Femtosecond Laser Holographic Processing: Manufacturing Micro Spiral Robots

Editor’s Note

Driveable Micro Robots have broad application prospects in non-invasive surgery, targeted delivery, detoxification, and are receiving widespread attention. However, these applications place higher demands on the performance of micro robots.

For example, in non-invasive surgery, people hope to safely manipulate micro robots to target diseased cells without contact. In targeted delivery, micro robots with strong loading capacity are essential for transporting various drugs within the body. In detoxification applications, a larger relative contact area and pollution-free mass production are key to rapidly removing toxic substances. Therefore, micro robots with features such as non-contact manipulation, strong loading capacity, and mass production are urgently needed.

Currently, various strategies have been proposed for manufacturing micro spiral robots, such as film self-rolling, laser direct writing, etc., but these methods still face issues such as complex manufacturing processes, time-consuming operations, and weak loading capacities of the processed spiral structures.

To address these issues, a team led by Associate Professor Li Jiawen from the School of Engineering Science at the University of Science and Technology of China proposed a method for generating locally enhanced annular optical fields using a femtosecond Bessel beam superposition interference approach, combined with dynamic holography, to rapidly fabricate micro hollow double-spiral robots and achieve wireless magnetic drive, providing a new avenue for the widespread application of magnetic drive micro robots with carrying capacity in the biomedical field.

This achievement, titled “Efficient Processing of Hollow Double-Spiral Micro Robots Using Femtosecond Laser Dynamic Holography,” was published in The Journal of Optical Precision Engineering and was selected as the cover article for Volume 29, Issue 9.

Paper Information

Song Bowen, Li Jiawen. Efficient Processing of Hollow Double-Spiral Micro Robots Using Femtosecond Laser Dynamic Holography [J]. The Journal of Optical Precision Engineering, 2021, 29(09):2101-2107. DOI: 10.37188/OPE.20212909.2101

Paper URL

http://ope.lightpublishing.cn/thesisDetails#10.37188/OPE.20212909.2101

Cover image of The Journal of Optical Precision Engineering, 2021, Volume 29, Issue 9

Q&A

Reporter of this Issue: Zang Chunxiu (Scientific Editor of The Journal of Optical Precision Engineering)

Interviewee: Li Jiawen (University of Science and Technology of China, Associate Professor)

Q1: Please introduce the technical route of this research and the results obtained.

A: There are several main aspects:

(1) Superimposing Bessel holograms generated according to the Bessel transfer function and simulating and experimentally measuring the produced optical fields. Then, using the superimposed holograms to process annular structures with different side lobe numbers (2-4), analyzing the influence of two different parameters on side lobe width and annular diameter.

(2) By introducing dynamic holographic processing methods, efficient and rapid processing of micro hollow double-spiral robots was achieved, with a micro robot width of 25µm and length of 100µm.

(3) Using capillary force self-absorption, loading of microparticles with diameters of 1µm and 2.5µm was achieved.

(4) Using a rotating magnetic field to drive micro robots in a microfluidic environment. Experiments showed that processing a single micro robot takes only 6s, and under the rotating magnetic field, the micro robot moves 400µm in a straight line within 7s.

Schematic diagram: Femtosecond laser dynamic holographic processing system

Q2: Femtosecond processing technology has been applied to the processing of metals, semiconductors, organics, and various complex structures. What advantages does this processing technology have when applied to the manufacturing of micro spiral robots compared to conventional processing methods?

A: The advantages of femtosecond laser processing technology for micro spiral robots compared to conventional manufacturing techniques are mainly reflected in:

(1) Ultra-high processing resolution: The three-dimensional forming technology used in this article is femtosecond laser two-photon polymerization technology, where the two-photon absorption of the material is a nonlinear effect that occurs only in the central area of the laser focus, achieving photochemical polymerization and breaking through the spatial resolution limit of optical diffraction, reaching the nanometer level, making it particularly suitable for high-quality processing of micro-nano-level robots, which conventional processing methods cannot achieve at the micron level.

(2) High processing efficiency: Femtosecond laser two-photon polymerization processing directly forms structures at the laser focus. In this article, a rotating structured light scans the light polymerizing material at high speed, enabling the rapid manufacturing of micro double-spiral robots at the hundred-micron level within 6s, greatly shortening processing time compared to previous conventional methods and improving processing efficiency.

(3) Multi-material processing: Two-photon polymerization can process a variety of materials, including photoresists, hydrogels, and various doped composite materials. Micro robots made from different materials can meet various application needs in the same environment. For example, micro robots made from biodegradable hydrogels can self-degrade in the body after completing targeted delivery tasks, reducing the harm of secondary retrieval or prolonged retention of micro robots in the human body; hydrogels doped with magnetic iron oxide particles can also be directly used for femtosecond laser two-photon polymerization processing. The use of such composite materials can simplify the processing complexity of magnetic drive micro robots, shorten processing time, and save production costs.

Q3: Why was the off-axis superposition interference method using Bessel light chosen in the experiment, and what advantages does it have over the coaxial interference method?

A: To manufacture micro spiral robots with carrying capacity, there are requirements for the morphology of the micro robots to have spiral characteristics and chambers. To quickly manufacture micro robots with these two characteristics, structured light processing is a solution. Common structured lights such as Bessel light, vortex light, and Airy light cannot meet the processing requirements. The coaxial interference of vortex light to obtain annular multi-focus structured light is more suitable for processing micro cage structures, thus there is an urgent need to develop a new structured light specifically for the efficient manufacturing of micro spiral robots with carrying capacity.

Therefore, our group focused on the study of Bessel light. Since Bessel light has an annular light intensity distribution, it can achieve the processing of hollow annular chambers, but the realization of spiral characteristics still needs to be solved. Inspired by coaxial interference, our group attempted off-axis superposition interference of Bessel light, analyzing the simulated light intensity distribution of the new optical field generated by off-axis superposition interference of Bessel light, and found that the new structured light retains the annular characteristics and uniformly distributes energy enhancement points along the circle, with the number of enhancement points depending on the number of superimposed Bessels. This special light field distribution is particularly suitable for the processing of micro spiral robots with hollow chambers.

Q4: What key issues does this research solve in the processing of micro spiral robots?

A: It solves the problem of low processing efficiency in the current processing of micro spiral robots. A new light field was generated, and the dynamic holography processing method was adopted, which, compared to traditional femtosecond laser point-by-point scanning strategies, processes a single micro robot in only 6s, improving processing efficiency by two orders of magnitude.

Q5: What other issues remain in the manufacturing of micro spiral robots?

A: The adjustable range of the spiral structure width of micro robots can still be optimized. The wider the spiral structure of the micro robot, the greater the thrust given to the forward direction with each rotation, thus increasing the controllable speed range of the micro robot. Currently, our processing method mainly relies on adjusting the laser energy to regulate the spiral structure width, where larger energy enhances the nonlinear polymerization process of two photons, resulting in a larger spiral width. However, this method has limited adjustable range, thus the large-scale adjustability of the spiral structure width remains a problem in the manufacturing of micro spiral robots.

Q6: How long did the R&D cycle take? What problems were encountered during the research, and how were they overcome or solved? What advice can this topic share with current researchers?

A: The entire R&D process took one and a half years, from September 2019 to April 2021 when it was accepted. The achievements of every researcher are hard-earned, and we encountered many problems during our research:

(1) In the early stages of the experiment, our group tried many methods for generating structured light, such as using local phase modulation to modulate Bessel light holograms, generating gap annular light, and successfully processing single spiral structures with dynamic holography. However, the micro spiral structures obtained using this method not only lacked chambers but also had low structural strength and were prone to deformation. Additionally, we generated petal-shaped structured light by coaxial superposition interference of vortex light, but this structured light is unsuitable for processing spiral structures.

Ultimately, we attempted the method of off-axis superposition interference of Bessel light, generating locally enhanced annular light, analyzing the effects of topological charge number, conical lens radius, and the number of superpositions on the generated structured light, and finally selecting suitable processing parameters.

(2) After the steps of processing structural forming, structural metallization, and structural magnetization, we encountered many issues during the magnetic drive testing of the micro robots. For example: After metallizing the micro robots, they need to be transferred from the glass substrate to the micro liquid environment, and operating the transfer of micron-level robots requires precision. Initially, we used commercial capillaries with diameters in the 500µm range to poke the micro robots down for transfer; however, this was still very difficult for micro robots with a diameter of 30µm, often resulting in being unable to find or even damaging the micro robots.

Ultimately, we thought of melting the capillary on the flame, combining rapid stretching and cooling at the melted point of the capillary to obtain capillaries with diameters as small as tens of microns, greatly improving the transfer rate of micro robots. Through this research topic, we hope that all researchers will possess both perseverance and a positive mindset to solve problems.

Author Introduction

1. First Author

Song Bowen, a master’s student in the Department of Precision Machinery and Precision Instruments at the University of Science and Technology of China, mainly engaged in research on femtosecond laser processing and light field control.

E-mail: ust-csbw@mail. ustc. edu. cn

2. Corresponding Author

Li Jiawen, University of Science and Technology of China, PhD, Associate Professor, obtained his PhD from the University of Science and Technology of China in 2011, postdoctoral researcher from 2011 to 2013 at the University of Science and Technology of China, visiting scholar at the Department of Nanoengineering, University of California, San Diego (UCSD) from 2015 to 2016, mainly engaged in research on femtosecond laser processing, 3D bioprinting technology and applications, micro-nano functional surfaces and devices, structural color mechanisms, and processing.

Email: jwl@ustc. edu. cn

Supervised by | Yuan Jingze, Zhao Yang

Edited by | Zhao Wei