Starting with basic metrics such as daily steps and calories burned, wearables – including smart bracelets and devices such as the Apple Watch range – now track more biomarkers than ever, from heart rate variability to related factors such as blood oxygen levels.
While devices are getting smaller, the market is only going in the other direction. The expansion in the last 10 years has been “amazing to see”, says Panicos Kyriacou, professor of biomedical engineering at City St George’s, University of London. “This growth is a combination of the advancements of technologies… but also it’s the appetite of the consumer market to absorb those technologies. People want to feel empowered,” he says.
As wearable technology becomes more popular and is shrunk down into ever smaller formats, it is shifting health tracking “from episodic check-ups to continuous, real-time feedback,” says health technology specialist Dr Mark Kovacs. “This empowers people to see trends, not just snapshots, and to make informed decisions daily (even minute by minute) rather than waiting for annual screenings.”
Shining a light
If the shrinking down of devices and growth of the market is an ongoing story, the main “protagonist” is photoplethysmography (PPG), according to Kyriacou. The optical technique involves shining a light into the skin, then using a photodiode to collect the reflected light as blood pulses through the tissue.
Often found alongside accelerometers, gyroscopes and other sensors, PPG signals from optical systems enable pulse oximetry, which is used to measure blood oxygen saturation. It can also be used to infer information about heart rate, vascular age and performance, blood pressure and other factors.
Available since the 1930s, the technology was shrunk down by the use of light-emitting diodes (LEDs). At first, wearables used one LED and one detector, Kyriacou says, before more were added to collect information on other biomarkers.
Advances were enabled by LED miniaturisation and reduced costs, with green, red and infrared wavelengths available on chips measuring 1mm by 1mm. “It’s such a small footprint and a very high spec… that enables manufacturers to place those optoelectronic devices in a footprint like a wristwatch, an ear phone, a ring,” he says.
“The physical constraints – they are not there anymore. The variability of wavelengths, or colours of light, is available now in abundance, so you can select the colour of light you want to use in your wearable.”
The Circular Ring 2 uses an array of sensors against the skin
Preprocessing of the signal also took up more space in the past, but can now be done by small chips in the products.
Computational power and speed of signal processing was another limit, but the advanced computational techniques used by modern wearables – including machine learning and AI processes – can be carried out on the cloud to avoid even more demand on the device themselves.
“If you had to do it on the watch, for example, you would have struggled – you wouldn't have enough juice, enough power or enough computational power on the watch and would have made it very big,” says Kyriacou. “[That] enables this miniaturisation of the smartwatches, the rings – and God knows where are we going next.”
Other important advances include improvements in micro-electromechanical systems, which have shrunk accelerometers and gyroscopes to millimetre scale while still capturing high-fidelity movement data, says Kovacs, along with improved power efficiency.
The next big thing
Wearable devices have not just shrunk more technology into smaller spaces. By using flexible printed circuit boards (PCBs), designers are also fitting them into new form factors such as rings, says Cassandra Cummings, CEO at electronic manufacturer Thomas Instrumentation in New Jersey.
“Most PCBs are made from a hard fibreglass material that could not be bent or folded into small areas. Flexible PCBs are a polyimide film that can be used for electronic circuitry and be bent, folded and moulded to unique shapes,” says Cummings, whose own irregular heart rhythm was spotted by a Fitbit bracelet several years ago, allowing doctors to diagnose the issue.
Other new wearables include smart earrings and many projects are developing smart fabrics for health-tracking clothes.
All companies in the industry – from technology giants such as Apple and Samsung to the smallest start-ups – are looking for the next big thing, says Kyriacou. “If you interact with the wearable healthcare industry, which I do, and I do collaborate with them, basically all the discussions on their R&D tables when we see them are: ‘What’s next? What’s the next biomarker? What’s the one that is going to be transformative, to make us unique as a commercial entity?’”
New features might investigate glucose or sweat, or combine inputs from multiple sources for more robust results.
“The future is multi-sensor integration for more holistic insight. We’re moving toward non-invasive glucose monitoring, hydration and hormonal status tracking via sweat or interstitial fluid sensors, and continuous blood pressure measurement without cuffs,” says Kovacs.
“We’ll also see better contextualisation, combining environmental sensors (air quality, UV exposure) with physiological metrics to give truly personalised recommendations.
“The next leap will be predictive analytics: not just telling you how you slept, but forecasting when you’re about to overtrain, get sick or experience a performance drop.”
Building trust
The coming years will likely bring even more features to even smaller devices. But an even more significant trend could be a bigger emphasis on trust and security, says Kyriacou. Wearable tech companies want to make their products more like medical devices, he says, so healthcare professionals trust the alerts they share.
“Consistent accuracy is still a problem,” he says. “They want to be acknowledged by the healthcare sector, that their wearables can be trusted.
“If you’re wearing your watch at home and one of your biomarkers seems to be odd or low – your heart rate, your blood pressure, your SpO2, your blood oxygenation or something – [you’ll] be able to ring the GP and say, ‘I think something’s not right with me, because my wearable tells me that my heart rate, my blood pressure – whatever biomarker is measured – is not within the range for me, for my age.’ And we want that GP to say, ‘Yeah, come in. We’ll have a look.’”
That culture shift from consumer technology towards medical devices involves a major role for biomedical engineers. “They’re basically working at the interface between engineering and medicine, or healthcare, so their primary focus… is to create solutions for healthcare,” says Kyriacou. “The definition for us engineers is reliable, robust, trustworthy solutions. So this is where we come, to bridge the gap.”
By developing better hardware, software and algorithmic models for wearables, biomedical engineers can ensure accuracy and repeatability. “Empowered” users can then use their devices for early screening, Kyriacou says, potentially leading to savings for the NHS and other healthcare providers.
Kovacs concludes: “We’re entering a phase where wearables will act less like passive recorders and more like active partners in health. We have entered the personalised, predictive and seamlessly integrated era of health and wellness monitoring.”
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