Martin Gröschl

Ultrasonic separation of suspended particles

The forces on suspended particles in acoustic fields are reviewed briefly and the theoretical modelling of ultrasonic separators based on piezoelectrically excited layered resonators is described. Two flow-through resonator chamber concepts for ultrasonic particle (bio-cell) separation are investigated: (a) the coagulation or sedimentation approach, (b) the so-called h-shaped ultrasonic separator. The h-shaped ultrasonic separator is analysed by combining for the first time the mathematical modelling of the laminar flow with the acoustic force based velocity field of the particles relative to the suspension medium. This allows a complete modelling of the resonators particle separation performance. Examples for separation chamber designs optimized by use of the mathematical model are presented and the calculated particle traces in the h-resonator are shown and compared with experimental results. For direct comparison of different ultrasonic flow through separator concepts a separation performance figure is introduced and its value is given for the two investigated separator concepts for the sample suspensions of polystyrene spheres, yeast and spirulina cells in (salt) water. The presented results are of importance for the state of the art design of acoustic cell filters for perfusion type bioreactors, as recently launched at the biotechnology market, as well as for the ultrasonic separation of plant (algae) cells under low gravity conditions, where the sedimentation concept fails.

Comparison between BAW and SAW sensor principles

A comparison is given between piezoelectrically excited bulk acoustic wave (BAW) and surface acoustic wave (SAW) elements with respect to their primary sensitivity functions and principal capabilities for sensor applications. The importance of mode purity for high dynamic range sensors is emphasized. Characteristic sensor examples are reviewed, and the special demands on the electronics for BAW and SAW elements in the sensor field are described (e.g., cable problem, wireless SAW sensors). For a fair evaluation, a performance figure, SQ, defined as the product of reduced sensitivity S and resonator Q-value, is introduced. The potential of alternative piezoelectric materials for future sensor developments is discussed briefly.

Selective Retention of Viable Cells in Ultrasonic Resonance Field Devices

A double‐chamber ultrasonic resonance field device was used for the separation and retention of animal cells. By controlling operational parameters such as flow and power input, the device can retain viable cells more efficiently, allowing for selective removal of nonviable cells and cell debris. A simple model describing the forces acting on spherical particles in a sound field (primary radiation force, Bernoulli force, secondary radiation force) is presented. Field stability increases with decreasing average flow rates and increasing power input. At very high field stability, as achieved with low flow rates and high power input, the selectivity for viable cells is reduced, due to the efficient retention of all types of particles. At high flow rates and resulting low field stability, selectivity is also reduced, due to poor separation efficiency, resulting in equally low retention of viable cells, nonviable cells, and cell debris.

Measurement of absolute blood flow velocity and blood flow in the human retina by dual-beam bidirectional Doppler fourier-domain optical coherence tomography.

PURPOSE The present experiments were undertaken to evaluate the validity of absolute flow velocity measurements with a dual-beam bidirectional Doppler Fourier-domain optical coherence tomography (FD-OCT) system. METHODS The flow velocities of diluted milk through a glass capillary were measured at 30 different preset velocities in the range of 0.9 to 39.3 mm/s by bidirectional Doppler FD-OCT. The flow through the capillary was controlled by two infusion pumps working in different flow ranges and based on different technical principles. In vivo the validity of the method for measuring blood flow in retinal vessels was tested at bifurcations. The continuity equation was verified at 10 retinal venous bifurcations of 10 young healthy subjects (mean age, 29 ± 3 years) by velocity measurements, using dual-beam bidirectional Doppler FD-OCT, and measurements of retinal diameters, using the Dynamic Vessel Analyzer. RESULTS Flow velocities as measured with bidirectional Doppler FD-OCT in the glass capillary were in good agreement with the preset velocities (r = 0.994, P < 0.001 each). No significant difference was found between flow in the trunk vessels before the bifurcation (11.3 ± 5.2 μL/min) and the sum of flows in the daughter vessels (10.7 ± 4.8 μL/min). A significant association was found between retinal vessel diameters and both retinal blood velocities (r = 0.72, P < 0.001) and retinal blood flow (r = 0.95, P < 0.0001). CONCLUSIONS Dual-beam bidirectional Doppler FD-OCT delivered accurate retinal blood velocity values and, thus, offers high potential for examination of retinal blood flow in ocular disease.

Measurement of the total retinal blood flow using dual beam Fourier-domain Doppler optical coherence tomography with orthogonal detection planes

We present a system capable of measuring the total retinal blood flow using a combination of dual beam Fourier-domain Doppler optical coherence tomography with orthogonal detection planes and a fundus camera-based retinal vessel analyzer. Our results show a high degree of conformity of venous and arterial flows, which corroborates the validity of the measurements. In accordance with Murrays law, the log-log regression coefficient between vessel diameter and blood flow was found to be ~3. The bloods velocity scaled linearly with the vessel diameter at higher diameters (> 60 µm), but showed a clear divergence from the linear dependence at lower diameters. Good agreement with literature data and the large range and high measurement sensitivity point to a high potential for further investigations.

Viscosity sensor utilizing a piezoelectric thickness shear sandwich resonator

This paper describes a novel quartz crystal sensor for measurement of the density-viscosity product of Newtonian liquids. The sensor element consists of two piano-convex AT-cut quartz crystals vibrating in a thickness-shear mode with the liquid sample in between. This special set-up allows suppression of disturbing resonances in the liquid layer. Such resonances are generated in the common single-plate arrangements due to compressional waves caused by spurious out-of-plane displacements of the shear vibrating finite plate. The primary measurands of the sensor are the fundamental resonance frequency and the associated resonance Q-value, which are influenced by the viscously entrained liquid contacting the quartz surface. The sensor allows the measurement of samples with viscosities from almost zero (air!) up to 200 cP with a sample volume of 130 /spl mu/l.

Response of Retinal Blood Flow to Systemic Hyperoxia as Measured with Dual-Beam Bidirectional Doppler Fourier-Domain Optical Coherence Tomography

Purpose There is a long-standing interest in the study of retinal blood flow in humans. In the recent years techniques have been established to measure retinal perfusion based on optical coherence tomography (OCT). In the present study we used a technique called dual-beam bidirectional Doppler Fourier-domain optical coherence tomography (FD-OCT) to characterize the effects of 100% oxygen breathing on retinal blood flow. These data were compared to data obtained with a laser Doppler velocimeter (LDV). Methods 10 healthy subjects were studied on 2 study days. On one study day the effect of 100% oxygen breathing on retinal blood velocities was studied using dual-beam bidirectional Doppler FD-OCT. On the second study day the effect of 100% oxygen breathing on retinal blood velocities was assessed by laser Doppler velocimetry (LDV). Retinal vessel diameters were measured on both study days using a commercially available Dynamic Vessel Analyzer. Retinal blood flow was calculated based on retinal vessel diameters and red blood cell velocity. Results As expected, breathing of pure oxygen induced a pronounced reduction in retinal vessel diameters, retinal blood velocities and retinal blood flow on both study days (p<0.001). Blood velocity data correlated well between the two methods applied under both baseline as well as under hyperoxic conditions (r = 0.98 and r = 0.75, respectively). Data as obtained with OCT were, however, slightly higher. Conclusion A good correlation was found between red blood cell velocity as measured with dual-beam bidirectional Doppler FD-OCT and red blood cell velocity assessed by the laser Doppler method. Dual-beam bidirectional Doppler FD-OCT is a promising approach for studying retinal blood velocities in vivo.

Ultrasound affects distribution of plasminogen and tissuetype plasminogen activator in whole blood clots in vitro

Ultrasound of 2 MHz frequency and 1.2 W/cm(2) acoustic intensity was applied to examine the effect of sonication on recombinant tissue-type plasminogen activator (rt-PA)-induced thrombolysis as well as on the distribution of plasminogen and t-PA within whole blood clots in vitro. Thrombolysis was evaluated quantitatively by measuring clot weight reduction and the level of fibrin degradation product D-dimer (FDP-DD) in the supernatant. Weight reduction in the group of clots treated both with ultrasound and rt-PA was 35.2% +/-6.9% which is significantly higher (p<0.0001) than in the group of clots treated with rt-PA only (19.9% +/-4.3%). FDP-DD level in the supernatants of the group treated with ultrasound and rt-PA increased sevenfold compared to the group treated with rt-PA alone, (14895 +/-2513 ng/ml vs. 2364 +/-725 ng/ml). Localization of fibrinolytic components within the clots was accomplished by using gel-entrapping technique and immunohistochemistry. Spatial distributions of t-PA and plasminogen showed clearly that ultrasound promoted the penetration of rt-PA into thrombi significantly (p<0.0001), and broadened the zone of lysis from 8.9 +/-2.6 microm to 21.2 +/-7.2 microm. We speculate that ultrasound enhances thrombolysis by affecting the distribution of rt-PA within the clot.

Surface analysis on rolling bearings after exposure to defined electric stress

Abstract This article gives an overview about classical and frequency converter-induced spurious bearing currents in induction machines and discusses typical damage patterns caused by the current passage. To investigate on the electric damage mechanisms, test bearings are operated in a test rig and exposed to specific (classical low-frequency, and high-frequency) bearing currents. The induced damages to the surfaces are analysed visually and with the help of an atomic force microscope, and compared for the different electric regimes applied. Further, the electrically damaged bearing surfaces are characterized by standard roughness parameters. The surface structure observable on certain test bearings shows good correlation to the structure found with a bearing that had failed in the field under similar electric conditions. One of the investigated electric regimes applying high-frequency currents proved capable of generating fluting patterns - as found in real applications - on the test rig. The experiments also indicate that high-frequency bearing currents, although in total dissipating less energy, are more dangerous to a bearing than continuous current flow. The presented method gives a good starting point for further investigation on electric current damage in bearings, especially regarding high-frequency bearing currents, and on bearing/grease lifetime under specific electric regimes.

In vivo tear film thickness measurement and tear film dynamics visualization using spectral domain optical coherence tomography.

Dry eye syndrome is a highly prevalent disease of the ocular surface characterized by an instability of the tear film. Traditional methods used for the evaluation of tear film stability are invasive or show limited repeatability. Here we propose a new non-invasive fully automated approach to measure tear film thickness based on spectral domain optical coherence tomography and on an efficient delay estimator. Silicon wafer phantom were used to validate the thickness measurement. The technique was applied in vivo in healthy subjects. Series of tear film thickness maps were generated, allowing for the visualization of tear film dynamics. Our results show that the in vivo central tear film thickness measurements are precise and repeatable with a coefficient of variation of about 0.65% and that repeatable tear film dynamics can be observed. The presented approach could be used in clinical setting to study patients with dry eye disease and monitor their treatments.