“For wearables to truly transcend portables, we will need to rethink the way in which devices communicate with each other,” according to the university. “The usual approach of using an antenna to radiate signals into the surrounding area while hoping to reach a receiver won’t cut it for wearables – this method of transmission not only demands a lot of energy but can also be unsafe from a cybersecurity standpoint. Moreover, the human body itself also constitutes a large obstacle because it absorbs electromagnetic radiation and blocks signals.”
Human body communication (HBC) is one of the wireless body area network (WBAN) techniques.
Instead of Bluetooth, Zigbee or similar antenna-based electro-magnetic technique, HBC interfaces to the skin with electrodes. “Some electric fields can propagate inside the body very efficiently without leaking to the surrounding area,” said the university. “By interfacing skin-worn devices with electrodes, we can enable them to communicate with each other using relatively lower frequencies than those used in conventional wireless protocols like Bluetooth.”
Binaural hearing aids were picked as a subject for the project because they work in pairs, have to be synchronised, and are naturally in contact with the skin. 10, 20 and 30MHz were selected as potential operating frequencies, plus 2.45GHz to compare results with Bluetooth.
The researchers had previously studied HBC on real people, and have now used numerical simulation to determine how the electric fields emitted from an electrode in one ear travel through the head to reach the opposite ear.
They first tested various human-body models to find one that was sufficiently representative, then explored the effects of various system parameters.
“We calculated the input impedance characteristics of the transceiver electrodes, the transmission characteristics between transceivers, and the electric field distributions in and around the head. In this way, we clarified the transmission mechanisms of the proposed HBC system,” said researcher Dairoku Muramatsu.
With the model calibrated, they could weed out less effective electrode structures and “they also calculated the levels of electromagnetic exposure caused by their system and found that it would be completely safe for humans, according to modern safety standards”, said the university.
Findings included that reactive impedance matching makes a big difference to communication effectiveness, and that signals outside the body attenuate sufficiently quickly to allow devices on separate people to operate at similar frequencies in the same space without interference.
“With our results, we have made progress towards reliable low-power communication systems that are not limited to hearing aids but also applicable to other head-mounted wearable devices,” said Muramatsu. “Not just this, accessories such as earrings and piercings could also be used to create new communication systems.”
Much more detail, including safety-related SAR (specific absorption rate), can be found in the paper ‘Transmission Analysis in Human Body Communication for Head-Mounted Wearable Devices‘, published in Electronics by MDPI. This can be read in full without payment, and is worth at least browsing as it is clearly written, easily understood and informative.