Polylactic Acid Stent Research Articles

Achieving high mechanical properties of biodegradable vascular stents by four-axis 3D printing system and heat treatment

This study fabricated biodegradable polylactic acid (PLA) stents with extremely smooth surfaces by a four-axis printing system. Isothermal heat treatment has been carried out on the 3D-printed stent in a salt bath. The crystallinity of PLA has increased by 212% after heat treatment. Compared to the untreated stent, the heat-treated stent has an 11% higher radial supporting stiffness and a 33% higher maximum radial supporting load. The stent can also withstand bending deformation more safely by eliminating residual strain through heat treatment. These results suggest that the isothermal heat treatment is an effective technique for enhancing the load-bearing capacities of 3D-printed PLA stents.

Finite element analysis of the mechanical performance of a two-layer polymer composite stent structure

Bioresorbable stents (BRS) are an alternative to existing bare-metal stents (BMS) and drug-eluting stents (DES), aiming to reduce the short- and long-term complications in cardiovascular interventions. A combination of biodegradable polymers in a composite scaffold is a promising solution to improve BRS flexibility and strength, which is currently limited by the material properties of polymers. In this study, a mechanical performance of a composite BRS comprised of two layers with four different combinations of polylactic acid (PLA) and polycaprolactone (PCL) was examined with finite element analysis (FEA). Stent crimping and expansion with the balloon inside an atherosclerotic artery was simulated, and stent performance parameters were evaluated and compared between different composite layer configurations. The single-material PCL stent showed the highest recoil and failed to provide support to the artery, while the stiffer single-material PLA stent experienced significantly lower recoil, but also the highest dogboning during expansion, which could increase arterial wall damage. Nearly identical performance was observed for both mixed-material configurations, with recoil and foreshortening closer to single-material PLA stent and dogboning during expansion between both single-material stents. They showed the highest dogboning after balloon deflation due to a combination of permanent deformation at the ends of the stent and lower stiffness at the middle, which allowed for higher recoil at the middle but not at both ends of the stent. The combination of biodegradable polymers into composite BRS shows potential for targeted stent designs with performance tailored to specific needs.

Structure design and mechanical performance analysis of three kinds of bioresorbable poly-lactic acid (PLA) stents

Vascular stent implantation has become an important choice for the treatment of severe cardiovascular and cerebrovascular blockage. Rational design is vital to ensure the mechanical properties of the vascular stents, which are important both to the implantation and service as for clinical treatment of coronary heart disease. Therefore we proposed a wholly new non-uniform honeycomb stent E and compared with an inverted honeycomb-like shaped stent F and a honeycomb-like shaped stent G. To evaluate their properties, a finite element method (FEM) was used to simulate the implantation process (crimp, crimp recoil, expand, and expand recoil) of these three different kinds of stents. Results showed that the stent E exhibits better mechanical behaviour than the other two stents F and G as far as radial strength and axial shortening performances and that the distribution of equivalent stress among the stent E is more uniform than that among the other two stents F and G. After that, a three-point bending method was used to study the bending flexibility of these three vascular stents. Stent E shows high bending stiffness compared with stents F and G due to the existence of additional support bridges in its structure. This study can be helpful to the rational design of optimizable PLA stents for its practical clinical performance and therefore possibly improve the prognosis of patients.

Fabrication of Edge Rounded Polylactic Acid Biomedical Stents by the Multi-Axis Micro-Milling Process

This paper presents the first try to fabricate degradable polylactic acid (PLA) biomedical stents with round edges by the multi-axis micro-milling process. Conventionally biomedical stents are produced by laser processing. Post-processing operations are usually required to handle sharp edges and thermal defects of the stent due to laser processing. A computer graphics software package was used to design the strut structures with round corners of the PLA stent. A PLA tube was first created using injection molding, and a degradable biomedical stent was then fabricated through micro-milling by using a five-axis computer numerical control (CNC) machine tool. This study investigated the error in the rotation center that can occur during five-axis micro-milling. Data obtained from experiments on center-of-rotation errors were substituted into homogeneous coordinate conversion formulas. Center-of-rotation errors in the five-axis machine tool were compensated for improving the milling precision (A and C axes) to be within 5 μ m. Furthermore, milling parameter optimization experiments were conducted, which determined the optimal conditions for milling PLA to be a spindle speed of 60,000 rpm, feed per tooth of 0.005 mm, and feed rate of 600 mm/min, and achieved the minimum burr 0.01 mm and the average surface roughness (Ra) 0.4 μ m. These optimal cutting parameters will be used in the following actual stent processing experiments. Finally, the error compensation and optimal parameters were combined in a CAM software package and layered spiral micromilling to machine the actual stent. The experimental results revealed that the combination of five-axis micro-milling led to the successful fabrication of a degradable biomedical stent (stent diameter = 6 mm, strut width = 0.3 mm, and radius at round corners = 0.1 mm). The machined actual stent had cross-section height and width errors within 0.01 mm, and arc depth of cut variation within 6 μ m. In addition, the PLA stent machining results indicated a rebound of approximately 33% at the strut round edge machining. This work may also open up future possibilities for complex three-dimensionally structured biomedical stents for better performance and special functionality.

Radial Compressive Property and the Proof-of-Concept Study for Realizing Self-expansion of 3D Printing Polylactic Acid Vascular Stents with Negative Poisson's Ratio Structure.

Biodegradable stents offer the potential to reduce the in-stent restenosis by providing support long enough for the vessel to heal. The polylactic acid (PLA) vascular stents with negative Poisson’s ratio (NPR) structure were manufactured by fused deposition modeling (FDM) 3D printing in this study. The effects of stent diameter, wall thickness and geometric parameters of arrowhead NPR structure on radial compressive property of 3D-printed PLA vascular stent were studied. The results showed that the decrease of stent diameter, the increase of wall thickness and the increase of the surface coverage could enhance the radial force (per unit length) of PLA stent. The radial and longitudinal size of PLA stent with NPR structure decreased simultaneously when the stent was crimped under deformation temperature. The PLA stent could expand in both radial and longitudinal direction under recovery temperature. When the deformation temperature and recovery temperature were both 65 °C, the diameter recovery ratio of stent was more than 95% and the maximum could reach 98%. The length recovery ratio was above 97%. This indicated the feasibility of utilizing the shape memory effect (SME) of PLA to realize the expansion of 3D-printed PLA vascular stent under temperature excitation.

Electrospun Poly-lactic Acid/Chitosan Nanofibers Loaded with Paclitaxel for Coating of a Prototype Polymeric Stent

Electrospun composite fibers of poly-lactic acid (PLA), chitosan (Ch) and paclitaxel (PTX) were fabricated for surface covering of a polymeric prototype PLA stent by means of single nozzle electrospinning approach to prepare a low cytotoxicity drug-eluting stent. Different concentrations of the drug (40 %, 60 %, 80 %, 100 % and 120 %) and chitosan (3 %, 5 %, 7 % and 9 %) were incorporated to reach the optimum composite fibers. The electrospun composite fibers were subjected to detailed analyses including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), tensile test, MTT assay, cell culture and in vitro drug release. Results have confirmed a proper physical encapsulation of PTX in the polymeric matrix in which no chemical bonding was detected between the polymers and the drug. Among the fabricated composite fibers, specimens including 40 % and 60 % drug also exhibited an excellent cytotoxicity and controlled drug release. SEM images have proved the effect of paclitaxel in resisting cell adhesion and propagation on the fibers. Findings from this study suggest a novel polymer/drug coating that could be potentially applicable in surface covering of polymeric stents e.g. PLA stents.

Design and characterization of PLLA stents with Z-structure

Biodegradable intravascular stents could become totally degraded after supporting the stenosis diseased vascular without risks of thrombus and restenosis. As the earliest used biodegradable material on stents, poly-l-lactic acid (PLLA) has favorable mechanical and degradable performance. In this paper, poly-lactic acid was used as a stent material for the imitation of the S7 stent (Medtronic AVE, Minnesota, USA). The Z-structure stent was designed and constructed with textile methods by wounding and bonding the strut with a cylindrical mold. Poly-lactic acid Z-structure stents were made into different forms with two different bonding methods for the comparison of stent mechanical properties. In the meantime, polydiaxanon monofilament was used for the preparation of the Z-structure stent and WallStent to compare with poly-lactic acid stents. The stent radial force was tested by the platform compression method. Experimental results showed that the polydiaxanon Z-structure stent had a higher radial force than the poly-lactic acid ones because of better mechanical properties of the strut. Small-diameter stents expressed higher radial force, whereas stents with more crown rows had lower radial forces. Stents bonded with medical glue had higher radial force but lower fastness and inefficient self-expanded performance. The WallStent and Z-structure stent both had favorable elastic resilience performance, whereas the Z-structure stent performed better in compression resistance under large deformation

STENT THROMBOSIS: IS THE ACHILLES HEEL OF BIORESORBABLE SCAFFOLDS EMERGING? A META-ANALYSIS OF RANDOMIZED CLINICAL TRIALS

bioresorbable polylactic acid stents (BVS) are a promising technology but concerns regarding its radial strength are a matter of debate. Whether BVS are associated with an increased risk of stent thrombosis when compared to metallic drug eluting stents (DES) is not established. a systematic review

Treatment of coronary artery disease from the inside: Light at the end of the tunnel?

Surgical treatment of coronary heart disease has shown its life saving benefits in millions of patients for more than half a century. Attempts to create less invasive ways to achieve similar results have attracted great attention since the introduction of balloon angioplasty in 1977. The fascination with such techniques was hampered by a 30% recurrence rate and a 5% rate of abrupt closure, requiring emergency bypass surgery in most instances.Angioplasty lost much of its unpredictability with the introduction of stents in 1986. The use of potent anti-platelet regimes added further safety, but recurrence remained a definite thread until the introduction of anti-mitotic coatings during the early years of this century. Very late thrombosis, however, continues to haunt interventionist and patients. Vanishing poly-lactic acid stents time still fail to exhibit comparable mechanical properties at present. So far bypass surgery has not yet lost its attraction in complicated and diffuse disease, despite its much higher patient discomfort.

Development and characterization of a coronary polylactic acid stent prototype generated by selective laser melting

In-stent restenosis is still an important issue and stent thrombosis is an unresolved risk after coronary intervention. Biodegradable stents would provide initial scaffolding of the stenosed segment and disappear subsequently. The additive manufacturing technology Selective Laser Melting (SLM) enables rapid, parallel, and raw material saving generation of complex 3- dimensional structures with extensive geometric freedom and is currently in use in orthopedic or dental applications. Here, SLM process parameters were adapted for poly-L-lactid acid (PLLA) and PLLA-co-poly-ε-caprolactone (PCL) powders to generate degradable coronary stent prototypes. Biocompatibility of both polymers was evidenced by assessment of cell morphology and of metabolic and adhesive activity at direct and indirect contact with human coronary artery smooth muscle cells, umbilical vein endothelial cells, and endothelial progenitor cells. γ-sterilization was demonstrated to guarantee safety of SLM-processed parts. From PLLA and PCL, stent prototypes were successfully generated and post-processing by spray- and dip-coating proved to thoroughly smoothen stent surfaces. In conclusion, for the first time, biodegradable polymers and the SLM technique were combined for the manufacturing of customized biodegradable coronary artery stent prototypes. SLM is advocated for the development of biodegradable coronary PLLA and PCL stents, potentially optimized for future bifurcation applications.

Introduction to biodegradable polylactic acid ureteral stent application for treatment of ureteral war injury

• To study the operability and effectiveness of a biodegradable ureteral stent for clinical treatment of ureteral war injury using a canine model. • A device was designed and employed to generate firearm fragment wounds in unilateral ureters (on randomly chosen sides) of nine beagles (Group A). The wounded ureters were then debrided and sutured. • Intravenous pyelography (IVP) and radioactive renography were performed 40, 80 and 120 days postoperatively. In Group B, firearm fragment wounds were made to the bilateral ureters in nine beagles. A polylactic acid stent was placed unilaterally (on a randomly chosen side) whereas the ureter on the other side was debrided and sutured without stenting. • Both IVP and radioactive renography were performed 40, 80 and 120 days postoperatively. The operability and effectiveness of the biodegradable ureteral stent were studied thereafter. • In Group A, hydronephrosis and hydroureter occurred and worsened postoperatively on the wounded sides in all nine beagles. The ratio of the renal partial concentration indices (RPCI) between the kidneys (unwounded side : wounded side) increased. • The ratio of the kidney washout half-time between the kidneys (unwounded side : wounded side) decreased. In Group B, neither hydronephrosis nor hydroureter was found postoperatively in the stented ureters but both occurred in the unstented ureters in all nine beagles. • The ratio of RPCI between kidneys (stented side : unstented side) increased whereas the kidney washout half-time ratio between the stented and unstented sides decreased. Differences were significant. • In Group A, the new canine model for firearm fragment wounds was tested and proved to be operable and effective. In Group B, hydronephrosis and hydroureter were effectively prevented in ureters by biodegradable stent placement compared with the non-stented ureters where hydronephrosis and hydroureter occurred. The renal concentration capacity was effectively protected and the half-time of kidney washout was shortened.

Composite material stent comprising metallic wire and polylactic acid fibers, and its mechanical strength and retrievability

Although metallic stents are characterized by strong expanse of force, thin walls, and easy stent deployment, their removal from the body is usually difficult or impossible due to the difficulty of unraveling their mesh structure. A stent built of a composite material comprising a metallic wire and a polylactic acid (PLA) fiber, in which the metallic wire component could be unraveled after PLA fiber degradation in the body, should allow easy stent removal. To evaluate the mechanical strength and retrievability of a composite material stent comprising a metallic wire and a PLA fiber. We produced a composite material stent comprising a metallic wire and a biodegradable fiber (hybrid stent). As the metallic wire is not cross-linked with itself, but with the PLA fibers only, the hybrid stent can be easily unraveled after PLA fiber degradation. This stent was built with a 0.2-mm stainless-steel wire and a 0.23-mm PLA fiber knitted in the same textile as an Ultraflex stent. For comparison, an identical stent was built using PLA fiber only (PLA stent). The mechanical strength of these stents was tested by the radial expansive force response against circumferential shrinkage stress load. Change in radial force due to PLA fiber degradation was estimated by adding an artificial PLA degeneration process, by immersing each stent in a water bath at 80 degrees C for 48 hours. Retrievability of the hybrid stent after PLA degeneration was examined by hooking and pulling out the residual stainless-steel wire from a silicon tube. The hybrid stent exhibited a linear response in radial expansive force within the range of 15% diameter reduction. The PLA stent did not exhibit linear response at over 15% diameter reduction. Decrease of radial expansive force after PLA degradation was within 5% of the original force in the hybrid stent, but the PLA stent did not create effective radial expansive force. Hybrid stents, even after PLA degradation, exhibited a linear response in radial expansive force, within the range of 15% diameter reduction. The metallic component of the heat-processed hybrid stent was easily unraveled by pulling out the wire. The hybrid stent comprising a stainless-steel wire and a PLA fiber appears to provide effective radial expansive force and retrievability.

Efficacy and safety of pimecrolimus-eluting stents in porcine coronary arteries

We aimed to evaluate an effective dosage and safety profile of pimecrolimus as an anti-inflammatory drug for drug-eluting stents. In the dose finding study, coronary arteries of 20 domestic swine were randomly implanted with bare metal stents (ProKinetic and Guidant Vision), the ProKinetic stent with polylactic acid (PLLA), and pimecrolimus-eluting stents (32, 75, and 120 microg) over a period of 4 weeks. In addition, pimecrolimus (75 microg) and ProKinetic stents were randomly implanted into six swine over 3 months. In the safety study, the ProKinetic stent, the ProKinetic stent with PLLA, mid- (45 microg) and high-dose pimecrolimus (120 microg), and overlapping mid-dose stents were implanted over a period of 4 weeks. Mid-dose, ProKinetic stent, and ProKinetic stent with PLLA were implanted over a period of 3 months. The dose finding study revealed excellent luminal patency with low percent occlusion (approximately 29% vs. approximately 41%), injury (0.53-0.59 vs. 1.25), and inflammation (0.78-0.97 vs. 1.08) for the pimecrolimus group compared with the vision group. The safety study arm showed similar angiographic results for all tested groups, with a significantly larger minimal lumen diameter for pimecrolimus stents compared to PLLA stents. Except for the high-dose group and overlapping area of the overlapping group, promising morphometric results were found for pimecrolimus compared to bare metal stents. Present data suggest that pimecrolimus-eluting stents are safe and have a similar healing profile to bare metal stents. They may suppress inflammation, leading to a reduced intimal response and a milder inflammatory reaction in a porcine model.

An experimental study on role of biodegradable poly-lactic acid stent in repair of bile duct injury

目的 评价一种新型的可降解聚乳酸支架在胆管损伤胆管修复中的支撑作用和其安全性以及可行性.方法选择4条犬作为实验动物,麻醉后行胆囊切除,横断胆总管,造成胆管损伤模型.行胆管对端吻合,并放置可降解聚乳酸支架支撑.分别于10周、11周、13周和15周行胆道造影,并切取胆总管组织和肝组织做组织学检查.结果所有犬血胆红素正常.胆道造影检查显示胆管支架通畅,没有胆管狭窄.支架膨胀,完好支撑在胆管内,没有移位.支架内没有胆泥形成.组织学检查显示胆管黏膜未见异常,没有明显炎症反应和上皮增生.结论可降解聚乳酸支架能有效支撑胆管,安全,可行,为胆管损伤的支撑治疗提供了一个新的材料.但尚需要对其进行进一步观察和研究。