Mechanical properties and in vitro degradation of bioresorbable knitted stents

The development of bioresorbable composite polymeric implants with high mechanical strength

Implants for the treatment of tissue defects should mimic the mechanical properties of the native tissue of interest and should be resorbable as well as biocompatible. In this work, we developed a scaffold from variants of poly(glycolic) acid which were braided and coated with an elastomer of poly(glycolide-co-caprolactone) and crosslinked. The coating of the scaffold with the elastomer led to higher mechanical strength in terms of compression, expansion and elasticity compared to braids without the elastomer coating. These composite scaffolds were found to have expansion properties similar to metallic stents, utilizing materials which are typically much weaker than metal. We optimized the mechanical properties of the implant by tuning the elastomer branching structure, crosslink density, and molecular weight. The scaffolds were shown to be highly resorbable following implantation in a porcine femoral artery. Biocompatibility was studied in vivo in an ovine model by implanting the scaffolds into femoral arteries. The scaffolds were able to support an expanded open lumen over 12 months in vivo and also fully resorbed by 18 months in the ovine model.

Medical Textiles as Vascular Implants and Their Success to Mimic Natural Arteries

Vascular implants belong to a specialised class of medical textiles. The basic purpose of a vascular implant (graft and stent) is to act as an artificial conduit or substitute for a diseased artery. However, the long-term healing function depends on its ability to mimic the mechanical and biological behaviour of the artery. This requires a thorough understanding of the structure and function of an artery, which can then be translated into a synthetic structure based on the capabilities of the manufacturing method utilised. Common textile manufacturing techniques, such as weaving, knitting, braiding, and electrospinning, are frequently used to design vascular implants for research and commercial purposes for the past decades. However, the ability to match attributes of a vascular substitute to those of a native artery still remains a challenge. The synthetic implants have been found to cause disturbance in biological, biomechanical, and hemodynamic parameters at the implant site, which has been widely attributed to their structural design. In this work, we reviewed the design aspect of textile vascular implants and compared them to the structure of a natural artery as a basis for assessing the level of success as an implant. The outcome of this work is expected to encourage future design strategies for developing improved long lasting vascular implants.

A 5-fluorouracil-loaded polydioxanone weft-knitted stent for the treatment of colorectal cancer

In-stents restenosis caused by tumour ingrowth is a major problem for patients undergoing stent displacement because the conventional stents often lack a sustained anti-tumour capability. The aim of this paper was to develop a weft-knitted polydioxanone stent which can slow release 5-fluorouracil (5-FU). In order to determine the most suitable drug concentration, the 5-FU safe concentration in vivo and appropriate loading percentage in the membranes were investigated, and then 5-FU-loaded poly-l-lactide membranes at concentration of 3.2%, 6.4% and 12.8% were coated onto the stent using electro-spinning method, respectively. The morphology, chemical structure and in vitro drug release property of the coating membranes were subsequently examined. Their anti-tumour activity and mechanism were assessed in vitro and in vivo using a human colorectal cancer cell line HCT-116 and tumour-bearing BALB/c nude mice. The half maximal inhibitory concentration (IC50) and the median lethal dose (LD50) demonstrated that the 6.4% and 12.8% membranes had better anti-tumour effects than pure 5-FU due to the sustainable drug releasing property of the coated membranes on the stent. The membranes possessing appropriate drug loading doses, such as 6.4% or 12.8% also provided better anti-in-stents restenosis effects than other groups tested. Therefore, it is concluded that the drug-loaded stents have great potential for the use in the treatment of intestinal cancers in the future.

Electrospinning versus knitting: two scaffolds for tissue engineering of the aortic valve

Two types of scaffolds were developed for tissue engineering of the aortic valve; an electrospun valvular scaffold and a knitted valvular scaffold. These scaffolds were compared in a physiologic flow system and in a tissue-engineering process. In fibrin gel enclosed human myofibroblasts were seeded onto both types of scaffolds and cultured for 23 days under continuous medium perfusion. Tissue formation was evaluated by confocal laser scanning microscopy, histology and DNA quantification. Collagen formation was quantified by a hydroxyproline assay. When subjected to physiologic flow, the spun scaffold tore within 6 h, whereas the knitted scaffold remained intact. Cells proliferated well on both types of scaffolds, although the cellular penetration into the spun scaffold was poor. Collagen production, normalized to DNA content, was not significantly different for the two types of scaffolds, but seeding efficiency was higher for the spun scaffold, because it acted as a cell impermeable filter. The knitted tissue constructs showed complete cellular in-growth into the pores. An optimal scaffold seems to be a combination of the strength of the knitted structure and the cell-filtering ability of the spun structure.

The biocompatibility of rapidly degrading polymeric stents in porcine carotid arteries

The goal of this study was to evaluate the biocompatibility of materials for use in fully bioabsorbable vascular stents. 10:90 poly(L-lactic-co-glycolic acid) (10:90 L-PLGA), 85:15 poly(L-lactic-co-glycolic acid) (85:15 L-PLGA), polydioxanone (PDO), and poly-L-lactic acid (L-PLA) polymers were chosen as materials. Polymeric fibers were woven into a braided structure with a mass equivalent to or greater than that expected for a vascular stent, secured to balloon-expandable bare metal stents and implanted into porcine carotid arteries. The in vivo response was analyzed at 30 and 90 days by angiography, histopathology, and histomorphometry. All vessels were patent at 30 and 90 days. Injury score and neointima formation was mild for all samples. The faster-degrading 10:90 L-PLGA had the highest inflammatory response at 30 days, but was completely absorbed with minimal inflammation and neointimal formation at 90 days. PDO showed signs of partial absorption at 90 days, while 85:15 L-PLGA and L-PLA demonstrated minimal absorption at 30 and 90 days. The inflammatory response to these three groups was similar over the experimental period. Using a robust materials-testing platform, we demonstrated long-term patency and intravascular biocompatibility of bioabsorbable polymers with varying rates of resorption. The data point to biocompatibility of a polymeric stent in the vascular space that is fully absorbable in less than a year.

Approaches for prevention of restenosis

Coronary artery disease is characterized by a narrowing (stenosis) of the arteries that supply blood to the tissue of the heart. Continued restriction of blood flow manifests itself as angina and ultimately myocardial infarction (heart attack) for the patient. Heart bypass was once the only treatment for this condition, but over the years percutaneous coronary intervention (PCI) has become an increasingly attractive alternative to medical therapy and surgical revascularization for the treatment of coronary artery disease. A vascular stent is a medical device designed to serve as a temporary or permanent internal scaffold, to maintain or increase the lumen of a blood vessel. Metallic coronary stents were first introduced to prevent arterial dissections and to eliminate vessel recoil and intimal hyperplasia associated with PCI. Further advancement in the treatment of coronary artery disease is the development of drug-eluting stents that dramatically reduce the incidence of in-stent restenosis to less than 5%. Local drug delivery offers the advantages of allowing a relatively high local concentration of drug at the treatment site while minimizing systemic toxic effect. This review describes approaches for prevention of restenosis. It focuses on drugs for prevention of restenosis, bare metal stents, and drug-eluting stents. It also describes recent advances in bioresorbable stents. One of the chapters is dedicated to our novel composite bioresorbable drug-eluting fibers, designed to be used as basic elements in drug-eluting stents.

Intravascular bioresorbable polymeric stents: a potential alternative to current drug eluting metal stents

Stent implantation following angioplasty is the standard treatment of coronary artery disease necessitating interventional procedures. The use of stents as a platform for local drug delivery is a popular strategy to achieve local pharmacological treatment to the diseased artery. Drug eluting stents (DES) are now largely preferred to bare metal stents when stent implantation is necessary. Lately, there have been several reports questioning the long-term safety of DES. An alternative to these drug eluting metal stents are bioresorbable polymeric stents (BPS) because of the many advantages of bioresorbable material. However, the fundamental differences in polymeric and metallic materials make the development of such an alternative a significant challenge. This review discusses the different advantages of BPS and the many constrains and requirements of such devices. An up to date commented review of published data concerning BPS is presented. Considerations are given on using BPS as local drug delivery systems as well as on evaluating BPS performances.

DRUG-ELUTING BIORESORBABLE STENTS FOR VARIOUS APPLICATIONS

A stent is a medical device designed to serve as a temporary or permanent internal scaffold to maintain or increase the lumen of a body conduit. Metallic coronary stents were first introduced to prevent arterial dissections and to eliminate vessel recoil and intimal hyperplasia associated with percutaneous transluminal coronary angioplasty. The stent application range has expanded as more experience was gained, and encouraging results have been obtained in the treatment of vascular diseases. Stents are currently used for support of additional body conduits, including the urethra, trachea, and esophagus. The rationale for bioresorbable stents is the support of a body conduit only during its healing process. The stent mass and strength decrease with time, and the mechanical load is gradually transferred to the surrounding tissue. Bioresorbable stents also enable longer term delivery of drugs to the conduit wall from an internal reservoir and abolish the need for a second surgery to remove the device. The present review describes recent advances in bioresorbable stents, focusing on drug-eluting bioresorbable stents for various applications. Controlled release of an active agent from a stent can be used to enhance healing of the surrounding tissues, to increase the implant's biocompatibility, as well as to help cure certain diseases. Because a lot of research in this field has been done by us, examples for these functions are described based mainly on developments in our laboratories.

PLA stereocopolymers as sources of bioresorbable stents: Preliminary investigation in rabbit

The aim of the present work was to evaluate whether the degradation of PLA-based bioresorbable stents can be modulated via the configuration of repeating units as it is the case in other applications like osteosynthesis. The first obstacle was finding a stent design that could allow implantation in the aorta of a rabbit taken as a model of a human coronary artery. In the absence of guidelines other than those tentatively proposed in patents, several simple designs were considered that allowed us to evaluate the fate of the stents made of poly(lactic acid) stereocopolymers with L/(L + D) ratio of 0.92 (PLA92) and 0.50 (PLA50) up to 6 months post in vivo implantation. Our findings show the feasibility of bioresorbable stenting using PLA stereocopolymers and that PLA50 degraded faster than PLA92. Therefore, using stereocopolymers appears as a means to vary the degradation rate and adapt it to the artery remodelling process that is very much dependent on the release of the stenting stress protection.

Bioartifizielle Materialien in der Urologie

The scope of our research is the development of polymer-based bioabsorbable stents for urologic applications and in vitro testing of tissue reactions of cultured ureteral and urethral segments induced by implanted polymer stent prototypes. For these purposes a tissue cultivation model was developed using selected techniques of tissue engineering. Essential advantages of degradable over nondegradable urethral stents are elimination of the adverse extraction of epithelialized stents and the potential for recovery of organ-specific functionality. Moreover, the biocompatibility of a degradable urethral stent could potentially reduce the risk of restenosis due to hyperplasia and could be used, even repeatedly, for the treatment of a number of subvesical obstructions. For the treatment of tumor-induced strictures, application of degradable polymer stents coated with cytostatic drugs may be possible. The mechanical effect of the drug-loaded stent as a "place holder" could be complemented by adjuvant or palliative approaches such as local chemotherapy. We have developed and tested in vitro a degradable urethral stent incorporated with the model drug methotrexate for local drug delivery (LDD) by diffusion and during stent degradation.

Mechanical properties and in vitro degradation of self-reinforced radiopaque bioresorbable polylactide fibres

The aim of this study was to evaluate the effect of the radiopaque filler, barium sulfate (BaSO4), on the mechanical properties of self-reinforced bioresorbable fibres. The bioresorbable polymer was a copolymer of L- and D-lactide with an L/D monomer ratio of 96:4 (96L/4D PLA). The fibres were manufactured using an extrusion and a drawing process. Three different methods of processing the composites were studied. The materials were blended prior to extrusion. In the first method, the BaSO4 powder was mixed with the polymer granulates by hand (manual blending). The blend was then processed using a twin-screw extruder. The second and third methods utilized a single-screw extruder. In the second method, the BaSO4 powder was manually mixed with the polymer prior to extrusion. In the third method, the BaSO4 powder was mechanically attached on the polymer granulates (mechanical blending) prior to extrusion. The mechanical and chemical properties of the radiopaque bioresorbable fibres were measured after processing and during in vitro degradation. The fibres were gamma, plasma or EtO sterilized. There was no statistical difference in the mechanical properties of the fibres when manufactured using the twin-screw extrusion with manual blending or the single-screw extrusion with mechanical blending. The gamma sterilization markedly decreased the initial intrinsic viscosity of all fibres, whereas the plasma and EtO sterilization methods had no effect on the initial intrinsic viscosity. During in vitro testing, the loss in the intrinsic viscosity occurred at the same rate whether the fibres were loaded with the barium sulfate or not.

Mechanical properties and in vitro degradation of bioabsorbable self-expanding braided stents

The aim of this study was to characterize the mechanical and self-expansion properties of braided bioabsorbable stents. In total four different stents were manufactured from PLLA fibres using a braiding technique. The changes in radial pressure stiffness and diameter recovery of the stents were determined initially, and after insertion and release from a delivery device. The braided stents were compared to three commercially available metallic braided stents. The changes in physical and mechanical properties of the PLLA fibres and stents during in vitro degradation were investigated. After release from the delivery device, the PLLA stents did not fully recover to their original diameter. The radial pressure stiffness of the bioabsorbable stents was similar to that of the metallic stents. The in vitro degradation study showed that the stents would keep at least half of their initial radial pressure stiffness for more than 22 weeks.