Hyper-Miniaturisation of Monodisperse Janus Hydrogel Beads with Magnetic Anisotropy Based on Coagulation of Fe₃O₄ Nanoparticles

Kyouhei Aketagawa, Hirotada Hirama, Toru Torii

Department of Human and Engineered Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, Ka-shiwa-shi, Japan.

DOI: 10.4236/msce.2013.12001                

Abstract

We describe a simple and robust technique for fabricating monodisperse Janus hydrogel beads with magnetic anisotropy smaller than the microchannel used to produce them.

Keywords

Magnetic Janus Particles; Microchannels; Calcium Alginate Gel; Monodisperse; Osmotic Pressure; Coagulation

1. Introduction

Janus particles are anisotropic microparticles with hemispherically distinct geometries. Janus particles with magnetic anisotropy have attracted much attention and can be used in electronic paper [1,2], rheological probes [3], self-assembly [4-6] and biomedical applications [7-9] because they can be rotated and precisely manipulated by an external magnetic field [1-9]. The mono-dispersity of the Janus particles enhances the performance of such applications. Therefore, many studies have focused on monodisperse biphasic droplet formation techniques using microchannels for the fabrication of Janus particles with magnetic anisotropy [2,6-8].

With respect to electronic paper using Janus particles, a decrease in the thickness of the electronic paper can allow for operation at a low electric voltage, and the thickness depends on the size of the particles [10]. For cell encapsulation, the cell is strongly affected by the size of the capsules [11]. Therefore control of the particle size is needed. However, there are limits to the minimum monodisperse biphasic droplet size that can be fabricated using microchannels because the microchannel size has a dominant effect on the monodisperse droplet size [12-15] and the risk of clogging a smaller microchannel is higher [2,16]. Moreover, convective mixing across the interface inside the small biphasic droplets is enhanced, leading to fabrication of Janus particles with a blurred boundary or completely homogeneous particles [3,17,18]. It is therefore difficult to fabricate small Janus particles with magnetic anisotropy.

In our previous work, a new shrinkage-gelation technique was developed in which small monodisperse alginate-inorganic composite particles were fabricated from larger sodium alginate droplets containing inorganic nanoparticles [19-21]. In this method, monodisperse sodium alginate droplets containing inorganic nanoparticles formed in microchannels are placed at the interface between a high-viscosity oil with a polymeric surfactant and a high-strength hydrogel containing calcium chloride. Because there is a difference in osmotic pressure between the alginate droplets and the Ca²⁺-loaded hydrogel, water molecules in the droplets transfer to the hydrogel, [22] resulting in the shrinkage of the droplets. The droplets then solidify due to transfer of calcium ions from the Ca²⁺-loaded hydrogel to the droplets [23,24].

In this study, we report a simple and robust technique for fabricating monodisperse Janus hydrogel beads with magnetic anisotropy at a size that is smaller than the microchannel used to produce them based on the coagulation of Fe₃O₄ nanoparticles and the shrinkage-gelation technique, which can avoid the problems of clogging microchannels and fabricating Janus particles with a blurred boundary due to convective mixing.

2. Experimental

We fabricated a microchannel made of polydimethylsiloxane (PDMS) via multilayer photolithography [25]. As shown in Figure 1, the microchannel consisted of a Yshaped dispersed phase channel measuring 100 μm in width and depth and a sheath-flow junction. The sheathflow structure was composed of two continuous phase channels measuring 100 μm in width and depth and a channel measuring 200 μm in width and depth.

We first prepared a Ca²⁺-loaded agarose gel plate. A solution of 2 w/v% agarose S (Wako Pure Chemical Industries, Ltd., Japan) was prepared in calcium chloride aqueous solution (0.5, 1.0, 1.5 or 2.0 mol/L) at approximately 95˚C, and the solution was poured into a polystyrene dish and solidified at 25˚C. Subsequently, an oil/ hydrogel interface was formed by pouring 0.8 mL/well of sunflower oil (Showa Sangyo Co., Ltd., Japan) supple

Figure 1. Fabrication method of small Janus hydrogel beads with magnetic anisotropy. (Top) Two-phase parallel stream forms biphasic droplets. (Bottom) Small magnetic Janus alginate gel beads are fabricated on the Ca²⁺-loaded agarose gel plate.

mented with polymeric surfactant (polyglycerol esters of fatty acids, SY GLYSTER CRS-75, Sakamoto Yakuhin Kogyo Co., Ltd, Japan) onto the agarose gel plate.

In the method presented here, we induce coagulation by electrolyte addition during the fabrication of Janus hydrogel beads with magnetic anisotropy. When Fe₃O₄ nanoparticles are suspended in an aqueous solution, electric double layers form spontaneously around the nanoparticles and a repulsion barrier generated by the electric double layers prevents the nanoparticles from aggregating [26]. Because the thickness of the double layers is inversely proportional to the concentration of ions in the aqueous solution, the repulsion between the nanoparticles decreases as the concentration of electrolyte added in the aqueous solution increases, resulting in aggregation of the nanoparticles by the van der Waals force between them [26].

Hofmeister examined the concentrations of the thirteen types of sodium salts necessary to precipitate colloidal ferric oxide suspended in an aqueous solution and discovered that sodium citrate can aggregate and precipitate ferric oxide at the lowest concentration [27]. Based on these results, we examined the concentration of sodium citrate necessary to precipitate Fe₃O₄ nanoparticles measuring 10 nm in mean diameter (Ferrotec Corporation, Japan) suspended in water. In the case that the density of Fe₃O₄ nanoparticles was 1.0 w/v%, the minimum concentration of sodium citrate for completely precipitating Fe₃O₄ nanoparticles was approximately 0.01 mol/L.

The fabrication method for small Janus hydrogel beads with magnetic anisotropy is shown in Figure 1. A 1.0 w/w% sodium alginate aqueous solution containing 0.02 mol/L sodium citrate and a 1.0 w/w% sodium alginate aqueous solution containing 1.6 w/v% paramagnetic Fe₃O₄ nanoparticles were injected separately through two inlets resulting in a two-phase parallel stream in the microchannel due to the low Reynolds number. This stream then entered a continuous flow of mineral oil (SigmaAldrich Co., Ltd., USA) with a polymeric surfactant (SY GLYSTER CRS-75), and biphasic droplets were formed. Convective mixing immediately occurred across the interface inside the biphasic droplets. Next, sodium alginate droplets containing both Fe₃O₄ nanoparticles and sodium citrate were formed. The Fe₃O₄ nanoparticles aggregated in the presence of sodium citrate and electrolyte, and they precipitated at the bottom of the droplets (Figure 1, bottom). The droplets were placed on the Ca²⁺-loaded agarose gel plate, resulting in fabrication of small Janus alginate gel beads with magnetic anisotropy.

3. Results and Discussion

Figure 2(a) shows the monodisperse sodium alginate

Conflicts of Interest

The authors declare no conflicts of interest.

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