Recently, Prof. Gu Fuxing's group under the leadership of academician Zhuang Songlin at the School of Optoelectronics of the University of China has invented a laser capture technology based on the photothermal shock effect, called Photothermal-Shock Tweezers, which realizes the capture and arbitrary manipulation of micro- and nano-objects at the interface of solids and explores its nano-robotics applications. Autonomous nanorobots with powerful thrust under dry solid-contact conditions by photothermal shock" was published on November 11, 2012, and the results were published in the journal "Autonomous Nanorobots with Powerful Thrust under Dry Solid-Contact Conditions by Photothermal Shock". Autonomous nanorobots with powerful thrust under dry solid-contact conditions by photothermal shock" was published on Nov. 24 in Nature Communications. Dr. Gu Zhaoqi, Zhu Runlin and Shen Tianci were the co-authors, and Prof. Gu Fuxing was the corresponding author, while other collaborators included Prof. Liu Xu from Hebei University of Technology and Prof. Liu Jia from Auburn University, and Academician Zhuang Songlin supervised the whole research. The research was supported by the National Natural Science Foundation of China and Shanghai. The technology is expected to explore unprecedented applications in various fields such as nanomanufacturing, biomedicine, aerospace and military.
The photothermal impulse tweezers system can seamlessly inherit robotics in the macroscopic world and realize scenarios of intelligent robotic work in the microscopic world. The team used a metal nanosheet, combined with image recognition, deep learning, path planning, and feedback control to realize the world's first autonomous nanorobot with cleaning function. By recognizing the cleanliness of a selected area, the robot will repeat the cleaning cycle until it reaches a satisfactory level of cleanliness.
Laser trapping (trapping) is understood to be a powerful tool for manipulating the motion of objects in the nanoworld, having won the 1997 and 2018 Nobel Prizes in Physics for its wide range of applications in environments with suspended media, such as vacuums and liquids, but it remains challenging on solid contact surfaces. The researchers used a pulsed light source to heat a micro- and nano-object, and the absorbed energy of the light pulse was instantaneously converted into mechanical expansion, generating an extremely large instantaneous load inside the object, called a Photothermal Shock (PS). The instantaneous shock effect produces a force that far exceeds the normal vibration mode, just like the instantaneous lunge speed of snakes feeding far exceeds the normal crawling speed, so it can break the predicament of micro-nano resistance and realize the movement at the solid interface.
Schematic diagram of the impulse-momentum theorem, illustrating the visual comparison between a snake's lunge and a normal crawl.
The trapping property is at the heart of laser manipulation technology, as it allows the particle's movement to be mastered by spot position, enabling arbitrary motion control, rather than just stopping at lack of controlled actuation. Gold nanowires in the 532nm nanosecond pulse of Gaussian-type spot action, will move to the inside of the spot, until the center of the nanowire and the center of the spot in line with the center of the spot, which is a typical capture process. Through theoretical analysis, the researchers found the physical source of the photothermal shock driving force, which is called the photothermal gradient force because the expression includes a temperature gradient. When moving the spot, the equilibrium of the photothermal gradient force distribution is broken, and the nanowire re-moves toward the center of the spot, and by repeating the process all the time, the nanowire moves axially all the way with the spot. In addition, for the nanowire captured in the center of the spot, increasing the laser power will cause the nanowire ends to be squeezed by a larger photothermal gradient force and bent sideways, thus realizing lateral movement. This enables arbitrary movement of the nanowires in a two-dimensional plane. The figure below shows the team's use of multiple nanowires to form the Chinese character "冲" and the English word "SHOCK".
Photothermal impulse tweezers manipulate nanowires
Using palladium nanosheets as a chassis, the researchers built a more complex and versatile nanorobot, dubbed HOUbot because of its resemblance to a Chinese horseshoe crab (Figure 4a and video). The robot is able to move freely like a car, and performs higher degree of freedom and fine movements such as head pushing, independent tail swinging and poking. The robot is equipped with semiconductor nanowires that can be used for in situ moisture sensing. Due to its relatively large surface, the robot is highly loadable, with theoretical payloads on the order of milligrams (equivalent to the mass of an ant). By adopting an existing macro-mechanical design to equip additional on-board components or cargo, HOUbot can work like a macro-robot and is the world's first nano-robot that can perform specific tasks using traditional mechanical means.
Related schematic
The invention of photothermal tweezers has enabled laser manipulation to break through the interfacial resistance dilemma, complementing the application environment of light manipulation and enabling lasers to finally realize the ability to manipulate objects arbitrarily in a micro-nano environment comparable to the three worlds of land, sea, and air (vacuum/gas, liquids, and solids). Physically, the focus is on transient thermoelastic dynamics and tribology, especially non-destructive studies, which further reveal the understanding of mechano-dynamic processes in the microscopic realm. The technology can in principle be used in any wavelength range and with any absorbable material. In addition, through spatial light modulation and multi-robot collaboration, clusters of autonomous nanorobots can be realized to perform complex tasks that are currently unattainable by conventional means.
