Week of October 10, 2005

The Net: not so vulnerable

A key finding from researchers who have teased out the structure of the Internet during the past half-dozen years has been that that, though the Internet is resistant to random failures, attacks targeting the largest hubs could fragment the network (see Five percent of nodes keep Net together, Hubs increase Net risk).

A new study by researchers from the California Institute of Technology, the University of Adelaide in Australia, Internet2, The Institute of Physical and Chemical Research in Japan, and AT&T Labs, however, shows that research describing the scale-free nature of the Internet has not captured the whole picture.

In light of the new findings, it looks like the Internet is fairly resilient to attacks.

The study showed that the Internet's network of routers, which controls the flow of data between computers connected to the Internet, is different than the scale-free structure of Web sites and the connections between them. While scale-free networks have a few highly-connected sites, or hubs, in the center and many peripheral sites with far fewer connections, the physical router network that underpins the Internet has highly connected hubs at its periphery and less well-connected central hubs, making it resistant to targeted attacks.

The study took performance, constraints and trade-offs into account. In contrast, the scale-free approach takes a statistical physics perspective that deliberately omits particular factors of specific networks. The researchers based their analysis on the structure of the Abilene network, which is the backbone of the Internet2 academic network.

(The "Robust yet Fragile" Nature of the Internet, Proceedings of the National Academy Of Sciences, October 3, 2005)

Cheap solar cells get efficient

Plastic solar cells promise to dramatically lower the cost of generating electricity from sunlight -- once researchers figure out how to make them more efficient.

Plastic solar cells tend to capture from 1 to 3 percent of the energy contained in the sunlight that hits them, while standard, relatively expensive silicon solar cells capture as much as 30 percent.

Researchers from the University of California at Los Angeles and the National Renewable Energy Laboratory have improved the picture with a plastic solar cell that is 4.4 percent efficient. The key to the efficiency boost is giving the blend of plastics, or polymers, more time to dry. The longer drying time allows the polymers to self-organize, increasing polymer mixing and reducing electrical resistance.

Making plastic solar cells using the method is relatively easy, paving the way for inexpensive plastic solar cells with high enough efficiencies to be commercially viable.

The advance is the latest in a string of solar energy research improvements and breakthroughs, including a method of spraying on solar cell material, a material that increases efficiency by converting energy from the full spectrum of sunlight and a device that doubles potential solar efficiency by generating twice as much electricity per captured photon.

(High-efficiency Solution Processable Polymer Photovoltaic Cells by Self-organization of Polymer Blends, Nature Materials, October 9, 2005)

Swimming blood cells

Scientists have turned red blood cells into microbial cyborgs by equipping them with artificial filaments and using magnetic fields to cause the filaments to propel the cells.

The French National Center for Scientific Research (CNRS) and Harvard University researchers created the artificial microbes in order to better understand swimming at the microscopic level, where swimming through water is the equivalent of swimming through honey at the human scale.

The red blood cells' filaments consisted of one-micron-diameter magnetic particles strung together and attached with DNA molecules. A steady magnetic field acted to keep the artificial flagella extended while a second, oscillating magnetic field moved the filaments to produce the necessary swimming motions.

The method promises to be useful in precisely positioning microscopic devices and biological entities like blood cells and in pumping tiny amounts of fluids through biochips.

(Microscopic Artificial Swimmers, Nature, October 6, 2005)

Microbes drive sensor

Scientists are looking for ways to use simple chemistry techniques to make electronics that are smaller, faster and cheaper than today's chip-based devices. Researchers have recently begun using microbes in this effort, both as templates for wires and electrodes, and also as living components whose biological responses can play a role in the devices' operation.

Researchers from the University of Nebraska have made a prototype humidity sensor using Bacillus cereus bacterium coated with gold nanoparticles. The researchers coated 5-micron long bacteria with 30-nanometer-diameter gold nanoparticles and used a pair of thus bedecked bacterium to bridge gold electrodes. A micron is one thousandth of a millimeter and a nanometer is one thousandth of a micron.

The device acts as a humidity sensor because a decrease in humidity shrinks the bacterium, shrinking the spaces between the nanoparticles on its surface, which decreases the electrical resistance across the bacterium. Decreasing the humidity from 20 percent to 0 percent increased the current 40 fold, making for a relatively sensitive sensor.

The work paves the the way for sensors and other devices that combine the biological properties of living microorganisms with the electrical, optical and/or magnetic properties of nanoparticles. Such devices promise to be relatively inexpensive because they can be constructed using simple chemistry rather than chipmaking methods.

(Self-assembly of Nanoparticles on Live Bacterium: an Avenue to Fabricate Electronic Devices, Angewandte Chemie, October 10, 2005)

Bits and pieces

A laser-driven shape-shifting plastic device snares blood clots; a rotary motor brings molecular machines another step closer to reality; tiny solid-state electronic heaters and coolers bring temperature control to biochips.