Researchers at Virginia Tech and elsewhere have increased the adhesive bond of ordinary tape by 60 times while keeping it easily removeable using a method adapted from kirigami, the ancient Japanese art of paper cutting. This seemingly paradoxical combination of properties could dramatically change applications in robotic grasping, wearables for health monitoring and manufacturing for assembly and recycling.
Normally, when tapes are peeled off, they separate in a straight line along the length of the strip until the tape is completely removed. A team led by Michael Bartlett, an assistant professor in the Department of Mechanical Engineering, theorised that if the separation path were controlled, then perhaps adhesives could be made both strong and removable. They then tapped into the methods of a 2,000-year-old Japanese art form of kirigami to determine how to do it.
Kirigami, through folding and cutting, can transform a flat sheet of paper into a shape or even a three-dimensional object. Children often use a basic form of this method when creating paper snowflakes.
Bartlett’s team used kirigami’s principles to engineer a series of U-shaped cuts. ‘We realised that by using cuts, we could control how an adhesive separates,’ said Bartlett. ‘An engineered cut can force the adhesive separation path to go backwards at specific locations, which we call reverse crack propagation, making the adhesive very strong. But by peeling in the opposite direction, it always goes forward, making it easy to remove. This is quite unusual behaviour, but it is very useful to make strong yet releasable adhesives.’
Bartlett’s team found that applying these cuts made the bond of the tape stronger by a factor of 60 while still allowing for easy removal by peeling in the opposite direction. The team also found that the type of tape didn’t matter. Kirigami increased the bonds of every type of tape tested, from packaging tapes to medical tapes. In all cases, strong adhesive bonds become even stronger and normally weaker adhesives increased in strength, too.
‘What really matters is the shape and size of the cut,’ said former graduate researcher Dohgyu Hwang. ‘We do not have to rely on the specific adhesive material, but as long as the cuts are made at a characteristic size, which is defined by the physics of the adhesive, we found that this enhanced adhesion in every system we tried.’
According to the researchers, the approach can be highly customised. ‘By placing the cuts in specific locations, we can activate this reverse crack propagation to tune adhesion strength at any film location and it further enables the programming of adhesive strength in two directions simultaneously in a single region of a film,’ said Bartlett. ‘We also use a rapid digital fabrication approach, so we can quickly create highly customisable adhesives with tuneable strength. This is a very exciting methodology for the development of future adhesives.’
The team applied the same type of tape to two cardboard boxes, sealing them as one might for shipping. One of the tapes had the application of kirigami cuts and the other didn’t. Researchers then dropped a brick onto the lid of each box. For the unaltered tape, the bond was broken after two drops, collapsing the lid of the box and allowing the brick to enter. However, the altered tape resisted the drops, and on the third attempt, the brick even bounced off the lid.
‘This was a very exciting result,’ said graduate researcher Chanhong Lee. ‘We can use normal packaging tapes and make them stronger where needed, but still allow for them to be released. This is very useful for packaging that’s reusable as well, potentially playing a role in sustainability.’
In another test, the team boosted the sticky strength of objects other than tapes. After applying kirigami cuts to a glove used by football players to catch the ball, Lee placed his gloved hand flatly on a sheet of plastic. Both an altered and unaltered glove were abrasive enough for the researcher to hold the plastic at a 90° angle, but the unaltered glove lost its bond after a few seconds. The altered glove continued holding the plastic fast.
‘It is common to make adhesive bonds stronger but harder to remove,’ said Bartlett. ‘It’s also common to make those bonds less strong but easy to remove. The challenge is making it both stronger and still easy to remove, and that’s what we’ve achieved.’
The research has been published in Nature Materials.