-K1 cells cultured on a solvent-extracted 3DP-PDMS petri dish after 24 hrs. (d) Merged fluorescence micrograph of CHO-K1 cells stained with Calcein Green AM (5 μM) (green), Ethidium homodimer 1 (4 μM) (red) and Hoechst 33342 (1 μM) (blue). (e) Comparison of viability of CHO-K1 cells after 24 hrs of culture on a Sylgard-184 thermally cured PDMS disc, a solvent-extracted SL-printed 3DP-PDMS-S petri dish and an unextracted SL-printed 3DP-PDMS-S petri dish. Error bars denote SEM (n ≥ 3). Double asterisk (**) denotes p < 0.01 when using unpaired two-tailed Student’s t-test to determine statistical significance. (f) Bar graph of the number of live cells on Sylgard-184 PDMS disc and solvent- extracted SL-printed 3DP-PDMS-S petri dish after every 24 hours for 3 days. Error bars denote SEM (n ≥ 3). There was no statistical difference in the mean number of total live cells at the end of each day (unpaired two-tailed Student’s t-test).