Interview with Ray Baughman

We have for a very long time been inventing new types of artificial muscles, which convert electrical energy to mechanical energy. For each muscle type, we tried to run the artificial muscle in reverse to convert mechanical energy to electrical energy. Our effort was largely unsuccessful until we discovered our twistron harvesters, which use stretch-induced changes in the twist of a coiled carbon nanotube yarn to generate electricity.

For harvesters that are woven or sewn into a textile, we use solid-state electrolytes (called gel electrolytes). In our preliminary investigation of the breathing sensor, the self-powered twistron was used to generate an output voltage that was recorded using an oscilloscope, but we did not measure the output power. This output power will increase with increase in the amount of twistron yarn that is sewn into the deformed region of the textile.

For the breathing monitor, the twistron harvester produces electricity when the twistron is stretched by breathing. Hence, the twistron sensor should be placed at a location on the garment where breathing causes textile stretch. No feeling of wearer discomfort will occur. For the demonstration reported in our Science paper, wires connected the harvesters to the monitor. However, we could have used the twistron harvester to power a wireless transmitter that sends data to a remote location. We mention in our Science paper just 31 milligram of CNT yarn harvester could provide the average power needed to transmit a 2-kB packet of data over a 100m radius every 10 s for the IoT.

Just like the case for a coiled metal wire spring found in your hardware store or a coiled rubber band (like used to power a rubber band powered toy airplane), stretching  a coiled carbon nanotube yarn causes the yarn to increase the twist. Since the carbon nanotubes in the yarn are electrically conducting and have the nanoscale dimensions needed to electrochemically store electrical charge, this increased yarn twist forces stored charges closer together, thereby increasing yarn voltage. This increased yarn voltage, relative to a counter electrode, enables the harvesting of mechanical energy as electrical energy.

We believe that our twistron yarns provide a major advance for commercial needs for everything from harvesting the energy of human motion and powering sensors on the Internet of Things (IoT) to harvesting the enormous energy in ocean waves. The cost of making the carbon nanotube yarn in the laboratory is too expensive for using our twistrons for harvesting the energy of ocean waves to power cities. However, this cost is likely to be not prohibitively expensive for such applications as selfpowered sensors that are woven into clothing. In our initial work, these energy harvesters were used in the ocean to harvest wave energy, combined with thermally driven artificial muscles to convert temperature fluctuations to electrical energy, and sewn into textiles for use as self-powered respiration sensors. Other applications might eventually be feasible, like generating electrical energy from the deformation of automotive springs and the deformations of automotive tyres.

Relevant for future impact on people-using our twistron harvesters to harvest the energy of the ocean to light cities and to power sensors that collect medical data and transmit it seem especially important.

It might be possible in the future to use our twistron harvesters to light up clothing in attractive ways in response to body movements, without the use of a battery. While piezoelectrics in shoes can light up shoes, they cannot harvest meaningful energy to do the same thing when woven into clothing.

In our previous work, published in papers in Science and elsewhere, we have shown that our coiled polymer fibre muscles and our coiled carbon nanotube hybrid artificial muscles (which are the same as our twistron harvesters, except they are filled with an environmentally responsive guest) can generate mechanical energy from fluctuations in temperature or humidity. To provide a harvester system that generates electricity from this mechanical energy, we connect the twistron mechanical energy in series to our environmentally powered artificial muscles. Incidentally, our polymer artificial muscles (which can be cheaply made from fishing line or sewing thread) are being commercially developed for application in comfort adjusting clothing. Also, they are of interest in the fashion world to provide dramatic effects by morphing wearables.

The output power from our twistron harvesters increases with increasing amplitude of stretch and increasing frequency of stretch. We have made textiles that both harvest mechanical energy and store it for subsequent use (such as in sensors that sense body movements and biological functions and remotely communicate information, thereby eliminating the need for battery recharge or replacement).

We are continuing research and development with the goals of further improving twistron performance and using what we have discovered to make less expensive twistron harvesters.

For harvesting energy from ocean waves, the electrolyte exploited was ocean saltwater.  For harvesters that are woven or sewn into a textile, we use solid-state electrolytes (called gel electrolytes). There is a chemical potential difference between the nanotube yarn and the electrolyte, which causes either holes or electrons to be injected into the yarn, depending upon whether the pH of the electrolyte is low (like pH for 0.1 M for aqueous HCl) or high (like pH 13 for 0.1 M KOH), respectively.  Stretching the yarn causes the capacitance of the yarn to decrease, which increases the voltage associated with this injected charge, thereby enabling the harvesting of electricity.