Microfluidic Assembly of Magnetic Hydrogel Particles with Uniformly Anisotropic Structure

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Original entry: Darren Yang, AP225, Fall 2010

Reference

C.H. Chen, A.R. Abate, D. Lee, E.M. Terentjev, D.A. Weitz, “Editing Microfluidic Assembly of Magnetic Hydrogel Particles with Uniformly Anisotropic Structure,” Advanced Materials, 21, 3201–3204 (2009).

Keywords

Drop and bubble formation; Magnetic fluids and ferrofluids; Microemulsions; Micro-and nano-scale flow phenomena

Summary

The authors present a method to produce monodisperse magnetic particles from double emulsions formed using sequential flow-focus drop formation. This microfluidic drop formation allows the particles to be formed with high monodispersity and with consistently anisotropic internal structure. This structural anisotropy gives rise to magnetic anisotropy, allowing the particles to be rotated by a magnetic field.

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Background

Gel particles with the incorporation of an inorganic material are used in a wide range of application. For example, microparticles consisting of magnetic materials encapsulated in a polymer matrix are of great interest in drug deliver, image enhancement, and much more. There are various polymerization methods such as microemulsion and suspension polymerization were establish and implemented to produce these particles. However, these traditional techniques have limitation on size and morphology.

The authors present an alternative technique utilizing microfluidics devices. Magnetic hydrogel particles with uniform anisotropic internal structure were produced by a flow focusing drop make using double emulsions as templates.

Methods and Results

The droplets consist of the hydrophobic monomer core with magnetic material (black) that is encapsulated by a hydrophilic monomer droplet (white) suspended in fluorocarbon oil (gray). By increasing the inner flow rate, two magnetic cores could be encapsulated in the droplets, which demonstrates excellent control of emulsion morphology by the microfludics devices (Figure 1).

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Figure 1. Scheme of the PDMS device for forming double emulsion droplets. The scale bar for the two images corresponds to 50mm.


Once polymerized (under UV light), the magnetic gel particles were robust and could be washed, dried, and re-dispersed into water (Figure 2).

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Figure 2. a) Particles with a single magnetic core, and b) with two magnetic cores, in fluorocarbon oil. c) Single-core particles dried, and d) re-dispersed in water. The images in the second row show magnified views of the same particles. The scale bar in the top row is 50 mm and in the bottom row is 20 mm.


These particles, which has the properties of the biocompatibility of hydrogel, have the ability to be manipulated and rotated on the micro-scale using an externally applied magnetic field. This makes them useful for sorting applications, as contrast enhancers in magnetic imaging and for micro-mixing applications. The authors demonstrated the magnetic response of the particles, we observed their rotation in the presence of a magnetic field (Figure 3).

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Figure 4. The superposed vector field of fluid velocity a) was obtained by recording trace colloid movements over the full rotation cycle. The scale bar is 50mm; the X on the particle sketch marks the rotation axis. Plot b) shows the azimuthal average of flow velocity around the particle, showing that the highest flow is at 75mm from the rotation axis.