Researching MEMS sensors – the modern Lilliputians - at the University of Cambridge’s Department of Engineering
Malar Chellasivalingam, a PhD student in the University of Cambridge’s Department of Engineering, writes for the Cambridge Independent on her work investigating this tiny technology.
“Time’s up, everyone!” I heard my school principal say in the exam hall.
We’d been writing an essay about Gulliver and his voyage to Lilliput for the English test.
I was so deep into it that I kept writing. Other students were looking at me. I was entranced by how a shipwreck had pushed Gulliver ashore on an island and how he became a prisoner, tied up with ropes by the tiny Lilliputian inhabitants – and how he found himself unable to move or open his eyes.
If you have read Gulliver’s Travels, you will know the Lilliputians were able to walk on top of him in great numbers, while he was tied to the ground.
Little did I know then that I would later do scientific research about the Lilliputians. Today, I research MEMS sensors. People often ask me if it’s something related to memes they encounter on social media... they’re not. Instead, they have more in common with the Lilliputians that appeared so tiny to the huge Gulliver.
These MEMS devices are so small that they can even be inserted into our blood vessels.
The MEMS field started when the Nobel Laureate Richard Feynman challenged scientists and engineers in a conference held in the California Jet Propulsion Laboratory in 1959 by posing a question: “Why can’t we write the entire 24 volumes of the Encyclopaedia Britannica on the head of a pin?”
Feynman said he would reward $1,000 to the first person who could reduce the page of a book to 1/25,000 linear scale, readable by an electron microscope. And he offered the same amount to the first person to build a miniature motor no bigger than 1/64th-inch cube.
Not too many succeeded. But a young electrical engineer, William McLellan, created a tiny electric motor that weighed 250 micrograms and generated one-millionth of a horsepower.
Feynman paid up, but warned McLellan not to try the other challenge, as he had since got married and bought a house, so did not intend to make good on it. That is probably where the foundation for the world of MEMS started.
MEMS stands for Micro-Electro-Mechanical Systems, which consist of micro-sized moving parts that we can only see in a microscope. These tiny moving mechanical parts, responsive to electrical signals, are significantly different from other existing technologies. Today, scientists and engineers are trying to use MEMS to solve global issues.
In our top picture, you see how a MEMS sensor with micrometres dimensions looks under a microscope.
Here, you see how 32 such MEMS sensors can occupy a chip that measures around one centimetre on its side. We can make many chips that contain 32 such MEMS sensors at a time using fabrication technologies. This enables high-volume, cost-effective manufacturing of MEMS sensors.
My research uses MEMS to sense ultra-fine particles or nanoparticles that are less than 100 nanometres in diameter. We can find these nanoparticles in our houses, rooms, offices, etc but we can’t see them with the naked eye. We can also find these ultra-fine particles from vehicle exhausts outdoors. But why should we care about these ultra-fine particles?
The World Health Organization (WHO) says that if these ultra-fine particles exceed an annual average concentration of around 10 micrograms per metre cubed, it could be harmful to human health. The WHO highly recommends reducing the emission of these ultra-fine particles from their sources to prevent lung cancer, respiratory and heart diseases.
My PhD research focuses on detecting these ultra-fine particles from their sources so we can moderate their origins.
I started by learning the theory of MEMS sensors and designing them to make them highly sensitive to ultra-fine particles. When designing such a sensor, a MEMS designer needs to consider its size. According to physical laws, the force we need to lift an object increases based on its size. But the force that pulls the object downwards when we drop that object decreases according to size.
So a MEMS sensor designer must consider these various factors in order to design one that is efficient at detecting ultra-fine particles.
Suppose we need only a tiny amount of force or pressure to set these moving parts into motion. That translates to low energy consumption.
My research involves lab experiments where I test these designed MEMS sensors to determine their sensitivity to ultra-fine particles.
The experimental procedure encompasses targeting laboratory-generated ultra-fine particles onto the tiny surface of the MEMS sensors.
In this picture shown, you see a small black zig-zag opening or cavity, within which the MEMS sensor is placed. In this experiment, I was trying to direct the laboratory-generated silver nanoparticles onto that tiny opening to reach the surface of the MEMS sensors.
During this experiment, the generated nanoparticles deviated towards the periphery of the tiny opening. This challenge can be overcome by designing a suitable interface as demonstrated in subsequent experiments.
In that event, the electrical signals obtained from these sensors will provide an estimate of the accumulated mass of the nanoparticles on the sensor surface.
My ongoing work with these MEMS sensors covers different nanoparticles and determines the sensor’s accuracy in detecting these nanoparticles. If I can complete this work, I will have designed a MEMS chip sensor that can efficiently and accurately detect ultra-fine particles that are harmful to humans.
One of my recent experimental results has proved that the designed MEMS sensors can not only detect the ultra-fine particles, but they can also measure the weight of a single virus. That’s so exciting. It is great to think that I could contribute a small amount to solving some of the most significant issues that we face today – such as Covid-19 – through my research.
My work is ongoing – stay tuned for more results.
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