# Overview on the Evolution of Laser Welding of Vascular and Nervous Tissues
## 2 Laser Welding of Vascular and Nervous Tissues
### Welding Procedure  
As shown in Figure 1, laser welding occurs when the laser energy induces photomechanical and photothermal tissue modifications, mainly due to its capability for selective modulation of the biologic functions of cells. According to various studies [59; 60; 61], the tissue remodeling process is characterized by the equilibrium between extracellular matrix (ECM) remodeling and collagen biosynthesis. This balance is achieved through the thermal response of three main biologic structures: matrix metalloproteinases (MMPs) [62; 63], transforming growth factors (TGFs) [64; 65], and heat-shock proteins (HSPs) [66; 67]. When observing collagen biosynthesis, [PERSON] [67] and [PERSON] et al. [68] showed that HSP 47 is responsible for producing neocollagenesis that will benefit tissue remodeling. In turn, [PERSON] and [PERSON] [62] and [PERSON] et al. [69] stated that MMPs have an important role in vascular tissue remodeling, since their activation will act upon collagen degradation and ECM transformations. Additionally, positive results on collagen expression and an increased number of new growing vessels were recently reported by [PERSON] et al. [70] through the application of the 670 nm GaAIAs laser. Considering the ECM transformations and collagen biosynthesis, laser irradiation focuses on structural proteins, resulting in the protein denaturation process at temperatures between 60 \({}^{\circ}\)C and 65 \({}^{\circ}\)C [41; 44; 71; 72]. When the temperature exceeds 65 \({}^{\circ}\)C, the fusion with laser does not serve its purpose and cell degeneration takes place, primarily through vaporization at 100 \({}^{\circ}\)C, and finally through burn or disruption at higher temperatures [41; 44; 71; 72]. Thus, the optimization of the laser power and the exposure time is required to achieve the cross-linking fusion of the collagen fibrins, which is directly related to the success of the welding operation [41; 42; 44; 55]. Despite the precision and the efficiency of laser welding in vascular and nerve surgeries, thermal damage of the cells may occur, i.e., damage without possible regeneration, which results in a low tensile strength of the bonding area. Thermal tissue degeneration is directly related to laser parameters and biological conditions, mainly with the laser exposure time and the absorptive properties of the bonding region. Equilibrium of these factors is crucial to achieve improved results [47; 49]. Moreover, it has been concluded that variables, such as the laser wavelength, the depth of tissue penetration, the energy fluency, and the use of cooling solutions, also influence the success of the surgery [40; 44; 45]. Several strategies have been tested in laser-welding procedures in order to minimize the thermal  
Figure 1: Schematics of laser welding of a nervous tissue [40]. damage of the tissues, specifically, the use of thermal feedback systems, saline solutions, and different irradiation techniques. The use of thermal feedback systems is a very effective strategy for preventing thermal damage, as they enable the monitoring of the temperature in the weld region, with a very precise control of the laser parameters [42; 45; 47; 52; 73]. Regarding the use of saline solutions, although [PERSON] et al. [45] reported that it could decrease the bonding strength of the weld, many other studies demonstrated that it had no implications on this property, being an effective strategy for cooling down the welding site [41; 42; 57; 71]. Another strategy was tested by [PERSON] et al. [72], who compared different irradiation techniques in vascular repairs, specifically, laser-welding scanning and single-spot laser welding. According to these authors, the single-spot method may be more beneficial when applied through multiple laser pulses in the surgical area, enabling tissue to cool down between pulses, and therefore, decreasing the thermal damage.