# Light-Controlled Microbots in Biomedical Application: A Review
## 4 Aspects of Design
### Manipulation of Microbots
#### 4.1.1 Optical Tweezers (OT)  
Optical tweezers are the oldest and widely used method for micromanipulation using lights. Pioneered by [PERSON] in the early 1970s [48], this remarkable invention received a Nobel Prize in 2018. In an optical tweezer, a trap can be formed by focusing a laser beam in an inverted microscope with the help of an objective lens of high numerical aperture (NA). The dielectric microparticle needs to have a refractive index different from the medium in which it stays. With the incident laser beam, it will experience a force due \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Optical** & **Magnetic** & **Chemical** & **Biological** \\ \hline \multirow{4}{*}{Control} & Optical Tweezers & Helmholtz and & & \\  & Optoelectronic & Maxwell coils [129] & & \\  & Tweezers Electrother- & Single Position & & \\  & moplasmonic & Controlled & & \\  & Nanotweezer & Electromagnetic & & \\  & & coil [130] & & \\ \hline \multirow{2}{*}{Precision of control} & Highly accurate and precise & Precise with gradient, rotating, and oscillating & Easy targeting using enzyme surface & Less predictable and \\  & & magnetic fields [132] & functionalization & less accurate \\ \hline \multirow{2}{*}{Limitation of control} & Low penetration & Strong Magnetic Field & & \\  & without photo & results in heat in the & & \\  & damaging & microbots & & \\ \hline \multirow{2}{*}{Self-propulsion capabilities} & \multirow{2}{*}{–} & Not possible without & Possible & \\  & & external magnetic fields & self-propulsion in fluid & – \\  & & & medium & \\ \hline \multirow{4}{*}{Medical Applications} & Single-cell & Precision manipulation & & \\  & Single-cell & of cells e.g., Yeast, & & \\  & manipulation, Live & Mouse Embryonic & & \\  & RBC capture, etc. [133] & STEM cell, Neuron & & \\  & & Transport, etc. [102] & & \\ \hline \multirow{2}{*}{Imaging} & Optical Imaging & MRI & Radiography/Ultrasonic & \\  & PACT,PATER & & etc. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._to the momentum transfer from the refracted beams. Two forces will appear here with the incident photons. One pushes the particle away, also known as the scattering force, and the other one ensures the balance of the particle in the cross direction. Now, if the size of the particle is larger than the wavelength (a \(\gg\lambda\)) of the laser beam, Mie scattering takes place, and then simple ray optics is enough to compute the optical forces. Gaussian laser beams tend to have an intensity profile with higher intensity in the middle of the beam and get dimmed along the cross direction. Now, as light beams hit the dielectric particle, the momentum of the refracted light balances the sideways movement of the particle but at the same time, as some light gets absorbed by the particle, there is a net momentum transfer to the particle from the incident photons. This momentum transfer results in a net force pushing the dielectric particle away. To overcome this unstable situation, an objective with high NA is used. This mechanism sharply focuses the trapping laser beam, which then gets refracted through the dielectric particle, resulting in a momentum change or force in the opposite direction of the scattering component of the previous force. Now, this ensures the stability of trapping along the beam. However, if the particle is smaller than the wavelength of the laser beam (a \(\ll\lambda\)), then the scattering force component and the gradient force components can be calculated by assuming the particle is a point dipole using the following equations:  
\[ \begin{array}{c}F_{\mathrm{scatt}}=\frac{I_{0}\sigma n_{m}}{3\lambda^{4}} \\ \sigma=\frac{128\pi^{5}\phi}{3\lambda^{4}}\left(\frac{m^{2}-1}{m^{2}+2}\right) ^{2}\end{array}  \tag{1}\]  
Here, \(I_{0}\) is the intensity of incident light, \(\sigma\) is the scattering cross section of the sphere, \(n_{m}\) is the index of refraction of the medium, \(c\) is the speed of light in vacuum, \(m\) is the ratio \(\left(n_{p}/n_{m}\right)\), and \(\lambda\) is the wavelength of the incident beam.