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Mechanics in Medical Implant Industry
The major challenge in medical implant industry is the knowledge about human body. Had we know the human body and its functions better, we can make better and reliable implants. Below are two examples that I have learned.
Let's start from stent, a small, lattice-shaped, metal tube that is inserted permanently into an artery. The stent opens the narrowed artery so that an adequate supply of blood can be restored. See this FDA site for further detail.
Stent has revolutionized the treatments for cardiovascular disease and the interventional system. However, stent fractures are commonly observed in-vivo in the past years and has become a concern for patient wellness and therefore a challenge/opportunity for mechanical engineering. Both the engineering and the medical care societies have to work together to solve this issue. It is very surprising that little publications are available to study the key issues such as artery deformation, motion, its mechanical properties and its variations among patient age, race, and other factors. As a result, current stents, even they have been proven to be lifesavers for many patients, they are not necessarily a satisfactory product for a mechanical engineer. We can not wait for the medical care society to give us the information because they often concern and focus on different issues than us. In addition, they can not work alone to come up with the necessary equipments. Therefore, we need proactive to interact and help each other to get what we want. The day we know our interventional system better is the day that we can make better stents because stents can only be as good as our knowledge to the interventional system.
My next example is replacement heart valve. Mechanical heart valve is a very successful medical implant and is very well engineered from design, processing all the way to quality control. It is amazing how a ceramic material, i.e., pyrolitic carbon could be engineered to make a long lasting, great functional mechanical heart valve. However, there are also issues such as leaflet fluttering, backflow and hinge thrombosis that can eventually lead malfunction of the valve leaflets. The sound when leaflet closes also bothers some patients. The medication for thinner the blood is also made the patient life inconvenience and subjected to other threatens such as internal or external bleeding. Therefore as alternatives, flexible heart valves are developed, yet the durability is an issue there.
I hope these two issues convince you that the roles of mechanical engineering are different in medical implants from traditional industry. In latter case, often a design space is known or anticipated. In medical implant industry, to date, it is unfortunate that many key issues are answered only during clinical studies at the potential cost of the patient lives. We have to develop and constantly improve the design space for medical implants and develop the associated in-vitro tests in the lab. Our ultimate goal is to break the parts in the lab to prevent them from fractures inside human body.
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Response of stents
I wrote a couple of papers in the Journal of Applied Mechanics a few years back on analytical and experimental evaluation of the response of a particular type of braided stent. Using these results to evalute the coupled response of stent and artery is a simple exercise, given the arterial response. Abstracts of the papers and links are given below.
Ravi
Paper 1: The mechanical response of a metallic stent is considered in this series of two papers. In Part I, the development of a test method for the characterization of the mechanical response of a metallic aortic stent subjected to internal or external pressure, and a model that captures the relationship between the pressure and diameter of the stent based on slender rod theory are described. The axial and radial deformation of a bare-metal stent were measured as the stent was subjected to loading ranging from an external pressure of about 80 mm of Hg to an internal pressure of about 160 mm of Hg. The pressure was applied using a polyethylene bag; the method of applying the pressure and measuring the strains was found to provide an accurate determination of the mechanical behavior of the stent. The stent was shown to exhibit two stiff limiting states corresponding to the fully collapsed and fully expanded diameters and an intermediate range between the two where the stiffness was an order of magnitude smaller than the typical stiffness of an aorta. A complete mathematical characterization of the pressure-diameter response of the wire stent was also developed; this model is a straightforward application of the theory of slender rods to the problem of the stent. Excellent agreement with the experimental measurements is indicated, opening the possibility for modeling of the coupled response of the stent and the vessel into which it is inserted. In Part II, we consider the effect of variations of pressure over the length of the stent that introduce changes in the diameter along the length of the stent which leads naturally to the formulation of the coupled problem of the stent within the blood vessel.
Paper 2: The main objective of the paper is to develop the mathematical analysis of the response of a metallic stent subject to axisymmetric loads over its length and to different boundary conditions. These situations introduce bending stresses in the stent and cannot be captured by a model of the stent that can be used to characterize the pressure-diameter relationship under axially uniform loading. The analysis presented here is based on an analogy between a thin-walled pressure vessel and a beam on elastic foundation; in the present application, we derive an equivalent beam model for the bending response of a stent. Using this model, we evaluate the shape of the stent exiting the catheter as well as the variation of the diameter along the length of the stent constrained by stiff end supports. This approach can be used to evaluate the coupled response of the stent and the blood vessel, if the mechanical properties of the blood vessel are known. The coupled problem and its implications in the design of stents are discussed.