Photovoltaic cells are becoming cheaper and more efficient each year, but there are still some questions regarding their long-term sustainability. Most technologies involve the use of elements that may be limited in supply, toxic, expensive, and difficult to recycle, which may ultimately limit our ability to produce them on the sorts of scales that a wholly renewable energy economy would require. One possible alternative to the traditional hardware is the use of biological materials, which are invariably comprised of abundant elements, and are produced in bulk by organisms simply as part of their normal life. The main downside of biologicals has been that they're far less stable than solid-state devices, which can last for decades. But a study released by Nature Chemistry indicates that it's possible to use an organism's own self-repair systems to keep proteins operating long past the end of their normal lifespan.

Compared to some of the best devices on the market today, the systems cells used to harvest sunlight during photosynthesis aren't very efficient. But they do have two major advantages. Since life evolved to rely on some of the most abundant elements around—primarily carbon, hydrogen, nitrogen, and oxygen—producing more of them and recycling damaged components is incredibly simple. It also partially eliminates the manufacturing issues, since bacteria will happily pump out more of the light-harvesting proteins each time they divide. That doesn't mean there's a requirement for some hardware to support the proteins, but this is generally simpler and cheaper than the hardware used to harvest light.

So, why aren't we all getting our power from bacteria-based devices? Because proteins have a fairly short lifespan in the cell, which devotes lots of energy to identifying damaged ones and destroying them, recycling their components in the process. Instead of decades of useful time, you'd be lucky to find a protein that lasted a week outside of the cell. That's why some people are looking into using entire bacteria for the production of energy.

The new research is based on our understanding of how damaged photosynthetic proteins are handled by bacteria. When one of these proteins is damaged (specifically, protein D1 of photosystem II), the entire complex ends up changing its structure. That pops it out of its normal membrane home, and makes the whole photosystem more prone to falling apart. The undamaged components can spontaneously form a new complex with an undamaged protein, and get right back to work; the damaged components are then recycled.

Since many of the steps involved occur spontaneously, the authors reasoned, it should be possible to do something similar outside the cell.

Of course, to determine whether it was effective, they first had to create a functional solar device that uses photosystem II proteins. The proteins are normally buried in a membrane, so they started with a collection of chemicals called phospholipids, which normally comprise part of a cell's membrane. These will spontaneously form spheres in water, which wasn't entirely convenient, so the authors added a protein that reshaped the membrane, flattening the sphere into a disk. Each disk contains a single copy of photosystem II.

Placed in a solution with carbon nanotubes, the opposite side of the disk will spontaneously stick to the surface of the tubes. Conveniently, the arrangement causes a key site in photosystem II—the place where a charge difference develops in response to light—right next to the surface of the carbon nanotube. Since nanotubes conduct currents very well, that allows them to harvest the charge difference to perform useful work.

The authors produced a cell that has a metal electrode to counter the carbon nanotube/protein/membrane mesh, with a solution containing charge carriers (both chemicals and other photosynthetic proteins). When exposed to light of the appropriate wavelength, a photocurrent was produced by the device. Unfortunately, as expected, they quickly succumbed to light-driven damage; within five hours, the level of the photocurrent dropped to half its peak value.

This is where the self-repair comes in. The authors simply added a bit of detergent to the solution, and its ability to generate a photocurrent plunged, suggesting the detergent had disrupted photosystem II. They then added a bit more proteins, and pulled the detergent back out of the solution by dialysis. As predicted, any damaged proteins were not incorporated into the newly reformed photosynthetic complexes. Performance went right back to its initial peak once the detergent was gone before declining again along a similar trajectory. This behavior kept going through at least four cycles of detergent addition, with the system regenerating to the same peak each time.

Even though the regeneration process takes eight hours (during which time no current is generated), the overall efficiency of the system went up to three times what it would have been if left running for the entire length of time. The authors also say that there seems to be no apparent limit to haw many times the system can survive the regeneration process, since any damaged proteins and lipids simply end up removed.

Don't expect to buy one of these any time soon, though. It still takes a lot of work to harvest the proteins from bacteria in the first place, and the system's efficiency isn't brilliant. But those are things that could potentially be optimized, and there's little doubt that the researchers are likely to be working very hard at doing so.

Nature Chemistry, 2010. DOI: 10.1038/NCHEM.822 (About DOIs).