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The Synthesis and Assembly of Polymeric Microparticles Using Microfluidics
By Dhananjay Dendukuri, and Patrick S. Doyle*
The controlled synthesis of micrometer-sized polymeric particles bearing features such as nonspherical shapes and spatially segregated chemical properties is becoming increasingly important. Such particles can enable fundamental studies on self-assembly and suspension rheology, as well as be used in applications ranging from medical diagnostics to photonic devices. Microfluidics has recently emerged as a very promising route to the synthesis of such polymeric particles, providing fine control over particle shape, size, chemical anisotropy, porosity, and core/shell structure. This progress report summarizes microfluidic approaches to particle synthesis using both droplet- and flow-lithography-based methods, as well as particle assembly in microfluidic devices. The particles formed are classified according to their morphology, chemical anisotropy, and internal structure, and relevant examples are provided to illustrate each of these approaches. Emerging applications of the complex particles formed using these techniques and the outlook for such processes are discussed.
The use of polymeric particles can be traced back to the ancient Mayans who used natural rubber – a suspension of polymeric microparticles – for a variety of applications. In the past century polymer science witnessed an explosive growth, resulting in the discovery and development of a number of new synthetic polymers. Dispersions of particles made from several of these polymers are now commonly used to provide effective protection, binding and finishing to a number of industrial products such as paper, metals and wood. Gradually, polymeric particles have also found use in high value biological and analytical applications including as column supports for chromatography, beads for flow cytometry and in the recovery of DNA and proteins. The use of polymeric particles has spread from applications requiring bulk quantities of particles to niche applications in photonics, diagnostics and tissue engineering where the properties of each individual particle are critical to their technological function. With this, the requirements on particle monodispersity,
chemistry, porosity, shape and size are becoming increasingly stringent.
Currently, the most common approach to the synthesis of dispersions of polymeric particles at the colloidal length scale is emulsion polymerization. In a typical indus- trial reactor, a monomer is emulsified in an aqueous solution containing a suitable surfactant and an initiator molecule. Upon heating this mixture, particles are first nucleated from surfactant micelles and then continue to grow in size until the desired diameter is reached. The reaction is termi- nated at an appropriate time to obtain particles of a desired size across the colloidal length scale; up to a few micrometers. The predominant shape obtained is a sphere. Although spherical shapes are sufficient and indeed desirable for many applications, there has been a growing realization of the necessity for custom-designed, non- spherical particles for several applications.
For instance, particle-based assays are expected to compete with and even replace standard substrate-based assays such as enzyme-linked immunosorbent assays (ELISAs) in the future. This is due to their ability to perform multiple protein measurements using a single sample while at the same time reducing sample volume requirements. In such applications, tight monodispersity standards and the ability to provide for multiplexing by encoding a unique identity into each particle are essential to provide accurate measurements. A number of recent studies have also explored the use of particles as building blocks for the synthesis of complex structures. One promising application for polymeric particles here is to use them to build photonic crystals through the assembly of individual particles. These crystals possess the ability to selectively filter out certain wavelengths of light. In such applications, anisotropic particles that exhibit preferential self-assembly in one direction expand the range of crystal structures formed and are essential to providing finely tunable photonic band gaps. Further, there is a requirement for the development of techniques to controllably assemble such particles into organized superstructures. In the bottom-up approach envisioned to build the materials and devices of the future, precisely shaped and patterned ‘patchy’ particles will be essential to function as encoded building blocks that self- assemble into the required superstructure. There is also a need for spherical monodisperse polymeric particles in the range of several micrometers and above for chromatography and liquid
[*] Prof. P. S. Doyle, Dr. D. Dendukuri 66-270, 77 Massachusetts Avenue Department of Chemical Engineering MIT, Cambridge MA 02139 (USA) E-mail: firstname.lastname@example.org
DOI: 10.1002/adma.200803386 Adv. Mater. 2009, 21, 1–16
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1
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