Author: University of Wisconsin-Madison April 7, 2021

The mother-of-pearl or mother-of-pearl found in red abalone and other mollusks has strong resistance to fracture. This is one reason why researchers are interested in natural materials, and why scientists at the University of Wisconsin-Madison wanted to find a way to measure its thickness without damaging the nacre. Photo Credit: Courtesy of Kats Lab / UW-Madison

Most people know about mother-of-pearl, which is a rainbow-colored biomineral, also called nacre, which comes from buttons, jewelry, musical instrument inlays, and other ornaments. For decades, scientists have also admired and marveled at the nacre, not only because of its beauty and optical properties, but also because of its extraordinary toughness.

"It is one of the most studied natural biological minerals," said Pupa Gilbert, a professor of physics at the University of Wisconsin-Madison, who has studied nacre for more than a decade. "It may not look much-just a shiny decorative material. But its fracture resistance is 3,000 times that of aragonite, which is a mineral made from it. It arouses the interest of materials scientists. Because making the material better than the sum of its parts is very attractive."

Popa Gilbert. Image source: University of Washington at Madison

Now, a new lossless optical technology will unlock more knowledge about the nacre, and in the process may lead to a new understanding of climate history. Gilbert, Mikhail Kats, professor of electrical engineering at the University of Wisconsin-Madison, their students and collaborators today described this technique called hyperspectral interference tomography in the Proceedings of the National Academy of Sciences.

Gilbert already understood how the nacre formed, arranged, resisted fracture, and how its layered structure recorded the temperature at which it was formed. The layered structure of this nacre reflects light and produces different colors according to the thickness of the layer. This has led to interest in finding a way to assess the thickness of the nacre, which does not involve destroying the mollusk shell where the nacre is deposited.

To meet this challenge, Gilbert turned to Kats and graduate student Jad Salman, experts in the study of optical phenomena.

For this project, Salman prepared 22 fresh red abalone shells for optical analysis. But obtaining the spectrum of the nacre is much more difficult than it seems.

"If you want to detect such a curved shell, it is difficult to get a good spectrum with a traditional spectrometer," Salman said.

This is why the team turned to a newer technique, hyperspectral photography, to image the entire spectrum of shells. In the early days, before purchasing their own hyperspectral camera, they imaged the shells at the industry partner Middleton Spectral Vision.

Researchers at the University of Wisconsin-Madison use a hyperspectral camera like this to record the spectrum of the nacre in the curved abalone shell, a method that cannot be achieved with traditional spectrometers. Image source: Courtesy of Kats Lab / UW-Madison.

"This is an imaging spectrometer, where each pixel in the image provides a complete spectrum," Salman said. "When we use the camera in the setup, we can easily extract reliable spectral data from the large, uneven surface of the cannonball in one go."

In addition to red abalone, the team also photographed the nacre of another species, namely the abalone shell from New Zealand, also known as rainbow abalone. Then, Salman used the sophisticated modeling software he developed to determine the thickness of the nacre on a pixel-by-pixel basis using hyperspectral data.

The team calls this combination of techniques hyperspectral interference tomography and expects it to be suitable for measuring other transparent layered structures found in plants, animals, geological samples, or synthetic materials.

Mikhail Kats, yes, and Jad Salman. Image source: University of Washington at Madison

For Gilbert, the new technology revealed the surprise of the red abalone; it showed for the first time that the thickness of the nacre becomes thinner as the mollusk ages. Because this thickness records the temperature at which seawater is formed, the team believes that it is possible to use this technique to analyze fossil mollusk shells to understand past climates.

"This project consists of several different parts, and each part is well understood," Kats said. "The power of this research is that we have brought all these experimental and theoretical expertise, and we are able to model not only well-designed and well-performing hierarchical structures, but also messy, disorderly biological structures. Modeling. We can get useful information from it in a way that biologists or paleoclimatologists can use."

Reference: "Hyperspectral Interferometric Tomography of the Nacre" Authors: Jad Salman, Cayla A. Stifler, Alireza Shahsafi, Chang-Yu Sun, Stephen C. Weibel, Michel Frising, Bryan E. Rubio-Perez, Yuzhe Xiao, Christopher Draves , Raymond A. Wambold, Zhaoning Yu, Daniel C. Bradley, Gabor Kemeny, Pupa UPA Gilbert, and Mikhail A. Kats, April 13, 2021, Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2023623118

Other research contributors at the University of Washington at Madison include Cayla Stifler, Alireza Shahsafi, Chang-Yu Sun, Michel Frising, Bryan Rubio-Perez, Yuzhe Xiao, Raymond Wambold, Zhaoning Yu, and Daniel Bradley. Stephen Weibel, Christopher Draves and Gabor Kemeny of Middleton Spectral Vision also participated in the study.

This research was funded by the Air Force Office of Scientific Research (FA9550-18-1-0146), the Department of Energy (DE-FG02-07ER15899) and the National Science Foundation (DMR-1603192).

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