Up to now, the closed loop problem has been addressed by molding in layers. “This layer-by-layer technique can only be used to mold a limited range of structures, and it requires precise alignment of each mold,” says Fourkas. “We realized that we could take advantage of a property of PDMS that is usually viewed as a problem, which is that it likes to stick to itself.”
The Fourkas team created a thin wall of PDMS in the original structures, effectively removing any closed loops. “For instance, on the Golden Gate, we would create a thin wall underneath the entire length of the bridge model. That would make it possible to remove the mold from the original object,” says Fourkas. Then, once the mold is free, the wall region in the mold can be closed off by gentle pressure, making it possible to create copies of the bridge that do not contain a wall.
“One of the exciting things about this technique,” says Fourkas, “is that it vastly increases the range of microscopic structures that can be created in a single molding step. This represents an important step towards the mass production of micromachines made from plastic.”
The Fourkas team also recently invented a successful method to incorporate a broad range of materials, including metal, into micro structures fabricated by multiphoton absorption polymerization (MAP).