Biodegradation Behavior And Biocompatibility Study Of JDBM Alloy For Potential Vascular Stent Application

Magnesium alloy based vascular stent is a promising candidate for the next generation biodegradable stent.The distinct advantages of biomedical magnesium alloy vividly described as“Fulfilling the Mission and Stepping Away”include:elimination of permanent irritation to blood vessel and blood flow;possibility of positive vessel remodeling;restoration of vasomotor function;feasibility of surgical revascularization for the stented target lesion,etc.The PROGRESS-AMS,BIOSOLVE-I and BIOSOLVE-II clinical trials proved the safety profile of magnesium alloy stents and gave promising results.However,some important questions remain unanswered.Our group has successfully developed a patented Mg-Nd-Zn-Zr alloy?abbr.JDBM?with good mechanical properties and uniform in vitro degradation behavior.In this dissertation,the in vitro cytocompatibility of JDBM was firstly evaluated as part of the biocompatibility evaluation.Results showed that JDBM had no adverse effect on the viability,cytoskeleton and apoptosis of primary human umbilical vein endothelial cells?HUVEC?.Besides,JDBM showed no adverse effect on viability,adhesion morphology and apoptosis of A7r5 smooth muscle cells.Degradation rate is one of the most important parameters of biomedical magnesium alloys.It determines not only the reduction speed of mechanical properties but also the increase of local pH and ion concentration of alloying elements,which can further affect the biocompatibility.However,the corrosion rates obtained from in vitro and in vivo tests are often inconsistent.This difference is caused by many factors,including pH buffering system,body fluid flow,concentration of inorganic ions,organic molecules,and cells,etc..Among these factors,the influence of cells on the in vitro degradation behavior of biomedical magnesium alloys has been rarely studied.In this dissertation,the influence of proteins,HUVEC and macrophages on in vitro degradation of JDBM biomedical magnesium alloy was investigated.Results showed that proteins could slow down the degradation rate of JDBM,while HUVEC could eliminate proteins'protection effect and accelerate the degradation rate of JDBM.Macrophages could also accelerate the in vitro degradation of JDBM in a uniform way via the increased secretion of reactive oxygen species?ROS?.Compared with biodegradable polymer stent,the complete biodegradation profile of magnesium alloy vascular stent and its degradation products remain unclear.Previously published reports showed that magnesium alloy stent struts were in situ replaced by degradation product composed of Ca,P and O.This aroused clinicians'worry about the risk of vessel calcification.Extended follow up examination of the degradation product is needed to answer the question“Is the degradation product degradable?”Besides,the final distribution of alloying elements is worthy of being investigated to see if element enrichment and persistent existence in organs happen.In this dissertation,implantation into carotid artery of New Zealand white rabbit was chosen as the animal model to investigate the safety,efficacy and in vivo degradation behavior of JDBM vascular stent.The longest follow up time was 20 months.Results proved the excellent safety profile of JDBM stent without the occurrence of acute,late or very late stent thrombosis.The liver and renal functions were normal after 1 month,4month and 12 month implantation.Reendothelialization of JDBM stent was completed after 4 week implantation,which is similar to the time course for reendothelialization of 316L stainless stent.Some corners and links of JDBM stent broke possibly due to stress corrosion cracking after 1 month implantation.The lumen area significantly decreased after 4 month and 12 month implantation as compared to that with 1 month implantation,while the neointimal area remained similar.The complete degradation time of Mg was estimated to be 3 month.The degradation product converted from Mg?OH?₂ and Mg phosphate to Ca phosphate with high Ca content,then to Ca phosphate with low Ca content.The time needed for complete conversion and metabolism of Ca phosphate was estimated to be 26 month.Thus,there is no potential risk of vessel calcification in the long term.There was no element enrichment in the rabbit organs and blood for Mg and Zn,while the concentration of Zr and Nd was higher in spleen/liver and spleen/liver/lung after 20 month implantation than the negative control?no stent implantation group?,respectively.No continuous element enrichment of Zr and Nd was observed in all organs.Part of Nd retained in situ in the blood vessel in the form of Nd?OH?₃ or Nd phosphate.Part of the JDBM vascular stent degradation products were broken into particles with a diameter of several microns by the vasoconstriction and vasodilation in the late stages of in vivo degradation.Whether these particles stay in situ permanently with the risk of causing chronic inflammation or can be metabolized or excreted needs to be investigated.As a preliminary study,the in vitro response of macrophages to JDBM degradation product particles was investigated.An electrochemical method was used for fast preparation of Mg alloy degradation products.The conventional immersion method produces a firmly adhered thin degradation product layer,which is difficult to be peeled off and cannot provide enough amount of degradation products for cell culture experiments.An electrochemical method was developed for fast preparation of JDBM degradation products with similar element composition to those prepared by immersion method.In vitro phagocytosis of JDBM degradation product particles with diameter of several microns was observed.Macrophage phagocytosis may be one possible route for the conversion and metabolism of degradation products of Mg alloy stent.