[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.

[Optimisation of the visualisation technique for optical paths through intraocular lenses for characterisation of multifocal imaging properties of Fresnel-zone plates]. / Optimierung der Visualisierungstechnik für Strahlenverläufe durch Intraokularlinsen zur Charakterisierung multifokaler Abbildungseigenschaften von Fresnel-Zonenplatten.

The utilisation of the diffractive properties of Fresnel zone plates offers the possibility of intraocular lens designs with multiple foci. Such intraocular lenses can be manufactured by two-photon polymerisation (2PP). This paper explains the underlying concept and shows the principles for visualisation of the focus properties of such implants.

[New concepts for pressure-controlled glaucoma implants]. / Neue Konzepte für druckgesteuerte Glaukomimplantate.

In industrialized countries glaucoma is one of the most common causes that leads to blindness. It is also the most common cause of irreversible blindness worldwide. In addition to local treatment of intraocular pressure and filtering glaucoma surgery, alloplastic implants are increasingly being used in glaucoma therapy. As long-term results published in the literature of commonly used implants are unsatisfactory, it seems useful to search for new concepts. In order to avoid the well-known short-term and long-term postoperative complications a pressure-controlled microstent with antiproliferative surface modifications was developed. Additionally, the functionality of such a microstent should be investigated using an animal glaucoma model. This paper describes the concept of a microstent which drains aquous humour from the anterior chamber into the suprachoroidal space. In addition, the glaucoma models described in the literature are discussed. Unfortunately, none of the methods could be reproduced permanently. First results show a correct implantation of a coated microstent with valve where the anti-proliferative effect could be demonstrated histologically. The promising results should lead to further investigations and the final goal will be the testing of the stent in the human eye.

Clearance of levofloxacin by an in vitro model of continuous venovenous hemodialysis (CVVHD).

Information about the elimination and the adequate dosing of levofloxacin during renal replacement therapy is scarce. The aim of this study was to characterize in vitro the elimination of levofloxacin during continuous venovenous hemodialysis (CVVHD) and to investigate whether the CVVHD clearances of creatinine and urea are correlated with the levofloxacin clearance in order to facilitate dosage adjustments. An in vitro model of CVVHD was established using five dialyzer membranes at varying dialysate flow rates applied in the clinical setting (8, 16, 25, 33 and 41 ml/min). Plasma and dialysate samples were drawn for determination of levofloxacin, creatinine and urea concentrations to evaluate clearances by CVVHD. During CVVHD, the clearance of levofloxacin varied between 9.02 and 33.30 ml/min, depending on the chosen setup. Positive correlations (p<0.001) were received for: dialysate flow rate (QD) and creatinine/ urea clearances (R(2)>0.93); QD and levofloxacin clearance (R(2) 0.59-0.71); levofloxacin and creatinine clearance (R(2) 0.69-0.75); and levofloxacin and urea clearance (R(2) 0.56-0.75) as well. When dosing critically ill patients, therefore, extracorporeal as well as total clearance of levofloxacin should be considered.

Validation of a levofloxacin HPLC assay in plasma and dialysate for pharmacokinetic studies.

An HPLC method with fluorescence detection suitable for routine determination of levofloxacin in plasma and dialysate has been validated. Sample preparation was assured by one-step protein precipitation for plasma or direct injection of the dialysate solution, respectively. Separation occurred on an YMC Pro C18 RP column (150 mm x 2 mm) with an acidic binary gradient mobile phase and detection at excitation and emission wavelengths of 296 and 504 nm. The assay was linear between 0.1 and 6 microg/ml for plasma and 0.1 and 5 microg/ml for dialysate with intra- and inter-day precision and accuracy lower than 10%. No degradation of levofloxacin was observed under the applied conditions for both matrices. The method was successfully applied to an in vitro pharmacokinetic study and patient samples as well.