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Flexible resonant acoustic sensors have attracted substantial attention as an essential component for intuitive human-machine interaction (HMI) in the future voice user interface (VUI). Several researches have been reported by mimicking the basilar membrane but still have dimensional drawback due to limitation of controlling a multifrequency band and broadening resonant spectrum for full-cover phonetic frequencies. Here, highly sensitive piezoelectric mobile acoustic sensor (PMAS) is demonstrated by exploiting an ultrathin membrane for biomimetic frequency band control. Simulation results prove that resonant bandwidth of a piezoelectric film can be broadened by adopting a lead-zirconate-titanate (PZT) membrane on the ultrathin polymer to cover the entire voice spectrum. Machine learning–based biometric authentication is demonstrated by the integrated acoustic sensor module with an algorithm processor and customized Android app. Last, exceptional error rate reduction in speaker identification is achieved by a PMAS module with a small amount of training data, compared to a conventional microelectromechanical system microphone.

Inkjet printing technology uses the low-cost direct deposition manufacturing technique for printing and is applicable in various fields including optics, ceramics, three-dimensional printing in biomedicine, and conductive circuitry. This study reviews the classifications and applications of inkjet printing technologies, with a focus on recent publications. The different design approaches, applications, and research progress of several inkjet printing techniques are reviewed. Among them, the piezoelectric inkjet printing technology is the main focus owing to its reliability and handling of a diverse range of inks. A piezo-driven inkjet printhead is activated by applying a voltage waveform to a piezoelectric membrane. The waveform ensures the formation of the designed droplet and a stable jet. A survey of various drivingvoltage waveforms is conducted, which can serve as a reference to the research community that uses piezodriven inkjet printheads. The challenges of printing quality, stability, and speed and their solutions as published in recent studies are reviewed. Technologies for producing high-viscosity inkjets are explored, and the applications of inkjet printing technology in textile, displays, and wearable devices are discussed.

Mechanical energy harvesting technology converting mechanical energy wasted in our surroundings to electrical energy has been regarded as one of the critical technologies for self-powered sensor network and Internet of Things (IoT). Although triboelectric energy harvesters based on contact electrification have attracted considerable attention due to their various advantages compared to other technologies, a further improvement of the output performance is still required for practical applications in next-generation IoT devices. In recent years, numerous studies have been carried out to enhance the output power of triboelectric energy harvesters. The previous research approaches for enhancing the triboelectric charges can be classified into three categories: i) materials type, ii) device structure, and iii) surface modification. In this review article, we focus on various mechanisms and methods through the surface modification beyond the limitations of structural parameters and materials, such as surficial texturing/patterning, functionalization, dielectric engineering, surface charge doping and 2D material processing. This perspective study is a cornerstone for establishing next-generation energy applications consisting of triboelectric energy harvesters from portable devices to power industries.