Micro-/Nanotechnology - Polymer Science

Polymersomes Produced by Microfluidics

Scientists have developed a new polymer-based microfluidic device capable of producing tiny polymer-based capsules continuously without clogging.

Polymersomes are tiny spherical capsules made up of a polymer bilayer and some enclosed contents, like a synthetic vesicle but much stronger and more easy to functionalize. The bilayer itself usually forms by self-assembly, and is made from a mixture of two (or more) polymers called a polymer diblock. Many scientists are occupied with the problem of how to produce these polymersomes as they are useful for delivery applications and have interesting physical properties for fundamental studies as well.

There are several ways to make polymersomes but most of them lead to a mixture of polymersome sizes, or to poorly filled capsules. A good way to make reproducibly sized polymersomes is by employing a microfluidic device in a so-called double-emulsion process which uses a core droplet that is surrounded by a layer of a second fluid, so that the inner fluid is completely separated from the outer fluid; these are often oil–water–oil mixtures. Droplets made in the device are effectively used as templates to make the polymersome. Microfluidic devices contain tiny channels into which fluids can be injected. Their small size means that it is possible to exploit effects that are important only at these small scales to make large numbers of droplets or particles at high throughput. However scientists using these devices to make polymersomes like this in the past have often been stymied by problems with clogging of the channels as the polymers precipitate out as local solvent concentrations change.

In a new study from US and German researchers, the clogging problem has been overcome to give a more continuously operating device that can produce polymersomes 50–100 μm across. Dave Weitz and his team at Harvard, Hamburg, and Mannheim have designed a new device with three injection junctions that all have tailored solvent-loving or solvent-hating properties. Two hydrophobic junctions are used for injecting organic solvents, and this means that the solvent being injected into the channels can be constantly varied to compensate for evaporation effects that would otherwise alter the solvent composition and lead to precipitation of the dissolved polymers. The poly(dimethyl siloxane) (PDMS) device can be printed easily onto a surface unlike common methods, and a glass-like coating on the PDMS prevents it from swelling on contact with organic solvents too.

Professor Wilhelm Huck, an expert in microdroplets and microfluidics from University of Cambridge and Radboud University Nijmegen says that this is the first time that people have made polymersomes in this way. He thinks that ”Because you have the ability to make monodisperse asymmetric polymersomes with controlled composition in either one or two layers, this looks promising for fluidic control.” However he points out that the process is not trivial, and there is much tuning to be done before it can be used regularly.

The researchers demonstrate their device in action with a mixture of polylactic acid and polyethylene glycol, but claim that the design principle could be used also for much smaller capsules, and for capsules in which one hemisphere has different properties from the other (commonly called Janus particles after the two-faced Roman God).

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