Microscopic lenses made of calcium: Regularly aligned hemispheres of calcium carbonate form an array of high-quality optical lenses. Kyubock Lee, Max Planck Institute of Colloids and Interfaces |
The body of the brittlestar Ophiocoma wendtii is
studded with tiny crystalline lenses made of calcium carbonate.
Microlenses like these are of great interest technologically, yet they
have always been extremely expensive to produce. However, scientists
from the Max Planck Institute of Colloids and Interfaces and colleagues
from other institutes took their cue from biology and came up with a
relatively simple and inexpensive method of producing calcium carbonate
lenses packed together in a regular arrangement.
Many
living creatures build minerals into their organism so perfectly as to
produce structures with truly astonishing properties. It is a
combination of calcium carbonate (CaCO3) and organic matter that makes
the seashells, corallites and sea urchin spines we find in our oceans.
The brittlestar Ophiocoma wendtii has developed the most perfect way of using minerals for biological purposes. Related to the starfish,
the brittlestar long remained something of a mystery to zoologists
because it was clearly sensitive to light without having any visible
eyes. Then, a few years ago, scientists discovered that the body of the
reef-dwelling creature is covered with tiny crystalline lenses made of
calcium carbonate, which together form a type of compound eye.
Microlenses
like these are used technologically in all sorts of applications that
require optical systems and measurements in anything smaller than the
millimeter scale. For instance, they are used in telecommunications to
conduct light signals through bundles of glass fibres. However, up to
now, it has been very tricky to produce these miniature optical lenses,
requiring the use of semiconductor technology, for example. Scientists
at the Max Planck Institute of Colloids and Interfaces working in
conjunction with colleagues from the University of Konstanz and partners
from South Korea have developed a surprisingly simple method of
producing such lenses—and Mother Nature inspired them.
Microlenses without a cleanroom environment
What
the team of scientists discovered is that—within a few minutes at room
temperature—tiny calcium carbonate structures form on a solution that
has been saturated with calcium, and that these structures grow into a
thin film within one or two hours. With the addition of an organic
surfactant, they then form uniform hemispheres.
“Compared
with the conventional method, which is a multi-step procedure requiring
a cleanroom environment, this enables us to very cheaply and easily
produce microlenses packed together in a regular arrangement,” says
Kyubock Lee who works at the Max Planck Institute of Colloids and
Interfaces in Potsdam and the KAIST in South Korea.
The researchers could observe clear multiple images of a micron-sized ‘A’ projected through the array of microlenses.
“The
high quality of the microlenses was a huge surprise to us,” says
Wolfgang Wagermaier, a materials scientist at the Max Planck Institute
in Potsdam. “This was the first time anyone had ever demonstrated such
optical properties in synthetically produced calcium carbonate
structures.”
The
lenses focus a beam of parallel rays of light, each of which is around
0.001 mm in size; they have a diameter of 0,006 mm and a focal length of
0.007 to 0.008 mm.
Brittlestar produces lenses with no image defect
Many
living beings build calcium carbonate into their skeletons as a basic
material, which shows that the mineral has no negative effect on their
organism. The scientists were also able to demonstrate that the lab-made
microlenses are compatible with biological substances; as a result,
they could potentially be used in fields such as cell research.
This
replication of the brittlestar’s optical lenses is a prime example for
the up-and-coming research field of bio-inspired materials production to
showcase. That is because the crystalline lenses of this sea dweller
have an astonishing quality: the crystals are aligned such that the
birefringence, or double refraction, that is typical of CaCO3, does not
affect them, meaning that no double image is produced. Moreover, they
have exactly the right shape to correct any spherical aberration, a
serious image defect.
Peter
Fratzl, who heads the Department of Biomaterials at the Max Planck
Institute of Colloids and Interfaces, says: “Deriving fundamental
principles for material synthesis from the way in which natural
materials are formed is usually an enormous challenge. Sometimes we also
get pleasant surprises, like this discovery of a relatively easy method
of producing optical elements modelled on nature.”
