Swarms of microscopic, magnetic, robotic beads could be scrubbing in next to the world’s top vascular surgeons—all taking aim at blocked arteries. These microrobots, which look and move like corkscrew-shaped bacteria, are being developed by mechanical engineers at Drexel University as a part of a surgical toolkit being assembled by the Daegu Gyeongbuk Institute of Science and Technology (DGIST) in South Korea. MinJun Kim, PhD, a professor in the College of Engineering and director of the Biological Actuation, Sensing & Transport Laboratory (BASTLab) at Drexel, is adding his team’s extensive work in bio-inspired microrobotics to an $18-million international research initiative from the Korea Evaluation Institute of Industrial Technologies (KEIT) set on creating a minimally invasive, microrobot-assisted procedure for dealing with blocked arteries within five years. 
Drexel's microswimmer robots (bottom) are modeled, in form and motion, after the spiral-shaped bacteria, Borrelia burgdorferi (top), that cause Lyme Disease. DGIST, a government-funded research entity in Daegu, South Korea, is the leader of the 11-institution partnership, which includes some of the top engineers and roboticists in the world. Drexel’s team, the lone representatives from the United States, is already well on its way to tailoring robotic “microswimmer” technology for clearing arteries. “Microrobotics is still a rather nascent field of study, and very much in its infancy when it comes to medical applications,” Kim said. “A project like this, because it is supported by leading institutions and has such a challenging goal, is an opportunity to push both medicine and microrobotics into a new and exciting place.” Kim’s microswimmers are chains of three or more iron oxide beads, rigidly linked together via chemical bonds and magnetic force. These chains are small enough—on the order of nanometers—that they can navigate in the bloodstream like a tiny boat. The beads are put in motion by an external magnetic field that causes each of them to rotate. Because they are linked together, their individual rotations cause the chain to twist like a corkscrew and this movement propels the microswimmer. 
Using magnetic fields (visual representation at right) generated by an electromagnetic device (left) Drexel engineers are able to control the movement of their micro-swimmer robots. By controlling the magnetic field, Kim can direct the speed and direction of the microswimmers. The magnetism involve also allows the researchers to join separate strands of microswimmers together to make longer strings, which can then be propelled with greater force. This research, which was recently reported in the ("Minimal geometric requirements for micropropulsion via magnetic rotation"), is one of the reasons Kim’s lab was chosen for the ambitious project. “Our magnetically actuated microswimmer technology is the perfect fit for this project,” Kim said. “The microswimmers are composed of inorganic biodegradable beads so they will not trigger an immune response in the body. And we can adjust their size and surface properties to accurately deal with any type of arterial occlusion.” Kim’s inspiration for using the robotic swimmers as tiny drills actually came from a malicious bacterium that wreaks havoc inside the body by doing just that—burrowing through healthy tissue. Borrelia burgdorferi, the bacteria that causes Lyme’s Disease, is classified by its spiral shape, which enables both its movement and the resultant cellular destruction. DGIST researchers are planning to harness this behavior in the microswimmmers to lead the way for a vascular probe by loosening the arterial plaque that is causing the blockage. The probe, which looks like a tiny drill, is being designed by Bradley Nelson from ETH Zurich, a pioneer in the field of microrobotic surgery. The team’s plan is to use a catheter to deliver the microswimmers and the drill directly to the blocked artery. From there, the swimmers would push their way into the blockage, then the drill would clear it completely. Once flow is restored in the artery, the microswimmer chains could disperse and be used to deliver anti-coagulant medication directly to the effected area to prevent future blockage.
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Drexel's microswimmer robots (bottom) are modeled, in form and motion, after the spiral-shaped bacteria, Borrelia burgdorferi (top), that cause Lyme Disease. DGIST, a government-funded research entity in Daegu, South Korea, is the leader of the 11-institution partnership, which includes some of the top engineers and roboticists in the world. Drexel’s team, the lone representatives from the United States, is already well on its way to tailoring robotic “microswimmer” technology for clearing arteries. “Microrobotics is still a rather nascent field of study, and very much in its infancy when it comes to medical applications,” Kim said. “A project like this, because it is supported by leading institutions and has such a challenging goal, is an opportunity to push both medicine and microrobotics into a new and exciting place.” Kim’s microswimmers are chains of three or more iron oxide beads, rigidly linked together via chemical bonds and magnetic force. These chains are small enough—on the order of nanometers—that they can navigate in the bloodstream like a tiny boat. The beads are put in motion by an external magnetic field that causes each of them to rotate. Because they are linked together, their individual rotations cause the chain to twist like a corkscrew and this movement propels the microswimmer. Using magnetic fields (visual representation at right) generated by an electromagnetic device (left) Drexel engineers are able to control the movement of their micro-swimmer robots. By controlling the magnetic field, Kim can direct the speed and direction of the microswimmers. The magnetism involve also allows the researchers to join separate strands of microswimmers together to make longer strings, which can then be propelled with greater force. This research, which was recently reported in the ("Minimal geometric requirements for micropropulsion via magnetic rotation"), is one of the reasons Kim’s lab was chosen for the ambitious project. “Our magnetically actuated microswimmer technology is the perfect fit for this project,” Kim said. “The microswimmers are composed of inorganic biodegradable beads so they will not trigger an immune response in the body. And we can adjust their size and surface properties to accurately deal with any type of arterial occlusion.” Kim’s inspiration for using the robotic swimmers as tiny drills actually came from a malicious bacterium that wreaks havoc inside the body by doing just that—burrowing through healthy tissue. Borrelia burgdorferi, the bacteria that causes Lyme’s Disease, is classified by its spiral shape, which enables both its movement and the resultant cellular destruction. DGIST researchers are planning to harness this behavior in the microswimmmers to lead the way for a vascular probe by loosening the arterial plaque that is causing the blockage. The probe, which looks like a tiny drill, is being designed by Bradley Nelson from ETH Zurich, a pioneer in the field of microrobotic surgery. The team’s plan is to use a catheter to deliver the microswimmers and the drill directly to the blocked artery. From there, the swimmers would push their way into the blockage, then the drill would clear it completely. Once flow is restored in the artery, the microswimmer chains could disperse and be used to deliver anti-coagulant medication directly to the effected area to prevent future blockage.
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This procedure could supplant the two most common methods for treating blocked arteries: stenting and angioplasty. Stenting is a way of creating a bypass for blood to flow around the block by inserting a series of tubes into the artery, while angioplasty pushes out the blockage by expanding the artery with help from an inflatable probe. “Current treatments for chronic total occlusion are only about 60 percent successful,” Kim said. “We believe that the method we are developing could be as high as 80-90 percent successful and possibly shorten recovery time.”
In this adiabatic elimination scheme, the movement of light through two outer waveguides is controlled via a "dark" middle waveguide that does not accumulate any light. (Image: Zhosia Rostomian, Berkeley Lab) Integrated electronic circuitry is approaching its limits because of heat dissipation and power consumption issues. Photonics, in which electrical signals moving through copper wires and cables are replaced by pulses of light carrying data over optical fibers, is a highly touted alternative, able to carry greater volumes of data at faster speeds, while giving off much less heat and using far less power. However, the crosstalk problem in coupled optical nanowaveguides has been a major technological roadblock. "When nanowaveguides in close proximity are coupled, the light in one waveguide impacts the other. This coupling becomes particularly severe when the separation is below the diffraction limit, placing a restriction on how close together the waveguides can be placed," Zhang says. "We have experimentally demonstrated an adiabatic elimination scheme that effectively cuts off the cross-talk between them, enabling on-demand dynamical control of the coupling between two closely packed waveguides. Our approach offers an attractive route for the control of optical information in integrated nanophotonics, and provides a new way to design densely packed, power-efficient nanoscale photonic components, such as compact modulators, ultrafast optical signal routers and interconnects." Zhang, who also holds an appointment with the Kavli Energy NanoSciences Institute (ENSI) at Berkeley, is the corresponding author of a paper describing this research in ("Adiabatic elimination based coupling control in densely packed subwavelength waveguides"). "A general approach to achieving active control in coupled waveguide systems is to exploit optical nonlinearities enabled by a strong control pulse," Zhang says. "However this approach suffers from the nonlinear absorption induced by the intense control pulse as the signal and its control propagate in the same waveguide." Zhang and his group turned to the adiabatic elimination concept, which has a proven track record in atomic physics and other research fields. The idea behind adiabatic elimination is to decompose large dynamical systems into smaller ones by using slow versus fast dynamics. "Picture three buckets side-by-side with the first being filled with water from a tap, the middle being fed from the first bucket though a hole while feeding the third bucket through another hole," says co-lead author Mrejen. "If the flow rate into the middle bucket is equal to the flow rate out of it, the second bucket will not accumulate water. This, in a basic manner, is adiabatic elimination. The middle bucket allows for some indirect control on the dynamics compared to the case in which water goes directly from the first bucket to the third bucket." Zhang and his research group apply this concept to a coupled system of optical nanowaveguides by inserting a third waveguide in the middle of the coupled pair. Only about 200 nanometers separate each of the three waveguides, a proximity that would normally generate too much cross-talk to allow for any control over the coupled system. However, the middle waveguide operates in a "dark" mode, in the sense that it doesn't seem to participate in the exchange of light between the two outer waveguides since it does not accumulate any light. "Even though the dark waveguide in the middle doesn't seem to be involved, it nonetheless influences the dynamics of the coupled system," says co-lead author Suchowski, who is now with the Tel Aviv University. "By judiciously selecting the relative geometries of the outer and intermediate waveguides, we achieve adiabatic elimination, which in turn enables us to control the movement of light through densely packed nanowaveguides. Until now, this has been almost impossible to do."
Proton permeation through flagellar motor stator complex MotA/B. Based on the model of the three-dimensional structure of MotA/B identified in this research, protons can permeate through the gate (green) of the motor by diffusion of hydronium ions (blue), which induces the formation of a water wire (red and white) that may mediate the proton transfer to the proton binding site (yellow). (Image: Yasutaka Nishihara and Akio Kitao) Bacteria such as and swim by rotating flagellar motors and filaments, which highly efficiently utilize the energy originating from the difference in ion concentration between the cell interior and exterior. Among the bacterial flagellar motors, some convert the energy by the permeation of protons through the motor stator, while others utilize sodium ions or multiple ions. However, the atomic structure of the bacterial flagellar motor remained unknown, and the mechanism of ion permeation had not been elucidated in detail. Project Researcher Nishihara Yasutaka at the Graduate School of Arts and Sciences and Associate Professor Akio Kitao at the Institute of Molecular and Cellular Biosciences constructed a three-dimensional model structure of the protein complex that comprises the flagellar motor stator MotA/B, and found that protons permeate through the transmembrane stator as hydronium ions, inducing a motion similar to a ratchet wrench (ratchet movement) limited to one directional rotation. Investigation of this type of highly efficient energy conversion mechanism is essential to understand biological mechanisms which can utilize energy efficiently.