A 3D Nanoprinted Normally Closed Microfluidic Transistor

Emerging applications in areas such as soft micro-robotics and biofluidic microsystems demand microfluidic circuitry at smaller scales. State-of-the-art additive manufacturing (or colloquially, “three-dimensional (3D) printing”) technologies are uniquely suited to enable such capabilities; however, current 3D printed microfluidic transistors are all based on “normally open” operations (i.e., fluid flow persists until a control pressure is applied to stop the flow). As both p-channel and n-channel transistors offer distinct benefits in varying electronics scenarios, we seek to provide such alternatives for microfluidics. Here we present the first 3D printed “normally closed” microfluidic transistor – fabricated in a $30-\mu \mathrm{m}$ -tall channel via two-photon direct laser writing (DLW) – which comprises a “free-floating” sealing element that can be actively displaced to permit source-to-drain fluid flow ( $Q_{SD}$ ). Theoretical and experimental results revealed that the sealing disc effectively blocked $Q_{SD}$ through the 3D microfluidic transistor until a gate pressure ( $P_{G}$ ) of sufficient magnitude led to displacement of the sealing disc, and in turn, unobstructed $Q_{SD}$ . These results suggest that the presented normally closed microfluidic transistor – the smallest reported to our knowledge – offers unique promise for fluidic processing applications in diverse chemical, biomedical, and soft robotics fields.