Multi-material additive manufacturing for microfluidic applications

Multi-material additive manufacturing has become a powerful technique to fabricate complex microfluidic devices by integrating multiple functional materials, enabling the seamless integration of structural, mechanical, sensing, and fluidic functionalities. This paper reviews the state-of-the-art in this research field. Specifically, different additive manufacturing technologies are presented. Extrusion-based techniques (e.g. fused filament fabrication, polyjet printing, direct ink writing) allow sequential deposition of various polymers through multiple nozzles, enabling the fabrication of microfluidic devices with embedded electrodes. However, extrusion methods generally offer limited resolution and require surface post-processing to achieve smooth fluidic channels. Conversely, light-based techniques (e.g. stereolithography, digital light processing and two-photon polymerization) yield micrometer-scale resolution but they are constrained to single-material fabrication. Several approaches to overcome this limitation are reviewed, including multi-vat printers, single-vat resin swapping systems, (micro)fluidic resin exchange, and wavelength-controlled dual-curing resins, to enable the fabrication of integrated microfluidic devices, which exhibit a variety of material properties (rigid, flexible, conductive, etc.) and functional elements. The remaining challenges include material compatibility and biocompatibility, preventing cross-contamination, and balancing high-resolution with throughput. The ongoing developments in materials and process optimization have resulted in improvements in the reliability of multi-material printing, driving the adoption of this technology for the fabrication of microfluidic devices, with the potential to be used in a variety of applications, including lab-on-a-chip technologies, clinical diagnostics, and point-of-care devices.