| Skin defect is the common tissue damage or disease in clinical. The small case is onlya slight wound, but the serious case can cause infection, water electrolyte disturbance,immune dysfunction, multiple organ failure, or even fatal death. The emergence oftissue-engineered skin for the treatment of skin defects, especially large area defects,provide an effective method. Tissue-engineered skin is the so-called construction of tissueengineering artificial skin substitute in order to achieve the role of temporary or permanentskin substitute itself. Bioreactors used in tissue-engineered skin product process isconducive to cell growth, differentiation, which can maintain the activity of the cells, andhelp cells to excrete extracellular matrix and reform scaffold composite. Bioreactors, bycontrolling the cell growth conditions and the stimulation of mechanical stress, can makecells grown in three-dimensional scaffolds, improve the efficiency of mass organization andmature, elevate the rate of industrialized production. Due to each type of tissue engineeringproduct has its own particularity, a tissue engineering product may require one or morecorresponding bioreactor system. Skin is on the surface of body, which only endures therelatively small mechanical stress, while the keratinization formation of the epidermis isneed to contact with air directly. It decides the special requirements of tissue-engineeredskin culture, that is, the gas-liquid interface and low shear stress. In order to guaranteeuniform supply of nutrition, it is also needed to construct a uniform flow field. Under thecondition of perfusion with these requirements, it’s only possible on the condition of aunified flow field under low flow rate. So we propose a new hypothesis that "thequasi-static plane flow field is the best culture mode of tissue-engineered skin"to meet thesespecial requirements of skin tissue culture. This idea will be to solve the disadvantage oftraditional reactor such as non-uniform flow field, larger volume and less limited culturespace. Bioreactors are commonly used following as: stirred, rotary, airlift and perfusionbioreactor. Among these devices, the advantages of perfusion bioreactor are moreoutstanding. Fresh culture medium is continuously added into the culture chamber, whilethe consumption of medium containing metabolic waste is constantly dropped out. Thisculture mode allows cells to grow in a relatively stable environment with a betterproliferation, which namely saves time, improves effort, reduces the chances of cellproduce pollution and increases the cell density of more than10times. But the perfusionbioreactor still exists smaller specific surface area, complex structure, non-uniform flowfield and poor extensibility. The non-uniform flow field makes the nutrient transfer limitedand distribution unevenly. The liquid in the scaffold flows uneasily, that is to say, it is easyto cause the cell death, meanwhile the different shear force in flow plays adverse impactson the cells, such as the cell growth, proliferation, differentiation In this subject wedesigned a new type of the tissue-engineered skin perfusion bioreactor which increases thespecific surface area, reduces the size and the line, and simplifies the structure. So it is easyto be operated and maintained for meeting the requirements of gas-liquid interface culture,lower shear force and uniformly flow field. This device prove efficiently to supply nutritionand exchange oxygen. It has strong extensibility, variable size and flexible layers. It can notonly be used for the basis research of laboratory, but also be used for industrial equipment.Computational fluid dynamics(CFD) is a method using computers combined withnumerical discretization to simulate and analyze the problem in flow. CFD can be also usedto analyze the flow distribution in the bioreactor, and calculate the fluid shear stress of thesurface inside the three-dimensional scaffold materials as well as fluid flow rate, pressure,and oxygen distribution. CFD, as a new and an effective means of test and analysis, isincreasingly applied in bioreactor design requirements and cell culture studies. This topicbased on the concept of quasi-static plane flow field in the perfusion type culture will beused to design a new type of bioreactor, in order to solve some deficiencies in otherbioreactors. It meets the basic requirements of tissue-engineered skin gas-liquid interfaceculture and low shear force, in large-scale cultivation. Using CFD, the flow field and thewall shear stress were studied to obtain the optimal structure and size. These results will beprovided an experimental platform for future application of tissue-engineered skin culturein the clinical transplanted. Objectives:According to ours hypothesis "quasi-static plane flow field is the best culture mode for tissue-engineered skin", a new perfusion type bioreactor of quasi-static plane flow field was designed in order to meet the special requirements of skin tissue culture. That is suitable for gas-liquid interface culture and low shear force, which is suitable to large-scale culture.The shear stress in the flow field of new bioreactor and on the surface of mesh scaffold and the tissue-engineered skin was simulated by CFD software to obtain the optimal structure and size of new type perfusion bioreactor platform.Methods:1. New type perfusion bioreactor of tissue-engineered skin was designed and preliminary simulated1.1.2D and3D numerical simulation of skin culture section in the new type bioreactor by CFD software FLUENT.1.2. The numerical method1.2.1The governing equation:Due to the low medium velocity, the maximum value was only about100ml/min. So the flow of the medium can be regarded as incompressible fluid laminar flow, the control equation is for:Continuity equation:(?)-u=0(1)In equation:u(x,t) as the velocity vector; p as the pressure; p for medium density; υ for the dynamic viscosity.In bioreactor gas-liquid interface exists, that is to say, there are two phase flow in the reactor. So it needs to start two phase flow model in the simulation. This article uses the VOF model to simulate gas-liquid two phase flow of the interface. VOF model tracking by solving a phase or multiphase is the interface between the volume ratio of continuous equation of (αq). For the first q phase, the equation is for:Inequation:mqp is the mass transfer of the q phase to the p phase; mpq is the mass transfer of the p phase to the q phase.The main phase volume ratio (αq) is calculated based on the following constraints:1.2.2Boundary conditions and some important parameters are:Sets the imports as the entry flow speed; exports set as the free flow; other is set to no slip wall boundary.Because of the gas-liquid interface, it needs to set up physical parameters of gas phase and liquid phase. Important physical parameters of gas phase air:density is1.225kg/m3; dynamic viscosity is1.7894×10-5Pa·s Important physical parameters of liquid culture medium are as follows:density is998.2kg/m3; dynamic viscosity is1.003×10-3Pa·s2. Numerical simulation and structure optimization of new type perfusion bioreactor of tissue-engineered skin2.1. By using CFD software FLUENT, the high weir in different shapes, different altitude and different slope was simulated to get the optimal flow characteristics, and the internal flow field under different flow in the chamber was numerically simulated after the optimization.2.2. The numerical method as above.Results:1.According to the hypothesis tha"quasi-static plane flow field is the best culture mode for tissue-engineered skin" we designed a new kind of perfusion bioreactor, it could be stacking and founds the quasi-static plane flow field to meet the culture requirements of the tissue-engineered skin.2.The results of numerical simulation from the whole flow field of culture chamber within new perfusion bioreactor showed that the discharge flow from high weir was less impact. Therefore the skin culture chamber and high weir are respectively numerically simulated. For different working conditions (mainly considering the two important factors including the screen width and height) of skin culture chamber, the results showed that among the working condition of numerical calculation the shear stress on the surface of the skin construct and mesh is less than3dyn/cm2in all the skin culture chamber, which met the requirements of tissue-engineered skin culture. 3.The different shapes, different altitude and different slope of high weir weresimulated and analyzed. The results showed that when the discharge part of high weir wasflat, the height was10mm and12mm, the angle as20°,the discharge field of high weirhad good flow characteristics.4.Under different flow rate after the optimization platform (the high weir dischargepart was plane, height was10mm or12mm, angle as20°), the numerical simulation ofinternal flow field was found that the smaller flow rate (50ml/min and60ml/min)existedcutoff flow in the two kinds of height weir, but the larger flow rate (80ml/min and100ml/min), did not exist cutoff flow.5.According to these results of numerical simulation, we got optimal structure and sizedata of the perfusion bioreactor.Conclusions:1.We propose a hypothesis that"quasi-static plane flow field is the best culture modefor skin tissue engineering culture". It implies a kind of low shear force and uniform planeflow field, is found in order to meet the special requirements of skin construct tissueculture.2.We design a new type perfusion bioreactor that may be widely used in production ofskin tissue engineering research in future. |
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