Medical monitoring devices that are flexible and can adapt to the wearer's shape are increasingly being introduced.

As the need for remote medical care increases during the COVID-19 pandemic and technological advancements improve sensor accuracy, the use of wearable medical monitoring devices is increasing across a variety of healthcare fields.

In particular, wearable sensing and monitoring devices for use in telemedicine are being actively developed.

Telemedicine has been growing rapidly over the past few years, and it has been given a further boost during the coronavirus pandemic as contact between doctors and patients has been limited.

Telemedicine allows patients to be monitored remotely from their own homes, freeing up hospital beds for more urgent patients. Wearable technology can be used for diagnostics in hospital settings, but it has even greater potential when used for telehealth monitoring.

These wearable sensing devices are made using flexible materials such as polymers, thin films, 2D materials, and other nanomaterials.

These sensing devices are integrated with other sensors, a power source (which can be a battery, solar cell, or energy harvester device such as a nanogenerator), communication technology (to transmit data to a central processing system), and other circuits.

Making electronic components small and flexible enough to use in wearable technology is not easy.

Therefore, various nanomaterials (especially 2D materials) are used to make these parts because they are cost-effective and easy to integrate.

But there's growing interest in another approach: printing sensors directly onto wearable devices. Sensors are the most important element in a monitoring system.

These printed sensors may use 2D materials and other nanomaterials, but they may also use other materials.

Printing sensors directly onto wearable health monitoring devices would be a much simpler and easier process that could help commercialize more wearable monitoring devices.

This is especially true when using existing, more economical and easier printing technologies.

■ Benefits of printed sensors

While a range of advanced deposition and nanodeposition techniques have emerged to fabricate thin-film sensors on substrates for wearable devices (which often use soft polymers for their flexibility and conformability), a variety of biomedical sensors can be printed using screen printing techniques.

Techniques such as inkjet printing and transfer printing can also be used, but screen printing is considered the best method for printing sensors.

Screen printing is widely available with related equipment, is relatively inexpensive, and is easier to use than complex deposition techniques.

Therefore, it provides a much more scalable and commercially feasible approach for manufacturing printed sensors as medical monitoring devices.

Screen printing can also use a variety of materials, from polymer solutions to conductive nanomaterial inks. Therefore, it provides a versatile platform capable of printing various sensors.

So why print sensors instead of other manufacturing techniques? Screen printing is a simple, fast, and efficient printing technique that can mass produce identical patterns in a single print.

From a large-scale operation and commercialization perspective, screen printing can be easily mass-produced at a lower cost compared to other techniques.

From a performance perspective, screen printing allows for high-resolution patterning and printing over large areas.

Screen printing also reduces waste compared to traditional manufacturing techniques because it can deposit materials in specific locations as needed. Everything can be done at once, especially in large-scale manufacturing.

Therefore, printing techniques can bring many manufacturing advantages. In addition, printing techniques can produce very thin sensors and can print on the surface of the device, which enables many design possibilities for medical wearables.

This approach is much simpler than integrating devices into the material matrix of a wearable.

Additionally, because they can be printed on demand, more customized sensors can be manufactured, and modifying the printing process to manufacture different sensors is much easier than on a conventional manufacturing line.

The range of materials that can be used for screen printing is also very wide. Depending on the type of sensor you want to manufacture, a variety of materials can be used.

A series of metals can be prepared into printable conductive inks, and then the sensors can be printed.

Metals commonly used in wearable medical sensors include gold, copper, platinum, nickel, aluminum, and silver.

In addition to metals, a range of nanomaterial compounds (graphene, carbon nanotube compounds), functional nanomaterial inks, and silver compounds are all being used as active sensing surfaces in printed medical wearable sensors.

These sensors require a platform on which they can be printed. This is often in the form of a conductive polymer, which allows the sensor to interact better with the skin or other components and provides high sensitivity and reliability.

PEDOT:PSS is the most widely used polymer platform for printed sensors. Other polymer materials such as polyacetylene, polypyrrole, polyphenylene, poly(p-phenylene vinylene), and polythiophene polyaniline can also be used.

■ Medical monitoring applications using printed sensors

Just as different materials can be used for printed medical sensors, different types of printed sensors can be used for different medical monitoring devices.

For example, let's look at a sensor that measures strain. Strain-based sensors can measure a certain type of movement. These wearable sensors are commonly used for monitoring human physiological signals and human joint motion.

Another example is measuring various biomolecules and biosignals. Non-strain-based sensors directly detect biomolecules of interest or directly measure physiological parameters exhibited by the patient.

Molecular sensing can detect various biomolecules in the blood. A representative example is blood sugar. It can also detect sweat from human skin. And now, using printed sensors, it is possible to measure the patient's breathing rate and heart rate, and even remote electrocardiogram (ECG) monitoring.

Wearables can also use sensor arrays, which combine multiple sensors to examine more complex tissues or tissues that require analysis from multiple stimulus angles.

Sensor arrays are useful for monitoring gait during walking, monitoring the sitting posture of wheelchair users, or monitoring skin.

Development of printed temperature sensors for use in wearable medical devices is also progressing rapidly.

Thermal sensing is very important for diagnosing illness. The human body suffers from heat stress when it is sick, and an increase in skin temperature and core temperature can indicate serious illness.

Printed temperature sensors can be used in medical wearables to diagnose chronic diseases such as cardiovascular, diabetes, and respiratory diseases, and can even diagnose cancer.

■ Conclusion

The adoption of healthcare monitoring wearables is expanding as an effective way to remotely measure various health factors of patients.

The most important role of these wearables is the sensing system for measuring the patient. To ensure that patients can comfortably wear medical monitoring devices for long periods of time, the sensors and other components must be flexible.

Flexible sensors can also be manufactured using extremely thin nanomaterials, but printing the sensors onto the surface of the device is much easier, requires less raw materials, and is more scalable.

A variety of printed medical wearables are already appearing (both in commercial and research stages). The printing techniques introduced in this article will contribute to expanding the distribution of medical wearables.

Among the many printing technologies, screen printing seems to be the most promising. Screen printing is versatile and can use a variety of materials. Using printed sensors manufactured in this way, various aspects related to human health can be monitored, from measuring heart rate to detecting signs of disease and measuring various biomolecules in the blood.

※ author

Written by / Liam Critchley
Provided by / Mouser Electronics

Liam Critchley is a writer, journalist and communicator specializing in chemistry and nanotechnology, who has worked to explain how molecular principles can be applied to a wide range of fields. He uses his knowledge to explain complex scientific topics in a simple way for both scientists and laypeople. He has published over 350 articles across a wide range of topics and industries, including chemistry and nanotechnology.

He is the Science Communications Manager for the Nanotechnology Industry Association (NIA) in Europe and has been writing articles for companies, associations and media websites worldwide for several years. He holds a Masters degree in Chemistry with a focus on nanotechnology and chemical engineering.

He is an advisory board member of the National Graphene Association (NGA) in the United States and the Nanotechnology World Network (NWN), an international organization, and a board member of GlamSci, a scientific society in the United Kingdom. He is a member of the British Society for Nanomedicine (BSNM) and the International Association for Advanced Materials (IAAM). He is also a peer reviewer for several academic journals.