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advances in nanomaterials Metamaterial flat lens works at UV wavelengths Scientists working at the National Institute of Standards and Technology recently demonstrated a flat lens that bends and focuses ultraviolet light to create 3D images of objects that float in free space. Developed in collaboration with researchers from the Maryland NanoCenter at the University of Maryland, Syracuse University, and the University of British Columbia, the device is fabricated from alternating nanolayers (Credit: Lezec/NIST.) Schematic of UV metamaterial flat lens showing alternating nanolayers of silver (green) and titanium dioxide (blue). When it is illuminated with UV light, a sample object placed on the slab is projected as a 3D image in free space on the other side of the slab. of silver and titanium dioxide. The resulting metamaterial lens has a negative refractive index, enabling it to project a 3D image of any object placed on the lens. Scientists say the device is easy to build and could lead to improved photolithography, nanoscale manipulation and manufacturing, and high-resolution 3D imaging, among other applications. The metamaterial has a negative index of refraction, meaning it causes light to flow essentially backward. Such a capability does not exist in nature— naturally occurring materials, such as air or water, have positive refractive indexes—but Russian physicist Victor Veselago postulated in 1967 that materials with negative electrical permittivity and negative magnetic permeability would have a negative index of refraction. He also theorized that a material with a refractive index of –1 could be used to make a flat (as opposed to curved) lens and that such a lens would be able to project 3D images into free space. It took more than 30 years after Veselago’s prediction to develop lenses that worked at microwave, infrared, and visible wavelengths. Making lenses that work at shorter UV wavelengths requires features as small as 10 nm. The NIST researchers adapted a design proposed by a group at Holland’s FOM Institute for Atomic and Molecular Physics, producing a sandwich of alternating nanometer-thick layers of silver and titanium dioxide they say is easy to make and has a negative index of refraction regardless of the angle of incidence of incoming light. They believe using other materials combinations may make similar lenses possible for use in other parts of the electromagnetic spectrum. n Research focuses on clays to build better bones As the US population ages, more than a million Americans a year are undergoing hip or knee replacement surgery, according to the National Institutes of Health. Add to that orthopedic injuries incurred by military veterans, and diseases such as osteoporosis and arthritis, and the importance of research in ways to help the body regenerate human bone becomes clear. Scientists studying that topic are now looking to modified clay materials as crucial foundations for bone ingrowth and regeneration. One team at North Dakota State University (Fargo) has developed a 3D mesh scaffold based on degradable, biocompatible nanoclay materials. The clay improves mechanical properties of the scaffold, allowing it to bear loads while bone regenerates, according to team leader Kalpana Katti, Distinguished Professor of Civil Engineering. “The biomineralized nanoclays also impart osteogenic or bone-forming abilities to the scaffold to enable birth of bone,” Katti says in a press release. The workers report using modified, amino acid-containing nanoclays to facilitate new bone growth in bioreactors designed to simulate flow of fluid and blood in the body during bone regeneration. Another research team working at Brigham and Women's Hospital (Boston) recently reported that layered clay can transform stem cells to bone cells without additional bone-inducing factors. The group, led by Ali Khademhosseini, BWH Division of Biomedical Engineering, says in a news release that synthetic silicates it has pioneered can "direct stem cell differentiation and facilitate functional tissue formation." The materials are simple or complex salts of silicic acids that have been widely used as food additives, fillers for glasses and ceramics, and other industrial applications. “Based on the strong preliminary studies, we believe that these highly bioactive nanoplatelets may be utilized to develop devices such as injectable tissue repair matrixes, bioactive fillers, or therapeutic agents for stimulating specific cellular responses in bonerelated tissue engineering,” researcher Akhilesh Gaharwar, BWH Division of Biomedical Engineering, says in the release. n Silica nanoparticles make Teflon tougher Well-known as a nonstick surface in applications from kitchen tools to aerospace and medical components, polytetrafluoroethylene (Teflon) is getting a 12 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 6


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