A thin flexible patch for generating electrical power from body heat has been developed by a team working at the Korea Advanced Institute of Science and Technology (KAIST). If this could be evolved into a production model it would be a vital component to boosting the endurance of wearable electronic devices; including those used in home healthcare applications.
Thermoelectrics
The KAIST thermoelectric (TE) patch is formed by interweaving glass fibres. This is then impregnated with liquid pastes based on bismuth telluride and antimony telluride. These materials exploit the difference in surface temperature between the skin on one side and air temperature on the other to produce an electrical current.
The KAIST thermoelectric (TE) patch is formed by interweaving glass fibres. This is then impregnated with liquid pastes based on bismuth telluride and antimony telluride. These materials exploit the difference in surface temperature between the skin on one side and air temperature on the other to produce an electrical current.
Creating body-mounted thermoelectric generators is an idea that has been indiscussion for some years - with the heat-converting thermocouples being printed onto a plastic or other flexible substrate. However the new KAIST prototype represents a step forward in performance as using glass fibres allows a hitherto unseen optimisation of thermoelectrical and physical performance.
The research which has spawned the new patch was published on 14 March in the journalEnergy and Environmental Science.
Glass fibre advance
Typically the thermoelectric effect has relied on an inorganic material like bismuth telluride. These give strong thermoelectric performance but need to be mounted on thick rigid ceramic or aluminium oxide substrates. Less rigid organic materials like carbon nanotubes can be engineered to have a thermoelectric effect however the rate of power generation is inferior to telluride compounds.
Typically the thermoelectric effect has relied on an inorganic material like bismuth telluride. These give strong thermoelectric performance but need to be mounted on thick rigid ceramic or aluminium oxide substrates. Less rigid organic materials like carbon nanotubes can be engineered to have a thermoelectric effect however the rate of power generation is inferior to telluride compounds.
The KAIST prototype's design allowed both antimony telluride and bismuth telluride to permeate through the glass fibre matrix and form a series of films of thermoelectric materials several hundreds of micron thick. This means a series of hundreds of thermoelectric material d
40mW
If a sample of the new material was made into a wearable 100mmx100mm pad, it is projected to generate 40mW of power given a 17.2°C difference between the skin and ambient temperature.
If a sample of the new material was made into a wearable 100mmx100mm pad, it is projected to generate 40mW of power given a 17.2°C difference between the skin and ambient temperature.
Besides being low weight, the glass fibres patch has a bend radius of 20mm and there is no observed detrimental impact on performance even after being bent 120 times. This allows the KAIST device to be easily fitted and retained on the skin.
Project leader Byung Jin Cho says: 'Our technology presents an easy and simple way of fabricating an extremely flexible, light, and high-performance TE generator. The glass fabric itself serves as the upper and lower substrates of a thermoelectric generator, keeping the inorganic TE materials in between. This is quite a revolutionary approach to design a generator.'
Powering wearables
One major technical challenge for consumer wearable electronics has been to liberate devices like smartwatches from the need for frequent recharging, thereby aiding a seamless integration into consumers' lives. While work to develop longer lasting flexible batteries is advancing, the first generation of these require a wearable device to make a trade-off between having a lower power draw - which often equates to lower functionality - or requiring frequent recharging which is inconvenient for the user.
One major technical challenge for consumer wearable electronics has been to liberate devices like smartwatches from the need for frequent recharging, thereby aiding a seamless integration into consumers' lives. While work to develop longer lasting flexible batteries is advancing, the first generation of these require a wearable device to make a trade-off between having a lower power draw - which often equates to lower functionality - or requiring frequent recharging which is inconvenient for the user.
For example, early versions of Google Glass now undergoing trials have a 570 mAh lithium ion battery mounted. This can be drained after between 3-5 hours of use, and quicker if power-hungry functions like video recording and playing are engaged. The battery then takes two hours to recharge.
Cheap and easily conformable body mounted thermoelectric generator would then provide an efficient means to replenish a battery during use.
Medical and beyond
For very low power applications the thermoelectric generator itself might be enough. For example, in a body mounted label with a simple biomonitoring function. A key early market for this will be healthcare, where organisations are investigating the distance monitoring of patients as a promising way to minimise the amount of time clinicians spend on physical visits and examinations. This is especially relevant for managing chronic conditions, such as blood pressure, in ageing populations.
For very low power applications the thermoelectric generator itself might be enough. For example, in a body mounted label with a simple biomonitoring function. A key early market for this will be healthcare, where organisations are investigating the distance monitoring of patients as a promising way to minimise the amount of time clinicians spend on physical visits and examinations. This is especially relevant for managing chronic conditions, such as blood pressure, in ageing populations.
The British military has also investigated the potential of using thermoelectric materials for its soldiers on operations.
Thermoelectric generators can work on any surface where there is a temperature difference this opens up opportunities for low-power sensors or other devices in a wide range of industries. Cho says: 'We expect that this technology will find further applications in scale-up systems such as automobiles, factories, aircrafts, and vessels where we see abundant thermal energy being wasted.'
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