Numerical modeling of a flat microdialysis probe for skin wound analysis

Summary form only given. Skin wound is the state at which the outer most skin layer (epidermis) is broken. Chemokines and cytokines are generated as the inflammatory response to wounding. Researchers have used various techniques to explore inflammatory effects of the skin, including skin blister, ultra-filtration and microdialysis [1]. Microdialysis is a diffusion-based sampling technique that is widely used in vivo in different clinical areas [2]. Microdialysis is a continuous sampling technique based on controlling the mass transfer rate of small molecules across a semipermeable membrane while excluding the larger ones. For biochemical monitoring, microdialysis systems are usually placed (inserted or implanted) inside the tissue of interest with an isotonic perfusion fluid flowing through the system and diffusional exchange occurring between the perfusate and the surrounding interstitial fluid (ISF). Because the dialysis process has a minimal effect on the surrounding fluid, it is viewed as a tool for continuous monitoring. Microdialysis probes consist of a very small membrane where solutes of a certain size can diffuse through. The membrane is implanted in the tissue and perfused with a saline solution matching the tissue physiological fluid. The fluid inside and outside the membrane equilibrates following the principle of diffusion, recovering the substances of interest. Skin microdialysis is preferred to other technique due to its minimal invasion during probe placement. However, this minimal invasion causes stress to an already damaged area, increasing the inflammatory response further [3]. In this study we have designed a new flat microdialysis probe geometry, which eliminates perforation and hence ideal for use on skin wound tissue, using COMSOL Multiphysics as modeling software. Flow rate plays and important role in microdialysis recovery and needs to be considered in designing an efficient microdialysis probe [4]. Typically, microdialysis flow rates range- between 0.5 and 6 μ1/min [2]. The probe design was developed varying the degree of bifurcation for both sharp corners and rounded corners. Velocity field was simulated at different angles (20 to 60°) and constant pressure, and in the case of rounded corners, at different radius of curvature as shown in Figure 1(a-b). Our results show that as the angle is decreased, the velocity increases, presenting a maximum velocity at 20 degrees. As for rounded angles our results show that larger radius leads to higher velocity at constant pressure. On that basis and in order to cover larger area of the wound, we have selected the angle of 30 degrees to model a complete probe design. We analyze four models, these models consist of; all corners are sharp, front corner was rounded, back corner was rounded, and all corners are rounded as shown in Fig. 2(A-D). Our results show that higher flow rate can be applied with the all corners rounded design. Table 1 shows the comparison between these models in term of maximum velocity. Figure 3 shows the relation between build up pressure in the bifurcation versus bifurcation degree at constant inlet flow rate of 2 μl/min. As the angle is increased, the pressure increases, which can lead to channel deformation. In conclusion we have designed and simulated a novel microdialysis probe that is not a concentric tube design like those currently used in most experiments, but rather a relatively small flat microdialysis patch which is able to sample fluid on the surface of wounds in a non-invasive manner and in a reasonable time frame, collecting markers of microbial infection, inflammation and growth factors.