KNN-based frequency-adjustable ferroelectric heterojunction and biomedical applications.

High-performance lead-free KNaNbO piezoelectric ceramics present a practical alternative to lead-containing counterparts by effectively reducing potential environmental hazards. This advancement is particularly relevant to the development of ferroelectric heterojunction devices for biomedical applications. Here, we design and fabricate a frequency-adjustable ferroelectric heterojunction based on the developed KNaNbO piezoelectric ceramics with a high piezoelectric coefficient (d = 680 pC/N). By leveraging flexible encapsulation, the heterojunction achieves miniaturization (φ = 13.3 mm, h = 2.28 mm) and suitability for implantation. After penetrating the rat skull, the ultrasound generated by the heterojunction at a frequency of 3 MHz reaches a focal depth of about 7.9 mm, a focal width of approximately 480 μm at -6 dB, and millimeter-scale continuous focal tuning (1.5 mm) within a narrow frequency range (2.7-3.3 MHz). Additionally, the implanted heterojunction enables long-term and high-precision transcranial neuromodulation, and consequently yields therapeutic effects in a myocardial infarction animal model. Collectively, this study highlights a viable strategy for developing and applying lead-free ferroelectric heterojunctions, expanding their potential in brain modulation, and providing new insights into clinical treatments of myocardial infarction.