# Light-Controlled Microbots in Biomedical Application: A Review
## 4 Aspects of Design
### Wavelength Dependency and Trapping Efficiency  
The wavelength dependence is very crucial when dealing with biocells. The specialized microbot to perform in vivo tasks must be designed considering the impact of the laser on the cells. The penetration of light through the skin for different wavelengths is visualized in Figure 8. Ideally, lasers of any wavelength can be used to trap microscopic particles, but only a few wavelengths are considered to be usable because of the factors of heating and photodamage of the cells. The first thing that comes to mind is how much heat will be produced in how large of an area. It is found that a laser at the near-infrared wavelength of 1064 nm can cause a temperature increase of up to 1.45 \({}^{\circ}\)C with a power increase of 100 mW in the sample [192]. It is also calculated that lasers of 1064 nm wavelength can cause a temperature rise 4.5 times higher than that of a 980 nm laser. However, at the same time, the samples are often surrounded by water and a 980 nm wavelength laser causes 3.5 times more heating than a 1064 nm laser [193]. Studies showed that high-intensity laser beams could penetrate cells, posing a risk of photodamage [194; 195; 196]. Usually, optical damage is characterized in two ways. The first is the efficiency of cloning and the other is the regular rotation of _Escherechia Coli_ bacteria [197]. It is reported that a trapped wavelength of 1064 nm laser with 88 mW power in ovary cells from a Chinese Hamster (CHO) has reduced the cloning rate by a significant amount, and only after twenty minutes of exposure, the cloning efficiency decreased to zero percent [195]. Another study was conducted to measure the stress response of _Caenorhabditis elegans_ during exposure to a high-power laser from an optical tweezer [198]. It is found that for the same laser power and exposure period, the gene expression and calculated temperature of the test sample depend on the wavelength of the laser. The stress response is measured by the expression of a heat shock-responsive gene within _C. elegans_. Another study also tried to quantify the damage of _Escherichia coli_ bacteria by measuring the rotation rate of bacteria with applied laser power, wavelength, and exposure period [197].  
A 980 nm laser exhibits a minimum risk of damage on a normalized scale, and both 830 nm and 1064 nm pose greater photodamage risk. Another research studied the effect of the 1064-nm laser on temperature, DNA structure, viability, and pH of CHO and human sperm cells in both continuous wave (CW) and pulse mode trapping [201]. For CW trapping, an average of 1 \({}^{\circ}\)C increase in temperature per 100 mW laser power is observed up to 400 mW. Damage to different cell types varies depending on the exposure time, wavelength, and power of the laser. For example, this research reveals that increasing laser power from 1 mW to 100 mW at 1 mm spot size ranged from 0.0025 \({}^{\circ}\)C to 0.26 \({}^{\circ}\)C and from 0.03 \({}^{\circ}\)C to 2.85 \({}^{\circ}\)C at depths of 15.9 and 4.9 mm in the adult and newborn brain, respectively [202]. Long-term exposure to the high-powered laser may cause physiological damage to the cells, and long-term use might pose a fatal risk.