Intelligent Machines

From the Labs: Nanotechnology

New publications, experiments and breakthroughs in nanotechnology–and what they mean.

New Circuit Element
The memristor could be useful for nonvolatile memory

An image from an atomic force microscope shows a circuit with 17 memristors lined up in a row. Every memristor shares the same bottom wire (top left to bottom right), and each has its own top wire. The memristors are between the wires.

Source: “The missing memristor found”
R. Stanley Williams et al.
453: 80-83

This story is part of our July/August 2008 Issue
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Results: Researchers at HP Labs have fabricated a memristor, or memory resistor–a fundamental electronic device that had been described theoretically but never produced until now. The amount of charge that flows through the device can be changed by exposing it to an electrical voltage. Applying a positive voltage lowers its resistance, and applying a negative voltage increases it. Furthermore, the change in resistance is proportional to the length of time the voltage is applied: the more the device is charged, the more electricity it conducts. Once set, the resistance stays the same until it’s reset.

Why it matters: Memristors could lead to ­nonvolatile memory chips that store more data than flash ­memory. They could also be used in processors designed to mimic aspects of the human brain. In the brain, learning depends on changes in the strength of connections between neurons. The memristor can be used to set the strength of connections between transistors, achieving a similar effect. Chips using memristors could be useful for face recognition and for controlling robot movement.

Methods: The new memristors consist of two layers of titanium dioxide sandwiched between two electrical contacts. One layer of titanium dioxide is an insulator, blocking the flow of electrons from one contact to the other. The other layer, which has fewer oxygen atoms than titanium dioxide normally does, conducts electricity.

When a voltage is applied, some of the oxygen ions move from the first layer into the oxygen-­deficient layer. That improves the conduc­tivity of the first layer, allowing electrons to pass through the memristor from one contact to the other.

Next steps: Since Hewlett-Packard doesn’t make ­memory chips, the tech­nology will probably be licensed to another company for product development. The HP researchers are working on a prototype that combines transistors and memristors to form a brainlike chip.

Nano RNA Therapeutics
Lipidlike materials improve delivery of RNA for silencing gene expression

Source: “A combinatorial of lipid-like materials for delivery of RNAi therapeutics”
Daniel G. Anderson et al.
Nature Biotechnology
26: 561-569

Results: By developing new techniques of chemical synthesis, researchers at MIT and Alnylam Pharmaceuticals in Cambridge, MA, were able to quickly make 1,200 different lipidlike molecules that serve as building blocks for nanoscale containers called liposomes. Some of the resulting liposomes proved effective at introducing RNA into a variety of different cells in rodents and primates. The RNA was able to block the action of certain genes and of microRNA, which serves to regulate genes.

Why it matters: Silencing genes with RNA–a technique known as RNA interference, or RNAi–is a potentially powerful therapy for genetic diseases, viral infections, cancer, and even heart attacks, but it’s been difficult to deliver RNA to cells, since the body’s immune system breaks it down quickly. The new delivery agents skirt the body’s defenses.

Access to such a large collection of lipidlike materials will allow scientists to better understand how to design effective delivery nanoparticles. And the new synthesis methods could help researchers find delivery agents that are even more effective, which could increase the chances of success for RNA-based therapies, as well as other drugs.

Methods: The researchers developed a one-step process for making molecules that resemble the lipids typically used for the commercial manu­facture of liposomes. Then they developed ways to screen the ability of liposomes made from these mole­cules to deliver RNA directly to cells in a petri dish and to living tissue in animals.

For in vivo tests, the researchers added cholesterol and an ether called polyethylene glycol to their molecules to help the liposomes avoid the body’s defenses.

Next Steps: Researchers at MIT are sorting through new molecules to find better delivery agents. They are testing their potential for targeting a variety of diseases and for delivering therapeutics other than RNA.

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