Blood Coagulation Time Measurement Using a 1μL of Whole Blood on a TE Mode BAW Resonator

This paper presents a possible way to blood coagulation time measurement using a TE (Thickness Extensions) mode BAW (Bulk Acoustic Wave) resonator which requires as low as 1 micro-liter of whole blood. The blood sample is placed on the top surface of a glass plate where a compressional ultrasonic transducer is fabricated on the bottom surface. The transducer is made of 8 μm thick zinc oxide (ZnO) thin film that has a thickness resonance frequency around 400 MHz. The transducer generates compressional (longitudinal) acoustic wave inside the piezoelectric thin film and glass substrate. The acoustic waves are mostly reflected and trapped inside the device from both sides of it; 1) the glass/liquid (blood) interface, and 2) the transducer/air interface. Most of the acoustic waves are reflected from the second interface because of the higher impedance mismatch, while the reflections from the first boundary are related to impedance (mechanical properties) of the liquid sample or blood. The acoustic impedance of blood changes due to the coagulation process. This affects the reflection coefficient and amplitude of the reflected waves from the blood/glass interface. Thus, the overall acoustic energy trapped inside the bulk film changes over the time which consequently affects the resonator parameters. The blood coagulation time was determined by monitoring the amplitude of the reflected sinusoidal acoustic waves at 400 MHz in the previous work using the same device. However, in this paper we demonstrate the resonance frequency shifts obtained by numerical modeling and practical measurements for a few liquid samples with different mechanical properties. The proposed method has a potential to be used in a low-cost portable coagulation time measurement cartridge which requires only 1μL of whole blood without centrifuging. A simple resonator can be implemented for tracking the resonating frequency to further reduce the size and cost of the device, to make it more suitable for patient self-testing applications.