Hydrodynamic Focusing Lithography

Flow lithography for structured microparticles: fundamentals, methods and applications

Structured microparticles, with unique shapes, customizable sizes, multiple materials, and spatially-defined chemistries, are leading the way for emerging 'lab on a particle' technologies. These microparticles with engineered designs find applications in multiplexed diagnostics, drug delivery, single-cell secretion assays, single-molecule detection assays, high throughput cytometry, micro-robotics, self-assembly, and tissue engineering. In this article we review state-of-the-art particle manufacturing technologies based on flow-assisted photolithography performed inside microfluidic channels. Important physicochemical concepts are discussed to provide a basis for understanding the fabrication technologies. These photolithography technologies are compared based on the structural as well as compositional complexity of the fabricated particles. Particles are categorized, from 1D to 3D particles, based on the number of dimensions that can be independently controlled during the fabrication process. After discussing the advantages of the individual techniques, important applications of the fabricated particles are reviewed. Lastly, a future perspective is provided with potential directions to improve the throughput of particle fabrication, realize new particle shapes, measure particles in an automated manner, and adopt the 'lab on a particle' technologies to other areas of research.

Fabrication of 3D concentric amphiphilic microparticles to form uniform nanoliter reaction volumes for amplified affinity assays

Reactions performed in uniform microscale volumes have enabled numerous applications in the analysis of rare entities (e.g. cells and molecules). Here, highly monodisperse aqueous droplets are formed by simply mixing microscale multi-material particles, consisting of concentric hydrophobic outer and hydrophilic inner layers, with oil and water. The particles are manufactured in batch using a 3D printed device to co-flow four concentric streams of polymer precursors which are polymerized with UV light. The cross-sectional shapes of the particles are altered by microfluidic nozzle design in the 3D printed device. Once a particle encapsulates an aqueous volume, each "dropicle" provides uniform compartmentalization and customizable shape-coding for each sample volume to enable multiplexing of uniform reactions in a scalable manner. We implement an enzymatically-amplified immunoassay using the dropicle system, yielding a detection limit of <1 pM with a dynamic range of at least 3 orders of magnitude. Multiplexing using two types of shape-coded particles was demonstrated without cross talk, laying a foundation for democratized single-entity assays.

Two-Photon Vertical-Flow Lithography for Microtube Synthesis

Two-photon vertical-flow lithography is demonstrated for synthesis of complex-shaped polymeric microtubes with a high aspect ratio (>100:1). This unique microfluidic approach provides rigorous control over the morphology and surface topology to generate thin-walled (<1 µm) microtubes with a tunable diameter (1-400 µm) and pore size (1-20 µm). The interplay between fluid-flow control and two-photon lithography presents a generic high-resolution method that will substantially contribute toward the future development of biocompatible scaffolds, stents, needles, nerve guides, membranes, and beyond.

Multiplexed immunoassay using post-synthesis functionalized hydrogel microparticles

In response to a growing demand for simultaneous detection of multiple proteins in a single sample, multiplex immunoassay platforms have emerged at the forefront of proteomic analysis. In particular, detections using graphically encoded hydrogel microparticles synthesized via flow lithography have received attention for integrating a hydrogel, a substrate that can provide enhanced kinetics and high loading capacity, into the bead-based multiplex platform. Currently, the method of microparticle functionalization involves copolymerization of antibodies with the gel during particle synthesis. However, its practical operation is too precarious to be adopted because antibodies are susceptible to aggregation due to incompatibility with hydrophobic photoinitiators used in the photo-induced gel polymerization. In this work, we present a multiplex immunoassay platform that uses encoded hydrogel microparticles that are functionalized after particle synthesis by conjugating antibodies with remnant active groups readily available in the hydrogels. The method not only precludes antibody aggregation but also augments the loading density of the antibodies, which translates into enhanced detection performance. In addition to multiplexing, our platform demonstrates high sensitivity, a broad assay range, and a fast detection rate that outperform the enzyme linked immunosorbent assay (ELISA).

Surface-templated hydrogel patterns prompt matrix-dependent migration of breast cancer cells towards chemokine-secreting cells

This paper describes a novel technique for fabricating spatially defined cell-laden collagen hydrogels, using patterned, non-adhesive polyacrylamide-coated polydimethylsiloxane (PDMS) surfaces as a template. Precisely patterned embedded co-cultures of breast cancer cells and chemokine-producing cells generated with this technique revealed matrix-dependent and chemokine isoform-dependent migration of cancer cells. CXCL12 chemokine-secreting cells induce significantly more chemotaxis of cancer cells when the 3-D extracellular matrix (ECM) includes components that bind the secreted CXCL12 chemokines. Experimental observations using cells that secrete CXCL12 isoforms with different matrix affinities together with computational simulations show that stronger ligand-matrix interactions sharpen chemoattractant gradients, leading to increased chemotaxis of the CXCL12 gradient-sensing CXCR4 receptor-expressing (CXCR4+) cells patterned in the hydrogel. These results extend our recent report on CXCL12 isoform-dependent chemotaxis studies from 2-D to 3-D environments and additionally reveal the important role of ECM composition. The developed technology is simple, versatile and robust; and as chemoattractant-matrix interactions are common, the methods described here should be broadly applicable for study of physiological migration of many different cell types in response to a variety of chemoattractants.

Designer hydrophilic regions regulate droplet shape for controlled surface patterning and 3D microgel synthesis

A simple technique is presented for controlling the shapes of micro- and nanodrops by patterning surfaces with special hydrophilic regions surrounded by hydrophobic boundaries. Finite element method simulations link the shape of the hydrophilic regions to that of the droplets. Shaped droplets are used to controllably pattern planar surfaces and microwell arrays with microparticles and cells at the micro- and macroscales. Droplets containing suspended sedimenting particles, initially at uniform concentration, deposit more particles under deeper regions than under shallow regions. The resulting surface concentration is thus proportional to the local fluid depth and agrees well with the measured and simulated droplet profiles. A second application is also highlighted in which shaped droplets of prepolymer solution are crosslinked to synthesize microgels with tailored 3D geometry.