药物洗脱支架在降低支架内再狭窄和靶血管重建方面优于裸金属支架，但是药物洗脱支架的由多因素导致的晚期支架内血栓发生率更高。其中一个重要因素是持续不可降解的多聚物涂层引起局部炎症反应及高敏反应，进一步导致愈合延迟和晚期支架内血栓的可能。

    第一代的药物洗脱支架在药物完全释放后，它的不可降解的多聚物涂层仍然持续存在。与裸金属支架相比，持续存在的多聚物涂层增加了局部血管炎症的可能性。为了和药物相结合和控制药物的洗脱，多聚物涂层通常是疏水性的，这种特性在体内会引起更高的界面张力，进一步导致机体的炎症反应。Cypher™支架就是第一代药物洗脱支架的典范，它的涂层由聚乙烯-乙烯醋酸(PEVA)、聚正丁酯甲基丙烯酸甲酯(PBMA) 混合物和活性药物（西罗莫司）组成。它的顶涂层由不含药物的PEVA-PBMA 混合物组成，用来控制药物的释放以及避免快速释放。Cypher支架置入后一周释放50%的西罗莫司、30天释放85%、90天完全释放。Taxus™支架是另一种第一代药物洗脱支架，它的碳氢化合物涂层使得药物在支架置入后2天内快速释放、10天内低水平释放、30天后就停止释放。

    最近研发的新一代药物洗脱支架主要着重于改善多聚物的生物相容性，通过应用可降解的多聚物和无多聚物涂层来提高支架的安全性。Endeavour Resolute支架携有三种独特多聚物构成的涂层： 疏水的 C19多聚物、水溶性的聚乙烯醇 (PVP) 和疏水的 C10多聚物。它模拟机体的生化特性，具有高度的生物相容性，最大限度了降低了宿主的排异反应。 可降解的多聚物涂层也是一个非常诱人的理念：通过正常的机体代谢途径，经过一定的时间，多聚物最终降解成小分子物质排出体外，在局部没有任何残留，消除了潜在的多聚物对血管的不良反应。EXCEL™支架 (吉威， 中国)就是这样一种典范：一种可降解的聚乳酸涂层的西罗莫司洗脱支架。这种多聚物在支架置入后6个月后完全降解成水和二氧化碳。EXCEL支架的涂层只限于血管壁侧，最大限度降低了多聚物对血管愈合的不良反应，同时使得药物呈单一方向的释放。中国牵头的几个注册研究证实了该支架的安全性和有效性。

    关于药物洗脱支架多聚物涂层技术最新的焦点集中在完全取消多聚物的应用。目前有两种支架： 无多聚物涂层他克莫司JANUS支架和 CoSTAR支架。两种支架都应用凹槽技术来装载药物，而不是象第一代药物洗脱支架那样，将药物涂在支架的表面。  JANUS支架完全不使用多聚物涂层； CoSTAR 支架应用聚乙醇酸共聚物 (PLGA)， 是一种具有专利的可吸收的多聚物，但局限于装载药物的支架凹槽内，整个支架表面是没有多聚物的。另一种令人激动的正在研发的无多聚物支架 是BioFreedom支架。

    药物洗脱支架技术的发展是一个持续性的进程，不断发展的聚合物是其最重要的特点。无论是研发生物可降解多聚物涂层，还是无多聚物涂层技术，药物洗脱支架远期的安全性需要进一步临床研究的证实。

英文：

    Drug eluting stents (DES) have been shown to be effective in reducing in-stent restenosis and need for target vessel revascularization compared with bare metal stents. However it is known to be associated with higher incidence of late stent thrombosis for which the cause is multifactorial. One of the purported cause of stent thrombosis is the presence of permanent nonbiodegradable coating polymer on the stent surface that can elicit inflammatory response and local hypersensitivity reaction， leading to delayed healing and possible late stent thrombosis.

    The first generation DES are based on biostable polymers that remain in the body well after the drug is exhausted. The unabated presence of these polymer coatings after completion of drug elution process increased the possibility of ongoing vascular inflammation compared with bare metal stents. They are also frequently hydrophobic to obtain uniform drug coating and diffusion controlled elution， and hydrophobicity is known to cause higher inter-facial tension in the aqueous body environment resulting in a foreign body inflammatory response.  The Cypher™ stent is the prototypical example of DES which employed a layered coating， consisting of a copolymer of polyethylene-co-vinyl acetate (PEVA) and poly-n-butyl methacrylate (PBMA) which carries the active agent， sirolimus; and a top thin， drug-free PEVA-PBMA coating that serves to control drug release and prevent burst effect. The Cypher stent releases 50% of its sirolimus content during the first week after implantation， 85% over 30 days and 100% by 90 days. The Taxus™ stent， another first generation DES， is coated with a hydrocarbon-based copolymer which provides a burst release of 2 days， followed by lower-level release for 10 days and no further release after 30 days.  

    The development of new generation DES in recent years have focused on improving the biocompatibility of the polymers， the use of biodegradable polymer and nonpolymeric drug coatings on the stents to improve its safety profile. The Endeavour Resolute stent carries a unique blend of 3 polymers which has a hydrophilic C19 polymer， water soluble polyvinyl pyyrolidinone (PVP) and a hydrophobic C10 polymer. It is designed to mimic the biological chemistry of the body and hence highly biocompatible， with minimal host response elicited. The use of biodegradable polymer for drug delivery is a very attractive concept as the polymers are broken down into molecules that are metabolized and removed from the body via normal metabolic pathways， leaving no residues after a period of time. That will potentially eliminate any adverse effects that the polymers may cause on the vessels. The EXCEL™ (JWMS， China) stent is one such good example in which the sirolimus-eluting stent is coated with a biodegradable polylactic acid (PLA) material. The polymer is degraded into water and carbon dioxide within 6 months of implantation. The abluminal coating of the EXCEL stent minimizes the possible adverse effects of polymer on vascular healing while allowing controlled unidirectional drug release. Several registry studies performed in China have demonstrated the safety and clinical efficacy of this stent.

    The latest focus now in DES polymer technology is to do away with the use of polymer completely. Two of such stents are the JANUS Carbostent， a nonpolymerised stent coated with tacrolimus， and the CoSTAR stent. Both stents employ reservoirs where drug can be loaded， rather plastered over the entire outer and inner surface area of the stent， as was done with the first generation DES.  While the JANUS uses no polymer at all， CoSTAR uses polylactic-co-glycolic acid (PLGA)， a proprietary bioresorbable polymer， but only within the small wells embedded in the stent where the drug is loaded， leaving the actual surface of the stent polymer-free.

The evolution of DES technology is a unrelenting process and is best evident by the changing landscape of polymer development. Whether the development of biodegradable polymer or nonpolymeric coating adds to the long term safety of DES will need to be validated with more clinical studies.