in which the stent itself dissolves over time. as of 2009, materials that had been explored for use included magnesium, polylactic acid, polycarbonate polymers, and salicylic acid polymers. resorbable stents have held the promise of providing an acute treatment that would eventually allow the vessel to function normally, without leaving a permanent device behind. one to three or more layers of polymer can be used in the coating, e.g., a base layer for adhesion, a main layer that holds and elutes (releases) the drug into the arterial wall by contact transfer, and sometimes a top coat to slow down the release of the drug and extend its effect. the first few drug-eluting stents to be licensed used durable coatings. the first generation of coatings appear to have caused immunological reactions at times, and some possibly led to thrombosis. this has driven experimentation and development of new coating approaches. the drug is mainly used to inhibit neointimal growth due to the proliferation of smooth muscle cells that would cause restenosis. much of the neointimal hyperplasia seems to be caused by inflammation. hence, immunosuppressive and antiproliferative drugs are used. sirolimus, paclitaxel, and everolimus were previously used for other medical applications and have been included in licensed des. vascular stents are class iii medical devices that are intended for use as a mechanical radial support to enhance vessel patency over the intended design life of the device. device properties and the intended use are dependent on results from extensive testing that involve mechanical tests on universal testing machines. types of mechanical test include bend testing, fatigue testing, radial loading, tensile testing, and torsion testing. depending on the type of vascular stent, its location and intended use, other tests may include crush resistance, kink resistance, corrosion, coating integrity, and more. history main article: history of invasive and interventional cardiology the first procedure to treat blocked coronary arteries was coronary artery bypass graft surgery (cabg), wherein a section of vein or artery from elsewhere in the body is used to bypass the diseased segment of coronary artery. in 1977, andreas grüntzig introduced percutaneous transluminal coronary angioplasty (ptca), also called balloon angioplasty, in which a catheter was introduced through a peripheral artery and a balloon expanded to dilate the narrowed segment of artery. as equipment and techniques improved, the use of ptca rapidly increased, and by the mid-1980s, ptca and cabg were being performed at equivalent rates. balloon angioplasty was generally effective and safe, but restenosis was frequent, occurring in about 30–40% of cases, usually within the first year after dilation. in about 3% of balloon angioplasty cases, failure of the dilation and acute or threatened closure of the coronary artery (often because of dissection) prompted emergency cabgs. charles theodore dotter and melvin judkins had proposed using prosthetic devices inside arteries in the leg to maintain blood flow after dilation as early as 1964. in 1986, puel and sigwart implanted the first coronary stent in a human patient. several trials in the 1990s showed the superiority of stent placement over balloon angioplasty. restenosis was reduced because the stent acted as a scaffold to hold open the dilated segment of artery. acute closure of the coronary artery (and the requirement for emergency cabg) was reduced, because the stent repaired dissections of the arterial wall. by 1999, stents were used in 84% of percutaneous coronary interventions (i.e., those done via a catheter, and not by open-chest surgery). early difficulties with coronary stents included a risk of early thrombosis (clotting) resulting in occlusion of the stent. coating stainless steel stents with other substances such as platinum or gold did not eliminate this problem. high-pressure balloon expansion of the stent to ensure its full apposition to the arterial wall, combined with drug therapy using aspirin and another inhibitor of platelet aggregation (usually ticlopidine or clopidogrel) nearly eliminated this risk of early stent thrombosis. though it occurred less frequently than with balloon angioplasty or other techniques, stents nonetheless remained vulnerable to restenosis, caused almost exclusively by neointimal tissue growth. to address this issue, developers of drug-eluting stents used the devices themselves as a tool for delivering medication directly to the arterial wall. while initial efforts were unsuccessful, the release (elution) of drugs with certain specific physicochemical properties from the stent was shown in 2001 to achieve high concentrations of the drug locally, directly at the target lesion, with minimal systemic side effects. as currently used in clinical practice, "drug-eluting" stents refers to metal stents that elute a drug designed to limit the growth of neointimal scar tissue, thus reducing the likelihood of stent restenosis. the first successful trials were of sirolimus-eluting stents. a clinical trial in 2002 led to approval of the sirolimus-eluting cypher stent in europe in 2002. after a larger pivotal trial (one designed for the purpose of achieving fda approval), published in 2003, the device received fda approval and was released in the u.s. in 2003. soon thereafter, a series of trials of paclitaxel-eluting stents led to fda approval of the taxus stent in 2004. both sirolimus and paclitaxel are natural products, making the drug-eluting stents a specific kind of application totally dominated by drugs directly derived from natural sources. the first resorbable stent tested in humans was developed by the igaki medical planning company in japan and was constructed from poly-l-lactic acid (a form of polylactic acid); they published their initial results in 2000. the german company biotronik developed a magnesium absorbable stent and published clinical results in 2007. the first company to bring a bioresorbable stent to market was abbott vascular which received a european marketing approval in september 2012; the second was elixir which received its ce mark in may 2013. in 2017, abbott pulled its bioabsorbable stent, absorb, from the european market after negative press regarding the device. boston scientific also announced termination of its renuvia bioresorbable coronary stent program as studies showed higher risk of serious adverse events. due to challenges in developing resorbable stents, many manufacturers have focused efforts on targeting or reducing drug-release through bioabsorbable-polymer coatings. boston scientific's synergy bioabsorbable polymer stent has been shown potential to reduce length of dual antiplatelet therapy post-implantation. microport's firehawk target eluting stent has been shown to be non-inferior to traditional drug-eluting stents while using one-third of the amount of equivalent drug. society and culture in 2012, a meta-analysis of clinical trial data was published, showing that, for people with stable coronary artery disease, des has no benefit compared to treatment with drugs. the new york times interviewed the study's main author, who said that more than half of patients with stable coronary artery disease were implanted with stents without even trying drug treatment and that he believed this happened because hospitals and doctors wanted to make more money. in 2013 the times of india reported that des were widely overused and that indian distributors used profits from high markups on des to bribe doctors to use them. in 2014 an investigation by the maharashtra food and drug administration found that high markups and bribery related to des was still widespread.