Superior lead-free piezoceramics for wearable multimodal ultrasound imaging arrays

Domain: physics
Published: 2025-12-02
Editorial outcome: major_revision

Abstract
Medical ultrasound technology provides zero-radiation, non-invasive, high-penetration and high-resolution evaluation of body organs for diagnosing various diseases. However, challenges remain for emerging ultrasonic bioelectronics in terms of novel material exploration, structural flexibility, wearable integration. Here, we present a 64-channel lead-free ultrasound line imaging array transducer developed from biofriendly potassium sodium niobate-based ceramics, which enables wearable multimodal imaging through the incorporation of multilevel material structure engineering and flexible device design. On the materials engineering aspect, the lead-free ceramics exhibits superior piezoelectricity (d33 ~630 pC N−1, k33 ~0.66) by restoring long range ferroelectric ordering and suppressing excessive polar nanoregions. In terms of device architecture, the miniaturized array integrates 64 lead-free 1-3 piezo-elements on a flexible printed circuit board and a bioadhesive hydrogel layer for acoustic coupling and robust adhesion to the skin, enabling portable, high-precision operation with a center frequency of 8.36 MHz and a −6 dB bandwidth of over 40%. In wearable applications for the human body, this array demonstrates real-time high-resolution vascular imaging, blood flow dynamics, and elastography of muscle dynamics, advancing the next generation of wearable ultrasound technologies.

Introduction
Ultrasound transducers based on piezoelectric materials are essential across industrial and medical fields, offering noninvasive operation, real-time imaging, and broad applicability to diverse scenarios1,2. In medicine, transducers are integral to visualizing tissue structure, assessing mechanical properties, and monitoring physiological process2–4. The advent of wearable and conformable ultrasound devices is transforming traditional healthcare by enabling portable, wireless systems for continuous monitoring and diagnosis3–8. Advanced techniques, such as ultrasound shear wave elastography (SWE), enable precise characterization of tissue stiffness, providing valuable insights into pathological conditions like acute liver failure9. Therefore, the development of wearable designs and multimodal functionalities are emerging as major trends for ultrasound transducers. These expanding applications further highlight the growing demand for transducers with high sensitivity, improved resolution, and greater durability, driving innovations in materials and device design.

Despite their widespread use, current ultrasound transducers predominantly rely on lead-based piezoelectric materials, such as lead zirconate titanate (PZT) and lead niobium magnesite-lead titanate (PMN-PT), due to their high piezoelectric and electromechanical performances. However, the toxicity of lead raises significant biosafety and environmental concerns, particularly for applications involving direct and prolonged human contact. Consequently, there is an urgent need to develop high-performance, lead-free alternatives10,11. Among the most promising candidates, potassium sodium niobate (KNN)-based ceramics have attracted substantial attention due to their promising properties enabled by phase boundary engineering. Recent progress in constructing rhombohedral-tetragonal (R-T) and rhombohedral-orthorhombic-tetragonal (R-O-T) multiphase coexistence has yielded a new category of KNN-based ceramics with enhanced piezoelectricity, achieving piezoelectric constant d33 exceeding 450 pC N−112. Notably, ultrahigh d33 values of up to 650 pC N−1 have been reported13. However, most KNN-based ceramics with multiphase coexistence demonstrate moderate piezoelectricity (typically below 600 pC N−1) and low electromechanical response, with further enhancement remaining challenging. Texture engineering has shown potential in boosting the piezoelectric performance of KNN-based ceramics beyond 700 pC N−1 by optimizing anisotropy, matching or even surpassing commercial lead-based ceramics14,15. However, such approaches remain complex in manufacturing, limiting scalability and industrial applicability. Similarly, the growth of high-quality ferroelectric single crystals with intricate compositions remains technically challenging. Simplified and scalable strategies are therefore essential to achieve high piezoelectric performance of KNN-based ceramics for practical applications.

In multiphase-coexisting KNN-based ceramics, piezoelectric properties are typically optimized through chemical modifications that adjust the phase transition temperatures, such as the R-O (TR-O) and O-T (TO-T) phase transition temperatures10,12. While effective at tuning phase behavior, these modifications often disrupt long-range ferroelectric ordering (LRFO) and promote the formation of polar nanoregions (PNRs). Although PNRs facilitate polarization rotation under external electrical fields and enhance piezoelectricity, their excessive presence diminishes ferroelectric polarization, ultimately degrading piezoelectric performance12,16. Moreover, KNN-based ceramics exhibit high sensitivity to composition, with minor deviations (for example, ±1 mol%) significantly impacting d33 values10,12,17. High-temperature sintering exacerbates this issue by causing volatilization of alkali metal elements, resulting in stoichiometric deviations and local compositional heterogeneities, which further promote undesirable PNRs formation and degrade performance18–20. Consequently, the delicate interplay between LRFO and PNRs is a critical factor in achieving high piezoelectric performance in KNN-based ceramics.

To address these challenges, we present a two-step sintering (TSS) method (Supplementary Figs. 1, 2 and Supplementary Note 1) as an effective strategy to restore LRFO and suppress excessive PNRs in multiphase-coexisting KNN-based ceramics. Using this method, chemically modified 0.96 K0.48Na0.52Nb0.94Sb0.06O3-0.04Bi0.5Ag0.5ZrO3-0.3% Fe2O3 (abbreviated as KNNS-BAZ) ceramics exhibited reduced volatilization and mitigated compositional deviations, resulting in repaired LRFO with fewer PNRs compared to conventional sintering (CS). This improvement led to enhanced polarization values and superior piezoelectric properties, achieving a high d33 value (~630 pC N−1) and an improved longitudinal-mode electromechanical coupling coefficient k33 (~0.66). Leveraging these advancements, we developed a lead-free ultrasound imaging array (LF-UIA) composed of 64 high-performance 1–3 piezo-elements with a compact configuration. The LF-UIA operates at a center frequency of 8.36 MHz with a −6 dB bandwidth of 40%, enabling high-resolution B-mode and Doppler-mode imaging of wrist vessels. Additionally, it demonstrated wearable real-time elastography capabilities, accurately detecting Young’s modulus of the biceps brachii during muscle contraction and relaxation. These results highlight the potential of TSS-optimized LF-UIA for advancing lead-free, wearable ultrasound technologies for multimodal imaging and real-time dynamic muscle assessment.

Results
Multimodal imaging integrates complementary imaging techniques to provide comprehensive structural and functional insights, enhancing diagnostic accuracy, sensitivity, and real-time monitoring capabilities for improved clinical decision-making. Additionally, muscles are one of the largest organs in the body, accounting for about 40% of total body weight, and are crucial for various physiological functions21. During physical activity, repetitive muscle contractions and relaxations induce dynamic variations in their Young’s modulus. Monitoring these changes provides critical insights into muscle physiology, offering potential applications in understanding biomechanical behavior and advancing personalized healthcare strategies. Compared to magnetic resonance elastography and computed tomography elastography, ultrasound elastography provides a faster, noninvasive alternative with high spatial resolution, minimal discomfort, and greater accessibility, particularly for point-of-care applications. Its versatility allows for the detection of both superficial and moderately deep tissue changes with high sensitivity9,22–25. As ultrasound probes becomes smaller, the technology holds the potential for continuous, wearable monitoring, eliminating the need for traditional clinician-operated systems2,9.

As a proof of concept, we designed a 64-channel LF-UIA for non-invasive multimodal imaging, capable of high-resolution B-mode, Doppler-mode imaging, and real-time SWE (Fig. 1a). The LF-UIA comprises 64 elements arranged in a conventional three-port transducer architecture, incorporating a functional KNNS-BAZ piezoelectric 1–3 composite layer (200 μm in thickness, with a resonant frequency of ~8 MHz), a matching layer (40 μm in thickness), and a backing layer (1 mm in thickness) (Fig. 1b)8,26,27. Its compact design, combined with a flexible printed circuit board (F-PCB) interface and a bioadhesive hydrogel coupling layer, ensures tight and stable contact with tissues, enabling portable and high precise operation. The bioadhesive hydrogel maintains high interfacial toughness (>500 J m−2) and low acoustic impedance (~1.54 MRayl), facilitating continuous, wearable imaging with the LF-UIA9,28. As the core functional material for acoustic-electric coupling, the piezoelectric composite elements directly determine the imaging capability of the device. Therefore, we propose here a strategy to enhance piezoelectricity in KNN-based lead-free ceramics by restoring LRFO, which creatively includes the following steps: (i) a high-performance ceramic matrix with lattice distortions and pre-established R-O-T multiphase coexistence was first synthesized, (ii) a TSS method with temperature fine-tuning was then explored to restore ferroelectric ordering and breakthrough current polarization values and piezoelectric performances (Fig. 1b and Supplementary Fig. 3). Leveraging materials engineering and device design, the LF-UIA enables multimodal ultrasound imaging, providing detailed insights into human physiological behaviors. When integrated with the Verasonics Vantage system, the host controller manages data acquisition, processing, and inter-component communication, while the Vantage hardware handles signal transmission, reception, and processing, enabling real-time multimodal operation of the LF-UIA (Fig. 1c). Finally, the LF-UIA is presented in Fig. 1d, its small and flexible design ensures portability and ease of use, while its robust adhesion to the skin ensures wearability. These methodologies are expected to advance lead-free ultrasound transducers a big step toward wearable multimodal medical imaging.

To evaluate the structural and compositional characteristics of ceramics prepared via CS and TSS, surface morphology and elemental distribution analyses were first conducted on thermally etched ceramics. Both CS and TSS ceramics demonstrated effective elemental detection and relatively homogeneous elemental distribution (Supplementary Figs. 4–6). Notably, TSS ceramics exhibited a more refined grain structure compared to CS ceramics, characterized by a narrower grain size distribution and reduced average grain size (Supplementary Figs. 7 and 8). This refinement resulted in a denser microstructure, evidenced by fewer pores and higher bulk density in the TSS ceramics (Supplementary Fig. 8). Elemental composition analysis at ten randomly selected surface points reveled that TSS ceramics exhibited Na/K and (K+Na)/Nb ratios closer to their theoretical values than CS ceramics (Supplementary Fig. 9). This improvement is attributed to reduced volatilization of alkali metal elements at the lower sintering temperature employed during TSS process29. The synergistic effects of refined grain structure, increased density, and improved compositional stoichiometry highlight the effectiveness of TSS in mitigating alkali metal loss and enhancing the structural uniformity of KNN-based ceramics.

Moreover, in the CS ceramic, only a broad dielectric anomaly peak was observed from −150 to 150 °C, indicating the convergence of the TR-O and TO-T peaks near 50 °C (defined as TR-O&O-T), suggesting R-O-T phase coexistence around room temperature (Fig. 2a)20. In contrast, the TSS ceramic exhibited distinct TR-O (~0 °C) and TO-T (~50 °C) peaks, with TO-T shifting to higher temperatures (Fig. 2b). This similar shift has been reported in KNN-based ceramics synthesized via various sintering methods, usually attributed to grain-size effect or stoichiometric deviations18,30,31. Meanwhile, the TSS ceramic exhibited increased dielectric permittivity and a higher Curie temperature (TC) compared to the CS ceramic, which result from improved density and compositional stoichiometry (Supplementary Fig. 10). To further analyze dielectric relaxation behaviors, the diffuseness degree (γ) was evaluated using a modified Curie–Weiss law (Supplementary Fig. 11)32. The calculated γ values were 1.69 and 1.62 for the CS and TSS ceramics, both comparable to values reported for typical KNN, barium titanate, and lead-magnesium niobate (PMN)-based relaxors. The slightly elevated γ values in the CS ceramic aligns with previous reports on non-stoichiometry-engineered KNN-based ceramics and is likely due to the increased presence of PNRs17,31,32.

X-ray diffraction (XRD) measurements combined with Rietveld refinements were performed to delve into the phase structures of the ceramics. Both CS and TSS ceramics exhibited a pure perovskite structure without detectable secondary phases, as indicated by low Sig (<2) and Rwp (<10%) values, confirming the reliability of the fitted results (Fig. 2c and Supplementary Fig. 12). These results further confirm that both CS and TSS ceramics feature a R-O-T phase coexistence at room temperature. Notably, TSS ceramics exhibited a higher fraction of the O phase and a lower fraction of the T phase compared to CS ceramics (Fig. 2d). The composition-induced multiphase coexistence and the resulting structural flexibility are known to flatten the Gibbs free energy, facilitating polarization rotation and enhancing piezoelectric properties (Fig. 2e)12,13. This phenomenon is attributed to the energy difference of the phases, where phase-field simulations have shown that the O phase possesses the lowest energy, while the T phase possesses the highest energy33. As a result, the TSS ceramics exhibit lower energy barriers and a more flattened Gibbs free energy, further contributing to their enhanced piezoelectric performance.

Furthermore, factors such as the size and spatial arrangement of the ferroelectric domains have a significant effect on the macroscopic properties of ferroelectric materials. Piezoelectric force microscopy (PFM) was therefore employed to elucidate the domain morphology and switching behaviors. CS ceramics demonstrated a typical domain structure consisted of randomly distributed nano- to micro-scale island-like domains (Fig. 2f, g and Supplementary Fig. 13a)13,34. In contrast, the TSS ceramics exhibited a distinctly different domain configuration, characterized by a coexistence of long-range ordered sub-micro stripe domains (width ~500 nm) and densely packed labyrinth-like nanodomains (Fig. 2h, i and Supplementary Fig. 13b). The labyrinth nanodomains resemble those typically found in sodium bismuth titanate-based ferrorelaxors, while the stripe domains reflect a partially restored LRFO35. The fragmentation of long-range ferroelectric macrodomains into nanoscale domains is usually attributed to random fields caused by local chemical inhomogeneities20,36,37. However, the improved compositional homogeneity in TSS ceramics effectively mitigated these random fields, promoting the re-establishment of LRFO and leading to the emergence of sub-micro stripe domains (Fig. 2j).

Then, switching spectroscopy PFM (SS-PFM) measurements were conducted to investigate the domain switching behavior in the TSS ceramics. Local switching responses were collected over a 20×20 mesh within a 2×2 μm region, revealing a spatial variation in coercive voltages (Fig. 2k and Supplementary Fig. 14). Regions with lower coercive voltages were distinctly separated from those with higher values, correlating with the hierarchical domain structure consisting of nanodomains and larger stripe domains, as observed in the amplitude image (Supplementary Fig. 14). These findings suggest the easy switching properties of nanodomains under external stimulus (for example, electric fields)19,34. The phase loops further confirmed these behaviors, where nanodomains exhibited lower coercive voltages compared to stripe domains. Both domains displayed rectangular phase loops with a phase contrast of approximately 200°, indicating complete and sufficient domain switching for both nanodomains and strip domains under the external electric field of 20 V (Fig. 2l). Additionally, asymmetric butterfly amplitude curves were observed, with stripe domains demonstrating higher amplitude responses than nanodomains, further highlighting their distinct switching characteristics. Transmission electron microscopy (TEM) characterizations provided additional evidence for this hierarchical domain structure, showing the coexistence of larger domains (50–200 nm) and nanodomains (<10 nm) (Fig. 2m and Supplementary Fig. 15). This combination of long-range ordered sub-micro domains and highly responsive nanodomains is essential for attaining high piezoelectricity as well as efficient polarization.

To elucidate the atomic-scale local structures contributing to the spontaneous polarization in TSS ceramics, high-angle annular dark-field scanning transmission electron microscopy (HADDF-STEM) was performed along the [110]c zone axis. This method effectively distinguishes the local symmetries (R, O, and T) based on the displacement of the center Nb5+ cation relative to the surrounding Na+/K+ cations, a key feature of KNN-based materials (Fig. 2n)38,39. As shown in Fig. 2o, polarization vectors extending from the Nb5+ cation toward the A-site cations were visualized using arrows. Two-dimensional Gaussian peak fitting analysis revealed that the polarization vectors in TSS ceramics were more heterogeneous compared to normal single-phase ferroelectrics, where R, O, and T PNRs (1–3 nm) were distinctly observed, with polarization directions identified as [1–11]c, [1–10]c, and [001]c directions, respectively. Furthermore, boxed areas in Fig. 2p demonstrated a gradual rotation of polarization vectors among the three symmetries, indicating a nanoscale R-O-T multiphase coexistence. This static slush nanopolar state, characterized by reduced polarization anisotropy and enhanced local symmetry flexibility, facilitates polarization rotation under external stimuli (electric, thermal, or mechanical), thereby contributing to piezoelectricity enhancements19.

The ferroelectric and electromechanical properties of CS and TSS ceramics were systematically analyzed to elucidate the underlying mechanisms contributing to the enhanced piezoelectric performance of TSS ceramics. Both CS and TSS ceramics presented typical ferroelectric hysteresis (P–E) loops and bipolar strain (S–E) curves (Fig. 3a, b), while TSS ceramics exhibited higher maximum polarization (Pmax) and remnant polarization (Pr) values, which are attributed to the restored LRFO. The Pr values reflect the alignment stability of ferroelectric domains with the applied electric field even after its removal. In CS ceramics, internal fields and excessive PNRs facilitated domain reorientation, leading to a reduction in aligned domains and lower Pr, as evidenced by the increased back-switching polarization (Pbs, calculated as Pbs = Pmax − Pr) (Supplementary Fig. 16)40. Despite similar coercive fields (Ec) due to comparable macroscopic structures, the J–E curves revealed an increased current density in TSS ceramics, indicative of enhanced domain switching activity (Supplementary Fig. 17). The positive strain (Spos) and unipolar strain (Suni) values were comparable for both ceramics, while the TSS ceramics exhibited higher negative strain (Sneg) value compared to CS ceramics, which is associated with enhanced non-180° domain switching (Fig. 3b and Supplementary Fig. 18)20,37. Furthermore, the dynamic ε–E curves further demonstrated the higher dielectric permittivity of TSS ceramic (Supplementary Fig. 19). Given the theoretical relationship of d33 = 2Q33εrP, the simultaneous increases in εr and Pr values contributed to the enhancement in d33 values of the TSS ceramic (Fig. 3c). In conclusion, the denser structure with moderate local heterogeneity enabling the TSS ceramics a high d33 value of 630 pC N−1 together with a TC of 210 °C, rendering them potential candidates for industrial lead-based piezo-materials (for example, PZT-5H) (Fig. 3d and Supplementary Table 1)13,20,34,41–47.

The electromechanical coupling coefficient, k, quantifies the conversion efficiency of a piezoelectric material between electrical and mechanical energy and vice versa, and is a key parameter for ultrasonic transducer applications, which were evaluated for both TSS and CS ceramics through longitudinal vibration mode resonance and anti-resonance frequencies (Fig. 3e, f and Supplementary Fig. 20). The TSS ceramics exhibited a higher k33 of 0.66, indicating enhanced electromechanical properties and ensuring high imaging sensitivity of the transducer (Fig. 3g). Phase angle analysis further revealed a higher polarization state in TSS ceramics, attributed to the increased collinear domains and reduced PNRs contents (Supplementary Fig. 20)20. Additionally, the TSS ceramics demonstrated favorable temperature stability, which ensures reliable performance during device processing and operation and promises practical applications in different environments (Fig. 3h). The excellent electrical fatigue resistance ensures the long-term reliability of the TSS ceramics for wearable biomedical applications (Supplementary Fig. 21).

To further elucidate the intrinsic origin of the enhanced piezoelectricity in TSS ceramics, we performed first-principles calculations based on density functional theory (DFT). As analyzed above, TSS ceramics effectively mitigate alkali metal loss and improve structure uniformity, both of which are crucial for the enhancement of piezoelectric properties. To examine this, A-site and oxygen vacancies were introduced into the KNN matrix to simulate the more pronounced volatilization of K/Na alkali metal elements in CS ceramics, resulting the model labeled KNND (Fig. 3i and Supplementary Fig. 22a). Figure 3j and Supplementary Fig. 22b presents the charge density distributions of KNN and KNND. Additionally, the piezoelectric coefficients e33 and e15 were calculated to assess the impact of these effects on the piezoelectric properties. In the KNND model, both e33 and e15 exhibited reduced values, indicating a deterioration in piezoelectric performance (Fig. 3k, l). This result further supports the improved piezoelectricity observed in TSS ceramics.

The fabrication of KNNS-BAZ ceramics using the TSS method, along with the preparation of 1–3 composites and their integration into the LF-UIA, is presented in Supplementary Fig. 23. To fabricate the linear array transducer, TSS ceramics were processed into 1–3 piezo-elements through the dice-and-fill technique, with a kerf of 15 μm and a pitch of 100 μm (Supplementary Fig. 24). The resulting 1–3 piezo-element, featuring high-performance piezo-micropillars embedded within the passive epoxy matrix, offers substantial advantages over bulk ceramics, including reduced acoustic impedance and enhanced longitudinal-mode electromechanical coupling1,3,4,9. Additionally, the 1–3 piezo-element also demonstrates mechanical stability under bending, supporting its applicability in flexible and wearable devices integration (Supplementary Figs. 25 and 26). The frequency-dependent impedance and phase angle spectra of a single array element confirm efficient conversion of electrical energy into mechanical vibrations (Fig. 4a). The LF-UIA demonstrates a center frequency of 8.36 MHz with a −6 dB bandwidth over 40% (Fig. 4b). Uniformity in array performance is essential for high-quality ultrasonic imaging, therefore, the center frequency, peak-to-peak sensitivity, and fractional bandwidth were measured across all elements, showing remarkable consistency (Fig. 4c and Supplementary Fig. 27). Additionally, the insertion loss (IL) was evaluated across a frequency range of 5–11 MHz, revealing a minimized IL of 25.6 dB at the resonant frequency, which ensures high ultrasonic sensitivity for high-resolution imaging (Fig. 4d)3,48.

As a 64-channel ultrasonic linear array, the working principle of LF-UIA is illustrated in Fig. 4e. Once triggered by the input signal with specific time sequence, the LF-UIA generates ultrasound waves that can be focused at different spatial positions, enabling a wide range of acoustic beam sweeps4. Finite element analysis simulations were performed to assess the beamforming capabilities of the linear array (Fig. 4f). The LF-UIA demonstrated excellent beam directivity, effectively focusing at 70 mm as the tilt angle increased from 0° to 20°. However, grating lobes became more pronounced at larger tilt angles, which is a common trade-off in array transducers. This beam-tilting capability enables active targeting over a wide spatial range, a feature not achievable with single-element transducers or annular arrays limited to direct focal depth adjustment beneath the transducers7,49.

The LF-UIA was then integrated into the Verasonics Vantage system and operated with the coherent plane-wave compounding method to facilitate real-time imaging and acoustic performance evaluation50. To assess its lateral and axial resolution, four tungsten wire phantoms with a diameter of 25 μm were imaged. The phantoms were laterally spaced at intervals of 1.5, 1.5, and 1 mm from left to right, and uniformly distributed over a 2 mm distance along the axial direction (Fig. 4g). The ultrasound image of the tungsten wire phantoms acquired by the LF-UIA was presented in Fig. 4h. At a depth of approximate 5 mm (marked by the white box), the LF-UIA achieved lateral and axial resolutions of 0.29 and 0.2 mm, respectively, at −6 dB (Fig. 4i, j). Overall, the LF-UIA demonstrates promising imaging resolution and quality over some current lead-free and lead-based transducer arrays (Supplementary Table 2). Notably, high-quality B-mode images of tungsten wire and anechoic cyst phantoms at depths exceeding 15 mm validated the promising imaging resolution and quality of the LF-UIA (Supplementary Fig. 28). Given the safety considerations of acoustic exposure to human tissues, we measured the acoustic pressure generated by LF-UIA under various input voltages and calculated a series of safety parameters accordingly, including the mechanical index, thermal index, and spatial peak pulse average (ISPPA) (Fig. 4k, l and Supplementary Fig. 29). All of these parameters are positively correlated with the input voltage, indicating that the acoustic output of the LF-UIA can be flexibly adjusted to ensure safe operation in compliance with U.S. Food and Drug Administration (FDA) restrictions.

The stability of LF-UIA was subsequently assessed under conditions relevant to continuous wearable use. When the device was worn for 4 h, the electrical impedance and phase spectra remained essentially unchanged (Supplementary Fig. 30). During this period, no significant thermal effects were observed in the device itself, and natural fluctuations in body temperature did not produce measurable impacts. These results confirm the stability of the device and demonstrate that minor physical activity and physiological temperature variations do not compromise device performance (Supplementary Fig. 31). The mechanical robustness of the LF-UIA was evaluated through cyclic bending tests. Even after 1000 cycles, the impedance spectra, center frequency, peak-to-peak sensitivity, and fractional bandwidth exhibited negligible changes (Supplementary Fig. 31), indicating the stable functional performance of LF-UIA under repeated mechanical deformation. Considering that the long-term operation of LF-UIA also depends on stable adhesion and acoustic coupling, the bioadhesive hydrogel was examined under ambient conditions (relative humidity 50–70%, 27 °C). The hydrogel retained its morphology and more than 75% of its initial mass after 3 days (Supplementary Fig. 32a, b), and exhibited strong tissue adhesion (on porcine skin) after 7 days (Supplementary Fig. 33). These results demonstrate that the hydrogel provides durable adhesion while maintaining reliable acoustic coupling over multi-day use. Together, these results validate that LF-UIA maintains stable electrical, mechanical, and adhesive performance under realistic physiological and environmental conditions, ensuring reliable operation during continuous and long-term wearable applications.

The real-time imaging capabilities of the LF-UIA connecting to the Verasonics system were demonstrated through in vivo arterial imaging (Fig. 5a). First, ultrasound Brightness (B)-mode imaging enabled precise visualization of the spatial distribution of radial artery (Fig. 5b). When operated in ultrasound Doppler imaging mode, the LF-UIA then effectively assessed both the spatial distribution and dynamics of blood flow, reflecting changes in blood volume (Fig. 5c). In Doppler mode, ultrasound beams directed into the flowing blood encounter moving scatterers (such as red blood cells), which reflect the ultrasound waves (Fig. 5d). As the red blood cells travel through the bloodstream, the frequency of these reflected waves is shifted, that is, a Doppler shift fD:

fD = fR − f0 = 2f0 V cosθ / c (1)

where f0 is transmit frequency, fR is transmit frequency, V is the flow velocity, c is the speed of ultrasound, and θ is the Doppler angle, that is, the angle between the axis of the ultrasound beam and the direction of flow. The blood flow velocity can be calculated by analyzing this frequency shift51. Thus, to evaluate the spatiotemporal resolution and Doppler sensitivity of the LF-UIA, further pulse wave Doppler measurements were conducted on both the radial artery in the hand and carotid artery in the neck, revealing four distinct feature points within 3 s (Fig. 5e and Supplementary Fig. 34). The peak flow velocity in the hand was estimated at approximately 60 cm s−1, while the neck demonstrated a higher peak velocity of up to 100 cm s−1. These velocities were calculated under an assumed 60° insonation angle using the LF-UIA. The corresponding pulse waveforms were extracted by tracking the temporal brightness variation within the spectrograms, reflecting relative flow intensity over time (Fig. 5f and Supplementary Fig. 34). The black traces represent the raw brightness-based envelope, and the overlaid red curves are smoothed representations highlighting systolic and diastolic dynamics. These results demonstrate that LF-UIA enables noninvasive, high-resolution assessment of arterial flow, supporting its potential for continuous vascular health monitoring and wearable diagnostics.

Wearable applications are one of the future aspects of emerging bioelectronics. Here, the wearable ultrasound imaging capabilities of the LF-UIA were further extended to SWE for assessing tissue mechanical properties, offering valuable insights into biomechanics in motion (Supplementary Note 2)9. To characterize the performance of LF-UIA in SWE, a commercial elasticity QA phantom (Model 049, CIRS Inc., Norfolk, VA, USA) was used to evaluate the effective depth limit. The signal-to-noise ratio (SNR) of shear wave propagation signals decreased with increasing acoustic radiation force impulse (ARFI) focal depth, falling to 6.606±2.88 dB at 25 mm (Supplementary Fig. 35), where reliable tracking was no longer feasible. Therefore, the effective depth limit for accurate SWE measurements with LF-UIA is defined as ~20 mm. The decline in SNR with depth is primarily attributed to acoustic attenuation, beam divergence, and electronic noise. Additionally, impedance mismatches in the current system may further degrade SNR by inducing signal reflections and transmission losses. Future improvements will focus on circuit optimization, impedance matching, and the implementation of advanced beamforming techniques to enhance deep imaging performance. For spatial resolution assessment, a dual-stiffness gelatin phantom was fabricated, and the boundary response was analyzed using an edge-shaped function approach. The derived point-spread function yielded a full width at half maximum of 0.6151 mm (Supplementary Fig. 36), demonstrating the ability of the system to resolve sub-millimeter variations in stiffness.

As depicted in Fig. 6a, the LF-UIA was positioned and secured on the arm (with a small amount of ultrasound gel acting as a coupling agent) to monitor changes in the modulus of the biceps brachii during movement. By generating sufficient ARFI, the LF-UIA induced shear waves that propagated through the tissue, enabling localized displacement tracking and subsequent analysis (Fig. 6b). To validate the SWE performance of the LF-UIA, the commercial elasticity phantom (CIRS Inc.) was used as a reference. The group velocity of the shear wave (Cg) in the phantom was determined to be 2.8 m s−1 using established commercial methods. The LF-UIA-generated shear waves and their propagation were captured and analyzed (Fig. 6c and Supplementary Movie 1), yielding a Cg value of 2.73 m s−1, closely approximating the true value (Fig. 6d). This result confirms the accuracy and reliability of the LF-UIA in SWE measurements.

The LF-UIA was then applied for real-time SWE to assess the mechanical state of the biceps brachii during contraction and relaxation. As shear wave velocity correlates with tissue stiffness, higher velocities were expected in contracted muscle states. The mean Cg (n=3) was measured to be approximately 2.85 m s−1 during contraction and 2.26 m s−1 during relaxation (Fig. 6e–g). The corresponding Young’s modulus (E) was calculated using the Eq. (2)9,52,53:

E = 3ρCg2 (2)

where ρ represents the density and is assumed to be 1000 kg m−3 in this work. Notable differences in Young’s modulus were observed, with the contracted biceps brachii exhibiting a higher modulus of approximately 24.96 kPa compared to 15.25 kPa in the relaxed state (Fig. 6h). These values are consistent with those reported in clinical practice, underscoring the accuracy of the measurements54,55. These results highlight the capability for the LF-UIA to perform accurate, real-time SWE for dynamic muscle state assessments, demonstrating its potential for applications in biomechanics and real-time tissue characterization, including the evaluation of liver cirrhosis, neck skin sclerosis, and other conditions (Fig. 6i)56,57.

Ultrasonic transducers are integral to medical applications and often require direct contact with the human body, making their biosafety a critical concern. Here, we assessed the biosafety of TSS-optimized KNNS-BAZ ceramics and its primary precursor, Nb2O5, in comparison with PZT and its precursor, Pb3O4, based on the composition and design of LF-UIA. Cytotoxicity was evaluated using live/dead staining and the Cell Counting Kit-8 (CCK-8) assay on mouse fibroblast cells (L929). As shown in Fig. 7a, cells cultured with leachates of Nb2O5 and KNNS-BAZ exhibited proliferation similar to that of controls, with the majority of cells displaying green fluorescence. In contrast, the Pb3O4 and PZT groups showed a significant number of red-stained dead cells, indicating substantial cytotoxicity associated with lead-based materials. The CCK-8 assay further confirmed that the cell viability in the KNNS-BAZ group exceeded 97%, while the viability in the Pb3O4 group was significantly lower than in the control (Fig. 7b). These results highlight the non-toxicity and biocompatibility of KNN-based ceramics and its components. Additionally, hemolysis experiments demonstrated that KNNS-BAZ and Nb2O5 possess good hemocompatibility, with hemolysis rates below 1.6%, well within the <5% threshold set by the International Organization for Standardization (ISO 10993-4) (Fig. 7c and Supplementary Fig. 37).

To further evaluate biocompatibility, subcutaneous embedding experiments were conducted using KNNS-BAZ, PZT and their respective precursors. One month post-implantation, blood biochemical analyses revealed that hepatic and renal function parameters, including alkaline phosphatase, creatine kinase, alanine aminotransferase, total bilirubin (T-bil-V), urea, and creatinine (CREA), remained within normal ranges in the KNNS-BAZ group (Fig. 7d–g and Supplementary Fig. 38). In contrast, significantly elevated levels of these biomarkers were observed in the lead-containing Pb3O4 and PZT groups. Histopathological evaluation of major organs (heart, liver, spleen, lungs, and kidneys) showed no notable pathological alterations across all experimental groups (Fig. 7h and Supplementary Fig. 39). Local tissue responses at the implantation site were assessed using hematoxylin and eosin (H&E) staining, which revealed no evidence of inflammatory cell infiltration or structural deformation in the KNNS-BAZ group (Fig. 7i). Furthermore, immunofluorescence staining and quantitative analysis of the pro-inflammatory cytokines TNF-α and IL-6 indicated markedly higher expression levels in lead-containing groups compared to KNNS-BAZ group, whose cytokine levels closely aligned with those of the control group (Fig. 7j–l). These results demonstrate the superior biocompatibility and minimal inflammatory response of KNN-based materials. Biocompatibility assessments of the constituent materials of the LF-UIA device, including the bioadhesive hydrogel, epoxy, backing layer (E-solder 3022), and F-PCB further confirmed negligible cytotoxicity (Supplementary Fig. 40), in line with previous reports on their safe use in biomedical devices1–5,9. Collectively, the LF-UIA exhibits reliable biocompatibility for biomedical applications.

Methods
Preparation of lead-free piezoceramics
0.96 K0.48Na0.52Nb0.94Sb0.06O3-0.04Bi0.5Ag0.5ZrO3-0.3% Fe2O3 ceramics were prepared using the CS and TSS methods. Raw materials of K2CO3 (99.0 %), Na2CO3 (99.8%), Nb2O5 (99.5%), Sb2O3 (99.99%), Bi2O3 (99.99%), Ag2O (99.7%), ZrO2 (99.0%), and Fe2O3 (99.0%) were weighted according to their stoichiometric ratios and ball-milled with alcohol for 24 h. After calcined at 850 °C for 6 h, the powders were mixed with 8 wt% PVA and pressed into pellets with a diameter of 10 mm at the pressure of 10 MPa. Then the PVA was burned off at 850 °C to obtain green samples. The CS ceramics were sintered at 1065 °C for 3 h, while the TSS ceramics underwent a TSS process: initially heated to 1200 °C for 10 min, followed by rapid cooling to 960 °C, where they were maintained 6 h. Subsequently, silver electrodes were printed on both surfaces of the ceramics and heated at 600 °C for 10 min. To assess the electric properties, the ceramics were poled under a direct current electric field of 30 kV cm−1 for 10 min in a silicone oil bath at room temperature.

Fabrication of LF-UIA
The ceramic/epoxy 1–3 piezo-element was fabricated using the dice-and-fill method (DS9260) with a kerf width of 15 μm and a pitch of 100 μm. The kerfs were filled with epoxy (ELIN OPTO E106-7A/B) and cured at 60 °C for 4 h. After curing, the piezo-element was lapped and polished to a thickness of 200 μm and sputtered with Au electrode on both sides. Electric poling was then performed at 30 kV cm−1 for 10 min. A silver epoxy mixture was applied as the matching layer, lapped, and polished to a thickness of 40 μm. A 1 mm-thick backing layer (E-Solder 3022) was cast on the opposite side of the piezo-element. Electrode separation for each array element was processed through sub-scratch dicing for 64-channel, achieving the pitch of 200 µm (about 1λ pitch) and optimizing the thickness-mode resonance (mainly longitudinal vibration represented by piezoelectric column k33) for improved performance in ARFI-based SWE. The resulting 1–3 piezo-elements with separated electrodes were then boned to a commercial-fabricated F-PCB with 64 copper trace channels using silver paste (E-Solder 3022), ensuring precise alignment. Finally, the assembly was coated with a protective layer of parylene C (DPX-C, Speciality Coating Systems), completing the fabrication of the LF-UIA.

Characterization of piezoelectric ceramics
The temperature-dependent dielectric constants and dielectric loss of CS and TSS ceramics from 1 to 100 kHz were obtained by the Dielectric Spectroscopy test system (TZDM-200-300, Harbin Julang Technology Co. Ltd., Harbin China) at −150 to 150 °C. The crystal structure of the CS and TSS ceramics was carried out using the XRD (XRD, Bruker AXS Inc., Madison, WI, USA) with Cu Kα radiation. For the measurement of microstructure and domain morphology, the CS and TSS ceramics were polished to 0.5 μm using diamond suspension. The microstructure of thermally etched ceramics was observed by a scanning electron microscope (SEM, Zeiss, Gemini 300). The domain morphologies of polished CS and TSS ceramics were recorded by the commercial PFM (PFM, MFP-3D, Asylum Research, Goleta, CA) in Dual AC Resonance-Tracking mode at a voltage of 2 V. Their switching properties were analyzed using the SS-PFM modes, applying a voltage of 20 V. TEM and HADDF-STEM were used to characterize the domain configuration and atomic scale local structure of TSS ceramics, respectively. Digital Micrograph was used to analyze the STEM images and determine the atomic occupancies at the A and B lattice sites.
The P–E loops and strain curves of CS and TSS ceramics were measured by the TF analyzer 2000 (aixACCT, Aachen, Germany) at 4 kV mm−1 and 1 Hz. The room temperature d33 values and εr were obtained by a quasistatic meter (ZJ-3A, China) and an LCR analyzer (HP4980, Agilent, U.S.A), respectively. The impedance and phase spectra of ceramics were measured by the impedance analyzer (E4991B, Keysight, USA). For the bipolar fatigue measurements, a function generator (DG992, Rigol) in conjunction with a high voltage amplifier (ATA-7020, Agitek) was employed to apply a sinusoidal electric field to the ceramics at a frequency of 50 Hz and an amplitude of 2Ec.

Performance measurements and imaging of the LF-UIA
The electrical impedance spectrum of the LF-UIA was measured at room temperature using an impedance analyzer (Agilent 4294A, Santa Clara, CA) to determine the resonant and anti-resonant frequencies. The center frequency and bandwidth were evaluated through pulse-echo experiments conducted with a pulse/receiver (JSR Ultrasonic DPR 300) and a cubic quartz reflector, with data acquisition facilitated by an oscilloscope (Keysight DSOX3024G) and analyzed via fast Fourier transform. To assess lateral and axial resolution, four tungsten wire phantoms with a diameter of 25 μm were used in ordered arrangements. Acoustic energy verification was performed using a hydrophone (3646, Precision Acoustics Ltd, UK) to measure the acoustic pressure generated by the LF-UIA.

Muscle ultrasound elastography
The LF-UIA was integrated with the Verasonics Vantage system to perform both ultrasound imaging and SWE in phantom studies and real-time muscle assessments (Supplementary Fig. 41). The system was controlled using MATLAB scripts (MATLAB R2022a release, the MathWorks, Inc.). In the SWE study, the ARFI was transmitted at 8 MHz with 2000 cycles to achieve a push duration of 250 μs. Subsequently, plane waves at three angles (−3°, 0°, and 3°) were transmitted at the same frequency to detect motion. The frame rate was set at 80 μs for all assessments. Particle velocity was calculated through autocorrelation of the in-phase/quadrature data, and spatiotemporal maps were constructed to visualize shear wave propagation. A region of interest, spanning the push row and five adjacent rows above and below (~0.5 mm in depth), was selected from the axial displacement matrix. The maximum particle velocity at each time point was identified, and the Cg was estimated via linear regression.

DFT calculations
The first-principles calculations were performed by the Vienna Ab initio Simulation Package (VASP), which is based on DFT. The charge density distributions and piezoelectric properties of KNN and KNND were predicted.

FEA simulations
Ultrasound beam steering simulations for the LF-UIA were performed using COMSOL Multiphysics 6.1 under conditions matching the experiment setup. The medium was modeled as homogeneous, with a sound speed of 1540 m s−1 and a density of 1000 kg m−3. The simulations accounted for wave propagation, reflections, and diffraction effects. Beam steering was achieved by applying linear phase delays across the array elements. The resulting acoustic pressure fields were calculated for steering angles ranging from 0° to 25°.

Biocompatibility evaluations
Mouse fibroblast cells (L929) were cultured in high-glucose Dulbecco’s Modified Eagle Medium (Hyclone, China) supplemented with 10% fetal bovine serum (Procell, China) and 1% penicillin/streptomycin (Hyclone). To evaluate cytotoxicity, Pb3O4, PZT, Nb2O5, and KNNS-BAZ powders were first sterilized via high-pressure steam in accordance with the ISO 10993-12:2012 standard (Biological evaluation of medical devices). Subsequently, these materials were immersed in DMEM medium at a weight-to-volume ratio of 0.2 g mL−1 and incubated at 37 °C with 5% CO₂ in a cell culture incubator for 24 h. After incubation, the supernatant was collected by centrifugation and filtration to obtain the conditioned medium. The cytotoxic effects were assessed using the CCK-8 assay and live/dead cell staining (FDA/PI) on days 1, 3, and 5 of cell culture.
The Experimental Animals Ethics Committee at Sichuan University accepted the animal study (approval number: KS2023352). Male SD rats (6-week-old, 180–220 g) were purchased from Chengdu Dossy Experimental Animals Co., Ltd and were kept at the West China Animal Management Centre of Sichuan University for 7 days to acclimatize to the environment (ambient temperature of 23 °C, 50% relative humidity, and 12 h light/dark cycle). The rats were randomly assigned into five groups (control, Pb3O4, PZT, Nb2O5, and KNNS-BAZ groups). Animals were anaesthetized with 4% isoflurane (Shenzhen RWD Life Science Co., Ltd. China), maintained at 2% during the surgical procedure. A subcutaneous implantation model was established in all rats. Briefly, a 5×5 cm2 operative area was obtained after shaving and disinfection. Surgical scissors were then used to make incisions in the skin on both sides of the rat spine. Implants (2 mg) from the same group were inserted, followed by suturing (n=3). For the control group, only skin incisions were made without implantation of materials, and the incisions were subsequently sutured. After surgery, the animals were housed individually in their original cages under the same conditions. Standard balanced food and water were provided ad libitum. At the 30-day endpoint, the blood samples from rats were collected for routine blood, liver and kidney function assessment. Then, the rats were deeply anesthetized with an intraperitoneal dose of sodium pentobarbital (100 mg kg−1) and transcardially perfused with PBS and 4% paraformaldehyde (PFA) in PBS. The implants, along with the surrounding skin tissues, were collected from each rat, and the heart, liver, spleen, lungs, and kidneys were also harvested. All samples were fixed in 4% PFA solution at 4 °C for 24 h. Following standard paraffin embedding and sectioning, the tissues underwent H&E staining. For immunofluorescence, antibodies against TNF-α and IL-6 were applied according to the protocol, and the sections were imaged using a fluorescence microscope (Pannoramic MIDI, 3D Histech Ltd). The quantitative analyses were performed using ImageJ software.

Hemolysis activity assay
The hemocompatibility of KNN-based powders was evaluated by a hemolysis test of rat blood. The rat blood was collected by heparin anticoagulant tubes, and centrifuged to obtain erythrocytes at 500×g for 10 min. The collected erythrocytes were washed with normal saline three times and prepared to 2% (v/v) erythrocyte saline solution. Respectively, Pb3O4, PZT, Nb2O5, and KNNS-BAZ powders (5 mg) were mixed with the erythrocyte saline solutions and incubated at 37 °C for 24 h. Normal saline and 0.1% Triton X-100 were separately set as negative control and positive control. The erythrocyte saline solutions were centrifuged at 1250×g for 15 min, and the optical densities (OD) of the supernatants at 540 nm were measured with a microplate reader (BioTek Instruments, Inc., USA). The hemolysis ratios were calculated according to the following formula:
Hemolysis ratio (%) = (ODsample − ODnegative control) / (ODpositive control − ODnegative control) × 100% (3)

Statistical analysis
The presentation of all data is mean ± s.e.m. (n ≥ 3). Unless otherwise specified, one-way ANOVA was employed for statistical analysis. *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001 were deemed as the thresholds for significant differences between groups.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors have developed a superior lead-free potassium sodium niobate (KNN)-based piezoceramic and fabricated it into a wearable multimodal ultrasound imaging array. This device demonstrates impressive capabilities, including high-resolution vascular imaging, blood flow dynamics monitoring, and muscle elastography. This material exhibits superior piezoelectric properties, and the device architecture is suitable for wearable applications, which is potentially important for advancing the next generation of wearable ultrasound technologies. However, there still remain some areas that need to be clarified and improved before publication:\n1. The Methods section states that the two-step sintering (TSS) process involves heating to 1200°C for 10 minutes, followed by cooling to 960°C and holding for 6 hours. This is a very specific parameter set, but the manuscript does not provide a rationale for selecting these exact temperatures and times. It is recommended that the authors supplement the manuscript with a brief study on the optimization of these TSS parameters, for instance, by showing the effect of different sintering conditions on key properties like the piezoelectric coefficient (d33) in the Supplementary Information.\n2. The manuscript repeatedly highlights the potential for \"continuous monitoring\" and \"continuous, wearable imaging\". To substantiate the claim of continuous monitoring capability, a long-term wearability test should be performed. For example, the device could be worn by a volunteer for 2–4 hours, with performance metrics (e.g., impedance spectra) measured periodically.\n3. In the context of long-term wear, it is unclear how the device's performance might be affected by thermal fluctuations. Will changes in body temperature cause a drift in the device's performance metrics? A clarification on the thermal stability of the integrated device during operation would be beneficial.\n4. The device relies on a bioadhesive hydrogel for acoustic coupling and skin adhesion. Could the authors comment on the stability of this hydrogel during prolonged wear? Specifically, is there a risk of the hydrogel drying out or losing its adhesive properties over time, which would negatively impact the acoustic coupling?\n5. While the device is shown to be flexible in photographs, its mechanical durability—a critical factor for wearable applications—is not quantitatively assessed. To strengthen the claims of its suitability for wearable use, a cyclic bending test is recommended. For example, the device could be subjected to over 1,000 bending cycles, with key performance metrics such as impedance, the center frequency, peak-to-peak sensitivity, and fractional bandwidth measured before and after cycling to evaluate its robustness.\n6. The biocompatibility evaluation is comprehensive for the core KNNS-BAZ ceramic material and its precursors. However, the final LF-UIA device is an assembly that also contains other materials, including epoxy, a backing layer, and a flexible printed circuit board (F-PCB). A complete biocompatibility assessment should ideally consider the entire device that comes into contact with the patient. It is advisable that the authors either test the final device or add a discussion addressing the biocompatibility of all constituent components.", "reviewer_2": "This article presents a comprehensive and detailed study on the integration of a lead-free piezoelectric material into a multi-element transducer dedicated to imaging and other medical applications.\nToday this issue is very important because the piezoelectric ceramics currently used contain lead, and for over 25 years, researchers have been trying to identify new compositions. To date, three major families have been identified, including KNN-based. This article makes a significant scientific contribution to understanding at the microstructural scale and improving the electromechanical properties of ceramics. The ceramic is then used to manufacture a composite to enhance these performances for integration into a 64-element transducer (linear array).\nThis transducer was then connected to a research ultrasound system to evaluate its usefulness and performance for various applications. Beyond these applications, its characteristics, depending on the intended application, are also discussed. Each time, the usefulness of this new transducer is clearly demonstrated through partial comparisons with transducers incorporating lead-based materials.\nFinally, a specific study on the toxicity of this new material was conducted in comparison with PZT (lead-based). The conclusions are quite interesting and further demonstrate the value of this new high-performance composition.\nThe quality of the data is unquestionable and well supported. This publication is very rich and has many aspects with sometimes a lack of specifications concerning the applications.\nThe distribution of data between the body text and the supplementary is well balanced.\nThe writing of this article is well organized, but several points often minor must be completed and improved before a publication.\nThis work thus shows that lead-free compositions are today competitive with PZT, but this then raises an important question that should be addressed in the discussion of this article. What is the situation today with regard to upscaling with a view to manufacturing on a larger scale and therefore marketing these ceramics, and then for transducer manufacturers to integrate them into their manufacturing process?\nSupplementary – Table 1 : A number of transducer characteristics are given to compare them to the presenet transducer, but there are also other lead-free transducers that have been published with their characteristics and which could also be added to this table (particularly in the IEEE UFFC journal)\nThe value of the thickness mode coupling coefficient is specified as kt=0.65 but given the dimensions of the sample used, the lateral dimensions are close to those of the thickness (2*2*6.4) and in this case it is not the kt that is measured (intermediate between the kt and the k33). This measurement must therefore be reviewed.\nThe manufactured transducer has a center frequency close to 8 MHz and is considered to deliver « high-resolution images ». Generally, it is rather from 20 MHz that high-frequency transducers will be considered to allow to give high-resolution images with spatial resolutions lower than those of this article. Precisely for the spatial resolutions, what is the focal length used to make the measurements?\nSeveral wearable applications are mentioned, particularly with photos such as 5.a. This may seem misleading because to date, in the work presented, the electrical connections with the Verasonics systems exist and a photo including the complete system would then be more representative.\nFig 2.a gray colors seem to be used it is necessary to harmonize the colors with the fig .2b.\nFig 3.d, there is a list of references to be added for the specified data on KNN-based and PZT-based.\nFig 4.a : the pulse-echo response is measured for one representative element ?", "reviewer_3": "This manuscript presents a 64-channel flexible ultrasound imaging array based on a lead-free KNN-based ceramic synthesized through a two-step sintering process. The authors demonstrate that this approach significantly enhances the performance of the ceramic, achieving a piezoelectric coefficient (d33) of approximately 630 pC/N, an electromechanical coupling coefficient (kt) of around 0.65, and a high Curie temperature of about 210 °C. The manuscript includes detailed characterization of domain structures and material properties using techniques such as PFM, TEM, and STEM, providing evidence of restored long-range ferroelectric order (LRFO) and reduced polar nanoregions (PNRs). In addition, the work showcases the array’s multimodal imaging capabilities—including B-mode, Doppler, and shear wave elastography (SWE)—achieving an axial resolution of approximately 0.2 mm, real-time muscle stiffness monitoring, and acoustic output within safe limits. The biocompatibility and biosafety of the material are also evaluated, indicating its potential as a safe and viable alternative to lead-based ceramics for biomedical applications.\nHere are some specific questions and comments:\nThe authors introduce a TSS process to restore LRFO and suppress excessive PNRs in multiphase coexisting KNN-based ceramics. A brief explanation and illustration of the process are provided in the Supplementary Information, indicating the use of a first-step temperature of 1200 °C for 10 minutes, followed by a second-step temperature of 960 °C for 6 hours. However, the rationale behind selecting these specific parameters (T1 = 1200 °C, t1 = 10 min; T2 = 960 °C, t2 = 6 h) is not discussed. It remains unclear whether these values were chosen based on previous optimization studies, phase diagrams, or empirical tuning. The authors are encouraged to clarify how these parameters were determined and whether this represents a standard protocol for KNN-based ceramics or a novel optimization developed in this work.\nGiven the intended use in wearable biomedical applications, it would be important to understand whether the TSS process affects the long-term mechanical and electrical stability of the ceramics. Have the authors evaluated the durability of the material under repeated mechanical deformation or prolonged electrical operation?\nFor the SWE application, what is the effective depth limit for reliable measurements in tissue? Additionally, what is the minimal detectable change in stiffness, or the resolution of the system in terms of elasticity measurements?\nSupplementary Table 1 provides a useful comparison of the LF-UIA device’s imaging performance (e.g., resolution, bandwidth) relative to other transducers. However, it remains unclear how the material-level properties of the TSS-based ceramics (such as piezoelectric coefficients, electromechanical coupling, dielectric constant, and loss tangent) compare to commonly used lead-free and lead-based materials, including PZT, PMN-PT, BT-based systems, and other KNN variants (e.g., textured, Li-doped, or single-crystal compositions). It would be helpful to include a table or a concise discussion summarizing key material parameters (e.g., d33, kt, TC, εr) alongside representative values from commercial or previously reported ceramics. Additionally, a brief comparison of imaging trade-offs (such as performance vs. biocompatibility) and material processing complexity or scalability would provide valuable context for readers and help position this work within the broader landscape of piezoelectric materials.\nSome acronyms like Kt (line 74) not defined upon first use. Standardize acronym introduction for clarity."}, "author_response": "Point-by-point Response to Reviewers’ Comments on Research Article, NCOMMS-25-39070\nDear Editor and reviewers:\nThank you for your letter and the reviewers’ comments concerning our manuscript entitled “Superior lead-free piezoceramics for wearable multimodal ultrasound imaging arrays” (Research Article, NCOMMS-25-39070). These comments and suggestions are all valuable and very helpful for revising and improving our manuscript. We have carefully studied the comments, performed supplementary experiments, given more discussion and explanation, and tried our best to correct and re-edit the manuscript and supplementary information that we hope to meet with approval. The main modifications are highlighted in the revised manuscript, and the point-to-point responses to the reviewers’ comments are listed as follows (the original comments are in black color and our responses are in blue):\nReviewer #1 (Remarks to the Author):\nIn this manuscript, the authors have developed a superior lead-free potassium sodium niobate (KNN)-based piezoceramic and fabricated it into a wearable multimodal ultrasound imaging array. This device demonstrates impressive capabilities, including high-resolution vascular imaging, blood flow dynamics monitoring, and muscle elastography. This material exhibits superior piezoelectric properties, and the device architecture is suitable for wearable applications, which is potentially important for advancing the next generation of wearable ultrasound technologies. However, there still remain some areas that need to be clarified and improved before publication:\nAuthors’ response: We sincerely thank you for the thoughtful and constructive evaluation of our manuscript. We greatly appreciate your recognition of our work in developing a high-performance lead-free KNN-based piezoceramic and its integration into a wearable multimodal ultrasound imaging array. Your positive comments regarding the material properties, device design, and its potential for next-generation wearable ultrasound technologies are truly encouraging. We also acknowledge and appreciate your insightful suggestions for clarification and improvement. We have carefully addressed each of the comments below and revised the manuscript accordingly to improve its clarity and scientific rigor.\n1. The Methods section states that the two-step sintering (TSS) process involves heating to 1200°C for 10 minutes, followed by cooling to 960°C and holding for 6 hours. This is a very specific parameter set, but the manuscript does not provide a rationale for selecting these exact temperatures and times. It is recommended that the authors supplement the manuscript with a brief study on the optimization of these TSS parameters, for instance, by showing the effect of different sintering conditions on key properties like the piezoelectric coefficient (d ) in the Supplementary Information.\nselected sintering parameter (1200°C/10 min/960°C/6 h) yields the highest d value. 33\nThe revised Supplementary Information is copied below:\nIn the revised Supplementary Information:\n“Supplementary note 1 | Two-step sintering\nDifferent from the traditional conventional sintering (CS), the TSS method consists of two steps: the green samples are first heated to a high temperature (T ) and kept for a 1\nshort time, and then the temperature is rapidly decreased to a lower temperature (T ) 2\nand kept for a longer time. Due to the shorter dwell time at high temperatures, the TSS method reduces the volatilization of alkali metal elements and promotes densification.\nMoreover, the TSS method has been reported to expand the sintering window of KNN-based ceramics, a development with significant implications for practical applications1.\nTo optimize the TSS parameters, the effects of both the sintering temperature and the holding time on the piezoelectric performance were systematically investigated. As shown in Supplementary Fig. 1, the d values maximized under the condition of 33\n1200°C for 10 min followed by 960°C for 6 h. Therefore, the TSS parameter of 1200°C/10 min/960°C/6 h was selected for TSS sample preparation (Supplementary Fig. 2).”\nSupplementary Fig. 1 | Piezoelectric performance of ceramics sintered with different TSS parameters. a, Dependence of d on sintering temperature T2. b, 33\nDependence of d on sintering temperature T1. c, Dependence of d on dwell time t1. 33 33\nd, Dependence of d on dwell time t2. The d values maximized under the condition 33 33\nof 1200°C for 10 min followed by 960°C for 6 h.\n2. The manuscript repeatedly highlights the potential for “continuous monitoring” and “continuous, wearable imaging”. To substantiate the claim of continuous monitoring capability, a long-term wearability test should be performed. For example, the device could be worn by a volunteer for 2–4 hours, with performance metrics (e.g., impedance spectra) measured periodically.\nAuthors’ response: We thank the reviewer for this insightful comment. Following the suggestion, we performed a wearability test in which the device was continuously worn for monitoring for 4 hours. The electrical impedance and phase spectrum were measured before and after the 4-hour period, and no obvious changes were observed (Supplementary Fig. 28). This result indicates that the device maintains stable electrical performance during continuous wear applications. In addition, the natural fluctuation of body temperature during this period did not induce any measurable effect on device performance. These findings demonstrate that our LF-UIA is well-suited for potential long-term, continuous monitoring applications. The corresponding impedance data have been added to the revised manuscript, which are also copied below:\nIn the revised manuscript:\n“The stability of LF-UIA was subsequently assessed under conditions relevant to continuous wearable use. When the device was worn for 4 h, the electrical impedance and phase spectra remined essentially unchanged (Supplementary Fig. 28). During this period, no significant thermal effects were observed in the device itself, and natural fluctuations in body temperature did not produce measurable impacts. These results confirm the stability of the device and demonstrate that minor physical activity and physiological temperature variations do not compromise device performance (Supplementary Fig. 28).” (Page 10, line 281)\nSupplementary Fig. 28 | Electrical impedance and phase spectrum of the selected element of LF-UIA before and after wearing for 4 hours. Minor physical activity and physiological temperature variations do not compromise device performance.\n3. In the context of long-term wear, it is unclear how the device's performance might be affected by thermal fluctuations. Will changes in body temperature cause a drift in the device's performance metrics? A clarification on the thermal stability of the integrated device during operation would be beneficial.\nAuthors’ response: We sincerely thank the reviewer for raising these important points.\nTo directly address the concern, we conducted a continuous wear test, in which the device was worn by a volunteer for 4 hours. The electrical impedance and phase spectrum were recorded before and after the test (Supplementary Fig. 28), and no noticeable changes were observed, confirming the device’s stable performance during continuous wear. Regarding thermal stability, on one hand, measurement results indicate that the device did not exhibit significant thermal effects during wear tests, as it operated at low voltages under FDA safe conditions due to the excellent electromechanical coupling coefficient of the KNN-based piezoelectric materials; on the other hand, the device showed no noticeable drift under natural fluctuations in human body temperature. Overall, this stability arises from the intrinsic material properties of the probe. The device operates at a temperature far below the Curie temperature of our synthesized piezoelectric material (T ~210°C), preventing any risk C\nof depolarization. Consequently, variations in body temperature do not impact the device’s functional performance. Hence, this result shows that our array is well-suited for long-term continuous monitoring. The corresponding data have been added to the revised manuscript, which are also copied below:\nIn the revised manuscript:\n“The stability of LF-UIA was subsequently assessed under conditions relevant to continuous wearable use. When the device was worn for 4 h, the electrical impedance and phase spectra remined essentially unchanged (Supplementary Fig. 28). During this period, no significant thermal effects were observed in the device itself, and natural fluctuations in body temperature did not produce measurable impacts. These results confirm the stability of the device and demonstrate that minor physical activity and physiological temperature variations do not compromise device performance (Supplementary Fig. 28).” (Page 10, line 281)\nSupplementary Fig. 28 | Electrical impedance and phase spectrum of the selected element of LF-UIA before and after wearing for 4 hours. Minor physical activity and physiological temperature variations do not compromise device performance.\n4. The device relies on a bioadhesive hydrogel for acoustic coupling and skin adhesion. Could the authors comment on the stability of this hydrogel during prolonged wear? Specifically, is there a risk of the hydrogel drying out or losing its adhesive properties over time, which would negatively impact the acoustic coupling?\nAuthors’ response: We thank the reviewer for raising this important point regarding the long-term stability of the bioadhesive hydrogel. To address this, we performed additional experiments to assess both the weight retention and adhesive properties of the hydrogel over time. As shown in Supplementary Fig. 30a, the hydrogel gradually developed wrinkles due to water loss when kept in ambient conditions (relative humidity 50-70%, temperature 27℃). Nevertheless, its morphology remained almost unchanged within the first three days. Supplementary Fig. 30b further quantifies the relative weight change, indicating that the hydrogel preserved more than 75% of its initial mass after three days, and retained approximately 50% after seven days. Additionally, adhesion tests (Supplementary Fig. 31) demonstrated that the hydrogel maintained strong adhesion on porcine skin even after 7 days of exposure. These results suggest that the hydrogel enables reliable skin adhesion and acoustic coupling for at least three days of continuous use, which is sufficient for the prolonged wear. In practice, the hydrogel can be replaced every three days to ensure stable and efficient operation of LF-UIA. Furthermore, encapsulation strategies (for example, applying an elastomeric membrane) could be incorporated in future designs to further suppress dehydration and extend the functional lifetime of the bioadhesive hydrogel [1].\nCorresponding results and discussion have been added to the revised manuscript, or as follows:\nIn the revised manuscript:\n“Considering that the long-term operation of LF-UIA also depends on stable adhesion and acoustic coupling, the bioadhesive hydrogel was examined under ambient conditions (relative humidity 50–70%, 27°C). The hydrogel retained its morphology and more than 75% of its initial mass after 3 days (Supplementary Fig. 30a,b), and exhibited strong tissue adhesion (on porcine skin) after 7 days (Supplementary Fig. 31). These results demonstrate that the hydrogel provides durable adhesion while maintaining reliable acoustic coupling over multi-day use. Together, these results validate that LF-UIA maintains stable electrical, mechanical, and adhesive performance under realistic physiological and environmental conditions, ensuring reliable operation during continuous and long-term wearable applications.” (Page 10, line 289)\nSupplementary Fig. 30 | Time-dependent morphology and mass retention of the bioadhesive hydrogel. a, Photographs showing the morphological changes of the bioadhesive hydrogel over 7 days. Scale bar, 2 cm. b, Relative mass of the bioadhesive hydrogel during the 7-day period.\nSupplementary Fig. 31 | Time dependent adhesion properties of the bioadhesive hydrogel on porcine skin. Representative photographs show that the hydrogels maintain strong adhesion with time, with robust adhesive properties still observed on day 7. Scale bar, 2cm.\nReferences:\n[1] C. Wang, X. Chen, L. Wang, M. Makihata, H.-C. Liu, T. Zhou, X. Zhao. Bioadhesive ultrasound for long-term continuous imaging of diverse organs. Science 377 (6605), 517-523 (2022).\n5. While the device is shown to be flexible in photographs, its mechanical durability— a critical factor for wearable applications—is not quantitatively assessed. To strengthen the claims of its suitability for wearable use, a cyclic bending test is recommended. For example, the device could be subjected to over 1,000 bending cycles, with key performance metrics such as impedance, the center frequency, peak-to-peak sensitivity, and fractional bandwidth measured before and after cycling to evaluate its robustness.\nAuthors’ response: We appreciate the reviewer’s constructive suggestion. Following this recommendation, we performed cyclic bending tests, in which the device was subjected to over 1,000 bending cycles (Supplementary Fig. 29). The key performance metrics—including impedance spectra, center frequency, peak-to-peak sensitivity, and fractional bandwidth were measured before and after the cycling test. No appreciable differences were observed in these parameters, confirming the mechanical robustness of the device. The excellent stability can be attributed to the structural design as the piezoelectric elements are firmly bonded onto the flexible printed circuit board (F-PCB), while the backing layer (E-solder 3022) provides additional mechanical support and a thin-film encapsulation (Parylene C, 10 μm) covers the device’s outermost layer. This structure effectively protects the active array from performance degradation under repeated bending. These results illustrate the device’s durability and its suitability for long-term wearable applications. Corresponding results and discussion have been added to the revised manuscript, or as follows:\nIn the revised manuscript:\n“The mechanical robustness of the LF-UIA was evaluated through cyclic bending tests. Even after 1000 cycles, the impedance spectra, center frequency, peak-to-peak sensitivity, and fractional bandwidth exhibited negligible changes (Supplementary Fig. 29), indicating the stable functional performance of LF-UIA under repeated mechanical deformation.” (Page 10, line 286)\nSupplementary Fig. 29 | Performance characterization of LF-UIA after bending for 1000 cycles. a, Center frequency. b, Fractional bandwidth. c, Peak-peak sensitivity. d, Electrical impedance. No appreciable differences were observed in these parameters, confirming the mechanical robustness of the device.\n6. The biocompatibility evaluation is comprehensive for the core KNNS-BAZ ceramic material and its precursors. However, the final LF-UIA device is an assembly that also contains other materials, including epoxy, a backing layer, and a flexible printed circuit board (F-PCB). A complete biocompatibility assessment should ideally consider the entire device that comes into contact with the patient. It is advisable that the authors either test the final device or add a discussion addressing the biocompatibility of all constituent components.\nAuthors’ response: We thank the reviewer for this valuable comment. In addition to the core KNNS-BAZ ceramic, we have also evaluated the cytocompatibility of the other constituent materials of the LF-UIA device, including the bioadhesive hydrogel, epoxy, backing layer (E-solder 3022), and F-PCB. The results confirm that all these materials exhibit excellent biocompatibility without inducing adverse cellular responses (Supplementary Fig. 38). Moreover, these materials (hydrogel, epoxy, E-solder 3022, and epoxy) are widely reported and frequently used in biomedical electronic devices, with well-documented biocompatibility [1-6]. We have added a clarification in the revised manuscript to highlight that not only the KNNS-BAZ ceramic but also the constituent materials of the LF-UIA device possess good biocompatibility, ensuring the overall safety of the assembled device.\nIn the revised manuscript:\n“These results demonstrate the superior biocompatibility and minimal inflammatory response of KNN-based materials. Biocompatibility assessments of the constituent materials of the LF-UIA device, including the bioadhesive hydrogel, epoxy, backing layer (E-solder 3022), and F-PCB, further confirmed negligible cytotoxicity (Supplementary Fig. 38), in line with previous reports on their safe use in biomedical devices1-5,9. Collectively, the LF-UIA exhibits reliable biocompatibility for biomedical applications.” (Page 13, line 383)\nSupplementary Fig. 38 | Biosafety assessments. Live/dead cells staining of the control group, bioadhesive hydrogel, backing layer, epoxy, and F-PCB leachate-treated samples on days 1, 3, and 5, along with corresponding cell viability assessment via CCK-8 assay (n = 3 biologically independent samples). Results are presented as mean ± s.e.m.. Scale bar, 100 μm.\nReferences:\n[1] H. Hu, X. Zhu, C. Wang, L. Zhang, X. Li, S. Lee, Z. Huang, R. Chen, Z. Chen, C. Wang, Y. Gu, Y. Chen, Y. Lei, T. Zhang, N. Kim, Y. Guo, Y. Teng, W. Zhou, Y. Li, A. Nomoto, S. Sternini, Q. Zhou, M. Pharr, F. Lanza di Scales, S. Xu. Stretchable ultrasonic transducer arrays for three-dimensional imaging on complex surfaces. Sci. Adv. 4 (3), eaar3979 (2018).\n[2] C. Wang, X. Chen, L. Wang, M. Makihata, H.-C. Liu, T. Zhou, X. Zhao. Bioadhesive ultrasound for long-term continuous imaging of diverse organs. Science 377 (6605), 517-523 (2022).\n[3] H. Hu, Y. Ma, X. Gao, D. Song, M. Li, H. Huang, X. Qian, R. Wu, K. Shi, H. Ding, M. Lin, X. Chen, W. Zhao, B. Qi, S. Zhou, R. Chen, Y. Gu, Y. Chen, Y. Lei, C. Wang, C. Wang, Y. Tong, H. Cui, A. Abdal, Y. Zhu, X. Tian, Z. Chen, C. Lu, X. Yang, J. Mu, Z. Lou, M. Eghtedari, Q. Zhou, A. Oberai, S. Xu. Stretchable ultrasonic arrays for the three-dimensional mapping of the modulus of deep tissue. Nat. Biomed. Eng. 7, 1321-1334 (2023).\n[4] C. Wang, B. Qi, M. Lin, Z. Zhang, M. Makihata, B. Liu, S. Zhou, Y.-h. Huang, H. Hu, Y. Gu, Y. Chen, Y. Lei, T. Lee, S. Chien, K.-I. Jang, E. B. Kistler, S. Xu. Continuous monitoring of deep-tissue haemodynamics with stretchable ultrasonic phased arrays. Nat. Biomed. Eng. 5, 749-758 (2021).\n[5] S. Zhou, G. Park, K. Longardner, M. Lin, B. Qi, X. Yang, X. Gao, H. Huang, X. Chen, Y. Bian, H. Hu, R. S. Wu, W. Yue, M. Li, C. Lu, R. Wang, S. Qin, E. Tasali, T. Karrison, I. Thomas, B. Smarr, E. B. Kistler, B. A. Khiami, I. Litvan. S. Xu. Clinical validation of a wearable ultrasound sensor of blood pressure. Nat. Biomed. Eng. 9, 865-881 (2024).\n[6] H.-C. Liu, Y. Zeng, C. Gong, X. Chen, P. Kijanka, J. Zhang, Y. Genyk, H. Tchelepi, C. Wang, Q. Zhou, X. Zhao. Wearable bioadhesive ultrasound shear wave elastography. Sci. Adv. 10 (6), eadk8426 (2024).\nReviewer #2 (Remarks to the Author):\nThis article presents a comprehensive and detailed study on the integration of a lead-free piezoelectric material into a multi-element transducer dedicated to imaging and other medical applications. Today this issue is very important because the piezoelectric ceramics currently used contain lead, and for over 25 years, researchers have been trying to identify new compositions. To date, three major families have been identified, including KNN-based. This article makes a significant scientific contribution to understanding at the microstructural scale and improving the electromechanical properties of ceramics. The ceramic is then used to manufacture a composite to enhance these performances for integration into a 64-element transducer (linear array). This transducer was then connected to a research ultrasound system to evaluate its usefulness and performance for various applications. Beyond these applications, its characteristics, depending on the intended application, are also discussed. Each time, the usefulness of this new transducer is clearly demonstrated through partial comparisons with transducers incorporating lead-based materials. Finally, a specific study on the toxicity of this new material was conducted in comparison with PZT (lead-based). The conclusions are quite interesting and further demonstrate the value of this new high-performance composition. The quality of the data is unquestionable and well supported. This publication is very rich and has many aspects with sometimes a lack of specifications concerning the applications. The distribution of data between the body text and the supplementary is well balanced. The writing of this article is well organized, but several points often minor must be completed and improved before a publication.\nAuthors’ response: We sincerely thank you for your thorough and thoughtful evaluation of our manuscript. We greatly appreciate the recognition of the scientific contributions of our work, particularly the development and integration of a lead-free KNN-based piezoelectric material into a 64-elemnt linear array transducer, and the comprehensive investigations spanning materials properties, device performance, and biological safety. We are pleased that you found the manuscript to be well organized, rich in content, and supported by high-quality data. We also acknowledge your comment regarding the occasional lack of specifications concerning some application aspects. We have carefully revised the manuscript to clarify and expand on the application contexts and performance parameters where appropriate. Thank you again for your constructive comments.\nThis work thus shows that lead-free compositions are today competitive with PZT, but this then raises an important question that should be addressed in the discussion of this article. What is the situation today with regard to upscaling with a view to manufacturing on a larger scale and therefore marketing these ceramics, and then for transducer manufacturers to integrate them into their manufacturing process?\nAuthors’ response: Thank you for raising this critical and forward-looking question regarding the scalability and manufacturability of lead-free piezoelectric ceramics, which indeed deserves attention in the context of potential commercialization and integration into industrial transducer manufacturing. Currently, KNN-based ceramics are considered promising candidates for large-scale production, owing to significant progress in several key areas such as powder synthesis, sintering optimization, and defect engineering. Nevertheless, certain challenges remain before full industrial adoption can be realized, such as ensuring stability and uniform poling across large batch productions, preserving mechanical robustness during device assembly (especially in composites and multilayer structures), and achieving cost-competitive fabrication at high volumes. Additionally, enhancing process tolerance, including controlling compositional fluctuations, grain size uniformity, and reproducibility of functional properties, is identified as a critical step toward large-scale manufacturing and market integration. In response to this valuable comment, we have added a more detailed discussion in the Discussion section to address the current state of upscaling KNN-based ceramics, which is also copied below:\nIn the revised manuscript:\n“Beyond the promising performance, the scalability and integration of lead-free ceramics into industrial manufacturing processes are critical for bridging the gap between laboratory research and practical applications. Recent advances in powder synthesis, sintering optimization, and defect engineering have enhanced the potential for scaling up KNN-based ceramics. However, challenges remain in ensuring stability and uniform poling across large batches, maintaining mechanical integrity during composite fabrication, and achieving cost-efficiency at industrial scales. Addressing these issues through pilot-scale production and collaborations with industrial partners is essential, alongside strategies to improve process tolerance for large-scale adoption.” (Page 14, line 407)\nSupplementary – Table 1: A number of transducer characteristics are given to compare them to the present transducer, but there are also other lead-free transducers that have been published with their characteristics and which could also be added to this table (particularly in the IEEE UFFC journal)\nAuthors’ response: We sincerely thank the reviewer for the valuable suggestion. The original intention of Supplementary Table 2 was to summarize and compare the performance of our LF-UIA with representative lead-based transducer arrays, in order to highlight that our KNN-based lead-free ultrasound imaging array achieves performance almost comparable to state-of-the-art lead-based systems. Indeed, some reports in journals such as IEEE T-UFFC remain unaccounted for. However, following your suggestion, we have now added relevant data on reported lead-free transducer arrays in Supplementary Table 2 to provide a more comprehensive comparison. The revised table offers a clear context for evaluating the performance of our LF-UIA in relation to both lead-free and lead-based alternatives.\nIn the revised Supplementary information:\nSupplementary Table 2 | Comparison of the performance of the LF-UIA device with existing ultrasound array devices.\nCenter Resolution (mm)\nTransducer Lead- -6 dB\nfrequency References\narray free bandwidth Lateral Axial\n(MHz)\nLF-UIA Yes 8.36 40% 0.29 0.20 This work\nKNN based\nYes 18.3 42% 0.166 0.105 24\nlinear-array\nHigh-\nfrequency Yes 27 48.5% - - 25\nlinear-array\nBCTZ based\nYes 2.9 80.5% 2.68 0.86 26\nphased array\nBAUS-E no 7.2 37.45% 0.13 0.36 27\ncUSBr-Patch no 7.1 70% 1 0.5 28\nStretchable\nultrasonic no 3 50% 0.56 0.50 29\narray\n3 68% 1.79 0.77\nBAUS no 7 75% 0.38 0.25 30\n10 78% 0.38 0.1924\nPhased array\nno 3.5 - 0.6 0.6 31\ntransducer\nThe value of the thickness mode coupling coefficient is specified as k = 0.65 but given t\nthe dimensions of the sample used, the lateral dimensions are close to those of the thickness (2*2*6.4) and in this case it is not the k that is measured (intermediate t\nbetween the k and the k ). This measurement must therefore be reviewed.\nt 33\nAuthors’ response: Thank you for your professional and constructive comment. We fully agree with your assessment that when the aspect ratio of the sample is low, the measured electromechanical coupling coefficient may not correspond to a pure thickness mode (k). According to the IEEE standard for piezoelectric measurements, a t\nreliable measurement of k requires a sample with a diameter-to-thickness (or lateral- t\nto-thickness) ratio greater than 10 to ensure a pure thickness vibration mode. In this work, the sample dimensions were 2 mm × 2 mm × 6.4 mm, with the polarization direction along the 6.4 mm length. Given this configuration, the measured vibration corresponds to the longitudinal mode (k ) rather than the thickness mode (k). Thus, 33 t\nthe previous reported coupling coefficient should be attributed to the longitudinal mode k , not k. Furthermore, we note that k measurements using samples with a thickness- 33 t 33\nto-width ration in the range of 3-5 have been widely reported and accepted in the literature [1-4]. In the context of 1-3 piezoelectric composites, it has also been demonstrated that the epoxy in the 1-3 structure confines the transverse vibration of piezoelectric rods and thus concentrates the energy on the longitudinal vibration [5].\nIn light of your comment, we have corrected the notation in the revised manuscript from k to k and re-calculated the coupling coefficient using the appropriate formula t 33\nfor k . We sincerely appreciate your observation, which helps to improve the accuracy 33\nand rigor of our work.\nReferences:\n[1] L. Zhang, C. Marcus, D. Lin, D. Mejorado, S. Joseph Schoen Jr, T. T. Pierce, V. Kumar, S. V. Fernandez, D. Hunt, Q. Li, I. I. Shuvo, D. Sadat, W. Du, H. Edenbaum, L. Jin, W. Liu, Y. C. Eldar, F. Li, A. P. Chandrakasan, A. E. Samir, C. Dagdeviren. A conformable phased-array ultrasound patch for bladder volume monitoring. Nat. Electron. 7, 77-90 (2024).\n[2] H. Lv, W. Liao, Y. You, R. Xiong. Inch-size molecular ferroelectric crystal with a large electromechanical coupling factor on par with barium titanate. J. Am. Chem. Soc. 144, 22352-22331 (2022).\n[3] H. Wang, S. Xia, M. Ma, Z. Li. Enhanced electrical properties with high Curie temperature in PMN-PZT single crystal by solid-state crystal growth. J. Am. Ceram. Soc. 108 (10), e20685 (2025).\n[4] A. Boonruang, T. Thongchai, Y. Jiang, C.E.M. Demore, S. Neale, A. Moldovan, T.W. Button, S. Cochran. Progress towards wafer-scale fabrication based on gel casting technique for 1-3 randomised piezocomposite μUS linear array. J. Eur. Ceram. Soc. 42, 5565-5574 (2022).\n[5] H. Hu, X. Zhu, C. Wang, L. Zhang, X. Li, S. Lee, Z. Huang, R. Chen, Z. Chen, C. Wang, Y. Gu, Y. Chen, Y. Lei, T. Zhang, N. Kim, Y. Guo, Y. Teng, W. Zhou, Y. Li, A. Nomoto, S. Sternini, Q. Zhou, M. Pharr, F. Lanza di Scales, S. Xu. Stretchable ultrasonic transducer arrays for three-dimensional imaging on complex surfaces. Sci. Adv. 4 (3), eaar3979 (2018).\nThe manufactured transducer has a center frequency close to 8 MHz and is considered to deliver high-resolution images. Generally, it is rather from 20 MHz that high-frequency transducers will be considered to allow to give high-resolution images with spatial resolutions lower than those of this article. Precisely for the spatial resolutions, what is the focal length used to make the measurements?\nAuthors’ response: Thank you for your valuable comment. It is indeed that transducers operating at center frequencies of 20 MHz or higher are typically classified as high-frequency and are often associated with high-resolution imaging. However, spatial resolution is not solely determined by center frequency, it also depends critically on transducer bandwidth, beamforming strategies, and system design. In this work, although the LF-UIA operates at a center frequency of approximately 8 MHz, it achieves high spatial resolution due to several key factors. First, the transducer exhibits a relatively broad fractional bandwidth, which enhances axial resolution. Second, the LF-UIA employs specifically designed matching and backing layers to improve both axial and lateral resolution. These structural and acoustic optimizations contribute significantly to imaging performance, even at a center frequency below 10 MHz.\nUltrasound transducers with center frequencies less than 10 MHz but capable of high-resolution imaging through structural design and system integration have been previously reported [1-3]. For spatial resolution measurements, we used the coherent plane-wave compounding [4] method. In this approach, each transmission is an unfocused plane wave and transmit focusing is achieved synthetically during image reconstruction by coherently summing multiple steering angles. Therefore, there is no specific physical focal depth as in conventionally focused transmit imaging.\nCorresponding details have been added into the revised manuscript, which are also copied below:\nIn the revised manuscript:\n“The LF-UIA was then integrated into the Verasonics Vantage system and operated with coherent plane-wave compounding method to facilitate real-time imaging and acoustic performance evaluation50.” (Page 9, line 265)\nReferences:\n[1] H. Hu, Y. Ma, X. Gao, D. Song, M. Li, H. Huang, X. Qian, R. Wu, K. Shi, H. Ding, M. Lin, X. Chen, W. Zhao, B. Qi, S. Zhou, R. Chen, Y. Gu, Y. Chen, Y. Lei, C. Wang, C. Wang, Y. Tong, H. Cui, A. Abdal, Y. Zhu, X. Tian, Z. Chen, C. Lu, X. Yang, J. Mu, Z. Lou, M. Eghtedari, Q. Zhou, A. Oberai, S. Xu. Stretchable ultrasonic arrays for the three-dimensional mapping of the modulus of deep tissue. Nat. Biomed. Eng. 7, 1321-1334 (2023).\n[2] C. Wang, X. Chen, L. Wang, M. Makihata, H.-C. Liu, T. Zhou, X. Zhao. Bioadhesive ultrasound for long-term continuous imaging of diverse organs. Science 377 (6605), 517-523 (2022).\n[3] S. Zhou, X. Gao, G. Park, X. Yang, B. Qi, M. Lin, H. Huang, Y. Bian, H. Hu, X. Chen, R. S. Wu, B. Liu, W. Yue, C. Lu, R. Wang, P. Bheemreddy, S. Qin, A. Lam, K. A. Wear, M. Andre, E. B. Kistler, D. W. Newell, S. Xu. Transcranial volumetric imaging using a conformal ultrasound patch. Nature 629, 810-818 (2024).\n[4] G. Montaldo, M. Tanter, J. Bercoff, N. Benech, M. Fink. Coherent plane-wave compounding for very high frame rate ultrasonography and transient elastography. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 56 (3), 489-506 (2009).\nSeveral wearable applications are mentioned, particularly with photos such as 5.a. This may seem misleading because to date, in the work presented, the electrical connections with the Verasonics systems exist and a photo including the complete system would then be more representative.\nAuthors’ response: We thank the reviewer for this insightful comment. We totally agree that, in the current stage, the wearable array was connected to the Verasonics system. The wearable prototypes shown in Figure 5a are therefore primarily intended to highlight the form factor and conformability of the device rather than to represent a fully standalone imaging system. We acknowledge that this distinction could be made clearer. Thus, we have rephotographed the image to display the complete system with the LF-UIA connected to the Verasonics Vantage.\nAt the same time, the integration with the Verasonics system has been essential to validate the acoustic and imaging performance of our device under well-controlled conditions. This provides a critical proof-of-concept step before translating the technology into more smart and wearable platforms. In the next stage, our team will keep working on developing customized electronics and wireless driving system for a truly wearable, untethered ultrasound array for functional ultrasound applications. Such future integration will enable continuous and multimodal functional imaging in a broader range of mobile and clinical settings.\nIn the revised manuscript:\n“The real-time imaging capabilities of the LF-UIA connecting to the Verasonics system were demonstrated through in vivo arterial imaging (Fig. 5a).” (Page 10, line 299)\nFig. 5 | Real-time PWD measurements using the LF-UIA. a, Photograph of the LF-UIA positioned on the wrist for real-time imaging radial artery. Scale bar, 5 cm. b, B-mode images capturing radial artery using the LF-UIA. c, Doppler images for visualizing spatial distribution and dynamics of radial artery blood flow with the LF-UIA. d, Schematic of the LF-UIA in Doppler mode for blood flow velocity measurements. e, PWD spectrum acquired using the LF-UIA from radial artery. f, Typical blood flow spectrum of radial artery during a cycle.\nFig. 2a gray colors seem to be used it is necessary to harmonize the colors with the Fig. 2b.\nAuthors’ response: Thank you for pointing this out. We agree that harmonizing the color schemes in Fig. 2a and Fig. 2b will improve the visual consistency and readability of the manuscript, as they both represent the dielectric properties. We have revised Fig. 2a to match the color scheme used in Fig. 2b, ensuring a uniform and coherent presentation. The updated figure has been incorporated into the revised manuscript and is copied below:\nIn the revised manuscript:\nFig. 2 | Multilevel structural origins of superior piezoelectricity in TSS ceramics.\na,b, Temperature-dependent dielectric constants and dielectric loss curves for CS (a) and TSS (b) ceramics. c, Rietveld refinements for CS and TSS ceramics around 45°. d, Phase fractions of CS and TSS ceramics derived from Rietveld refinements. e, Schematic illustrating the reduced energy barrier enabled by phase coexistence compared to a single-phase structure. f,g, PFM amplitude (f) and phase (g) images of CS ceramics. Scale bars, 500 nm. h,i, PFM amplitude (h) and phase (i) images of TSS ceramics. Scale bars, 1 μm. j, Schematic models of the domain configuration of CS and TSS ceramics. k, Coercive voltage mapping over a 2 μm × 2 μm area for TSS ceramics. l, Representative amplitude curves and phase loops for nanodomain and stripe domain. m, TEM images of TSS ceramics showing domain structures. Scale bar, 200 nm. n, Magnified image (top) and schematic (bottom) of the unit cell symmetries along [110] . c\no, Atomic-resolution HADDF-STEM polarization vector mapping along [110] . c\np, Enlarged HADDF-STEM image showing gradual polarization vector transitions.\nFig. 3d, there is a list of references to be added for the specified data on KNN-based and PZT-based.\nAuthors’ response: Thank you for your careful reading. We have added the appropriate references in the revised manuscript to properly credit the original sources, which are also copied below:\nIn the revised manuscript:\n“In conclusion, the denser structure with moderate local heterogeneity enabling the TSS ceramics a high d value of 630 pC N-1 together with a T of 210°C, rendering 33 C\nthem potential candidates for industrial lead-based piezo-materials (e.g., PZT-5H) (Fig. 3d and Supplementary Table 1)13,20,34,41-47.” (Page 8, line 217)\nFig. 4a: the pulse-echo response is measured for one representative element?\nAuthors’ response: Thank you for your insightful question. The pulse-echo response shown in Fig. 4b does correspond to one representative element. In fact, to evaluate the overall performance and consistency of the array, we measured the pulse-echo response for all 64 elements in LF-UIA. Fig. 4b presents one representative for clarity, and the center frequency and bandwidth values calculated from pulse-echo responses of all 64 elements are summarized in Fig. 4c and Supplementary Fig. 25, which show remarkable consistency of the elements in LF-UIA.\nReviewer #3 (Remarks to the Author):\nThis manuscript presents a 64-channel flexible ultrasound imaging array based on a lead-free KNN-based ceramic synthesized through a two-step sintering process. The authors demonstrate that this approach significantly enhances the performance of the ceramic, achieving a piezoelectric coefficient (d ) of approximately 630 pC/N, an 33\nelectromechanical coupling coefficient (kₜ) of around 0.65, and a high Curie temperature of about 210°C. The manuscript includes detailed characterization of domain structures and material properties using techniques such as PFM, TEM, and STEM, providing evidence of restored long-range ferroelectric order (LRFO) and reduced polar nanoregions (PNRs). In addition, the work showcases the array’s multimodal imaging capabilities—including B-mode, Doppler, and shear wave elastography (SWE)—achieving an axial resolution of approximately 0.2 mm, real-time muscle stiffness monitoring, and acoustic output within safe limits. The biocompatibility and biosafety of the material are also evaluated, indicating its potential as a safe and viable alternative to lead-based ceramics for biomedical applications.\nAuthors’ response: We sincerely thank you for your thorough evaluation and encouraging comments. We are very pleased that you recognized the significance of our work in developing a 64-channel flexible ultrasound imaging array based on a lead-free KNN-based ceramic, and the advance we have made in both material performance and imaging capabilities. We also appreciate your recognition of the potential impact of our approach in the field of biomedical ultrasound. Your positive feedback is highly motivating and greatly valued. Thank you again for your thoughtful and supportive review.\nHere are some specific questions and comments:\nThe authors introduce a TSS process to restore LRFO and suppress excessive PNRs in multiphase coexisting KNN-based ceramics. A brief explanation and illustration of the process are provided in the Supplementary Information, indicating the use of a first-step temperature of 1200 °C for 10 minutes, followed by a second-step temperature of 960 °C for 6 hours. However, the rationale behind selecting these specific parameters (T1 = 1200 °C, t1 = 10 min; T2 = 960 °C, t2 = 6 h) is not discussed. It remains unclear whether these values were chosen based on previous optimization studies, phase diagrams, or empirical tuning. The authors are encouraged to clarify how these parameters were determined and whether this represents a standard protocol for KNN-based ceramics or a novel optimization developed in this work.\nAuthors’ response: We sincerely thank you for your thoughtful comment. These specific parameters were determined through a series of optimization experiments designed to investigate the effects of the sintering temperature and holding time on the piezoelectric performance of the ceramics. As detailed in Supplementary Fig. 1, we systematically varied T1, T2, t1, and t2, and evaluated the resulting samples in terms of d values. The results clearly demonstrate that the d value reaches its maximum when 33 33\nthe TSS process is performed at 1200°C for 10 min followed by 960°C for 6 h.\nFurthermore, we have added a brief discussion in the revised Supplementary Information, which is copied below:\nIn the revised Supplementary Information:\n“Supplementary note 1 | Two-step sintering\nDifferent from the traditional conventional sintering (CS), the TSS method consists of two steps: the green samples are first heated to a high temperature (T ) and kept for a 1\nshort time, and then the temperature is rapidly decreased to a lower temperature (T ) 2\nand kept for a longer time. Due to the shorter dwell time at high temperatures, the TSS method reduces the volatilization of alkali metal elements and promotes densification.\nMoreover, the TSS method has been reported to expand the sintering window of KNN-based ceramics, a development with significant implications for practical applications1.\nTo optimize the TSS parameters, the effects of both the sintering temperature and the holding time on the piezoelectric performance were systematically investigated. As shown in Supplementary Fig. 1, the d values maximized under the condition of 33\n1200°C for 10 min followed by 960°C for 6 h. Therefore, the TSS parameter of 1200°C/10 min/960°C/6 h was selected for TSS sample preparation (Supplementary Fig. 2).”\nSupplementary Fig. 1 | Piezoelectric performance of ceramics sintered with different TSS parameters. a, Dependence of d on sintering temperature T2. b, 33\nDependence of d on sintering temperature T1. c, Dependence of d on dwell time t1. 33 33\nd, Dependence of d on dwell time t2. The d values maximized under the condition 33 33\nof 1200°C for 10 min followed by 960°C for 6 h.\nGiven the intended use in wearable biomedical applications, it would be important to understand whether the TSS process affects the long-term mechanical and electrical stability of the ceramics. Have the authors evaluated the durability of the material under repeated mechanical deformation or prolonged electrical operation?\nAuthors’ response: Thank you for your insightful comment regarding the long-term stability of the TSS-processed ceramics in the context of wearable biomedical applications. To address this important point, we have conducted additional fatigue tests to evaluate the durability of the materials under prolonged electrical cycling.\nSpecifically, the ceramics were subjected to bipolar electric field loading with an amplitude of twice the coercive field (2E ) for 106 cycles. As shown in Supplementary C\nFig. 21, both the remanent polarization and positive/negative strain values remained nearly unchanged after cycling, indicating excellent electrical stability of the TSS-processed ceramics. In addition, we have clarified this point in the revised manuscript to emphasize the long-term reliability of TSS-processed ceramics. These findings reinforce the suitability of the material for wearable and potentially implantable ultrasonic devices, where electrical endurance is critical.\nIn the revised manuscript:\n“The excellent electrical fatigue resistance ensures the long-term reliability of the TSS ceramics for wearable biomedical applications (Supplementary Fig. 21).” (Page 8, line 228)\nSupplementary Fig. 21 | P-E loops and bipolar strain curves of TSS ceramics before and after 106 bipolar fatigue cycles. a, P-E loops. b, Bipolar strain curves. Both the remanent polarization and positive/negative strain values remained nearly unchanged after 106 fatigue cycles.\nFor the SWE application, what is the effective depth limit for reliable measurements in tissue? Additionally, what is the minimal detectable change in stiffness, or the resolution of the system in terms of elasticity measurements?\nAuthors’ response: Thank you for your professional comments. First of all, to further determine the effective depth limit, we evaluated the signal-to-noise ratio (SNR) of the detected shear wave propagation signals at various focusing depths of the acoustic radiation force impulse (ARFI). A lower SNR indicates a reduced ability to reliably track the shear wave, thereby limiting the accuracy of the subsequent elasticity estimation. For this characterization, we utilized a commercial elasticity QA phantom (Model 049, CIRS Inc, Norfolk, VA, USA) to ensure the reliability and reproducibility of our measurements. The ARFI focus was systematically set to depths of 10 mm, 15 mm, 20 mm, and 25 mm. To maintain consistency across all measurements, the ARFI parameters were kept constant: a push sequence of 1000 cycle, a transmitted voltage of 45 V, and a 100% duty cycle.\nOur results, as illustrated in Supplementary Fig. 33, demonstrate a clear trend of decreasing SNR with increasing focal depth. Specifically, at a depth of 25 mm, the measured SNR was found to be significantly low, at an average of 6.606 ± 2.88 dB. At this level, the shear wave signal was substantially attenuated and difficult to distinguish from background noise, making reliable tracking of the shear wave propagation challenging, if not impossible. Therefore, based on this quantitative analysis, we define the effective depth limit for reliable SWE measurements with our current system configuration to be approximately 20 mm, beyond which the data quality is compromised.\nBesides, to quantify the minimal detectable change in stiffness, or more precisely, we fabricated and analyzed a side-by-side tissue-mimicking phantom. The phantom was constructed using gelatin (G2500, Sigma-Aldrich) with two different concentrations, 5% and 10%, to create a well-defined boundary between two regions of distinct stiffness. To ensure adequate acoustic scattering for ultrasound imaging, silicon carbide powder (S5631, Sigma-Aldrich) was incorporated into both gelatin mixtures at the same concentration. Using this phantom, we generated a shear wave speed (SWS) map, which is presented in Supplementary Fig. 34. To calculate the lateral spatial resolution, we first averaged the SWS values along the axial (depth) dimension to obtain a one-dimensional lateral profile across the stiffness boundary. An Edge Spread Function (ESF) was then derived by fitting this profile with a sigmoid model.\nSubsequently, the Point Spread Function (PSF) was obtained by taking the first derivative of the ESF. The spatial resolution of the system is defined by the Full Width at Half Maximum (FWHM) of this PSF curve.\nFollowing this analysis, the FWHM of the PSF was calculated to be 0.6151 mm. This value represents the spatial resolution of our device, indicating its ability to resolve changes in tissue stiffness over a sub-millimeter scale. Corresponding details have been added to the revised manuscript, which are also copied as follows:\nIn the revised manuscript:\n“To characterize the performance of LF-UIA in SWE, a commercial elasticity QA phantom (Model 049, CIRS Inc, Norfolk, VA, USA) was used to evaluate the effective depth limit. The signal-to-noise ratio (SNR) of shear wave propagation signals decreased with increasing acoustic radiation force impulse (ARFI) focal depth, falling to 6.606 ± 2.88 dB at 25 mm (Supplementary Fig. 33), where reliable tracking was no longer feasible. Therefore, the effective depth limit for accurate SWE measurements with LF-UIA is defined as ~20 mm. For spatial resolution assessment, a dual-stiffness gelatin phantom was fabricated, and the boundary response was analyzed using an edge-shaped function approach. The derived point-spread function yielded a full width at half maximum (FWHM) of 0.6151 mm (Supplementary Fig. 34), demonstrating the ability of the system to resolve sub-millimeter variations in stiffness.” (Page 11, line 324)\nSupplementary Fig. 33 | Effect of ARFI focal depth on shear wave propagation and signal-to-noise ratio (SNR). Measurements were performed in a commercial elasticity QA phantom (Model 049, CIRS Inc., USA) with ARFI focal depths of 10, 15, 20, and 25 mm under identical excitation conditions (1000-cycle push, 45 V transmit voltage, 100% duty cycle, n = 5). The SNR of shear wave signals decreased progressively with depth, reaching 6.606 ± 2.88 dB at 25 mm, where the signal became indistinguishable from background noise and reliable tracking was no longer feasible. Results are presented as mean ± s.e.m.\nSupplementary Fig. 34 | Quantification of spatial resolution in shear wave speed mapping. A dual-stiffness gelatin phantom (5% and 10% w/w) with added acoustic scatterers was used to generate shear wave speed maps across a well-defined stiffness boundary. The lateral profile was analyzed using an edge-spread function, from which the point-spread function was derived. The FWHM was 0.6151 mm, indicating sub-millimeter resolution in detecting stiffness variations.\nSupplementary Table 1 provides a useful comparison of the LF-UIA device’s imaging performance (e.g., resolution, bandwidth) relative to other transducers. However, it remains unclear how the material-level properties of the TSS-based ceramics (such as piezoelectric coefficients, electromechanical coupling, dielectric constant, and loss tangent) compare to commonly used lead-free and lead-based materials, including PZT, PMN-PT, BT-based systems, and other KNN variants (e.g., textured, Li-doped, or single-crystal compositions). It would be helpful to include a table or a concise discussion summarizing key material parameters (e.g., d , k, T , εᵣ) alongside 33 t C\nrepresentative values from commercial or previously reported ceramics. Additionally, a brief comparison of imaging trade-offs (such as performance vs. biocompatibility) and material processing complexity or scalability would provide valuable context for readers and help position this work within the broader landscape of piezoelectric materials.\nAuthors’ response: We sincerely thank you for this insightful and constructive suggestion. We fully agree that comparing the material-level properties of our TSS-prepared KNN ceramics with those of commonly used lead-based and lead-free materials will help position this work more clearly within the broader landscape of piezoelectric materials for ultrasound applications. While our initial focus was on demonstrating the device-level imaging performance of the LF-UIA, we recognize that the underlying material properties are critical to understanding both the advantages and trade-offs of the proposed system.\nIn response to the comments, we have added a new supplementary table (Supplementary Table 1) summarizing key material parameters (d , k /k/k , T , ε) 33 p t 33 C r\nfor our TSS-prepared ceramic and representative lead-free and lead-based materials. This table provides a clear and concise comparison across widely studied compositions. Furthermore, we have added a short discussion in the revised manuscript to address the trade-offs in piezoelectric materials. These additions provide broader context and have helped significantly improve the completeness and positioning of our work. Thank you again for your thoughtful suggestions.\nIn revised manuscript:\n“First, with materials as the cornerstone, the piezoelectric properties of KNN-based ceramics could be further enhanced to approach those of single-crystal materials. This improvement not only addresses the biocompatibility and environmental limitations associated with lead-based ceramics, but also enables higher imaging precision and sensitivity, especially in detecting subtle muscle contractions and early-stage tissue changes. Beyond the promising performance, the scalability and integration of lead-free ceramics into industrial manufacturing processes are critical for bridging the gap between laboratory research and practical applications. Recent advances in powder synthesis, sintering optimization, and defect engineering have enhanced the potential for scaling up KNN-based ceramics. However, challenges remain in ensuring stability and uniform poling across large batches, maintaining mechanical integrity during composite fabrication, and achieving cost-efficiency at industrial scales. Addressing these issues through pilot-scale production and collaborations with industrial partners is essential, alongside strategies to improve process tolerance for large-scale adoption.” (Page 14, line 403)\nIn revised Supplementary information:\nSupplementary Table 1 | Comparison of d , k /k/k , T and ε values among this 33 p t 33 C r\nwork and other lead-free and lead-based materials.\nSystem d (pC N-1) k /k/k T (°C) ε References\n33 p t 33 C r\nTSS-prepared k ~0.52\np\n630 210 3323 This work\nKNNS-BAZ k ~0.66\n650 k ~0.52 175 4588 7\np\nAS\nKNNS-BAZ-Fe/In 536 k ~0.56 225 3432 8\np\nKNNS-BNZ 440 k ~0.48 259 2067 9\np\nKNN-BNZ-LF 345 k ~0.48 314 1250 10\np\n5BZ/3BLT/92NKN 252 k ~0.51 256 1580 11\np\nKNNS-BNS 195 k ~0.53 300 678 9\np\nKNN-0.05LN 175 k ~0.34 401 483 12\np\nLF4T 416 k ~0.61 253 1570 13\np\nKNN-3T 700 k ~0.76 242 - 14\np\nT-3.5BHT 680 k ~0.725 260 - 15\np\nTextured KNN-5Sb 807 k ~0.88 245 - 16\n2\n261 k ~0.66 412 275 17\nt\nsingle crystal\nK Na NbO\n0.7 0.3 3\n167 k ~0.52 432 - 18\nt\nsingle crystal\nBCTZ-0.1SiC 638 k ~0.58 95 - 19\np\nBTS-BCT 700 - 40 - 20\nPZT8 225 k ~0.64 300 1000 21\np\nPZT4 410 k ~0.60 250 2300 21\np\nPZT-5H 590 k ~0.75 190 3400 21\np\nModified PMN-PT 1200 k ~0.55 - 3500 22\nt\n[001]-oriented\n2100 k ~0.94 132 7800 23\nSome acronyms like k (line 74) not defined upon first use. Standardize acronym t\nintroduction for clarity.\nAuthors’ response: Thank you for pointing out the inconsistency in acronym usage. Upon careful review, we realized that the electromechanical coupling coefficient in this work (measured by preparing piezoelectric rods) should be denoted as k rather than 33\nk, since the measurement corresponds to the longitudinal mode. We have revised the t\nmanuscript accordingly by replacing all instances of k with k and ensuring that all t 33\nacronyms, including k (longitudinal-mode electromechanical coupling coefficient), 33\nare defined upon their first appearance for clarity and consistency.\nIn the revised manuscript:\n“Recent progress in constructing rhombohedral-tetragonal (R-T) and rhombohedral-orthorhombic-tetragonal (R-O-T) multiphase coexistence has yielded a new category of KNN-based ceramics with enhanced piezoelectricity, achieving piezoelectric constant d exceeding 450 pC N-1 12.” (Page 2, line 47)\n33\n“This improvement led to enhanced polarization values and superior piezoelectric properties, achieving a high d value (~630 pC N-1) and an improved longitudinal- 33\nmode electromechanical coupling coefficient k (~0.66).” (Page 3, line 75)\n33"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all the previous comments and concerns raised during the review process. I believe the work now meets the standards of Nature Communications and recommend it for publication.", "reviewer_2": "In agreement with my comments given in the first review, the new version of the article has been really improved with the correction/modification of minor points and the addition of comments to clarify several discussions.\nThis substantial and very interesting work in terms of review and novelty can be published as is.", "reviewer_3": "1- Thank you for clarifying the rationale and providing the supporting optimization data for the TSS process.\n2- Thank you for sharing the electrical fatigue data, which makes the case for long-term electrical stability clear. My original question also referred to repeated mechanical deformation, which is particularly relevant for wearable use where bending and flexing are unavoidable. Have the authors assessed mechanical durability (for example, bending or flexural fatigue), or could they comment on how the material would be expected to perform under such conditions?\n3-Thank you for the detailed and quantitative characterization of the SNR, depth limit, and the spatial resolution. It would also be valuable if the authors could briefly comment on the main factors that limit SNR at increasing depth and how these might be improved or mitigated in future implementations."}, "author_response": "Point-by-point Response to Reviewers’ Comments on Research Article, NCOMMS-25-39070A\nDear Editor and reviewers:\nThank you for your letter and the reviewers’ comments concerning our manuscript entitled “Superior lead-free piezoceramics for wearable multimodal ultrasound imaging arrays” (Research Article, NCOMMS-25-39070A). These comments and suggestions are all valuable and very helpful for revising and improving our manuscript. We have focused on addressing the mechanical durability of the materials especially under repeated bending relevant to wearable applications, as well as the factors limiting signal-to-noise ratio (SNR) at increasing depths and potential strategies for improvement in future implementations. The main modifications are highlighted in the revised manuscript, and the point-to-point responses to the reviewers’ comments are listed as follows (the original comments are in black color and our responses are in blue):\nReviewer #1 (Remarks to the Author):\nThe authors have addressed all the previous comments and concerns raised during the review process. I believe the work now meets the standards of Nature Communications and recommend it for publication.\nAuthors’ response: We sincerely thank you for your thoughtful evaluation and positive feedback. We are pleased to hear that the revisions have addressed the previous comments and that the manuscript now meets the standards of Nature Communications. We greatly appreciate your time and effort in improving the quality of our work.\nReviewer #2 (Remarks to the Author):\nIn agreement with my comments given in the first review, the new version of the article has been really improved with the correction/modification of minor points and the 1\naddition of comments to clarify several discussions.\nThis substantial and very interesting work in terms of review and novelty can be published as is.\nAuthors’ response: We sincerely thank you for your positive feedback and for acknowledging the improvements made in the improvements made in the revised manuscript. We appreciate the time and effort spent reviewing the work, and we are pleased to hear that the revisions have addressed the concerns. Your support and recognition of the novelty and quality of this work are highly valued.\nReviewer #3 (Remarks to the Author):\n1- Thank you for clarifying the rationale and providing the supporting optimization data for the TSS process.\nAuthors’ response: Thank you for your helpful comments and for acknowledging the clarification and supporting data we provided for the TSS process. We greatly appreciate your constructive feedback, which has improved the manuscript.\n2- Thank you for sharing the electrical fatigue data, which makes the case for long-term electrical stability clear. My original question also referred to repeated mechanical deformation, which is particularly relevant for wearable use where bending and flexing are unavoidable. Have the authors assessed mechanical durability (for example, bending or flexural fatigue), or could they comment on how the material would be expected to perform under such conditions?\nAuthors’ response: Thank you for your valuable comments and for pointing out the importance of mechanical durability, especially in the context of wearable applications. In this work, due to the intrinsic rigidity of ceramic materials, the TSS ceramics were first processed into 1-3 piezoelectric composites to enhance their mechanical compliance for wearable applications. To address your concerns, we conducted bending tests on the TSS ceramic-based 1-3 composites. As shown in Supplementary Fig. 25, the composite withstood large bending curvatures while maintaining its structural integrity, demonstrating their suitability for use in flexible and wearable devices. In 2\naddition, COMSOL simulations further confirmed the mechanical stability of the 1-3 composite structure under different bending conditions (Supplementary Fig. 26).\nTo further assess mechanical durability under repeated deformation, we also performed cyclic bending tests on the LF-UIA device, in which the TSS ceramic-based 1-3 composites were integrated as the core functional materials. The device was subjected to 1000 bending cycles, and key performance parameters, including impedance spectra, center frequency, peak-to-peak sensitivity, and fractional bandwidth were measured before and after the test. As shown in Supplementary Fig. 31, no appreciable changes were observed in these parameters, confirming the mechanical robustness of both the LF-UIA device and TSS ceramic-based 1-3 composites under repeated mechanical deformation.\nFurthermore, the use of 1-3 piezoelectric composites to improve the mechanical flexibility of ceramics for wearable device integration is well supported in the literature [1-4], as it effectively combines mechanical compliance with electromechanical performance. We have added the relevant details to the revised manuscript, which are also copied below:\n“Additionally, the 1-3 piezo-element also demonstrates mechanical stability under bending, supporting its applicability in flexible and wearable devices integration (Supplementary Fig. 25,26).” (Page 9, line 249)\nSupplementary Fig. 25 | Mechanical flexibility of TSS ceramic-based 1-3 3\npiezoelectric composites under bending. a, Experimental setup for bending tests. Scale bar, 2 cm. b, Optical image of the composite before bending. Scale bar, 1 cm. c, Bending for 5°. Scale bar, 1 cm. d, Bending for 10°. Scale bar, 1 cm.\nSupplementary Fig. 26 | COMSOL simulations of TSS ceramic-based 1-3 composites under different bending angles. a, Simulated stress distribution at a bending angle for 5°. b, Simulated stress distribution at a bending angle for 10°.\nSupplementary Fig. 31 | Performance characterization of LF-UIA after bending 4\nfor 1000 cycles. a, Center frequency. b, Fractional bandwidth. c, Peak-peak sensitivity. d, Electrical impedance. No appreciable differences were observed in these parameters, confirming the mechanical robustness of the device.\nReferences:\n[1] C. Wang, X. Chen, L. Wang, M. Makihata, H.-C. Liu, T. Zhou, X. Zhao. Bioadhesive ultrasound for long-term continuous imaging of diverse organs. Science 377 (6605), 517-523 (2022).\n[2] Y. Zeng. X. Sun, J. Zhang, C. Chang; B. Liu, C. Gong, J. Ji, B. Zhang, Y. Wang, M. Ren, R. Wodnicki, H. Liu, Q. Zhou. High-frequency wearable ultrasound array belt for small animal echocardiography. IEEE T. Ultrason. Ferr. 71, 1915-1923 (2024).\n[3] H. Hu, X. Zhu, C. Wang, L. Zhang, X. Li, S. Lee, Z. Huang, R. Chen, Z. Chen, C. Wang, Y. Gu, Y. Chen, Y. Lei, T. Zhang, N. Kim, Y. Guo, Y. Teng, W. Zhou, Y. Li, A. Nomoto, S. Sternini, Q. Zhou, M. Pharr, F. Lanza di Scales, S. Xu. Stretchable ultrasonic transducer arrays for three-dimensional imaging on complex surfaces. Sci. Adv. 4 (3), eaar3979 (2018).\n[4] S. Zhou, X. Gao, G. Park, X. Yang, B. Qi, M. Lin, H. Huang, Y. Bian, H. Hu, X. Chen, R. S. Wu, B. Liu, W. Yue, C. Lu, R. Wang, P. Bheemreddy, S. Qin, A. Lam, K. A. Wear, M. Andre, E. B. Kistler, D. W. Newell, S. Xu. Transcranial volumetric imaging using a conformal ultrasound patch. Nature 629, 810-818 (2024).\n3-Thank you for the detailed and quantitative characterization of the SNR, depth limit, and the spatial resolution. It would also be valuable if the authors could briefly comment on the main factors that limit SNR at increasing depth and how these might be improved or mitigated in future implementations.\nAuthors’ response: Thank you for your insightful comments. The reduction of SNR with increasing depth is primarily related to acoustic attenuation, beam divergence, and electronic noise. In addition, in our current implementation, part of the SNR reduction may originate from impedance mismatch between the transducer and the acquisition system, particularly in the connecting cables and front-end electronics. This mismatch 5\ncan lead to signal reflection and reduced energy transfer efficiency. In future work, we plan to optimize the circuit design and improve impedance matching in the interconnection to mitigate this issue. Furthermore, we plan to investigate alternative beamforming strategies, such as adaptive beamforming and coherent compounding, which may enhance the effective SNR at greater depths.\nIn the revised manuscript:\n“The decline in SNR with depth is primarily attributed to acoustic attenuation, beam divergence, and electronic noise. Additionally, impedance mismatches in the current system may further degrade SNR by inducing signal reflections and transmission losses. Future improvements will focus on circuit optimization, impedance matching, and the implementation of advanced beamforming techniques to enhance deep imaging performance.” (Page 12, line 331) 6"}, {"round": 3, "reviewer_comments": {"reviewer_3": "Thank you for thoroughly addressing all my comments. The revised manuscript has been greatly improved and I have no further concerns."}, "author_response": "Point-by-point Response to Reviewers’ Comments on Research Article, NCOMMS-25-39070B\nDear Highly Respected Editorial Team:\nThank you for your letter and the reviewers’ comments concerning our manuscript entitled “Superior lead-free piezoceramics for wearable multimodal ultrasound imaging arrays” (Research Article, NCOMMS-25-39070B). We would like to express our sincere gratitude to your valuable time and constructive comments. We are pleased that the revisions have addressed all previous concerns and appreciate the reviewers’ positive evaluation of our work.\nReviewer #3 (Remarks to the Author):\nThank you for thoroughly addressing all my comments. The revised manuscript has been greatly improved and I have no further concerns.\nAuthors’ response: We sincerely thank you for your positive feedback and for recognizing the improvements made in our revised manuscript. We appreciate your thoughtful comments and constructive suggestions during the review process, which have significantly enhanced the quality and clarity of our work."}]

Decision letter