UNIVERSITY PARK, Pa. — Materials can store information about their past — like a crease in a piece of paper that has been unfolded is a "memory" of being folded — that can be retrieved or read out and used for various purposes. In everyday life, combination locks must remember the turns of the dial to open, and the memory of specialized materials is used to make airplanes safer, electronics more efficient and bridges stronger and more resilient. Now, researchers at Penn State have demonstrated that ordinary adhesive tape has a specialized type of material memory capable of storing a sequence of multiple memories that can be fine-tuned to have different strengths or be erased to make way for new memories.
A paper describing the research was published recently in the New Journal of Physics .
"Many materials or systems have a property called return-point memory that allows them to remember a sequence of events," said Nathan Keim, associate professor of physics in the Penn State Eberly College of Science and the leader of the research team. "A common example is a combination lock that must remember the sequence of turns of the dial in order to open."
Return-point memory generally relies on the input that causes the deformation to be alternating, Keim explained. The dial on the combination lock is first turned clockwise to a certain number and then must be turned back past zero in a counterclockwise direction to form the next memory. The defining characteristic of return-point memory is that if at any point you reverse the steps, the system returns to its previous state.
"We were interested if there was a system that could demonstrate this ability to remember a series of events without alternating the input," Keim said. "With a combination lock, if after the first turn, you return to zero and turn clockwise again, the memory will be lost."
The researchers set out to develop a way to add memories without losing previous ones.
"We found that we could store the sequence of multiple memories with a single-directional input in ordinary adhesive tape," Keim said. "And not only that, but that the strength of the memories is tunable — meaning we can adjust how strong the memories are — and they can be erased to reset the system."
The team built an automated device that can peel back a piece of tape to a designated distance after it has been gently placed on a surface, then lay the tape back down.
"Ordinary tape is pressure sensitive," said Sebanti Chattopadhyay, postdoctoral scholar in physics and first author of the paper. "The harder you press it down, the more firmly it adheres to a surface. We found that peeling the tape partway results in a line of strong adhesion at the stopping point that remains when you lay the tape back down. You can then repeat this multiple times by peeling the tape successively shorter distances establishing multiple lines or memories."
The team's device is fitted with an instrument that measures the force required to peel the tape. Memories are retrieved by peeling the tape past the lines, which results in a spike in the force required to peel the tape at each line.
"Peeling past the lines erases them and resets the system," Chattopadhyay said. "But, we can also tune the strength of the memories, making them require different amounts of force to peel past, which means that each line could represent different information. We can even make some strong enough to persist after resetting the system."
The researchers explained that the act of peeling the tape up increases the pressure at the stopping point, creating the line.
"When we peel and hold the tape before laying it back down, the line of adhesion becomes stronger, allowing us to control the strength of the memories," Chattopadhyay said.
An important aspect of the tape's memory is that the last memory formed will always be the first one read, the researchers explained.
"This fact allows a simple type of mechanical computation," Keim said. "It's similar to a test used for working memory in neuroscience, called a one-back comparison. Subjects are presented with a series of stimuli and have to compare each one with the previous stimulus. Because the last memory formed in the tape is always the one you encounter first during peeling, we can always compare a memory to the one that directly preceded it."
Understanding these different types of material memories could lead to devices that could perform simple mechanical calculations, according to the researchers.
"There has long been an interest in developing devices that don't need electricity and don't have the same vulnerabilities as electronic computers," Keim said. "We don't expect that these devices will be made with adhesive tape, but we are driven by a desire to understand the fundamental science underlying the various types of memories that materials can form and how they might apply in future systems. As this understanding grows, we may find ways to use it that we can't yet imagine."
In addition to Keim and Chattopadhyay, the research team included Carys Chase-Mayoral, an undergraduate student at Dickinson College who was part of the Sustainable Physics Research Experience for Undergraduates program in the Department of Physics at Penn State. Chase-Mayoral won a poster prize for this work from the Division of Soft Matter Physics at the American Physical Society's Global Physics Summit held this March in Denver.
The Human Frontier Science Program funded the research, and Chase-Mayoral was supported by the U.S. National Science Foundation.
