Guided fluidic self-assembly of microtrains using railed microfluidics

In this article, we introduce 'railed microfluidics,' as an agile method to guide and assemble microstructures inside fluidic channels. The guided movement of microstructures in microfluidic channels was done by fabricating grooves ("rails") on the top surface of the channels and also creating complementary polymeric microstructures that fit with the grooves. Using the rails as guiding mechanism for the microstructures, we built complex 1D and 2D microsystems without wasting a single microstructure. Complex structures composed of more than 50 microstructures (each sized smaller than 50 μm) were fluidically selfassembled with zero error. Furthermore, we were able to use the rails to guide microstructures through different fluid solutions, successfully overcoming strong interfacial tension between solutions. This process allows for efficient and practical heterogeneous complex assembly. We envision that railed microfluidics will add vital flexibility in microfluidic design and guided selfassembly.