Fabrication and manipulation of polymeric magnetic particles with magnetorheological fluid

Abstract

Polymeric magnetic microparticles have been created using a microfluidic device via ultraviolet (UV) polymerization of double emulsions, resulting in cores of magnetorheological (MR) fluids surrounded by polymeric shells. We demonstrate that the resultant particles can be manipulated magnetically to achieve triggered rupture of the capsules. This illustrates the great potential of our capsules for triggered release of active ingredients encapsulated in the polymeric magnetic microparticles.

Introduction

Functional composite particles, which are made of an inorganic material incorporated to a polymer network, are of great interest to biotechnology, optics and cosmetics [1]. Specifically, microparticles with a magnetic core are widely used in many applications such as treatment of hyperthermia [2], drug delivery [3], contrast enhancement for magnetic resonance imaging [4] and fluid flow modifications [5]. The magnetic properties of these materials make them susceptible to a non-uniform external field and enable their manipulation under controlled magnetic field. The high potential of these particles can be further realized by improving both the process of particle fabrication as well as the development of particles with new structures.

Microfluidic technologies offer an approach to create particles using emulsions as a template. With microfluidic devices, emulsion drops are fabricated one drop at a time. One way to prepare these drops is by co-flowing two immiscible fluids for droplet formation; the resultant particles have high size uniformity. In addition, the size and shape of the created droplets can be conveniently varied by controlling the flow rates of the fluids. Moreover, the approach also enables fabrication of higher-order emulsions such as water-in-oil-in-water (w/o/w) double emulsions. If in the emulsion one of the phases is a polymer, the emulsions could be used as a template for generation of polymeric particles [6].

To provide a magnetic behavior to the particles, one approach is to directly coat polymeric particles with a layer of magnetic nanoparticles through surface chemistry techniques [7] or dispersion polymerization [7], [8]. Another approach is to encapsulate magnetic nano- or micro- particles suspended in a carrier fluid in a polymeric matrix that form the particles. Common magnetic particle suspensions include magneto-rheological (MR) fluids and ferrofluids. MR fluids are suspensions of magnetic particles in a carrier liquid with some additives to stabilize the medium. Particle sizes are between 1 and 10 μm, which are too heavy for Brownian motion to keep them suspended, so due to the different densities between the particles and the carrier fluid, particles tend to sediment in MR fluids. Due to the high sensitivity of the magnetic particles, MR fluids exhibit a rapid response when a magnetic field is applied. Typical response time is on the order of milliseconds [9]. The fast response time makes MR fluids suitable as dampers and shock absorbers in applications ranging from helicopters to prosthetic legs [10], [11]. From the point of view of the inner structure, in the absence of an external field, a MR fluid is a suspension. When a magnetic field is applied; a magnetic dipole moment is induced in the particles parallel to the magnetic field, and, as a result, particles tend to align to form ordered structures. The application of this field causes the microstructure of the fluid to change from a liquid-like state to a more structured state, giving rise to significant variations not only in viscosity but also in its inner order as exhibited by the particle rearrangement. At sufficiently high particle concentration, the original medium turns from an isotropic state to an anisotropic state, where particles aggregate to form chain-like structures. However, ferrofluids primarily consist of nano-size particles suspended in a carrier liquid so they are suspended by Brownian motion and generally will not settle. When a magnetic field is applied, Brownian motion prevents formation of a solid structure, so the mechanical properties do not change as much as in a MR fluid.

In this work, polymeric magnetic microparticles are prepared using a microfluidic device via UV polymerization of double emulsions in which the core is a MR fluid and the shell is a polymer. We demonstrate magnetically triggered release of the MR fluids from the core-shell microparticles.