Biodegradable polymeric stents for vascular application in a porcine carotid artery model: English version.

Over the past years the development of biodegradable polymeric stents has made great progress; nevertheless, essential problems must still be solved. Modifications in design and chemical composition should optimize the quality of biodegradable stents and remove the weaknesses. New biodegradable poly-L-lactide/poly-4-hydroxybutyrate (PLLA/P4HB) stents and permanent 316L stents were implantedendovascularly into both common carotid arteries of 10 domestic pigs. At 4 weeks following implantation, computed tomography (CT) angiography was carried out to identify the distal degree of stenosis. The PLLA/P4HB group showed a considerably lower distal degree of stenosis by additional oral application of atorvastatin (mean 39.81 ± 8.57 %) compared to the untreated PLLA/P4HB group without atorvastatin (mean 52.05 ± 5.80 %). The 316L stents showed no differences in the degree of distal stenosis between the group treated with atorvastatin (mean 44.21 ± 2.34 %) and the untreated group (mean 35.65 ± 3.72 %). Biodegradable PLLA/P4HB stents generally represent a promising approach to resolving the existing problems in the use of permanent stents. Restitutio ad integrum is only achievable if a stent is completely degraded.

[Coupled Analysis of Fluid-Structure Interaction of a Micro-Mechanical Valve for Glaucoma Drainage Devices]. / Gekoppelte Analyse der Fluid-Struktur-Interaktion eines mikromechanischen Ventils für Glaukomdrainageimplantate.

BACKGROUND: Glaucoma is the leading cause of irreversible blindness worldwide. In therapeutically refractory cases, alloplastic glaucoma drainage devices (GDD) are being increasingly used to decrease intraocular pressure. Current devices are mainly limited by fibrotic encapsulation and postoperative hypotension. Preliminary studies have described the development of a glaucoma microstent to control aqueous humour drainage from the anterior chamber into the suprachoroidal space. One focus of these studies was on the design of a micro-mechanical valve placed in the anterior chamber to inhibit postoperative hypotension. The present report describes the coupled analysis of fluid-structure interaction (FSI) as basis for future improvements in the design micro-mechanical valves. MATERIALS AND METHODS: FSI analysis was carried out with ANSYS 14.5 software. Solid and fluid geometry were combined in a model, and the corresponding material properties of silicone (Silastic Rx-50) and water at room temperature were assigned. The meshing of the solid and fluid domains was carried out in accordance with the results of a convergence study with tetrahedron elements. Structural and fluid mechanical boundary conditions completed the model. The FSI analysis takes into account geometric non-linearity and adaptive remeshing to consider changing geometry. RESULTS: A valve opening pressure of 3.26 mmHg was derived from the FSI analysis and correlates well with the results of preliminary experimental fluid mechanical studies. Flow resistance was calculated from non-linear pressure-flow characteristics as 8.5 × 10(-3) mmHg/µl  · min(-1) and 2.7 × 10(-3) mmHg/µl  · min(-1), respectively before and after valve opening pressure is exceeded. FSI analysis indicated leakage flow before valve opening, which is due to the simplified model geometry. CONCLUSIONS: The presented bidirectional coupled FSI analysis is a powerful tool for the development of new designs of micro-mechanical valves for GDD and may help to minimise the time and cost expended on manufacturing and testing prototypes. Further optimisation of the FSI model is expected to ensure further convergence between the simulation and the results of experimental investigations.

Biodegradable polymeric coatings on cochlear implant surfaces and their influence on spiral ganglion cell survival.

To improve the electrode-nerve interface of cochlear implants (CI), the role of poly(L-lactide) (PLLA) and poly(4-hydroxybutyrate) (P(4HB)) as potential coating matrices for CI was assessed both in vitro and in vivo in terms of degradation behavior and effects on spiral ganglion neurons, the main target of the electrical stimulation with a CI. Growth rates of fibroblasts on the polymers were investigated and a direct-contact test with freshly isolated spiral ganglion cells (SGC) was performed. In addition, the effects of the polymer degradation inside the inner ear were evaluated in vivo. The polymer degradation was assessed by use of scanning electron microscopy in combination with an energy-dispersive X-ray analysis. In vitro, no influence of the polymers was detected on fibroblasts' viability and on SGC survival rate. In vivo, SGC density was decreased only 6 months after implantation in the basal and middle turns of the cochlea in comparison to normal-hearing animals but not between implanted groups (coated or uncoated). The analysis of the electrode models showed that in vivo P(4HB) is characterized by a gradual degradation completed after 6 months; whereas, the PLLA coatings burst along their longitudinal axis but showed only little degradation within the same time frame. In conclusion, both polymers seem to justify further evaluation as possible coating for CI electrodes. Of the two options, due to its excellent coating adhesion/stability and optimal degradation behavior, P(4HB) may prove to be the more promising biodegradable polymer for designing a drug delivery system from the surface of CI electrodes.

Experimental investigation of modern and established carotid stents.

PURPOSE: The design and material determine the mechanical properties of stents. In vitro parameters such as radial force, flexibility and wall adaptation of different stents were investigated in order to obtain evidence for clinical use. MATERIALS AND METHODS: A total of 8 stents, including 2 hybrid stents with a combination of closed/open-cell design (Sinus Carotid RX, Cristallo Ideale), 3 closed-cell stents (Adapt, Carotid Wallstent, Xact Carotid) and 3 open-cell stents (Vivexx Carotid, Protégé Rx and Precise) with a diameter of 8 mm and a length of 40 mm, were investigated. The radial force, the bending stiffness of the stent system and of the stent, and the collapse pressure were measured. The wall adjustment of the stents was documented by fluoroscopy and assessed in a step and curve model. RESULTS: The bending stiffness of the stent systems declined significantly in the expanded state, whereby the Xact Carotid stent showed the highest value (291.1 N/mm2) in contrast to 31.6-39.4 N/mm2 for the Sinus Carotid and Cristallo Ideale. The radial force on expansion of the stents to 7 mm was lowest for the Adapt (0.009 N/mm) and highest for the Precise (0.068 N/mm). The collapse pressure was highest for the Carotid Wallstent (0.48 bar), compared with the other stents (0.1 - 0.2 bar). The best wall adjustment in the curve model was shown by the Precise, the Sinus Carotid Rx and the Vivexx Carotid Stent. The diameter change from 5 to 7 mm was smoothly adapted by the Cristallo Ideale and the Carotid Wallstent. The Adapt showed poor vessel wall adaptation in both step and curved vessels. CONCLUSION: As a result of their design, the Sinus Carotid Rx and the Cristallo Ideale show the best wall adjustment, with comparable radial force and high flexibility, whereby the Cristallo Ideale has the advantage that it has a closed-cell design in the middle third of the stent. The other stents should be taken into consideration with their specifications in individual cases. KEY POINTS: The investigation of carotid stents provides an objective comparison of mechanical properties. Conclusions about the respective optimum uses of stents can be drawn from this. The hybrid stents show a good balance of properties for wide applicability. Citation Format: • Wissgott C, Schmidt W, Behrens P et al. Experimental Investigation of Modern and Established Carotid Stents. Fortschr Röntgenstr 2014; 186: 157 - 165.