Flexible amplifiers for vital-sign monitoring

Amplifiers based on flexible thin-film electronics can provide healthcare monitoring systems with high signal-to-noise ratios.


Flexible amplifiers for vital-sign monitoring
Amplifiers based on flexible thin-film electronics can provide healthcare monitoring systems with high signal-tonoise ratios.

Kris Myny
I n a hospital, the vital signs of a patient are typically monitored by attaching several electrodes to different locations on the body. The analogue electrical signals from the body are then captured and transferred via long cables to a processing unit for data analysis and subsequent visualization on information screens. In this arrangement, the conversion of analogue signals to digital data occurs only at the processing unit positioned far away from the electrodes. This creates some challenging requirements for the electrodes interfacing the body, as well as for the cables transmitting the weak electrical signals towards the processing unit. A key figure of merit in such a system is the aspect ratio between the captured electrical biomedical signal and the noise generated between signal acquisition and digitization. Noise is inevitably added to the signal from various sources, including the body-electrode interface, body movements, the electronic equipment and the cabling.
One improvement proposed for this system is the introduction of active electrodes rather than passive electrodes 1,2 . In an active electrode, the electronics appear closer to the body-electrode interface by means of integrating an operational amplifier directly onto the electrode. This allows for signal amplification prior to the connection of the electronic processing unit, improving the output impedance and the signal-tonoise ratio, and thus reducing artefacts from cables and electrode movements. Despite these advantages, the active electrodes can be bulkier and more expensive than the passive electrodes. Now, writing in Nature Electronics, Tsuyoshi Sekitani and colleagues show that the advantages of passive and active electrodes can be combined by using a 2-μm-thick electronic amplifier directly on the electrodes 3 . With the setup, the researchers obtain electrocardiogramsa recording of the electrical activity of a person's heart -in which the ultraflexible amplifier improves the obtained signal quality by subtracting the noise, and allows all of the essential electrocardiogram components to be extracted from the biomedical electrical signal.
The 2-μm-thick amplifier is based on organic electronics (Fig. 1a), which are part of a family of emerging thin-film transistor technologies that include lowtemperature polycrystalline silicon (LTPS), amorphous silicon and amorphous metal oxide-based electronics (such as indium gallium zinc oxide) 4 . These transistor technologies can be manufactured directly on flexible substrates due to their low manufacturing temperature requirements (below 350 °C). Several polymeric materials can be used as substrates for such flexible electronics, resulting in an ultrathin, flexible and lightweight technology that is well suited to wearable devices. This relatively simple transistor technology, compared to mainstream silicon chip technologies, is comprised of only a small number of thin layers of semiconductors, dielectrics and metals, enabling rapid prototyping and leading to a potential low cost per unit area. Currently, the main application of flexible electronics is in displays, where the transistors act solely as switches and drivers to control the display frontplane, which is comprised of large arrays of liquidcrystal-display pixels or light-emitting-diode pixels. Beyond displays, the technology is promising for flexible-integrated-circuit applications like radio-frequency identification tags or sensor tags.
Sekitani and colleagues -who are based at Osaka University and the National Institute of Advanced Industrial Science and Technology in Osaka -integrated several organic thin-film transistors into a differential amplifier to be used in combination with passive electrodes. The design and realization of circuits with organic transistors is challenging due to the absence of an n-type transistor and presence of intrinsic parameter variations, which limit the signal processing capabilities of the corresponding circuit. The researchers address the issue of parameter variation by means of a post-fabrication mismatch compensation technique. This technique offers the possibility to finalize the circuit by adding a certain number of parallel load and/or drive transistors depending on the measured variation, which advantageously reduces the on-current mismatch and therefore improves the behaviour of the amplifiers. Combined with an optimized p-type load circuit, required due to the absence of an n-type transistor, the amplifier resulted in a common-mode rejection ratio at 50 Hz of 45 dB, and is able to amplify the obtained weak biomedical signals. The researchers illustrate the capabilities of their approach by using the differential amplifier to measure a weak heartbeat signal and subsequently extracting the correct specific waveforms of an electrocardiogram signal (Fig. 1b).
Although silicon complementary metal-oxide-semiconductor (CMOS) chip technologies result in better-performing signal processing, the form factor and potential lower manufacturing cost of ultrathin flexible transistor technologies make them appealing for such healthcare applications. The work of Sekitani and colleagues demonstrates, in particular, the significant potential of these technologies to record weak bio-potential signals. Nonetheless, to penetrate the commercial market, essential performance improvements need to be made. For example, it will be necessary to further suppress intrinsic mismatch and flicker noise in order to further enhance amplifier performance such that it offers the ability to process the even weaker signals necessary for an electroencephalogram -a recording of brain activity.
Different thin-film transistor technologies, such as amorphous metal oxides or LTPS may also impact future results in a beneficial way, due to their higher charge carrier mobility and therefore resulting bandwidth, or the coexistence of a p-type and n-type transistor, improving the gain and noise suppression of the bespoke circuit. Another important direction for research in this field will be the employment of architectural studies to optimize the partitioning between silicon CMOS and thin-film transistors, whereby even the conversion of the analogue signal to digital may be executed with thin-film transistors.
Finally, a limiting factor of silicon CMOS is the restriction in the number of inputoutput pins on a chip, making the use of thin-film transistor-based multiplexers a viable pathway for more complex biomedical patches. ❐