Bug Power |

Now, this is thinking outside the box. Dr. Adam Driks (Loyola, Illinois) is a microbiologist who has been studying spores for a while. A spore is a protective shell (actually, concentric shells) that some microbes form to keep themselves preserved during periods of environmental stress. It was originally thought to be like a walnut shell protecting the valuable components within. But, Driks has been studying Bacillus spores and found that when the relative humidity changes, so does the spore itself.

I have been interested in spores, too, specifically Bacillus thuringiensis (BT) for nearly four decades. Because BT produces a toxin that can eradicate mosquitoes and other flying bugs. We’ve developed methods of production and preservation, so that we can administer BT to keep flies at a minimum near certain waste treatment facilities (those with fixed beds- called trickling filters). But, Driks had other ideas- and this time chose a slightly different strain for this research.

In concert with Drs. L. Mahadevan (Harvard), X. Chen, and O Sahin (both the latter are at Columbia now), they found a way to harness the energy and action of a spore as it reacts to changes in humidity. You see, while the microbe may shrink into a spore (protecting its essence, the genetic code), it swells up as soon as there is moisture. And, shrink back again when that water isn’t present.

If a sliver of silicon (coated with the spores) gets wet (from a person exhaling, for example), the silicon sliver will bend and straighten with that change in the local humidity. Knowing this, the researchers then took that same microbial strain (Bacillus subtilis, a microbe that lacks an exosporium, which would make a tight packing density difficult) and coated it on a rubber sheet. As alternating humid and dry air (85% and 15% relative humidity) hit the sheet, the sheet flipped back and forth, which enabled a rotating magnet to produce electricity.

For their next step in the development, they sought out mutant strains that could develop twice as much energy. The goal is to generate renewable electricity. And, they reported their test results in Nature Nanotechnology. Amazingly, the response to changes in humidity required less than a 1/2 second (actually the system responded a little faster to the presence of water than to its absence).

They developed what they termed a hygrovoltaic generator (shown above). They coupled the spore-coated rubber sheet to an (electromagnetic) generator, contiguous to a container of water. Alternating the humid and dry air moved the magnet, which then afforded current to pass through a 300 kOhm resistor. While not enough to run an air-conditioner, 0.7 microWatts of power were developed from some 300 mg of spores. (This resulted in some 233 mW/kg power to spore density.)

Now, comes the job for the engineers. To scale up this concept and make it a viable source of renewable power.