Mix-and-shake leads to instant microcapsule

InTech, Jul 2005

WHEN IT COMES TO NANOTECHnology, microcapsules play a big role. There is now a new way to make the tiny hollow spheres, said chemical engineers at Rice University.

Microcapsule research is one of the most active fields in applied nanotechnology, with dozens of companies either developing or using the tiny containers, which are usually smaller than living cells, to deliver everything from drugs and imaging agents to perfumes and flavor enhancers.

The new approach for making microcapsules involves mixing a solution of polymer and salt with tiny particles of silica that contain just a few hundred atoms apiece, said Michael Wong, Rice assistant professor of chemical and biomolecular engineering and chemistry.

The usual method to make a microcapsule consists of depositing layers of a coating onto a template or core, which they remove to form the hollow center of the structure. The core usually burns out with high heat processes or dissolves with harsh chemicals. Both processes can damage the microcapsules and their cargo.

"Our process takes place almost instantaneously, at room temperature, under normal pressure, in water, and at mild pH values,"Wong said/The spheres naturally become hollow during the self-assembly, which is highly unusual and is an advantage over existing methods."

Wong's approach has advantages over other microcapsule production methods that involve spraying techniques. While these techniques can scale up, it is difficult to adjust the materials properties of the resulting microcapsules.

"We've shown that we can tailor the properties of our self-assembled microcapsules-make them smaller, larger, thicker, or thinner-simply by changing the ingredients we start with or by adjusting the mixing procedure," Wong said. "The underlying chemistry is so easy to perform that anyone who can pour, mix, and shake can make these microcapsules in less than a minute."

Wong's process involves'self-assembly,' meaning the hollow spheres form spontaneously when the nanoparticle building blocks mix with polymer and salt. Because the process takes place in water, any chemical or drug suspended in the water gets trapped inside the hollow sphere when it forms.

Besides encapsulating drugs, flavor compounds and other molecular cargo, Wong's team hopes to develop their microcapsules for drug delivery. They are already exploring ways, like using changes in pH or temperature, to trigger the microcapsules to open and release drugs. In addition, they've made magnetic microcapsules by using iron oxide nanoparticles instead of silica. This could allow doctors to use magnets to precisely position drugs prior to their release.

"We can also use fluorescent nanoparticles called quantum dots to make glowing hollow spheres, which could be useful for combined drug delivery and imaging," Wong said.