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Brittlestars: Fabricating Microlenses with Perfect Geometry

Brittlestars: Fabricating Microlenses with Perfect Geometry

A. Ali Eren

Nov 1, 2014

The unity underlying nature manifests itself in many different forms. Sometimes various "things" work towards accomplishing only one task while sometimes only one "thing" is utilized in many different tasks. We can already see countless examples of both phenomena with our naked eyes; however, the developing science and technology let us observe many more interesting examples in the micro and nano scale. This article aims to describe one little example of this miraculous work of art in which many things are made from one thing and to show that the more we study nature in detail the more we admire all that have been granted to us.

Brittlestars form a large group of sea animals that are similar to starfish. There are more than 2,000 species of brittlestars. However, this article will focus on two of them, Ophiocoma pumila (Figure 1a) and Ophiocoma wendtii (Figure 1b). In spite of their similar appearance, these two kinds of brittlestars have one main difference. While O. pumila is insensitive to light, O. wendtii is highly light sensitive. For example, the latter has different colors at day and night, as shown in Figure 1b, left and right respectively. More interestingly, O. wendtii can sense shadows of predators and quickly move into dark areas such as a cave or underneath a rock.

To understand the mechanisms behind the difference in light sensitivities of these two species, Joanna Aizenberg and her colleagues investigated1 the microstructure of both brittlestars' outer skeletons with an electron microscope and came up with a striking result: The top surface of O. wendtii's skeleton has very well ordered lens-like hemi spherical elements (Figure 1f). The cross section image of one of those hemispheres actually looks like a compound lens made up from two hemispheres with different diameters (Figure 1g). On the other hand, O. pumila's skeleton had a typical stereom (sponge-like calcite) structure (Figure 1e). These images strongly suggest that the lenses in O. wendtii's skeleton are responsible for the relatively high light sensitivity. However, understanding how that really happens require further investigation.

It is well-known that spherical lenses suffer from a problem called "spherical aberration," which means that the light rays that are closer to the optical axis are focused at a different point than the ones that are away from the axis. A quick solution to this problem is to use two lenses, whose diameters have a certain ratio, back to back; this helps to correct the aberration originating from the first one with the second one. Interestingly, when Aizenberg et. al. calculated1 the optimum compound lens configuration for O. wendtii's skeleton, which has the minimum aberration, their result matched the original lens structure perfectly (the orange outline in Figure 1e). They were also able to locate the focal point of these lenses (d = 4-7 um* below the lens) with the same method. Their further electron microscopy studies showed optically sensitive nerve bundles exactly at that location. All these results clearly show that O. wendtii's skeleton has the perfect geometry to collect and focus light to improve its light sensitivity. However, there is one big question about these lenses: their material.

Calcite, a kind of calcium carbonate (CaCO3), is a common ingredient of the shell or the skeleton of marine organisms. Interestingly, the birefringence property of calcite makes it very unfavorable as a lens material. In a birefringent material the speed of the light depends on the direction it travels with respect to the crystallographic axes of the material. As a result, if one looks through it, they will observe a doubly refracted image (Figure 2). Being the most famous example of birefringent crystals, calcite's refractive index is 1.64 parallel to one crystallographic axis and 1.49 in the perpendicular direction. Therefore a regular calcite lens cannot focus light on a single spot, unless it is oriented along a special crystallographic axis (c-axis to be specific), which would be along the diagonal of the prism in Figure 2.

At this point we are not surprised to learn that the optical axis of the O. wendtii's lenses, and the c-axis of the calcite crystal that they are made of, indeed overlap. We are not surprised because we already had a strong feeling that these lenses should work. However, it is quite surprising that these little creatures can grow single crystals of calcite with a specific crystallographic orientation. As Kenneth Towe states in the context of a similar study, "This precise orientation of crystals is the big mystery of biomineralization. Organisms know how to do it; we do not yet know how they know."3

Biomineralization, the controlled deposit of inorganic minerals by living organisms, is a very active research field attracting many scientists from various disciplines, including biology, physics, chemistry, and material science. In general, controlling crystal structures at small length scales is a very challenging task. Scientists spend millions of dollars to build state-of-the-art facilities for single crystal materials synthesis. They work in clean rooms, under an ultra high vacuum and at extremely high temperatures. On the other hand, from brittlestars to large whales, almost all living creatures have biominerals, such as bones and shells, manufactured in chemically dirty environments and at decent temperatures. Organisms are apparently equipped more efficiently than our laboratories are.

A. Ali Eren has a Ph.D. in Physics and lives in the USA. He studies physical chemistry of biological processes.

References

*1 um (micron) is one thousandth of a millimeter. Human hair is approximately 100 micron thick.

1. Aizenberg, Joanna, et al. "Calcitic microlenses as part of the photoreceptor system in brittlestars." Nature 412.6849 (2001): 819-822.
3. Towe, Kenneth M. "Sea urchins as crystallographers." Science 311.5767 (2006): 1554-1555.

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