Friday, February 8, 2008

Knee Power

A new human-powered generator tries to capture walking energy.

Power walking: This knee brace (above) sports a generative-braking power system that converts energy expended while a person is walking into electricity.
Credit: Greg Ehlers, Simon Fraser University
See the knee brace in action.

Engineers who design wearable devices that harvest human energy for power face a daunting dilemma: how do you collect a significant amount of power without making the user expend a lot of extra effort? Gadgets like hand-crank generators and windup radios require manual work from a user, and existing shoe-mounted generators produce less than one watt of power.

A team of engineers has developed a modified knee brace that captures energy that would otherwise have been lost while the wearer walks. The generator produces about five watts--enough to power 10 cell phones simultaneously.

"If you want power, go where the muscles are," says Max Donelan, a professor at Simon Fraser University, in British Columbia, who led the research. "We thought, maybe there's a smart, selective way to do energy harvesting when muscles are normally decelerating in the body." Donelan's research appears in the February 8 issue of the journal Science.

Donelan looked to the legs, which have the largest muscles in the body, and capitalized on a careful understanding of how humans use energy to walk. During an average stride, a person uses her muscles to bend at the knee and swing her leg forward, like a pendulum bob. This is positive work. At the end of the swing, she executes negative work to decelerate her moving leg. She places her foot on the ground, and by then her other leg has begun its swing.

Donelan and his team concentrated on harvesting energy from the end of the stride using their bionic knee brace. When the brace's generator is engaged, it collects power while slowing down the motion of the leg. As a result, the brace reduces the human effort required at the end of the swing phase.

If the mechanism were continuously engaged, however, it would also impede acceleration at the beginning of the swing and require more energy from the wearer. To solve this problem, Donelan installed a sensor in the device to monitor the knee angle and switch the generator on and off. According to his research, this "generative braking" approach requires only one-eighth the metabolic power of a continuously operating mechanism.

"What's extremely clever about this device is that it only tries to capture mechanical energy when the muscles would be primed to slow the body down," says Lawrence Rome, a biology professor at the University of Pennsylvania. Rome, who did not work on the knee brace, recently designed a backpack that converts walking energy into electricity. "[Donelan's knee brace is] a smart device, and it only works when you're trying to brake yourself," says Rome. "It lets the reverse torque of the generator do the work of the muscle."

If Donelan's approach to energy harvesting sounds familiar, it's because he uses the same strategy employed by hybrid automobiles. When a driver applies the brakes of a hybrid, the electric motor begins to act as a generator. The generator slows down the car and at the same time converts kinetic energy into electricity, which is then used to recharge the battery. Conventional braking systems rely on friction to slow down, and the car's kinetic energy is dissipated as heat.

"Walking is like stop-and-go driving," Donelan says. "Within every stride, the muscles are accelerating and then decelerating the body. Hybrid cars take energy and give it to the battery."

Donelan's prototype weighs in at just over three pounds, and he is currently developing a lightweight model that could be used by prosthetics manufacturers and the military. Demand for human-energy harvesters like Donelan's knee brace and Rome's backpack is increasing, thanks in part to the proliferation of small electronics like cell phones and handheld GPS units, especially in the military.

"A soldier with a 24-hour mission [might have to] carry nearly 30 pounds of batteries with him," Donelan says. "They have to power everything from GPS to communications to night vision."

In addition, Donelan says that his knee brace has potential in medical markets: it could augment a paralyzed limb or power a prosthetic. "You could take a healthy limb and use it to power the injured limb," he says.

A Window into Alzheimer's

Advances in imaging shed light on how the disease develops.

Instant plaques: Because Alzheimer's disease develops over the course of decades, many scientists assumed that its hallmark brain lesions, called amyloid plaques, were slow to appear. A new imaging technique called multiphoton confocal microscopy reveals that on the contrary, plaques can form in a single day. This image of a mouse's cortex shows a mature plaque (large blue splotch) with a brand-new "microplaque" (small blue dot within the white square) developing beside it. Reimaging the same brain area over several subsequent days showed the plaque growing.
Credit: Melanie Meyer-Luehmann et al., courtesy of Nature Publishing Group

An innovative imaging technique has revealed that the plaques that develop throughout the brains of Alzheimer's patients can form overnight, and they are likely a cause rather than a symptom of the disease.

Plaques, a defining hallmark of Alzheimer's disease, are brain lesions that result from the abnormal accumulation of a protein called amyloid-beta. Since the symptoms of the disease progress over the course of decades, plaques were generally thought to appear and accumulate slowly.

"The notion was that since the disease plays out over a long period of time, individual lesions in the disease process would also have that same tempo," says Bradley Hyman, director of the Alzheimer's unit at Massachusetts General Hospital's MassGeneral Institute for Neurodegenerative Disease. But his study's results, which appear in this week's Nature, suggest that plaques can develop in a single day.

Hyman's team harnessed a fledgling imaging technique called multiphoton confocal microscopy to peer into the brains of living mice. The technique generates images using rapidly pulsed lasers that penetrate deep into living tissue without damaging it. By cutting out tiny sections of skull and replacing them with glass, the researchers created windows into the brains of mice that were genetically engineered to develop amyloid plaques. They could then repeatedly observe the same area of brain, and thus follow plaque formation over time.

"This gives us an opportunity to apply a time stamp to the events that are occurring," says Hyman. "So rather than simply having an individual snapshot of a pathophysiologic event, we can watch the process evolve."

While groups have applied multiphoton confocal microscopy to living brains before, Hyman's group is the first to apply the technique to the study of a neurodegenerative disorder. "It really pushes the technology forwards," says Steven Finkbeiner, associate director of the Gladstone Institute of Neurological Disease at the University of California, San Francisco, who was not involved with the study.

Besides revealing the surprisingly fast pace of plaque formation, the study addresses a long-standing debate over the role of amyloid plaques in the development of Alzheimer's disease.

A long-established hypothesis posits that amyloid plaques themselves bring about damage to neural tissue, causing the disease's symptoms--most notably behavioral changes, memory loss, and dementia. But some scientists counter that plaques are not correlated strongly enough with the disease to be a convincing culprit for its symptoms. Rather than causing the symptoms of Alzheimer's, plaques could themselves be symptoms--stemming from some other, yet unknown mechanism.

"There's always been a lot of debate," says Juan Troncoso, codirector of the Alzheimer's Disease Research Center at Johns Hopkins School of Medicine, who was not involved with the study. "What happens first, and what's responsible for what? Is the damage to the nerve cells first, and then the plaque, or vice versa?"

Because the new imaging technique followed plaque formation in detail over many days, it could address this chicken-and-egg conundrum as previous approaches could not. "When you only have single snapshots of the process, it's hard to be sure how to interpret causation," says Hyman.

Hyman's team found that plaque formation was indeed the first step in the process, with amyloid-beta protein depositing into an aggregate that appeared quickly and continued to grow. Next, immune cells called microglia were activated and flocked to the area. In the ensuing days, a halo of damage began to appear around the plaque. Nearby neurons became distended and twisted into abnormal, corkscrew-like shapes, likely hampering their ability to transport critical cell components and communicate with one another.

"The bottom line," says Troncoso, "is that this study establishes that at least in the mouse, the plaque is the first step." This kind of investigation would not be possible in humans for ethical reasons, and there's no guarantee that the mechanism observed in mice is the same one that takes place in the brains of human Alzheimer's patients. But Troncoso says that the results are relevant nonetheless. "These animal models are our best available tool to try to understand these types of processes," he says.

Finkbeiner agrees that Hyman's results implicate amyloid plaques as the instigators of the neural damage that surrounds them. "I think this study clearly establishes that the dystrophy that you see in association with plaques does occur after the plaque forms," he says. But he contends that there is still no powerful evidence that such damage is to blame for the primary symptoms of Alzheimer's.

"I don't doubt for a minute that dystrophy does have deleterious consequences for the neurons involved," says Finkbeiner. "But it probably doesn't explain the majority of symptoms that people get with Alzheimer's disease."

Hyman maintains that the local damage associated with plaques could very well underlie the systemic disruption in neural function that characterizes the disease. "Ultimately, the types of changes that we see, I think, lead to a breakdown in the connections of the brain," he says.

If that is the case, preventing amyloid buildup is likely to be a key strategy in treating Alzheimer's. According to Troncoso, since the study "strongly suggests that amyloid is a very early event in the development of Alzheimer's disease, the corollary would be that it becomes the therapeutic target of choice."

Hyman plans to probe the plaque formation process in more detail, investigating how the amyloid-beta protein develops into a full-blown plaque, and how it brings about the observed changes in neighboring neurons.

Large-Scale Rewritable Holograms

A new material allows researchers to write and erase 3-D images for displays.

Holographic car: A new material could make possible very large holographic displays that can be erased and rewritten. Eventually, the displays could be as big as cars. The car pictured here is on a prototype display that's 10 centimeters on a side.
Credit: Savas Tay, University of Arizona
See a video of a hologram.
Listen to a description of the technology.

A holographic display based on a new material can be repeatedly written to and erased. Rewritable holograms have been possible at small sizes, such as for holographic memory devices. But it's been difficult to make these materials at a scale large enough for displays. The new material, developed by researchers at the University of Arizona and at Nitto Denko Technical Corporation, in Oceanside, CA, could eventually allow for life-sized displays of people and objects the size of cars that could be refreshed every few minutes.

Existing high-end holographic images can be full-color and extremely detailed, but they've been restricted to still images that can't be rewritten. Stereoscopic displays, in which a different two-dimensional image is shown to each eye, are the basis of 3-D movies, but they lack some of the realism of holograms. The new display can produce holographic images, which are easier to view than stereoscopic images and can be of higher quality. But the display is better than typical holographic images in that it can be updated.

The University of Arizona researchers developed a new polymer-based material that encodes information using electric fields. The material contains two components. When light strikes the film, one of these components, a polymer, absorbs photons and generates electrons and their positive counterparts, called holes. The polymer is also a good conductor of holes, but not of electrons. As a result, the holes can easily move away from the illuminated areas where they were generated, whereas the electrons stay put. This separation of charges creates patterns of tiny electric fields within the material. These electric fields change the way that light moves through the different parts of the film.

The second component of the material, a dye, responds to the electric fields in two ways. The dye molecules change their polarization and physically rotate depending on the nature of the fields in each part of the film. These changes locally affect the index of refraction, which has to do with how a material bends and reflects light. When the researchers shine a laser through the film, the dye alters the path of the light, projecting a pattern that the eye interprets as a three-dimensional image. "It comes out of thin air--you feel like you could touch it," says Nasser Peyghambarian, a professor of materials science and engineering at the University of Arizona, who led the work.

To erase the image, the researchers expose the film to uniform light, which redistributes the electrons and holes, removing the electric fields and the changes in the material that they had produced.

Researchers have tried making rewritable holographic displays in the past, but they faced a number of problems. Materials failed to produce bright images, for example, or the images faded quickly. Peyghambarian's new materials can preserve an image for hours and produce very bright images. The materials can also be easily made in large areas. The prototype holographic film created with the new material is 10 centimeters on a side, but because it was made using well-known polymer processing techniques, it should be relatively easy to scale it up to much larger sizes, says Joseph Perry, a professor of chemistry and biochemistry at Georgia Tech.

The process currently takes a few minutes to write and erase an image--much too long for video. But it might be possible to significantly increase the write and erase speed, says Perry. There are two key limitations right now. One is how fast the electric fields can be established, which is determined by how fast the holes can move. Next, once the fields are in place, it takes some time for the dye molecules to rotate. One way to improve the speeds is to amplify the other property of the dye that changes the behavior of light--the change in polarization. Right now, this is a small effect, but the polarization changes very quickly--fast enough to change the image in real time, Perry says.

For many applications, the new approach will face stiff competition from a growing number of 3-D technologies that can already display video and, like the new approach, do not require that the viewer wear special equipment. That could limit the applications of the new display to those that don't require fast updates, such as maps, says Brian Schowengerdt, a research scientist at the Human Interface Technology Laboratory at the University of Washington. Peyghambarian's approach could also have an advantage for very large displays, since the other technologies are difficult to make that size. These could be used for high-end marketing displays, says Neil Dodgson, the director of studies in computer science at Emmanuel College, part of the University of Cambridge.

"People already spend a lot of money on holograms," Dodgson says. "An updatable one would be a fantastic advertising medium."