Printed sensors required for wearable skin patches

4th January 2021
Alex Lynn

Wearable technology with sensing functionality, such as wearable skin patches, is emerging as a highly promising application for printed/flexible electronics. Deployed either as skin patch or embedded within an e-textile, these devices enable parameters such as heart rate and temperature to be recorded in real-time and then transmitted wirelessly.

With such clear benefits, IDTechEx has estimated that the wearable skin patch market for healthcare applications will reach around $4 bn by 2030. Printed/flexible electronics is extremely well suited to wearable skin patches due to its low weight, flexibility/conformality, and potential for high-throughput, low-cost manufacturing of these disposable items.

Need for continuous monitoring

A major trend in the healthcare sector is a shift towards continuous monitoring, which brings multiple benefits. Firstly, rather than attending a doctor's surgery or hospital for a check-up, the patient can remain at home. The current COVID-19 situation has emphasised this benefit since patients can have their temperature measured remotely without having to interact with healthcare professionals and thus risk transmitting or acquiring the virus.

An additional benefit is that continuous monitoring enables data to be acquired with a far higher temporal resolution than would be possible with in-person visits since a skin patch can be worn at all times. This additional data, coupled with analysis via machine learning, should facilitate improved diagnosis and preventative medicine.

Monitoring electrical signals

Monitoring electrical signals is an important use case for wearable skin patches. This can include electrocardiograms (ECG) for investigating the heart, electromyography (EMG) for recording electrical signals from muscles, and even detecting brain activity in epileptic patients.

These are typically manufactured using screen-printed stretchable conductive inks and electrodes, with the latter made from a mixture of silver and silver chloride. Conductive traces are typically printed in a wave-like pattern to increase stretchability, while most of the electronics are currently contained in a rigid case. Electrodes and conductive traces are also being integrated within e-textiles, either on a stretchable substrate that is subsequently attached or printed directly onto the fabric.

Pressure and strain sensing

Aside from monitoring electrical signals, both pressure and strain can be measured using printed/flexible sensors. For example, incorporating a conformal thin film piezoresistive sensor can continuously measure the pressure being applied by bandages and alert medical staff if the bandage needs adjusting. Maintaining an appropriate pressure is important: too loose and it is entirely pointless, too tight and it unnecessarily restricts blood flow to the affected region.

Another application being trialed is monitoring joint movement using printed capacitive strain sensors incorporated, for example, into knee and elbow sleeves. This is useful for those receiving physiotherapy since the amount and extent of movement can be continuously assessed.

Wound monitoring

Currently in its early stages of development, wound monitoring is another promising medical application for printed/flexible electronics. At present, the only way to check a wound is to remove the dressing - by integrating sensors and electronics, the healing process can be monitored in real-time without the dressing being removed.

This sensing can involve measurement of temperature and pH, ideally with spatial resolution to monitor how healing is progressing. Multi-parameter sensing, with multiple functional materials printed onto the same surface to measure different variables simultaneously, is being developed and is ideally suited to this application.

Flexible hybrid electronics

At present most of the electronics for signal processing and communication used in wearable skin patches and e-textiles are contained within a rigid plastic box. While this provides protection and is required to enclose a conventional rigid PCB, it adds bulk and makes the patch or clothing uncomfortable to wear.

As such, much effort is being devoted to placing as much of the electronics as possible onto the stretchable substrate, rather than using a rigid PCB. This approach is termed flexible hybrid electronics (FHE), which is an emerging manufacturing methodology that utilises the best aspects of conventional and printed/flexible electronics. It combines printed interconnects and flexible/stretchable substrates with mounted components and is ideally suited to wearable electronics.

While FHE is still in the relatively early stages, it is a highly promising technology that will enable devices to break away from the form factor constraints imposed by rigid PCBs. Indeed, IDTechEx forecast that across all applications FHE will be around a $3.2 bn market by 2030.

To summarise, the transition towards continuous healthcare monitoring and interest e-textiles for fitness tracking provide substantial opportunities for printed/flexible electronics. This includes both printed sensors to detect the biological parameters and flexible hybrid electronics to process the signals and handle wireless communication.

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