Su 8 2050 photoresist

Microfluidics devices were generated on-site, using a technique described below, which is based on an earlier Drop-Seq protocol^{37 ,128 ,129 (link)}. Soft lithography was performed using SU-8 2050 photoresist (MicroChem) on a 4″ silicon substrate, to generate a 90 μm aspect depth feature. The wafer masks were subjected to silanization overnight using chlorotrimethylsilane (Sigma), before being used for the fabrication of microfluidics. Silicon-based polymerization chemistry was used to fabricate the Drop-Seq chips. In short, we prepared a 1:10 ration mix of polydimethylsiloxane (PDMS) base and cross-linker (Dow Corning), which was degassed and poured onto the Drop-Seq master template. PDMS was cured on the master template, at 70 °C for 2 h. After cooling, PDMS monoliths were cut and 1.25 mm biopsy punchers (World Precision Instruments) were used to punch out the inlet/outlet ports. Using a Harrick plasma cleaner, the PDMS monolith was then plasma bonded to a clean microscope glass slide. After the pairing of the PDMS monolith’s plasma-treated surfaces with the glass slide, we subjected the flow channels to a hydrophobicity treatment using 1H,1H,2H,2H-perfluorodecyltri-chlorosilane (in 2% v/v in FC-40 oil; Alfa Aesar/Sigma) for 5 min of treatment. Excess silane was removed by being blown through the inlet/outlet ports. Chips were then incubated at 80 °C for 15 min.

The PDMS microfluidic devices are fabricated according to the methods described in Mazutis et al.^{34 (link)}, with minor modifications and details specific to our experiments. SU8-on-Si wafer is prepared using spin coated SU-8 2050 photoresist (MicroChem, MA) layer of a desired thickness that is UV etched using the photomask. PDMS is baked at 65 °C for 3 h after pouring onto the SU-8 master. To remove the debris after hole punching, we sonicate the PDMS slab in an isopropanol bath, dry with pressurized air, then bake at 75 °C to ensure all isopropanol is evaporated. After plasma bonding of PDMS slab to glass, the PDMS-on-glass device is heated on a 75 °C hot plate for ~10 min and then incubated at 65 °C overnight in an oven to enhance bonding. To make the channel surfaces hydrophobic, PTFE-syringe-filtered Aquapel (PPG Industries) is injected into the channel, incubated ~60 s at room temperature, and then blown out of the channels using pressurized nitrogen. To remove traces of Aquapel, PDMS device is incubated at 60 °C for ~2 h.

Gelatin type B (225 bloom) from bovine skin, selenium (99%, powder), sulfur (99.9%, powder), glutaraldehyde solution (Grade I, 50%) were obtained from Sigma Aldrich. Trioctylphosphine (TOP, 90%) was obtained from Acros Organics. SU-8 2050 photoresist was purchased from MicroChem, USA. RTV615 Silicone Potting Compound was obtained from Momentive Performance Materials (Waterford, NY, USA). Recombinant human tissue inhibitor of metalloproteinases 2 (TIMP2) was obtained from Sino Biological (Beijing, China).

The ZnO growth chip and the ZNI chip were fabricated using a standard soft lithography process. The SU-8 2050 photoresist (MicroChem) was spin-coated on the silicon wafer. Following ultraviolet (UV) exposure and development, the mold was treated with trimethylchlorosilane and then filled with PDMS (RTV615) prepolymer at a 10:1 ratio of base polymer to cross-linker. Next, the mold filled with PDMS prepolymer was de-bubbled in a vacuum manner. The PDMS replicas were peeled off after curing at 80°C for 2 hours. The ZnO growth chip consisted of 4 channels with a width of 200 μm and a height of 75 μm, and the ZNI chip consisted of the main microfluidic channel (height: 10, 20, 30, 40 μm) and the HB structure (height: 5 μm) (Figure 2B).