Your heart on your sleeve

22 January 2020



Advances in wireless technologies, low-power electronics and the internet of things are driving innovations in wearables at a tremendous pace. Emma Green speaks to Yasser Khan, a doctoral researcher in the Department of Chemical Engineering at Stanford University, about the key considerations when selecting materials for these devices.


Wearable technology is a relatively new but rapidly growing market. Usually equipped with smart sensors connected to the internet to create a data exchange, the first consumer device to be produced was Pulsar’s Calculator Wristwatch in 2000, which achieved global success. Since then, there has been continued interest in the area, not only for general health purposes but also for addressing unmet medical needs. Consumer and medical devices do not only have different goals but also use different technologies.

If you were to take apart a consumer wearable device, such as an Apple Watch or Fitbit, you would notice that they are mainly comprised of rigid printed circuit boards and silicon chips. While this is the norm for consumer devices, academia adopts an alternative approach, giving the technology a dramatically different look and feel.

Yasser Khan, a PhD student at Stanford University, is at the forefront of research in this area. “We want wearables to be thin, light and comfortable – in essence, imperceptible,” he says. “The device will act as a second skin, and will be able to relay sensory information such as touch and temperature.”

Blending wearables into the skin is no easy feat and demands specific component parts in order to be successful. “We need materials that are soft, flexible, stretchable, biocompatible and self-healable,” explains Khan. “These properties are at odds with conventional silicon-based electronic materials.”

There are a number of different options for these materials, many of which have been explored recently within academia. “Organic and inorganic nanomaterials are promising for all these skin-like properties,” says Khan. “In the past few years, researchers around the world have shown fascinating skin-inspired plastic and elastomeric wearables.”

Another key consideration when manufacturing wearables is ensuring that materials can become semiconductors and insulators as well as having metallic properties, opening up numerous possibilities for devices. “These three properties allow us to create electronic devices, sensors and circuits,” says Khan. “We are now living in exciting times; after decades of research, we now have the materials to design wearables that are truly skin-like and imperceptible.”

There are a number of challenges to overcome when making these devices, ensuring that they have the desired look, feel and performance. “Existing rigid and hard electronics are meant to be robust to ensure good electronic performance,” explains Khan. “In the case of soft, skin-like wearables, the mechanical requirements of flexibility and stretchability fundamentally oppose pristine electronic properties. Adding flexibility and stretchability to electronic materials degrades their electronic performance.”

Stretch of imagination

To address this issue, Khan and other researchers at Stanford are investigating the use of intrinsically stretchable polymer materials, which can help to balance both worlds by being soft, mechanically robust and having high performance. This has led to the creation of flexible bioelectronic, biophotonic and biochemical sensors.

One piece of work by the team involved the development of a flexible sensor for oximetry, the optical technique for determining oxygen saturation in blood and tissue. This is important for monitoring the health of patients with any type of condition that can affect blood oxygen levels, such as chronic obstructive pulmonary disease, especially while they’re in the hospital.

Traditional pulse oximeters use expensive optoelectronic components that are bulky, rigid and restrict sensing locations to fingertips or earlobes. Flexible organic optoelectronics are thus particularly advantageous for oximetry because of their inherent mechanical flexibility.

“In 2014, we demonstrated the first allorganic optoelectronic oximeter sensor for pulse oximetry,” says Khan. “This transmission-mode sensor demonstrated that oximetry can be performed with organic optoelectronics, but to realise the true potential of organic optoelectronics for oximetry, a reflection-mode operation is essential to allow sensor placement on different parts of the body.”

Khan and other researchers at Stanford have taken this research forward in their most recent work. “We demonstrated a flexible reflectance oximeter array that senses reflected light to determine the oxygen saturation,” explains Khan. “We also used printing techniques to fabricate the sensor on flexible plastic substrates, making the sensor comfortable to wear and efficient at extracting a high-quality biosignal.”

Unlike fingertip oximeters, which are commonly used in healthcare, the sensor researchers developed can detect blood oxygen levels at nine points in a grid and can be placed anywhere on the skin, opening up new possibilities for oximetry.

“The grid arrangement of the sensor enables the mapping of oxygen concentration across an area rather than at a single point,” explains Khan. “It could potentially be used to map oxygenation of skin grafts or to look through the skin to monitor oxygen levels in transplanted organs.”

Following transplantation, surgeons need to determine whether all parts of an organ are getting enough oxygen. “If only one sensor is used, surgeons need to move it around to measure oxygenation at different locations,” says Khan. “With our array, we can know right away if there is a point that is not healing properly. No existing device provides this level of functionality in a flexible form.”

“Our research team is currently merging impedance spectroscopy and oximetry to design a smart bandage to electrically and optically determine tissue health.”

Smart bandage

During his PhD studies, Khan has also worked on a ‘smart bandage’ that uses electrical currents to detect early tissue damage from pressure ulcers, or bedsores, before they can be seen by human eyes. “We printed an array of electrodes onto a thin, flexible film,” explains Khan. “These discharged a very small current to create a spatial map of the underlying tissue based upon the flow of electricity at different frequencies, a technique called impedance spectroscopy.”

As a cell begins to die, the integrity of the cell wall starts to break down, allowing electrical signals to leak through. Khan and the team saw that the impedance response of tissue was strongly correlated with tissue health, which gave them an idea for developing a novel device. “Our research team is currently merging impedance spectroscopy and oximetry to design a smart bandage to electrically and optically determine tissue health,” says Khan. “This will have an unprecedented level of bioelectronic and biophotonic sensing capability.”

In light of these exciting findings and the high level of interest in wearables more generally, there are set to be a number of developments over the next few years. Although devices have varying goals, they all have two key functions: sensing biosignals, and processing and transmitting the data. Materials most suitable for one function are not necessarily the most suitable for the other. For example, soft materials are well-suited to interfacing with the skin and thus provide excellent biosignals. On the other hand, processing and transmitting the sensor data demands high-performance electronics, which remain beyond the capabilities of many soft polymer-based materials.

Achieving these functions is a challenge but not impossible, and companies are already making serious inroads in this area. “I believe, in the future, we will have a hybrid approach, where soft electronics will be interfaced with biology, and highperformance silicon electronics will be used for data processing and computation,” says Khan. “Similar to Elon Musk’s Neuralink neural interface – soft electrodes to interface with neurons and silicon chips to process the neural signals.” 


Wearable device opportunities in modern healthcare

As healthcare shifts towards a patient-centred outcome-based delivery model, wearable devices are being transformed into important players in the healthcare system. As such, this transformation requires cross-disciplinary collaboration and should be driven by three main factors.

A shift to disease prevention

There is increasing recognition that early diagnosis of a disease helps in providing the best medical care and health outcomes for the patients. In particular, there is a general agreement that damage to organs can be reversed if the disease is caught and treated at an early stage. Hence, early diagnosis, in conjugation with prevention of disease, has attracted more attention and is considered a more cost-effective approach to bring better health outcomes, as well as improving the quality of life. The principle of preventive medicine includes prevention of a disease before it occurs (primary prevention) and reducing the intensity of a disease that has already arisen, thereby controlling progression or preventing relapse after recovery (secondary prevention).

Personalisation of medical care

Effective early diagnosis relies on the application of routine monitoring of complex parameters of individuals, which involves the uninterrupted monitoring of human physiological indexes, such as the body temperature, heart-beat pulse or respiration rate, and skin moisture, as well as some informative biomarkers. Monitoring these informative physiological indexes and biomarkers seems not only to provide for timely diagnosis of disease (especially for high-risk populations, such as carriers of hereditary and inevitable diseases), but is useful for gauging pervasive and personalised physiological activity indexes, and treatment to improve the quality of life and the efficacy of treatment.

Intelligent interpretation of a large volume of health-related data

Big-data tools collect billions of data points from wearable devices that can be used for health management in descriptive analytics, measuring what has happened (for example, frequency, costs and resources); predictive analytics, which use the descriptive data to forecast likely outcomes; and prescriptive analytics, which provide the ability to make proactive decisions considering pre-empting predictions. These analytics can help clinicians to make an accurate diagnosis, predict the health condition at an early stage and intervene during the initial stages of an illness. They can also refer to historical medical data related to clinical assessments and lab data to create a continuous inflow to quickly implement changes.

Source: Advanced Healthcare Materials 

A flexible oximeter array that can detect blood oxygen levels at nine sperate points.
Khan displays the sensors of an oximeter printed on flexible plastic substrates.


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