Power Walking
Knee brace collects energy from its user’s stride.

Source: “Biomechanical Energy Harvesting: Generating Electricity during Walking with Minimal User Effort”
Max Donelan et al.
Science 319: 807-810

Results: Scientists at Simon Fraser University in British Columbia have designed a knee brace that can harvest as much as 13 watts of power from the energy of its wearer’s strides, enough to charge 30 phones simultaneously.

Why it matters: With the proliferation of small gadgets that need to be charged, engineers are looking for alternatives to electrical outlets for charging. Researchers had already made shoe-­embedded generators that are easy to use, but they don’t collect more than a watt of power, making them impractical.

Methods: The researchers looked at the biomechanics of the human gait and saw that at the end of a stride, a person must actually exert energy to slow his or her moving leg. When the brace’s generator is engaged, it helps slow the leg for the wearer, capturing energy in much the same way that a hybrid car harvests power from braking.

Rather than forcing the wearer to work harder to produce extra energy, the brace reduces the effort exerted at the end of a stride. A sensor in the device monitors the angle of the knee to turn the generator on and off so that it doesn’t impede motion in the early part of a stride, when the knee is accelerating.

Next steps: The prototype device weighs just over three pounds; a spinoff company, Bionic Power, is developing a lightweight model.

Better Phase-Change Memory
Improved technology could compete with flash

Source: “A Multi-Level-Cell Bipolar-Selected Phase-Change Memory”
Ferdinando Bedeschi et al.
International Solid-State Circuits Conference, February 3-7, 2008, San Francisco

Results: Researchers at Intel and STMicroelectronics have developed an algorithm that doubles the amount of data that can be stored in a single phase-change memory cell, which represents bits as distinct arrangements of a material’s atoms.

Why it matters: For the past decade, flash memory has pro­vided compact, sturdy storage for small devices such as iPods and cell phones. But flash chips, which store data as electric charge, may soon reach the limits of their capaci­ty. Phase-change memory is among the alternatives that engineers have been pursuing. Previously, a phase-change memory cell could represent only one bit of data at a time. But the researchers found a way to store two bits in each cell, doubling the capacity of phase-change memory and making it competitive with flash.

Methods: A typical phase-change memory cell uses a type of glass that can switch back and forth between amorphous and crystalline states; the crystalline state represents a 1, the amorphous state a 0. The researchers created two-bit memory cells by giving the glass two more states in between amorphous and crystalline. To write data to a memory cell, an electrode heats it until a crystalline filament forms in the amorphous material. The bigger the filament grows, the more current passes through the memory cell. In the amorphous state, the memory cell represents two 0s. In the semi­amorphous state, it represents a 0 and a 1. In the semicrystalline state, it represents a 1 and a 0. In the crystalline state, with memory cell resistance at its lowest point, it represents two 1s.

Next steps: The phase-change chips were made using existing fabrication processes that yield memory cells larger than those of a flash memory chip. Future versions of phase-change memory should use newer processes that produce smaller cells, but researchers need to make sure that the technique holds up as the cell sizes shrink.