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To solve the problem of a single function encountered by wearable electronic devices, a multifunctional wearable piezoelectric MEMS device is presented, which performs motion monitoring and disease and health warning according to respiratory frequency, functions as an earphone based on the inverse piezoelectric effect of the PZT film. The rigid-flexible coupling sealed vibration membrane allows the piezoelectric MEMS device to eliminate acoustic loss or avoid airflow loss. By placing the device in a simple wearable mask, the inactive, walking, and running states may be possible to be recognized and monitored through respiratory frequency. In the inactive state, the device can identify diseases when abnormally fast or slow respiration occurs. By placing the device in the surgical mask when going out, an early warning of something affecting lung health can be obtained by excessive respiration frequency when wearing the mask for too long. Moreover, the device can also be utilized as an earphone. Higher than 55 dB SPL can be obtained in the full frequency range of the human ear under 2 V, meeting the basic needs of people's hearing. This simple strategy represents a significant step toward future applications of the wearable piezoelectric MEMS device due to its portability, multifunctioning, convenience, and mass production.

MEMS speakers are attracting more and more interests due to their potential applications in portable and wearable electronics. However, insufficient sound pressure level (SPL) associated with the reduced size is a main challenge for most existing MEMS speakers. This paper presents the design, fabrication and characterization of a high-SPL piezoelectric MEMS speaker based on high-piezoelectric constant ceramic PZT. The ceramic PZT, bonded on an SOI substrate, was thinned down to only 5 μm successfully. The microfabricated piezoelectric MEMS speakers exhibited significantly enhanced SPLs. At a distance of 10 mm, a maximum SPL of 119 dB was achieved in open air by such a single MEMS speaker with a footprint of 6.7 mm × 6.7 mm under a low driving voltage of only 10 Vpp. The SPLs at frequencies as low as 400 Hz still reached over 90 dB. These SPLs are comparable to those of conventional loud speakers or micro-speaker arrays with significantly larger sizes.

While microelectro-mechanical (MEMS) resonators are increasingly being considered in biochemical sensing applications, the complexity of the system results in a need for a system level optimization in which the readout circuit and the MEMS components should carefully be co-optimized. Here, we present systematic optimization approach that highlights the trade-offs between the MEMS resonator and the accompanying oscillator-based integrated readout circuit. A system figure of merit is formulated to account for resonator modal frequency, equivalent electrical characteristics, sensitivity, readout circuit bandwidth, frequency resolution and power consumption. While the methodology can be adapted to study of various MEMS-based resonators coupled to oscillator-based readouts, for clarity, we analyze here the specific case of polymer-coated piezoelectric microbridge resonators sensitive to volatile organic compounds.

Integration of materials other than silicon and its compounds into micromachined transducers is a rapidly emerging area of MEMS research technology. For example, future innovations and improvements in inertial sensors for navigation, highfrequency crystal oscillators and filters for wireless applications, microactuators for RF applications, chip-scale chemical analysis systems, and countless other applications can be significantly impacted by the successful miniaturization and integration of piezoelectric materials and subsystems onto silicon substrates. Piezoelectric materials are high energy density materials that scale favorably upon miniaturization. In this article, a cursory review of the fabrication technology for piezoelectric MEMS transducers is presented along with a discussion on the challenges and opportunities in the field.