Fig. 1. Schematic diagram of piezoelectric effect and inverse piezoelectric effect in quartz crystal, the bottom panels display the corresponding positions of the positive charge center C_Q+ and the negative charge center C_Q- in the quartz crystal. (a) Structure of quartz crystal without applied stress or electric field; (b) structural deformation of quartz crystal under mechanical pressure; (c) structural change of quartz crystal induced by an external electric field

Fig. 2. Mechanism of acoustic-electric conversion in piezoelectric materials. (a) Initial state of the piezoelectric material without external excitation; (b) ultrasound produced by piezoelectric materials when subjected to an alternating voltage; (c) piezoelectric materials driven by ultrasound to generate electrical signals

Fig. 3. Piezoelectric coefficients and relative dielectric constants of piezoelectric materials^[22-32]

Fig. 4. Transcranial ultrasound stimulation. (a) Therapeutic ultrasound devices for brain applications; (b) precise thermoablation of brain tissue for incisionless neurosurgical treatment; (c) focused ultrasound stimulation combined with microbubbles to open the blood-brain barrier to facilitate drug delivery; (d) neural activity noninvasively modulated by low-intensity focused ultrasound stimulation^{in specific brain regions}

Fig. 5. Implantable in vivo ultrasonic transducer^[52]. (a) Implanted piezoelectric ultrasonic transducer (ImPULS); (b) cross-sectional schematic of the ImPULS; (c) dopamine release triggered by ImPULS; (d) changes in dopamine signals before and after the ImPULS stimulation

Fig. 6. Wireless ultrasound-powered stimulation electrode^[37]. (a) Application of the Sm-PUEH device in deep brain stimulation; (b) structural schematic of the Sm-PUEH; (c) application of the Sm-PUEH device for pain relief

Fig. 7. Neural dust^[55]. (a) Schematic diagram of the neural dust system, shown here for stimulating the sciatic nerve of a rat; (b) neural dust fully implanted and affixed to the rat sciatic nerve; (c) skin over the closed surgical site after neural dust implantation; (d) external transceiver

Fig. 8. Neuromodulation using nanoscale stimulators^[58]. (a) Schematic illustration of blood–brain barrier opening and deep brain stimulation induced by functionalized piezoelectric nanoparticles under ultrasound; (b) preparation process of functionalized piezoelectric nanoparticles

Fig. 9. Ultrasound-activated piezoelectric nanostickers promoting stem cell differentiation and traumatic brain repair^[59]. (a) Regulating the neuronal differentiation of neural stem cells by ultrasound-activated piezoelectric nanostickers; (b) using the piezoelectric nanostickers to induce the neuronal differential of stem cells for traumatic brain treatment; (c) experimental procedure of traumatic brain treatment in animal models using the piezoelectric nanostickers; (d) therapeutic performance of the piezoelectric nanostickers in treating traumatic brain injury

Fig. 10. Flexible ultrasound transducer array and ultrasound imaging^[44]. (a) Schematic diagram of the structure of flexible ultrasound transducer array patch; (b) schematic diagram of ultrasound imaging; (c) flexible ultrasound transducer array patch attached to the temporal window; (d) volumetric imaging of major cerebral arteries obtained through the temporal window (left and right show different viewing angles)

Fig. 11. Ultrasonic brain-computer interface^[67]. (a) Schematic diagram of the functional ultrasound brain-machine interface; (b) ultrasonic brain-computer interface controlling movements in eight directions; (c) memory-guided brain-computer interface task controlling point movement on the display

Fig. 12. Ultrasonic sensing technology^[69]. (a) Schematic diagram of the principle of wireless intracranial physiological signal sensing by metagels using an ultrasound transducer; (b) injection-based delivery, microscopic structure, and degradable properties of metagels; (c) changes in intracranial pressure, temperature, and pH detected by metagels through ultrasound emission and reception by an ultrasound transducer