Su 8 3050

Microfluidic chips were designed in AutoCAD (Autodesk) and a transparency mask was manufactured (Advance Reproductions). SU-8 3050 (MicroChem) was spin coated onto a clean silicon wafer to a thickness of 100 µm and exposed through the mask using a contact mask aligner. After development, polydimethylsiloxane (PDMS) was poured over the master mold, degassed in a desiccator and cured in an oven at 80 °C for 2 h. PDMS slabs were cut from the substrate, holes were punched for tubing connections and the slab was bonded to a clean glass slide using oxygen plasma. Finally, microfluidic channels were treated with Aquapel (Pittsburgh Glass Works) and dried in an oven at 80 °C for 30 min.

The proposed microfluidic device is composed of a polydimethylsiloxane (PDMS) microstructure layer and an indium-tin-oxide (ITO) microelectrode layer. The PDMS microstructure layer was fabricated by using the standard soft lithography technique. After fabricating a 50-μm thick positive mold on a 3-inch silicon wafer (ePAK, Austin, TX, USA) by using SU-8 3050 (Microchem, Westborough, MA, USA), 20 g PDMS (Dow Corning, Midland, MI, USA) and a curing agent in a ratio of 10:1 were used to replicate the mold to form the microfilter and collection structure, as shown in Figure 1A. Transparent ITO glass was used to fabricate the microelectrode structure. The fabrication process can be found in a previous publication [27 (link)]. A PDMS replica was bonded onto the ITO glass via O₂-plasma activation (PDC-MG, Chengdu, China). The microfluidic device had four reservoirs with a diameter of 4 mm and a long microchannel with a varied length, a width of 6 mm and a depth of 50 μm. The device consisted of a series of octagonal microposts arrayed at an angle in the channel, and the gaps between microposts were 25 (the first stage) and 14 μm (the second stage). The distance between the two ITO electrodes (d) was wider than the microposts, varying from 300 to 800 μm.

The device was fabricated using standard photolithographic methods. In brief, electrodes were patterned on a glass substrate. The device was passivated using SU-8 2002 (Microchem, Newton, MA) and apertures were patterned to expose the electrodes below. The channel was fabricated by patterning SU-8 3050 (Microchem, Newton, MA).

The master used to sequentially trap and release microdroplets was fabricated using a photolithographic process. In brief, a 50 μm-thick negative photo-resist (SU-8 3050, MicroChem) was spin coated onto a silicon wafer. This was then soft baked for 25 min at 95 °C. The mask was placed onto the wafer, exposed under UV light in order to induce polymerisation and post-baked at 95 °C for 5 min^{26 (link)}. Finally, in order to remove any excess photo-resist, the master was developed in Propylene glycol methyl ether acetate (PGMEA) (Sigma–Aldrich).
Microfluidic devices were fabricated using a 10:1 ratio of elastomer PDMS to curing agent (Sylgard 184, DowCorning, Midland, MI) and cured for 3 h at 65 °C. PDMS was cut, peeled off the master and holes of 0.75 mm were punched on the PDMS. The PDMS was then bonded on a glass slide after treatment with a plasma bonder (Diener Electronic, Ebhausen, Germany).