Polymer selection for Eustachian tube stent application based on mechanical, thermal and degradation behavior

The novel concept of stenting the Eustachian tube was established to provide an effective and safe therapy of Eustachian tube dysfunction. Biodegradable polymer stents are being developed to restore impaired tube function. As the supporting effect may be required for different time periods, PLA-co-PEG copolymers, PLLGA, PDLLA and PDS, having shorter degradation times compared to PLLA, were evaluated as potential stent materials. Since tensile tests and thermal analyses of solvent cast films from PLA-co-PEG copolymers showed comparable properties to PLLA, stent samples were manufactured from these materials. Mechanical stent testing revealed an increase of elastic recoil and slight decrease of collapse pressure compared to PLLA. In a short term accelerated degradation study a considerable percentage molar mass reduction and an increasing degree of crystallinity depending on PEG content was found. Based on the results obtained, the tested polymers offer a promising, faster degradable alternative to the established stent material PLLA.

PALMAZ–SCHATZ STENT-OPENING MECHANICS USING A SIMPLE APPROACH INVOLVING THE BALLOON–STENT AND STENT–ARTERY CONTACT PROBLEM: APPLICATION TO BIOPOLYMER STENTS

We used the finite element method-based toolbox COMSOL Multiphysics to address the important question of biopolymer coronary stent mechanics. We evaluated the diameter of the stent, the immediate elastic recoil, the dogboning and the foreshortening during deployment while using an idealized model that took into account the presence of the balloon and the coronary artery wall (equivalent pressure hypothesis). We validated our model using the well-known mechanics of the Palmaz–Schatz metal stent and acquired new data concerning a poly-L-lactic acid (PLLA) stent and some other biodegradable co-polymer-based stents. The elastic recoil was relatively high (26.1% to 31.1% depending on the biopolymer used) when taking into account the presence of both the balloon and artery. The dogboning varied from 31% to 46% for the polymer stents and was 62% for the metal stent, suggesting that less arterial damage could be expected with biopolymer stents. Various strut thicknesses were tested for the PLLA stent (114, 180 and 250[Formula: see text][Formula: see text]m) and no significant improvement in elastic recoil was observed. We concluded that the stent geometry has a greater impact on the scaffolding role of the structure than the strut thickness, or even the mechanical properties of the stent.

Mechanical Performance of PLLA Stent

Stent implantation is widely used to treat blocked lumen. Stents were meshed structure made of polymers and metal alloys, including stainless steel, cobalt chrome and nitinol [1]. Clinical studies had demonstrated that stents helped to scaffold the diseased lesion up to one year when tissue adapted to the stented environment [2]. However, the permanently implanted stents inside artery were associated with complications such as stent fracture, tissue inflammation, in-stent restenosis and thrombosis [3]. Currently, biodegradable stents are attracting more attention due to its potential long-term efficacy in treating blocked lumens. The detailed characterizations of biodegradable stents are essential for the desired clinical outcomes.

A New Approach for Polymer-Free Coating with Paclitaxel Microparticles on Fully-Bioabsorbable Poly(l-Lactic Acid) Stent and Studies of Drug Release Behavior

A first clinically-used, fully-bioabsorbable poly(L-lactic acid) (PLLA) stent was coated with microcrystals of paclitaxel (PTX), antiproliferative agent through seeding and/or crystal growing technique to investigate the drug properties on the stent surface and drug release behavior from the stent. PTX particles subject to only seeding process was peeled off after stent compression, while less PTX was coated on the stent subject to only crystal growth without seeding. PLLA stent with both processes could stably maintain an increased amount of PTX on its surface. The maximum amount of initial release (10 μg/cm2) was found in the sample only with seeding, which decreased to 3 μg/cm2 or below after one week. Also in the PLLA stent only with crystal growth, release amount decreased after one week. In the PLLA stent with seeding and crystal growth, formation of needle-like PTX crystals on the surface resulted in decreased initial release and lower solubility after hydration in phosphate buffered solution compared to other types of stent with different procedures. It is suggested that both seeding and crystal growth are required for stable application of PTX on fully-bioabsorbable PLLA stent and needle-like crystals containing water have lower aqueous solubility resulting in decreased PTX release.

The Effect of Static and Dynamic Loading on Degradation of PLLA Stent Fibers

Understanding how polymers such as PLLA degrade in vivo will enhance biodegradable stent design. This study examined the effect of static and dynamic loads on PLLA stent fibers in vitro. The stent fibers (generously provided by TissueGen, Inc.) were loaded axially with 0 N, 0.5 N, 1 N, or 0.125–0.25 N (dynamic group, 1 Hz) and degraded in PBS at 45 °C for an equivalent degradation time of 15 months. Degradation was quantified through changes in tensile mechanical properties. The mechanical behavior was characterized using the Knowles strain energy function and a degradation model. A nonsignificant increase in fiber stiffness was observed between 0 and 6 months followed by fiber softening thereafter. A marker of fiber softening, β, increased between 9 and 15 months in all groups. At 15 months, the β values in the dynamic group were significantly higher compared to the other groups. In addition, the model indicated that the degradation rate constant was smaller in the 1-N (0.257) and dynamic (0.283) groups compared to the 0.5-N (0.516) and 0-N (0.406) groups. While the shear modulus fluctuated throughout degradation, no significant differences were observed. Our results indicate that an increase in static load increased the degradation of mechanical properties and that the application of dynamic load further accelerated this degradation.

The Effect of Static and Dynamic Loading on Degradation of PLLA Stent Fibers

PLLA is a commonly used biodegradable polymer in stent designs because it is non-toxic and easily eliminated from the body. However, very little is known about the effect of loading conditions on the degradation rate. Rajagopal and Wineman developed a model of polymer degradation which is driven by load applied to the fiber [1]. Soares et. al. further developed this model for use with PLLA stent fibers under tensile loading conditions [2]. In this model the degradation rate is linearly related to deformation through the radius in the (IB, IIB) plane. Both models predict that greater deformation will induce a higher degree of degradation.

Influence of CO2 Blowing Agent on Porous Bioresorbable Stent Structure

Bioresorbable stents with limited functional lifetimes and with drug delivery capabilities are desired. Various methods have been investigated to induce porosity in bioresorbable polymeric stent fibers, thereby to permit increased drug reservoir capacity versus polymer-coated metal stents. We developed microporous surface layers on PLLA fibers to serve as the drug reservoir, but found that impurities, the use of chemicals, and multiple step procedures associated with our, and other published methods limited utility. Thus we investigated theoretically attractive CO 2 blowing methods, in which gas under pressure and temperature induces porosity. We report the results of initial studies of CO 2-induced porosity in PLLA stent fibers.

Comparison of Acute Recoil between Bioabsorbable Poly-L-lactic Acid XINSORB Stent and Metallic Stent in Porcine Model

Objective. To investigate acute recoil of bioabsorbable poly-L-lactic acid (PLLA) stent.Background. As newly developed coronary stent, bioabsorbable PLLA stent still encountered concern of acute stent recoil.Methods. Sixteen minipigs were enrolled in our study. Eight PLLA XINSORB stents (Weite Biotechnology Co., Ltd., China) and eight metallic stents (EXCEL, Jiwei Co., Ltd. China) were implanted into coronary arteries. Upon quantitative coronary angiography analysis, acute absolute recoil was defined as the difference between mean diameter of inflated balloon (X) and mean lumen diameter of stent immediately after deployment (Y), while acute percent recoil was defined as (X−Y)/Xand expressed as a percentage. Intravascular ultrasound (IVUS) was performed immediately after implantation and 24 hours later to compare cross-sectional area (CSA) between two groups and detect stent malapposition or collapse.Results. Acute absolute recoil in XINSORB and EXCEL was0.02±0.13 mm and −0.08±0.08 mm respectively (P=0.19). Acute percent recoil in XINSORB and EXCEL was0.66±4.32% and −1.40±3.83%, respectively (P=0.45). CSA of XINSORB was similar to that of EXCEL immediately after implantation, so was CSA of XINSORB at 24-hours followup. Within XINSORB group, no difference existed between CSA after implantation and CSA at 24-hours followup. No sign of acute stent malapposition was detected by IVUS.Conclusions. The acute stent recoil of XINSORB is similar to that of EXCEL. No acute stent malapposition or collapse appeared in both kinds of stent. This preclinical study was designed to provide preliminary data for future studies of long-term efficacy and safety of XINSORB stent.

A Retrievable Polymer Urethral Stent for Treatment of Urethral Strictures

This paper reported a newly designed retrievable urethral stent used in treatment of urethral strictures. The spiral urethral stent is made of Poly (l-lactide) (PLLA), adding barium sulphate (BaSO4) to certain content, which can be displayed precisely under X-ray after implanted in body. The stent was clinically tested in 32 patients with recurrent urethral strictures and was left in the urethra for 5 to 12 months. The response to the effect of the stent was assessed using symptom scores, peak flow rate (Qmax), residual urine volume (RUV), stent degradation, infection and outcome. During the treatment, there were not any inflammations found. The mechanical properties of the stent are almost the same as those of the PLLA stent without BaSO4. Improvements in patient symptom score, mean Qmax and RUV were significant at the 18 month follow-up. The infection was not observed. The stent was taken out easily through urethral forceps. In no patient was the stent obstructed by incrustation for up to 1.5 years.