While emulsions consist of droplets of one liquid encased within a second immiscible liquid, it is also possible to make double emulsions: these consist of droplets that in turn hold finer droplets that are suspended in a continuous phase, such as water in oil in water.
While double emulsions are hard to make with the uniformity demanded of pharmaceutical formulations using present methods, researchers have now uncovered a new method of making them using microfluidics that could overcome this obstacle.
At present, double emulsions are generally made using vigorous mixing techniques or by forcing single emulsions through membranes or nozzles, but in all cases a broad size distribution is obtained. Okushima et al report in Langmuir (10.1021/la0480336) a microfluidic approach that can make precisely controlled droplets.
They used a microfluidic device with a series of T junctions that could be used to make various forms of double emulsions, including water in oil in water emulsions. For example, to make water droplets within an oil phase, the junctions were designed so that water flowed into a hydrophobic channel, causing it to pinch off. This stream then flowed into a second hydrophilic channel, thus creating the double emulsion, with a very narrow distribution in the size of the droplets.
Water-oil-water emulsions have been of interest to the pharmaceutical industry as possible formulation routes for the delivery of peptide- and protein-based drugs. For example, various groups have looked at using polymer (poly-lactide-co-glycolide; PLGA) microspheres manufactured using a water-oil-water technique for this purpose, including for the delivery of the osteoporosis/Paget's disease drug clodronate.
With a slight adjustment of the flow conditions, it was also possible to encapsulate two water droplets from different sources within a single oil droplet.
Meanwhile, by reversing the order of the channels they were also able to make oil-water-oil emulsions.