A woman uses Apple Watch that is on display at an electronics store in Omotasando in Tokyo April 24, 2015. REUTERS/Thomas Peter

By Jeremy Wagstaff

(Reuters) - Forget 'wearables', and even 'hearables'. The next big thing in mobile devices: 'disappearables'.

Even as the new Apple Watch piques consumer interest in wrist-worn devices, the pace of innovation and the tumbling cost, and size, of components will make wearables smaller - so small, some in the industry say, that no one will see them.

Within five years, wearables like the Watch could be overtaken by hearables - devices with tiny chips and sensors that can fit inside your ear. They, in turn, could be superseded by disappearables - technology tucked inside your clothing, or even inside your body.

"In five years, when we look back, everything we see (now) will absolutely be classified as toys, as the first very basic steps of getting this right," says Nikolaj Hviid, the man behind smart earbuds called the Dash.

Developed by Munich-based Bragi GmbH, the Dash is a wireless in-ear headphone that looks like a discreet hearing aid. Packed inside is a music player, 4 gigabytes of storage, a microphone to take phone calls - just nod your head to accept - and sensors that monitor your position, heart rate and body temperature.

Nick Hunn, a consultant who lays claim to the term 'hearables', reckons the Dash is just the start. He predicts smartwatches will dominate wearable sales for the next three years, hearables will then overtake and, by 2020, will account for more than half of a $30 billion wearable device market.

This rapid shift is being driven, he says, by a new generation of chipsets using Bluetooth wireless communication and using far less power than their predecessors. Designers now realize "the ear has potential beyond listening to music - it's an ideal site for measuring a variety of vital signs," Hunn wrote in a recent report.

EYEBALL POWER

A parallel revolution in sensors is making this possible.

Kow Ping, whose Hong Kong company Well Being Digital Ltd provides algorithms and reference designs on wearable sensing to companies like Philips, Motorola, Haier and Parrot, says chipmakers have invested heavily in reducing the power consumption and size of sensors.

An accelerometer, which measures things like position, motion and orientation, for example, is now 1 square millimeter. "A few years ago," he says, "it was two or three times as big and two or three times less refined."

When they can harvest energy from the body's heat or motion they'll be even smaller, autonomous and ubiquitous.

Andrew Sheehy of Generator Research calculates that, for example, the heat in a human eyeball could power a 5 milliwatt transmitter - more than enough, he says, to power a connection from a smart contact lens to a smartphone or other controlling device.

And Ping's company is working with a top Asian university to add sensors to a sports bra which could harvest energy from relative motion. In five years, he says, "there will be people building sensors into every existing wearable device or apparel."

BUTLER FEEDBACK

Bragi's Hviid calls these 'disappearables'. And while medical and fitness top the list of what these devices might measure, he and others are looking beyond that. A dozen sensors in your pants, he suggests, could advise on how to improve your posture or gait when trying to impress a suitor.

"It's more like a butler ... they do some basic stuff that you really want, but there are deeper experiences in there," Hviid says.

Sheehy points beyond the personal, as parallel advances in machine learning and artificial intelligence "come together and lead to some remarkable use cases:" a politician's contact lens, for example, might provide real-time feedback from a sample of voters, allowing for a speech to be tweaked on the fly.

A lot of this technology is already here.

Google is working with Novartis on a contact lens to measure glucose levels in tears. The healthcare group has also invested in Proteus Digital Health, a biotech start-up which promises edible embedded microchips, the size of a grain of sand, which are powered by stomach juices and transmit data via Bluetooth.

"We're looking at a major technological revolution of a similar magnitude to the mobile revolution," says Sheehy.

"VERY TRICKY"

Not everyone agrees that disappearables are necessarily just around the corner. Wearables still need to gain widespread acceptance - remember Google Glass - and the technology still needs to finessed.

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