Elastomeric Particles DOI: 10.1002/anie.201402471 Nucleation and Growth Synthesis of Siloxane Gels to Form Functional, Monodisperse, and Acoustically Programmable Particles** C. Wyatt Shields IV, Danping Sun, Kennita A. Johnson, Korine A. Duval, Aura V. Rodriguez, Lu Gao, PaulA. Dayton, and Gabriel P. López* Dedicated to Professor George M. Whitesides on the occasion of his 75th birthday Abstract: Nucleation and growth methods offer scalable means of synthesizing colloidal particles with precisely speci- fied size for applications in chemical research, industry, and medicine. These methods have been used to prepare a class of silicone gel particles that display a range of programmable properties and narrow size distributions. The acoustic contrast factor of these particles in water is estimated and can be tuned such that the particles undergo acoustophoresis to either the pressure nodes or antinodes of acoustic standing waves. These particles can be synthesized to display surface functional groups that can be covalently modified for a range of bioanalytical and acoustophoretic sorting applications. Biofunctional, polymeric particles in suspension have been used to isolate proteins, [1] sort cells, [2] and deliver drugs, genes, or small molecules. [3] An emerging use of these particles is in acoustofluidic systems, which use standing pressure waves for rapid filtration, [4] sorting, [5] and flow cytometry. [6] While the majority of laboratory and commercial particles display positive acoustic contrast behavior by focusing along the nodes of an acoustic standing wave, we have shown that elastomeric silicone particles can display negative acoustic contrast behavior and can be used to isolate small bioanalytes and cells to the pressure antinodes for sorting. [1a, 7] Previously, we have synthesized elastomeric particles using rapid homog- enization techniques; however, these methods typically suffer from poor control over particle size and size dispersity. [7b] As an alternative, we and other groups have synthesized elastomeric particles using microfluidics, which can provide narrow size distributions, but suffer from low particle production rates. [1a, 8] We have implemented methods for the nucleation and growth of alkoxysilane and silicon alkoxide monomers to develop a class of functional, monodisperse, and acoustically programmable (FMAP) particles that can be synthesized in bulk (Figure 1). These particles 1) can display ample surface groups (for example, vinyl and acrylate) for high-yield conjugations; 2) exhibit narrow size distributions (less than 15 % CV); and 3) exhibit tunable acoustic behaviors (that is, positive or negative acoustic contrast factors, f) for versatile manipulations in acoustofluidic systems. FMAP particles can display f > 0 or f < 0 behavior by migrating to the node(s) or antinodes of an acoustic standing wave, respectively. The f is dependent on the density and compressibility of the particle and its suspending fluid; particles with higher densities and Figure 1. Silicone-based particles formed by nucleation and growth synthesis can a) contain various functional groups (for example, hydroxy, vinyl, and acrylate) for surface modification, b) exhibit narrow size distributions, and c) display tunable acoustic properties. [*] C. W. Shields IV, D. Sun, Prof. G. P. López Department of Biomedical Engineering Duke University, Durham, NC (USA) E-mail: gabriel.lopez@duke.edu C. W. Shields IV, Dr. K. A. Johnson, Dr. L. Gao, Prof. G. P. López NSF Research Triangle Materials Research Science and Engineering Center (USA) Dr. K. A. Johnson, A. V. Rodriguez, Prof. P. A. Dayton Department of Biomedical Engineering University of North Carolina, Chapel Hill, NC (USA) K. A. Duval, Dr. L. Gao, Prof. G. P. López Department of Mechanical Engineering and Materials Science Duke University, Durham, NC (USA) [**] This work was supported by the NSF’s Research Triangle MRSEC (DMR-1121107) and an NSF Graduate Research Fellowship (1106401) to C.W.S. The authors thank Alexander A. Doinikov for his assistance with the theoretical considerations of particles in an acoustic field. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201402471. . Angewandte Communications 8070  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2014, 53, 8070 –8073