# A Photolysis-Assist Molecular Communication for Tumor Biosensing
## 2 System Model
### Photolysis Reaction  
Photodissociation, also known as photolysis, is a reaction in which a photon breaks down a chemical bond in a chemical molecule. This photon's energy must be high enough for the bond to break beyond its dissociation barrier. Because photon energy is inversely related to wavelength, photons in the infrared spectral region do not have enough energy to produce direct photodissociation of molecules. The photolysis process is fast and represented by the first-order differential rate equation:  
\[\frac{dC}{dt}=-JC, \tag{5}\]  
where \(C\) is the concentration of a particular degradable molecular \(X\), and \(J\) is the photolysis rate coefficient of the molecule. The rate is determined by chemical and environmental factors, such as the photoadsorption properties of the medium, the intensity of the light radiation, the reactive of the target chemical, etc. An accurate description of these photolysis processes is done by calculating the photolysis rate coefficient, the \(J\) value, in [31]:  
\[J=\int_{\lambda}\sigma(\lambda)\varphi(\lambda)F(\lambda)d\lambda, \tag{6}\]  
For each photoactive molecule with an absorption cross-section \(\sigma(\lambda)\) and a photolysis quantum yield \(\varphi(\lambda)\), the \(J\) value is given by integrating the product over wavelengths \(\lambda\). Here \(F\) is the actinic flux. The photolysis reaction that causes human skin to produce vitamin D daily following exposure to sunshine is the one that has received the most attention. During exposure to sunlight, provitamin D3 is converted to previtamin D3, which is rapidly thermally isomerized inside the plasma membrane to cholecalciferol [32]. Photolysis reaction is one of the most well-known nonenzymatic reactions in our bodies. According to [32], The transition in 7-DHC exhibits a broad absorption line shape spanning the wavelength range  
Figure 2: The information molecules and synchronization molecules with different diffusion coefficients are transmitted along with symbol sequences 1010, where information molecules are released at the start of symbol “1”, while the synchronization molecules are released during the symbol “0”. \(t/s\) denotes that the unit of time is seconds. of 260 and 290 nm, the optimal irradiation range is in the absorption band of 280-295 nm. That energy breaks a covalent bond, turning provitamin D3 to previtamin D3, which will thermodynamically convert to Vitamin D3 without the need for enzymes. The sensitive molecules to the specified wavelength as information carriers can be chosen from naturally occurring molecules such as chromophores and provitamin D3 or bioengineered ones. The chromophores are visible light-sensitive, while the provitamin D3 is UV-sensitive. However, molecules with a specified wavelength absorption can be engineered, as in the study [33]. It is not harmful to use visible light inside the human body for medical purposes [7]. If shorter wavelengths are employed, specific parameters must be followed to ensure the application's safety. UV light is safe, but the amount of radiation energy and the length of exposure should be considered to ensure it is not hazardous. Therefore, the American National Standards Institute (ANSI) defined the standard ANSI Z-136.1 [34], which gives the maximum permissible exposure limits (MPEs) for users. The minimum MPE for the UV light is 3.0 mJ/cm\({}^{2}\) over 8 h. Therefore, if the energy and the exposure time are safe for the cornea, they should be safe inside the body as well. While the thermal process is the main cause of UV radiation damage [35], to design a safe UV generator is necessary to generate nonthermal radiation by using a pulsed beam instead of a continuous one. The energy will be released in a short time, and this compression delivers the light beam more rapidly, which gives more power using less energy [36]. The shorter the pulse, the less energy is consumed, which greatly reduces the risk of tissue burn.