Building new materials in drops
Production of emulsions from viscous fluids
We developed a microfluidic filter that enables the production of emulsion drops of a defined size from fluids with viscosities up to 800 times higher than that of water at a high throughput. We form a crude emulsion through mechanical shaking and inject it through a single inlet. The polydisperse emulsion drops are pushed through an array of regularly arranged posts; thereby, they are broken into much smaller drops with a narrower size distribution, as shown in Figure 1. The average drop size is determined by the spacing between adjacent posts and the device height and can thus easily be tuned with the device design. The simplicity of the device makes its operation robust. Furthermore, if the device is operated at Capillary numbers > 0.03, the drop size distribution is nearly independent on the flow rate of the fluids; the microfluidic filter than therefore easily be up-scaled.
Figure 1: Optical microscopy images of the microfluidic filter. A crude emulsion is injected into the device through a single inlet. The drops consist of a fluid with a viscosity 800 times higher than that of water. They are broken at the posts into many much smaller drops that exit the device through a single outlet.
Photo- and Thermo-Responsive Polymersomes for Triggered Release
Polymersomes are vesicles consisting of amphiphilic block-co-polymers. They can be designed to have a low permeability for encapsulants during storage which makes them attractive delivery vehicles. However, their low permeability makes the release of encapsulants often inefficient. To overcome this difficulty in their release, it would be desirable to build capsules with a low permeability during storage and a high permeability once applied. To simultaneously satisfy these seemingly contradictory requirements, polymersomes must be made responsive. In collaboration with Shin-Hyun Kim, I assemble thermo-responsive polymersomes consisting of a thermo-responsive block-co-polymer with a non-responsive block-co-polymer using double emulsion drops as templates. We assemble monodisperse water/oil/water double emulsion templates using glass capillary devices. We disperse the amphiphilic block-co-polymer in the oil phase; these polymers assemble at the two liquid/liquid interphases. Upon spontaneous removal of the oil through a dewetting process, polymersomes form. The thermo-responsive block-co-polymer imparts thermo-responsiveness to them. We further functionalize thermo-responsive polymersomes by imbedding hydrophobic gold nanoparticles into the hydrophobic part of their membrane. These photo-responsive polymersomes disintegrate when illuminated with a laser and thus instantaneously release all the encapsulants, as shown in Figure 2.
Figure 2: (a) Schematic illustration of thermo-and photo-responsive polymersomes. (b) Confocal microscopy images of photo-responsive polymersomes that disintegrate upon illumination with laser light and thereby instantaneously release all the encapsulants.
Assembly of thermoresponsive capsules with an upper critical solution temperature
Most thermo-responsive capsules are composed of polymers with a lower critical solution temperature (LCST), such as poly(N- isopropylcrylamide) (PNIPAM). These capsules collapse if heated above their LCST. However, for many applications that require triggered release, it would be beneficial to have capsules that swell if heated. We are working on assembling thermo-responsive capsules from polymers that have an upper critical solution temperature (UCST). We produce these capsules from double emulsion templates assembled using microfluidic drop makers. We dissolve encapsulants in the inner aqueous phase that also contains high concentrations of dextran. We dissolve monomers in the middle, aqueous phase; since monomers are immiscible in dextran, the two aqueous phases become immiscible if the concentrations of dextran and the monomers are sufficiently high. We use a perfluorinated oil, containing a perfluorinated surfactant as an outer phase. We can closely control the size and shell thickness of the water/water/oil double emulsion templates by controlling the channel dimensions and the relative flow rates of the fluids, as shown in Figure 3. We solidify the shell of the double emulsion by polymerizing the monomers contained in it and obtain thermo-responsive capsules of a well-defined size and composition.
Figure 3: Confocal micrographs of thermoresponsive capsules consisting of a shell of polymers that have an upper critical solution temperature (UCST) and rhodamine B labelled dextran core. The shell thickness is tailored by controlling the flow rate of the inner and middle phases.