An optical signal, represented by the red arrow, comes into contact with the metamaterial and interprets the ultrasound waves, resulting in an altered optical signal that is processed to produce a high-quality image. Image Credit: Texas A&M University.

Ultrasound technology could soon experience a significant upgrade that would enable it to produce high-quality, high-resolution images thanks to the development of a new key material by a team of researchers that includes a professor in Texas A&M University’s Department of Biomedical Engineering. The material, which converts ultrasound waves into optical signals that can be used to produce an image, is the result of a collaborative effort by Texas A&M Professor Vladislav Yakovlev and researchers from King’s College London, The Queen’s University of Belfast and the University of Massachusetts Lowell. The engineered material, known as a “metamaterial,” offers significant advantages over conventional ultrasound technology, which generates images by converting ultrasound waves into electrical signals, Yakovlev explains. The material, he notes, consists of golden nanorods embedded in a polymer known as polypyrolle. An optical signal is sent into this material where it interacts with and is altered by incoming ultrasound waves before passing through the material. A detection device would then read the altered optical signal, analyzing the changes in its optical properties to process a higher resolution image.

Illustration: Rod-shaped chemotherapy drug nanoparticles bind more efficiently to receptors on cancer cells. Image Source: UC Santa Barbara, Credit: Peter Allen

Bioengineering researchers at University of California, Santa Barbara have found that changing the shape of chemotherapy drug nanoparticles from spherical to rod-shaped made them up to 10,000 times more effective at specifically targeting and delivering anti-cancer drugs to breast cancer cells. Their findings could have a game-changing impact on the effectiveness of anti-cancer therapies and reducing the side effects of chemotherapy, according to the researchers. “Conventional anti-cancer drugs accumulate in the liver, lungs and spleen instead of the cancer cell site due to inefficient interactions with the cancer cell membrane,” explained Samir Mitragotri , professor of chemical engineering and Director of the Center for BioEngineering at UCSB. “We have found our strategy greatly enhances the specificity of anti-cancer drugs to cancer cells.”  To engineer these high-specificity drugs, scientists formed rod-shaped nanoparticles from a chemotherapeutic drug, camptothecin, and coated them with an antibody called trastuzumab that is selective for certain types of cancer cells, including breast cancer. The antibody-coated camptothecin nanorods were 10,000-fold more effective than tratsuzumab alone and 10-fold more effective than camptothecin alone at inhibiting breast cancer cell growth.

New work by theorists at Rice and Tsinghua universities shows defects in polycrystalline forms of graphene will sap its strength. The illustration from a simulation at left shows a junction of grain boundaries where three domains of graphene meet with a strained bond in the center. At right, the calculated stress buildup at the tip of a finite-length grain boundary. (Credit: Zhiping Xu/Tsinghua University)

Graphene, the single-atom-thick form of carbon, has become famous for its extraordinary strength. But less-than-perfect sheets of the material show unexpected weakness, according to researchers at Rice University in the U.S. and Tsinghua University in China. The kryptonite to this Superman of materials is in the form of a seven-atom ring that inevitably occurs at the junctions of grain boundaries in graphene, where the regular array of hexagonal units is interrupted. At these points, under tension, polycrystalline graphene has about half the strength of pristine samples of the material. Calculations by the Rice team of theoretical physicist Boris Yakobson and his colleagues in China could be important to materials scientists using graphene in applications where its intrinsic strength is a key feature, like composite materials and stretchable or flexible electronics. Graphene sheets grown in a lab, often via chemical vapor deposition, are almost never perfect arrays of hexagons, Yakobson said. Domains of graphene that start to grow on a substrate are not necessarily lined up with each other, and when these islands merge, they look like quilts, with patterns going in every direction.

Sometimes the best discoveries come by accident. A team of researchers at Washington University in St. Louis, headed by Srikanth Singamaneni, PhD, assistant professor of mechanical engineering & materials science, unexpectedly found the mechanism by which tiny single molecules spontaneously grow into centimeter-long microtubes by leaving a dish for a different experiment in the refrigerator. Once Singamaneni and his research team, including Abdennour Abbas, PhD, a former postdoctoral researcher at Washington University, Andrew Brimer, a senior undergraduate majoring in mechanical engineering, and Limei Tian, a fourth-year graduate student, saw that these molecules had become microtubes, they set out to find out how.  To do so, they spent about six months investigating the process at various length scales (nano to micro) using various microscopy and spectroscopy techniques.  “What we showed was that we can actually watch the self-assembly of small molecules across multiple length scales, and for the first time, stitched these length scales to show the complete picture,” Singamaneni says. “This hierarchical self-organization of molecular building blocks is unprecedented since it is initiated from a single molecular crystal and is driven by vesiclular dynamics in water.” Self-assembly, a process in which a disordered collection of components arrange themselves into an ordered structure, is of growing interest as a new paradigm in creating micro- and nanoscale structures and functional systems and subsystems. This novel approach of making nano- and microstructures and devices is expected to have numerous applications in electronics, optics and biomedical applications.

Video Caption: A team of researchers at Washington University in St. Louis, headed by Srikanth Singamaneni, PhD, assistant professor of mechanical engineering & materials science, unexpectedly found the mechanism by which tiny single molecules spontaneously grow into centimeter-long microtubes by leaving a dish for a different experiment in the refrigerator.

This scanning electron microscope (SEM) image shows a nanobeam probe, including a large part of the handle tip, inserted in a typical cell. (Image Credit: Stanford University)

If engineers at Stanford University have their way, biological research may soon be transformed by a new class of light-emitting probes small enough to be injected into individual cells without harm to the host. Welcome to biophotonics, a discipline at the confluence of engineering, biology and medicine in which light-based devices – lasers and light-emitting diodes (LEDs) – are opening up new avenues in the study and influence of living cells.  The team’s work is the first study to demonstrate that tiny, sophisticated devices known as light resonators can be inserted inside cells without damaging the cell. Even with a resonator embedded inside, a cell is able to function, migrate and reproduce as normal. The researchers call their device a “nanobeam,” because it resembles a steel I-beam with a series of round holes etched through the center. This beam, however, is not massive, but measure only a few microns in length and just a few hundred nanometers in width and thickness. It looks a bit like a piece from an erector set of old. The holes through the beam act like a nanoscale hall of mirrors, focusing and amplifying light at the center of the beam in what are known as photonic cavities. These are the building blocks for nanoscale lasers and LEDs. “Devices like the photonic cavities we have built are quite possibly the most diverse and customizable ingredients in photonics,” said the paper’s senior author, Jelena Vuckovic, a professor of electrical engineering. “Applications span from fundamental physics to nanolasers and biosensors that could have profound impact on biological research.” At the cellular level, a nanobeam acts like a needle able to penetrate cell walls without injury. Once inserted, the beam emits light, yielding a remarkable array of research applications and implications. While other groups have shown that it is possible to insert simple nanotubes and electrical nanowires into cells, nobody had yet realized such complicated optical components inside biological cells.

Computer simulations show that when a light particle (blue wave on left) hits a crystal of a high-pressure form of silicon, it releases two electron-hole pairs (red circles/green rings), which generate electric current. (Stefan Wippermann/UC Davis photo)

Using an exotic form of silicon could substantially improve the efficiency of solar cells, according to computer simulations by researchers at the University of California, Davis, and in Hungary. Solar cells are based on the photoelectric effect: a photon, or particle of light, hits a silicon crystal and generates a negatively charged electron and a positively charged hole. Collecting those electron-hole pairs generates electric current. “Conventional solar cells generate one electron-hole pair per incoming photon, and have a theoretical maximum efficiency of 33 percent. One exciting new route to improved efficiency is to generate more than one electron-hole pair per photon,” said Giulia Galli, professor of chemistry at UC Davis. “This approach is capable of increasing the maximum efficiency to 42 percent, beyond any solar cell available today, which would be a pretty big deal,” said Stefan Wippermann, a postdoctoral researcher at UC Davis. “In fact, there is reason to believe that if parabolic mirrors are used to focus the sunlight on such a new-paradigm solar cell, its efficiency could reach as high as 70 percent,” Wippermann said. Galli said that nanoparticles have a size of nanometers, typically just a few atoms across. Because of their small size, many of their properties are different from bulk materials. In particular, the probability of generating more than one electron-hole pair is much enhanced, driven by an effect called “quantum confinement.” Experiments to explore this paradigm are being pursued by researchers at the Los Alamos National Laboratory, the National Renewable Energy Laboratory in Golden, Colo., as well as at UC Davis. The researchers simulated the behavior of a structure of silicon called silicon BC8, which is formed under high pressure but is stable at normal pressures, much as diamond is a form of carbon formed under high pressure but stable at normal pressures.

An up-close look at the “hyperbolic metamaterial waveguide,” which catches and ultimately absorbs wavelengths (or color) in a vertical direction. (Image credit: University of Buffalo)

University at Buffalo engineers have created a more efficient way to catch rainbows, an advancement in photonics that could lead to technological breakthroughs in solar energy, stealth technology and other areas of research. Qiaoqiang Gan, PhD, an assistant professor of electrical engineering at UB, and a team of graduate students developed a “hyperbolic metamaterial waveguide,” which is essentially an advanced microchip made of alternate ultra-thin films of metal and semiconductors and/or insulators. The waveguide halts and ultimately absorbs each frequency of light, at slightly different places in a vertical direction (see the figure to the right), to catch a “rainbow” of wavelengths. Gan is a researcher within UB’s new Center of Excellence in Materials Informatics. “Electromagnetic absorbers have been studied for many years, especially for military radar systems,” Gan said. “Right now, researchers are developing compact light absorbers based on optically thick semiconductors or carbon nanotubes. However, it is still challenging to realize the perfect absorber in ultra-thin films with tunable absorption band. “We are developing ultra-thin films that will slow the light and therefore allow much more efficient absorption, which will address the long existing challenge, he added.” The research could lead to advancements in an array of fields. For example, in electronics there is a phenomenon known as crosstalk, in which a signal transmitted on one circuit or channel creates an undesired effect in another circuit or channel. The on-chip absorber could potentially prevent this.