Conventional acoustofluidic manufacturing platforms are inherently limited by their operating principles and acoustic wavelength. This study introduces a novel MEMS platform that harnesses the lateral vibration modes of GHz Bulk Acoustic Wave (BAW) resonators-modes typically regarded as parasitic. By engineering the resonator's symmetry, we enhance and utilize the powerful acoustofluidic effects generated by these lateral modes, creating a new platform for sub-wavelength acoustofluidic fabrication. This platform unlocks two interesting physical mechanisms: high-gradient interfacial forces and structured acoustic streaming vortices. We demonstrate the platform's versatility through three state-of-the-art applications: (1) Generation of attoliter-scale (10−18 L) droplet arrays using an “acoustic nano-scissors” effect, producing droplets more than three orders of magnitude smaller than existing acoustic methods; (2) Maskless, template-free fabrication of complex 3D nanostructures with sub-200 nm vertical features; (3) Stable and dynamic patterning of nanoparticles as small as 30 nm using “acoustic streaming tweezers”. This work establishes a novel, purely acoustofluidic pathway for achieving subwavelength manufacturing and manipulation.

To address the critical safety monitoring requirements of large-scale energy storage systems based on lithium-ion batteries, this paper proposes a novel ultrasonic hydrogen gas sensor based on piezoelectric micromachined ultrasonic transducers and phase-detection methodology. For the first time, it offers an ultrasonic solution with high resolution and miniaturized size that meets the requirements. The complete sensor assembly, including the transducers and their enclosure, signal conditioning electronics, and connectors, measured 60 mm (W) & times; 55 mm (H) & times; 24 mm (D). The various factors influencing sensor resolution are analysed, and their impact is quantified. The experimental results validate a low hydrogen gas concentration resolution of 50 parts per million (ppm) with a full-scale error (0-20000 ppm) of 5%. Test results of low cross-sensitivity with carbon monoxide (CO) and longterm drift experiment spanning six months further demonstrated the sensor's suitability for application in lithium-ion battery thermal runaway safety monitoring scenarios.

Low-frequency characteristics serve as primary metrics for evaluating the acoustic performance of modern microspeakers. However, conventional microspeakers exhibit flat low-frequency in-ear responses due to their inherent diaphragm displacement-sound pressure relationship, which deviates from ideal human auditory characteristics and fails to meet active noise cancellation requirements. To address this limitation, this study proposes a piezoelectric MEMS speaker that is based on ultrasonic modulation-demodulation principles. This is achieved by dynamic acoustic impedance, where the ultrasonic acoustic source and valve actuators work synchronously. The speaker achieves superior low-frequency performance, featuring a low-frequency sound pressure level of 134 dB at 20 Hz, following the IEC 711 standard. The total harmonic distortion remains below 2% in the 20 Hz to 1 kHz range. The speaker measures only 4 mm x9 mm in size, and this size is possible due to its source-valve decoupling architecture. Combined with high performance and miniaturization, this technological breakthrough overcomes the performance limitations of conventional speakers and establishes a new paradigm for microspeakers. [2025-0163]

Precise, parallel, and dynamically reconfigurable patterning of sub-100 nm nanoparticles plays a vital role in materials science, biochemistry, and physics. However, achieving this via acoustofluidics has remained elusive, due to the severe attenuation of acoustic radiation forces and the interference of uncontrolled acoustic streaming effects. To overcome this, we introduce GHz acoustic streaming micro-vortex arrays (GASMA) for the stable, arrayed assembly of sub-100 nm nanoparticles. By deliberately exciting specific lateral modes of GHz bulk acoustic wave (BAW) resonators, we generate a highly structured, periodic acoustic field on the device surface. When coupled into the adjacent liquid, this field gives rise to ordered micro-vortex arrays. These periodic micro-vortices rapidly transport nanoparticles to acoustic streaming potential wells (ASPWs)-the stagnation zones between adjacent vortices-where they are stably trapped. We demonstrate the precise parallel patterning of nanoparticles as small as 30 nm and validate GASMA's versatility by manipulating diverse materials, including polystyrene (PS) nanoparticles, gold nanorods, and extracellular vesicles (EVs). Critically, by simply modulating the driving frequency, we achieve dynamic, on-demand reconfiguration of these nanoparticle arrays. In summary, GASMA addresses several key limitations in manipulating sub-100 nm nanoparticles, offering a promising acoustofluidic platform for applications in tunable optoelectronics, nanofabrication, and biomedicine.

The precise fabrication of three-dimensional (3D) surface nano-topographies remains a significant challenge in advanced manufacturing. Conventional acoustic methods are inherently limited by wavelength-dependent resolution and primarily restricted to two-dimensional (2D) patterning. Here, we demonstrate a novel fabrication strategy based on the enhanced lateral mode of solidly mounted resonator (SMR), which enables the customizable, area-specific fabrication of periodic 3D surface nanostructures. By harnessing Lamb wave propagation and acoustofluidic effects, we fabricate surface nanostructure arrays with vertical feature sizes below 200 nm and inter-feature spacing controlled to within 5 µm. Compared with conventional surface acoustic wave (SAW) methods, this technique improves precision by overcoming wavelength-imposed limitations, while also allowing the rapid generation of diverse, customizable architectures without the need for complex transducer arrays. Furthermore, by tuning the device geometry, nanostructures with diverse symmetries—including circular, square, hexagonal, and octagonal—can be fabricated in less than 10 s. This work establishes a rapid, efficient, and maskless route for generating high-fidelity nanotopography for large-scale replication, holding significant potential for applications in advanced manufacturing, biomedicine, microelectronics, and photonic devices.

In the field of nano-fluidics, the generation and manipulation of minuscule droplets with volumes ranging from attoliters (aL) to femtoliters (fL) represents a crucial frontier. Such ultrasmall droplets exhibit immense potential in single-molecule detection, targeted drug delivery, and fundamental research into nanoscale biochemical processes, owing to their unique physicochemical properties, such as low Reynolds number flow and interface-dominated mass transport. Furthermore, ordered liquid-patterned arrays hold promise for applications in optically tunable nano-lenses. However, generation and manipulation of attoliter-scale droplets have long posed significant challenges, particularly for open-interface operations like dispensing, merging, splitting, and patterning into arrays. This study introduces acoustic nano-scissors generated by lateral modes of high-frequency bulk acoustic waves. The induced acoustofluidic effect in thin liquid films forms shear forces between the adjacent wave peaks and wave valleys, thereby successfully cutting the liquid into attoliter-scale droplets at an open interface. This approach could produce droplets with volumes more than three orders of magnitude smaller than those from existing acoustic solutions. Furthermore, the acoustic nano-scissors could generate ordered attoliter droplet arrays with specific patterns, with fast droplet splitting and merging controlled by switching on and off the device. This work provides a novel and flexible solution for various applications requiring attoliter droplet arrays on open interfaces.

Rapid advances in compact electronic devices have led to severe thermal management challenges in inherently confined internal spaces, where pump characteristics are required for air coolers to maintain their cooling effectiveness. In the present work, for the first time, a silicon-based piezoelectric jet pump is developed as a MEMS cooler. This MEMS air cooler with backflow suppression is realized by incorporating a piezoelectric MEMS actuator array with a multi-orifice outlet configuration. With a compact footprint of 9 × 9 × 1.7 mm3, the cooler achieves a back pressure of 300 Pa and a volume-normalized flow rate of 2.3 L ⋅ min−1 ⋅ cm−3. With a power consumption level of only 11 mW, it removes ∼500 mW of heat flow rate at a heater–cooler distance as small as 1 mm. In addition, quiet operation can be achieved by extending the driving frequency to the ultrasonic range. This MEMS air cooler represents an ideal solution with a compact size, high flow rate, low power, and low noise, showing strong potential for revolutionizing the landscape of miniaturized active cooling.

Monitoring of mechanical forces on the rotator cuff in real time after surgical repair may allow the development of personalized rehabilitation programs for patients undergoing recovery. However, there are no currently available modalities for continuous monitoring of the healing process during rehabilitation; previously reported implantable sensors are either too bulky or stiff without sufficient sensitivity. In this study, a strategy employing a stretchable and implantable sensor is proposed for biomechanical monitoring of the rotator cuff. The sensor is made of a patterned boomerang on thin piezoelectric composite sheets, optimized for biosafety, suitability, and functionality. It allows strain and force sensitivities measurements of up to 755 V ε −1 and 80 V N −1 , respectively. To the best of the knowledge, the sensor achieves the highest sensitivity among reported flexible piezoelectric strain sensors, although it has the smallest size. In a cadaveric study, the sensor is used to monitor tension changes in the rotator cuff during shoulder movements. In an in vivo study, the sensor exhibits excellent biocompatibility and functionality in a rabbit model. High‐accuracy motion poses recognition is realized using machine learning, illustrating the potential applicability of the device for real‐time monitoring of tendon healing.

Compact electronics, such as smartphones and tablets, face significant overheating issues; these issues are a byproduct of enhanced performance. However, thermal management in these devices still primarily relies on inefficient passive cooling methods since today's active cooling solutions fail to meet the stringent size and power consumption requirements. To address this difficulty, in this work, a piezoelectric micromachined synthetic jet (SJ) air cooling chip is introduced. The active cooler features a small footprint of 6 x 6.7 x 2.1 mm and a low power consumption of only 69 mW; this value is approximately one-fourth that of a typical fan. Moreover, the cooler has a convective heat transfer coefficient as high as 72 W/m(2)K, and the cooling coefficient of performance (COP) is the highest among all competing miniaturized air coolers. In device design, the piezoelectric actuator adopts a partially mechanical decoupled structure, enabling effective generation of an SJ with a restricted area; in the working mechanism, the pulsating nature of the SJ and its vertical impingement on the target ensures sufficient disturbance of the thermal boundary layer. The microscale cooling technology has emerged because of its minimal size, high energy efficiency, and substantial cooling capacity and shows significant promise for the active cooling of compact electronics.

This work introduces a miniaturized ambient temperature sensor using piezoelectric micromachined ultrasound transducers (PMUTs) and indirect time-of-flight (iTOF) measurement. Leveraging phase-difference calculations at high ultrasound frequencies, the method overcomes size and resolution limitations of conventional direct TOF approaches. A compact 3-cm prototype demonstrates immunity to thermal interference and achieves a high temperature resolution of 0.02 degrees C. This iTOF-PMUT platform enables reliable ambient sensing for portable applications.

In conventional piezoelectric micromachined ultrasonic transducers (PMUTs), the backside acoustic energy is often used inefficiently, resulting in up to half of the energy being wasted. Vacuum encapsulation can improve the energy utilization efficiency, but this technique is not compatible with state-of-the-art devices such as cantilever-based PMUTs. A closed back cavity provides an alternative method for effectively utilizing the backside acoustic energy. This paper investigates the effects of a closed back cavity on PMUT performance through theoretical analysis, simulations, and experimental verification. Increasing the cavity depth produces a periodic modulation of several key PMUT metrics, such as the relative frequency deviation and quality factor. The optimal cavity depth for PMUTs that ensures a robust resonant frequency and high quality factor is defined as a function of the acoustic wavelength. A closed back cavity also provides an effective method for continuously tuning the quality factor, and thus the bandwidth, of PMUTs. This work paves the way for air-coupled PMUTs with adjustable performance for various applications.

Piezoelectric MEMS speakers, an emerging technology with great promise, face significant challenges in performance evaluation and rational design. Their broadband nature means that responses at every frequency point across the whole operating bandwidth contribute to performance, yet there is no widely recognized weighting approach for fair evaluation. This absence of quantitative criteria makes objective comparisons of different designs difficult, slowing the adoption of new design concepts; and it leads to ambiguous design goals without response balance across frequency bands. Additionally, the current design methods rely on labor-intensive simulations, further prolonging the development process. To address these challenges, two figures of merit (FOMs) obtained via theoretical deduction are proposed in this study. These FOMs facilitate the evaluation of key metrics, such as sound pressure level and energy efficiency over a wide frequency range, enabling quantitative comparisons among various speaker designs. On the basis of FOMs, the design process can be simplified into a single-objective optimization problem, significantly streamlining the speaker design. Using this method, piezoelectric MEMS speakers with ultra-high FOMs and superior performance are demonstrated. The normalized SPLs at 1 and 10 kHz reach an impressive 76.6 and 86.6 dB/mm²/Vrms, respectively, with normalized sensitivities of 91.2 and 91.5 dB/mm2/mW. This achievement validates our FOM theory, representing a notable advancement in the field.

The impacts of acoustic leakage on omnidirectional MEMS microphones have been thoroughly studied, yet the effects on directional MEMS microphones remain largely unexplored. For the first time, we analyze and discuss the relationship between acoustic leakage and frequency response of directional MEMS microphones. This relationship is predicted by models, verified by experiments and finally compared between omnidirectional and directional microphones. It is discovered that the impact degree of acoustic leakage on frequency response is far less with directional microphones. The discovery indicates that MEMS directional microphones with added perforations in packaging can be less concerned with the acoustic leakage issue than omnidirectional ones, and they should have different frequency response calibration and compensation schemes.

In this article, a 2.4-GHz micro-electromechanical systems (MEMS)-based oscillator is designed and demonstrated based on g(m)-boosting Colpitts structure. A dynamic self-body-biasing scheme is proposed to shorten startup time without introducing any extra units or additional parasitics to there sonant tank. Darlington cell is also employed to enhance negative transconductance and improve the oscillation stability, especially under a low supply voltage operating condition. The oscillator is integrated with a 2.4-GHz MEMS resonator with high Q characteristic. The tail current source is designed at the boundary of the current- and voltage-limited regime for a superior phase noise performance. The measurement result shows that the phase noise at 10- and 100-kHz offsets is-112.92 and-138.23 dBc/Hz, respectively. A figure-of-merit of 226 dBc/Hz is achieved. The measured startup time is similar to 5 mu s. The core oscillator consumes a dc power of 0.98 mW from a 0.7-V supply voltage

This letter introduces a micro-electromechanical system (MEMS) resonant pressure gauge based on aluminum nitride piezoelectric resonators that can operate in an atmospheric environment. The pressure is measured in terms of the change in the resonant frequency of a double-ended tuning fork (DETF) resonator, where the aluminum nitride thin film serves as both a piezoelectric layer and a passive layer. The resonator exhibits a resonant frequency of 719 kHz and a Q value of 1764 in air. To our knowledge, this Q value surpasses those of previously reported in-air MEMS DETF resonators that operate in the 100 kHz frequency range. The electronic oscillator circuit for the resonant sensor exhibits a phase noise level of -120 dBc/Hz. The measured sensitivity is 16.8 Hz/kPa within the pressure range of 0-90 kPa, which is consistent with the simulation results obtained from finite element modeling. The resolution of the sensor is 14.9 Pa based on Allan variance measurements. This work thus demonstrates a new class of MEMS sensors for use in harsh environments.

AbstractContinuous and quantitative monitoring of knee joint function has clinical value in rehabilitation assessment and the timing of return to play for anterior cruciate ligament injury patients. However, the existing approaches, including clinical examination, arthrometry and inertial solutions, can only be used for qualitative, off‐line and low‐quality evaluations, respectively. Burgeoning Kirigami stretchable sensors could be a disruptive candidate solution, but they usually suffer from structural buckling issues when used for large strain applications, such as knee joint motion capture where the buckling degrades sensor reliability and repeatability. Here, we propose a buckling‐resistant stretchable and wearable sensor for knee joint motion capture. It enables continuous and precise motion signal capture of the knee joint and provides high wearing comfort and reliability. Clinical tests were conducted on 30 patients in the field, tracking data provided by the sensor from their initial hospitalization to later surgery. And the full rehabilitation of one subject was recorded and analyzed. The test results show that our sensor can dynamically assess knee function in real time and recommend the best timing for return to play, which paves the way for personalized and telerehabilitation.

For the first time, we develop piezoelectric micromachined ultrasound transducer (PMUT) array with the cells having suspended vibrational membranes and the array having the same-dimension cells in plane but with various mass loads for tuning their frequency range. The suspended membrane of PMUT, facilitated by four through-holes at the edge, offers distinct advantages of low constraint and high sensitivity. The resonant frequency of equidimensional suspended PMUT with the same diameter becomes adjustable through various mass loads. The suspended PMUT array containing same-dimension cells with different mass loads exhibits superior acoustic performance and broader bandwidth compared to traditional PMUT array. According to this strategy, this paper presents our PMUT array, with central frequency be in around 10MHz, has a 1.5X higher displacement and 1.8X higher bandwidth than traditional PMUT array. It promises significant advancements in PMUT-based ultrasound imaging.

Implantable medical devices are increasingly being used to perform electrophysiological stimulation, thus calling for wireless implantable neural stimulators for which wireless power transfer is needed. Compared with radio frequency and inductive coupling wireless power transfer methods, acoustic wireless power transfer features lower attenuation, a smaller size and a higher safe power threshold. This paper demonstrates the design, fabrication, assembly and characterization of a mm-sized acoustic wireless implantable neural stimulator based on a piezoelectric micromachined ultrasound transducer. Ex vivo experiments in water are conducted to characterize the power transfer link, showing improvement of the power transfer efficiency by more than two times with a matching circuit. Following FDA guidelines, the wireless implantable neural stimulator achieves a 3.24 μW output power in tissue. The feasibility for rat sciatic nerve stimulation is successfully demonstrated in vivo by the implantable prototype with a size of 5×5.5×2.5 mm3. The proposed solution has the potential to shrink the entire implanted system to a monolithic transducer-IC chip, paving the way toward an acoustic wireless implantable neural stimulator with higher levels of biocompatibility, integration and miniaturization.

Microelectromechanical system (MEMS) cantilever resonators suffer from high motional impedance (Rm). This paper investigates the use of mechanically coupled multi-cantilever piezoelectric MEMS resonators in the resolution of this issue. A double-sided actuating design, which utilizes a resonator with a 2.5 μm thick AlN film as the passive layer, is employed to reduce Rm. The results of experimental and finite element analysis (FEA) show agreement regarding single- to sextuple-cantilever resonators. Compared with a standalone cantilever resonator, the multi-cantilever resonator significantly reduces Rm; meanwhile, the high quality factor (Q) and effective electromechanical coupling coefficient (Kteff2) are maintained. The 30 μm wide quadruple-cantilever resonator achieves a resonance frequency (fs) of 55.8 kHz, a Q value of 10,300, and a series impedance (Rs) as low as 28.6 kΩ at a pressure of 0.02 Pa; meanwhile, the smaller size of this resonator compared to the existing multi-cantilever resonators is preserved. This represents a significant advancement in MEMS resonators for miniaturized ultra-low-power oscillator applications.

By leveraging the benefits of a high energy density, miniaturization and integration, acoustic-wave-driven micromotors have recently emerged as powerful tools for microfluidic actuation. In this study, a Lamb-wave-driven micromotor is proposed for the first time. This motor consists of a ring-shaped Lamb wave actuator array with a rotor and a fluid coupling layer in between. On a driving mechanism level, high-frequency Lamb waves of 380 MHz generate strong acoustic streaming effects over an extremely short distance; on a mechanical design level, each Lamb wave actuator incorporates a reflector on one side of the actuator, while an acoustic opening is incorporated on the other side to limit wave energy leakage; and on electrical design level, the electrodes placed on the two sides of the film enhance the capacitance in the vertical direction, which facilitates impedance matching within a smaller area. As a result, the Lamb-wave-driven solution features a much lower driving voltage and a smaller size compared with conventional surface acoustic-wave-driven solutions. For an improved motor performance, actuator array configurations, rotor sizes, and liquid coupling layer thicknesses are examined via simulations and experiments. The results show the micromotor with a rotor with a diameter of 5 mm can achieve a maximum angular velocity of 250 rpm with an input voltage of 6 V. The proposed micromotor is a new prototype for acoustic-wave-driven actuators and demonstrates potential for lab-on-a-chip applications.