Wearable electronics and sensors are becoming increasingly popular and will play an important role in the future of smart technology including those within sport and medical applications. Such devices require materials that can accurately perform their intended purpose, with stretchable structures and a size and topology that enables them to be mounted to skin or clothing without causing discomfort to the user.
Piezoresistive materials (characterised as displaying a change in electrical resistance when strain is applied) are mostly used for measuring mechanical strain associated with displacements and deformations of parts in buildings and machines. Lately, applications of these materials have expanded into areas such as sport and medical science. Piezoresistive materials can be used in devices such as wearable sensors to monitor heart rate and other small body movements, whilst withstanding mechanical deformations and other variables such as temperature and humidity.
Practical applications require sensor devices that are low-cost as well as being suitable for mass production with cross compatibility between technologies. Requirements can be met by techniques such as roll-to-roll printing, which enables efficient printing of both the substrate and electrical component of a device on suitable substrates.
Researchers at the University on Oulu and the VTT Technical research centre of Finland created innovative, fully printed sensor devices using roll-to-roll printing methods. The electrodes and the connecting inkjet-deposited piezoresistive carbon nanotube micropattern (SWCNT) were printed alongside the polydimethylsiloxane (PDMS) substrate to create the sensor. To investigate the functionality of the developed sensor device, researchers needed to understand performance under different scenarios, such as stress and strain in combination with variables such as temperature and humidity – mimicking potential real-world use cases. This required highly accurate and precise sample measurements to understand mechanical deformation as well as running simultaneous electrical measurements to ensure the sensors performed their intended purpose.
The researchers used the Linkam TST350E* for mechanical testing of the material, including stress, strain and deformation, as well as repetitive cyclical testing to see how the sensor responded to repetitive movement and investigating electrical patterns for the intended use cases. Lead author, Henri Ervasti, notes this scientific instrument was selected “due to the stages’ ideal size for small sample testing, good clamping mechanism, precise and sufficiently fast movement and potential for cyclical testing. Combined with a sourcemeter it allowed us to precisely log electrical properties of our samples when they were subjected to different kinds of strains.”
The Linkam THMS600 was used to investigate temperature dependence of resistance of the SWCNT pattern.
Illustration taken from reference paper [1]. A-C shows the subsequent process steps of sensor preparation; D shows sensor mounted on the Linkam TST350E.
During the research, the researchers also investigated optimum sensor printing patterns. Typically, straight line patterns are used for strain sensors that require higher sensitivity and gauge, whereas other geometries, such as zig zag patterns are used for higher durability and low sensitivity to strain. Here, the researchers tested both straight and zigzag SWCNT micropatterns within the sensors. Mechanical testing validated that pattern design has a large impact on response characteristic of piezoresistive materials, with zigzag structures utilised in design to decrease the stress in patterns for such applications.
The research demonstrated that this developed stretchable printed material and structure showed high sensitivity to tensile strain (up to 400 gauge factor), pressure (∼0.09 Pa–1 sensitivity) and bending deformation (<10 mm radius); suitable for the intended application.
When considering both temperature and humidity, the sensor network became less resistant with increasing temperature, related to electron movement dependence on temperature. Between 10-65% humidity, resistance increased as water on the sensor acted as a weak reductant and decreased conductivity. The researchers note that temperature and humidity have a smaller influence on the device than the piezoresisitivity and could be calibrated to remove these influences if needed.
Researchers mounted the device to skin and monitored radial artery cardiac pulses, finger flexing and detected chest movement during breathing. All of which have strong medical and sport applications, with the sensor providing a low-cost, easy to manufacture and versatile solution within wearable electronics, as well as providing highly sensitive monitoring required of such applications. Additionally, results suggest that inkjet deposition of nanomaterials in this way can also be used with other printing technology, which expands applications across projects from infrastructure to aeronautics.
*Linkam has since replaced our TST350E with the Modular Force Stage (MFS). The MFS system is a new and improved version of the popular TST350 tensile stage with increased sensitivity, resolution and a modular concept. The modular design allows users to have an additional level of control over their experiments with the ability to change grips, heater type and force ranges.
Thank you to Krisztian Kordas and Henri Ervasti for their contributions. For full research details see [1].
For more information on Linkam’s MFS, and our products to control temperature and humidity around the sample for microscopy or spectroscopic characterisation, contact info@linkam.co.uk.
Reference:
[1] Ervasti et al., Inkjet-Deposited Single-Wall Carbon Nanotube Micropatterns on Stretchable PDMS-Ag Substrate–Electrode Structures for Piezoresistive Strain Sensing. ACS Appl. Mater. Interfaces 2021, https://doi.org/10.1021/acsami.1c04397