6. Cricket. If you think the sound of crickets is an essential part of any romantic summer evening, you have good reason. Male crickets are trying to attract females by a species-specific chirp at a fixed frequency of about 4.5 kHz. But how do the females find them once they hear the call?
Female crickets are equipped with special receptors or tympani on their front legs that are used to compare the pressure fluctuations between right and left sides of the animal. The pressure is carried up into the cricket by means of auditory trachea. Pressure fluctuations reach each tympanum directly as well as indirectly, from the other side of the body. Phase shifts between these two sources strongly modulate the tympanum-oscillations and allow the female to determine the direction of the source of potential romance.
Crickets also have many tiny hairs or cerci along their abdomens that measure pressure fluctuations and scientists believe their purpose is to sense attacks. The sensitivity and precision of these hairs are remarkable, and inspired a research group from the University of Twente in the Netherlands to mimic their operation.
The team was part of CICADA, a European Union project aimed at developing a bio-inspired perception system. They attached hundreds of 0.9 millimeter long plastic wires to sockets in silicon wafer sheets to form a receptor array. The wires rotate in their sockets as they are moved by minute air pressures and the smallest movements are registered by the flexibly-suspended plate. The electrical capacity of the plate changes as a result and this measure is fed into a central computer.
Further refinement allowed measurement at an even finer level of precision. Harmen Droogendijk discovered that it was possible to adjust the spring stiffness of the wires electronically. He investigated the alternating voltage needed to relax each wire at the required moment, enabling it to be extra sensitive to the related frequencies. The effect was significant and increased sensitivity at the adjusted frequency tenfold.
Gijs Krijnen, the physicist who led the project with colleague Remco Wiegerink, believes that their device will be a useful prototype for technologies like hearing aids and sensors as well as measuring devices for airflow in aeronautics. Indeed, the device already reaches levels of precision in measuring air pressure and particle velocity previously unmatched, according to Krijnen.
The innovation here is an example of successful scale change and use of modularity in order to improve an existing technology. The exploration of nature at the micro scale has informed the improvement and it has the potential for creating a new paradigm for sensing in several different technologies.
7. Mantis Shrimp. This crustacean has compound eyes that can distinguish between left- and right-circularly polarized light.
Polarization is the changing of the orientation of a light ray's oscillations. All light rays are examples of electromagnetic radiation and, as such, radiation is emitted in all directions out the "sides" from the direction of travel of the wave. Polarizing sunglasses block some of this radiation traveling in certain planes, and therefore cut out glare.
Many animals can see polarized light, including ants, bees, fruit flies and some fish. Humans, typically, cannot. It's not known why the Mantis Shrimp has evolved this extraordinary capability, but secret signaling and better underwater vision are possibilities.
The shrimp has an array of tiny hairlike folds, or microvilli, within a light-sensitive cell of each eyelet that functions as a quarter waveplate. Waveplates refract light and change the frequency of emitted wavelengths. A quarter waveplate creates a quarter-wavelength phase shift.
The membrane of every microvillus in the light-sensitive cell is made of a birefringent material. This means that light is doubly refracted by this material and, because the microvilli are arrayed in parallel ranks in diameters smaller than the wavelengths found in light, the rays finally emitted are not predisposed to one color or another. This capability makes it possible for Mantis Shrimps to convert linearly polarized light to circularly polarized light and vice versa.
This so-called "achromatic phase retardation," or wavelength-independent phase shift, achievable over the broad visible spectrum, is a very big deal within the optical technology sector; in display technologies, communications systems, and optical pick-up systems. Manmade quarter-wave plates perform this essential function in CD and DVD players and in circular polarizing filters for cameras, for example. They are limited, however, to a narrow wavelength range (certain colors), which is why mimicking the Mantis Shrimp is useful. Since the discovery of this natural waveplate by a team from the University of Bristol in Britain in 2009, many have been working on perfecting its synthetic version.
One such team, from National Taipei University of Technology in Taiwan, and Pennsylvania State University in the United States, has reported encouraging results in this synthesis. They have constructed an array of vertical nanorods made from alternating thin films of different refractive properties. This array mimics the two types of birefringence found in the Mantis Shrimp eyes. The films were laid down using a well-known process, the Oblique Angle Deposition (OAD) method. They claim that their methodology can be applied to design a waveplate for any wavelength range.
The innovation here is one of expanding the capabilities of current technology using a model from nature (widening the wavelength range of optical devices), and perfecting the methods and materials needed for more efficient manufacture (thin film deposition). Although the reason for the shrimp's optical capabilities are still a bit of a mystery, their usefulness is not and this is a very good example of making the inspirational practical.
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