The word ‘piezo-’ derives from the Greek word piezin, which means ‘to press.’ Many researchers in the semiconductor field will be familiar with the term ‘piezoelectricity,’ which refers to the electrical polarisation that occurs in proportion to mechanical strain exerted on certain crystals. Alternatively, ‘piezoresistivity’ describes the change of electrical resistance when mechanical stress is applied.
Materials with these qualities display remarkable attributes. What follows is an examination of piezoelectric and piezoresistive qualities in greater depth, and a discussion of how piezo-properties are influencing the future of electronic materials.
Piezo-powered electronics
The piezoelectric effect is observed in both naturally-occurring and man-made materials. As these materials produce an electric current when placed under mechanical stress, they are well-suited to numerous applications – from industrial, such as sonar and pressure sensors, to defense, including military sensors and course-changing bullets, to consumer electronics.
Piezoelectric actuators are one example of a mechanism that uses this phenomenon. These systems convert mechanical stress, (such as a voltage or force), into a desired controlling motion, making them ideal for consumer electronics. For example, piezoelectric speakers are deployed in gadgets that need to produce sound from a small device, such as mobile phones and musical greeting cards. Piezoelectric transducers, meanwhile, are central to clinical applications. When an electrical pulse is applied across a piezoelectric material, the material changes dimension, causing vibrations. This conversion also works in reverse, and vibrations can be converted into electrical signals. These mechanisms are commonly used in ultrasound imaging, and increasingly in ultrasonic surgery -– a technique that requires high levels of accuracy to result in improved recovery times and minimal damage to surrounding tissue.
With piezoelectric mechanisms being increasingly used in modern technology, there is a greater demand for researchers to enhance their understanding of piezoelectric materials and how they can support emerging technologies.
Piezoresistive performance in sensors
Piezoresistive materials display a change in electrical resistance when strain is applied, and are mostly used for measuring mechanical strain, such as deformations of parts in buildings. However, wearable sensors that can monitor small body movements are emerging as another potential application.
To deploy piezoresistive materials across wearable sensor technology, devices must be cost-effective and suitable for mass production. Researchers at the University of Oulu and the VTT Technical Research Centre of Finland have used roll-to-roll printing methods to develop sensor devices, by printing electrodes and connecting inkjet-deposited piezoresistive carbon nanotube micropatterns (SWCNT) alongside polydimethylsiloxane (PDMS) substrate. The sensor’s functionality was then tested against variables to understand how it will perform in real-world conditions.
A mechanical testing stage was used to exert stress, strain, and deformation, and repetitive cyclical testing allowed researchers to examine how the sensor responded to repetitive movement. A temperature-controlled stage was also used to examine the resistance of the SWCNT pattern in relation to temperature.
It was found that the sensor network became less resistant alongside higher temperatures, as electron movement is dependent on temperature. Increased humidity also increased resistance, as the water on the sensor decreased conductivity. However, temperature and humidity have a smaller influence on the device than the piezoresistivity, meaning sensors could be calibrated to remove these influences.
The team also investigated optimal sensor printing patterns. Traditionally, straight line patterns are used for sensors that require higher sensitivity, while zig-zag patterns can be used to improve durability and reduce sensitivity to strain. Researchers tested both SWCNT micropatterns, determining that pattern design has a large impact on response characteristic of piezoresistive materials.
The sensor was also mounted onto skin, and monitored radial artery cardiac pulses, finger flexing, and chest movement during breathing, at the level of sensitivity required.
These experiments showed that the developed stretchable printed material and structure demonstrated high sensitivity to tensile strain, pressure, and bending deformation, making it ideal for use as a low-cost, versatile, and easy-to-manufacture wearable sensor. Results suggest that this method of inkjet deposition of nanomaterials can also be used with other printing technology, holding promise for other applications.
What’s next for piezoelectric and piezoresistive materials?
There is no shortage of applications for piezoelectric and piezoresistive materials, from consumer goods, such as phones, to clinical applications, such as ultrasound scanning and wearable sensors. Materials that possess piezoelectric and piezoresistive qualities are extremely versatile, and as researchers explore new ways to use these materials, piezoelectric and piezoresistive materials could be expected to earn a firm place in the development of many new technologies.
Clara Ko is director of sales and marketing at Linkam Scientific Instruments.