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Biomedical Imaging and Instrumentation

Biomedical Imaging and Instrumentation - Poster Session B

Poster G8 - Fabrication Of Elastomer Composite Matching Layer For Wearable Ultrasound Patch

Thursday, October 24, 2024

2:45 PM - 3:45 PM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Adoniram Johnson

Undergraduate student
University of Maryland Baltimore County
Baltimore City, Maryland, United States

Co-Author(s)

SP

Spencer Pak

Graduate Research Asst
University of Nebraska–Lincoln, United States
AM

Andrea Mikulasova

Graduate Research Asst
University of Nebraska–Lincoln, United States
BH

Benjamin D. Hage

Post-Doc Rsch Assoc
University of Nebraska–Lincoln
Lincoln, Nebraska, United States
GB

Greg Bashford

Professor
University of Nebraska–Lincoln, United States

Primary Investigator(s)

Eric Markvicka, PhD

Robert F. and Myrna L. Krohn Assistant Professor of Biomedical Engineering
University of Nebraska–Lincoln, United States

Introduction: Aneurysmal subarachnoid hemorrhaging affects 6 out of 100,000 individuals globally annually. This can cause a vasospasm, a constriction of the artery that reduces blood flow, which results in stroke. Ultrasound provides a noninvasive way for users to receive instantaneous feedback on the blood flow velocity in the middle cerebral artery (MCA) and the internal carotid artery (ICA). By taking the Lindegaard ratio of the velocities (MCA/ICA), a vasospasm can be predicted if the result is greater than 3. Ultrasound requires a trained professional, limiting how frequently measurements can be taken. A wearable ultrasound patch can continuously monitor a patient without a trained professional, allowing for measurements to occur more often. Ultrasound uses acoustic waves to detect internal structures. As they pass through materials with different acoustic impedances, some energy is reflected, reducing the wave's ability to penetrate to the intended structure. The transducer's acoustic impedance is much greater than the skin's. This causes a significant reflection, which could be minimized through a matching layer that has an acoustic impedance equivalent to the geometric mean of the two impedances. Typical matching layers are made from rigid composites, which can achieve a wide range of acoustic impedances, but are not flexible or stretchable enough to be used on an ultrasound patch that conforms to the curvature of the human body. In this work, we discuss the development of elastomer composite matching layers that achieves high acoustic impedances between 1.92 - 2.25 MRayls while maintaining its flexibility and stretchability.

Materials and

Methods: A silicone elastomer (ExSil 100) is utilized as the base material of the matching layer. A surfactant was used to mix the hydrophobic elastomer and hydrophilic suspension fluid. The impedance of the elastomer is increased by adding a nanoparticle suspension, which is a combination of the high density fluid low viscosity polytungstate (LVP) and tungsten nanoparticles (NPs). This is due to the impedance being the product of density and speed of sound. The LVP and NPs increase the density of the matching layer. Furthermore, the LVP encases the NPs, helping the elastomer maintain its flexibility and stretchability, reducing the stiffness caused by the addition of rigid NPs. Through different volume loadings of the NP suspension, various acoustic impedances can be achieved. Four different volume loadings (10%, 15%, 20% and 25%) of tungsten NPs in LVP were tested. The NP suspensions were then added to the silicone elastomer at a volume loading of 30%. The Wood's model is utilized to determine the impedance of the matching layer at various volume loadings of NP suspension. From a given density and compressibility of the elastomer, the speed of sound at any volume loading can be calculated. To verify the results of the model, an experiment is set up where the matching layer is placed between two ultrasound transducers in a water bath. The speed of sound is measured from the time it takes for the wave to travel through the material at a given distance, and the acoustic impedance and attenuation are calculated.

Results, Conclusions, and Discussions: The suspension fluid and NP mixture has been optimized using LVP and tungsten nanoparticles. The acoustic impedances increased from 1.92 to 2.25 MRays as the volume loading of NP in LVP was increased from 10% to 25%. This compares well with the Wood’s model, which predicts an acoustic impedance of 1.80 to 2.10 MRayls for the same volume loading. The average error between Wood's model prediction and the experimental results was 7.26%. The attenuation for these volume loadings ranged from 6.09 db/mm to 6.7 db/mm. This is relatively low compared to other materials from previous work such as eutectic gallium - indium in a silicone elastomer, which was shown to have an attenuation of approximately 33 db/mm at a 50% volume loading. Future work includes fabrication of a wearable ultrasound patch utilizing the described matching layer in order to observe how the matching layer improves the readings of the patch. This work demonstrates the feasibility of a novel NP suspension as a method of increasing the acoustic impedance in hyper elastic materials. Such materials are necessary as acoustic matching layers in soft, conformal wearable ultrasound sensors. The matching layer will improve device efficiency and resolution, enabling more accurate prognosis of health issues in patients.

Acknowledgements (Optional): This work was supported by the National Science Foundation (grant # 2050587),
University of Nebraska - Lincoln Smart Materials and Robotics Laboratory and the
Biomedical Imaging and Biosignal Analysis Laboratory.