Microfluidic Assembly of Magnetic Hydrogel Particles with Uniformly Anisotropic Structure

Abstract

Functional gel particles produced through the incorporation of an inorganic material are used in a wide range of applications ranging from cosmetics to optics and biotechnology. Such particles with uniform morphologies are desirable because of their consistent and reproducible behavior. Particularly, microparticles consisting of magnetic materials encapsulated in a polymer matrix are of great interest in drug delivery, image enhancement, hyperthermia, sorting, separation, and immunoassays due to their ability to be manipulated remotely. Due to the sensitive response to an external field and its gradients, and bio-comparability, magnetic gel particles with anisotropic features are attractive in many domains of technology, such as electronic paper, rheological probe, and biomedical applications. Various polymerization methods, including microemulsions and suspension polymerization, were established and implemented to synthesize both polymeric and composite particles. However, these bulk methods do not offer control of particle morphology to achieve uniform distributions of inorganic inclusions in the polymer network, especially for magnetic materials. To generate particles with various controlled morphologies, several chemical methods were developed, based on manipulation of growth and nucleation of molecular species during precipitation, or the template-assisted manipulation of spherical particles. However, these techniques impose limitations on size and morphology, as well as material selection, due to the requirements for the initial particle formation. Thus, fabrication of anisotropic magnetic gel particles with uniform anisotropic features requires new techniques. Microfluidic devices provide an alternative technique for the generation of monodisperse droplets by coflowing two immiscible fluids to induce the drop formation. Many studies have successfully demonstrated microfluidic synthesis of microgels, polymeric particles, and composite particles for different purposes. However, most of them focus on making particles with homogeneous internal structure. Very few studies have been reported in which a microfluidic device was used to produce anisotropic magnetic gel particles, apart from recent important work. Hwang et al. have demonstrated magnetic manipulation of their homogenous particles; however, the control of particle rotation was still lacking. Recently, poly(dimethylsiloxane) (PDMS) microfluidic double emulsion devices have provided a straightforward and robust approach to forming highly monodisperse double emulsion droplets. The double emulsions generated by PDMS drop makers can be functionalized to form composite gel particles with anisotropic inhomogeneous internal structure for advanced applications. The use of microfluidics techniques using double emulsion templates allows making hydrogel particles with uniform and well-defined anisotropic features. In this paper, magnetic hydrogel particles with uniform anisotropic internal structure were produced by a flow focusing dropmaker using double emulsions as templates. These particles provide excellent rotational control by applying an external field. Moreover, with the advantage of double emulsion core-shell structure, the inorganic magnetic inserts were fully covered by a biocompatible polymer network, which allows the particles to be used in biomedical applications. Our device is made from a PDMS elastomer using softlithography methods. We coat the channels with a sol–gel layer that is functionalized with photoreactive silanes. This chemical treatment allows the wettability of the channels to be spatially patterned, and thus allows the formation of double emulsion droplets. The sol–gel mixture is prepared by combining tetraethylorthosilicate (TEOS) (0.2mL), methyltriethoxysilane (MTES) (0.2mL), (heptadecafluoro-1,1,2,2-tetrahydrodecyl) triethoxysilane (0.1mL), photoiniator-silane (0.2mL), water adjusted with HCl (0.2mL, pH 2), and methanol (2.6mL). The PDMS channels are first treated with plasma to form silanol groups and are then immediately bonded to a glass slide. These channels are filled with the photoreactive sol–gel mixture. The device is then placed on a hotplate set to 225 8C, which results in evaporation of the solvent and curing of the sol–gel on the channel walls. The sol–gel is fluorinated by fluorosilanes and is, therefore, very hydrophobic by nature. To spatially pattern the wettability, we graft hydrophilic patches of polyacrylic acid onto the channels. To accomplish this, we fill the sol–gel coated channels with an aqueous monomer solution comprising acrylic acid (0.2mL) with NaIO4 water (0.8mL, 5mM), ethanol (1mL), C O M M U N IC A IO N www.advmat.de