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Biopolymer Processing in Medical Application as Vascular Stents

Biopolymer Processing in Medical Application as Vascular Stents. Reviewed by: AGNES Purwidyantri Student ID No: D0228005. Biodegradable Polymers as Drug Carrier Systems. Polyesters Lactide/Glycolide Copolymers Have been used for the delivery of steriods, anticancer agent, antibiotics, etc.

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Biopolymer Processing in Medical Application as Vascular Stents

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  1. Biopolymer Processing in Medical Application as Vascular Stents Reviewed by: AGNES Purwidyantri Student ID No: D0228005

  2. Biodegradable Polymers as Drug Carrier Systems • Polyesters • Lactide/Glycolide Copolymers • Have been used for the delivery of steriods, anticancer agent, antibiotics, etc. • PLLA is found as an excellent biomaterials and safe for in vivo (Lactic acid contains an asymmetric α-carbon atom with three different isomers as D-, L- and DL-lactic acid) • PLGA is most widely investigated biodegradable polymers for drug delivery. • Lactide/glycolide copolymers have been subjected to extensive animal and human trials without any significant harmful side effects

  3. Biodegradable Polymers as Drug Carrier Systems • Poly(amides) • Natural Polymers • Remain attractive because they are natural products of living organism, readily available, relatively inexpensive, etc. • Mostly focused on the use of proteins such as gelatin, collagen, and albumin

  4. Biodegradable Polymers as Drug Carrier Systems • Polymer Processing • Drug-incorporated matrices can be formulated either compression or injection molding • Polymer & drug can be ground in a Micro Mill, sieve into particle size of 90-120 µm, then press into circular disc • Alternatively drug can be mixed into molten polymer to form small chips, then it is fed into injection molder to mold into desired shape

  5. What is a Stent? • A small tubular mesh usually made of either stainless steel or Nitinol. • Inserted into stenotic arteries to keep the lumen patent often used after PTCA. • Used at various sites including the coronary, renal, carotid and femoral arteries. • Non-arterial uses e.g. in bronchus, trachea, ureter, bile duct.

  6. Palmaz “Corinthian” Iliac artery stent Gianturco-Roubin II Stent

  7. History • The concept of vascular stents is accredited to Charles Dotter in 1969, who implanted stainless steel coils in canine peripheral arteries. • Not followed up in humans because of haemodynamically significant narrowing. • Not in clinical practice until 1980s. • Market leader is the Palmaz stent designed by Julio Palmaz in 1985. • Initially, 18 grafts placed in canine vessels, with patency rates approaching 80% at 35 weeks.

  8. Plaque Formation and Morphology • Smoking, high BP, toxins etc cause damage to the vascular endothelium. • LDL and fibrin pass through and collect in the sub-endothelium. • Monocytes adhere to the damaged endothelium, migrate to the sub-endothelial space and engulf LDL – FOAM CELLS. • SMC migration and CT formation. • Two main types of plaque: • Atheromatous (athere: gruel, oma: tumour) • Fibrous (like atheroma but with connective tissue cap)

  9. CVD statistics • Heart and circulatory disease is the UK's biggest killer.  • In 2001, cardiovascular disease caused 40% of deaths in the UK, and killed over 245,000 people. • Coronary heart disease causes over 120,000 deaths a year in the UK: approximately one in four deaths in men and one in six deaths in women.

  10. Revascularisation techniques • Coronary Artery Bypass Graft (CABG) • Percutaneous Transluminal Coronary Angioplasty (PTCA) • Stents

  11. PTCA CABG • Major surgery • Complications • Stroke • Mediastinitis (1-4%) • Renal dysfunction (8%) • Minimally invasive procedure • Percutaneous access either in the brachial or femoral arteries. • A guide wire is advanced to the stenotic region. • A balloon is advanced along the wire and inflated/deflated several times to fracture the plaque and open the lumen.

  12. Complications of PTCA • Plaque rupture, may lead to: • Thrombus formation • Intimal flap • Arterial rupture • Acute closure • Sub-optimal result • Restenosis • Requires further intervention to make vessel patent

  13. Stenting vs. PTCA • Prevents acute closure • Tacks back intimal flaps • Less restenosis: • 30–50 % restenosis with PTCA (coronary arteries). • Coronary stents are associated with fewer repeat revascularisation procedures • Rates of death and MI are low and are not significantly different between stents and PTA.

  14. Stent Failure- Stenosis (20-30% • ishear stress • Intimal Hyperplasia • i lumen • h shear stress • If baseline shear stress not restored – continuing intimal hyperplasia and RESTENOSIS

  15. Factors Which Contribute to In-stent Restenosis • Thrombus/platelet/fibrin adherence to stent struts. • Metabolic disorder/smoking/atherogenic diet. • Small lumen diameter. • Stress concentration at end of stent. • Flow disturbance within stented region. Thrombus in Human Coronary Artery

  16. Improving Vascular Stents (1) • Thrombus • Anticoagulants • Heparin – systemically or coated on stent. • Inhibition of the GP IIb-IIIa receptor: • Prevents platelet aggregation. • Available as Abciximab. • Associated with h incidence of MI. • PTFE coated stents.

  17. Intimal hyperplasia in stented Canine iliac artery. After insertion of stent plus PTFE graft material.

  18. Improving Vascular Stents (2) • Small diameter artery • Combination of local and systemic medication and covered stents. • Intimal hyperplasia • Brachytherapy: • Use of ionising radiation to stop cellular proliferation. • Delivery: Radioactive stents, catheter radiation. • 10% restenosis but may cause necrosis. • Anti-proliferative agents e.g. rapamycin (Sirolimus)

  19. Improving Vascular Stents (3) • Mechanical and flow disturbances: • Compliance Matching Stent (CMS) • This stent is rigid in the middle and becomes more compliant near its ends. • This compliance is achieved by parabolic and cantilevered struts. • The middle struts are straighter, providing some resistance to recoil and support for the atherosclerotic plaque.

  20. Compliance Matching Stent Parabolic and canti- levered struts cause ends to be most compliant. Straighter struts in middle provide stiff support for plaque. Transition in between.

  21. Compliance Matching Stent • The gradual change from rigid to compliant with the CMS reduces stress concentration at the stent edges. • The geometry of this stent also fosters more laminar flow through the stent. • Less flow disturbance means less intimal hyperplasia.

  22. Bioabsorbable Stents • Durable polymer coatings on drug-eluting stents have been associated with chronic inflammation and impaired healing. Potential advantages of bioabsorbable polymer stents: may • Reduce DAPT duration • Reduce risk with DAPT interruption • Decrease stent thrombosis • Reduced Polymer Load • Short-term Polymer Exposure

  23. SYNERGY Stent AbluminalBioabsorbable Polymer Bioabsorbable polymer (PLGA) Applied only to the abluminal surface (rollcoat) Thin strut (0.0029”) PtCr Stent Current Durable Polymer Abluminal Bioabsorbable Polymer Durable PermanentPolymer + Drug 360° AroundStent PLGA BioabsorbablePolymer + Everolimuson Abluminal Side of Stent

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