New
research allows each kirigami motif to bend into multiple shapes. Credit:
Northwestern University.
Paper
snowflakes, pop-up children's books and elaborate paper cards are of interest
to more than just crafters. A team of Northwestern University engineers is
using ideas taken from paper-folding practices to create a sophisticated
alternative to 3-D printing.
Kirigami
comes from the Japanese words "kiru" (to cut) and "kami"
(paper) and is a traditional form of art in which paper is precisely cut and
transformed into a 3-D object. Using thin films of material and software to
select exact geometric cuts, engineers can create a wide range of complex structures
by taking inspiration from the practice.
Research,
published in 2015, showed promise in the kirigami "pop-up"
fabrication model. In this iteration, the ribbon-like structures created by the
cuts were open shapes, with limited ability to achieve closed shapes. Other
research building on the same inspiration mainly demonstrates that kirigami can
be applied at a macroscale with simple materials like paper.
But new
research published today (Dec. 22) in the journal Advanced Materials advances
the process a step further.
Horacio
Espinosa, a mechanical engineering professor in the McCormick School of
Engineering, said his team was able to apply concepts of design and kirigami to
nanostructures. Espinosa led the research and is the James N. and Nancy J.
Farley Professor in Manufacturing and Entrepreneurship.
"By
combining nanomanufacturing, in situ microscopy experimentation, and
computational modeling, we unraveled the rich behavior of kirigami structures
and identified conditions for their use in practical applications,"
Espinosa said.
The
researchers start by creating 2-D structures using state-of-the-art methods in
semiconductor manufacturing and carefully placed "kirigami cuts" on
ultrathin films. Structural instabilities induced by residual stresses in the
films then create well-defined 3-D structures. The engineered kirigami
structures could be employed in a number of applications ranging from
microscale grippers (e.g. cell picking) to spatial light modulators to flow
control in airplane wings. These capabilities position the technique for
potential applications in biomedical devices, energy harvesting, and aerospace.
Typically,
there has been a limit to the number of shapes that can be created by a single
kirigami motif. But by using variations in the cuts, the team was able to
demonstrate film bending and twisting that result in a wider variety of
shapes—including both symmetrical and asymmetrical configurations. The
researchers demonstrated for the first time that structures at microscales,
using film thicknesses of a few tens of nanometers, can achieve unusual 3-D
shapes and present broader functionality.
For
example, electrostatic microtweezers snap close, which can be harsh on soft
samples. By contrast, kirigami-based tweezers can be engineered to precisely
control the grabbing force by tuning the amount of stretching. In this and
other applications, the ability to design cut locations and predict structural
behavior based on computer simulations takes out trial and error, saving money
and time in the process.
As their
research advances, Espinosa says his team plans to explore the large space of
kirigami designs, including array configurations, in order to achieve a larger
number of possible functionalities. Another area for future research is the
embedding of distributed actuators for kirigami deployment and control. By
looking into the technique further, the team believes kirigami can have
implications in architecture, aerospace and environmental engineering.