Taking a page from ancient East Asia, scientists have developed a way to increase the flexibility and stretchability of piezoelectric materials, which convert mechanical energy to electrical energy and vice versa. Such materials could be used for new applications, and to make current devices more efficient, as described in a study published in a recent issue of the Journal of Materials Research.

A team at Johns Hopkins University took inspiration from the art of kirigami, which is similar to origami but involves a careful pattern of cuts that allows a new structure to be formed. An everyday example of kirigami would be a paper snowflake, which is created by folding up a piece of paper, cutting it, and unfolding.

The researchers took thin films of polyvinylidene fluoride (PVDF), a material that is already used commercially in piezoelectric devices such as energy harvesters and sensors, says Sung Hoon Kang, an assistant professor in mechanical engineering, who authored the study along with doctoral candidate Lichen Fang and others. By slicing these with a laser cutter in three different patterns, they could tune the material’s properties in a number of desirable ways. These patterns include a unidirectional cut, a square cut, and a fractal cut, which makes the material increasingly stretchable, while retaining its piezoelectric properties. Although PVDF is flexible, it is not stretchable without this innovation.

The researchers were able to design sheets to meet desired goals, like creating sheets that could harvest energy at specific wind speeds for wind energy. By varying the size and pattern of the cuts, they were able to get spikes in energy production at low, medium, and high wind speeds in a wind tunnel.

The cut samples provided 15 times more compliance and stretchability than uncut material, and a tunable Poisson’s ratio. The team showed the resonant frequency of these kirigami-cut sheets could be controlled and reduced to a frequency that is relevant to typical environmental vibrations. “We could customize this system based on what people want,” Kang says. 

He envisions using the technique to optimize wearable piezoelectric devices, which often have folds and curves. Such materials “can follow the contour of the body much more easily,” he adds. It could also be useful for fashioning materials used to power and recharge small devices. This could be particularly important when there are design constraints such as space and weight. Prior to now, for example, to harvest energy under low wind speed conditions, one would need to add additional mass to piezoelectric materials.

These sheets are a type of architectured material, the study of which is rapidly emerging and growing. Architectured materials are “characterized by specific and periodic structural features which are larger than what is typically considered a microstructural length scale, such as a grain size, but smaller than the size of the final component made of the architectured material,” as the International Union of Technical and Applied Mechanics explains.

“The proposed method is simple but powerful, which could find broad applications in portable self-powered devices and wearable sensors by harvesting energy from the ambient environment such as wind and vibration,” says Jie Yin, an assistant professor in the department of mechanical engineering at Temple University.

To make the devices, the researchers obtained sheets of PVDF and cut 0.2 mm holes in them using a laser, in patterns drawn using a computer program (SolidWorks). They conducted a laser intensity study to ensure the cut made it all the way through but was not too wide, and noted that the cuts did not cause a short-circuit between the top and bottom, by measuring the resistance between two electrodes.

Kirigami allows them to make more with less. “If we add these cuts, we can utilize previously unavailable energy,” Kang says.

Read the abstract in the Journal of Materials Research.