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Light-harvesting smart tagging for mobile touchscreens

A team has created a wireless tag that smartphones can read through the touchscreen.

Thanks to contactless payment, near-field communication (NFC) is now an everyday technology. Despite the integration of NFC into smartphones, the manufacturers of the RF devices and tags that use the protocol have had difficulty making the technology spread much further.

That ambition is probably going to take a further knock now that a system able to piggyback off the touchscreens designed into practically every smartphone and tablet is now close to commercialisation.

A team from Benelux research institutes Imec and TNO, working together with card-games company Cartamundi, published a description (in the journal Nature Electronics) of a communications link that uses capacitive coupling rather than the RF signals of NFC.

This avoids the need to make and assemble a loop antenna, which tends to increase the size of the tag way beyond that needed by the processing electronics. This version uses an array of capacitors instead and results in a tag as small as one square centimetre.

What the researchers call the C-touch is not the first tag of its kind. The same team demonstrated an example last year and there have been a few other attempts in the past to make capacitive coupling practical for cheap tags.

One of the big problems is power. A regular RFID tag harvests energy from the electromagnetic field produced by a nearby reader. Compared to capactive coupling, this is very efficient. Although engineers often struggle to deal with capacitive coupling in any circuit, trying to harvest energy from an array of capacitors is tricky.

The earlier tag managed it with a power draw of less than 1nW. The novel aspect of the updated version - and one that could make the technology far more popular - is the connection through a touchscreen, which in most consumer-level implementations today is basically an array of capacitors. According to the researchers, no hardware modifications are needed for the touchscreen electronics to read the tag – it’s basically just a firmware update to make a tag reader.

As many use-cases will have people touch a tag to the screen when everything is powered, this provides the opportunity to power the tag by harvesting energy from the light it provides, rather than the coupling itself. For situations where there is no light coming from the screen, there is also the option to include a battery. The prototype design has a thin-film battery that can be fabricated onto the same substrate as the electronics.

Kris Myny, principal scientist and R&D team leader at Imec says the group is now testing how well the tags work with a variety of touchscreens used in mobile devices made by companies such as Apple, Samsung and Huawei. A further step is to make the link bidirectional, so that users could download data into the tag rather than just using it as way of sending an ID code to the handset.

Cartamundi expects to use the tag technology in its games – letting printed cards interact with mobile devices. Gaming is already an area where NFC has expanded beyond mobile payments, although it is far from widespread. If manufacturers can be sure that a tag technology like this will work with pretty much any smartphone, they will not have to worry whether those devices have built-in NFC readers.

As well as cost, that chicken-and-egg problem has been one of the factors holding back the proliferation of smart tags into everyday life.

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