Next Generation Ultrasound Transducers: The Role of Passive Layers and Hydrogel Couplants

The performance of ultrasound technologies, including biomedical imaging, therapeutics, and industrial inspection, is fundamentally dependent on the efficient transmission of acoustic energy between the transducer and the target medium. This is conventionally achieved through the use of acoustic couplants. Traditional gel couplants are limited by dehydration and lack of mechanical support, rendering them incompatible with evolving ultrasound interfaces. Dry and semi-dry couplants have been explored; however, their poor tunability and higher attenuation relative to water-based gels restrict their utilization. In recent years, hydrogels have emerged as a promising class of materials to address such requirements. Due to their high water content, they provide an acoustic impedance closely matching soft tissue to minimize reflection losses; furthermore, their physical and mechanical properties are highly tailorable. This review critically analyzes the evolution of couplants, focusing on advanced material design strategies of hydrogels to overcome the inherent limitations of traditional couplants and meet the emerging demands of next-generation transducers.