Free-Standing All-Polymer Ultrasound Transducer—A Wearable Solution for Medical Applications

Ultrasound (US) technology is one of the most widely used and quickly evolving diagnosis and treatment modalities, allowing real-time human body deep tissues monitoring, imaging or stimulation. Nevertheless, commercial-off-the-shelf US transducers are large, bulky, and rigid, making it challenging to cover curved surfaces like the elbow, knee, breast, and are inadequate for prolonged tissue monitoring. In addition, classical US probes are made of lead-based piezoelectric materials and are expensive to fabricate. In this context, the development of flexible/stretchable US transducers has attracted much attention as they could allow for a better mechanical match and conformable interfacing with human skin [1]. While the strategy usually adopted is to integrate rigid piezoelectric components (piezoelectric ceramic transducers) into a flexible substrate [2], our group recently proposed a fully printed, lead-free, polymer piezoelectric transducer [3]. This US transducer was fabricated by printing of Ag and P(VDF-TrFE) on a flexible polyimide substrate, showing optimal performance in the medical ultrasound range (15 MHz). The flexibility of polyimide is not enough to provide a conformal interfacing with skin, causing displacement defects risk. In addition, the use of Ag for the electrodes limits the device’s stability over time because of its oxidation in humid environments and the mechanical stability because of its brittleness.<br/>To overcome those limitations and with a view to sustainability, we made an intrinsically flexible all-polymer US transducer with a low-cost fabrication technology. We developed a US transducer based on a free-standing P(VDF-TrFE) film with top and bottom screen printed PEDOT:PSS electrodes. The P(VDF-TrFE) film was fabricated with an adjustable height film coater, making it easy to tune the film thickness ( ≈30 μm) and thus its resonance frequency in the medical range (≈7 MHz). PEDOT:PSS, a conductive polymer widely used in bioelectronics and in conformable skin patches, was selected thanks to its biocompatibility, transparency, flexibility . The transducer was polarized with a high voltage custom built set up at voltages equal to 1.5 times the coercive field, resulting in a piezoelectric coefficient value in the range 25-35 pC/N, coherent with PVDF state of the art. Pulse-echo and frequency responses of the device were analyzed, showing optimal performance in the frequency range 6-8 MHz, promising for biomedical applications. The all-polymer free-standing device offers an improvement both in conformability and stability when interfacing with the skin.<br/>Moreover, it can be easily implemented in an array format, potentially enabling advanced imaging capabilities and opening new possibilities for diagnostic and therapeutic uses.<br/>This approach is also promising for scale-up of production to large volumes, keeping material costs low.<br/>[1] Zhang, Lin, et al. "An emerging era: conformable ultrasound electronics." Advanced Materials 36.8 (2024): 2307664.<br/>[2] Hamelmann, Paul, et al. "Fetal heart rate monitoring implemented by dynamic adaptation of transmission power of a flexible ultrasound transducer array." Sensors 19.5 (2019): 1195.<br/>[3] Keller, Kirill, et al. "Fully Printed Flexible Ultrasound Transducer for Medical Applications." Advanced Materials Technologies 8.18 (2023): 2300577.