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

Abstract

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.

FIG. 1.

(a) The IS-printer assembly demonstrating the arrangement of the motorized light engine stage connected to a power supply and other essential electrical components. (b) A computer-aided design (CAD) model of the IS-printer, equipped with a motorized light engine stage and microfluidic co-flow system. (c) The IS-3DP printing procedure consisted of an aqueous two-phase system using PEGDA as a photopolymer and dextran as a blocking solution. The printing procedure includes a simple two-step loop, the layer formation followed by polymerization. The position of the light engine is adjustable and includes a configurable mask for focus and pattern control. (d) A dual-syringe pump system connected to the microfluidic device and synchronized with the light engine, enabling control of the printing layer thickness through flow rate adjustments. LE, light engine. (e) Image of the fabricated microfluidic chip that hosts the vertically aligned ATPS co-flow. The main microchannel is 1 mm in width, 2 mm in depth, and 40 mm in length.

FIG. 2.

The IS-3DP printing procedure. The procedure includes two key steps: step 1, layer formation, followed by step 2, polymerization. Layer formation uses a specific combination of flow rates of the two phases (shown in navy and yellow colors). By initiating masked light exposure in the polymerization step, the multi-phase flow-established printing layer can be polymerized through. These two steps can be repeated to form three-dimensional structures.

FIG. 3.

(a) Phase diagram showing operation zones with varying flow rates. Flow rate combinations used for testing are marked with a black checkmark. (b) Flow profile analysis in-silico (COMSOL) against experimental. The color coding represents the interface phase change in the simulation with Dex representing phase 1, denoted in red, and PEGDA representing phase 0, denoted in blue. The experimental data show the average normalized interface height and error bar from four technical repeats.

FIG. 4.

(a) Experimental data showing three different printed layer thicknesses under different inlet flow rate ratios. Channel walls are boxed in red, while the printed structure is boxed in green. Two different masks with same rectangular shape but different lengths were used: mask 1 for thickness 1 (bottom panel) and mask 2 for thicknesses 2 and 3 (top and middle panel). (b) Interface height in the ATPS co-flow profile was included to compare with the thickness of the polymerized structure.

FIG. 5.

(a) A COMSOL simulation comparing interface height in two scenarios, the distance between the interface and the channel wall and the distance between interface and the edge of an internal structure. (b) 3D image of a three-layer printed structure in the microchannel using the IS-3DP strategy. The 3D perspective image was generated through z-stacking of ten image slices. The thicknesses of the first, second, and third printed layers were measured to be 194, 299, and 314 μm, respectively.