Jürgen Burger

Oxygen and glucose distribution after intracorneal lens implantation.

Purpose. Insertion of an implant in the cornea to achieve corneal multifocality has been suggested as a solution for presbyopia. However, unresolved issues related to nutrient transport need to be resolved. Our aim was to find the best lens position and influence lens transport properties in order to optimize nutrient supply to corneal cells. Method. An axisymmetric corneal model was built to simulate the nutrient transport in the cornea. Oxygen and glucose concentrations were calculated for normal cornea and intracorneal lens wearing conditions. The simulation considers the different tissue layers (epithelium, stroma, and endothelium) as well as layer and solute concentration dependent consumption. Results. The minimum oxygen tension in the cornea was found to be higher when the lens was placed at 3/4 of the corneal thickness. Moreover, in this position, the influence of the inlay diffusivity was smaller than at more anterior or posterior placements. The diffusivity of the inlay affects the way nutrients will be transported through the cornea. The threshold where glucose may diffuse through or around the implant was found to be 1/100th of the stromal diffusivity. Conclusions. Computational methods are especially attractive to study nutrient transport in the cornea due to the difficulties associated with in vivo or in vitro measurements. The exact parameters that dictate the corneal metabolism are not known. However, the combined analysis of oxygen and glucose distribution is valuable in order to predict the complex physiological changes that arise under intracorneal lens implantation.

Safety of Active Implantable Devices During MRI Examinations: A Finite Element Analysis of an Implantable Pump

The goal of this study was to propose a general numerical analysis methodology to evaluate the magnetic resonance imaging (MRI)-safety of active implants. Numerical models based on the finite element (FE) technique were used to estimate if the normal operation of an active device was altered during MRI imaging. An active implanted pump was chosen to illustrate the method. A set of controlled experiments were proposed and performed to validate the numerical model. The calculated induced voltages in the important electronic components of the device showed dependence with the MRI field strength. For the MRI radiofrequency fields, significant induced voltages of up to 20 V were calculated for a 0.3T field-strength MRI. For the 1.5 and 3.0T MRIs, the calculated voltages were insignificant. On the other hand, induced voltages up to 11 V were calculated in the critical electronic components for the 3.0T MRI due to the gradient fields. Values obtained in this work reflect to the worst case situation which is virtually impossible to achieve in normal scanning situations. Since the calculated voltages may be removed by appropriate protection circuits, no critical problems affecting the normal operation of the pump were identified. This study showed that the proposed methodology helps the identification of the possible incompatibilities between active implants and MR imaging, and can be used to aid the design of critical electronic systems to ensure MRI-safety

Lateral Force Measurements In A Scanning Force Microscope With Piezoresistive Sensors

We measured the torsion of a small scanning force microscope cantilever beam that is induced by the lateral force acting on its tip during the scanning across a sample surface. The detection is based on the piezoresistive principle. Two resistor loops are suitably arranged on the beam and sense differences in the stress distribution. The silicon cantilever has a spring constant of k/sub b/ = 2.7 N/m, a torsional rigidity of k/sub t/ /spl equiv/ 5000 N/m and a resonant frequency of 40 kHz. The integrated silicon tip has a length of 6 /spl mu/m. The torsional sensitivity of the device is demonstrated by imaging in friction mode a grating of 15 nm in height, yielding an amplified (G=2000) output signal of 300 mV.

Ultra-thin layer packaging for implantable electronic devices

State of the art packaging for long-term implantable electronic devices generally uses reliable metal and glass housings; however, these are limited in the miniaturization potential and cost reduction. This paper focuses on the development of biocompatible hermetic thin-film packaging based on poly-para-xylylene (Parylene-C) and silicon oxide (SiOx) multilayers for smart implantable microelectromechanical systems (MEMS) devices. For the fabrication, a combined Parylene/SiOx single-chamber deposition system was developed. Topological aspects of multilayers were characterized by atomic force microscopy and scanning electron microscopy. Material compositions and layer interfaces were analyzed by Fourier transform infrared spectrometry and x-ray photoelectron spectroscopy. To evaluate the multilayer corrosion protection, water vapor permeation was investigated using a calcium mirror test. The calcium mirror test shows very low water permeation rates of 2 × 10−3 g m−2 day−1 (23 °C, 45% RH) for a 4.7 µm multilayer, which is equivalent to a 1.9 mm pure Parylene-C coating. According to the packaging standard MIL-STD-883, the helium gas tightness was investigated. These helium permeation measurements predict that a multilayer of 10 µm achieves the hermeticity acceptance criterion required for long-term implantable medical devices.

Silicon micromachined ultrasonic transducer with improved power transfer for cutting applications

This work presents a light and powerful silicon based ultrasonic micro-cutter. In order to achieve high cutting efficiency as well as good controllability when driven by commercially available control systems, important design parameters haven been identified. They have been verified by FEM-simulation as well as experiments via laser Doppler vibrometer measurements and cutting tests. The samples have been manufactured cost-effectively by microfabrication batch processing and their cutting ability has been successfully demonstrated on chicken tissue, while driven in a typical frequency range from 50 kHz to 100 kHz, generating tip displacements up to 36 μmpp.

Validation of Intra-Operative Measurement Apparatus to Determine the Stiffness Properties of Spinal Motion Segments

The load-displacement behavior of spinal motion segments is commonly determined from in-vitro experiments on cadaveric spines. However, clinically, it is often desirable to quantify the patient specific biomechanical properties of the spine in-vivo. Load-displacement measurement requires direct access to the appropriate anatomy, which is typically available in spinal surgeries that aim to correct lumbar spinal instability or scoliosis. We propose an approach to measure the spinal load-displacement behavior for use during these surgeries.Copyright