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DNA-Assisted Molecular Robots Autonomously Swarm in Response to Chemical and Physical Signals

DNA-assisted molecular robots, with the potential to autonomously group together in response to physical and chemical signals, have been developed by a research team from Hokkaido University and Kansai University. The research opens the door for creating futuristic nano-machines.

Microtubules (MT) interact with each other through DNA molecules while moving around on the kinesin motors. (Image credit: Keya J. J. et al., Nature Communications, January 31, 2018)

The dimensions of the smallest ever “swarm robot” developed across the globe are a length of 5 mm and a diameter of 25 nm. This robot displays swarming behavior similar to motile organisms such as ants, fish, and birds.

Swarm robots are one of the most elusive subjects in robotics. Fish schools, ant colonies and bird flocks show fascinating features that cannot be achieved by individuals acting alone. These include the formation of complex structures, distinct divisions of labor, robustness and flexibility, all of which emerge through local interactions among the individuals without the presence of a leader.

Akira Kakugo, Hokkaido University research group

Taking cue from these properties, scientists have worked to design micro-scale swarm robots.

In this research, Kakugo and his colleagues have developed a molecular system that contains three vital components of a robot: information processors, sensors, and actuators. They took cellular proteins known as kinesins and microtubules as the actuator, and DNA as the information processor. Microtubules are filamentous proteins acting as the railways in the cellular transportation system. Kinesins are motor proteins running on the railways by absorbing chemical energy acquired from hydrolysis of adenosine triphosphate (ATP). The researchers adopted a reverse approach and developed a system in which the microtubules advance randomly on a kinesin coated surface.

One of the main difficulties in swarm robotics is the development of a huge number of individual robots with the ability to perform programmable self-assembly. The researchers overcame this problem by injecting DNA molecules into the system that have the potential to hybridize upon having a complementary series. The chemically synthesized DNA molecules with certain programs in their sequences are conjugated to the microtubules labeled with green or red fluorescence dye.

Then, the researchers observed the movement of the DNA-conjugated microtubules sliding on a kinesin-coated surface. At first, five million microtubules advanced without interacting with one another. Then, the team added a single-strand linker DNA (l-DNA), programmed to start the interactions between the DNA-attached microtubules. When the l-DNA was introduced, the microtubules start to assemble and form groups of a considerably larger size than the microtubules. Upon adding another single-strand DNA (d-DNA) programmed to disintegrate the swarms, the microtubule groups disappeared instantly. This indicated that grouping of a huge number of microtubules can be reversibly controlled by selective provision of the input DNA signal in the system.

In addition, they coupled a photosensitive sensor to the system—azobenzene affixed to the DNA molecules. They used isomerization of the azobenzene that takes place reversibly as a result of irradiation of ultraviolet or visible light to switch the interaction between DNA molecules on or off. This allowed photo-irradiation-induced switching between the solitary and swarm state of the microtubules. The researchers also showed that the groups of microtubules move with a rotational or translational motion based on the rigidity of the microtubules.

This is the first evidence showing that swarming behavior of molecular robots can be programmed by DNA computing. The system acts as a basic computer by executing simple mathematical operations, such as AND or OR operations, leading to various structures and complex motions. It is expected that such a system contributes in developing artificial muscles and gene diagnoses, as well as building nano-machines in the future.

Akira Kakugo, Hokkaido University research group

Swarming behavior programmed by DNA computing

The movie shows the motility, swarming and dissociation of the flexible (low rigidity) microtubules. The images show the flexible microtubules forming swarms with circular motions (top), and them dissociating in response to the d-DNA input signal (bottom). (Keya J. J. et al., Nature Communications, January 31, 2018)

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