Simulation of Mass Transfer Phenomenon in a CAD Drug Eluting Stent System

Coronary artery disease is the most common type of heart diseases and the leading cause of death worldwide due to heart disease. It occurs when the arteries that supply blood to the heart become narrowed or blocked by a buildup of cholesterol and other material at the inner wall of the artery. Limitation of blood flow to the heart causes ischemia of the myocardial cells. Myocardial cells may die from lack of oxygen and this is called a myocardial infarction - or more commonly: a heart attack. Treatment options include medication, surgery or catheter-based procedures. Several types of catheter-based procedures are available. During balloon angioplasty, a special balloon catheter is passed into the narrowed segment of the artery and expands the balloon, which thus opens the artery and compresses the blockage against the wall of the artery. Stents are very small metal mesh-tubes that can be inserted via a balloon catheter into the narrowed segment of the artery. When the balloon is inflated, the stent expands and is embedded into the artery vessel wall, which thus opens the previously narrowed segment of artery. The balloon is then deflated and removed along with the catheter, and the stent is left behind to serve as a metal framework for the artery. In case of drug eluting stents a certain amount of anti-flammating drug is loaded in the coating over the base stent. This drug is released at the wall of diseased artery so that restenosis cannot take place at the place of artery where the stent has been implanted. In this thesis a drug eluting stent was studied where there was a biodegradable coating over a bare metal stent in which there was some amount of therapeutic drug. The degradation of the biodegradable coating layer thickness was determined with respect to time which was actually representing the remaining drug concentration in the coating layer. Then using this variable drug concentration as the drug concentration at initial tissue layer concentration profile of drug in tissue layer with respect to time and position was determined using finite volume algorithm, where this algorithm was coded using MATLAB programming language.

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Chapter Three:
OBJECTIVE WITH SPECIFIC AIMS AND RESEARCH SIGNIFICANCE:
3.1 Objectives of Study with Specific Aims:
Coronary artery disease is the most common type of heart diseases and the leading cause of death worldwide due to heart disease. The drug-eluting stents is generally considered as the best catheter-based therapy for coronary artery disease. However, the mass transfer of the drug from DES to the tissue is not fully understood till date. Hence, the main objective of this thesis is to simulate the overall mass transfer phenomenon occurs in CAD-DES in the treatment of coronary artery disease. When a biodegradable coating (containing drug at the porous place of the coating) is given on the main structure of the stent, then this coating is degraded with time and drug releasing property from the stent also become changed. With degraded coating, the diffusion resistance of the drug is changed. At the same time the characteristic diffusion length and drug concentration also become changed. Thus, the thesis work focus on the change in coating resistance as well as the equilibrium drug concentration with respect to time and drug concentration changing due to changed equilibrium drug concentration with respect to time and position (radial) in the artery wall. These changes are observed by solving two partial differential equations for concentration and coating thickness where initial and boundary conditions are assumed. The simulated model is intended for the following specific aims:
- To show the decay of drug coating thickness (including the change in concentration of therapeutic drug in the coating) with time.
- To show the change in drug concentration in the artery tissue layer with time both in radial and longitudinal directions (i.e., a two-dimensional concentration profile).
- The model will finally be generalized incorporating an extra-component of drug mass transfer from the drug coating through the bare stent into the external blood flow in artery channel.
3.2 Research Significance:
The main target of this research work was to develop a model which will be able to predict drug mass transfer in CAD-DES. The significance or importance of this model has been briefly indicated below:
- The time taken by drug coating to decay completely can reasonably be determined using the simulated model. This will help to determine the safe working period of a drug eluting stent (DES). The model can be employed to study the effect of the properties of DES on their safe working period.
- The distribution of therapeutic drug molecules in the artery tissue along axial and radial directions after the release from stent can be studied using the simulated model. This will greatly help to evaluate the therapeutic performances of the various DES.
- The time taken by therapeutic drug molecules to cover the entire artery tissue can also reasonably be determined from the simulated model. This will help to evaluate the activity fastness of a DES.
- The simulated model can also be used to optimize several parameters related to DES and drug concentration profile.
Overall, the design, operation and performance of DES-based angioplasty would be benefited immensely from the present study.
Chapter Four:
RESEARCH METHODOLOGY:
Mass transport refers to the movement of mass, i.e. the species of interest which is drugs in the case of a DES, within a defined system. This transport of species may be provoked by concentration gradients between two points, but quite often in systems, especially in the vasculature, overpowering complex flow dynamics will ultimately be responsible for the mass transport outcome. In the absence of a free flowing system the presence of these concentration gradients induces diffusion, e.g. between the DES and the artery wall. Mass transport can be broken up into two types within the human vasculature. Firstly blood side mass transport (BSMT) refers to species transport within the vessel lumen and is su