Source: “Three-Dimensional, Flexible Nanoscale Field-​Effect Transistors as Localized Bioprobes” Bozhi Tian et al.
Science 329: 830-834

Results: Researchers at Harvard have made biocompatible nanoscale probes that use transistors to take precise electrical and chemical readings inside cells. The tips of the probes are about the size of a virus.

Why it matters: To create complex bioelectronics such as neural prosthetics designed for fine control of artificial limbs, researchers need to create better interfaces with single cells. Existing electrodes can take intracellular measurements. But to be accurate, they must be large in comparison to the cell and can damage it.

This work also represents the first time digital devices, in the form of transistors at the tips of the probes, have been integrated with cells.

Methods: Using a process that they developed, the researchers grow millions of V-shaped silicon nano​wires at a time. The tip of each V acts as a very small transistor that can be inserted into a cell to send and receive electrical signals. The probe is more sensitive than a passive electrode, and it can enter cells without damaging them both because it’s so small and because it’s coated with a double layer of fatty molecules, just like a cell membrane. When placed near the membrane, the cell will actually pull the electrode inside. The electrical and chemical activity inside the cell changes the behavior of the transistor to produce a reading.

Next steps: The researchers want to incorporate circuits made from the nanoprobes into medical devices, including scaffolds for making artificial tissues. These circuits could “innervate” artificial tissue, mimicking the role of nerves to measure and respond to electrical signals propagating through the nervous system. The researchers also aim to take advantage of the electrodes’ ability to send electrical signals in addition to recording them. Applications could include neural interfaces with two-way communication between muscles and the ​nervous system.

Capturing Lost Energy

Device harvests power from heat as well as light in solar radiation.

Source: “Photon-enhanced thermionic emission for solar concentrator systems” Jared W. Schwede et al.
Nature Materials 9: 762-767

Results: A device built by researchers at Stanford University converts both the light and the heat in the sun’s radiation into an electrical current.

Why it matters: Conventional solar cells can use only a narrow band of the sun’s energy; the rest of the spectrum is lost as heat. The most common type of silicon solar cells convert 15 percent of the energy in sunlight into electricity. But Stanford researchers realized that the light in solar radiation could also enhance the performance of a device called a thermionic energy converter, which usually uses only heat. They say that such devices could in theory convert solar energy with 50 percent efficiency.

Methods: A thermionic energy converter consists of two electrodes separated by a small space. When one electrode is heated, electrons jump across the gap to the second electrode, generating a current. The Stanford researchers found that when they replaced the metal typically used to make the top electrode with a semiconducting material like the ones used in solar cells, photons hitting that electrode also drove current in the device. The Stanford prototype converts about 25 percent of the light and heat energy in radiation into electricity at 200 °C. Conventional thermionic energy converters require temperatures around 1,500 °C, which is impractical for many applications, and conventional solar cells don’t function well above around 100 °C.

Next steps: The researchers are working to make the device more efficient by testing different semiconducting materials for use as the top electrode. They’re also redesigning the system to work in conjunction with a solar concentrator that would raise temperatures to 400 to 600 °C. That would produce enough excess heat to harness with a steam engine.