Soft, Multifunctional, Robust Film Sensor Using a Ferroelectret with Significant Longitudinal and Transverse Piezoelectric Activity for Biomechanical Monitoring

Kirigami-Inspired Stretchable Piezoelectret Sensor for Analysis and Assessment of Parkinson's Tremor

Human muscle activity contains rich information that can reflect human movement patterns and conditions of diseases or physical abnormalities. Flexible pressure sensors enable the assessment of muscle tremors in Parkinson's disease (PD) through Force Myography (FMG). Here, an easily fabricated, ultra-sensitive, and stretchable piezoelectret pressure sensor is presented. Utilizing an effective integration of Kirigami structure and piezoelectret air gap, the sensor achieved a dynamic sensitivity of ≈725 pC/N (@5 Hz), measurement repeatability of <2.5%, measurement hysteresis of <1%, a pressure detection limit of <15 Pa, a response time of ≈2.5 ms, stable output within ±3% over 40 000 cycles, and output decay of <2.5% after 1000 cycles of complex deformation, meeting non-distorted measurement conditions up to 20 Hz. Successful monitoring and assessment of hand muscle tremors are demonstrated. Furthermore, using a 1×3 sensor array enabled tremor localization, achieving a high accuracy rate of 99.5% with machine learning algorithms. Additionally, the sensor facilitated the experimental quantification and assisted scoring of the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS), with an accuracy of ≈85%. The sensor demonstrates potential for assisting in the diagnosis and rehabilitation monitoring of Parkinson's disease.

Recent Advances in Ferroelectret Fabrication, Performance Optimization, and Applications

The growing demand for wearable devices has sparked a significant interest in ferroelectret films. They possess flexibility and exceptional piezoelectric properties due to strong macroscopic dipoles formed by charges trapped at the interface of their internal cavities. This review of ferroelectrets focuses on the latest progress in fabrication techniques for high temperature resistant ferroelectrets with regular and engineered cavities, strategies for optimizing their piezoelectric performance, and novel applications. The charging mechanisms of bipolar and unipolar ferroelectrets with closed and open-cavity structures are explained first. Next, the preparation and piezoelectric behavior of ferroelectret films with closed, open, and regular cavity structures using various materials are discussed. Three widely used models for predicting the piezoelectric coefficients (d33) are outlined. Methods for enhancing the piezoelectric performance such as optimized cavity design, utilization of fabric electrodes, injection of additional ions, application of DC bias voltage, and synergy of foam structure and ferroelectric effect are illustrated. A variety of applications of ferroelectret films in acoustic devices, wearable monitors, pressure sensors, and energy harvesters are presented. Finally, the future development trends of ferroelectrets toward fabrication and performance optimization are summarized along with its potential for integration with intelligent systems and large-scale preparation.

Bioinspired Tilted Magnetized Flakes as a Self-Powered and Antislip Smart Outsole for Healthcare Monitoring and Human-Machine Interaction

Footwear smart devices capable of reliably capturing body actions and conveniently transmitting human-made information are of great interest to advance healthcare monitoring, human-machine interactions (HMIs), etc. while remaining challenging. Herein, we present a self-powered, antislip, and multifunctional smart outsole based on the gecko toe-inspired tilted magnetized flakes (TMFs) and underlying flexible coils. With the pressure-induced flake deflection and the built-in magnetic moment alignment, the TMF can produce a variable magnetic field to induce the voltage signals in coils for precise pressure perception and linear velocity sensing. The TMF-based smart outsole can thus serve as a real-time footwear recorder to monitor various body actions for exercise analysis and to track the abnormal landing speed for alerting potential injuries. The gecko toe-like flakes also enable the excellent antislip capability of the outsole with a much higher friction coefficient than the standard one of the low slip risk. By programming the magnetic moment alignments of the TMFs, a single-circuit outsole can further output multiple signals as encoded instructions for controlling the racing game. Along with excellent abrasion resistance and environmental immunity, the proposed outsole exhibits great potential as a convenient platform for reliable healthcare monitoring and efficient HMI.

Advances in Piezoelectret Materials-Based Bidirectional Haptic Communication Devices

Bidirectional haptic communication devices accelerate the revolution of virtual/augmented reality and flexible/wearable electronics. As an emerging kind of flexible piezoelectric materials, piezoelectret materials can effortlessly convert mechanical force into electrical signals and respond to electrical fields in a deformation manner, exhibiting enormous potential in the construction of bidirectional haptic communication devices. Existing reviews on piezoelectret materials primarily focus on flexible energy harvesters and sensors, and the recent development of piezoelectret-based bidirectional haptic communication devices has not been comprehensively reviewed. Herein, a comprehensive overview of the materials construction, along with the recent advances in bidirectional haptic communication devices, is provided. First, the development timeline, key characteristics, and various fabrication methods of piezoelectret materials are introduced. Subsequently, following the underlying mechanisms of bidirectional electromechanical signal conversion of piezoelectret, strategies to improve the d33 coefficients of materials are proposed. The principles of haptic perception and feedback are also highlighted, and representative works and progress in this area are summarized. Finally, the challenges and opportunities associated with improving the overall practicability of piezoelectret materials-based bidirectional haptic communication devices are discussed.

Recent Advances in Tactile Sensory Systems: Mechanisms, Fabrication, and Applications

Flexible electronics is a cutting-edge field that has paved the way for artificial tactile systems that mimic biological functions of sensing mechanical stimuli. These systems have an immense potential to enhance human-machine interactions (HMIs). However, tactile sensing still faces formidable challenges in delivering precise and nuanced feedback, such as achieving a high sensitivity to emulate human touch, coping with environmental variability, and devising algorithms that can effectively interpret tactile data for meaningful interactions in diverse contexts. In this review, we summarize the recent advances of tactile sensory systems, such as piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. We also review the state-of-the-art fabrication techniques for artificial tactile sensors. Next, we focus on the potential applications of HMIs, such as intelligent robotics, wearable devices, prosthetics, and medical healthcare. Finally, we conclude with the challenges and future development trends of tactile sensors.

Leveraging Ferroelectret Nanogenerators for Acoustic Applications

Ferroelectret nanogenerator (FENG), renowned for its remarkable electromechanical conversion efficiency and low Young's modulus, has gained significant attention in various acoustic applications. The increasing interest is attributed to the crucial role acoustic devices play in our daily lives. This paper provides a comprehensive review of the advancements made in using FENG for acoustic applications. It elaborates on the operational mechanism of FENG in acoustics, with a special focus on comparing the influence of different fabrication materials and techniques on its properties. This review categorizes acoustic applications of FENG into three primary areas: acoustic sensing, acoustic actuation, and acoustic energy harvesting. The detailed descriptions of FENG's implementations in these areas are provided, and potential directions and challenges for further development are outlined. By demonstrating the wide range of potential applications for FENG, it is shown that FENG can be adapted to meet different individual needs.