Thermoelastic stress analysis of vascular devices

The research described in this plenary paper deals with the application of experimental techniques to medical devices used in the treatment of vascular disease. Vascular disease is a medical condition where fatty material narrows the artery. It is reported that over 4 million deaths can be attributed to the disease each year in Europe alone. Vascular disease can occur at locations throughout the arterial network. A blockage located in the carotid artery can cause what is known as a stroke, or if located in one of coronary arteries may lead to a heart attack. A major advance in the treatment of the disease was made when the Percutaneous Transluminal Coronary Angioplasty (PTCA) was introduced by Gruntzig [5] in 1977. The procedure offered a non-invasive, cost effective and rapid treatment for cardiovascular disease and was soon adapted to treat diseased vessels elsewhere in the body. During a typical procedure access to the vascular system is gained via the femoral artery at the groin. From this point a cardiologist uses endovascular techniques to navigate a catheter, with a polymer balloon deflated and tightly wrapped around its tip, through the artery network to the site of the blockage. The balloon is positioned across the blockage and is momentarily inflated. As the balloon inflates it exerts a radial force on the accumulated atherosclerotic plaque material, displacing it and thereby restoring patency to the vessel. Angioplasty balloons are used at locations throughout the arterial network and therefore range in size depending on the intended site of operation. They are typically constructed from rigid polymeric materials and are designed to exhibit low compliance at inflation high pressures; the balloon material is anisotropic. Fig. 1 shows a commercially available balloon. Studies have found the incidence of restenosis post angioplasty to be as high as 30 - 50%. To combat thisa fine mesh, metallic, cylindrical component known as a intravascular stent may be implanted as a follow up procedure to the angioplasty. The device is permanently implanted and acts as a scaffold within the artery (see Fig. 2). Two principal categories of stents exist: balloon expandable and self-expanding. The former are plastically deformed into position using an angioplasty balloon and are typically constructed from a stainless steel alloy. The latter, self-expanding stents (see Fig. 2) are deployed from the tip of a catheter and expand elastically into position, to provide support to the vessel wall. Self-expanding stents are constructed from a NiTi alloy commonly known as Nitinol. Nitinol is classed as a superelastic material as it can elastically recover strains in excess of 8% via a stress induced (reversible) transformation from a parent austenite microstructure to a martensitic phase. The stress-strain curve follows a non-linear path with significant thermal variations occurring as the material undergoes the phase change. Thermoelastic Stress Analysis (TSA) [1] is a non-contacting technique that provides full field stress information and can record high-resolution measurements from small structures. The work presented in this paper summarises the application of TSA to angioplasty balloons and Nitinol vascular stents carried out at the University of Southampton [2-6] and identifies the significant challenges in apply the technique to such devices. The use of high resolution optics is described along with a calibration methodology that allows quantitative stress measurements to be taken from the balloon structure. The paper then describes new work that assesses the effect of geometrical variations on balloon performance. Typical data from a balloon is shown in Figure 3. A brief account of a study undertaken to characterise the thermoelastic response from Nitinol is also included and it is demonstrated that thermoelastic data can be obtained from a stent at high resolutions (see Figure 4). Further work on stents conducted at body temperature is described