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[Feature article]Encryption/decryption and microtarget capturing by pH-driven Janus microstructures fabricated by the same femtosecond laser printing parameters

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Release Date: 2021-02-25 Visited: 

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1. Introduction

The microstructure composed of two parts with different chemical or physical properties is called Janus structure. Because of its simple structure, Janus structure is a kind of micro nano intelligent response structure with wide application prospect. However, the preparation of Janus structure usually depends on multi-step processing, different materials or /and different processing parameters, which will increase the process difficulty, cost and time, thus hindering its further application. Recently, Professor Zhang Li's research group of the Chinese University of Hong Kong published a research paper entitled " PH-driven microactuator based on Janus construction spontaneously formed with same femtosecond laser printing parameters and its application" in the International Journal of extreme manufacturing (IJEM), introducing a new processing method of intelligent microstructure. By employing the scattering effect of light, the researchers have prepared the sub-micron diameter pH fast response Janus structure in a single material through the same processing parameters, and demonstrated the application of the intelligent unit in information encryption/decryption and micro object capture.

2. Background

After millions years of survival competition, organisms in nature have evolved a variety of stimulus response mechanisms to adapt to environmental changes. By imitating this stimulus response mechanism, researchers have prepared a variety of intelligent micro nano functional structures. Among them, Janus structure has attracted wide attention because of its simple structure. But its preparation often depends on complex processing steps, different materials or processing parameters, which increase the processing cost. In addition, most of the Janus structures have uniform deformation in the array, which makes it difficult to customize their structural orientation within a certain range and lack flexibility. In recent years, as a three-dimensional high-precision micro nano processing method, femtosecond laser printing has been widely used in the fields of micro fluid, micro optics, intelligent response structure and so on. However, in the past studies, different materials or processing parameters were used to fabricate smart response microstructures by femtosecond laser, which increased the process complexity.

3. Recent Advances

Researchers developed a pH-sensitive hydrogel material which expands when pH>9. The polymerization degree of this pH sensitive material is positively correlated with the absorptive light intensity in the threshold range. As shown in fig.1b, the higher the polymerization is, the smaller the deformation under pH environment is; the lower the polymerization is, the larger the deformation under pH stimulation is. On this basis, the researchers proposed a double scanning strategy (fig.1a). The laser energy, step length and other parameters of the two scanning are exactly the same, but the structure formed by the first scanning will scatter and refract the light of the second scanning so that the aggregation degree of the structure formed by the second scanning reduces. Due to the proximity of the two scan, the structure formed by the two scanning was polymerized into a hydrogel micropillar (fig.1e) with a diameter of about 800 nm. Under the action of pH, the microstructure has ultra-high response speed (< 0.2 s) and stable response characteristics (fig.1f).

Figure 1. Janus micropillars fabricated by femtosecond laser dual-scanning. Scale bars: 15 μm (c), 4 μm (d, e).

The fabrication method of Janus structure based on femtosecond laser printing is highly flexible, so its spatial position can be patterned. Based on the pH responsive microstructure, the application of information hiding and displaying can be realized (Fig.2). As shown in Fig. 2c, when pH < 9, the pH responsive Janus microstructure is hidden in the normal structure array, and "CUHK" is not displayed; when pH > 9, the Janus microstructure expands and bends while the normal micropillar is not bent, so "CUHK" is displayed. Then, based on the pH controlled microstructure, researchers designed micro grippers array and demonstrated its application in micro object capture and release (Fig.3).

In addition, due to the flexibility of femtosecond laser processing, the pH controlled microstructure can also be integrated into the microchannel, which can be applied in microfluidic technology.

Figure 2. Applications of Janus microactuators in information encryption/ decryption. Scale bars: 15 μm.

Figure 3. Demonstration of the microparticle trapping. Scale bars: 10 μm.

4. Perspectives

This paper proposes a simple and flexible dual-exposure strategy for fabricating pH-driven geometry-switchable Janus micropillars. By utilizing the light scattering of printed structure, inhomogeneity forms spontaneously with an individual material and the same processing parameters which is different from current strategies to construct Janus structures based on different material, multi-step manufacture and/or different processing parameters. The bending effect of these Janus micropillars can be triggered by pH stimulus resulting in a microactuator with feature size at submicron scale (~ 1 μm) and fast response time (~0.2 s). By taking full advantage of the flexibility of laser writing, the pillars number, spatial location, and bending direction can be easily controlled, leading to a variety of dynamic ordered patterns. The resultant Janus microactuators hold potential in information encryption/decryption and microparticle capture, as demonstrated with the proof-of-concept experiments. The combination of femtosecond laser direct writing and environmentally sensitive smart materials opens up a new avenue to fabricate functional microactuators with great scalability and flexibility toward numerous applications in smart display, intelligent sensor, microobject manipulation, filtration, and transportation.

5.  About the Authors

Dr. Lao Zhaoxin received his B. Eng from Hefei University of Technology in June 2012. In June 2017, he graduated from USTC and received his PhD degree in Instrument Science and Technology. Then, he worked as a post-doc in CAS Key Laboratory of Mechanical Behaviorand Design of Materials at USTC. In August 2018, he was selected to “Hong Kong Scholars Program”. He joined Prof. Zhang's group in Jan. 2019. Now he is working on fs-laser fabrication and micro/nanoself-assembly.

Li Zhang is an Associate Professor in the Department of Mechanical and Automation Engineering (MAE) at The Chinese University of Hong Kong (CUHK). He is also director of the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences (CAS) - CUHK Joint Laboratory of Robotics and Intelligent Systems. Dr. Zhang received the Ph.D. degree from the University of Basel, Switzerland, in 2007. From 2002 to 2006, he was also with the Laboratory for Micro- and Nanotechnology (LMN), Paul Scherrer Institute (PSI). He joined the Institute of Robotics and Intelligent Systems (IRIS, Prof. Brad Nelson's group), Swiss Federal Institute of Technology (ETH) Zurich, Switzerland as a postdoctoral fellow in 2007, and as a senior scientist from 2009 to 2012. He then joined CUHK in 2012 as an Assistant Professor.

Dr. Zhang's main research interests include milli-/micro-/nanorobotics and their biomedical applications, and functional materials for sensors, actuators and practical applications. Dr. Zhang is a senior member of IEEE, who has won several awards or in the Finalist from IEEE international conferences including ICRA, IROS, CASE, ROBIO, ICARM and NANOMED. Since 2004 he has authored and co-authored over 200 papers, including Science Robotics, Nature Machine Intelligence (News & Views), Science Advances, Nature Communications, Advanced Materials, ACS Nano, AFM, TRO, IJRR, SoRo, Annual Review of Control, Robotics, and Autonomous Systems, as the corresponding author. He won the Hong Kong Research Grants Committee (RGC) Early Career Award in 2013, CUHK Young Researcher Award 2017, United College Early Career Research Excellence Merit Award 2018, and CUHK Research Excellence Award 2019-20. He currently serves as Associate Editor/Editorial Board Member of several top journals, such as IEEE TRO, IEEE/ASME T-MECH, IEEE T-ASE, and so on. Dr. Zhang is a Distinguished Lecturer appointed by IEEE NTC twice.

 Representative publications:

1. Zhang L, Yu J, Yang L, MicroroboticsFrom individual to swarm, Science Press, China, 2020.

2. Wang Q, Zhang L, External Power-Driven Microrobotic Swarm: From Fundamental Understanding to Imaging-Guided Delivery, ACS Nano, 2021, 15, 149.

3. Jin D, Zhang L, Microrobotics - Embodied intelligence weaves a better future. Nature Machine Intelligence, 2020, 2, 663.

4. Wang B, Kostarelos K, Nelson B. J, et al., Trends in Micro‐/Nanorobotics: Materials Development, Actuation, Localization, and System Integration for Biomedical Applications, Advanced Materials, 2020, 33, 2002047.

5. Zhang Y, Zhang L, Yang L, et al. Real-time tracking of fluorescent magnetic spore–based microrobots for remote detection of C. diff toxins. Science Advances, 2019, 5, eaau9650.

6. Yu J, Jin D, Chan K F, et al. Active generation and magnetic actuation of microrobotic swarms in bio-fluids. Nature Communications, 2019, 10, 5631.

7. Jin D, Yu J, Yuan K, et al. Mimicking the structure and function of ant bridges in a reconfigurable microswarm for electronic applications. ACS Nano, 2019, 13, 5999.

8. Yang L, Yu J, Zhang L. Statistics-based automated control for a swarm of paramagnetic nanoparticles in 2-D space. IEEE Transactions on Robotics, 2019, 36, 254.

9.  Yu J, Wang B, Du X, et al. Ultra-extensible ribbon-like magnetic microswarm. Nature Communications, 2018, 9, 3260.

10. Yu J, Yang L, Zhang L, Pattern generation and motion control of a vortex-like paramagnetic nanoparticle swarm. International Journal of Robotics Research, 2018, 37, 912.

11. Yan X, Zhou Q, Vincent M, et al. Multifunctional biohybrid magnetite microrobots for imaging-guided therapy. Science Robotics, 2017, 2, eaaq1155.

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