Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 149827

149827 - Acoustic Properties of Stretchable Liquid Metal-Elastomer Composites for Matching Layers in Wearable Ultrasonic Transducer Arrays 

Introduction

Wearable ultrasound devices have recently emerged as a promising solution for long-term monitoring of deep tissue organs and hemodynamics. These devices, typically in the form of patches with piezoelectric elements embedded in a soft and stretchable substrate, are designed  to conform to the skin, allowing for continuous monitoring. While these devices provide improved wearability compared to conventional ultrasound probes, stretchable ultrasound devices lack matching layers, leading to suboptimal energy transmission into the body and decreased resolution. 

Matching layers are located between the piezoelectric element and imaging medium to improve sound transmission by reducing losses due to acoustic impedance mismatch. Conventional acoustic matching layers, composed of rigid particles in an epoxy matrix, are incompatible with large-area stretchable ultrasound devices. To address this limitation, we introduce a soft and elastically deformable material for acoustic matching layers consisting of liquid metal (LM) microdroplets dispersed in an elastomer matrix. The volume loading of the liquid metal droplets can be adjusted to control the acoustic impedance of the elastomer composite, while remaining soft and highly deformable. 

Results and Discussion

In this study, we explored the effects of LM droplet size and volume loading on composite acoustic impedance and signal attenuation. Eutectic gallium indium (EGaIn) droplets were dispersed in an elastomer matrix (Sylgard 184; Dow) at volume loadings of 50%, 60%, and 70%. Composites with LM microdroplets on the order of 1 micron O(1), 10 microns O(10), and 100 microns O(100) were created. The density of each composition was measured gravimetrically while the speed of sound (SOS) through the composite was measured using a pitch-catch method with a 2.25 MHz emitter transducer and a 3.5 MHz receiver transducer. The SOS for all emulsion sizes increased as the volume loading was increased from 50% to 70%. The velocities agreed well with the Wood model for SOS in emulsions for O(1) and O(10) droplets. The acoustic impedance was then calculated by multiplying the SOS and density measurements for each sample. Attenuation was calculated by comparing the intensity of the acoustic wave through the composites to the signal intensity in water. The O(1) emulsions at 50% volume loading had the lowest measured attenuation. However, attenuation rose sharply as volume loading increased. Attenuation remained relatively constant for O(10) and O(100) composites. 

The LM-elastomer composites were placed under mechanical strain and acoustic signal perpendicular to the strain direction was measured to determine the effects of droplet elongation on impedance and attenuation. The acoustic impedance under strain was relatively unchanged for the O(1) and O(100) composites while impedance dropped slightly for the O(10) composites. The change in droplet shape from spherical to ellipsoidal had little influence on signal attenuation.

An LM-elastomer matching layer consisting of O(1) microdroplets at a 50% volume loading was incorporated into a stretchable ultrasound device. The device consisted of a 2x2 piezoelectric element array with a 2.75 MHz center frequency for Doppler ultrasound measurements. The stretchable Doppler ultrasound device was able to detect motion in a phantom model mimicking arterial blood flow. 

Conclusion

Here, we present a soft and stretchable acoustic matching layer created by embedding LM microdroplets into an elastomer matrix. The high density and fluidity of the LM emulsions led to a soft composite with an impedance as high as 4.8 Mrayl, over 440% greater than the impedance of the unfilled elastomer. Additionally, negligible changes to impedance and attenuation were observed when the composite was strained to values seen in wearable devices (<30%). A stretchable ultrasound device with an LM-elastomer matching layer was able to detect motion in a phantom model.

Presenting Author: Ethan Krings University of Nebraska-Lincoln

Presenting Author Biography: Ethan is a PhD student at the University of Nebraska-Lincoln studying Mechanical Engineering and Applied Mechanics. He is a research assistant in the Smart Materials and Robotics Lab at UNL where his current research focuses on developing gallium-based liquid metal composites for use in flexible electronics and wearable health monitoring devices.

Authors:

Ethan Krings University of Nebraska-Lincoln
Benjamin Hage University of Nebraska-Lincoln
Greg Bashford University of Nebraska-Lincoln
Eric Markvicka University of Nebraska-Lincoln