Wearable gadgets such as smart watches, or wristbands, represent a user-friendly and cost-effective platform for the tracking of physiological parameters, such as heart rate and blood pulse oxygenation levels. However their rigid and opaque nature hinder the development of skin-conformable sensors that provide continuous and accurate data for telemedicine. Dr Emre Ozan Polat and his team from Kadir Has University in Istanbul, Turkey, use the unique flexibility, strength, and transparency of graphene and related materials to design discreet wearable sensors that provide accurate, real-time monitoring of clinical data.
The latest generation of wearable gadgets are based on a technique known as photoplethysmography (PPG), which works by sending light through the skin to collect vital health information. Given that more than 40% of all countries have fewer than ten doctors per 10,000 people (World Health Organization Global Reports), wearable devices in clinical settings offer telemedicine solutions that can help to improve the provision of public health, especially in developing countries.
Human skin provides a unique interface for wearables to extract physiological parameters. The main aim of wearable technology has revolved around user-friendly operation; hence, some more invasive approaches including the use of implantable wearables have not gathered a large amount of user interest. A recent review by Dr Emre Ozan Polat, published in Advanced Material Technologies, reports the increasing use of PPG methods in wearable devices due to its user-friendly and non-invasive operation for continuous extraction of vital health parameters. In his recent work, Polat covers the routes to the skin and garment integration of optoelectronic components, and analyses the measurement sites for accurate extraction of heart rate (HR) and respiration rate (RR). The study highlights the key statistics on wearables, and it gives a complete outlook on skin-conformable devices that are still in the research and development phases.
Polat and his team from the Kadir Has University, Istanbul, are currently working on the development of multifunctional wearable devices that combine the latest nano-material technologies with traditional electronics. Although wearable devices have been around for some time, their widespread use has so far been limited by the need for rigid materials, which impact on the functionality of the gadgets and their aesthetic appeal. Polat and his team work on novel approaches that combine the advanced sensing properties of graphene and related materials (GRM) within mechanically flexible device structures to realise discreet health-and-fitness trackers.
Polat’s team investigates suitable graphene and related materials (GRM) as sensing platforms that can bypass the technological limitations of the rigid semiconductor-based wearable sensors.
Non-invasive and continuous monitoring for public health
PPG technology allows for non-invasive monitoring of vital signs by sending light of specific wavelengths to the skin. Changes in light intensity due to absorption in blood vessels can be linked to changes in the HR, RR, blood pulse oxygenation (SpO2), and related cardiovascular parameters. PPG devices typically use rigid semiconductor-based light sources and light sensors to collect vital parameters from the skin. Current PPG wearables commonly employ green LEDs (540nm) to measure HR, since this wavelength minimises signal interference from movement while providing enough tissue-penetration depth (dermis and epidermis) to extract the vital signs. However, the presence of rigid components cannot provide continuous skin contact during exercise and the wearer’s movements can create ‘motion artefacts’ in the measurements. Although the data processing to cancel motion artefacts has become an important part of the wearable market, additional hardware requirements limit the continuous and long-term use of wearables and case-specific motion artefact reduction algorithms remain insufficient under complex real-world conditions.
Polat and his colleagues are working on the development of optimum device specifications to provide clinical grade data from wearable devices. The team currently investigates suitable GRMs as a sensing platform that can bypass the technological limitations of the rigid semiconductor-based wearable sensors. In this way, Polat aims to upgrade the feel and form factor of the currently available wearables while providing clinical accuracy for diagnosis through telemedicine networks.
Flexible optoelectronics: from niche to ubiquity
Graphene has emerged as a viable material for wearable optoelectronic sensors due to its key properties of transparency, conductivity, and flexibility. When combined with semi-transparent light-absorbing layers, graphene offers broad wavelength sensitive photodetectors (from 300 to 2,000nm), that are mechanically robust.
Polat and his colleagues from ICFO, The Institute of Photonic Sciences, have previously reported that flexible graphene photodetectors can operate at broad wavelength spectrum with operating speeds of less than a millisecond. This is a key requirement for the optical extraction of vital health parameters.
The ability to develop humanitarian technologies on unconventional substrates opens up many opportunities, from smart wearable technologies to new generation display technologies such as electronic papers.
The team demonstrated the HR measurement by using flexible photodetectors in PPG and have found a strong correlation to the devices that are used in clinical settings. The HR measurements at multiple wavelengths (red and infrared) yielded the optical extraction of blood pulse oxygenation, and the authors also reported the simultaneous extraction of the RR, which is extremely important for diagnosing respiratory problems.
Currently, Polat is working on combining the GRM-based light-sensing technologies with new integration methodologies, to enable ubiquitous light sensors for a wide application range from defence to health and energy harvesting. The research and development in flexible electronics has enabled the use of unconventional substrates such as thin films and foils of polymers.
Plant-based substrates, such as paper, are increasingly gathering attention due to their biocompatible and biodegradable properties. In this regard, Polat has previously reported novel applications that use graphene as an optically reconfigurable medium on standard printing papers. Similarly, silk is currently considered to be usable as a substrate for flexible and wearable optoelectronics. According to Polat, the ability to develop technologies on unconventional substrates would open up a plethora of opportunities, from smart wearable technologies to new-generation display technologies such as foldable electronic papers.
What are the next steps for your research team?
We have come a long way to demonstrate the active usage of nanomaterial-based technologies in flexible and wearable applications. The next stage in our research is to empower our approach with new materials to measure multiple vital signs for complete medical information that is needed to diagnose through telemedicine networks. In addition to telehealth, integration methodologies that we develop for transparent and flexible nanomaterials will serve a wide range of optoelectronic applications, from defence to energy harvesting.