Then, we used standard soft lithography with polydimethylsiloxane (PDMS) to produce the microfluidic devices from the patterned SU-8 molds. We prepared the PDMS prepolymer (Sylgard 184, Ellsworth Adhesives) in a 10 : 1 ratio (elastomer base: curing agent, w/w). Our chips consisted of two PDMS layers with the microfluidic channels serving two functions: 1) distributing the liquid (in the lower 5 mm thick layer) through four distinct channels that collectively distribute the sample into each well, and 2) removing excess liquid during injection (in the top 5 mm thick layer, unused in the results shown in this work). We punched the chambers and inlet and outlet ports in the corresponding PDMS layers using 6 mm and 1.2 mm biopsy punches, respectively. We activated and bonded together the two PDMS layers and a glass slide (75x25 mm Corning) using O 2 plasma (PDC-002, Harrick Plasma).