DOI 10.5162/IMCS2012/P2.2.12
Design and Implementation of a High-Frequency Surface
Acoustic Wave Sensor Array for Pheromone Detection in
an Insect-inspired Infochemical Communication System
S. Thomas, S. Li Tai Leong, Z. Rácz, M. Cole, and J.W. Gardner
Microsensors & Bioelectronics Laboratory, University of Warwick, Coventry, United Kingdom,
J.W.Gardner@warwick.ac.uk
Abstract
This paper reports on the design and development of a novel pheromone detection biosensor module
that forms part of an infochemical communication system. Infochemical communication in insects is
based upon the use of sex pheromones for the tenacity of attracting conspecific mates. Our system
involves three functional blocks: a chemoemitter module comprising a high-precision syringe pump
and a micro-machined artificial gland; a chemoreceiver module comprising an array of four polymercoated dual Surface Acoustic Wave Resonator (SAWR) sensors with associated oscillator drive
circuitry; located inside a small odour chamber. Chemical information takes the form of insect
pheromone compounds that are released inside the odour chamber by a silicon-based microevaporator and then detected by the sensor array. The design and fabrication of a high frequency 262
MHz oscillator circuit is described giving stable Surface Acoustic Wave (SAW) signals with reference
SAWRs used to offset drift.
Key words: SAWR Sensors, pheromone, infochemical communication, high frequency, oscillator
circuit.
Introduction
It is of fundamental significance for a wide
variety of living organisms to exchange
molecular information and process it in a unique
way. Intraspecific communication in insects is
achieved by the use of semiochemicals; these
odour cues are able to modify behavior such as
conspecific mate finding, location of food, and
other social interactions [1]. This is due to the
fact that, for example, moths utilize their
olfactory system that enables them to translate
odour signals to a complex behavioural pattern
within the antenna lobe, which is at the bottom
of the detecting organ (antenna). The work
presented
here
mimics
chemical
communication in insects in which a female
moth releases a sex pheromone and the male
moth detects it remotely and arises from an EUfunded Framework project called “iCHEM” [2].
Pheromones play an important role in the
behaviour of an insect by triggering a preencoded behaviour that enables the male to
locate the calling female. These pheromone
components, usually released by the female
moths as a blend of different compounds, are
biosynthesized in the insect‘s pheromone
gland.
Fig. 1. Schematic block diagram of the biosynthetic
insect-inspired communication system.
In order to accomplish species-specificity, these
pheromone components are blended with
variation in molecular chain length, unsaturation
level, functional group and total number of
compounds. The olfaction system of the moth
Spodoptera littoralis has been studied
extensively in recent years, both from a
behavioral and neurophysiological view [3].
Since (Z,E)-9,11-tetradecadienyl acetate (ZE9,11- 14:OAc) forms the main component in the
sex pheromone of this moth [4], we have
employed this pheromone compound for the
establishment of insect-based communication
protocol in our work.
IMCS 2012 – The 14th International Meeting on Chemical Sensors
1440
DOI 10.5162/IMCS2012/P2.2.12
Figure 1 shows a schematic block diagram that
is analogous to pheromone communication of
insects. Our experimental set-up consists of a
chemoemitter that generates a set of semiochemical signals, an odour chamber simulating
a wind tunnel and a piezoelectric chemoreceiver containing an array of SAWR sensors
coated with different types of polymers. The
differential signal generated by the SAWR
sensors produce varied responses allowing us
to detect and decode the pheromone mixtures.
Insect-inspired Communication System
I.
Biosynthetic Pheromone Emission
The artificial pheromone-emitter gland is driven
by a syringe pump and an evaporation
controller system used to control a micromachined evaporator [5] for the release of
pheromone
into
the
chamber.
This artificial gland imitates the functionality of
the biological mechanism that allows the moth
S. littoralis to release pheromones in specific
dosing. Our chemoemitter is crucial for the
labeling of information before they are
transmitted into the real world based on relative
proportions between compounds [5].
As the normal solution of pheromone Z,E-9,1114:OAc is highly concentrated, hexane was
used as the solvent for diluting the pheromone.
The microevaporator temperature was set to a
temperature of 90°C and different volumes of
this pheromone solution was delivered to the
chip by a syringe pump at a flow rate of 5
µL/sec, in order to input the odour stimulus into
the wind tunnel.
II.
Fig. 2. Photograph of a dual 262 MHz SAW
resonator sensor with surface mount amplifier, filter
and buffer circuitry.
The X-propagating ST-cut quartz based gas
phase dual SAW Resonator sensors with
aluminum electrodes are designed to resonate
at 262 MHz in a feedback loop of a simple radio
frequency (RF) oscillator circuit. The design
specifications of the SAWR are 60.5 pairs of
interdigitated transducers (IDTs) with 3 µm
finger width, acoustic aperture of 720 µm and a
cavity length of 1764 µm. 550 reflector gratings
surround the IDTs for creating a standing wave
pattern. Figure 2 shows the photograph of the
dual 262 MHz SAWR sensor with surface
mount oscillator circuitry.
Chemoreception using SAWR Sensors
A highly-sensitive (ppb level), polymer-coated
RF-based SAWR oscillator array was employed
to detect the semiochemical concentrations in
the gas phase, which electronically simulates
the complex olfactory system of the night-active
insects. More specifically, the SAWR sensor
array mimics the insect’s antenna with different
types of sensilla designed for detection of
various olfactory stimuli [4]. The SAW resonator
based sensors are not only highly sensitive, but
also simpler in operation having only frequency
output and so require a much simpler interface
circuitry for data processing.
The dual SAWR arrangement enables the
generation of differential signals that help
ameliorate common mode variations, e.g. drift.
Fig. 3. Schematic diagram of the SAWR sensor
coated with a polymer based functional layer and
exposed to gas molecules.
Four different polymers, namely polycaprolactone (PCL), poly(9-vinylcarbazole)
(PVK), poly(styrene-co-butadiene) (PSB) and
poly(ethylene-co-vinyl acetate) (PEVA) were
recognized as the optimal polymers that were
stable, reasonably sensitive and selective to the
insect pheromones. Therefore, one side of each
dual SAWR was functionalized with these
polymers (schematic shown in figure 3) while
the other side served as a reference, thus
enabling the decryption of the encoded
information. Figure 4 [A] shows an optical
microscopy image specifying the locus of the
polymer coating on a dual SAWR sensor and
figure 4 [B] shows the magnified view of the
polymer coating.
IMCS 2012 – The 14th International Meeting on Chemical Sensors
1441
DOI 10.5162/IMCS2012/P2.2.12
Fig. 4 [A]. Optical micrograph specifying the locus of
the polymer coating on one side of a dual SAWR
sensor [B]. Magnified view of the polymer coating.
III.
resonator. Therefore the circuit employs a
commercial high gain monolithic microwave
integrated circuit (MMIC) wide-band amplifier
µPC2710 (NEC), having a gain of 33 dB, with a
maximum input power of 10 dBm and a
maximum output power of 13.5 dBm allowing
the device to be operated safely with a loop
loss of 3.5 dB or more. The low pass filter
consisting of passive elements suppresses the
oscillations at other spurious frequencies. A
commercial bipolar MMIC wide band amplifier
µPC2711 (NEC) forms the buffer stage with a
gain of 13 dB in order to measure the
differential analogue oscillator output signal.
Oscillator Circuit Design
The SAWR sensors were driven by amplifier
based independent feedback-oscillator circuits
consisting of surface mount amplifiers, a low
pass filter, and the buffer circuitry. The oscillator
Printed Circuit Board (PCB) is a four layer
board with separate grounding planes which
takes account of the phase shifts associated
with the RF signal path within the resonatorcircuitry loop. It also reduces the RF cross talk,
thereby improving noise sensitivity. A schematic
layout of the RF oscillator circuit is shown in
Figure 5 comprising low pass filter, amplifier
and buffer stages.
Fig. 6. Attenuation vs. frequency plot for a 262 MHz
SAW resonator Sensor.
Experimental Setup
Fig. 5. Schematic diagram of circuitry to drive the
SAWR sensor and buffer the read-out signal.
The SAWR devices are used as the frequency
and phase selective elements in the feedback
loop of the oscillator circuit because they
provide a maximum signal transmission at the
designed frequency (262 MHz) with very low,
typically 12 dB attenuation as indicated by the
scattering parameter analysis in figure 6. In
addition
to
the
SAWR
attenuation,
approximately 2 dB is also lost through the
filters and cables in the circuit. The amplifier
gain should always exceed the total signal loss
in the feedback loop of the oscillator circuit
elements by a typical gain margin of 5 dB [2],
for maintaining a stable operation of the SAWR
Fig. 7. Test chamber containing pheromone microevaporator and array of four polymer-coated SAWR
sensors.
A prototype of the insect-based pheromone
detection system has been constructed as
shown in the figure 7. The chemoemitter
module consists of a neMESYS high-precision
dual channel syringe pump (cetoni GmbH,
Germany) that drives a micro-machined artificial
gland, releasing pheromone into the 14x14x40
3
cm Perspex odour chamber. The polymer
coated 4-sensor dual SAWR array, with the
related RF oscillator circuitries and interfaces
make up the chemoreceiver module. Using a
IMCS 2012 – The 14th International Meeting on Chemical Sensors
1442
DOI 10.5162/IMCS2012/P2.2.12
commercial FQ4 interface instrument (JLM
Innovation, Tubingen, Germany) and a desktop
computer within the experimental system, the
output of each sensor was measured and
recorded independently.
Results
In this paper we have shown the response of a
SAWR sensor array to a biosynthetic
pheromone compound and the development of
a custom high-frequency oscillator circuit to
drive four dual SAWR sensors. The differential
frequency response of the polymer coated
SAWR sensors to the pheromone was
successfully measured as shown in Figure 8.
Fig. 8. Differential frequency response of four
different polymer-coated SAWR sensors to
pheromone Z,E-9,11-14:OAc.
In random repeated sequences, 5, 10, 15 and
20 µl of a solution of Z,E-9,11-14:OAc (10%
volume in hexane) were introduced into the
odour chamber using the microevaporator and
the responses obtained were approximately
linear (see Figure 9) and can be classified using
a simple linear statistical classifier. Moreover, it
can be also observed that variations in blend
concentrations are clearly visible and separable
as well. This also proves that the pheromones
can be transmitted and detected by the different
SAWR sensors. Thus proportionately-encoded
signals can be transmitted in our infochemical
communication system.
Conclusions
We have presented an integrated chemoemitter
and chemoreceiver system that simulates
infochemical communication in insects, thereby
forming a new branch of expertise for
information transmission using only molecules.
The differential sensor outputs can be directly
fed into a field-programmable gate array for
neuromorphic signal processing permitting
autonomous robot control for search and
rescue, and has become an intricate scheme
for encoded message transmission. In addition
to providing the required selectivity that is
necessary for pheromone blend specificity, the
SAW sensors offer low-cost and low-power,
and offer a route to full CMOS integration with
the driving, data acquisition and processing
circuitry.
Future development of this work is being
directed towards testing and optimizing the
capability of the system to generate and detect
a precise mix of pheromone compounds in
programmable
ratios
of
concentration.
Moreover, further advances include the
integration of the olfactory sensor drive and
interface circuitry into an application-specific
integrated circuit (ASIC) chip boosting them to
full system-on-chip (SoC) implementation of
chemoreceiver system.
Acknowledgements
This work was supported by the EC 6th
Framework Programme, and FET project no.
FP6-032275 called “Biosynthetic Infochemical
Communication”.
References
[1] E. Hartlieb, et al., “Appetitive Learning of Odours
With Different Behavioural Meaning in Moths,”
Physiology & behavior, vol. 67, no. 5, 671-677
(1999).
[2] iCHEM Project, EU FP6 STREP project, see
www.go.warwick.ac.uk/iCHEM
[3] R. J. Fan and B. S. Hansson, “Olfactory
Discrimination Conditioning in the Moth
Spodoptera littoralis,” Physiology & behavior, vol.
72, no. 1-2, 159-165 (2001).
[4] P. Anderson et al., “Increased Behavioral and
Neuronal Sensitivity to sex Pheromone After Brief
Odor Experience in a Moth,” Chemical senses,
vol. 32, no. 5, 483-491 (2007); doi:10.1093/
chemse/bjm017
[5] W. P. Bula et al., “Artificial Gland for Precise
Release of Semiochemicals for Chemical
Communication,” 14th International Conference
on Miniaturized Systems for Chemistry and Life
Sciences, 671-673 (2010).
Fig. 9. Response of four polymer coated SAWR
sensors to different volumes of odour pulse.
IMCS 2012 – The 14th International Meeting on Chemical Sensors
1443