Issue 29

L. Petrini et alii, Frattura ed Integrità Strutturale, 29 (2014) 364-375; DOI: 10.3221/IGF-ESIS.29.32 365 K EYWORDS . Bioresorbable alloys; Degradable scaffolds; Polymeric coating; Computational studies; Experimental validations. I NTRODUCTION n the last years stent implantations (weather coronary stent or not) have steadily grown both in terms of number and of variety of implantations (type of blood vessels involved). As a consequence, a reduction in the number of surgical operations, which are more invasive for the patient (as, for example, the coronary by-pass surgery), was observed. As known, a bare metal stent (BMS) is a metallic mesh (obtained by laser cutting of a tube made of stainless steel, titanium or cobalt alloy) that is designed to expand a stenotic blood vessel (that is a vessel clogged by atherosclerotic plaque), thus enhancing the blood flux in the downstream vessel. During the implant the stent is fixed on the ending part of a catheter, which consists of an "inflatable balloon". The catheter is inserted in an artery through a skin cut. Once in position the balloon is inflated with a consequent dilatation of the stent, which exerts a radial force on the stenotic blood vessel. Then, the balloon is deflated and the catheter extracted: due to the permanent deformation, the stent remains in place and withstands the natural contraction of the blood vessel. Once in the vessel, the stent is enveloped by a new tissue, called neointima. It has been observed that in some cases the neointima proliferates in an uncontrolled way (hyperplasia) and tends to re-obstruct the vessel, thus vanishing the effect of the implantation. To avoid the neointimal proliferation, drug eluting stents (DES) have been introduced in the clinical practice from the beginning of 21st Century. They are metallic stent coated by a polymer which locally releases an anti-inflammatory drug. Recent studies claimed that in some cases DES may lead to the generation of thrombi after a long period (late stent thrombosis), which are extremely dangerous for the patient [1,2]. Moreover, they prevent late vessel adaptive or expansive remodeling, and hinder surgical re-vascularization. For this reason, starting from the clinical consideration that vessel scaffolding is only required transiently [3], a new generation of stents is under study [4]. It consists of bioresorbable scaffolds (BRS): the stent should remain in the body for the time required to support the vessel tissues, then it "vanishes" as it is dissolved in the human body. In this way the development of inflammatory phenomena (the reason for the hyperplasia of the neointima) is largely reduced, therefore avoiding the risk of restenosis. Moreover, thanks to the fairly long lasting mechanical action, the blood vessel is remodeled and remains opened also after the stent is fully dissolved. Moreover, the vessel can recover the natural vasomotor response, reintervention is facilitated, the dual anti-platelet therapy is shorter, the risk of strut fracture-induced restenosis is reduced and mechanical instability of the device when the vessel grows in size in pediatric patients is avoided. From the side of bioresorbable materials, they are required to be biocompatible, degradable at an appropriate rate, easy to manipulate, sterilisable and storable. From the side of bioresorbable scaffolds, different properties are required corresponding to the different phases of the vessel treatments. In the first phase of artery revascularization, BRS have to mimic DES, so they have to show good deliverability, flexibility, ductility, minimum recoil, high acute radial strength. In the second phase of restoration of the vessel natural vasomotor response, they have to perform the transition from active support to passive implant, showing structural discontinuity and radial strength loss. In the last phase of resorption, the passive implant has to benignly disappear. Up to now biodegradable stents made of polymers or magnesium or iron alloys have been proposed [5,6]. However, all the solutions have some limitations. The polymers have low mechanical properties, which lead to devices that cannot withstand the natural contraction of the blood vessel: the restenosis appears just after the implant, and can be ascribed to the compliance of the stent. The magnesium alloys have much higher mechanical properties than polymers. Unfortunately, they dissolve too fast in the human body: the duration in a blood vessel is about few weeks and not 6-10 months, as should be to withstand the vessel remodeling. Lastly, the iron alloys degrade too slowly [7,8]. In this work we present some results of an ongoing study aimed at the development of a hybrid bioresorbable stent (HBS) made of a magnesium alloy that is coated with a polymer having a high corrosion resistance. The mechanical action on the blood vessel is given by the magnesium stent for the desired period, being the stent protected against early onset of corrosion by the coating. The coating will dissolve in a longer term, thus delaying the exposition of the magnesium stent to the corrosive environment. We dealt with the problem exploiting the potentialities of a combined approach of experimental and computational methods (both standard and ad-hoc developed) for designing magnesium alloy, coating and scaffold geometry from I