(E-176) Development of a Tungsten and Liquid Metal Matching Layer for Wearable Transcranial Doppler Ultrasound Device

Saturday, October 14, 2023

9:30 AM – 10:30 AM PDT

Location: Exhibit Hall - Row E - Poster # 176

Presenting Author(s)

MS

Megan Santamore

Undergraduate Research Assistance
Princeton University
Bel Air, Maryland, United States

Last Author(s)

EK

Ethan Krings

Graduate Researcher
University of Nebraska- Lincoln, United States

Primary Investigator(s)

EM

Eric Markvicka

Professor
University of Nebraska- Lincoln, United States

Introduction::

Transcranial Doppler (TCD) ultrasound is a type of ultrasound used to measure blood flow velocity in cerebral blood vessels. TCD ultrasound is used by radiology technologists to monitor post-op subarachnoid hemorrhage (SAH) patients that are at high-risk for developing a vasospasm, a condition in which there is persistent contraction of the blood vessels, thereby decreasing blood flow, which can be detected via TCD ultrasound. However, this imaging is limited by radiology technologist availability and hospital scheduling. Thus, a method to continuously monitor the blood flow velocity of post-op, high-risk SAH patients is needed to ensure that any change in a patient’s condition is immediately addressed. Recent studies have shown the efficacy of flexible ultrasound devices for long-term patient monitoring. However, these devices lack the impedance matching layer used by rigid ultrasound devices, which leads to lower ultrasound signal transmission compared to rigid devices. Acoustic impedance is dependent on matching layer density and compressibility. Most flexible materials are low density which leads to an acoustic impedance that is too low for use in a flexible ultrasound matching layer. Here, we introduce a method for incorporating tungsten-filled liquid metal (LM) droplets into an elastomer matrix to create a high-impedance matching layer that remains stretchable and conformal for long-term wearable ultrasound.

Materials and Methods:: Our theoretical models indicate that incorporating tungsten (W) particles into LM greatly increases the acoustic impedance of LM-elastomer composites. However, current methods for introducing W particles into LM causes unwanted LM oxidation which incorporates air voids into the LM-W mixture. These air voids drastically decrease the LM-W density which decreases acoustic impedance and increases signal attenuation. Taking inspiration from mussels in nature, the protocol outlined here takes advantage of polydopamine (PDA). When PDA is mixed with tungsten microparticles, a PDA adhesion layer is formed around the dense tungsten microparticles. When silver nitrate is added to this solution, its reduction and immobilization by the PDA yields the formation of a secondary silver shell around the PDA-coated tungsten microparticles. When these particles are mixed with liquid metal, the silver shells are able to alloy more effectively with the gallium in EGaIn, thereby preventing the need for oxide-mediated mixing. However, experimentally, imperfect silver coatings prevented ideal silver-gallium alloying. Thus, this protocol was altered such that an oxide-mediated EGaIn and tungsten foam was formed and then diluted, such that the liquid metal used in these many rounds of dilutions could act as a filler in the oxidation air pockets formed during the initial oxidation-mediated mixing process.

Results, Conclusions, and Discussions:: Typically, oxide-mediated mixing of LMs and microparticles produce foams with air pockets incorporated into the mixture during manual stirring. This results in a mixture that is less dense than pure EGaIn leading to unfavorable acoustic properties. By implementing this silver coating and liquid metal mixing and dilution protocol, it was found that LM-W mixtures could be produced that were more dense than pure EGaIn, which has a density of 6.250 g/cm3. Specifically, when LM-W mixtures consisting of 25% tungsten microparticles per mass were diluted to mixtures of 3.34% tungsten microparticles per mass, the density of the mixture was reported to be 6.343 g/cm3. Additionally, when LM-W mixtures consisting of 33% tungsten microparticles per mass were diluted to mixtures of 2.25% tungsten microparticles per mass, the density of the mixture was reported to be 6.379 g/cm3. Thus, these results indicate that air pockets formed by oxide-mediated mixing of these microparticles into LM can be filled by EGaIn following several rounds of dilutions and hand-mixing, as the density of these mixtures ultimately surpasses that of pure EGaIn. The acoustic impedance of a material is determined by the density and speed of sound through a material. Pure EGaIn has an acoustic impedance of 16.875 MRayls, while the tungsten and EGaIn dilutions were determined to have acoustic impedances closer to 20.5 MRayls. Further study is needed to determine what the ideal acoustic impedance of the matching layer should be for cerebral blood vessel imaging; however, this work presents an effective protocol for tuning the density, and therefore acoustic impedance, of EGaIn such that the appropriate acoustic impedance liquid metal matching layer can be implemented.

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