Fast multi-resolution 3D printing of microfluidics: enabling 2 μm channels and ultra-compact mixers.

Microfluidic devices with ultra-fine features are critical for applications in biomedical diagnostics, chemical analysis, and lab-on-chip systems, but achieving high-resolution negative features with fast print times remains a significant challenge due to limitations in conventional 3D printing techniques. Motivated by the need for rapid fabrication of precise, compact microfluidic structures to enhance performance and miniaturization, we present an efficient multi-resolution 3D printing technique designed to fabricate microfluidic devices with exceptionally high-resolution negative features. For instance, we achieve fully enclosed channels with cross sections as small as 1.9 µm × 2.0 µm, a two-order-of-magnitude reduction in cross-sectional area compared to the 18 µm × 20 µm channels reported in our previous work (Gong et al., Lab Chip 17, 2899, 2017). Our method utilizes a dual-optical-engine approach comprising a Very High Resolution Optical Engine (VHROE) and a Main Optical Engine (MOE), each employing distinct pixel resolutions and LED wavelengths. The VHROE, with a pixel pitch of 0.75 µm and a 365 nm LED, delivers unparalleled resolution, while the MOE, with a pixel pitch of 15 µm and a 405 nm LED, ensures efficient coverage for larger areas up to 38.9 mm × 24.3 mm. Custom ultraviolet (UV) short-pass filters are used to tailor each LED spectrum, optimizing performance for each optical engine. Both engines are mounted on an XY stage to achieve multi-resolution imaging in the XY plane. Depth-wise (Z-axis) multi-resolution is achieved by formulating a photopolymerizable resin incorporating two UV absorbers possessing distinct absorption spectra such that the different light spectra from the VHROE and MOE encounter disparate levels of absorption, resulting in 1/e penetration depths of 2 µm and 20 µm, respectively. This enables true multi-resolution printing in all three dimensions. Our method balances speed and resolution by selectively deploying the VHROE for ultra-fine features and the MOE for bulk structures within a single 3D print. To demonstrate the versatility of this technique, we fabricated intricate microfluidic structures, including a triply-periodic minimal surface (TPMS) with 7 µm pores embedded within a 150 µm × 150 µm cross section enclosed channel, and an ultra-compact microfluidic mixer with a printed volume of only 0.017 mm³ (17 nL) and a print time of 21 minutes. These examples underscore the potential of our multi-resolution 3D printing method for advancing microfluidic device fabrication.