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Your body the powerplant
(New Scientist Via Thomson Dialog NewsEdge) IT'S one of those days. Your retinal implants say it's twilight, even though you've just finished lunch. Your heart has been skipping beats, yet the new love of your life is nowhere in sight. You've had three blood-sugar alerts and one heart-attack warning from your implanted sensors all false alarms, as usual. Even that pesky bladder-control valve seems to be leaking again. Sigh. If only your doctor had been able to schedule your battery-change surgery sooner but you have to wait your turn.
This cyborgian dystopia is not yet reality, but it's not pure science fiction, either. Millions of people with heart ailments already have implanted pacemakers or defibrillators, and tens of thousands use cochlear implants to counter deafness. A deluge of ideas for other implantable devices is flooding in, including brain stimulators to block hunger signals and combat obesity, or to tackle Parkinson's disease or chronic pain, plus sensors to watch for the molecular warning signs of cancer or a heart attack. There's one big fly in the ointment, though: it's too damn hard to change the batteries.
People fitted with battery-powered pacemakers need surgery every few years just to get the battery replaced. That can cost as much as $20,000 a time and patients don't much enjoy having their chest cavity opened to have the work done. Add a few sensors and actuators, and you've got a bionic future that's just plain insupportable.
Maybe there's a better way. Researchers are working hard to harness the body's inner power not some mystical life force, but the chemical energy locked up in the body's own food stores and convert some of that into electricity. The hope is that medical devices can be made to behave like benign parasites, stealing just enough of this energy to run themselves without you, the host, even noticing. We're talking about a matter of tens of microwatts to a few milliwatts for many applications. The same could be done with the waste heat energy that our bodies pour out, or the kinetic energy of our pulsating muscles. Several of these so-called energy-scavenging systems have already reached the prototype stage, and more are under development. It is possible that within a few years many medical devices will be able to do away with batteries entirely. Eventually, your body (or should that be iBody?) might even deliver enough electrical power to run gadgets like your cellphone or MP3 player.
The most elegant power source for implanted medical devices would be the one your body already uses as its energy supply: glucose. There's an enormous amount of energy there the food you eat each day packs as much energy as a thousand AA batteries so diverting a little of it to power implants shouldn't cause any distress. "It's the electronic version of a tapeworm," says Thad Starner, an energy-scavenging specialist at the Georgia Institute of Technology in Atlanta. "It sits there, it's harmless, it takes such a tiny amount of food from you that it doesn't matter, and it does something useful for you."
Engineers are developing biofuel cells that strip electrons off glucose at one electrode and deliver them to oxygen at the other, creating an electrical potential that can drive a current through an external circuit. The glucose and oxygen are supplied by the body, so the fuel cell itself need consist of little more than the two electrodes and a pair of contacts to feed out the current. The current it delivers is constrained by the surface area of the electrodes but, that apart, the cell can be as small as you like. For a pacemaker or a sensor, which is likely to require only a few tens of microwatts, a glucose biofuel cell could easily be small enough to sit in the chest cavity or in muscle tissue, perhaps attached to the device it is to power. Incidentally, the relationship between power and electrode surface area means you are unlikely ever to be able to use a glucose fuel cell as an aid to losing weight. To do this, the fuel cell would have to consume a substantial fraction of the 100 watts that your body gets from a normal diet, and this would require electrodes with an area of thousands of square centimetres. Just get on a bike instead.
Most of the experimental implantable biofuel cells built so far use enzymes to catalyse the reaction at each electrode, because this yields much more power than non-enzymatic electrodes. There is a drawback, though. The enzymes last only a matter of hours, or a few days at most, before they begin to break down, causing the power output to tail off rapidly. As a result, enzymatic fuel cells have yet to make the jump from the lab into practical use.
To extend the lifetime of the cells, some researchers have tried using more durable enzymes, taken from bacteria found in hot springs. Another promising approach is to house the enzymes within membrane-like pockets in the electrode surface. "It's a straitjacket for the enzyme," says electrochemist Shelley Minteer of St Louis University in Missouri. "It holds it in and doesn't allow it to unravel." Using these nanostructured electrodes, Minteer has kept her enzymes functioning for up to two years.
Even Minteer doesn't expect her fuel cells to last forever. "Whether we're going to be able to operate them for three years or five years or 10 years, they will have a lifetime," she cautions. So if you're going to have to change them anyway, why not just stick with a battery?
Indeed, the task of developing implantable fuel cells that outperform batteries is getting more difficult all the time, as smaller, more powerful batteries come on the scene. A research team led by Adam Heller at the University of Texas, Austin, is developing a case-free battery with a zinc anode and a silver chloride cathode, each coated with a film to hold the reactants in and keep out unwanted cross-reactants. Heller hopes eventually to produce a battery that is less than 1 cubic millimetre in size about one-thirtieth of the size of a comparable conventional battery. While it would only last a few days, that would not matter as he envisions it being used in a disposable patch-style sensor, similar to a type of blood-sugar monitor already on the market for people with diabetes. The user would slip the sensor electrodes under the skin almost painlessly, because they are the size of fine hairs and secure the patch in place. Days later they would just remove the patch, pulling out the electrodes, and replace it with a new one.
If tapping the body's chemical energy proves too difficult, what about scavenging a little of its plentiful kinetic energy? This is present in a variety of forms: the regular, repetitive motions of breathing and heartbeat, for example, or voluntary movements of the limbs, such as walking or bending the arms. Industrial engineers have already produced practical devices that harvest energy from vibrating machinery. Now the race is on to apply this technology inside the body to power medical devices.
Move your bodyIn December 2006, the UK's Department of Trade and Industry announced a 1 million initiative
funded half by government, half by industry for a consortium to develop an in-body microgenerator along these lines. Much of the design work is being done by Perpetuum, a spin-off company from the University of Southampton which has already built a scaled-up prototype five times the projected size of the device and begun testing it in the laboratory. The company's CEO, Roy Freeland, won't reveal how the generator works, other than to say that Perpetuum is looking at two systems: one that uses the heartbeat and another powered by limb movements. "It's commercially sensitive at this stage," he explains.
It is likely that the consortium's new generator works by harnessing the inertia of a moving mass either to force the charged plates of a capacitor together or to move a conducting coil through a magnetic field. The ultimate goal is to build a device about 6 millimetres across by 20 or 30 millimetres long roughly the size of a cigarette butt that will generate 100 to 150 microwatts, says Martin McHugh, business development manager for Zarlink Semiconductor in Caldicot, UK, which is leading the project. That sort of output is easily enough to run a cardiac pacemaker or biosensor. The group hopes to have its generator on the market within five years.
For more power-hungry applications, this kind of inertial generator looks like a poor bet, however. Steve Beeby, who directed the European Union's Vibration Energy Scavenging project at the University of Southampton, estimates that producing 10 milliwatts from an inertial generator might require a 20-gram mass moving through 5 centimetres too unwieldy a device to be implanted.
Another way to convert motion into electricity is through piezoelectrics. Merely bending a piezoelectric material generates a voltage, so on the face of it this looks like a promising option for building a generator. In practice, however, many experts are pessimistic about the prospects of piezoelectric materials for powering medical devices, citing their fragility and limited power output. "You can certainly generate energy, but the amount is not really high," says Shad Roundy, a mechanical engineer specialising in energy scavenging at the company LV Sensors in Emeryville, California.
Some are persevering with piezoelectrics, though. In April, a team led by Zhong Lin Wang at Georgia Institute of Technology described a nanogenerator
made of a forest of piezoelectric zinc oxide nanowires topped by a conducting plate. When the plate is pressed down onto the wires they bend, causing current to flow. Because the wires are so thin just 40 nanometres in diameter they are very flexible and bend easily to produce a comparatively large current.
So far, Wang has extracted no more than a few picowatts of electrical power from his generator, but he is hoping for more. At the moment, some of the "trees" in his wire forest are much taller than others, and only the tallest fewer than 1 per cent actually make contact with the plate and produce electricity. Wang expects better fabrication techniques to improve that dramatically. "It's not a critical challenge," he says. "Within two to three years we should make this a useful device to deliver power. This could be two orders of magnitude better than piezoelectrics can currently offer." He also needs to improve the reliability, as the device now fails after a few months for reasons he does not fully understand.
Once those problems are solved, Wang envisions forming his material into a thin fabric whose flexing would scavenge energy from the flow of body fluids such as blood as it pulses through blood vessels. It could also be implanted into, say, the chest cavity and generate electricity from the beating heart without actually needing to be in contact with the organ. "When you make the nanostructure so small, a small force can make it deform to a large degree," he says.
While biofuel cells and mechanical generators have yet to make it past the prototype stage, thermoelectric generation has already yielded at least one product. The body loses a large amount of energy as heat at the rate of about 100 watts if you are doing nothing more strenuous than reading this article, but at many times that rate in the course of hard physical work. Some of that heat energy or, more precisely, the temperature difference between your skin and the air, or between warmer and cooler parts of your body can be used to make electrons flow, thus generating electricity. This thermoelectric effect has been known for almost 200 years, and is being tested to tap large amounts of waste heat in factories and other industrial plants.
What could work in the factory, though, is not so easy to reproduce in the body. The small temperature differences found around the body generate only tiny voltages, which are difficult to step up to useful levels, and the power output is low. One consumer product, the Seiko Thermic
wristwatch, drew a few microwatts to power its electronics from the heat of the skin. However, the watch was bulky and expensive, and Seiko no longer makes it.
The Thermo Life Energy Corporation in Riverside, California, which produces thermoelectric generators to tap waste heat in industry, is also developing generators that could run off temperature differences of just a few degrees the sort of difference that arises across the skin. By connecting 1000 thermogenerators in series to boost the power output, Ingo Stark, Thermo Life's chief technology officer, says the prototypes deliver about 100 microwatts from a 5 C temperature difference. That's enough to run a cardiac pacemaker or biosensor.
Other teams are managing to generate usable amounts of power from even smaller temperature differences. Rama Venkatasubramanian and colleagues at RTI International in Research Triangle Park, North Carolina, who use nanoscale materials to construct their devices, have been able to generate about 144 microwatts from a 1-centimetre-square device and a temperature difference of just 0.9 C, a difference that is easy to find around the body. The device uses thin films of thermoelectric semiconductor and this means that they can be made very small. "Some of our devices you can barely see they are that thin," Venkatasubramanian says. The team hopes eventually to be able to tap even smaller temperature gradients. The snag, though, has come from the need to step up the low voltages from these materials to the sort of level required for running pacemakers and other devices. Losses during this conversion slash the 144 microwatts scavenged to 67 microwatts of usable power, but that is still sufficient to power a pacemaker. They have other hurdles to clear, though. "It has to be made rugged and reliable," says Venkatasubramanian. "And the cost has to be significantly reduced."
As far as cost goes, salvation may lie with the computer industry. That's because the thermoelectric effect can be run in reverse to create a temperature gradient from an electric current the so-called Peltier effect. The amount of heat generated by microprocessor chips is enough to fry them unless some way is found to remove the excess heat. For chip designers, Peltier coolers are an attractive option, and Venkatasubramanian predicts that this means thermoelectrics could soon become big business. "When the technology gets implemented in chip cooling, the volume will be so huge that it will drive the cost down for some of these energy-harvesting applications," he says.
If the cost comes down far enough, Venkatasubramanian thinks his thin-film technology could eventually lead to slap-on thermoelectric patches that produce enough power to run your cellphone or iPod. "I would venture to guess and I have talked with physiologists that from the neck area and the palms you potentially lose between 10 and 20 per cent of your body's heat, so you should be able to tap into about 10 to 20 watts," he says. Covering that whole area would likely be too uncomfortable, but even if a patch covered just 10 per cent of the neck and palm area, and if only 1 per cent of that 10 per cent could be converted to electricity, the patch would yield 10 or 20 milliwatts easily enough to trickle-charge a rechargeable battery.
"That's the holy grail, isn't it?" says Beeby. "When you talk about energy harvesting, this is what people want to do." Right now there's a gaping chasm, two or three orders of magnitude, between the power that energy scavengers provide, and what is needed for them to be useful day to day. But look back a few years, and there is a similar gap between the power needed by the earliest cellphones and today's energy-efficient designs. If energy scavenging continues to improve its output, and consumer electronics continue to scale back their power needs, it shouldn't be too long before your iBody delivers all the electrical punch you could need, relieving you of the burden of batteries. More power to you.
Copyright 2007 Reed Business Information - UK. All Rights Reserved.
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