Proceedings of SerbiaTrib '13

Energy absorption during impact or loading isinherent property of the foams. Fig. 1 shows stress–strain behavior of the foams in compression, ingeneral [1]. From Fig. 1, it can be seen that threezones exists: I. Elastic region: deformation of the porewalls; II. A plateau of nearly constant flow stress andlarge strain (10–50%) and III. Densification regionwith steep increase of flow stress where the plasticdamage occurs. Relatively wide region of constantflow stress during compressive loading explains thefact that while foams are in this interval, anyincreasing strain hardly entails increasing stress. Iftension is observed, the stress–strain behavior offoams is approximately similar to ductile metals(curve b in Fig. 1).Figure 1. Stress - strain behaviour of foams.3. MAGNESIUM ALLOYS ASDEGRADABLE IMPLANTSThe application of Mg and its alloys fordegradable implants started with ligatures for bloodvessels (Huse, 1878, pure magnesium) and plates,arrows, wire, sheets, rods (Payr, 1905, puremagnesium) [2]. Mg alloys have been tried indifferent medical areas, such as: pure Mg for band,suture from woven Mg wires, fusiform pins (in1940); Mg–Al2%-wt. pure magnesium wires forclotting aneurysms (dog studies in 1951); Mg–Al2%-wt. for intravascular wires (rat studies in1980) [2]. The potential of magnesium alloys asbioabsorbable / biodegradable implants forbiomedical applications has been extensively studiedas emerging direction. Research activities related tobiomedical magnesium alloys have been pursued intwo main directions, orthopedic and cardiovascularimplants, by investigating different aspects ofalloying system design, novel structures, degradationrate control, and surface modification methods.Magnesium alloys are currently considered forapplications as load-bearing implant devices suchas plates, screws and pins for repairing bonefracture. Other metals currently used for boneimplants, such as stainless steels and titaniumalloys, have elastic modulus that are much higherthan natural bone, leading to unwanted stressshielding. The elastic modulus of magnesium andmany magnesium alloys are much closer to bone.The advantage of Mg alloys is favorable elasticresponse during loading (such as shown in Fig. 1).Also, the second surgery is avoided due to thedegradation of the implant after its function in thebody is finished. For example, compared with poly-96L/4D-lactide, the magnesium alloys AZ31 andAZ91 enhanced the osteogenesis response andincrease newly formed bone [4]. Investigationsshowed that Mg–6Zn, Mg–Ca and Mg–Mn–Znalloys gradually degrade within a bone and hadgood biocompatibility both in vitro and in vivo [4].Highly important direction of research isdegradable coronary stents. Degradable vesselstents promote stable vessel regeneration, unlikepermanent stents [5, 6, 7]. However, as a vesseldefect gets larger, stronger and degradablematerials are paramount for stable vesselregeneration. Vessel scaffolding is necessary onlyfor a certain, limited time, than the permanent stenthas no known advantage. A stent is a miniaturemesh tube, made of a biocompatible metal,biodegradable metal or polymer, placed inside of ablood vessel (cardiovascular, neurovascular andperipheral blood vessels) or a natural conduit(gastrointestinal, urinary and biliary tracts). Thestent acts as a scaffold, pushing against the internalwalls of the conduit/vessel to open a blocked areaand thereby enables natural flow and prevents thevessel from collapsing, narrowing or closing. Stentsdiffer greatly in their design, dimensions andmaterial, depending on application. Coronary stentsare now the most commonly implanted medicaldevice for angioplasty, with more than 1 millionimplanted annually. Currently used metallic stentspermanently remain in the artery and are associatedwith limitations such as continued mechanicalstress, transfer to the tissue, and continuedbiological interaction with the surrounding tissue.Also, within 6 months, 30-35% patients suffer fromrestenosis. They are associated with late stentthrombosis and artifacts when non-invasivetechnologies such as MRI and MSCT are used. Thestent presence is required for a period of 6 - 12months during which arterial remodelling andhealing is achieved. After this period the stentpresence within the blood vessel cannot provideany beneficial effects. With the development ofbiodegradable implants, the concept of biomaterialshas shifted from purely mechanical replacementdevices towards true biological solutions. Bioabsorbablestents (Fig. 2) aim to mechanicallyprevent vessel recoil without the permanentpresence of an artificial implant. The advantages ofbioabsorbable stents are to leave no stent behind,94 13 th International Conference on Tribology – Serbiatrib’13