Battery-free e-ink screen grabs screenshots from smartphones

University boffins from Washington and Massachusetts joined forces with those at Intel to create an e-ink screen which can catch a smartphone screenshot – without needing a battery.

The screen catches energy over the NFC standard, the pay-by-bonk technology built into the latest models of smartphone. That power usually drives a cryptographic chip and communicating radio, but the team used it to update an e-ink screen instead.

Regular readers might recollect UK startup PragmatIC doing something very similar back in 2011, and they'd be right. PragmatIC didn’t use e-ink, so the image faded after a few seconds, but it did manufacture the whole thing though a printing process so it was incredibly cheap.

NFC, in contrast to RFID, uses an induction coil in the reader to power a tag. That allows the tag to perform a proper cryptographic authentication, but also provides spare power for hijinks like this.

According to their research paper (PDF, surprisingly readable) the smartphone (filling the role of reader) transmits 200mW of energy: 17.7mW of that gets picked up by the reading coil of which 8.35mW is consumed in processing the NFC communications, leaving 9.4mW to spare*.

That 9.4mW is chucked into a battery (which could be replaced with a capacitor if one wanted a battery-free device, though the difference is debatable) and accumulated for 2.3 seconds, which provides enough power to refresh the e-ink screen.

The mono screenshot mounts up to 5.67KB, so at an average of 13.3Kb/s it takes 3.4 seconds to transmit the image – providing plenty of time to charge up the battery/capacitor.

NFC can ramp up a good deal faster than that, and one could compress the image too, but those would both increase the power consumption of the tag as well as reducing the time available to power it, risking the functionality.

Interesting as the prototype is, there remains the question of why one would want such a thing. Taking a screenshot sounds useful, but a pen and paper would be much more flexible and a good deal cheaper, and who remembers to capture the data before their phone battery goes dead?

It's top boffinry none the less, and the team will be presenting the work at Ubicomp in Zurich next month, but we're hoping that PratmatIC could meet up with (printed e-ink people) Plastic Logic to make a version costing pennies – they're both in Cambridge after all – allowing for a disposable screen for electronic graffiti, though even then it's hard to think of the killer application for such a device. ®

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* Yes, we also counted 9.35mW rather than 9.4, but given the complexity of the experiment we're not going to call them on their numbers and will assume it's a simplification issue.