(401b) Microfluidics-Based Lithography for Fabricating Complex Microparticles and Their Directed Assembly

Microfluidic devices offer the ability to finely control physical and chemical conditions which is advantageous for materials synthesis. Several groups have used multi-phase microflows to produce microparticles and capsules. Surface tension limits these particles to be spheroids. The Doyle group recently showed that more complex shapes and also patterned chemistries within a particle can be attained by coupling microfluidics to photolithography [1]. Continuous-flow lithography (CFL) [1] and the more refined stop-flow lithography (SFL) [2, 3] can generate 2-D extruded particles of any desired shape. To date, SFL has only been demonstrated with limited chemistries and thus the resultant microparticles have limited functionality. In particular, the particles generated using SFL are 2D and any feature added in the third dimension would enhance the practical use of these particles. Further, up until now, SFL has been used to generate polymeric particles. The ability to generate charged or magnetic particles would not only demonstrate the versatility of the process, but also provide the option for the directed assembly of these particles under an applied field. In this work, we demonstrate the ability to generate curved particles, having curvature in the plane orthogonal to the plain of projection of light. We also present the synthesis of anisotropic magnetic microparticles and their assembly into predictable macrostructures under an applied magnetic field.

Polymeric particles of complex architecture are widely used for applications such as photonic materials [4], MEMS [5], biomaterials [6] and self-assembly [7]. Introducing complexity in particle design is important since particle shape can significantly influence particle function [8]. An important facet of shape complexity is the introduction of curvature. It has been shown by molecular simulations that the curvature of particles can be tuned to regulate assembly as demonstrated by viral assembly [9]. Experimental evidence has also been provided for the preferential standing positions of concavo-convex particles with clear analogy to brachiopod or pelecypod shell orientation in moderately turbulent water [10]. Techniques for generating truly 3D non-convex (concave) particles with precise control over particle curvature will therefore by interesting. The Thomas group recently introduced concave curvature in polymeric particles using holographic interference lithography (HIL) which to the best of our knowledge is the only work of its kind. However, at present, their technique is only capable of creating single chemistry particles [11]. The literature is flooded with the generation of spherical magnetic particles. Any attempt at synthesizing non-spherical magnetic particles has been confined to the generation of modifications of spheres i.e., disks and plugs [12]. Magnetic micro-particles of anisotropic shapes have however, not been synthesized by hitherto known techniques. The ability to generate anisotropic magnetic particles and assemble them into predictable macrostructures would find application in a wide range of areas, from photonics [13] to tissue engineering [14].

Microfluidic devices for SFL are made out of PDMS using standard techniques [1]. SFL consists of three steps stop (stopping the flow of polymer solution), polymerize (UV flash through a mask) and flow (flushing out generated microparticles using a pressure pulse). For generating curved microparticles, photocurable solution (polymerization fluid) and tuning fluid are co-flowed and then stopped. Depending on the relative surface energies, curvature develops at the interface between the immiscible fluids. Ultraviolet light is then projected through a mask, solidifying polymeric particles having the shape on the plane of projection of light determined by the mask and the shape in the plane orthogonal to the plane of projection of light determined by the equilibrium curvature. The section of the tuning fluid exposed to UV light does not get polymerized. The chemical programmability of this technique is demonstrated by synthesizing Janus, patched and capped polymeric particles. Finally, the particles generated are grouped into different classes based on 5 different anisotropic axis depending on the chemistries used, curvature and patchiness. Anisotropic magnetic particles were generated using our SFL system by passing a suspension of magnetic beads in poly (ethylene glycol) diacrylate (PEG-DA) through the microfluidic device and flashing UV light through a transparency mask. The magnetic particles are essentially 2D extruded with a shape in the plane of projection of light given by the shape on the mask and straight walls along the plane of projection of light. These particles assemble into different macrostructures on the application of a magnetic field based on the initial shape and aspect ratio of the particles.

This facile approach offers a new method for generating curved microparticles for myriad applications, including photonics, tissue engineering, scattering functions of aerosols, and cosmetics. It also demonstrates the ability to synthesize magnetic microparticles of different shapes and helps envision their assembly for a bottom synthesis of desired macrostructures with potential applications in a variety of fields ranging from bioengineering to photonics and advanced material synthesis.

References

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