Smart Microgel Capsules from Macromolecular Precursors
Original entry: Darren Yang, AP225, Fall 2010
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,” J. Am. Chem. Soc., 132, 3201–3204 (2009).
Drop and bubble formation; Magnetic fluids and ferrofluids; Microemulsions; Micro-and nano-scale flow phenomena
Multiple layers Microgel particles and capsules are usually fabricated using droplet microfluidics; in those existing methods, emulsion templating forms layers of dissimilar polarity. The authors present a method that fabricates functional microgel capsules that consist of two miscible yet distinct layers. Briefly, they used microfluidic devices to template micrometer-sized drops that are loaded with prepolymerized precursors and solidify them through a polymer-analogous reaction.
These drops-in-drops emulsions are useful for many applications such as encapsulation and controlled release. Traditional method such as droplet microfluidics is a powerful method to form monodisperse double and higher order emulsions. However, such method is impossible to create distinct core−shell particles that consist of very similar materials with this technique, because their miscibility allows rapid interpenetration of the different layers in the preparticle droplets. Thus, in this paper, the authors present a method that can prevent marked interpenetration of the miscible layers. This method basically gelling the miscible layers from prepolymerized precursors, which do not intermix on the time scale of the experiments, and this allows us to produce microparticles which consist of distinct layers of poly(N-isopropylacrylamide) (pNIPAAm) and related polymers.
Methods and Results
The core−shell particles were proceed in a stepwise manner (Figure 1a). First, monodisperse microgel particles were created to serve as the core material. Then, they are wraped into monodisperse polymer shells using a microfluidic device. In the first junction, semidilute solution of cross-linkable pNIPAAm chains was added as the shell phase. In the second junction, oil was added to form bilayered pregel drops. Finally, the pNIPAAm chains in the shell phase were cross-linked to prevent interpenetration between the monodisperse microgel particles (Figure 2).
Figure 1. (a) Schematic of a microfluidic device forming aqueous poly(N-isopropylacrylamide) (pNIPAAm) droplets that are loaded with a well-defined number of prefabricated particles of a similar material, pNIPAAm or polyacrylamide. (b, c) Adjusting the flow rates of the inner particle phase (red-tagged pNIPAAm), the middle polymer phase (green-tagged pNIPAAm), and the outer oil phase controls the number of core particles in each shell. (d) Spatially resolved intensity profiles of the red and green fluorescence in the single-core particle.
Figure 2. UV-Induced Crosslinking of Dimethylmaleimide.
The author then demonstrated that this method can fabricate particles that consist of a non-thermoresponsive core nested in a thermoresponsive shell. It was accomplish by incorporating non-thermoresponsive polyacrylamide (pAAm) particles into thermoresponsive pNIPAAm shells. The behavior of these pAAm−pNIPAAm particles upon increase of the temperature to 35 °C is shown in Figure 3a and detailed in Figure 3b.
Figure 3. Thermoresponsive behavior of core−shell microgels.
Soft Matter Connection
A soft matter aspect for the formation of distinct core−shell structures is to prevent intermixing of the two phases. The authors' strategy to solve this problem is to use prepolymerized precursors in the shell phase, which encapsulates the cross-linked core particles. Since the diffusivity of polymer chains in semidilute solutions and networks is generally slow, so on the time scale of the experiments there is no interpenetration of the shell material into the core.