Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 1 2 PROCESS TECHNOLOGY SILK POLYMER MATERIALS A team of researchers at Beihang University, China, is mixing silk with polymers for potential use in biomed- ical implants. By combining silk with synthetic compounds, they are getting closer to developing new implantable compositematerials with the best prop- erties of both. Potential applications could include structures that hold bone in place after surgery or replacements for cartilage cushions in the knee. With their new combination, the team seeks to develop versatile materials for use in medicine and, potentially, other fields as well. The new research focuses on silk fabric woven from a long, single thread. Silkworms’ cocoons can contain fibers nearly 5,000 feet long, and when used whole in fabric, such a fiber can more effectively distribute mechanical stress than a series of shorter, discrete ones, the researchers say. They combine this fabric with a polymer matrix, often an epoxy, which is used in adhesives. To- gether, the fabric and the polymer form a laminate, which can then be cut into desired shapes. They say that the properties of these new materials could make them a better match for the tissues within the human body than what is being used today. For instance, they are collabo- rating with orthopedic doctors to de- vise structures resembling cages that temporarily hold vertebrae in place as they fuse after surgery, a task current- ly accomplished using primarily met- al. The silk composites’ hardness and stiffness is more compatible with bone, making them potentially more resilient yet more comfortable than metal struc- tures, they say. The scientists are also looking to supplement silk with other types of fi- bers, such as carbon. They say potential uses for these new structural materials could be extremely versatile—in the hu- man body, tennis rackets, or even on airplane engines. ev.buaa.edu.cn. HARDENING BULK METALLIC GLASS A new heat treatment technique out of Japan’s Tohoku University com- bines rapid heating and asymmetri- cal cooling processes in metallic glass. The technique enables researchers to induce a gradient of local glassy struc- ture, resulting in an apparent work hardening behavior. Although bulk metallic glasses have attracted much attention due to their high strength, wide elastic limit, and excellent thermoplasticity, a major disadvantage in terms of their mechan- ical properties has been a lack of ductil- ity due to an intrinsic work softening at room temperature. The local free volume concentra- tion related to the rejuvenation state controls the shear band angle and the maximum effective shear stress. Hence, shear band propagation is prohibited and the formation of a complete shear plane transecting the whole specimen is blocked. The generation of plastic strain is accompanied by an increase in the critical shear stress, which results in a sustainable apparent hardening. The team investigated a tailored hardening mechanism and established an experimental link between the gra- dient of the rejuvenation state and me- chanical properties in zirconium-based metallic glass. www.tohoku.ac.jp/en. Structures like this one are made by combining silk fabric with epoxy to create laminates that can be formed into shapes useful in the medical industry. Courtesy of Jiao Wen & Juan Guan. A schematic illustration of the asymmetrical rapid cooling method. Courtesy of Tohoku University. Scientists from Pohang University of Science and Technology and Korea Advanced Institute of Science and Technology, both in South Korea, developed the source technology for smart windows that change colors according to the amount of moisture, without needing electricity. The team developed a variable color filter using a metal- hydrogel-metal resonator structure and chitosan-based hydrogel, then combined it with solar cells to make a self- powering humidity sensor. www.postech.ac.kr/eng. BRIEF