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Shantanu Chakrabartty’s laboratory has worked to create sensors that can operate with the least amount of energy. His lab has been so successful in building smaller, more efficient sensors, that they have encountered an obstacle in the form of a fundamental law of physics.
Sometimes, however, when you encounter what appears to be an impenetrable roadblock, you just have to switch to quantum physics and walk through it. This is what Chakrabartty and other researchers at Washington University’s McKelvey School of Engineering in St. Louis did.
The development of these self-powered quantum sensors by Chakrabartty’s lab, Clifford W. Murphy’s professor at Preston M. Green’s Department of Systems and Electrical Engineering, was published online Oct. 28 in the journal Nature Communications.
The roadblock that inspired this research is the threshold effect.
“Imagine there is an apple hanging from a tree,” Chakrabartty said. “You can shake the tree a little, but the apple doesn’t fall. You have to pull enough to shake the apple. “That tug is like a threshold energy.” It’s the minimum amount of energy needed to move an electron over a barrier. “If you can’t move the electron beyond the barrier, you can’t create current.
But the phenomenon of quantum mechanics that occurs in nature moves electrons through barriers all the time. The research team took advantage of this to build a self-powered device that, with a small initial energy input, can run on its own for more than a year.
Here’s how it’s built:
The device is simple and inexpensive to build. All you need are four capacitors and two transistors.
From these six parts, Chakrabartty’s team built two dynamic systems, each with two capacitors and a transistor. Capacitors hold a small initial charge, around 50 million electrons each.
They added a transducer to one of the systems and coupled it to the property they were measuring. In one application, the team measured environmental micro-movement using a piezoelectric accelerometer, a type of transducer that transforms mechanical energy (such as the movement of molecules in air) into electrical signals.
“As long as you have a transducer that can generate an electrical signal, it can self-power our sensor data logger.”
Shantanu Chakrabartty
This is what you need to know:
Quantum physics. At least some of the more unusual properties of subatomic particles, especially tunneling.
Imagine a hill, said Chakrabartty. “If you want to get to the other side, you have to physically climb the hill. Quantum tunneling is more like crossing a hill. “
The beauty of this, he said, is that when the hill has a certain shape, you get truly unique dynamic properties that could last for years.
In this case, the “hill” is actually a barrier called the Fowler-Nordheim tunnel barrier. It is placed between the plate of a capacitor and a semiconductor material; it is often less than 100 atoms.
By building the barrier in a certain way, Chakrabartty said, “you can control the flow of electrons. You can make it reasonably slow, down to one electron every minute, and still keep it reliable. ”At that rate, the dynamic system works as a time indicator – without batteries – for more than a year.
This is how it works:
To measure ambient motion, a tiny piezoelectric accelerometer was attached to the sensor. The researchers mechanically shook the accelerometer; its motion was then transformed into an electrical signal. This signal changed the shape of the barrier, which, thanks to the rules of quantum physics, changed the speed at which electrons crossed the barrier.
To make sense of what happened, the process must be read as a sort of Rube Goldberg machine in reverse.
The probability of a certain number of electrons crossing the barrier is a function of the size of the barrier. The size of the barrier is determined by the energy produced by the piezoelectric transducer, which in turn is determined by the magnitude of the acceleration – how much it shook.
By measuring the voltage of the sensor capacitors and counting how many electrons were missing, Darshit Mehta, a doctoral student in Chakrabartty’s lab and lead author of the paper, was able to determine the total acceleration energy.
Of course, to be put into practice, these highly sensitive devices would likely have to travel – on a truck, tracking ambient temperature in vaccine cold chain management, for example. Or in the blood, by monitoring glucose.
That is why each device is actually two systems, a detection system and a reference system. In the beginning, the two are almost identical, only the detection system was connected to a transducer while the reference system was not.
Both systems were designed so that electrons tunneled at the same speed, destined to exhaust their capacitors identically if there were no external forces at play.
Because the sensing system was affected by the signals received by the transducer, its electrons tunneled at different times than the reference system. After the experiments, the research team read the voltage across the sensing and reference system capacitors. They used the difference of the two voltages to find the true measurements from the transducer.
For some applications, this final result is sufficient. The next step for Chakrabartty’s team is to overcome the computational challenge of more accurately recreating what happened in the past: how exactly were electrons affected? When did an electron tunnel pass through the barrier? How long did the tunnel take?
One of the goals of Mehta’s doctoral thesis is to use multiple devices to reconstruct the past. “The information is all stored on the device, we just have to do some intelligent signal processing to solve this problem,” said Chakrabartty.
Ultimately, these sensors hold promise for everything from continually monitoring glucose levels within the human body, to possibly recording neural activity without using batteries.
“Right now, the platform is generic,” Chakrabartty said. “It just depends on what it pairs with the device. As long as you have a transducer that can generate an electrical signal, it can self-power our sensor data logger.”
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