As shown in Fig. 1(f), the team employed the thermal embossing technique to wrap a specially prepared linear transducer around a 6 mm endoscopic ultrasound (EUS) probe, which could receive reflected echoes from nearby metal cylinders and water tanks at low voltages (<20 Vpp) (Fig. 1(g)), demonstrating its initial usability under bending conditions. By optimizing the process, a 128 elements’ linear transducer for high-resolution imaging achieved transmit−receive bandwidths of 107% and 78%, respectively, as shown in Fig. 1(h). The pulse-echo efficiency of the transducer was measured to be 0.2. The uniformity of transmit transfer performance of each component was found to be within ±10% (Fig. 1(i)). Connecting the 128 elements’ linear transducer to the Verasonics Vantage machine enabled the acquisition of B-mode images of a commercial tissue phantom with good resolution, as illustrated in Fig. 1(j). To demonstrate the scalability of the technology for larger areas, an array with an extremely large active aperture of 91.2 × 14 mm² was prepared (Fig. 1(k)), and was used for blood pressure monitoring, where 95% of the elements were functioning properly (Fig. 1(l)). Optimal locating of the carotid artery position was realized through the optimization of the array design and the array was perfectly attached to the subject's neck. As shown in Fig. 1(m), the array obtained clear echoes of the blood vessel wall and effectively distinguished the time-varying pulsation of the proximal and distal wall echoes caused by heartbeats.

In recent years, the application of flexible electronic technology^[1−3] combined with ultrasonic imaging^[4] has blossomed, resulting in a shift in the application place of ultrasonic imaging technology from medical institutions to households. The utilization of this application has expanded from the physician’s positioning and guidance to real-time patient monitoring^[5], while the range of application scenarios continues to widen. The transformative changes have made possible through the development of flexible transducer arrays^{[6, 7]}. Prof. Sheng Xu's team at the University of California San Diego has pioneered the development of various flexible transducer arrays based on island-bridge structures^[8] and serpentine electrodes^[9]. These transducer arrays incorporate rigid 1−3 composite piezoelectric transducers on an island, featuring thin electrodes between the islands to provide mechanical flexibility, metallic electrodes of serpentine structure to provide stretchability of the device, and elastomer materials are used to encapsulate the overall structure, ensuring structural stability and impedance matching to the skin tissue^[10]. These flexible transducer arrays have found extensive applications in human blood pressure detection^[11], cardiac imaging^[12], blood flow doppler imaging^[13], tissue modulus detection^[14], and tissue deep hemoglobin detection^[15]. However, this approach does have some inherent limitations, such as a smaller functionalized drive area, the absence of a transducer backing layer^[16], lower resolution due to reduced integrated density, and potential concerns regarding heavy metal hazards associated with lead, etc.

This technology that enables the fabrication of flexible large-area transducer arrays is poised to find extensive applications in the field of wearable ultrasound. The skin-adapted fully flexible design not only paves the way for all-weather wearable monitoring of ultrasound devices but also holds promise in the field of cylindrical probes such as endoscopes. With its actively adjustable resonance frequency and wide bandwidth design, it enables the probing of human tissues at various depths and facilitates imaging at different resolutions. Moreover, the scalability for large-scale production underscores the limitless possibilities for future commercial utilization in the future.

To address the above inherent problems of these flexible transducer arrays, prof. Gelinck and his team from Holst Center, The Netherlands, proposed an ultrasonic transducer-on-foil technology to address the inherent issues of flexible transducer arrays (Nat. Commun.https://doi.org/10.1038/s41467-024-47074-1)^[17]. This technology is based on piezoelectric polymer thermal embossing and utilizes a flexible polydimethylsiloxane (PDMS) printing film with hexagonal column microstructures. The process involves thermally imprinting piezoelectric polymers (P(VDF-TrFE)) laminated on polyimide feelers with patterned electrodes to form homogeneous uniform microstructures (Fig. 1(a)). Subsequently, as shown in Fig. 1(b), the transducer preparation was completed by laminating P(VDF-TrFE) and depositing common electrodes on top of the structure and encapsulating them. This transducer has excellent flexibility with a bending radius of curvature up to 1 mm (Fig. 1(c)). It offers adjustable resonant frequency, large effective transmitting and receiving areas, and better acoustic impedance matching with the tissue, resulting in higher transmission coefficients and increased bandwidth. This technology focuses on the preparation of transducers with a resonant frequency of 5−10 MHz, which is widely applicable for imaging of the abdomen and neck as well as pediatric diagnostics and transesophageal, transrectal and transvaginal endoscopic imaging. With a total thickness of only 100 μm, the bare P(VDF-TrFE) achieves a transmission coefficient of 0.58 on tissue, eliminating the need for a matching layer or backing to achieve typical bandwidths for medical imaging. Surface sound field testing using a hydrophone demonstrated the transducer's functionality, and the frequency response and transmission/reception efficiency were obtained (Figs. 1(d) and 1(e)), yielding bandwidths of 65% and 64% at −6 dB, respectively. The modified KLM toolbox is used to simulate the transducer elements, which can predict the performance of acoustic devices and provide a theoretical basis for the design of similar transducers.