Microlens Arrays from a Test Tube

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

Scientists at the Max Planck Institute of Colloids and Interfaces collaborated with researchers from the University of Konstanz, the Korea Institute of Geoscience and Mineral Resources, and the KAIST in South Korea to develop the optically functional micrometer-sized 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 seashells and bones, grow in aqueous media at ambient conditions in a genetically programmed way. The researchers discovered that, under atmospheric conditions, micrometer-sized 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 the researchers to synthesize hemispherical-shaped, uniform-sized structures.

“This is such a simple and cheap process for the fabrication of high-quality microlens arrays, and you just need 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 MPI of Colloids and Interfaces and at KAIST.

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

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

The biocompatible CaCO₃is a base material for the skeletons of a large number of biological organisms. The scientists demonstrated the biocompatibility of the CaCO₃ microlens arrays by seeding fibroblasts, the common connective cell tissue in animals, proving the viability of the cells of the array.

The scientists 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 the MPI of Colloids and Interfaces. “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.”

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