Wearable power: The heat is on you

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  • Published: Sep 15, 2016
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Wearable power: The heat is on you

The digital labcoat

Wearable integrated thermocells based on gel electrolytes have been developed that are powered by body heat. Credit: Angew Chem/Wiley

Wearable integrated thermocells based on gel electrolytes have been developed that are powered by body heat.

The concept of wearables is not just a buzz phrase for technophilic lifestyle magazine it is set to become the smart-apparel niche of the Internet of Things as devices woven into the fabric of our clothes, whether writer, sailor, soldier or spectroscopist, become woven into the fabric of society. Such devices will provide data and connectivity for countless applications from medical diagnostics and communication between patient and healthcare workers to environmental protection, whether in military zones or industrial plant.

Thermocell gels

Now, wearable integrated thermocells based on gel electrolytes have been developed that are powered by body heat alone and could solve one of the major problems of all portable electronic devices - stayed active. Jun Zhou, of the Huazhong University of Science and Technology in China and colleagues point out that at its simplest wearable technology may be nothing more sophisticated than a smart phone display integrated into clothing textiles, but the addition of personal and environmental sensors for various conditions, whether temperature, orientation, location, ambient chemicals, and disease biomarkers, may take the basic idea to the next level. If a reliable and sustainable power source can be found. Wearable batteries are a burden, solar panels too and they also rely on it being daytime...and sunny. Conventional fuel cells have been investigated but they commonly need an organic material or hydrogen to function. The main problem with the systems investigated so far is bulkiness and lack of comfort. Wearable means wearable but that premise is cancelled out if the lightweight athletic monitor requires a battery back pack!

The Chinese scientists hope that information will flow, for as long as the body is warm, if they can tap body heat to run wearable devices. Writing in the journal Angewandte Chemie, they have now introduced a flexible, wearable thermocell based on two different gel electrolytes. The device draws in the thermal energy generated by muscle activity and metabolism and released through the skin as we go about our daily activities.

Thermal glove

Of course, the relatively small temperature gradient between skin (approximately 32 degrees Celsius) and the ambient temperature (which might be 18-24 degrees Celsius indoors) means using body heat to do meaningful work, i.e. generate electricity, could be difficult. Earlier thermoelectric generators, such as those based on semiconductors, produce too little energy, are costly, or are too brittle for use in wearable systems. Zhou and colleagues have therefore turned to thermocells with electrolyte gel solutions that can be readily integrated into extensive wearable systems.

The new system exploits the thermogalvanic effect whereby two electrodes in contact with an electrolyte solution, or in the present case an electrolyte gel, are at different temperatures, a potential difference between the two electrodes is generated. The ions of a redox pair in the gel electrolyte can rapidly switch between two different charge states, accepting or releasing electrons at the electrodes at different temperatures. By combining two types of cells containing two different redox pairs, the team was able to draw a current from their system. The first cell type contains an iron(II)-iron(III) redox pair, the second type of cell contains the complex ions [Fe(CN)6]3−/[Fe(CN)6]4−. This particular choice of redox pairings, in cell type 1 means that the cooler electrode gives the negative potential, while in type 2, the cold end gives a positive potential.

By patterning these two types of cells into chess-board type pattern and connecting the cells to each other by metal plates alternating above and below, they could be wired up in series. In a proof of principle, the team has integrated this chessboard power supply into a glove. Hand heat is perfectly adequate to generate the necessary temperature gradient between upper and lower plates and produces 0.7 volts and about 0.3 microwatts with a 5 degrees Celsius differential. The team hopes that optimisation will boost this power output or make the requisite temperature differential smaller.

Related Links

Angew Chem Int Edn 2016, online: "Wearable Thermocells Based on Gel Electrolytes for the Utilization of Body Heat"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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