Robotic Chia Pets

by Bryan Bergeron, Editor
July 2010

As a young experimenter, I remember trying to use the ground return current from the city power station to power something — anything. By driving copper stakes in the ground about two meters apart, I was able to create a 500 mV AC source. At the time, it wasn’t enough to power any electronic device I had on hand. Today — thanks to developments in energy harvesting, including devices designed to work at very low voltages — 500 mV is huge. The only problem is, it’s AC and not DC.

There are natural sources of DC voltage that can and have been harvested, including plants. It’s been documented since at least the 1970s that plants generate a potential between the plant tissue and the soil. More recently, with the advent of ultra-low-power devices, experimenters have used plant ‘batteries’ together with charge pumps to power digital counters and other electronic circuitry.

For example, third place in the 2010 International Design MSP430 Ultra-Low Power Challenge (sponsored by Texas Instruments) was a device run by a plant battery.  The experimenter, Carlos Cossio, connected five potted plants in series to achieve a voltage of 3.2V open circuit — enough to power the MSP430 and some associated hardware.

The voltage generated by a plant is due to the pH gradient in its tissues, as opposed to some sort of chemical reaction. In other words, this isn’t one of those lemon juice as an electrolyte battery, but a different kind of battery based on a different mechanism. Because the potential is between the soil and plant tissue, you have to use separate pots if you want to connect the plants in series to create a higher voltage.

As you might expect, plant batteries have a high impedance and relatively low voltage, and the voltage varies from one plant species to the next. The common jade plant is an excellent voltage source, in case you’d care to experiment on your own. For contacts, you need a nail or other conductor stuck in the potted soil, and a straight pin/alligator clip to get at the tissue of the plant. I imagine that a long-term battery connection needs to be sealed from microbes that could destroy the plant tissue.

From the title of this editorial, you can probably guess where I’m going with this ... plant-powered robots. I’m talking about small, plant-powered motors, sensors, and control electronics. There’s a body of literature out there on BEAM robotics that suggests this is doable. I don’t envision jade-covered mechatronics circling underfoot, but perhaps a small carpet roamer powered by hundreds of smaller plants. You’ll have to design the system so that daily watering doesn’t destroy the onboard electronics and motors.

Before you start building your motorized chia pet robot, I suggest you read the excellent article, Ultra-Low Voltage Nanoelectronics Powered Directly, and Solely, from a Tree, by Carlton Himes, Eric Carlson, and others, published in the IEEE Transactions on Nanotechnology, 2009. There are full reprints on the Web.

By the way, in case you’re thinking about ways of cashing in on this area, there’s at least one patent that has been issued by the US patent office on plant batteries. Check the patent office (http://www.uspto.gov) for details. You can also check our sister publication, Nuts & Volts, for an upcoming article by Carlos Cossio on his entry in the Texas Instruments contest.

If you’ve seen The Matrix, you know that animals also generate electrical potentials. There may come a day when we each have communicators or at least mp3 players sewn into our skin, with electrodes taking power from our muscle tissue. Perhaps small rodents will provide electrical power as well as cortically-directed control of robots. However, plants are a better bet in the near-term — they’re more politically correct. SV

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