Recent Research (Literally) Stretches the Meaning of “Sensor” and “Battery”

February 04, 2020 by Nidia Trejo

Several universities are researching new, stretchable materials that can act as a wearable sensor or battery.

While we associate sensors and batteries with physical pieces of hardware, like an SoC, several universities are quite literally stretching our understanding of these terms.

A few recent examples include hydrogel sensing sheets, a stretchable battery, and sticky wireless sensors. In some instances, the researchers who have developed these materials are exploring the chemistry and physics that make circuit boards tick and applying those principles to wearable technology.



Mechanical and biomedical engineers at the University of Toronto and McGill University teamed up to create a "second skin" of hydrogel that will act as a sensor. They call the substance Artificial Ionic Skin (AISkin) because it mimics the flexibility and durability of human skin.


Researcher Binbin Ying stretching AISkin

Researcher Binbin Ying (pictured) stretching AISkin, a material he and his research partner Xinyu Liu believe will advance wearable electronics and robotics. Image used courtesy of Daria Perevezentsev

The AISkin is made of a positively charged sheet and a negatively charged sheet of hydrogel, creating a 3-D network of polymers. Researchers believe that by overlaying these positive and negative ions, they can simulate a "sensing junction."

When the material is exposed to changes in strain, temperature, or humidity, the ions can move in a controlled manner—similarly to a diode—and produce an electrical signal output, like current or voltage.  

This skin outperforms human skin in stretch capacity, reaching 400% because of the crosslinking in the polymer network. Yamada et al. also found that AISkin uses an external energy source and showed promise toward self-generating energy. 

The researchers behind AISkin see a promising future for the material in wearables. For instance, Professor Xinyu Liu envisions that the new material could be used for biometric measurement in devices like Fitbits, or even as a skin-like touchpad that could stick to the surface of a user's hand. 

Liu also posited that AISkin could be used to help athletes measure their body metrics during training or to help doctors measure the progress of patients engaging in muscle rehabilitation. Liu explains, "If you were to put this material on a glove of a patient rehabilitating their hand, for example, the health care workers would be able to monitor their finger-bending movements."


Demonstration of AISkin

Demonstration of AISkin as it could be used for muscle rehabilitation. Image used courtesy of Binbin Ying

AISkin might also find a place in soft robotics with robots composed of polymers.


Stretchable Battery

Lithium-ion batteries, which use polymers as electrolytes, have been abuzz for several years. But they face a major pitfall: because these polymer electrolytes exist as a flowable gel, they can possibly leak or combust.

Researchers at Stanford University aim to address this issue with a solid, yet stretchable polymer that can carry an electric charge between battery poles.

Stanford claims that the stretchable battery prototype, about the size of a thumbnail, can store half as much energy as a conventional battery of the same size. Yet, in lab tests, the researchers found that the stretchable battery maintained constant power, even when it was stretched and deformed beyond half its starting size.


Demonstration of the stretchable battery. Image used courtesy of Stanford University

The research team developing the stretchable battery hopes to boost the material's energy density in future iterations.

Though the stretchable battery has never been used outside the lab, the team sees this material being used to power stretchable sensors (like AISkin) and another Stanford-born material, BodyNet.



In addition to the stretchable battery, Stanford researchers have also made strides in stretchable sensors, similar to AISkin, last year. The material, BodyNet, is a rubber sticker printed with metallic link that includes an antenna and sensor.

This material is designed to adhere to skin and track a person's heartbeat along with other biometrics. The sensor then wirelessly beams these measurements to a receiver attached to the wearer's clothing. 


BodyNet and receiver

BodyNet sticker and receiver attached to clothing. Image used courtesy of Bao Labs

Recognizing that a stretchable antenna on a rubber sticker could weaken or destabilize a signal, the battery-powered receiver uploads data from BodyNet to a smartphone or computer via Bluetooth.

When designing BodyNet, researchers wanted to avoid using any clunky batteries or circuits that might prevent comfortable stretch with the skin or otherwise hinder the user's experience. According to a Stanford article on the material, the 14-person team instead used a variation of radiofrequency identification (RFID) technology that ordinarily controls keyless entry to locked rooms.

"When a person holds an ID card up to an RFID receiver, an antenna in the ID card harvests a tiny bit of RFID energy from the receiver and uses this to generate a code that it then beams back to the receiver," Stanford engineer Tom Abate explains.

Likewise, the BodyNet sticker employs an antenna to draws some incoming RFID energy from a receiver (attached to the wearer's clothing) in order to power the sensors. From there, measurements on the rubber stickers are beamed back to the receiver. 



BodyNet is a rubber sticker printed with metallic link that includes an antenna and sensor. Image used courtesy of Bao Labs

While researchers at the University of Toronto, McGill University, and Stanford University have not yet mentioned exactly how these stretchable materials may or may not play into circuit board design, they have explicitly stated AISkin, the stretchable battery, and BodyNet are built for use in electronics.



Do you design for wearable devices? Have you observed any surprising changes to your design requirements in recent years? Share your experiences in the comments below.

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