A team headed by Edwin L. Thomas and Patrick S. Doyle at MIT in Cambridge, Massachusetts (USA) has now developed a new method for the large-scale synthesis of three-dimensionally patterned polymer particles with morphological characteristics in the submicrometer range. As described in the journal Angewandte Chemie, with the use of stop-flow interference lithography, the team has even been able to produce Janus particles, microparticles with two chemically different hemispheres.

"Our new method is a combination of phase mask interference lithography and mirofluidic flow lithography, unifying the strengths of these two methods," explain the researchers. Liquid precursors of a polymer whose formation is induced by light are introduced into a microfluidic system (a system of channels that are just a few micrometers wide). The bottom portion of the device is a phase mask with a periodic surface structure.

This arrangement is irradiated through a transparency mask that defines the shape of the resulting particles. In a test sample these were triangles with sides of 60 micrometres. Once the parallel light rays pass through the strictly periodic surface structure of the phase mask, the result is a complex three-dimensional distribution of light intensity within the liquid (interference). In regions of high intensity, the polymer precursors are cross-linked to form three-dimensional structures in a solid hydrogel. In this way, the researchers were able to give the triangular particles a knobby, lattice-like structure.

Because this method works continuously, it can attain a very high throughput: Liquid flows in and polymerizes to form particles that are immediately rinsed away when the next portion of liquid follows-all in less than a second. In contrast to other techniques, the liquid does not need to be deposited in an even layer on a support and developed stepwise.

In addition, within a microchannel, it is possible to allow two different liquids to flow side by side without mixing. If the transparency mask is adjusted so that the light irradiates a region around the boundary between the two liquids, the process results in Janus particles with two chemically different hemispheres.

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