Printed sensors are a rapidly evolving technology that offers low-cost manufacturing, flexible film form factors, and large-area sensing, making them suitable for the Internet of Things (IoT), Industry 4. 0. Emerging applications such as continuous health monitoring. Printed and flexible sensors make up the largest market for printed electronics outside of displays.
According to IDTechEx, a well-known research company in the United Kingdom, the market size of fully printed sensors will grow to $4.9 billion by 2032. Although the largest glucose test paper on the market is gradually being replaced by continuous glucose monitoring (CGM) schemes, the rise of many new applications and technologies will continue to drive the continued growth of the printed and flexible sensor market.
Printed sensors cover a wide range of technologies and applications, from image sensors to wearable electrodes. Each type of sensor seeks to offer a value proposition that differs from existing technologies, enabling product manufacturing with printing. As they move towards widespread adoption, they all face different challenges in terms of technology and commercialization. This article allows us to focus on piezoresistive and piezoelectric sensors to see how far their technology has developed and what bottlenecks have been encountered in terms of marketing.
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The challenges of printed piezoresistive sensors
Printed piezoresistive force transducers have a history of applications and are now widely used in automotive occupancy sensing, musical instruments, industrial equipment, and some medical equipment. While these markets have been commoditized, these industries are still innovating to achieve new, differentiated, higher-value applications.
For example, a 3D touch panel, which can measure the force applied, can recognize more complex HMI gestures than existing capacitive touch panels. Suppliers will continue to target applications such as smartphones, computer games and automotive interiors.
The challenge with differentiated piezoresistive sensors is that many applications do not require complex functions such as 3D touch or proximity sensing. In addition, revenue can fluctuate significantly with various product cycles, requiring a very aggressive application of the development layout. Relatively low technical complexity may also mean lower barriers to entry and barriers to value. This allows some vendors to move higher up the value chain and offer fully integrated solutions.
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New applications for printed piezoelectric sensors
Piezoelectric sensors are different from piezoresistive sensors, when they are deformed by external forces in a certain direction, polarization will occur inside, and positive and negative opposite charges will appear on its two opposite surfaces. Like piezoresistive sensors, piezoelectric sensors can also be used for force sensing, but they are more expensive to manufacture and less simple to integrate. Therefore, manufacturers are mainly targeting applications that take advantage of their unique capabilities, especially sensitivity to high-frequency vibrations.
One application of piezoelectric sensors is condition monitoring, with the aim of enabling preventive maintenance. By connecting an inconspicuous thin-film piezoelectric sensor to an industrial device, vibration can be detected and worn parts are algorithmically identified based on changes in amplitude and frequency. A similar approach can be applied to large structures, such as bridges, tunnels, etc., to provide early warning of damage and potential collapse. Another particularly innovative application was developed by France's CEA-LITEN, which utilizes printed piezoelectric sensors for battery monitoring. Printed thin-film piezo actuators generate high-frequency ultrasonic waves that propagate through the battery and then detect these signals by a piezoelectric sensor with a very similar structure. By monitoring the propagation time and frequency correlation of the waves, the sound density can be monitored in real time, thus monitoring the battery condition.
The difficulty in commercializing printed piezoelectric sensors is that their performance is between two simple, proven technologies – affordable piezoresistive pressure sensors and sensitive rigid ceramic piezoelectric sensors. Despite extensive R&D, printed piezoelectric sensors are still struggling for market penetration so far.
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The difference between the two sensors
Because they are both printed and flexible, piezoelectric and piezoresistive sensors have many advantages: they are lightweight, cost-effective, easy to produce over large areas, and easy to spatially resolve pressure sensing. In addition, their thin-film form allows the sensor to be installed in locations where rigid material sensors are difficult to install.
In contrast, piezoresistive sensors are relatively simple to manufacture and low cost to use
material, and relatively easy to calibrate. Piezoelectric sensors rely on piezoelectric polymers, form crystal regions with dipoles as they solidify, and additional manufacturing steps, combined with a more challenging calibration process, mean that piezoresistive sensors are still by far the more favored choice among printed pressure sensors.
However, despite these drawbacks, printed piezoelectric sensors have some distinct advantages – including the ability to detect high-frequency vibrations. Because piezoelectric sensors have small shape changes because they do not require compression elastomers compared to piezoresistive materials, the power consumption requirements are extremely low.
Despite the challenges, more drivers are driving the adoption of multiple types of printed and flexible sensors, most importantly the increasing IoT and Industry 4. 0 Deployments require a broad network of low-cost discreet sensors that require wireless connectivity. In addition, the thin-film profile and conformality of printed and flexible sensors enable integration into smaller devices, giving designers greater freedom to differentiate and potentially new use cases.
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Source: Intelligent Manufacturing Vertical and Horizontal 2022.02
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