ULTRASONIC DRY COUPLING THROUGH TISSUE

Abstract

Ultrasonic transcutaneous energy transmission (UTET) is a promising new technology for delivering electrical power to active biomedical implants.  A key piece of technology required to make UTET devices viable is a “dry” acoustic coupling that can be worn daily for long periods of time by a patient.  The dry coupling must provide a low-reflectance, low-loss, transmission pathway for acoustic energy. For other ultrasound applications such as diagnostic imaging, a coupling gel is typically used to acoustically interface a transducer with the patient. However, in a more permanent application like UTET coupling gel is likely dry out or wash away over time and so a solid-state solution is needed.  The solid coupling must be comfortable, must not lead to skin irritation or other skin problems and must provide good acoustic coupling without requiring large and uncomfortable contact force with the skin.

We report on our investigations into the use of soft silicones for this application.  Silicones are highly biocompatible, and at less than 2 MHz they exhibit low acoustic losses.  Soft silicones have a “sticky” quality to them that provides good acoustic coupling to skin (<20% loss) with very low contact force (<0.5 N).  We present the results of studies into the relationship between contact force and acoustic reflectance for soft silicone couplings in a dry ex vivo porcine skin model. Silicone can be made with a wide array of mechanical properties. As a result, the acoustic impedance of the coupling material can be matched to skin, reducing losses due to reflection. We present two methods used to impedance match to skin; one by increasing the material’s speed of sound, the other by increasing its density. Material property effects on the UTET link efficiency are simulated using a KLM model and the simulation is compared to empirical measurements of the efficiency.