Vital Signs

The advanced device can pick up signals other sensors cannot and uses transistors to form an inverter capable of detecting strokes. Dr Paddy KL Chan, Associate Professor at the Department of Mechanical Engineering, leads the team which, in collaboration with Nanjing University, developed the device.

“The core breakthrough here is that our inverter uses a high-performance transistor, which includes high carrier mobility, ultra-low contact resistance and sub-threshold swing,” said Dr Chan. “These three parameters are the important indicators of transistor performance. It is not the ECG sensor but the transistor which is crucial in this device.

“We used two transistors to form an inverter, which has a unique function that can amplify the signal while at the same time filtering out noise. Conventionally, as the electrophysiological signal from the human body is in the range of millivolt to microvolt, this weak signal would require a more sophisticated set-up to measure properly. But, if a device can amplify the target signal while at the same time suppressing the noise, it can significantly improve the quality of the data and make it suitable to be measured by a basic – or even portable – set-up.”

Powered by a simple button battery, the device can be worn by patients night and day, and even in the shower. The signal amplification is outstanding, with a high gain of more than 10,000 which enables it to detect electrophysical signals – known as the f-wave – with a frequency of between 300 and 600 beats per minute, which indicates atrial fibrillation.

“The f-wave can be considered as the ‘signature’ of patients with atrial fibrillation,” said Dr Chan. “The capability to detect the high signal is down to the organic field effect transistors (OFETs) having an ultra-low sub-threshold swing. This enables our ECG sensor to pick up signals from patients which conventional sensors cannot.

“The sub-threshold swing is a vital parameter in transistor or inverter operation as it implies how much voltage change is needed to turn the device from the ‘off’ state to the ‘on’ state. Our record low sub-threshold swing device ensures low operating power and high sensitivity.”

Miniaturisation

Dr Chan’s research team have been focussing on organic field effect transistors for more than 10 years, and one of their main aims has always been to advance the miniaturisation and contact effect in these devices.

“Developing high-performance devices to be wearable electronic applications has been a clear goal for me and my group,” he said. “This is why in 2016, we developed a saliva glucose sensor and temperature sensor; in 2018, we developed a C-reactive protein sensor on a medical catheter and also a flexible optical sensor; and in 2020 we developed a conformal skin heater and tissue impedance sensor.”

His initial breakthrough in developing the staggered structure monolayer OFETs – the material used in the latest experiment – was published in Advanced Materials in 2020, and a patent was filed for the innovation in the US. In this latest work, his team have advanced the application of the monolayer OFETs to flexible substrate for wearable electronic applications.

When the team started working on this particular ECG sensor, a colleague of Dr Chan’s in Nanjing University sent his PhD student to HKU to join the team for 18 months to learn the skills to make high performance transistors.

“This student, Zongzong Lou, worked closely with my students – especially postdoctoral fellow, Boyu Peng – on developing the high-gain low-voltage sensor,” said Dr Chan. “When they showed me the performance of the integrated device one evening, I knew we had achieved exactly what we needed for sensing electrophysiological signals from the human body. After that, we started to perform a lot of tests, not only ECG but also including electromyography and electroencephalography.”

The team would like to see the device in public use, and to that end they are working to bring it from laboratory scale to mass production scale. “But first we will work on making the device even smaller,” said Dr Chan, “and instead of using one inverter we plan to use a more advanced and sophisticated circuit to do the electrophysiological signal sensing.”