Lock in amplifier

The PDMS channel and gold electrodes are treated with oxygen plasma using a plasma chamber (100W power for 60 seconds at 60 cm³ per min of oxygen under vacuum). Immediately after, the PDMS is aligned using a stereomicroscope and placed above the microelectrodes with focusing regions positioned between outer electrodes (Fig. 1c). After soft baking for 1 hour at 60°C, syringe tubing was inserted into PDMS inlet and outlet holes and syringe needles are inserted into the opposite end to facilitate media infusion using a syringe and the NE-300 syringe pump (Southpointe Surgical Supply, Coral Springs, FL, USA). Silver conductive epoxy components (Digi-Key Electronics, Thief River Falls, MN, USA) are combined to connect microfluidic devices with custom printed circuit boards (PCB, Sunstone Circuits, Mulino, OR, USA) and baked at 60°C for 1 hour. The PCB then connects with a custom Veroboard which facilitates transimpedance amplification for signal detection using HF2TA current amplifiers (Zurich Instruments, Zurich, SUI) from outer electrodes and inputs an AC voltage signal at a 303 kHz frequency to the middle electrode using a lock-in amplifier (Zurich Instruments, Zurich, SUI). PS particles (1, 3, 5, 7, and 9 μm diameters, 2.5% w/v, Spherotech, Lake Forest, IL, USA) are diluted in 1X phosphate buffered saline (PBS) and flow is driven through a syringe pump.