With a light-spinning device inspired by the Japanese art of paper cutting, Ann Arbor’s University of Michigan researchers can view microscopic twists in the internal structures of plant and animal tissue without X-rays.
The approach is the first that can fully rotate terahertz radiation in real time, and it could help in medical imaging, encrypted communications, and cosmology. The researchers are most interested in using terahertz rays to identify biological tissues through the twists in their structures, called their “chirality.” A tissue’s chirality affects how much it absorbs radiation.
Terahertz radiation is the band of electromagnetic waves that have a range of uses including infrared radiation and the millimeter scanners used in airport security. It can travel about a quarter of an inch into the body, but unlike X-rays, it’s non-ionizing, which means it doesn’t free up potentially damaging electrical charges in the body.
“Our bodies have a lot of twisted structures that are close enough to the surface for terahertz photons to penetrate: vessels, ligaments, muscle fibers, molecules, and even some helical bacteria,” says Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering at U-M.
Kotov believes it may be possible to gain medically relevant information about the working behaviors of these tissues using terahertz imaging. However, as with X-rays, it is difficult to tell the difference between soft tissues in terahertz scans.
The team of U-M researchers gathered everyday biological materials to look for differences in the absorption of clockwise- or counter-clockwise-rotating radiation in the terahertz spectrum. They studied a maple leaf, a dandelion flower, pork fat, and the wing case of a beetle. While the leaf and fat showed no difference in absorption of clockwise or counter-clockwise radiation, the flower and wing case preferentially absorbed the one over the other, revealing microscopic twists in their structures.
This technique, called circular dichroism spectroscopy, was impractical in the terahertz range until now because it couldn’t be done in real time.
The new device is essentially a plastic ribbon printed with a gold herringbone pattern and sliced with staggered rows of tiny cuts. The incisions are influenced by the Japanese art of kirigami, which uses arrangements of cuts to create 3-D structures from paper. When the ribbon is stretched, the cuts open up and the slices of ribbon twist. The gold lines then guide the radiation, twisting it in turn.
The team proposes the same design could be scaled for other types of radiation as well, with larger patterns interacting with microwaves or radio waves, or shrinking the pattern down to manipulate infrared light.
In addition to imaging living tissues, terahertz circular dichroism spectroscopy could also aid the development of new medicines based on large biological molecules such as proteins and antibodies.
Wonjin Choi, a Ph.D. student in materials science and engineering and co-first author on the study, anticipates other uses for the kirigami devices. One of these uses includes flying kirigami devices to satellites so they could determine new information about the earliest stars.
The study was supported by the Defense Advanced Research Projects Agency and the Department of Defense’s Vannevar Bush Fellowship.