Micro!uidic fabrication of smart microgels from macromolecular precursors

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Authorship

Title of Original Work: Mirofluidic Fabrication of Smart Microgels from Macromolecular Precursors

Journal of Original Work: Polymer, Volume 51, Pages 5883-5889, October 16, 2010

Authors: Sebastian Seiffert, David A. Weitz

Author of Review: Joseph Muth - AP 225 - Fall 2012 - 11/27/2012

Figure 1. Typcial reaction map for pNIPAAm polymer precursors

Overview

Stimuli-responsive (smart) microgels are micron sized polymer particles that change shape in response to enviornmental stimuli. This responsiveness makes them useful in the fields of drug delivery, catalysis, sensing, and photonics. The most commonly used smart microgel material is poly(N-isopropylacrylamide) (pNIPAAm) because it has a lower critical solution temperature (LCST) around 32°C. Both the LCST and Tg of (pNIPAAm) are readily tunable by changing its composition or its particle geometry. As such controlling each of these aspects independently is critical to optimizing the characteristics of the microgel.

One way to achieve independent optimization is by using microfluidic techniques to template pre-fabricated precursor polymers. In this way simultaneous solidifcation and polymerization are avoided because gelation occurs via a polymeric analogous route instead of by monomeric chain growth. Using prefabricated precursor polymers, allows the molecular structure to be tuned through polymer synthesis, while microfluidics allows the shape to be accurately and precisely controlled. In this work, the author explores various ways to exploit this smart microgel fabrication technique.

Experimental

Figure 2. Typical microfluidic device for the production of monodisperse polymer precursor droplets. The droplets are gelled after UV exposure.

The formation of microgels from precursor polymers is illustrated by the sequence of reactions in Figure 1, where pNIPAAm is prepared from N-isopropylacrylamide and dimethylmaleimide. The resulting pNIPAAm is then photocrosslinked. Molecular weight is controlled by running the pNIPAAm precursor is controlled by running the copolymerization step in the presence of sodium formate. This synthesis route allows the weight average molecular weight to be controlled between 100,000-2,000,000 g/mol.

Once the precursor polymers are formed, microfluidic devices are used to create monodisperse drops (Figure 2). The polymer precursor drops are then gelled via photocrosslinking. By separating particle formation from the polymer synthesis step, highly tailored, functionalized polymers can effectively be manufactured in a variety of particle geometries with minimal loss of functionality.

Examples

Figure 3. Florescence intensities for various ratios of red and green dyed polymer precursors. The reported intensities correspond exactly to the relative dye mixing ratios.

To illustrate the monodispersity and functionality of such systems, four different smart microgels were made with different ratios of precursor tagged with red and green dye. The ratios of red to green die were as follows: 15/0, 10/5, 5/10, 0/15 g/L. The gelled particles were then imaged with fluorescence microscopy and the relative intensity of each die was measured. Figure 3 shows that these results exactly reflect the initial fractions of red and green die within experimental error.

Microfluidic manipulation of precursor polymers can also be used to form more complex geometries besides sphere. In this way the spatial distribution of functionalization can be controlled. Janus microgels can be formed in which there are two distinguishable sides to the droplets. Janus microgels are formed by using a three polymer system within the microfluidic device as shown in Figure 4. At the first junction, the red, green, and untagged polymer solutions form a coflowing stream with a striped structure. At the second junction, oil breaks up the stream. As the oil passes through the stream, it creates convective flow. The convective flow disrupts the striped morphology, and forms a core-shell type structure, with oppositely colored hemispheres and an uncolored center. UV light is used to gel the polymer precursor drops in the core-shell state.

Fluid-in-Fluid emulsification can be extended to form higher order emulsions to template core shell microparticles. Core-shell morphologies are typically formed by layering alternate polarity precursors. However, if layers of similar polarity are desired, a step-by-step process must be employed. Initially a hydrogel microparticle is formed. Then a microfluidic device such as the set-up in Figure 5, is used to wrap the preformed hydrogel particles in aqueous polymer. The hydrogel in polymer stream is then split with an oil jet and the coated hydrogels are photocrosslinked. Thus microfluidic techniques enable the creation of both hetero-polar and homo-polar microparticles.

Figure 4. A) Example of a microfluidic device used to create Janus emulsions. B) Representative crosslinked microgels produced via the Janus technique. Janus microfluidics enable the polymer precursors to be segregated within the crosslinked droplet.

Conclusion

Smart microgels can be readily produced via microfluidic droplet templating and polymer precursor synthesis. By separating the polymer synthesis from droplet production, both functionality and morphology can be optimized. Using a variety of microfluidic technqiues, monodisperse, homogeneous spheres can be formed, Janus microparticles can be synthesized, multi-emulsion drops can be produced, and homo-polar microgels can be created. All of these strategies have practical applications in areas where the controlled release of substances is critical.