The Rise of the Memory Sensor
The Rise of the Memory Sensor
Engineered for agility, a robotic snake surveys submerged constructions.
Accidental discoveries in university labs have often led to groundbreaking technologies, from penicillin to graphene. At the University of California, Berkeley, an unexpected observation in vanadium dioxide (VO₂) research has now led to what researchers call a “memory sensor,” or MemSensor—a device that can both sense and retain information about environmental conditions, all without external power.
Professor Junqiao Wu began studying VO₂ nearly two decades ago for its unusual ability to undergo phase transitions—switching from an insulating to a conductive state near room temperature. Early applications included smart roof coatings designed to help buildings stay cool in the summer and warm in the winter.
While testing VO₂ for durability in wet conditions, Wu’s team made a surprising discovery. When a thin film of VO₂ was dipped into saltwater, its electrical resistance and even its color changed, signaling a phase transformation.
“Normally the color change for vanadium dioxide signifies phase transformation. In the past, people thought you could drive that by increasing temperature,” Wu explained. “Now you can do the same at room temperature, just by dipping the material into salted water.”
The finding was intriguing enough that Wu recruited doctoral student Ruihan Guo to dedicate her research to understanding it.
“I said you have one project and only one project: to get to the bottom of this effect and explain what causes this,” Wu recalled.
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The researchers coined the term MemSensor to capture the device’s dual nature. Unlike memristors, which require an applied voltage, their system uses the inherent electric field at the interface between VO₂ and the salted liquid.
The nematode C. elegans uses specialized neurons to remember salt exposure and guide its movement toward or away from environments while foraging for food. Mimicking this adaptive behavior, the new memsensor navigates a small robotic boat through varying salt gradients. Image: UC Berkeley
The MemSensor also bears resemblance to a dosimeter, which records cumulative exposure to radiation. Instead of radiation, the MemSensor can record exposure to ions in liquid such as sodium chloride. More profoundly, it can “remember” the concentrations of salt in the area even after it has been removed from the solution.
Achieving this point with the material was only the first hurdle. The researchers then had to learn how to consistently produce high-quality VO₂ films. Once they were able to fabricate pristine material, the complexity of the system posed another challenge: a solid VO₂ layer, an indium electrode, and an ion-rich liquid solution that interacted all at once.
“The system is quite complex,” Guo explained. “We spent a lot of time exploring what ions play what role and did a lot of comparative experiments to eliminate secondary effects.”
After years of experiments and revisions, their persistence paid off. The device reliably sensed salt exposure and retained that information in its material state. The challenge then became locating an application that could help advance this technology.
In speaking with his colleagues in biology, Wu learned about the nematode Caenorhabditis elegans. This millimeter-long worm is beloved by biologists because of its simple yet instructive behaviors.
One of its defining traits is chemotaxis: the ability to navigate toward or away from salt concentrations based on past experiences. This remarkable capability arises from the worm’s unique neurons called ASEL, which efficiently combine sensing with memory.
“In the past, people tried to mimic this behavior by using very complex electrical circuits,” Guo said. “But since our device can already hold memory inside the sensor, we thought we might be able to mimic chemotaxis.”
To do this, Wu and Guo built small VO₂-based “boats” that could respond to salt concentrations in solution, moving in a way that paralleled the worm’s navigation strategy.
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“Nothing here is complicated, just one thin layer of inorganic material with two elements; vanadium and oxygen,” Wu emphasized. “Yet, it can mimic some intelligent behavior of life.”
The implications extend far beyond worms. Robotics, specifically aquatic robots, could benefit from adaptive navigation systems. Environmental monitoring devices could track cumulative exposure to chemicals. Mechanical engineers might even use VO₂ to record stress or strain in structures.
“Think about tree rings. That’s a climate MemSensor,” Wu said. “The tree memorizes and records the climate of the past 100 years, saving us to record the climate in real time. That’s what we’re trying to do in materials.”
Neuromorphic computing, a process of building circuits that function more like brains, is another promising area for the MemSenor because of its ability to reflect the input of ion exposure into resistance changes.
Wu and Guo stress that this is only the beginning. They envision expanding the concept of mem-sensing beyond VO₂ and sodium chloride, exploring materials that could record exposures to light, temperature, or stress.
For now, the MemSensor is still in its infancy, proof of concept born from curiosity and persistence. The principle, however, demonstrates elegance, as materials themselves can both sense and remember.
“We found something unexpected,” Wu reflected. “In the end, we not only explained it but also showed an application that will hopefully inspire many to work with this material and this concept.”
Cassandra Kelly is a technology writer in Columbus, Ohio.
Professor Junqiao Wu began studying VO₂ nearly two decades ago for its unusual ability to undergo phase transitions—switching from an insulating to a conductive state near room temperature. Early applications included smart roof coatings designed to help buildings stay cool in the summer and warm in the winter.
While testing VO₂ for durability in wet conditions, Wu’s team made a surprising discovery. When a thin film of VO₂ was dipped into saltwater, its electrical resistance and even its color changed, signaling a phase transformation.
“Normally the color change for vanadium dioxide signifies phase transformation. In the past, people thought you could drive that by increasing temperature,” Wu explained. “Now you can do the same at room temperature, just by dipping the material into salted water.”
From serendipity to science
The finding was intriguing enough that Wu recruited doctoral student Ruihan Guo to dedicate her research to understanding it.
“I said you have one project and only one project: to get to the bottom of this effect and explain what causes this,” Wu recalled.
You Should Read: The Rise of Self-Healing Robots
The researchers coined the term MemSensor to capture the device’s dual nature. Unlike memristors, which require an applied voltage, their system uses the inherent electric field at the interface between VO₂ and the salted liquid.
The nematode C. elegans uses specialized neurons to remember salt exposure and guide its movement toward or away from environments while foraging for food. Mimicking this adaptive behavior, the new memsensor navigates a small robotic boat through varying salt gradients. Image: UC Berkeley
Achieving this point with the material was only the first hurdle. The researchers then had to learn how to consistently produce high-quality VO₂ films. Once they were able to fabricate pristine material, the complexity of the system posed another challenge: a solid VO₂ layer, an indium electrode, and an ion-rich liquid solution that interacted all at once.
“The system is quite complex,” Guo explained. “We spent a lot of time exploring what ions play what role and did a lot of comparative experiments to eliminate secondary effects.”
After years of experiments and revisions, their persistence paid off. The device reliably sensed salt exposure and retained that information in its material state. The challenge then became locating an application that could help advance this technology.
In speaking with his colleagues in biology, Wu learned about the nematode Caenorhabditis elegans. This millimeter-long worm is beloved by biologists because of its simple yet instructive behaviors.
One of its defining traits is chemotaxis: the ability to navigate toward or away from salt concentrations based on past experiences. This remarkable capability arises from the worm’s unique neurons called ASEL, which efficiently combine sensing with memory.
“In the past, people tried to mimic this behavior by using very complex electrical circuits,” Guo said. “But since our device can already hold memory inside the sensor, we thought we might be able to mimic chemotaxis.”
To do this, Wu and Guo built small VO₂-based “boats” that could respond to salt concentrations in solution, moving in a way that paralleled the worm’s navigation strategy.
Discover the Benefits of ASME Membership
“Nothing here is complicated, just one thin layer of inorganic material with two elements; vanadium and oxygen,” Wu emphasized. “Yet, it can mimic some intelligent behavior of life.”
Nature-inspired innovation
The implications extend far beyond worms. Robotics, specifically aquatic robots, could benefit from adaptive navigation systems. Environmental monitoring devices could track cumulative exposure to chemicals. Mechanical engineers might even use VO₂ to record stress or strain in structures.
“Think about tree rings. That’s a climate MemSensor,” Wu said. “The tree memorizes and records the climate of the past 100 years, saving us to record the climate in real time. That’s what we’re trying to do in materials.”
Neuromorphic computing, a process of building circuits that function more like brains, is another promising area for the MemSenor because of its ability to reflect the input of ion exposure into resistance changes.
Wu and Guo stress that this is only the beginning. They envision expanding the concept of mem-sensing beyond VO₂ and sodium chloride, exploring materials that could record exposures to light, temperature, or stress.
For now, the MemSensor is still in its infancy, proof of concept born from curiosity and persistence. The principle, however, demonstrates elegance, as materials themselves can both sense and remember.
“We found something unexpected,” Wu reflected. “In the end, we not only explained it but also showed an application that will hopefully inspire many to work with this material and this concept.”
Cassandra Kelly is a technology writer in Columbus, Ohio.
