(E-178) Design of a Mass - Spring Matching Layer for a Wearable Ultrasound Patch

Ultrasound is a non-invasive diagnostic technique used to image and measure internal components of the body such as arteries and various organs. It can also be used to monitor blood flow through the cerebral arteries, which is crucial in the early detection of  medical problems such as a stroke or vasospasms. Currently, ultrasound probes are rigid and handheld, which does not allow for continuous monitoring of blood flow through the cerebral arteries.A wearable ultrasound patch could allow for continuous monitoring of the cerebral arteries which can provide early detection of medical emergencies. Current ultrasound technology requires the use of a matching layer to mediate the difference in acoustic impedance between the transducer and the skin; however, existing matching layers are rigid and therefore, can not be used in a wearable ultrasound patch. Here, we designed and fabricated an stretchable matching layer that utilizes a mass-spring system to tune acoustic impedance. The matching layer consists of liquid metal film, which acts as the mass, applied onto an elastic silicone substrate, which acts as the spring. The acoustic impedance of the matching layer can be adjusted by altering the thicknesses of the liquid metal film and the silicone substrate. 

Computer modeling to determine the optimal thickness of each layer was derived from equations in” Novel Multi-Layer Polymer-Metal Structures For Use in Ultrasonic Transducer Impedance and Backing Absorber Applications” Toda, et.al. The material properties of the LM mass and elastomer spring needed for the computer model were found through literature review and experimental data.  

The matching layer consists of 3 components : A 160 micron outer stiffer PDMS (Sylgard 184) layer, a 10 micron Eutectic Gallium Indium (EGaIn) layer, and a 120 micron inner PDMS layer. To make the inner PDMS layer, an injection molding technique will be used. A CNC  mill, Bantam (Peeksville, NY) will be used to make the top and bottom portions of the mold out of Delrin. Then a laser cutter, LPKF(Tualatin, OR)  will be used to cut a spacer out of steel shim stock. Next, a film of EGaIn will be applied onto the first PDMS layer using an airbrush. Afterwards, the LPKF will be used to laser ablate the EGaIn film into a grid geometry. Finally the last PDMS layer will be spin coated on top. Characterization of the matching layer will be performed using acoustic resonance spectroscopy. Results from the matching layer characterization will then be used to refine the computer model. Matching layer modeling, fabrication, and characterization will be repeated until the required matching layer properties are met.

From computer modeling the optimal thickness of each layer was found. A 160 micron outer stiffer PDMS layer. A 10 micron EGaIn layer, and a 120 micron inner PDMS layer proved to be the optimal geometry for the matching layer. This matching layer should operate at 2.4 MHz and 58 MRayl. Future work would include a double matching layer that would increase the operating bandwidth  of the matching layer and make it suitable for ultrasound over a wider range of frequencies.