(741d) Hydrodynamic Focusing Lithography (HFL)

Anisotropic multifunctional particles hold great potentials as building blocks [1] for dynamic meso-structures such as self-assembled tissues [2] and 3D electrical circuits [3], and have shown elegant ways to create complex 3D structures via particle folding in given stimuli [4]. Of particular interest, multifunctional particles with unique barcodes have been suggested as diagnosis tools for rapid screening of biomolecules [5]. For these applications, particle design is at least as important as size and requires a fabrication technique with precise control over shape and chemical patchiness [6, 7]. Methods currently used to generate multifunctional particles include microcutting [8], droplet-based microfluidics [9], photo resist-based lithography [10], and the PRINT method [11]. The morphology of particles prepared by microcutting [8] and droplet-based microfluidics [9] has been limited to spheres and cylinders. Although multilayer lithography [10] overcomes this limitation, the use of photoresist materials renders this approach suboptimal for many applications. While the PRINT method [11] has its strength in producing small sub-micrometer particles, to date multiphasic particles beyond a 1-D stripe have not been synthesized. Furthermore, during multifunctional particle synthesis, the technique needs multiple steps and does not provide flexibility as particle shapes are restricted to the pre-defined stamping molds. Here, we introduce a new method called hydrodynamic focusing lithography (HFL) that harnesses flow focusing to create stacked flows in two-layered channels for particle synthesis. Contrary to our prior flow lithography techniques [12, 13] to produce multilayered particles, here the fluid interface can be perpendicular to the UV light propagation direction and precise mask alignment at the interface is no longer needed. This change in geometry also allows us to polymerize 2-D arrays, compared to 1-D in the prior method, which can increase throughput dramatically. In HFL, multiple flows can be stacked by increasing the number of inlets entering sequentially from the bottom layer of the device. Using such multi-flow stacking, we synthesized multiphasic particles with various aspect ratios. We also showed that multiple monomer streams can be simultaneously stacked in both the z- and y-direction leading to more complex particles than before. Another application of flow layering in flow lithography is the use of tuning fluids to easily and rapidly vary particle height in a channel with a fixed height. Finally, we demonstrate that particles prepared by HFL can be patterned with proteins on a specific layer. HFL is a compelling method as the technique is compatible with other flow lithographic methods such as Stop Flow Interference Lithography (SFIL) [14] and Lock Release Lithography (LRL) [15]. For example, the combination of HFL and LRL can lead to chemical patterning in all dimensions as LRL can provide chemical anisotropy of particles in x?y dimension. We believe that HFL can provide a powerful way to reach new complex particles.

References

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