Do you have noisy neighbours? If so, you will be pleased to know that
special wallpaper could one day blot them out for good. The paper would
sense the noise and neutralise individual frequencies automatically. Though
it is still too early to start planning the redecoration of your home, soundproof
surfacing is more than a fanciful notion. This is largely thanks to research
begun by the Pentagon to make its submarines and aircraft vanish from the
screens of enemy radar and sonar detectors.
Much of this 鈥榮tealth鈥� technology, which originated in the early 1980s,
followed research on substances that have been dubbed 鈥榠ntelligent鈥� or 鈥榮mart鈥�.
Some of these materials have already crept into everyday life, as labels
for food packaging that change colour if the products have been stored at
too high a temperature and as sunglasses that are indistinguishable from
ordinary spectacles in the shade but that darken in dazzling sunlight.
What makes materials smart is their ability to 鈥榬eact鈥� to changing circumstances.
For example, the materials in the wing of an aircraft might soon be able
to alter their shape 鈥� and thereby the profile of the wing itself 鈥� to overcome
the effects of turbulence or stiff headwinds. This would allow the aircraft
to retain its speed and stability instead of wobbling and slowing down .
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Researchers have already developed artificial muscles and ligaments.
When embedded in a host structure, such as the arm of a robot, they respond
to electrical and thermal signals by bending or flexing. There are also
piezoelectric materials that deform when they are subject to an electric
field or, alternatively, generate a voltage when squeezed. Both properties
are ideal for triggering activity in 鈥榮mart structures鈥�, the structures
or structural elements made from smart materials. And there are polymeric
gels, which are jellies containing strands of polymers and a melange of
charged molecules, or ions. When voltages are applied across the gels, or
when one side of the gel is made more acid than the other, or when the temperature
is altered, the ions become implanted in the strands and make the polymers
change shape.
Research on smart materials divides into two camps. Europe and the US
are trying to introduce new properties and functions into existing materials,
the 鈥榮tructures鈥� camp. Japan, meanwhile, has returned to first principles
and is pioneering the development of smart materials from scratch, the 鈥榤aterials鈥�
camp.
In the US, policy-makers are coming to terms with the end of the Cold
War. They want to transfer military technology, such as smart materials,
to the civilian sector. 鈥榃hat鈥檚 been developed for defence 鈥� for national
safety 鈥� we should adapt it for the safety of civilians,鈥� said Eleanor Sabadell
of the directorate of engineering at the National Science Foundation in
Washington DC. Speaking in May this year at the first European conference
on smart structures and materials in Glasgow, Sabadell was confident of
a major shift in White House policy to make the most of smart materials.
鈥楤ecause we are much more urbanised, we must take care of the urban infrastructure
with a much keener eye on survivability. Therefore, we should look at those
structures, in roads, buildings, bridges and piping, and invest in them
heavily to make them safer.鈥�
The structures camp wants to make buildings, bridges and other edifices
more responsive to changes affecting them by incorporating smart materials
into them. 鈥業t鈥檚 a process of system integration,鈥� says Brian Culshaw, professor
of optoelectronics and founder of the Smart Structures Research Institute
at the University of Strathclyde. 鈥榊ou can introduce more sophisticated
reactions into existing structures.鈥�
For example, fibre optic sensors in bridges, buildings and aircraft
could show where areas of stress were developing. Light would normally pass
uninterrupted through a fibre from one end of a structure to the other.
But if corrosion developed or a crack opened, the light would be deviated
or cut off altogether. Monitoring the patterns of light emerging from the
fibres would enable safety engineers to check structures more easily and
pinpoint structural defects more quickly.
Caroline Dry of the Architecture Research Center at the University of
Illinois in Urbana-Champaign, Illinois, has taken material smartness one
step further. She has developed 鈥榮elf-healing鈥� fibres that not only sense
cracking or corrosion in concrete but repair it automatically. These hollow,
porous fibres, which are made of glass fibre and polypropylene, could snake
throughout the concrete structures. Undue flexing of the concrete would
tear the fibres, causing them to release compounds that would fill cracks.
Other fibres would be wrapped around the steel reinforcement bars that strengthen
concrete. They would be sensitive to the changes in acidity that can cause
the bars to corrode, and some of the fibre coating would dissolve, releasing
a chemical to halt further decay.
In Japan, which joined the field in the late 1980s, the approach has
been completely different. 鈥楾hey took three steps backwards and looked at
functions of materials generally, and how they might be improved,鈥� says
Peter Gardiner, the director of the Strathclyde institute. 鈥楾he drive was
to bring the human into part of the system. They asked: 鈥榃hat parts of a
machine perform human functions?鈥� In 1987, Japan鈥檚 Science and Technology
Agency set up a working party on smart materials that reported its findings
to the government three years later. According to Toshinori Takagi, the
chairman of the working party: 鈥榃e discussed smart materials at a very fundamental
level . . . with people from all fields, from pharmacy and ceramics through
to mechanical engineering.鈥� Japan is now beginning to develop materials
that are capable of sensing their environment, identifying stimuli that
are important and responding to them. 鈥楤y assembling such materials into
systematised 鈥榮mart structures鈥�, we can make high-performance systems that
are self-running,鈥� he says. 鈥楾hey will be tailored to provide combinations
of sensor, information processor, effector, and feedback/feedforward systems
within the materials themselves.鈥� Takagi likened materials with this range
of functions to neurons, the cells in the nervous systems of living things.
Seizo Miyata, a researcher in the Department of Material Systems Engineering
at the Tokyo University of Agriculture and Technology exemplifies the Japanese
approach. He and colleagues are developing thin films of chemicals with
optical properties for use as visual displays in microscopic devices and
to simulate 鈥榥erve impulses鈥� in intelligent materials. They have turned
Langmuir-Blodgett films, which are just one molecule thick, into light-emitting
layers by doping them with rare metal ions. For example, Miyata has doped
organic dyes, such as BABA (behenamidobenzoic acid) and DBBA (4-dodecyl-benzoyl-2鈥�-benzoic
acid), with europium ions, which make the dye films fluoresce in laser light.
By further altering the structure of the films, either by making them
several layers thick or by changing the orientations of their molecules,
Miyata is trying to develop a network of devices of varying optical properties
that will sense and transmit light at different wavelengths. He describes
this work as the rudimentary stages of an artifical neural network.
On the medical front, Japanese researchers are developing artificial
pancreatic cells that automatically keep the blood-sugar concentrations
of diabetics at safe levels. The researchers, at the Tokyo Women鈥檚 Medical
College and at the Science University of Tokyo, are working with a polymer
that expands or contracts in glucose solutions depending on the concentration
of the glucose. At low concentrations, the strands of polymers wind themselves
into tangled balls that can encapsulate molecules of insulin. The idea is
that by injecting these balls loaded with insulin into the bloodstream,
it is possible to mimic the performance of healthy pancreatic cells. When
sugar levels begin to rise, insulin will seep out of the balls; once the
levels have stabilised, the balls will clam up again.
Another area of medical research is under way at Queen Mary and Westfield
College at the University of London. Garth Hastings, assistant director
of the Interdisciplinary Research Centre in Biomaterials at the college,
foresees the development of what he calls 鈥榤enotic鈥� materials. Instead of
implants that simply exist alongside live tissue, menotic materials would
integrate with host tissue, eventually becoming indistinguishable from it.
For example, in the early days of hip replacement surgery there was a huge
mismatch between the properties of the implanted material and those of the
bone. This meant that bone would waste away if the implant took too much
of the strain, but would be destroyed if it was overloaded. 鈥榃e must look
at how we can build new intelligence into implants, so they sense the new
load environment,鈥� says Hastings.
Another aim is to develop an implant for ligaments that would slowly
degrade, transferring increasing levels of stress to the host tissue as
it heals. At Queen Mary鈥檚, Hastings and colleagues are testing components
made of carbon fibres in the repair of bone fractures of the forearms on
more than 30 patients. 鈥楾hey do seem to have an 鈥榠ntelligent鈥� response,
and allow more load to be applied so that the normal healing processes are
allowed to proceed faster than if the fractures were treated in the normal
way through the implant of a rigid metal plate.鈥� Looking further into the
future, Hastings says that it might be possible to use effects such as temperature
changes to trigger drug release. He suggests capsules that only release
antibiotics when the higher temperature of an area of tissue indicates inflammation.
Smart structures are showing great promise, but could they be exploited
more quickly, and how should engineers and doctors decide when and where
to use them? Gardiner says that people should examine what he dubs 鈥榯he
responsibility chain鈥�. In the creation of anything, he says, 鈥榮omeone specifies
what鈥檚 needed, someone designs it, someone makes it and someone maintains
it鈥�. If smart materials are to take off, he says, every link in the chain
must benefit financially from using the technology.
Further reading: The proceedings of the First European Conference on
Smart Structures and Materials, edited by B. Culshaw, P. T. Gardiner and
A. McDonach, are available from IOP Publishing, Techno House, Redcliffe
Way, Bristol BS1 6NX. Price 拢50.
* * *
1: Flying starts for smart materials
Earlier this year, defence contractors in the US expressed an interest
in using smart materials in the fins of Sidewinder missiles. Existing Sidewinders
follow a preset path with the help of sets of rigid fins, at the front and
back, that alter position in accordance with instructions from an on-board
computer.
The contractors now plan to fit piezoelectric elements in the fins that
will make them twist in flight. They claim that the new design reduces aerodynamic
drag on the fins by more than 90 per cent. The piezoelectric elements are
attached in parallel strips to plates of fibreglass, and the plates flex
when voltages are applied to the elements. At a voltage of 600 volts per
millimetre of piezoelectric material, the plates will bend through 44 degrees
per metre of plate and twist through 31 degrees.
The use of smart materials in civil aircraft is a little further away,
but progress is rapid. Gareth Knowles and colleagues at the Grumman research
centre in the US have devised and tested a system for deforming the modified
Boeing 707 wing fitted to the 8C JSTAR, an aircraft of the US Air Force.
The system will make the wing more responsive to changing flight conditions.
The devices that deform the wing form a zigzag pattern in the cross-section
of the wing. The 鈥榣ines鈥� in the zigzags extend and contract to alter the
cross section of the wing. The aim is to make the lines from a smart material
called Terfenol, but this is not readily available yet and so the team
used small motors to make the lines for its first rig.
Preliminary analysis suggests that the design could reduce drag on the
JSTAR wing by between 6 and 26 per cent. 鈥楢ll aircraft are designed to give
optimal performance around just one set of flight conditions. We want to
make aeroplanes which work optimally in other conditions,鈥� says Knowles.
Once the investigators at Grumman find a practical system for deforming
the wing, they will have to develop an intelligent feedback system that
enables a computer to modify the shape of the wings during flight in response
to wind, vibration and turbulence sensors.
But application of smart materials in aerospace is not confined to aircraft
and missiles. Fred Nitzsche and colleagues at the DLR, the German air and
space research institute, have been investigating the scope for embedding
piezoelectic materials in helicopter blades to control unpleasant vibrations
as the helicopter flies forwards. After testing methods for individual blade
control, they focused on a system in which piezoelectric materials in the
blades would generate voltages reflecting the degree to which they were
being squeezed and compressed by the vibrations. To this extent, the materials
would act as 鈥榮ensors鈥�, monitoring the scale of the vibrations. In principle,
using this information, it should be possible to supply responsive voltages
that deform the piezoelectric materials exactly the right amount to damp
down each vibration. For the moment, however, the researchers have found
that the piezoelectric materials are simply not powerful enough to counteract
the strong vibrations generated in the helicopter blades.
Satellites and space stations are also beginning to make more of smart
materials. One key aspect in the functioning of space stations is the need
to keep receivers, such as antennas and satellite dishes, free from vibrations
that would disrupt reception and the interpretation of signals. Often, power
and control modules in satellites are connected to antennas by scaffolds
called 鈥榯russes鈥�. Aerospace engineers have attempted to fit the trusses
with smart materials that sense the size of disruptive vibrations and damp
them out. One concept is to fit the smart materials into the truss as spokes.
Fibre optic sensors all along the truss would gauge the scale of the vibrations
and activate the smart materials accordingly.
* * *
2: Materials smart enough to disappear
Coatings technology provides one of the best examples yet of an area
where defence research on smart materials is translating easily into civil
technology. Vijay Varadan, professor of engineering science and electrical
engineering at Pennsylvania State University, pioneered systems for the
military that would help submarines avoid sonar and aircraft avoid radar.
Varadan鈥檚 鈥榓ctive acoustic coatings鈥� make submarines invisible by generating
signals that 鈥榓bsorb鈥� the reflections from sonar detectors. He achieves
this by coating the submarine with a double layer of smart polymer. The
coating senses incoming sonar waves and feeds instructions to an electronic
feedback controller lying underneath it. The controller gauges the phase
and direction of the arriving sonar waves by noting the slight time lapse
between reception of the waves at the two polymer layers. From this information,
the electronic controller activates a piezoelectric cell to generate waves
180 degrees out of phase with the arriving sonar beam, and the vessel 鈥榙isappears鈥�.
Varadan says the same technology can be used to cut down the noise
from car engines to benefit people inside and outside the vehicle. Likewise,
he says, it is possible to cut out echoes in concert halls and lecture
theatres. 鈥楾he feedback control would match and get rid of the echo,鈥� he
says. The logical extension of this, he adds, is coatings that enable occupants
of houses and flats to mask the sound made by noisy neighbours.
For a boat, Varadan and colleagues developed a way of reducing drag
with a shape-memory alloy that responds to temperature changes by reshaping
the vessel鈥檚 skin. The changing shape of the hull generates eddies in the
water called 鈥榣ove waves鈥� that enable the boat to glide more smoothly through
the water. These waves also gently rub the side of the boat and prevent
crustaceans adhering to its hull. Thus, said Varadan, it could be possible
to reduce reliance on toxic antifoulants such as tributyl tin paint, which
harms marine life if it leaches into the sea.
There could be many civil spin-offs from antiradar smart technology
based on ferroelectric substances or polymer-based chiral materials. These
provide highly efficient shielding for aircraft against electromagnetic
interference. 鈥榊ou apply a voltage to the coating and it changes the absorption,
and you can mimic whatever background you want,鈥� says Varadan.
He likens the technology to 鈥榓ntennae in the skin of the aircraft鈥�.
In military terms, it enables aircraft in flight to automatically locate
and keep track of ground-based radar stations without sending back any detectable
signals.
In civil applications, similar techniques could provide the basis for
smart windows that can be programmed to trap incoming heat in winter and
deflect it in summer. They could also spawn windows that change colour.
At the heart of the smart coatings are conducting polymers based on
siloxane. Varadan and colleagues tailor the properties of the material by
attaching 鈥榗hiral鈥� chemical groupings on the molecular side chains branching
out from the central spine of the material. A chiral compound is one that
differs from its mirror image. Each member of a chiral pair is capable of
twisting polarised light to the same degree, but one twists it to the left
and the other to the right. Together, they cancel one another out so light
passes through a solution of them unaffected. Thus, to be effective optically,
the siloxane must be modified to have the activity of only one of the twins
that form a chiral pair. 鈥榃e always put only a one-handed chiral in. Then
by changing perhaps the pressure on the siloxane or the temperature, we
can change the pitch of the helical structure of the siloxane and thus alter
its refractive index,鈥� says Varadan.