In situ 3D polymerization (IS-3DP): Implementing an aqueous two-phase system for the formation of 3D objects inside a microfluidic channel.

Rapid prototyping and fabrication of microstructure have been revolutionized by 3D printing, especially stereolithography (SLA) based techniques due to the superior spatial resolution they offer. However, SLA-type 3D printing faces intrinsic challenges in multi-material integration and adaptive Z-layer slicing due to the use of a vat and a mechanically controlled Z-layer generation. In this paper, we present the conceptualization of a novel paradigm which uses dynamic and multi-phase laminar flow in a microfluidic channel to achieve fabrication of 3D objects. Our strategy, termed "in situ 3D polymerization," combines in situ polymerization and co-flow aqueous two-phase systems and achieves slicing, polymerization, and layer-by-layer printing of 3D structures in a microchannel. The printing layer could be predicted and controlled solely by programming the fluid input. Our strategy provides generalizability to fit with different light sources, pattern generators, and photopolymers. The integration of the microfluidic channel could enable high-degree multi-material integration without complicated modification of the 3D printer.