EUREKA DECEMBER 2005 COVER FEATURE STORY

Crickets inspire fine hair sensors

Study of how insects detect tiny air movements has led to the development of artificial analogues for aerospace and human prosthetics. Tom Shelley reports



A study of the hairs used by crickets to detect small air movements has led directly to the development of very small, sensitive and light weight artificial systems in silicon and plastic for use in aerospace systems and hearing aids.

While the development devices use capacitative sensors rather than neurons, other researchers have taken the actual insect neurons and grown them on Field Effect Transistors with a view to making even more sensitive and address the problem of signal processing.

These were just two of the plethora of ideas revealed by Professor George Jeronimidis of the Centre of Biomimetics at the University of Reading when he gave the keynote address to a Sensor Technology Transfer Event organised by the DTI and held at their conference centre in London.

Adult crickets have around 1000 or more hairs, 100 to 1500 microns long on organs called cerci, which allows them to detect air movements down to 1mm/s or less, indicating the possible approach of predators. The high sensitivity comes about because the tilting hairs apply pressure to neurons at their bases, greatly enhanced by mechanical lever amplification. The hairs are designed to be able to tilt in one plane and not another, giving the insects a knowledge of airflow direction as well as magnitude.

A Euro project called CICADA, Cricket Inspired perCeption And Decision Automata, has been initiated to imitate this with a view to producing artificial mechanical analogues of similar sensitivity.

Researchers in the Transducers Science and Technology group in the MESA Research Institute in the University of Twente in The Netherlands have been making analogous sensors with micron sized sensing hairs made from the polymer SU8. The manufacturing process starts with a highly conductive silicon wafer, which acts as the common lower plate of a group of sensing capacitors. A 100nm thick silicon nitride layer is deposited by LPCVD. A one micron thick layer of sacrificial polysilicon is then also deposited by LPCVD and patterned to form etch stop trenches. A second one micron thick silicon nitride layer is deposited and patterned to form the membranes and suspension springs followed by the sputtering of 20nm of chromium to form the upper sensing capacitor plates. The final layer of SU-8 is spin coated on and illuminated and developed to create the hairs. The process is completed by dry etching the sacrificial polysilicon, releasing the sensor structures. In use, the tilting hairs change the values of capacitance between the chromium areas and the substrate. The final devices are not quite as sensitive as those used by the crickets, since the hairs all have to be the same length, and can only reliably measure airflows of around 1m/s rather than 1mm/s.

Taking the idea on further, other researchers have been taking live neuron cells, including those from crickets, and growing them on Field Effect Transistors to make more sensitive connections between external sensors, either mechanical or biological in origin and electronics. The idea is not new - it has been around for the last two decades, but Professor Jeronimidis believes it is not 20 years away from exploitation but now only two or three years. The biocompatibility issues have now been mostly overcome, and the sensors would be very low cost and disposable, so keeping the cells alive for long periods would not be a problem.

Pointers

* It is possible to make extremely small and sensitive airflow sensors inspired by those associated with hairs on cricket cerci.

* Further developments include the use of live cricket neurons grown on Field Effect Transistors

Natural world shows the way

It is not just crickets, but other insects, spiders, fish, shellfish, plants and humans that have optimised designs through millions of years of evolution from which engineers can learn.

At the DTI event, Professor Jeronimidis declared that biological sensors are generally worthy of study because of their very highly integrated hardware and software systems able to capture and process important information from noisy environments. His list included: chemical (most animals and some plants), vibration (spiders and scorpions), infrared (beetles), air flow (various insects), strain (insects, arthropods and mammalian bone), pressure (fish), touch (most animals and some plants), electrical (fish), and magnetic (fish and birds).

Apart from the crickets, he made particular mention of sensors in bone, which capture loadings in order that the mechanical structure can then optimise itself in order to carry external load. He said that plants also had mechanical sensors that were, "Less well known", including those used by climbing plants to detect stems that they can climb up.

Biological sensors are often extremely small. As an example, he cited spiders 1mm long, which have several thousand sensors, including lyre shaped vibration sensors with slits in the chitin external skeleton to enhance deformation and act as external amplifiers so that the animal can detect the slightest vibration. Experimental mechanical amplifiers for detecting strains in composites derived from sensors on locust legs were the subject of Eureka's April 2002 cover feature story. We have also reported on autonomous devices to explore the human gut inspired by intestinal worms and leaf based fuel cell designs in December 2003. In January 2004 we revealed that a Chinese study of how earthworms eased their passage through soil by inducing small surface electric currents could be used to reduce machine-earth adherence in agricultural and construction equipment. In May the same year, we discovered that Russian researchers had been developing advanced under water vehicles based on earlier English studies of dolphins.

The UK's network to promote these ideas is BIONIS, the BIOmimetics Network for Industrial Sustainability, www.extra.rdg.ac.uk/eng/BIONIS, headquartered at the University of Reading, initially founded with 60,000 from EPSRC but presently funded by Swedish company LUL AB -The initials are those of award winning entrepreneur Lars Uno Larsson. This contrasts with the government support for BIOKON, the German Biomimetics Network, www.biokon.net, which started with 4.7 million DM funding for 3 years, and is currently supported by the German Federal Ministry for Education and Research within the framework of the program "Research for Sustainability" which is to receive 800 million euros over the next five years.

Centre for Biomimetics at the University of Reading
CICADA: Cricket Inspired perCeption And Decision
BIONIS
BIOKON

For more technical developments see www.eurekamagazine.co.uk

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