When sensors become substance
Pressure sensors today might seem pretty finite devices, with tightly defined performance ratings, size, weight and cost per unit. However, the future of sensors is beginning to look dramatically different to this traditional, regimented approach.
From aerogels, thin film pressure sensors to graphene, carbon nanotube technology, and even light itself, deep integration of sensors and meshing of technologies is likely to deliver startling results in the very imminent future.
Part of the stimulus for these developments is the explosion in wearable technology, which has opened up a mass-market for wearable sensors of all kinds. While the consumer-facing segment of this market continues to expand, researchers are also discovering a host of industrial benefits to these new technologies too, offering a wide range of sizeable market opportunities.
Carbon nanotubes take the stage
Researchers are currently testing a new range of fabric coatings based on carbon nanotubes that could function as enormously flexible pressure sensors - both in terms of their sensitivity rating, but also in literal terms.
As well as creating a whole new class of sensors, the innovation could result in the launch of truly ‘smart’ textiles, an opportunity that extends from consumer goods right through to industrial sensors. In addition, the ability to create flexible sensors that could be worn on or next to the skin, whether as standalone skin patch devices or as part of a wearable device offers significant opportunities, with one analyst report predicting that this niche market alone will be worth more than $10bn per year by 2023, and approaching $15bn per year by 2028.
The team of engineers from the University of Delaware has been successfully creating flexible carbon nanotube composite coatings on a wide range of fibres, including cotton, nylon and wool. As well as being extremely flexible, the coatings can also measure an incredibly wide range of pressure, from fingertip pressure through to being run over.
The technique creates a nerve-like electrically conductive nanocomposite coating on fibre materials by electrophoretic deposition (EPD) of polyethyleneimine functionalized carbon nanotubes. The carbon nanotubes provide the sensing capability by creating measurable electrical changes in the fabric when it is squeezed. The coatings are just 250 to 750 nanometers thick, and very light, especially in contrast to existing methods of creating ‘smart’ fabrics or wearable sensors, which often involve weaving metal strands into the fabric, which usually adds stiffness, or otherwise compromises the tactility of the original.
Aerogel innovation
That’s not the only exciting sensor research waiting in the wings however. A Swedish research team from Linköping University managed recently to create a thermoelectric aerogel, which can simultaneously sense pressure and temperature. The dual-parameter PNG aerogel sensor also decouples the two signals, so that both parameters can be measured independently.
Aerogels are increasingly used in technical clothing that is designed for extreme environmental performance, as aerogels can pack astonishing levels of insulation into a very compact space, making them ideal for winter sports and demanding expeditions. Additionally aerogel composition can be precisely modified to suit the use case, so even the most esoteric environmental performance requirements can be met, most importantly at a scalable price point. Adding the inherent ability to sense pressure and temperature potentially cuts manufacturing complexity and cost in applications that required separate sensors before, but also offers the possibility of new applications where multiple stimuli tracking might be a potent value add.
Silicone trips the light fantastic
Another promising area is the use of light to measure pressure, in one example by analyzing changes in the amount of light traveling through microscopic tunnels embedded in polydimethylsiloxane, a common type of silicone. The tunnels - known as a photonic tunnel-junction arrays - act as a sensor when the flow of light through them is measured - pressure interrupts the flow, creating a measurable effect.
The silicone is much more robust in use than comparable substances, but is also non-toxic and extremely malleable, so can conform to almost any shape required. Traditional pressure sensors, such as piezoresistive sensors, for example, often used in applications such as accelerometers and air-pressure sensors, can suffer from electromagnetic interference from power sources and nearby speakers, etc, and also contain metal components, which can be subject to corrosion.
The use of silicone mitigates many of these issues, as well as adding additional benefits - sensor is particularly sensitive, and can be sized as needed, covering almost any surface area. The researchers envisage the technology could be placed on display panels to create touch screens, or wrapped around robots to create a tactile ‘skin’.
Still space for piezoelectric
There are certainly many other promising approaches to this particular tactility problem, with another team recently publishing a paper that takes a more traditional approach, proposing a piezoelectric-based sensor that measures surface textures with extremely high accuracy. Piezoelectric-based pressure sensors are particularly common in industrial applications, especially dynamic environments, as they are low cost, almost immune to overload, solid-state design that doesn’t deteriorate with age, and also require little space, so the mass load they add is minimal.
The team claim to have achieved high accuracy by detecting signals from both touch and sliding - as a human does - but also deploys multiple receptors. This meshing of inputs means that sliding speed can be calculated using the time interval between two receptor signals and the distance between them, as opposed to existing systems that usually employ a single receptor, or rely on an additional external speedometer.
The team from Korea's Daegu Gyeongbuk Institute of Science and Technology (DGIST) teamed up with colleagues from ASML Korea, Dongguk University-Seoul, Sungkyunkwan University and the University of Oxford, and added soft material to their piezoelectric sensor to measure depth, thus sensing in three dimensions. Although the 3D-sensing aspect is still under development, the researchers believe that the sensor could be incorporated into devices including robots and smart phones, so that they can automatically detect surface textures.
Thin film benefits
Elsewhere in Korea, researchers from the Korea Advanced Institute of Science and Technology (KAIST) have used much-vaunted thin film technology to create flexible yet industrially viable pressure sensors. Thin film technology has been used extensively for temperature sensors, and the market for thin film pressure sensors is beginning to mature. One analyst firm (Transparency Market Research) predicts that the global thin film sensor market is expected to reach US$3,093.3 Mn by 2026, expanding at a CAGR of 3.1% from 2018 to 2026, in part driven by a rise in high-value pressure sensor demand.
The argument for thin film sensors - in this case hierarchical nanocomposite (HNC) film - is that they are highly scalable in size, very robust in challenging environments, and perform well in comparison to conventional film sensors. However, one of the technical challenges to overcome has been the gradual degradation of flexible pressure sensors by mechanical stress and deformation, especially in industrial-grade sensing applications. However, the Korean team believe they have solved the issue by developing non-air gap sensors, based on a transparent nanocomposite insulator containing metal nanoparticles and a nanograting substrate which can increase transparency as well as sensitivity by concentrating pressure.
The result is a transparent, highly sensitive, flexible force touch sensor that is mechanically stable against repetitive pressure - according to the researchers. The tests included bending to the radius of a ballpoint pen, as well as successfully demonstrating a wearable pulse-monitoring device in real-time.
Fabrics of the future
While some of these potential applications may seem slightly esoteric, the fact remains that pressure sensors will increasingly be - quite literally - part of the fabric of everyday life. Coupled with the exponential rise in connected IoT (internet of things) devices, that are not only connected to online networks, but can also interrogate and interpolate other connected sensors, the future for more integrated sensors looks bright indeed, certainly in pure numbers terms.
However, this fundamental change in the way we use, perceive and interact with everyday items will have a series of profound effects. For device manufacturers and designers there will be increasingly strict regulatory controls around permitted emissions, as well as the broadening of the challenges around battery life, durability and recyclability. But there will also be a host of ethical choices to make too, around questions concerning privacy and data control. We are already beginning to see some of these challenges arise, and in addition we can begin to appreciate some of the exciting new applications and use cases created by overlaying and intermeshing the new datasets and streams available.
Flexibility is certainly the watchword for the future!
Martin Keenan is the Technical Director at Avnet Abacus. Avnet Abacus assists and informs design engineers in the latest technological advances and provides guidance through the challenges of bringing new products to life. This includes navigating evolving market conditions, such as the current MLCC shortage, and avoiding manufacturing pitfalls before they occur.
