Glen Guhr

Monitoring blood coagulation with QCM and SH- SAW sensors

Investigations concerning a coagulation sensor based on shear horizontal surface acoustic waves (SH-SAW) are described. As a first step, experimental work was focused on a simpler system: a quartz crystal micro balance (QCM) also called thickness shear mode resonator (TSM). In the literature the suitability of such devices for the characterization of coagulation process is already described. Coagulation tests like Quick and activated partial thromboplastin time (aPTT) have been performed at a resonance frequency of 9 MHz. In a further step, smaller changes in resonant frequency with smaller concentration of fibrinogen could be shown with plasma dilution tests. While performing measurements with different sample volumes the coagulation process was still detectable with five microlitres of blood plasma. A special measurement cylinder was designed and tested to separate the sample from environmental influences. Performing all these tests with plasma or whole blood samples leads to similar results. For determination of the coagulation time, the steepest decrease of resonance frequency vs. time was found as a reproducible value. Measurements have been done at the overtones of the TSM in order to study whether the hemostasis is still measurable at frequencies common to SAW devices. The coagulation process could be measured up to 243 MHz (overtone 27) although the decay length decreases with higher frequencies. Tests using SAW devices with SH-SAW resonator filters (36°rotYX-LiTaO3 substrate) have been done at 168 MHz. The electrolytic character of the plasma caused damage to the electrode structure which could be avoided at later tests. Afterwards the blood coagulation process was successfully measured with SH-SAW. Finally a specially adapted SH-SAW sensor was designed, constructed and tested. Compared to TSM resonators vibrating on the fundamental mode, sensitivity was greatly increased.

Monitoring changes of viscoelasticity during blood coagulation with acoustic sensors

In our previous work we demonstrated the general suitability of surface and bulk acoustic wave (SAW and BAW) structures for recording the continuous change of fluidic behavior in the course of blood coagulation processes [Guhr et al.,2005]. Special advantage can be taken from the direct impact of fluid consistence on SAW and BAW device parameters (e. g. resonator frequency, quality factor, impedance). Beside the ability of continuous detection of coagulation process further tests can be realized with SAW and BAW devices as was shown in an acoustic version of thrombelastography [Guhr et al.,2006]. In this paper, we make the proposal to describe the observed features of blood in the frame of non-Newtonian behavior. Therefore we distinguish between acoustic monitoring of shear viscosity and shear elasticity changes during the whole process of haemostasis. The goal is to get more detailed insight into the ongoing biological process. That approach is suggested from modeling of resonance-antiresonance behavior of thickness shear-mode resonators (TSM), also called quartz crystal microbalance (QCM). The response of resonance and antiresonance parameters can be significantly different from each other, depending on the shear viscosity and elasticity of the loaded fluid. For example, the frequency shifts of both resonances have the same or the opposite sign when changing the viscosity or the elasticity, respectively. The results of modeling the QCM response to viscoelastic loading are used for the interpretation of time dependent measurement signals of the coagulation process.

3F-1 Thrombelastography Using Acoustic Sensors

The purpose of the present paper is to show that thrombelastography can be carried out by means of a quartz crystal microbalance (QCM) also known as a thickness shear mode resonator (TSM). A 9 MHz AT cut quartz was taken for the characterization of whole blood samples from healthy donors. To initiate fibrinolysis effects a special agent was used. The coagulation process was measured by monitoring the resonant frequency of the TSM. This frequency is affected by viscosity changes taking place during the coagulation process. The experimental setup was improved by covers made of PMMA (polymethyl methacrylate) and PDMS (polydimethylsiloxane). These covers seal the electrode in the TSM which is in contact with the coagulating blood sample. The use of such covers allows deleterious environmental effects such as evaporation and drying of the sample to be avoided. The ability of TSM to produce thrombelastograms was successfully demonstrated. The use of such devices allows state of the art measurement setups to be greatly simplified. No complicated mechanical or moving parts must be produced nor is an optical detection unit needed. In addition, direct monitoring of the viscosity results in additional findings at the end of fibrinolysis curves

Novel sensor combining impedance spectroscopy and surface acoustic waves to detect blood coagulation time and hematocrit value

In this paper a new tool simultaneously to detect viscoelastic and dielectric properties of human fluids is presented. Therefore, one-port SAW resonators (SAW … surface acoustic wave) with fundamental modes of 85, 170 und 340 MHz were developed. Shear horizontal polarized surface acoustic waves (SH-SAW) are used to detect the viscoelastic properties of coagulating blood and blood plasma samples. The electrode structure of these sensors additionally can be used simultaneously to detect the dielectric behavior of the whole system by impedance spectroscopy. Therefore, the sensor response in frequencies ranging from kHz to MHz is evaluated. The combination of both methods offers the detection of clinical relevant blood parameters like the blood coagulation time and the hematocrit value within one measurement. Such measurements can be carried out in less then 60 s. Realized as disposable sensors, these devices can be used both in point-of-care and in laboratory devices.

Surface acoustic wave resonators as novel tools for multiparametric blood analysis

In this paper a new tool to assess viscoelastic and dielectric properties of human fluids is presented. Shear horizontal polarized surface acoustic waves (SH-SAW) are used to detect the viscoelastic properties of coagulating blood and blood plasma samples. One-port SAW resonators, with fundamental modes of 85, 170 und 340 MHz were developed. Additionally, their electrode structures can be used simultaneously to detect the dielectric behavior of the whole system by impedance spectroscopy while the frequency ranges from kHz to MHz. The combination of both methods offers the detection of clinical relevant blood parameters like the blood coagulation time and the hematocrit value within one measurement.

More comprehensive model of quartz crystal microbalance response to viscoelastic loading

Using a one-dimensional approach the electric admittance |Y| of a quartz crystal microbalance (QCM) is calculated as a function of frequency depending on material properties as well as the acoustic impedance of an arbitrary loading. Several solutions for the shift of specific frequencies are derived and methods presented to either suppress or compute the influence of parasitic effects. Measurements of high and complex viscosities are possible. The model has been confirmed experimentally.

A surface acoustic wave sensor for detection of cell adhesion

In this paper preliminary results for the detection of the cell adhesion process with shear-horizontal polarized surface acoustic wave devices (SH-SAW) are presented. The murine fibroblast cell line L929 was investigated. Therefore a special measurement setup was developed. It enables us to perform long term measurements at stable ambient conditions. Different amounts of cells where seeded. According to the number of cells different slopes of resonant frequency increase vs. time were found.

Active mixing in microfluidic systems using surface acoustic waves

The growing demand for miniaturization and acceleration in chemical and biological analysis requires new methods for pumping and mixing in Micro Total Analysis Systems and Lab-on-Chip. The interaction of surface acoustic waves (SAW) with liquids atop lead to the excitation of longitudinal pressure waves in the liquid. These high frequency pressure waves generate an acoustic streaming which allows mixing, pumping and even atomization of liquids. However, several issues concerning assembly, signal generation, power durability and handling of such acoustic devices turned out to be rather difficult. In this study, we present our solutions for these issues.

Complex loadings on thickness shear mode resonators

An analytical aproach is presented to describe acoustic thickness shear mode resonators in the full frequency range, including all ovetones. The absolute value of loadings with complex properties (e. g. complex viscosity or shear modulus) can be determined by curve fitting. Loadings with complex behaviour like polymer layers have been studied and can be described. The appearance of layer resonances in thick metal layers have been predicted and proven at even numbered overtones which has generally been considered to be not possible.

Combination of surface acoustic wave measurement and impedance spectroscopy for detection of cell adhesion process

In this study, a new way of investigating a surface acoustic wave (SAW) based sensor device is reported. For that, SAW sensors with a fundamental mode of about 85 MHz and a metallization of gold were developed. Realized as one port resonators, these sensors allow the continuous detection of electric and viscoelastic properties of the same living cell culture in one measurement step. For obtaining the electric properties of the cells the sensors response in a low frequency range up to 10 MHz (impedance spectroscopy) can be investigated. For the detection of viscoelastic properties, the change of the sensors resonant frequency and its impedance are monitored. All signals are detected vs. time and can be compared face to face.