MICROFLUIDICS AS A FORMULATION TOOL

Anderson H.C. Shum and David A. Weitz

In this study, microfluidic techniques are compared with conventional mixing techniques for formulating emulsions in a controlled manner. Microfluidics has shown the following characteristics in the formulation of emulsions, both simple and multiple:

- highly monodisperse in size

- high degree of controllability in size and shape

These characteristics were shown in the present study to:

- improve emulsion stability

- increase encapsulation efficiency

- increase loading capacity

Part 1: Emulsion stability of simple emulsions

While polydisperse emulsion droplets grew in mean droplet size from 25 microns to 30 microns in 4 hours (Figure 1), monodisperse droplets of similar size remain monodisperse and show no Ostwald ripening in the course of a week, after going through temperature cycles of 45°C, 23°C and 50°C.

Figure 1: Droplet size evolution of polydisperse emulsion droplets at 50°C.

Figure 2: Droplet size evolution of monodisperse emulsion droplets at 45°C, 23°C, 50°C.

Part 2: Encapsulation efficiency of double emulsions

Microfluidics has shown itself to be a highly versatile technique for controlling the overall size as well as the shell thickness of the double emulsions.

The advantages of using microfluidics for formulating double emulsions are as follows:

- narrow size distribution of both inner drops and outer drops (Figure 4)

- high encapsulation efficiency (Figure 5)

- high loading (Figure 6)

Figure 3: Controlling the shell thickness and the overall size of double emulsions in glass microcapillaries.

Figure 4: Size distributions of the inner and the outer drops are much narrower than those prepared by conventional two-stage mixing techniques

Figure 5: Leakage of fluorescent dye, as a model encapsulant, is significantly lower in the formulation process using microfluidics, in comparison with the conventional two-stage mixing technique

Figure 6: High loading capacity was achieved by compartmentalizing the inner drop with a very thin shell in microfluidics.

Conclusion

The microfluidic technique for formulating emulsions demonstrated
- improved emulsion stability

- high degree of control over size and shape
- narrow size distribution
- high encapsulation efficiency
- high loading
The emulsion formulation technique offers unprecedented opportunities for new applications e.g. using such droplets as reaction vessels for carrying out reactions in a well-controlled environment or as precursors for engineering novel functional materials.

ACKNOWLEDGEMENTS
We thank BASF and NSF (MRSEC) for providing financial support. Thanks also to Andy S. Utada, Alberto Fernandez-Nieves, Carlos J. Martinez (Purdue), Yaqian Liu (BASF) and Christian Holtze (BASF) for helpful discussions.

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For questions or comments, please contact:

Anderson H.C. Shum
School of Engineering and Applied Sciences (SEAS)
Harvard University
Rm 202, 40 Oxford Street
Cambridge, MA 02143
617-495-1958
shum@fas.harvard.edu