Effect of Ultra-Conformable Coatings on CMUT Behaviour in Biological Environments

Abstract

Capacitive‑micromachined ultrasonic transducers (CMUTs) are promising wireless‑power sources for deep‑seated active implanted medical devices; yet their membranes are not made for implantation, and therefore require the application of a biocompatible coating to protect the patient as well as the membranes. This work investigates a thin (≈ 65 μm), medical‑grade silicone (PDMS) encapsulation as a simple, biocompatible protection layer. Three PCB‑mounted CMUT batches were examined: (i) uncoated (A1), (ii) coated with MED‑1000 (B1), and (iii) coated with MED2‑4213 (C1). Digital holographic microscopy (DHM) quantified membrane displacement in air (dry) and in 5 mL phosphate‑buffered saline (wet). In PBS the uncoated CMUT showed severe corrosion and a mean displacement of 1.05 ± 0.96 nm, whereas both silicone‑coated devices retained a mean displacement of ≈ 2.5 nm (≈ 44 % of the dry value) with no visual damage. A brief hydrophone test confirmed that the coating does not attenuate the dominant ~10 MHz acoustic component. The thin PDMS layer therefore provides corrosion protection, preserves membrane dynamics, and offers acoustic‑impedance matching (≈ 1.5 MRayl), supporting its use as a low‑cost encapsulant for CMUT‑based wireless power transfer in liquid environments.