Microlens arrays from a test tube

Ashley N. Paddock, ashley.paddock@photonics.com

Simple calcium carbonate precipitation at ambient conditions can produce microlens arrays of uniform size and focal length. The process offers a cheaper alternative to lithographic techniques used to create inorganic-based materials.

Scientists at Max Planck Institute of Colloids and Interfaces collaborated with researchers from the University of Konstanz, the Korea Institute of Geoscience and Mineral Resources, and KAIST (formerly the Korea Advanced Institute of Science and Technology), both in South Korea, to develop the optically functional micro-meter-size hemispherical CaCO₃ structures. To create the microarrays by simple mineral precipitation, they used a saturated calcium solution and CO₂ in air, along with an organic surfactant that regulates mineral formation.

Biominerals, such as sea-shells and bones, grow in aqueous media at ambient conditions in a genetically programmed way. The researchers discovered that, under atmospheric conditions, micrometer-size CaCO₃ structures form in a few minutes at the surface of a calcium-saturated solution. In less than two hours, the precipitate forms a thin film. The addition of organic surfactants allowed hemispherically shaped uniform structures to be synthesized.

“This is such a simple and cheap process for the fabrication of high-quality microlens arrays, and you just need [a] calcium-saturated solution with a small amount of the organic surfactant, while alternative lithographic techniques require multiple steps and a cleanroom,” said Kyubock Lee, a researcher who worked on the project at Max Planck and at KAIST.

Multiple images of a micron-size “A” were observed through the array of microlenses. The collimated light can be focused into a 1-µm spot by passing through 6-µm CaCO₃ micro-lenses with a focal length ranging from 7 to 8 µm.

“It was very surprising when we observed that hemispherical CaCO₃ structures work as micron-size convex lenses with such a high quality,” said Wolfgang Wagermaier, a researcher at Max Planck. “These optical properties have not been demonstrated before with synthetic CaCO₃ structures.”

The biocompatible CaCO₃ is a base material for the skeletons of a large number of biological organisms. The biocompatibility of the CaCO₃ microlens arrays was demonstrated by seeding fibroblasts, the common connective cell tissue in animals, proving the viability of the cell array.

They also observed that a single cell can cover multiple microlenses, signifying that CaCO₃ microlens arrays combined with optics have potential for cell biology research.

“Usually, it is very challenging to fully understand and mimic the biologically controlled formation of such advanced mineral structures, although it is an eventual goal of biomimetic materials research,” said Peter Fratzl, head of the biomaterials department at Max Planck Institute. “The fabrication of CaCO₃ microlens arrays demonstrates that the principles of self-assembly and organic controlled formation of mineral could be realized to produce advanced optical devices.”