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A fragile microparticle, such as a red blood cell, bacteria, ovum, sperm, DNA, or others, can be easily broken down when physically held by tweezers during a specific procedure or application. This fragile microparticle must be carefully handled to be applied for specific uses, such as physical measurement analysis, including elasticity, stiffness, micro rheology, and others [1–3]. To prevent this fragile microparticle from being easily broken down or flawed, a novel micromanipulation method without physical contact was explored using optical tweezers [4 - 5]. Optical tweezers use an intensely focused beam of light to hold and manipulate a fragile microparticle contactlessly, a process called optical trapping [6]. Optical trapping was discovered by Arthur Ashkin in 1990, and he was awarded the Nobel Prize in Physics in 2018 [7]. Optical tweezers work by tightly focusing a laser beam through an objective lens to form a laser spot in the sample [8]. When the objective lens focuses the laser beam, an optical force is generated, trapping the fragile microparticle in the laser spot. Figure 1 shows the optical trapping of a fragile microparticle of Lactobacillus exposed to the optical trap from (a) to (f) viewed from the monitor camera. The single rod-shaped cell is trapped, with its central axis aligned along the beam propagation direction. 

Optical Trapping Parameter Condition for Stable Trapping

1. Laser Power

The optical trapping procedure, particularly when handling the fragile microparticle, must consider several parameters to achieve successful, stable optical trapping. Stable and successful optical trapping means that the fragile microparticle being studied can be held and not escape from the laser spot during the monitoring period. The first parameter to consider is the laser power used for optical trapping [11 - 12]. The laser power is directly proportional to the optical force generated to hold the fragile microparticle. This means that using high laser power will produce a strong optical force. Since the optical trapping procedure focuses on holding and controlling the fragile microparticle, higher laser power is not recommended, as it might damage or destroy it. However, if the laser power is too low, the resulting optical force will be weaker, and the fragile microparticle cannot be held or controlled. Hence, it is required to use a laser power within a suitable range for optical trapping of the fragile microparticle. In addition, the wavelength selection for the trapping laser is also crucial in a specific application. For example, an infrared laser is preferable for trapping a biological sample in water, as the heating effect can be suppressed.

2. Particle Size

The second parameter to consider for the optical trapping of the fragile microparticle is its size [12]. The researcher must consider the size of the fragile microparticle that needs to be held or controlled by the optical tweezers. Since the optical trapping produced a pico-newton force, choosing the correct size for the trap becomes crucial. If the selected size of the fragile microparticle is larger, there is a probability that the process of holding and confining becomes harder as the optical force is weaker. From another perspective, if the selected fragile microparticle size is too small, it is also challenging to hold, as the small fragile microparticle would experience a larger optical force and be pushed away from the laser spot. The specific optical tweezers model, such as OTKB/M from Thorlabs, has a limited resolution of ~500nm, meaning that fragile microparticles smaller than 500nm cannot be observed during optical trapping. Despite this limitation, the use of dye and spectroscopy techniques combined with optical tweezers might help to observe the fragile particle during optical trapping.

As a conclusion, optical trapping of the fragile microparticle is possible when the technical aspects of the optical trapping conditions, such as laser power and size, are considered. Even though the trapping conditions for each fragile microparticle would differ, it is the researcher's skills to determine the appropriate optical trapping conditions. For future work, the researcher might explore the possibility of assisting gamete fertilisation, a comprehensive study of red blood cell deformation, and the manipulation of microorganisms, such as bacterial motion, using optical trapping.

Reference:

1. O. V Grishin, I. V Fedosov, and V. V. Tuchin, "Stiffness of RBC optical confinement affected by optical clearing," in Saratov Fall Meeting 2016: Optical Technologies in Biophysics and Medicine XVIII, 2017, vol. 10336, p. 103360U. doi: 10.1117/12.2267942.

2. T. Tsuji, K. Doi, and S. Kawano, "Optical trapping in micro- and nanoconfinement systems: Role of thermo-fluid dynamics and applications," Journal of Photochemistry and Photobiology C: Photochemistry Reviews, vol. 52, no. October 2021, p. 100533, 2022, doi: 10.1016/j.jphotochemrev.2022.100533.

3. W. N. S. Aziz, S. K. Ayop, and S. Riyanto, "The potential of optical tweezer (OT) for viscoelastivity measurement of nanocellulose solution," J Teknol, vol. 74, no. 8, pp. 45–48, 2015, doi: 10.11113/jt.v74.4722.

4. M. S. M. Yeng, S. K. Ayop, and I. R. Mustapa, "Depth - Dependent Optical Stiffness Toward Water - Air Interface," International Journal of Engineering and Technology, vol. 7, pp. 80–84, 2018.

5. M. S. M. Yeng, S. K. Ayop, I. R. Mustapa, and K. Sasaki, "Optical Stiffness of an Optically Trapped 4-Cyano-4’-Pentylbiphenyl (5CB) in the form of a Microdroplet in Water," Central Asia and the Caucasus, vol. 23, no. 1, pp. 3008–3016, 2022.

6. M. F. M. Yusof, S. K. Ayop, F. L. Supian, and Y. Juahir, "Optical trapping of organic solvents in the form of microdroplets in water," Chem Phys Lett, vol. 749, no. January, p. 137407, 2020, doi: 10.1016/j.cplett.2020.137407.

7. A. Ashkin, G. Boyd, and J. B. Labs, "Arthur Ashkin : Father of the optical tweezers," Proceeding of the National Academy of Sciences of United States of America, vol. 118, no. 7, pp. 1–4, 2021.

8. M. Xie, "Principle of optical tweezers trapping," Autonomous Robot-Aided Optical Manipulation for Biological Cells, pp. 3–13, 2021, doi: 10.1016/b978-0-12-823449-5.00003-4.

9. M. S. M. Yeng, S. K. Ayop, and K. Sasaki, "Optical Trapping of a Single Chloroform Microdroplet," J Teknol, vol. 85, no. 3, pp. 117–123, 2023, doi: https://doi.org/10.11113/jurnalteknologi.v85.19303.

10. D. Wang, X. Li, and J. Ng, "Enhancing gradient force over scattering force for nano-trapping through compensating for aberration," New J Phys, vol. 25, no. 12, 2023, doi: 10.1088/1367-2630/ad1698.

11. M. Y. Hamid, S. K. Ayop, W. N. S. Aziz, and Y. Munajat, "Spatial Distribution of an Optically Trapped Bead in Water," Buletin Optik 2016, vol. 33, no. 2, pp. 2–5, 2016, doi: 10.15011/jasma.33.330211.

12. W. N. S. Aziz, S. K. Ayop, M. Y. Hamid, and Y. Munajat, "Simple Determination of the Stiffness of an Optical Trap Using Video Microscopy and Particle Tracking," Buletin Optik 2016, vol. 1, no. 2, pp. 1–6, 2016.