Sensor 1. The base and lid had the same design as reported previously. Briefly, the fluidic chip was made of a base unit and lid, that when placed together created a RPS sensing zone, termed sensor 1. The base contained a surface channel and using this method, commercial stereolithography (SLA) printers and resins reproducibility produce channels ≥ 100 m. The lid had a ridge that matched the shape and dimensions of the channel and extended 1 mm out from the lids surface. Optical images of the channel are shown in figure . A PDMS gasket between the lid and base sealed the components preventing any leakage. The PDMS layer was typically 1mm in thickness and when placed between the lid and base acted as a spacer to prevent the ridge entering the channel. The shape or dimensions (internal volume) of the sensing zone were controlled by compressing the PDMS layer, forcing the ridge on the lid deeper into the channel (figure ). Changing the volume of the sensing zones allowed the sensitivity of the RPS sensor to be tuned. The tuneability of sensor 1 was controlled by turning a single screw on the back of the holder, which in turn increased the force on the lid (figure ). Upon tightening the screw in the back of the holder, the blockage magnitude of the same sized particles increased. Thus, a device printed with initial channel dimensions of 100×100×500 m can measure particles from 2 to 30 m in diameter (see figure ).