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FEATURE WEARABLE TECHNOLOGY


➤ conductive yarns woven into a textile to allow communication and powering of these devices. Bossuyt’s team collaborated with Philips


Research to make a wearable device for treating repetitive strain injury with light therapy. The washable, breathable band can be wrapped around the wrist, with integrated blue LEDs. ‘Blue light applied on the skin frees nitric oxide molecules, which were being transported to the muscle leading to the pain. This small molecule has the effect that it relieves the pain,’ said Bossuyt. Challenges remain for making higher resolution displays and minimising the space between LEDs, while keeping the stretchability. ‘Now, we’re looking to smart garments that use integrated LEDs for visibility. We’ve also made a garment with solar cells to capture energy,’ added Bossuyt.


In any photonics-based wearable, the main challenge is the battery, particularly because lighting applications have high power consumption. The wristband uses a mobile phone lithium-polymer battery, which is still relatively large and bulky. But thin-film solar cells integrated into the fabric itself, as with the Holst Centre’s ‘solar shirt’, make device charging a possibility, as well as storing electricity for later use.


Band aid-like optics


The new world of printed and flexible electronics has opened the market for analysing an increasing array of medical functions.


Detailed monitoring of sensitive information allows for early-warning of signs of trouble, and thus preventive measures to be taken against a range of skin and blood diseases. If it’s easy to use, and cheap to the point of being disposable, so much the better. Last year, engineers at UC Berkeley developed a band- aid-like pulse oximeter for


measuring pulse rate and blood oxygen levels2


Pulse oximeter comprised of organic light-emitting diodes (OLEDs) that are detected by the organic photodiode (OPD)


blood absorbs more infrared light, while darker, oxygen-poor blood absorbs more red. Comparing the transmitted wavelengths reveals how much oxygen is in the blood. The Berkeley sensors use red and green organic LEDs (OLEDs) and compare wavelengths transmitted to an organic photodiode (OPD).


Now, we’re


looking to smart garments that use integrated LEDs for visibility


.


Current wearable devices can be comparatively large and cumbersome. Rather than silicon, the Berkeley team used organic optoelectric sensors on a flexible plastic substrate to make a disposable device that performed just as well as conventional models. Now, they are developing a second-generation optical sensor that will allow placement beyond the conventional fingertips or earlobes, for integration with patches on the wrist or chest.


A standard pulse oximeter sends red and


infrared light through a fingertip or earlobe, and a sensor on the other side detects how much light is transmitted. Bright, oxygen-rich


22 ELECTRO OPTICS l FEBRUARY 2016


In the second-generation reflection mode, OLEDs and OPDs are placed on the same side of the tissue, and reflected light is used for data on oxygenation. ‘The big plus for these devices is that we can fabricate them on flexible substrates. If you use a flexible OPD bent around your finger, the conformal interface to the skin reduces background noise, essentially increasing the signal-to-noise ratio,’ explained Yasser Khan who, along with Claire Lochner and Adrien Pierre, formed the team that conducted the study in Ana Claudia Arias’ lab. A positive response from the medical community has brought offers of clinical trials to the


project. ‘The idea that these sensors are flexible, and can be used in places other than the finger or the earlobe, has been a key point for this positive response,’ said Khan. From the fabrication and assembly point of


view, the photonic sensor array is challenging. But from the measurement side, it offers a simple, easy-to-use, non-contact technique, as opposed to the complex contact-based technique of conventional electrode arrays. Indeed, flexible electronics labs are not focused


on organic photonics alone: complementary to the exciting potential of light-based wearables are other electronic sensors engineered to a flexible, wearable substrate. Arias’ lab has also developed


a ‘smart bandage’ that detects pressure ulcers, or bedsores, before damage reaches the skin’s surface, using a 3D-printed array of electrodes on a thin film3


.


The technique – in which a current is run between the electrodes and electrical impedance measured as a function of frequency to create a map of tissue damage – could have applications for the monitoring of other conditions such as subcutaneous damage, or bone healing. ‘There are challenges with optimising the


process for printing the electrodes, and with the materials for the flexible substrate,’ said Michel Maharbiz, UC Berkeley associate professor of electrical engineering and computer sciences and head of the smart- bandage project. ‘The key is that we are at this really nice convergence of electronics becoming smaller and low power, and a massive information infrastructure.’ This convergence is prompting people to try new applications for all sorts of parameters. Much of the work to build very small sensors is driven by clinically important applications. ‘We’re experiencing a revolution in wearables, with interest in understanding how this could be brought to bear in clinic,’ added Maharbiz. Today’s wearable technologies are yesterday’s science fiction and fairy tales. And the scientists and engineers who are weaving these magnificent fabrics offer new designs encompassing media, sports, and medical applications, well worthy of public awe. l


References 1


2 3 www.tobii.com


www.nature.com/ncomms/2014/141210/ ncomms6745/abs/ncomms6745.html


www.nature.com/ncomms/2015/150317/ ncomms7575/full/ncomms7575.html


@electrooptics | www.electrooptics.com


Yasser Khan/Arias Research Group, UC Berkeley


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