Inspired by the eyes of the lowly blowfly, researchers in the US have developed a new way to make highly efficient lenses that could boost the performance of solar cells. Their "bioreplication" technique creates surfaces that mimic fly corneas – and they say it could be automated to mass-produce lenses and other useful structures.

Nature often finds very efficient solutions to engineering problems, and the field of biomimicry is devoted to taking the best from nature and recreating it in the lab. One important way of doing this is bioreplication, whereby an actual biological structure is used as a mould to create replicas. Although bioreplication methods have been improved over the past decade, Akhlesh Lakhtakia of Penn State University, main author of the blowfly study, explains, "All the techniques currently available are not conducive to mass replication."

The team was inspired to use blowfly eyes when faced with the problem of designing a solar cell with maximum possible efficiency. Each eye has a hemispherical shape and complex hexagonal surface structure, and the researchers believe that stamping the surface of a solar cell with such a pattern would increase its efficiency. "These eyes are perfect for making solar cells because they would collect more sunlight from a larger area rather than just light that falls directly on a flat surface," explains Lakhtakia.

Coating corneas

Lakhtakia worked with colleagues from Penn State's Materials Research Institute and the Universidad Autónomia de Madrid in Spain to find a way of making large moulds or dies of the structures that still retained the desired micron-sized features and details. They began by securing an array of nine fly corneas on a glass substrate, and filling their backs with a silicone-based organic polymer to prevent any material that was subsequently applied from seeping behind the eyes. The corneas were coated in 250 nm of nickel using a technique developed at Penn State called conformal-evaporated-film-by-rotation.

The nickel-coated array was then strengthened and formed into a mould by depositing nickel onto its back using a form of electroplating called electroforming. The result is a master template that is half a millimetre thick, but the team says that could be made thicker. The template can be used as either a die to stamp a pattern or as a mould.

Crucially, the team showed that the templates could be used to create polymer replicas of the fly's eye that include faithful reproductions of features as small as few microns in size. Another important property of the template is that it is strong enough to be used multiple times in the stamping and casting of polymer materials. "We are now working on automating the process," says Lakhtakia.

Towards industrial bioreplication

Lakhtakia and colleagues now want to make a larger template that includes 30 blowfly corneas in order to show that the technique can be adapted to large-scale or industrial bioreplication.

As well as being used to create better solar cells, Lakhtakia believes that the technique could be adapted for use by police to "read" fingerprints on irregular surfaces that are currently too difficult or complex to examine in detail.

The study of insect eyes is a current hot topic – researchers from Bielefeld University, Germany, have made an artificial bee eye, complete with camera, with the aim of investigating the navigational, sensing and processing skills of bees. The group hopes to extend the study in the future to include UV capability to replicate the colour vision of the insects.

Both experiments are described in full detail in Bioinspiration & Biomimetics.