A portable blood plasma clot micro-elastometry device based on resonant acoustic spectroscopy

Abstract

Abnormal blood clot stiffness is an important indicator of coagulation disorders arising from a variety of cardiovascular diseases and drug treatments. Here, we present a portable instrument for elastometry of microliter volume blood samples based upon the principle of resonant acoustic spectroscopy, where a sample of well-defined dimensions exhibits a fundamental longitudinal resonance mode proportional to the square root of the Young's modulus. In contrast to commercial thromboelastography, the resonant acoustic method offers improved repeatability and accuracy due to the high signal-to-noise ratio of the resonant vibration. We review the measurement principles and the design of a magnetically actuated microbead force transducer applying between 23 pN and 6.7 nN, providing a wide dynamic range of elastic moduli (3 Pa-27 kPa) appropriate for measurement of clot elastic modulus (CEM). An automated and portable device, the CEMport, is introduced and implemented using a 2 nm resolution displacement sensor with demonstrated accuracy and precision of 3% and 2%, respectively, of CEM in biogels. Importantly, the small strains (<0.13%) and low strain rates (<1/s) employed by the CEMport maintain a linear stress-to-strain relationship which provides a perturbative measurement of the Young's modulus. Measurements of blood plasma CEM versus heparin concentration show that CEMport is sensitive to heparin levels below 0.050 U/ml, which suggests future applications in sensing heparin levels of post-surgical cardiopulmonary bypass patients. The portability, high accuracy, and high precision of this device enable new clinical and animal studies for associating CEM with blood coagulation disorders, potentially leading to improved diagnostics and therapeutic monitoring.

FIG. 1.

Example data set showing the (a) magnitude and (b) phase of $\tilde{\mathrm{I}}\left(\mathrm{\omega }\right)$ obtained from a RASOV measurement of normal platelet-poor plasma (NPP).

FIG. 2.

(a) Schematic of the signal and device components of the CEMport. (b) An enlarged view schematic of the sample microwell. Insets display photographs of the 500 μm bead (small red arrow) and the sample microwell. (c) Photographs of the CEMport with major components indicated. Left: overall view of the portable cart. Right: layout within the humidified chamber. Blood clots prepared within the plastic microwell are slid into a plastic mount; the use of plastic avoids induced currents during operation of the solenoid electromagnet. A small air gap and vibration-damped mounting post ensure that the microwell and displacement sensor are mechanically isolated from the solenoid during RASOV.

FIG. 3.

CEMport measurements of agarose samples with varying maximum strain amplitudes by adjusting the solenoid voltage. (a) Top: raw displacement data versus time; bottom: corresponding amplitude and phase spectral data and their fitting to the Lorentzian model. (b) Fitted f₀ versus maximum strain amplitude. The number of successful fits is indicated for each data point (e.g., 4/10 means 4 out of 10), where only data with R² > 0.85 are considered successful.

FIG. 4.

Calibration of RASOV using agarose gels of varying concentrations. (a) Elastic moduli measured by a texture analyzer were fitted to a power-law function. (b) Resonant frequencies measured by RASOV were fitted to a linear function. The functional description of RASOV’s characteristic is obtained by combining the two modeling fittings (mean ± standard deviation).

FIG. 5.

Box-and-whisker plot of consecutive RASOV measurements of elasticity of a 3.06 mg/ml agarose gel (left with blue data circles) and a 3.62 mg/ml agarose gel (right with red data triangles). Measurement times were within a window of 18 min (left) and 12 min (right). The bottom, middle, and top solid lines of each box are defined by the first (25%), middle (50%), and third quartiles (75%) of corresponding data distribution. Horizontal dashed lines across the boxes are mean values of elasticity.

FIG. 6.

CEMport measurements of normal platelet-poor plasma (NPP) versus heparin concentration. The inset displays a magnified view up of low concentrations on a linear scale to include the zero concentration data point.