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INTERFACING MICROFLUIDIC CHIPS: DUAL CONNECTOR-RESERVOIR FIXTURE

Daniel Alcaide Martín, Jean Cacheux, Sergio Dávila & Isabel Rodríguez

Madrid Institute for Advanced Studies in Nanoscience (IMDEA Nanoscience), Ciudad Universitaria de Cantoblanco, C/Faraday 9, Madrid 28049, Spain

1- Why is this useful?

Microfluidic devices need to be connected to fluidic pumps for regulation of the flow during the device operation1. Connecting and disconnecting devices is a tedious and time-consuming operation that often causes air bubbles which are detrimental for the fluidic experiment and a real nuisance for the time it takes to eliminate them.

Furthermore, in microfluidic experiments dealing with biomolecules or cell cultures, volumes are of concern as these materials are typically limited and/or costly. Hence, it will be very useful if the reagent filling or replacement process and the connecting and disconnecting operations to microchips are minimized to avoid both bubbles and reagents waste. Reducing the reservoir volume to the volumes needed for the experiment minimizing dead volumes will also allow saving expensive reagents.

With this aim, we have designed a fixture to make practical fluidic connections to a microchip from a pressure controller for fluidic control and device operations. It allows for easy opening and closing operations and for easy re-filling or replacement of the reagents into the microchannels without moving any tubing connection.

Here, we present a dual connector cum reservoir fixture as a practical and effective means to making fluidic connections onto polydimethylsiloxane (PDMS) microfluidic based chips. The fixture is completely built by stereolithography (SLA) 3D printing and includes two components: a piece including a reservoir with an O-ring slit and a cone shaped outlet as chip connector and, another piece that closes the reservoir and has a cone shaped inlet or air pressure connector.

This Chips and Tips builds on a previous approach,2 dealing with microchip fluidic fixtures using magnets. However, in this case, the reservoirs and release connection are moved off the chip which would be more practical to work with the chip on the microscope stage for real time observations. Moreover, it is adaptable for any chip design and particularly for microchips made in soft PDMS.

 

2- What do I need?

  • 3D design software.
  • SLA 3D printer.
  • Cubic magnets.
  • O-rings.

 

3 – What do I do?

We first digitally design the parts of the dual connector: the reservoir-chip connector and the reservoir-seal air pressure adaptor. See the drawing in Figure 1. The corresponding F3D files can be here downloaded.

In our design, the reservoir sits aside from the PDMS chip and it is connected to it through a standard tube fitted on to the connecting outlet. The reservoir-seal encloses the reservoir. Four permanent magnets are inserted in each of the two components to produce a magnetic compressive force onto the O-ring and a tight seal to allow for applying a controlled pressure to cause the reagent to flow at the desired rate into the microfluidic chip.

Once the components are printed, the magnets are inserted into the lateral slots. Then, the plastic tubes are connected to the chip inlets and outlets and to the reservoir chip connector. The reservoir is filled with the correct volume of reagent (in this design is 0.2 ml) and the reservoir- seal piece is placed on top. Finally, through the air pressure inlet, tubing is connected to a pressure controller. When air pressure is applied, the reagent flows through the chip. The closing system formed by the O-ring seal and magnet force is able to handle at least 1 bar working pressure without any leakage.

Figure 2 – Fluidic system set-up showing .two dual connector-reservoir fixtures connected to a microchip and to a pressure controller.

Acknowledgements

This work was performed within the framework of the EVONANO project funded by the European Union’s Horizon 2020 FET Open programme under grant agreement No. 800983.

References

  1. Interfacing of microfluidic devices – Chips and Tips. https://blogs.rsc.org/chipsandtips/2009/02/27/interfacing-of-microfluidic-devices/.
  2. Reusable magnetic connector for easy microchip interconnects – Chips and Tips. https://blogs.rsc.org/chipsandtips/2011/06/27/reusable-magnetic-connector-for-easy-microchip-interconnects/.

 

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Solvent Extraction of 3D Printed Molds for Soft Lithography

Jonathan Tjong1, Alyne G. Teixeira1 and John P. Frampton1,2

1School of Biomedical Engineering, Dalhousie University, Halifax, NS, Canada

2Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, Canada

Why Is This Tip Useful?

Casting polydimethylsiloxane (PDMS) in molds produced from additive manufacturing (i.e., 3D printing) enables rapid prototyping of parts with microscale features without the need for conventional photolithography. Whereas conventional photolithography followed by soft lithography involves the use of silicon substrates and photomasks, which can be costly and may require special preparation and processing (e.g., application and removal of photoresist in a clean room environment), the emergence of stereolithographic 3D printers allow for the rapid manufacture of masters for PDMS casting in almost any laboratory space. Stereolithographic 3D printers use photopolymer resins that when cured can withstand temperatures as a high as 200 °C without plastic deformation, which occurs with thermoplastics such as polystyrene that have glass transition temperature around 95-105 °C (Lerman et al.). The ability to withstand such high temperatures opens the possibility for PDMS and other silicone elastomers to be cured quickly and also provides the possibility for greater control of the mechanical properties of the elastomers (Johnston et al.). However, the components within the 3D printed resin mold such as residual photoinitiators and unreacted oligomers may interfere with the curing of PDMS, resulting in incomplete curing at the interface of the printed mold and the PDMS part. Here, we demonstrate a simple treatment to remove these unwanted materials through solvent extraction.

What Do I Need?

  • Stereolithographic 3D printer and appropriate resin
  • Leak-proof, sealable container large enough to hold the 3D-printed mold
  • Dishwashing detergent
  • 95% ethanol or isopropanol
  • Orbital shaker table
  • Uncured PDMS base and curing agent (10:1 w/w)
  • Post-curing UV lightbox

What Do I Do?

  1. After cleaning and post-curing of the 3D-printed mold in the UV lightbox, place the mold in the container and add enough solvent to submerge the part.
  2. Seal the container and leave on a shaker table for 24 hours.
  3. Discard the old solvent and add new solvent. Seal and agitate for another 24 hours.
  4. Remove the part from the solvent and allow to air dry at room temperature.

What else should I know?

The exact composition of photopolymer resins for stereolithography may vary significantly between different manufacturers; therefore, it may be necessary to adjust the protocol (e.g., the type of solvent). In addition, larger prints will likely require more solvent and a longer duration of solvent extraction to account for the increased migration time of unwanted components from the print to the free solvent.

To demonstrate our procedure, we printed two sets of 5 identical molds with basic geometric features. One set used 1 mm thick outer walls, while the other set used 3 mm thick outer walls (Figure 1). The molds were designed using Onshape (Onshape, Cambridge, MA) CAD software and printed using a B9Creator v1.2 (B9Creations, Rapid City, SD) stereolithographic 3D printer. For all prints, we used B9-R2-Black resin from B9Creations. This resin is documented by the manufacturer as having a heat deflection temperature of 65 °C at 0.45 MPa determined through ISO 75-1/2:2013 standards (B9Creations). As no significant mechanical load would be placed on the resin molds (with a maximum depth of 6 mm for the PDMS chamber), we decided this resin was suitable for our test molds. After printing, excess resin was removed from the molds by submerging and agitating in an approximately 1:10 mixture of Dawn dishwashing detergent and water in a 1 L container. This was followed with additional cleaning by rinsing with excess isopropanol using a wash bottle until no visible evidence of uncured resin was present on the surface (approximately 10-20 mL per part). The molds were then post-cured in a UV lightbox for 20 minutes.

Once post-cured, the molds were each placed in new, 15 mL polypropylene centrifuge tubes (Falcon® Corning, Corning, NY) with 10 mL of the test solvents (reverse osmosis-treated (RO) water, isopropanol, 95% ethanol, or methanol), and exposed to the two 24-hour extraction procedures listed in the “What Do I Do” section. After extraction, the molds were briefly rinsed with RO water and allowed to air dry for 1 hour. Then, approximately 0.4 mL or 1 mL of premixed 10:1 uncured PDMS and curing agent were added to the 1 mm and 3 mm molds, respectively. The PDMS parts were heat-cured in a dry oven at 65 °C overnight. The cured PDMS parts were then carefully removed from the mold using a stainless-steel spatula. Images were taken using the default settings on an iPhone 7 camera.

Resin molds that did not undergo extraction or that underwent extraction in water or methanol produced PDMS parts with major defects. For these extraction conditions, fragments of semi-cured and traces of uncured PDMS remained at the PDMS-mold interface (Figure 2A-C). Extraction with methanol appeared to weaken the cured resin, with significant softening and the appearance of cracks on these molds (Figure 2C), and while the cured PDMS easily released from the methanol-extracted molds, this left artifacts on the PDMS surface. We found that resin parts pretreated with either isopropanol or 95% ethanol performed well as molds for PDMS. The cured PDMS parts easily released from the substrates, with no visible traces of uncured PDMS (Figure 2D-E), and the PDMS parts we retrieved cleanly replicated the features of the resin mold (Figure 3). In addition to producing defects in the mold itself, extraction with methanol also led to bubbles forming in the PDMS as it cured (Figure 3C). Overall, PDMS casts were most easily released from the 1 mm-thick molds compared to the 3 mm-thick molds, but this may simply be due to the lower aspect ratio (h/l) of the 1 mm-thick molds.

Take Home Message

When casting PDMS parts from molds produced by stereolithography, incomplete curing and defects in the PDMS part can be minimized by extracting residual photo-initiators and oligomers present in the mold using either isopropanol or 95% ethanol.

 

 

 

References

B9Creations. Black Resin Material Properties. 2018, pp. 9–11, https://cdn2.hubspot.net/hubfs/4018395/Material Data Sheets/B9Creations Black Material Properties.pdf.

Johnston, I. D., et al. “Mechanical Characterization of Bulk Sylgard 184 for Microfluidics and Microengineering.” Journal of Micromechanics and Microengineering, vol. 24, no. 3, 2014, doi:10.1088/0960-1317/24/3/035017.

Lerman, Max J., et al. “The Evolution of Polystyrene as a Cell Culture Material.” Tissue Engineering – Part B: Reviews, vol. 24, no. 5, 2018, pp. 359–72, doi:10.1089/ten.teb.2018.0056.

 

 

 

Figures and Legends

 

Figure 1. Design features of molds produced by stereolithography. Top panels in (A) and (B) are the Onshape renderings. Bottom panels in (A) and (B) are parts printed in the B9Creations B9-R2-Black resin.

Figure 2. Molds produced by stereolithography following extraction in various solvents. Fragments of partially cured PDMS and uncured PDMS remain on the surface of molds that have not undergone extraction, as well as those extracted in RO-water and methanol. Isopropanol and 95% ethanol extraction produce molds that can be re-used numerous times for PDMS curing.

Figure 3. PDMS parts obtained from curing in resin molds extracted using various solvents. Extraction of residual photo-initiators and oligomers present in the mold prior to soft lithography using either isopropanol or 95% ethanol results in clean PDMS parts that are free of defects.

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