(440d) Adaptive, in-Situ 3D Printing Using Multiphase Microfluidic Control Enables Multi-Materials Integration

The development of 3D printing technologies has heralded a new era in microfabrication, enabling complex designs inaccessible to conventional soft lithography methods. Among various 3D printing techniques, Digital Light Processing (DLP) is the most popular method to fabricate micro-scale structures due to its capability to create high-resolution features (tens of micrometers) at a much lower cost. For example, DLP-based techniques have been reported to create microfluidic devices with complex 3D structures and fluid-actuating parts such as on-chip pumps and valves. However, the focus of most studies remains predominantly on the creation of mechanical structures using photo-curable resins. The integration of functional materials in DLP-type 3D printing, such as conductive polymers and functional hydrogels, is limited. This is because DLP 3D printing relies on the layer-by-layer manufacturing of a prime material in a resin bath. While multi-material printing methods exist, they typically require the frequent substitution of the primary photopolymer material—a process that is not only labor-intensive but also potentially detrimental to the print's quality, as each introduction of new material requires the prints to be cleansed with solvents. Additionally, the spatial integration of multiple materials is limited, with the embedding of functional materials within the existing void features particularly challenging.

To address this challenge, we report an innovative DLP-based 3D printing approach, termed adaptive in-situ 3D printing. This approach facilitates the dynamic modulation of the printing layer in the third dimension, the Z-direction, within a microfluidic channel, leveraging the distinct advantages of multiphase laminar flow. Instead of using the VAT and build plate to create the printing layer, we use flow focusing of a photo-curable resin pre-polymer stream in a microchannel to precisely define the printing layer. By modulating the thickness of the resin pre-polymer stream, we can dynamically adjust the height of the print layer based on the design to minimize the fabrication time and the unintended crosslinking. To ensure the precise pattern on the XY-plane, an innovative self-focusing step is incorporated, leveraging image-based feedback alongside precise z-axis control to fine-tune the focal printing plane. This is complemented by an automated flow system for exchanging printing reagents, facilitating the sequential DLP printing of diverse functional materials within the microfluidic channel. Our approach enables the formation of intricate three-dimensional structures embedded with a broad spectrum of photopolymerizable materials in void spaces—overcoming one of the significant challenges faced by conventional stereolithography and soft-lithography methods.

We demonstrate the utility of our technique by integrating an ion-exchange polyelectrolyte and conducting polymer electrode in a microfluidic channel to achieve a field-flow fractionation device for biomolecule extraction. The integration of materials directly on-chip using our 3D printing method significantly enhances the electric field's coupling efficiency within the microchannel and make it possible to drive the electrophoretic biomolecule separation with low-power batteries. We envision that our technique could open new avenues for sophisticated on-chip functionalities in microfluidic sensor and biomedical applications.