Fred K. Forster

Optimization of a circular piezoelectric bimorph for a micropump driver

Piezoelectric bimorph actuation has been successfully used in numerous types of microdevices, most notably micropumps. However, even for the simple case of circular geometry, analytical treatments are severely limited. This study utilized the finite-element method to optimize the deflection of a circular bimorph consisting of a single piezoelectric actuator, bonding material and elastic plate of finite dimensions. Optimum actuator dimensions were determined for given plate dimensions, actuator-to-plate stiffness ratio and bonding layer thickness. Dimensional analysis was used to present the results for fixed- and pinned-edge conditions in a generalized form for use as a design tool. For an optimally-thick actuator, the optimum actuator-to-plate radius ratio ranged from 0.81 to 1.0, and was independent of the Youngs modulus ratio. For thin plates, a bonding layer minimally affected the optimum dimensions. The optimized actuator dimensions based on a model of an actual device were within 13% of the fixed-edge condition.

A planar microfabricated fluid filter

Abstract Many blood tests must be performed on plasma without cellular matter present. In the standard laboratory protocol, pure plasma is obtained through centrifugation. In order to produce a miniaturized blood sensor, a method to separate plasma other than centrifugation is needed. We describe the design, fabrication, and testing of a fluid filter that fulfils this need, and also has some features of general interest. Results from a particular device show that we can easily remove 16 μm diameter spheres from the fluid. This filter can be reusable, can potentially remove particles as small as 0.1 μm, and is easily fabricated.

Measurement of fluid turbulence based on pulsed ultrasound techniques. Part 1. Analysis

The pulsed ultrasonic Doppler velocimeter has been used extensively in transcutaneous measurement of the velocity of blood in the human body. It would be useful to evaluate turbulent flow with this device in both medical and non-medical applications. However, the complex behaviour and limitations of the pulsed Doppler velocimeter when applied to random flow have not yet been fully investigated. In this study a three-dimensional stochastic model of the pulsed ultrasonic Doppler velocimeter for the case of a highly focused and damped transducer and isotropic turbulence is presented. The analysis predicts the correlation and spectral functions of the Doppler signal and the detected velocity signal. The analysis addresses specifically the considerations and limitations of measuring turbulent intensities and one-dimensional velocity spectra. Results show that the turbulent intensity can be deduced from the broadening of the spectrum of the Doppler signal and a mathematical description of the effective sample-volume directivity. In the measurement of one-dimensional velocity spectra at least two major complicacations are identified and quantified. First, the presence of a time-varying, broad-band random process (the Doppler ambiguity process) obscures the spectrum of the random velocity. This phenomenon is similar to that occurring in laser anemometry, but the ratio of the level of the ambiguity spectrum to the largest detected velocity spectral component can be typically two to three orders of magnitude greater for ultrasonic technique owing to the much greater wavelength. Secondly, the spatial averaging of the velocity field in the sample volume causes attenuation in the measured velocity spectrum. For the ultrasonic velocimeter, this effect is very significant. The influence of the Doppler ambiguity process can be reduced by the use of two sample volumes on the same acoustic beam. The signals from the two sample volumes are cross-correlated, removing the Doppler ambiguity process, while retaining the random velocity. The effects of this technique on the detected velocity spectrum are quantified explicitly in the analysis for the case of a three-dimensional Gaussianshaped sample-volume directivity.

Low-order modeling of resonance for fixed-valve micropumps based on first principles

Micropumps that utilize fixed-valves, i.e., valves having no moving parts, are relatively easy to fabricate and inherently reliable due to their simplicity. Since fixed-valves do not close, pumps based on them need to operate in a well-designed resonant mode in order to attain flow rates and pressures comparable with other designs. However, no methodology currently exists to efficiently investigate all the design parameters including valve size to achieve optimal resonant response. A methodology that addresses this problem is 1) the determination of optimal parameters including valve size with a low-order linear model capable of nonempirical prediction of resonant behavior, and 2) the independent determination of the best valve shape for maximal valve action over a target Reynolds number range. This study addresses the first of these two steps. The hypothesis of this study is that the resonant behavior of a fixed-valve micropump can be accurately predicted from first principles, i.e., with knowledge only of geometric parameters and physical constants. We utilized a new low-order model that treats the valves as straight rectangular channels, for which the unsteady solution to the Navier-Stokes equations is exact and with which the problem was linearized. Agreement with experiment using pump-like devices with valves replaced by straight channels was found to be excellent, thereby demonstrating the efficacy of the model for describing all aspects of the pump except actual valves. Agreement with experiment using pumps with Tesla-type valves was within 20 percent. With such accuracy and without the need for empirical data, the model makes possible reliable, efficient investigation and optimization of over 30 geometric and material parameters.

Improvements in Fixed-Valve Micropump Performance Through Shape Optimization of Valves

The fixed-geometry valve micropump is a seemingly simple device in which the interaction between mechanical, electrical, and fluidic components produces a maximum output near resonance. This type of pump offers advantages such as scalability, durability, and ease of fabrication in a variety of materials. Our past work focused on the development of a linear dynamic model for pump design based on maximizing resonance, while little has been done to improve valve shape. Here we present a method for optimizing valve shape using two-dimensional computational fluid dynamics in conjunction with an optimization procedure. A Tesla-type valve was optimized using a set of six independent, non-dimensional geometric design variables. The result was a 25% higher ratio of reverse to forward flow resistance (diodicity) averaged over the Reynolds number range 0<Re⩽2000 compared to calculated values for an empirically designed, commonly used Tesla-type valve shape. The optimized shape was realized with no increase in forward flow resistance. A linear dynamic model, modified to include a number of effects that limit pump performance such as cavitation, was used to design pumps based on the new valve. Prototype plastic pumps were fabricated and tested. Steady-flow tests verified the predicted improvement in diodicity. More importantly, the modest increase in diodicity resulted in measured block-load pressure and no-load flow three times higher compared to an identical pump with non-optimized valves. The large performance increase observed demonstrated the importance of valve shape optimization in the overall design process for fixed-valve micropumps.

Ultrasonic attenuation in articular cartilage

Previous studies have utilized articular cartilage from joints as a model to investigate the influence of various constituents in a connective tissue matrix on ultrasonic properties. These studies have assumed a degree of homogeneity of articular cartilage taken from the same joint. However, tactile loads on articular cartilage vary significantly with location in a joint, and the effects of mechanical load on the connective tissue matrix and the resulting effects on ultrasonic properties are not known. This work reports the variations in acoustical properties of bovine articular cartilage from the stifle (knee) joint both among different joints and within each joint. A pulse-echo transmission technique was used to measure acoustic attenuation in the frequency range of 10 to 40 MHz. The attenuation coefficient was characterized by the integrated attenuation (mean value) over the frequency bandwidth considered. Integrated attenuation averaged over each joint varied among joints from 3.2 to 7.5 NP/cm (6.0 +/- 2.0, mean +/- s.d.). Additionally, a linear regression (r = 0.59) of all the data versus location along the patellar groove indicated that within joints integrated attenuation increased from proximal to distal locations by 6% to 60% (32 +/- 25, mean +/- s.d.). The variations observed among joints and along the patellar groove within a given joint suggest that studies utilizing articular cartilage to determine the role of connective tissue constituents on acoustic properties require control for joint and location. An additional outcome of this study was the observation that damage to the load-bearing surface of articular cartilage may be detectable ultrasonically through characteristics of the acoustic reflection from the articular surface.

Measurement of fluid turbulence based on pulsed ultrasound techniques. Part 2. Experimental investigation

An extensive experimental programme in both laminar and turbulent flow was undertaken to examine the validity of all of the major implications of the model of the pulsed ultrasonic Doppler velocimeter for turbulent flow developed in part 1 of this investigation. The turbulence measurements were made in fully developed flow at the centre of a 6·28 cm diameter pipe. The Reynolds number of the flow ranged from 6000 to 40000. The carrier frequency of the ultrasonic velocimeter was 4·7 MHz. Measurements of the turbulence intensity and of the one-dimensional velocity spectra made with the ultrasonic velocimeter are compared with the analysis and with the actual quantities as measured by a hot-film anemometer. The experimental results are in agreement with theoretical predictions. Measurements of one-dimensional turbulence spectra with reduced ambiguity spectra made by the two sample volume methods described in part 1 are presented. The results verify the analysis and indicate that an improvement in the useful dynamic range of the velocity power spectrum of nearly three orders of magnitude can realistically be achieved.

A new high intensity focused ultrasound applicator for surgical applications

Improved high-intensity focused ultrasound (HIFU) surgical applicators are required for use in a surgical environment. We report on the performance and characteristics of a new solid-cone HIFU applicator. Previous HIFU devices used a water-filled stand-off to couple the ultrasonic energy from the transducer to the treatment area. The new applicator uses a spherically-focused element and a solid aluminum cone to guide and couple the ultrasound to the tissue. Compared with the water-filled applicators, this new applicator is simpler to set up and manipulate, cannot leak, prevents the possibility of cavitation within the coupling device, and is much easier to sterilize and maintain during surgery. The design minimizes losses caused by shear wave conversion found in tapered solid acoustic velocity transformers operated at high frequencies. Computer simulations predicted good transfer of longitudinal waves. Impedance measurements, beam plots, Schlieren images, and force balance measurements verified strong focusing and suitable transfer of acoustic energy into water. At the focus, the -3 dB beam dimensions are 1.2 mm (axial)/spl times/0.3 mm (transverse). Radiation force balance measurements indicate a power transfer efficiency of 40%. In vitro and in vivo tissue experiments confirmed the applicators ability to produce hemostasis.

Ultrasonic assessment of skin and surgical wounds utilizing backscatter acoustic techniques to estimate attenuation

Backscatter acoustic techniques at high ultrasonic frequencies (10-40 MHz) were utilized to investigate the ultrasonic attenuation coefficient of normal skin and the relationship between attenuation and the healing of surgical wounds ranging in age from 9 to 49 days. The attenuation coefficient was calculated with measurements from depths of 0.5 and 1.0 mm, primarily in the reticular dermis. The values for control skin ranged from 6.0 Np cm-1 at 10 mHz to 19.6 Np cm-1 at 40 MHz and a corresponding slope of 0.45 Np cm-1 MHz-1. Wound attenuation initially increased with wound age from approximately 15% of control at day 9 to 30% of control at day 34 but did not continue to increase through day 49. During the same period, however, total collagen (as a percentage of wet weight) increased at a constant rate from approximately 45% of control at day 9 to 70% of control at day 49. Thus the attenuation coefficient in healing wounds over the time period studied may be sensitive to more than total collagen content in the tissue. It may be affected by other competing factors as wounds mature such as intermolecular cross-linking, collagen fiber bundle size, and structural arrangement of fiber bundles or other tissue constituents such as proteoglycans and elastin. The observations that wound integrated attenuation using backscatter techniques over the range of 10-40 MHz substantially increased in early wounds, did not increase beyond day 34, and remained significantly lower than control values over the entire healing period studied are in agreement with the results of an independent study of the same tissue using transmission techniques at 100 MHz.

Biochemical and acoustical parameters of normal canine skin

The scanning laser acoustic microscope (SLAM) at 100 MHz and backscattering acoustic technique (BAT) at 10-40 MHz were used to examine the normal canine skin. Skin specimens from four animals and from four locations on the animal were analyzed biochemically and morphologically as well as acoustically. At 100 MHz, the mean ultrasonic speed obtained with the SLAM was 1632+or-34 m/s and the mean attenuation coefficient at 25 MHz was 13+or-4 Np/cm. The biochemical analyses yielded a collagen concentration of 20+or-2% of the net weight or 65+or-12% of the dried defatted weight and a water concentration of 60+or-3% of the wet tissue.<<ETX>>