(J-369) Stereolithographic 3D-printing of multimaterial microfluidic actuators using tunable resins with PDMS-like elastic modulus

Introduction:: Stereolithographic 3D-printing (SLA) allows for the fabrication of high-resolution microfluidic and lab-on-a-chip devices¹. SLA photopolymers often yield rigid plastic parts that cannot match the mechanical compliance of elastomers like poly(dimethyl siloxane) (PDMS). This shortcoming makes it difficult for researchers to 3D-print microfluidic actuators with dynamic, movable elements. To formulate tunable resins for SLA, we used a co-polymer of poly(ethylene glycol) diacrylate (PEGDA, Mw~258, often used in SLA-printed microfluidics^2,3) and poly(ethylene glycol methyl ether) methacrylate (PEGMEMA, Mw~300) monomers. By altering (or “tuning”) the ratio of PEGMEMA to PEGDA, the elastic modulus of the printed plastics can be adjusted to match that of PDMS. This resin blend, referred to as PEGDA-co-PEGMEMA, can facilitate 3D-printing of high-performance microfluidic actuators, such as microvalves and micropumps, and the adoption of these components for complex 3D microfluidic devices. These actuators contain a flexible PEGDA-co-PEGMEMA membrane embedded within rigid PEGDA housing and are 3D-printed with a custom multimaterial 3D-printing protocol; Print-Pause-Print (PPP)⁴.




Materials and Methods:: The co-polymers are based on a tunable blend of PEGDA-258 [poly(ethylene) glycol diacrylate, MW~258] and PEGMEMA-300 [poly (ethylene) glycol methyl ether methacrylate,MW~300]. PEGDA photopolymers are prepared by mixing PEGDA monomer (Sigma-Aldrich) with 0.6 wt% phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (Irgacure-819, BASF, IL) as the photo-initiator and 0.6 wt% 2-isopropylthioxanthone (ITX) (PL Industries, PA) as the photo-absorber compound. We prepared PEGDA-co-PEGMEMA resins by mixing 0.6 wt% of Irgacure-819 and 0.6 wt% of ITX in a monomer precursor containing a varying volumetric ratio of PEGDA and PEGMEMA at M_w~300 (Sigma-Aldrich). We used a desktop DLP-based SLA 3D-printer (Pico 2 HD, Asiga) with an X-Y pixel resolution of 27 µm, equipped with a 385 nm UV-LED light source that has a projection light intensity of 85 mW/cm². To measure the Young’s modulus of the printed polymers, we 3D-printed dog-bone specimens and recorded their stress-strain curves using Instron 5584H Load Frame equipped with a 2 kN load cell by following the standard ASTM protocol for evaluating plastics. The multimaterial 3D-printing procedure is a Print-Pause-Print (PPP) protocol. In PPP, the print job is paused at a desired location, and the semi-printed part is cleaned with a pad to absorb the resin. Next, the resin is switched to another resin, and the print job continues afterwards.

Results, Conclusions, and Discussions:: Fig. 1 shows the Young’s modulus of polymerized PEGDA-258 with varying amount of PEGMEMA-300. As shown, increasing PEGMEMA in PEGDA-co-PEGMEMA photopolymers from 0 to 35% decreased the Young’s modulus of the plastic by a factor of ~22, achieving values around 40 MPa for 35% PEGMEMA. A further increase in the PEGMEMA content from 35% to 40% yielded plastics with a Young’s modulus of ~2.24 MPa, similar to PDMS (as reported in refs.^5,6). The decrease in Young’s modulus can be attributed to the increase in the number of mono-acrylate PEGMEMA chains that creates sliding chains with higher mobility in the co-polymer (higher mobility).

Next, using 20% and 40% PEGDA-co-PEGMEMA we 3D-printed microvalves and micropumps by embedding a flexible PEGDA-co-PEGMEMA membrane within a PEGDA rigid housing (Fig. 2). To test the valve performance, we injected blue-dyed liquid at various inlet pressures into the microchannel, and we swept the pneumatic pressure between 0 and 4 psi using a pressure source device. As seen in Fig. 2c, the valve made from 40% PEGDA-co-PEGMEMA performs better (lower actuation pressure) than PEGDA-258 microvalves. Similarly, we tested the performance of a 3D-printed peristaltic micropump by using the flexible co-polymer at various v/v ratios (Fig. 3). The micropumps are operated by sequentially opening and closing the membranes at a time interval of T₀ to push the liquid in one direction. The performance of the peristaltic micropumps is shown in Fig. 2c. From the comparison, we can see that using 40% PEGDA-co-PEGMEMA enhances the pressure delivery of the micropumps.

In conclusion, we have shown the use of tunable co-polymers, PEGDA-co-PEGMEMA to 3D-print high-performance microfluidic actuators. The increase in performance is attributed to the improved elasticity of the membrane that determines valve closing pressure, and the overall pressure delivery of the pump. We envision that PEGDA-co-PEGMEMA can benefit the field of microfluidics by enabling the integration of high-performance actuators into 3D-printed chips.






Acknowledgements (Optional): : This work was supported by a grant from the NIGMS (R21GM137161).


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