Wearable technologies, like the Apple Watch, have been ‘standard issue’ for many of us, but the current technology has its limits. Japanese and US researchers have now developed stretchable/flexible sensors, batteries and transmitters that can be assembled into a wireless ‘smart skin’. This technology will open new horizons in precision, and remote, medicine.

Shortcomings of current wearables

Current commercial wearables are good at monitoring our heart rate or blood oxygen levels by photoplethysmography (PPG), our heart’s electrical activity by electrocardiography (ECG), and body activities such as exercise, sleep, and stress levels.

But strapped usually to your wrist, the area of skin contact is limited and the device is monitoring your body from the body’s periphery. This can mean that some of their readings are not as accurate as they could be if the skin exposure area was larger or the device was affixed more closely to the body’s core.

While engineered to a relatively small physical size, the wearable is still rigid and bulky. It can bounce around on the body during physical activity, breaking the interface (through the skin) with the body, which further reduces the accuracy of its data readings.

There are also many people who find current wearables uncomfortable or distracting, including children and people with dementia.

Lastly, there are many serious medical conditions which current wearables are not capable of monitoring, such as the progression of tumours on the skin. As the skin is our largest organ, sensors which can have a more direct, intimate interface with the skin also open up the possibility of new forms of continuous monitoring, such as sweat chemicals.

“Comfort of wear” the key

In a recent article, one of the leading group of researchers on ‘skin contact’ wearables described their key design objective as follows:

“In addition to their sensing capability, it is very important to design for the “comfort-of-wear” of wearable devices. Comfort-of-wear can be defined by how the devices worn by the wearers can minimize the irregularity caused by the device’s physical existence in their daily activities. This concept, among others, regulates the wearer’s willingness to wear a wearable device. Although the comfort-of-wear of current rigid wearable devices (e.g., wristwatches or rings) is partially achieved by their small size, it is not enough for expanding the usage of wearable devices. ….We predict that comfort-of-wear will be one of the deciding criteria for the further proliferation of next generation healthcare monitoring devices.”

The design parameters for ‘skin contact’ wearables

Like the clothes we wear in everyday life, effective wearable tech should not interfere with body movement. This depends on a combination of two factors: materials which have a lower bending stiffness will cause less interference with body movement and the device must be highly stretchable, otherwise will may cause discomfort (e.g., tightness) when the body is extended or contracted, or the device may fracture.

There are several technologies which can be used. Ultrathin polymer substrates (e.g. as used to laminate sensitive areas like the cortex) can be used to envelop or encase thin-film sensor and communication devices such as transistors, photodiodes, and light-emitting diodes, without harming their electronic function. These materials are measures in microns (μm) : visualizing a micron, a human red blood cell is 5 microns and an average human hair has a diameter of 100 microns. Researchers have fabricated a transistor with an overall thickness of 2 μm using a 1.2 μm foil as the substrate, resulting in a device insensitive to bending. Another technology is nanomesh, which can even be moulded to the ridges of your fingerprints.

Stretchable conductors can be achieved by using liquid metals, and conductive polymers. The researchers have been able to achieve stretchability of over 100% without impacting the wearables electrical performance.

A drawback of thinness is the risk of tearing and difficulty in handling (the medical version of the ‘unspooling glad wrap’ challenge). However, this could be solved by future gel materials which are skin-like and self-healing:

“The next frontier in soft materials for future devices can be found in hydrogels. Hydrogels are composed of a crosslinked porous polymer network and water molecules occupying the spaces between the polymer chains. As a result, hydrogels are soft and moist and have physical properties such as human tissue. Additionally, hydrogels can have the ability of skin-like self-healing when supramolecular bonds, such as strong hydrogen bonds, are introduced. This, along with its chemical structure, allows hydrogels to adhere intimately to human skin. Therefore, hydrogels are expected to be applied as an adhesive layer to bridge the gap between skin and wearable devices.”

Breathability is another important factor for “comfort-of-wear”. The breathability of the skin mainly refers to water vapor permeability. If the breathability is low, the area where the device is worn is effectively sealed, and skin irritation or rashes may occur after prolonged use. Also, we all have experienced the problem of band-aids etc falling off if we are sweating too much. Precision engineering of the ultra-thin polymers can allow for holes which align with and are the size of skin pores.

Lastly, communications devices generate heat, which can be uncomfortable for the wearer. This is not such a problem with current rigid wearables because they are usually worn over or with clothing and sitting up on your wrist, they dissipate heat to the external environment. But wearables directly ‘glued’ to your body could be uncomfortable with any perceptive heat generation. Some transmitters are low power and therefore low heat, but light-emitting displays or actuators can generate more heat. The device design may need to manage (‘ration’) the operating time for these parts of a wearable.

Powering ‘smart skin’

The biggest challenge for 'skin contact' wearables is power, as the researchers explain:

“Although the use of compliant electronic materials can effectively improve the comfort-of-wear, some components in the whole systems can be difficult to realize in a soft or thin form, which include power supplies and processors. Currently, lithium-ion batteries are powering most of the wearable devices on the market, which contribute to the increase in hardness, volume, and weight of the overall wearable device. Another consideration is that lithium-ion batteries are potentially dangerous if punctured,”

One solution is self-powered devices – or rather devices powered by our own bodies. The human body generates thermal, chemical and mechanical energy, and this can be captured to power wearables. Thermal energy can be harvested through thermoelectric generators (TEG) and pyroelectric generators (PEGs), which can come in a (less efficient but still viable) soft form that can be embedded directly into the on-skin wearable device.

Kinetic energy from our body movements can be converted into electrical energy through contact charging and electrostatic induction – and this too can be done by using stretchable composite materials, or shaping them to a fibre structure with breathability.

It is even possible to use redox enzymes as catalysts to convert the chemical energy of metabolites present in biological fluids (e.g., sweat) into electricity.

Another source of power for 'skin contact' wearables can be light, thin, and flexible solar cells laminated into the wearable device.

But, of course, we are not always moving, or sweating or the sun is not always out, and a stable source of power is needed for the on-skin wearable. The researchers’ answer was to develop a ‘stretchable’ battery. All of the components of the battery are intrinsically stretchable films, with total thickness of about 270 μm.

Where’s the chip?

Computing devices, including wearables, depend on chips – and even if miniaturised, they are hard, inflexible things.

The researchers’ solution is to locate the chip away from the ‘skin contact’ wearable, e.g. in a hand held reader or a smartphone, which is then wirelessly connected to the wearable. This, in turn, requires reliance on communications technologies which don’t require a transmitter/receiver at both ends of the transmission pathway and significant amounts of powering in the ‘skin contact’ wearable. This means Bluetooth, Wi-Fi and Zigbee are out, but radio frequency identification (RFID) or near field communication (NFC) are viable. The RFID and NFC antennas can be made of stretchable materials and they too can be laminated into the ‘skin contact’ wearable. Integrating a stretchable high-frequency diode to convert wirelessly transmitted alternating current (AC) DC power allows the wireless system embedded in the ‘skin contact’ wearable to acquire direct current power that can operate various electronic devices.

However, RFID and NFC have short transmission distances, and it may be necessary to have a more traditional transmitter on-body to gather the signals from the wearables over the body to retransmit onto a hand held device for the wearer or the doctor.

The researchers note that a simpler solution may be available in the near term:

“Another approach to eliminating rigid elements is to eliminate the use of electrical components. These systems display sensing results by using a chemical method of coloration. A personal UV sensor using photochemistry and sensing of sweat using a colorimetric method have been proposed. These devices are made by composites of UV-sensitive dyes or reactive materials with soft materials, which are soft and can conform to the human body.”

A working example

Healthcare professionals utilize tumour shrinkage as a key metric to establish the efficacy of cancer treatments. However, the use of metal pincer-like callipers to measure soft tissues is not ideal, and radiological approaches cannot deliver the sort of continuous data needed for real-time assessment.

Researchers at Stanford University, Georgia Tech, USC Viterbi School of Engineering and the University of Tokyo have created a small, autonomous device with a stretchable/flexible sensor that can be adhered to the skin to measure the changing size of tumours.

In an article outlining the new device (called FAST for “Flexible Autonomous Sensor measuring Tumors”), the researchers say that it offers at least three significant advances:

“First, it provides continuous monitoring, as the sensor is physically connected to the mouse and remains in place over the entire experimental period. Second, the flexible sensor enshrouds the tumour and is therefore able to measure shape changes that are difficult to discern with other methods. Third, FAST is both autonomous and non-invasive. It is connected to the skin, not unlike a band-aid, battery operated and connected wirelessly.”

FAST is low cost (especially compared to a CAT scan), at US$60.

Read more: Engineering the Comfort-of-Wear for Next Generation Wearables