CONVEX microfluidic devices: a new microscale agile manufacturing pipeline for material extrusion additive manufacturing

Abstract

This study is the first to report the fabrication of complex microfluidic devices based on CONtinuously Varied EXtrusion (CONVEX) of extruded filament in material extrusion additive manufacturing (MEAM). A range of complex geometries and channel widths (100–400 µm) were developed by direct GCode scripting including passive mixers of hexagonal, diamond, zigzag and variable-width zigzag (V-zigzag) and hydrodynamic flow focusing components. For each design, a single layer of filament was deposited as the nozzle moved in the X or Y direction, while simultaneously controlling the extrusion volume and printing speed to achieve seamless Y- or cross-junction channels. The novel V-zigzag toolpath design required deposition at varying printing speed along the path, to create the zigzag structure with variable width (200% of nozzle diameter) at pre-determined locations. The passive mixer regions were selectively exposed to acetone for 10 s to reduce the surface roughness of channels before embedding in the polydimethylsiloxane (PDMS). Device structural and fluid flow properties were investigated to generate insights on the impact of manufactured geometry on performance. Microscopic analysis showed the combination of novel manufacturing and chemical treatment reduced the surface roughness of all designs by two orders of magnitude compared to typical values for MEAM parts. Fluid mixing dynamics of microfluidic devices with 400 µm channel widths were measured from 1–1000 µl.min^-1. V-zigzag mixers achieved complete mixing rapidly irrespective of flow rates after only 15 mm following two liquids coming into contact along the flow direction. By contrast, the mixing performance progressively decreased for the other designs as the flow rate increased from 50 to 100 µl.min^-1, highlighting the important effect of geometry. It was established that the variable-width microscale modification in V-zigzag enhances mixing by promoting directional changes in fluid flow within the channel, affording better mixing performance even at high flow rates compared to a conventional zigzag design. The resilience and robustness of this manufacturing strategy is demonstrated by pushing the boundaries in AM to produce channels with cross-section of 100 × 100 µm with high repeatability. Case studies demonstrated the applicability of the newly developed microfluidic devices for a wide range of microfluidic applications including fluidic-chip droplet generator and flow focusing printhead capabilities to precisely control the width of multi-material fluid sheaths.