A passive flow was created between the zoospore inlet and the device outlet driven by the height difference of water inside the inlet and outlet tubing. The layout of the channels was rationally designed to facilitate the attraction of zoospores. Firstly, we used narrow 'necks' to restrict the flow rate and higher width in the attracting area to slow down zoospores. Secondly, an L shape was designed for the path of the passive flow, thus increasing the chance of zoospores entering the straight part. Thirdly, a funnel shape was created at the entry of the sensing channel to facilitate the entrance of zoospores. The fluid dynamics computation presented in Figure showed a relatively high velocity (>5 cm/s) within the constrictions 1 and 2 when a pressure difference of 100 Pa was applied, slowing to 1 cm/s in the wider part. A linear gradient of the attractant was created inside the sensing channel, 5, with a flux that led to a gradient penetrating some distance into the sampling stub, 4.