With the rising population density and increasing number of cars on the roads, monitoring air quality is essential. As technology advances, sensors play a crucial role in detecting harmful substances in the air. However, most sensors are bulky, costly, and limited to industrial use. The University of Malta is working to develop smaller, more affordable, and accessible alternatives.
Among many other molecules, our air may contain volatile organic compounds (VOCs) that pose a risk to human health. These VOCs may originate from various sources – emitted by plastics (giving off a distinct smell) or by adhesives manufactured in wood and furniture. When the air’s VOC levels rise, human health may be compromised without us noticing anything. If our smart devices were equipped with smart sensors, we could receive alerts when the air quality is below an acceptable limit.
Current sensors found in industrial and commercial settings are typically based on chemo-resistive sensors, such as metal oxide sensors that detect VOCs by measuring resistance changes. These are low-cost sensors but struggle with precision and selectivity, especially in the presence of multiple gases. High-accuracy sensors also exist but are expensive, bulky, and non-portable.
Dr Ing. Barnaby Portelli is working on the PolyMEMSens Project, which aims to bridge the gap by developing small, portable sensors that maintain accuracy. The goal is to create affordable devices to integrate into smart systems, making air monitoring more accessible and widespread, especially in smart buildings and beyond.
‘The current project I’m working on combines polymer structures for gas sensing with microelectromechanical systems (MEMS) devices,’ electrical engineer Portelli tells THINK. He is a Research Support Officer for Microelectronics and Nanoelectronics at UM’s Faculty of Information and Communication Technology. The PolyMEMSens project is a collaboration between UM and Sabancı University in Turkey, funded by MCST-TÜBİTAK Fund 2023.
Vibrations at the Micro Scale
Portelli’s MEMS prototypes have a vibrating structure at their heart, covered with a special polymer. ‘This vibrating structure is operated by a piezoelectric material, where electrical signals drive its oscillations. As the polymer absorbs VOCs, the vibrating structure’s mass increases, and the oscillation frequency changes. The sensor then translates this frequency change to determine VOC concentration and eventually send alerts about air quality,’ Portelli says. ‘Think about placing a ruler at the edge of a table like in school and flicking it, letting it oscillate. That’s the same thing that happens in our sensor on a micro-scale. However, due to the small dimensions of the vibrating structure, the oscillation frequency is approximately 100000 times higher.’
When voltage is applied, the piezoelectric material generates a force that keeps the vibrating structure oscillating at a steady frequency. The term ‘piezoelectric’ combines ‘piezo’, meaning movement, and ‘electric’, referring to the voltage driving it. The small structure enables high-frequency oscillations, improving gas detection accuracy and sensitivity.
Portelli’s project focuses on tiny devices, the largest being just two millimetres wide and some as small as 0.01 millimetres. These microdevices operate at a range of frequencies, with piezoelectric material as the core component, converting electrical voltage into mechanical movement. This enables precise control and functionality within the sensors.
‘The sensor works by leveraging the unique properties of piezoelectric materials, which generate mechanical movement from electrical voltage and convert mechanical vibrations back into electrical signals. In practice, the sensor uses two piezoelectric components: one applies voltage to create movement, while the other picks up the resulting vibrations and converts them into a voltage signal. This feedback loop allows us to measure the oscillation frequency of the system,’ Portelli says.
Besides ruler-like vibrating structures, Portelli is also testing circular diaphragms [similar to a drum]. ‘Instead of a traditional cantilever [ruler-like], I’m also testing a diaphragm, which behaves like a flexible membrane, moving up and down in a wave-like motion. I’m currently comparing this design to the cantilever, manufacturing both types to evaluate their performance and determine the most effective configuration,’ Portelli says. His research should show whether cantilever or diaphragm structures work better for picking up VOCs.
Sensitive Polymer Coating
In both cases, the surface of the cantilevers and diaphragms is covered by a special polymer. The polymer absorbs specific VOCs and nothing else, so no other gases or humidity can stick to its surface. This characteristic is vital for accuracy.
While Portelli at UM is responsible for engineering the sensors, the polymer comes out of the expertise of the Turkish partner. ‘The polymer must be precisely engineered to target a specific VOC while avoiding interference from other substances like carbon dioxide or humidity, which could compromise accuracy. On my end, I’m focusing on refining the mechanical design, ensuring the sensor’s performance remains optimal and reliable. These elements create a highly specialised sensor tailored for precise gas detections,’ Portelli says.
When VOCs adhere to the polymer sensor, the oscillation shifts, enabling detection of VOC concentration. While some polymers can absorb various gases, this project targets a specific VOC. The challenge is distinguishing VOCs from moisture, as many polymers also absorb water vapour. The Turkish team tackles this by designing a polymer that selectively absorbs the target gas while reducing interference.
The Roadmap to Completion
The project began in January 2024 with a review of previous studies before moving into the design phase, where Portelli is developing various structures. Manufacturing these designs requires specialised equipment not available at UM, where the Europractice comes into play – a European collaborative initiative funded by the European Union that connects institutions and universities. This partnership allows resource sharing.
However, the timeline for manufacturing such prototypes is lengthy. ‘I sent the designs for manufacturing in September, and I expect to receive the completed devices by February. The manufacturing process is complex and carried out by only a few companies worldwide. We’re fortunate to have access to these facilities through this collaboration. While simulations are helpful, they can’t fully verify the designs, so sending them for production is necessary,’ Portelli says.
Once Portelli receives the prototypes, he plans to visit the partners in Turkey to discuss further development. After testing and verification, this phase is expected to wrap up by mid-year 2025 and conclude the project by the end of the year.
Smart Sensors Underway
‘In the meantime, we’re also establishing a testing strategy. We’re sourcing reference gases with known VOC levels to calibrate and verify the accuracy of the sensors in a controlled environment. By March 2025, when we visit Turkey for testing, we aim to finalise this process. Our Turkish partners have made significant progress, successfully developing a polymer that can absorb the targeted VOC instead of humidity, which was the issue in earlier phases. They’ve reported promising results, and we’re eager to continue refining the sensor together,’ Portelli adds.
Although the sensor is still being prototyped, its usage will be profound once it is ready. It can plug into the Internet of Things and may become widely available. In fact, less sophisticated technology is already being used. ‘Current air purifiers on the market can reduce VOCs using activated-carbon-based filters, often equipped with VOC sensors. However, these lack accuracy and reliability. We aim to develop more precise sensors that provide real-time feedback on how effectively an air purifier removes VOCs and the current VOC levels in a home,’ Portelli says. Beyond residential use, these sensors have potential applications in building management systems. They could help control ventilation based on environmental conditions, automatically increasing airflow when VOC levels rise to maintain safe indoor air quality.
The potential to integrate compact, accurate sensors into everyday devices like mobile phones could revolutionise air quality monitoring. Imagine real-time alerts, personalised recommendations, and greater control over the air we breathe right at our fingertips. From enhancing home air purification to improving property management and empowering individuals to make informed decisions, these advancements are not just timely but essential in today’s world.
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