​How stretchable sensors will bring great change to wearables

Fitting the human form

Electronics circuitry is hard and rigid. We are not. And so a sea change is required if we want to make wearables that actually fit the human form.

Wrist-worn wearables only work as well as they do because there's a thin layer of relatively static human putty above our bones in this area. Try to produce a sensor for the neck or joints and you'll discover that today's wearables, small computers strapped onto pliant flesh, aren't up to the job.

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Several stretchable solutions are in the works, though. Here are some of the most exciting and innovative projects to look out for.

What are stretchable electronics?

How stretchable sensors will change wearable tech

Making a device flexible and stretchable, just like our skin, demands a re-think of the fundamentals of electronics. There are millions of transistors inside a fitness tracker or smartwatch. They sit on a hard, rigid silicon wafer, and there's a bed of plastic underneath. Each part is stiff by design.

"If you're looking at human interfaces on the body you're looking at the desirability to create a soft form factor," says Flexterra's Phil Inagaki. "That is a way different challenge than curving your display, putting a glass plate over it and saying you've got a curved TV."

Inagaki was the public face of the Polyera Wove Band, a wrist-worn wearable that launched in 2015. It is a band with an E Ink display that folds around your wrist and flattens out when you take it off.

In 2017 Polyera folded into a new company called Flexterra after reported funding problems, and Inagaki says the company will never produce the Wove Band. But the technology that made the band truly flexible is alive and well.

"The special sauce is the transistor material surrounded in an amorphous or polycrystalline based transistor," says Inagaki. "We have organic semiconductors and organic dielectrics which stack up to build the full working transistor." (Transistors are the brain cells of electronics.)

"That transistor becomes highly mechanically robust because it's really made of plastic-like materials whereas other transistors are based on materials like silicon, which are brittle," says Inagaki.

The technical terms may sound intimidating if you haven't taken a course in electronics, but the fundamentals are simple. Companies like Flexterra replace the core ingredients of circuitry with bendable "plastic-like" alternatives.

A thin transparent film of transistors is the result, one much more hardy than the printed circuit board of the average gadget. It can withstand "very substantial bending or rolling over 100,000 times" as well as the impact of a 150g acrylic ball dropped from a 30cm height, says Inagaki.

This kind of technology enables wearable electronics that don't need to be armoured behind a shell of plastic metal or glass just to survive daily life.

However, Flexterra is focusing on display makers at present. It showed off a 6.8-inch flexible e-reader in partnership with E Ink earlier this year. But Inagaki says this same technology could be used for other purposes including worn displays and sensors: "Now the fundamental materials technologies are proven to work, those are things I think will gain a lot more traction in the next 12-24 months."

Putting the stretch in sensors

How stretchable sensors will change wearable tech

Researchers at Melbourne's RMIT University are taking similar concepts in a slightly different direction, with a process for making stretchable, not just bendable, sensors.

This involves heating metal oxide on a bed of platinum and silicon wafer to 400 degrees, adding a layer of stretchy silicone and then peeling it off the silicon wafer. Then finally a gas chamber is used to remove the platinum layer. The metal oxide suspended in the silicone is all that's left. Again, it's a thin transparent film with an embedded circuit.

It sounds like a non-stretchy metallic circuit entombed in silicone, but under the microscope Dr Madhu Bhaskaran from RMIT's School of Engineering discovered a "micro-tectonic" effect. Rather than breaking after a stretch, the metal oxide formed plates that slid over each other, maintaining their integrity.

The team has tested this technology with sensors for monitoring UV levels and identifying harmful gases, and the stretchy substrate will let them adhere to your skin much more comfortably than current designs.

L'Oreal showed off a comparable UV patch, the UV Sense, at CES 2018 in January, but as it's not stretchable you have to wear it on a fingernail. Even small, simple devices are not really compatible with the human body unless they are flexible and stretchable.

"We started doing this research around six years back," says Bhaskaran. "The project was driven by our curiosity around trying to understand why we cannot make the same materials which make our everyday electronic devices stretchable. What makes our electronic devices rigid?"

These ideas are intuitive. You might imagine they had already been solved in a world where AI-run homes and driverless cars are an impending reality. However, adoption of this tech is in its infancy.

"I could see UV sensors being deployed in six years time but other applications might take longer as they also depend on regulations and so on," says Bhaskaran.

Medical uses for stretchable sensors

How stretchable sensors will change wearable tech

These ideas around stretchable sensors are not new. John A. Rogers of Northwestern University's School of Engineering published a paper on flexible silicon electronics in 2005. In the past he has outlined the concept of a soft web of sensors laminated onto the brain itself, to monitor its electrical activity.

Rogers currently works with the Shirley Ryan AbilityLab, developing a series of stretchable sensors that look a little like plasters.

There are sensors for the forearm, for the legs and neck, used to let medical professionals monitor the rehabilitation of stroke and traumatic injury patients.

"Stretchable electronics allow us to see what is going on inside patients' bodies at a level traditional wearables simply cannot achieve. The key is to make them as integrated as possible with the human body," says Rogers.

Some of the sensors in these patches are familiar. They use an accelerometer and gyroscope to monitor movement and a wireless chip to transmit data to a phone or tablet. However, they also can also directly monitor muscle, speech (vocal cord activity) and swallowing using "mechano-acoustic" vibration sensors.

It's around the throat that the sensors' stretchy properties become vital.

"If you think of the conventional form factor of a wearable, something going on a wrist, strapped in place that way, it's just completely incompatible with the throat," says Rogers.

The idea of wearing sensor plasters around your larynx may not seem appealing. But think of it in the context of stroke recovery, where a patient may need to re-learn speech. Rogers's neck sensor provides a metric for progress, which could help in the rehabilitation process.

SensorSkin

How stretchable sensors will change wearable tech

There's a lighter side to stretchy sensors too. Tacterion makes SensorSkin, a fabric that acts as both a resistive and capacitive touch sensor.

It can be wrapped around objects and surfaces and will sense swipes like a laptop trackpad as well as varying degrees of pressure. SensorSkin's capacitive component can also detect your hand hovering above it too. It was initially developed by co-founder Michael Strohmayer at the German Aerospace Center.

SensorSkin has a textile-like surface. It could be used to make a clothing-like panel the interface for a wearable. You can "hit the sensor patch with a hammer and it will not break," says Strohmayer.

There are no mechanical parts to this polymer-based touch sensor. And as it is bendy and stretchy, SensorSkin can be used to coat objects with relatively complicated shapes.

The possible uses for this tech are wildly diverse, easy as it is to start thinking of jackets that control music streamed from your phone, or rejecting calls with a swipe of a lapel. For example, at the IDTechEx Show! in November 2017 Tacterion demonstrated the fabric wrapped around a metal bar, functioning as a pressure sensor, with obvious physiotherapy applications.


Tacterion is a startup promoting this technology, run by CEO Daniel Strohmayer, brother to inventor Michael. As with most developers of stretchable and flexible interfaces and sensors, when, where and if we'll see them in a product sold at Amazon relies on a, most likely, larger company buying into their ideas.

"If, some day, people are playing with the next-generation controller of a gaming console saying 'Tacterion inside' at home, or their cars carrying a label reading 'Tacterion inside'," says Daniel Strohmayer, "we would be extremely pleased."


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