Nanoengineered, 3D-printed swiming microrobots

Nanoengineers at the University of California, San Diego used an innovative 3D printing technology they developed to manufacture multipurpose fish-shaped microrobots -- called microfish -- that swim around efficiently in liquids, are chemically powered by hydrogen peroxide and magnetically controlled. These proof-of-concept synthetic microfish will inspire a new generation of "smart" microrobots that have diverse capabilities such as detoxification, sensing and directed drug delivery, researchers said. The technique used to fabricate the microfish provides numerous improvements over other methods traditionally employed to create microrobots with various locomotion mechanisms, such as microjet engines, microdrillers and microrockets. Most of these microrobots are incapable of performing more sophisticated tasks because they feature simple designs -- such as spherical or cylindrical structures -- and are made of homogeneous inorganic materials. In this new study, researchers demonstrated a simple way to create more complex microrobots. 3-D-Printed Microfish 3-D-printed microfish contain functional nanoparticles that enable them to be self-propelled, chemically powered and magnetically steered. The microfish are also capable of removing and sensing toxins. (Illustration: J. Warner, UC San Diego Jacobs School of Engineering) The research, led by Professors Shaochen Chen and Joseph Wang of the NanoEngineering Department at the UC San Diego, was published in the Aug. 12 issue of the journal ("3D-Printed Artificial Microfish"). By combining Chen's 3D printing technology with Wang's expertise in microrobots, the team was able to custom-build microfish that can do more than simply swim around when placed in a solution containing hydrogen peroxide. Nanoengineers were able to easily add functional nanoparticles into certain parts of the microfish bodies. They installed platinum nanoparticles in the tails, which react with hydrogen peroxide to propel the microfish forward, and magnetic iron oxide nanoparticles in the heads, which allowed them to be steered with magnets. "We have developed an entirely new method to engineer nature-inspired microscopic swimmers that have complex geometric structures and are smaller than the width of a human hair. With this method, we can easily integrate different functions inside these tiny robotic swimmers for a broad spectrum of applications," said the co-first author Wei Zhu, a nanoengineering Ph.D. student in Chen's research group at the Jacobs School of Engineering at UC San Diego. As a proof-of-concept demonstration, the researchers incorporated toxin-neutralizing nanoparticles throughout the bodies of the microfish. Specifically, the researchers mixed in polydiacetylene (PDA) nanoparticles, which capture harmful pore-forming toxins such as the ones found in bee venom. The researchers noted that the powerful swimming of the microfish in solution greatly enhanced their ability to clean up toxins. When the PDA nanoparticles bind with toxin molecules, they become fluorescent and emit red-colored light. The team was able to monitor the detoxification ability of the microfish by the intensity of their red glow. "The neat thing about this experiment is that it shows how the microfish can doubly serve as detoxification systems and as toxin sensors," said Zhu. "Another exciting possibility we could explore is to encapsulate medicines inside the microfish and use them for directed drug delivery," said Jinxing Li, the other co-first author of the study and a nanoengineering Ph.D. student in Wang's research group. How this new 3D printing technology works The new microfish fabrication method is based on a rapid, high-resolution 3D printing technology called microscale continuous optical printing (µCOP), which was developed in Chen's lab. Some of the benefits of the µCOP technology are speed, scalability, precision and flexibility. Within seconds, the researchers can print an array containing hundreds of microfish, each measuring 120 microns long and 30 microns thick. This process also does not require the use of harsh chemicals. Because the µCOP technology is digitized, the researchers could easily experiment with different designs for their microfish, including shark and manta ray shapes. "With our 3D printing technology, we are not limited to just fish shapes. We can rapidly build microrobots inspired by other biological organisms such as birds," said Zhu. The key component of the µCOP technology is a digital micromirror array device (DMD) chip, which contains approximately two million micromirrors. Each micromirror is individually controlled to project UV light in the desired pattern (in this case, a fish shape) onto a photosensitive material, which solidifies upon exposure to UV light. The microfish are built using a photosensitive material and are constructed one layer at a time, allowing each set of functional nanoparticles to be "printed" into specific parts of the fish bodies. "This method has made it easier for us to test different designs for these microrobots and to test different nanoparticles to insert new functional elements into these tiny structures. It's my personal hope to further this research to eventually develop surgical microrobots that operate safer and with more precision," said Li.
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More efficient chips based on plasmonics are a step closer

By using the tip of a scanning tunneling microscope (STM), A*STAR researchers and their collaborators have generated electromagnetic waves known as surface plasmon polaritons in a gold grating and demonstrated that the direction of travel of these waves can be controlled ("Electrically-excited surface plasmon polaritons with directionality control."). This demonstration is a step toward the development of plasmonic chips, so called because they use plasmons — collective excitations of electrons in a conductor — rather than electrons to transfer and process data. Such chips promise to be much faster and potentially more energy efficient than current electronic chips. experimental setup used to investigate the directional excitation of surface plasmon polaritons in a one-dimensional gold grating Experimental setup used to investigate the directional excitation of surface plasmon polaritons in a one-dimensional gold grating. The probe of a scanning tunneling microscope is used to excite a surface plasmon polariton and the resulting light is analyzed by an inverted optical microscope. Joel Yang and Zhaogang Dong at the A*STAR Institute of Materials Research and Engineering, together with colleagues at the A*STAR Institute of High Performance Computing and other institutes in Singapore, investigated controlling the traveling direction of plasmons in a gold grating both theoretically and experimentally. In the experiments, they moved the STM tip relative to the edge of the gold grating and observed the generated light using an inverted microscope (see image).“The STM tip acts as a point source of surface plasmons,” Yang explains. “When placed on a metal film, electrons that tunnel across the gap can excite plasmons, although inefficiently.” Yang likens the excitation of plasmons in gratings to dropping pebbles in a swimming pool with swimming lanes demarcated by floats. “What is interesting is that depending on how far we drop the pebble from the barrier for each lane, we can get waves that preferentially move away from the barrier and even across lanes. By adjusting the position just by a small amount — in our case by about 100 nanometers — we can turn on waves that propagate in the opposite direction, namely toward the barrier and beyond.” This control of direction stems from the surface plasmon polariton reflected from the grating edge interfering with the one at the STM probe. By modeling this process on a computer, the researchers found a good match with the experimental results. The result provides point sources of surface plasmon polaritons. This could prove useful for developing ways to replace wires between chips with optical connectors, which will greatly speed up chip-to-chip communication in integrated circuits based on plasmonics rather than electronics. The researchers intend to investigate the optical characteristics of the plasmon source when the electrically excited plasmons are coupled to plasmonic waveguides, opening the way to plasmonic counterparts of electronic components. “Potentially, we hope to achieve logic gates, which underpin all processing circuits, based on electrically driven plasmons,” says Dong.
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Finding 'Goldilocks' nanoparticles for catalysis

A*STAR scientists have used first-principles computer simulations to explain why small platinum nanoparticles are less effective catalysts than larger ones ("Platinum nanoparticle during electrochemical hydrogen evolution: Adsorbate distribution, active reaction species, and size effect"). platinum nanoparticle catalyst First-principles simulations reveal distribution of absorbed hydrogen atoms (cyan) on a small platinum nanoparticle catalyst during hydrogen evolution reaction. (© ACS) Platinum nanoparticles are used in the catalysis of many reactions, including the important hydrogen evolution reaction used in fuel cells and for separating water into oxygen and hydrogen. Improved effectiveness of platinum nanoparticles to catalyze this reaction had been experimentally shown with decreasing nanoparticle size until it fell below about 3 nanometers. There was no clear explanation for why catalytic activity was reduced at this scale. Teck Leong Tan and colleagues at the A*STAR Institute of High Performance Computing, and collaborators at the Ames Laboratory in the USA, performed first-principles calculations of platinum nanoparticles for the hydrogen evolution reaction. Based on these calculations, they produced a map of the intermediate compounds — in this case adsorbed hydrogen atoms — that form on the nanoparticles. They also estimated the contribution made by each catalytic active species to the overall activity. An effective catalyst must not bind to reaction intermediates too weakly because reactants will fail to bind to its surface. Too strong an adherence will cause difficulty for reaction products to detach from the catalyst surface. The binding energy of an effective catalyst should be ‘just right’, lying somewhere between these two extremes. The researchers found that edge sites on small platinum nanoparticles bind too strongly to hydrogen atoms and become inactive catalytically, but face sites continue to bind with hydrogen with an appropriate energy level and remain catalytically active. The increased ratio of edge sites to face sites as nanoparticle size reduces explains the observed fall in catalytic activity for small nanoparticles. It also suggests that the nanoparticle shape could be tailored to optimize the nanoparticle’s catalytic activity. The simulation results augur well for the potential of this technique. “Experimentalists have long been trying to visualize the structure of nanosized catalysts and the adsorbate distribution in real-time during reactions,” explains Tan. “However, this is often difficult to achieve. Our first-principles computational method provides an accurate model of the catalyst structure with adsorbate coverage and thus allows researchers to visualize what is going on in catalysts during a reaction.” The computational method can be applied to nanoscale catalysts besides platinum, and the team is keen to explore its potential to predict the performance of nanoparticles of other elements.
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