Ho Cheung Shum

Designer emulsions using microfluidics

We describe new developments for the controlled fabrication of monodisperse emulsions using microfluidics. We use glass capillary devices to generate single, double, and higher order emulsions with exceptional precision. These emulsions can serve as ideal templates for generating well-defined particles and functional vesicles. Polydimethylsiloxane microfluidic devices are also used to generate picoliter-scale water-in-oil emulsions at rates as high as 10 000 drops per second. These emulsions have great potential as individual microvessels in high-throughput screening applications, where each drop serves to encapsulate single cells, genes, or reactants.

Microfluidic fabrication of monodisperse biocompatible and biodegradable polymersomes with controlled permeability.

We describe a versatile technique for fabricating monodisperse polymersomes with biocompatible and biodegradable diblock copolymers for efficient encapsulation of actives. We use double emulsion as a template for the assembly of amphiphilic diblock copolymers into vesicle structures. These polymersomes can be used to encapsulate small hydrophilic solutes. When triggered by an osmotic shock, the polymersomes break and release the solutes, providing a simple and effective release mechanism. The technique can also be applied to diblock copolymers with different hydrophilic-to-hydrophobic block ratios, or mixtures of diblock copolymers and hydrophobic homopolymers. The ability to make polymer vesicles with copolymers of different block ratios and to incorporate different homopolymers into the polymersomes will allow the tuning of polymersome properties for specific technological applications.

Double emulsion templated monodisperse phospholipid vesicles.

We present a novel approach for fabricating monodisperse phospholipid vesicles with high encapsulation efficiency using controlled double emulsions as templates. Glass-capillary microfluidics is used to generate monodisperse double emulsion templates. We show that the high uniformity in size and shape of the templates are maintained in the final phospholipid vesicles after a solvent removal step. Our simple and versatile technique is applicable to a wide range of phospholipids.

Multicompartment Polymersomes from Double Emulsions

Polymersomes are vesicles which consist of compartments surrounded by membrane walls that are composed of lamellae of block copolymers; these are important for numerous applications in encapsulation and delivery of active ingredients such as food additives, drugs, fragrances and enzymes [2] . Polymersomes are typically prepared by precipitating block copolymers from their solvents through addition of a poor solvent for the copolymers, or by rehydrating a dried film of the copolymers. The unfavorable interactions between blocks in the copolymer and the poor solvent induce formation of aggregate structures ranging from micelles, wormlike micelles and vesicles. However, the resultant polymersomes are highly polydisperse and have poor encapsulation efficiency. Recently, a new approach has been developed to fabricate monodisperse polymersomes by using double emulsions as templates. Water-in-oil-in-water (W/O/W) double emulsions with a core-shell structure are first prepared in capillary microfluidic devices. Diblock copolymers, dissolved in the oil shell phase, assemble onto the walls of the polymersomes upon removal of the oil by evaporation 7] after adhesion of the diblock copolymeradsorbed interfaces. This approach leads to polymersomes with high size uniformity and excellent encapsulation efficiency; it also enables precise tuning of the polymersome structures. Advances in techniques for fabricating polymersomes have led to controlled spherical polymersomes with a single compartment. However, non-spherical capsules with multiple compartments also have great potential for encapsulation and delivery applications. By storing incompatible actives or functional components separately, polymersomes with multiple compartments can achieve encapsulation of multiple actives in single capsules and reduce the risk of cross contamination. Moreover, multiple reactants can be encapsulated separately to allow reaction upon triggering. By tuning the number of compartments containing each reactant, the stoichiometric ratio of the reactants for each reaction can be manipulated. These multi-compartment polymersomes will create new opportunities to deliver not only multiple functional components, but also multiple reactants for reactions on demand. In addition, with the versatility of synthetic polymer chemistry to tune properties such as polymer length, biocompatibility, functionality and degradation rates, non-spherical polymersomes with multiple compartments can be tailored for specific delivery targets. However, polymersomes that have been reported to date are almost exclusively spherical in shape, and have only one compartment; since most conventional polymersome fabrication processes rely on self-assembly of the block copolymer lamellae, little control over the size and structure of the resultant polymersomes is achieved. With the conventional emulsion-based methods, non-spherical droplets are also not favored because interfacial tension between the two immiscible phases favors spherical droplets, which have the smallest surface area for a given volume. Recent advances in microfluidic technologies enable high degree of control in droplet generation, and ease in tuning the device geometry; this offer a new opportunity to fabricate double emulsion with controlled morphology, which serve as templates for fabricating the nonspherical multi-compartment polymersomes. However, such investigations have not, as yet, been carried out. In this work, we demonstrate the generation of non-spherical polymersomes with multiple compartments. We use glass capillary microfluidics to prepare W/O/W double emulsions with different number of inner aqueous drops. These emulsions are initially stabilized by the amphiphilic diblock copolymers in the oil shells, which consist of a mixture of a volatile good solvent and a less volatile poor solvent for the copolymers. As the good solvent evaporates, the copolymers at the W/O and the O/W interfaces are attracted towards each other to form the membranes. As a result, neighboring inner droplets adhere to one another; this leads to formation of multi-compartment polymersomes, as schematically illustrated in Scheme 1. We also use a modified glass capillary device for generating double emulsions with two distinct inner phases containing different encapsulants; this process leads to the fabrication of non-spherical polymersomes with multiple compartments for separate encapsulation of multiple actives. A glass capillary microfluidic device is used to generate double emulsions with controlled morphology. (See Fig. S1 in Supporting Information) Due to the high degree of control afforded by microfluidics, the number of inner droplets in a W/O/W double emulsion system can be controlled by varying the flow rates of the three phases independently; 12] an example of the process is shown in Fig. 1A. The thickness of the double emulsion shells can be adjusted by changing the flow rates; however, as long as the flow rates are not altered enough to change the number of inner droplets of the double emulsion templates, change in shell thickness does not affect the morphology of the final polymersomes since all solvents in the shells is removed in subsequent steps. To prepare the double emulsion templates, multiple inner droplets are dispersed in drops of a mixture of chloroform and hexanes (36:65 v/v) with 10 mg/mL poly(ethylene-glycol)-b-poly(lactic acid), (PEG(5000)-bPLA(5000)); the drops-in-drops are suspended and stabilized in a poly(vinyl alcohol) (PVA) solution. A homopolymer, PEG, is added [] Prof. D. A. Weitz, Dr. H. C. Shum, Dr. S. H. Kim School of Engineering and Applied Sciences, Department of Physics and Kavli Institute for Bionano Science and Technology, Harvard University, Cambridge, Massachusetts 02138 (USA) Fax: (+1) 617-495-0426 E-mail: weitz@seas.harvard.edu Homepage: http://www.seas.harvard.edu/projects/weitzlab/

Microfluidic generation of multifunctional quantum dot barcode particles

We develop a new strategy to prepare quantum dot (QD) barcode particles by polymerizing double-emulsion droplets prepared in capillary microfluidic devices. The resultant barcode particles are composed of stable QD-tagged core particles surrounded by hydrogel shells. These particles exhibit uniform spectral characteristics and excellent coding capability, as confirmed by photoluminescence analyses. By using double-emulsion droplets with two inner droplets of distinct phases as templates, we have also fabricated anisotropic magnetic barcode particles with two separate cores or with a Janus core. These particles enable optical encoding and magnetic separation, thus making them excellent functional barcode particles in biomedical applications.

Multiple Polymersomes for Programmed Release of Multiple Components

Long-term storage and controlled release of multiple components while avoiding cross-contamination have potentially important applications for pharmaceuticals and cosmetics. Polymersomes are very promising delivery vehicles but cannot be used to encapsulate multiple independent components and release them in a controlled manner. Here, we report a microfluidic approach to produce multiple polymersomes, or polymersomes-in-polymersome by design, enabling encapsulation and programmed release of multiple components. Monodisperse polymersomes are prepared from templates of double-emulsion drops, which in turn are injected as the innermost phase to form the second level of double-emulsion drops, producing double polymersomes. Using the same strategy, higher-order polymersomes are also prepared. In addition, incorporation of hydrophobic homopolymer into the different bilayers of the multiple polymersomes enables controlled and sequential dissociation of the different bilayer membranes in a programmed fashion. The high encapsulation efficiency of this microfluidic approach, as well as its programmability and the biocompatibility of the materials used to form the polymersomes, will provide new opportunities for practical delivery systems of multiple components.

Droplet Microfluidics for Fabrication of Non-Spherical Particles

We describe new developments for controlled fabrication of monodisperse non-spherical particles using droplet microfluidics. The high degree of control afforded by microfluidic technologies enables generation of single and multiple emulsion droplets. We show that these droplets can be transformed to non-spherical particles through further simple, spontaneous processing steps, including arrested coalescence, asymmetric polymer solidification, polymerization in microfluidic flow, and evaporation-driven clustering. These versatile and scalable microfluidic approaches can be used for producing large quantities of non-spherical particles that are monodisperse in both size and shape; these have great potential for commercial applications.

25th Anniversary Article: Double Emulsion Templated Solid Microcapsules: Mechanics And Controlled Release

How droplet microfluidics can be used to fabricate solid-shelled microcapsules having precisely controlled release behavior is described. Glass capillary devices enable the production of monodisperse double emulsion drops, which can then be used as templates for microcapsule formation. The exquisite control afforded by microfluidics can be used to tune the compositions and geometrical characteristics of the microcapsules with exceptional precision. The use of this approach to fabricate microcapsules that only release their contents when exposed to a specific stimulus--such as a change in temperature, exposure to light, a change in the chemical environment, or an external stress--only after a prescribed time delay, and at a prescribed rate is reviewed.

Microfluidic Melt Emulsification for Encapsulation and Release of Actives

A microfluidic melt emulsification method for encapsulation and release of actives is presented. Using a water-in-oil-in-water (W-O-W) double emulsion template, solid capsules can be formed by freezing the middle shell phase. Actives encapsulated inside the solid shell can be controllably and rapidly released by applying a temperature trigger to melt the shell. The choice of the shell materials can be chosen to accommodate the storage and release temperatures specific to the applications. In addition, we have also demonstrated the same concept to encapsulate multiple actives in multicompartment capsules, which are promising as multifunctional capsules and microreactors.

Fabrication and characterization of monodisperse PLGA-alginate core-shell microspheres with monodisperse size and homogeneous shells for controlled drug release.

Monodisperse PLGA-alginate core-shell microspheres with controlled size and homogeneous shells were first fabricated using capillary microfluidic devices for the purpose of controlling drug release kinetics. Sizes of PLGA cores were readily controlled by the geometries of microfluidic devices and the fluid flow rates. PLGA microspheres with sizes ranging from 15 to 50μm were fabricated to investigate the influence of the core size on the release kinetics. Rifampicin was loaded into both monodisperse PLGA microspheres and PLGA-alginate core-shell microspheres as a model drug for the release kinetics studies. The in vitro release of rifampicin showed that the PLGA core of all sizes exhibited sigmoid release patterns, although smaller PLGA cores had a higher release rate and a shorter lag phase. The shell could modulate the drug release kinetics as a buffer layer and a near-zero-order release pattern was observed when the drug release rate of the PLGA core was high enough. The biocompatibility of PLGA-alginate core-shell microspheres was assessed by MTT assay on L929 mouse fibroblasts cell line and no obvious cytotoxicity was found. This technique provides a convenient method to control the drug release kinetics of the PLGA microsphere by delicately controlling the microstructures. The obtained monodisperse PLGA-alginate core-shell microspheres with monodisperse size and homogeneous shells could be a promising device for controlled drug release.