Background
Wearable auditory sensors are important for human-machine interaction and sound recognition systems in the Internet of Things. This article introduces a skin-attachable acoustic sensor that offers improved sensing accuracy, a flat frequency response, and a good linear range. The sensor uses a polymer-built thin-film diaphragm to achieve high-fidelity sound sensing. Its ultra-small, thin form factor is compatible with flexible substrates and maintains sound detection quality across a wide temperature range. When connected to commercial mobile devices, the sensor can successfully recognize sounds, demonstrating potential for auditory electronic skin.
Highlights
- The sensor is highly conformable and attachable to flexible surfaces and human skin. By designing a complex capacitive diaphragm structure and using polymer materials, the sensor achieves high sensitivity within a small area and can detect sounds within the human audible frequency and sound pressure ranges. The sensor provides high fidelity and can capture various sounds with minimal distortion, including sounds below typical human perception thresholds.
- The device is ultra-compact and thin, adheres stably to flexible substrates and skin, and retains stable response across different temperatures. Compared with smartphones and professional recording microphones, the sensor demonstrates high fidelity. Importantly, the sensor can accurately perceive sound on human skin, offering potential for applications such as speech recognition.
Figures and Analysis
Figure 1. Structure of the skin-attachable acoustic sensor. a) Cross-sectional schematic of the acoustic sensor. b) Top-view optical microscope image of the diaphragm structure used in the skin-attachable acoustic sensor (scale: 500 μm). c) Cross-sectional scanning electron microscope image of the sensor, showing the suspended diaphragm, support regions, and perforated backplate (within the dashed box in a). The specimen for this image was prepared by focused ion beam cutting of the suspended diaphragm upper structure (scale: 10 μm). d) Photograph of the skin-attachable acoustic sensor mounted on a 4 mL glass vial (outer radius: 7.5 mm). Diaphragm diameters shown are 1600, 1200, and 800 μm respectively.
Figure 2. Acoustic response of the high-fidelity skin-attachable acoustic sensor. a) Electromechanical coupling equivalent circuit model corresponding to the sensor.
Figure 3. Comparison of sensing response and operational stability. a) Sensitivity per sensing area compared with previously reported flexible acoustic sensors. b) Dynamic range and bandwidth compared with previously reported flexible acoustic sensors. Human auditory ranges are plotted quantitatively using pain/hearing thresholds and sound ranges from reference data. c) Normalized sensitivity as a function of temperature shows the sensor's temperature stability from 25 to 120 °C.
Figure 4. High-fidelity demonstration. a) Photograph of the experimental setup used for sound quality comparison in an anechoic chamber. b-d) Recorded and measured output waveforms and spectra from a commercial smartphone (iPhone XR) (b), the skin-attachable acoustic sensor (c), and a professional recording microphone (AT2035, Audio-Technica) (d).
Figure 5. Applications in flexible electronics and auditory electronic skin. a) Photograph of an ultra-compact wearable acoustic sensor (diaphragm diameter: 1600 μm; overall device area: 9 mm2; thickness: 420 μm) mounted on a flexible substrate for auditory electronic skin. b) Normalized sensitivity versus bending radius (flat to 5 mm) showing no sensitivity loss under bending. c) Recorded and measured output waveforms and spectra from sensors attached to a curved surface (bending radius: 5 mm; left) and a flat surface (right) while a user says "electronic skin." d) Photograph of the speech recognition demonstration setup showing the skin-attachable acoustic sensor connected to a readout circuit, directly interfaced with a mobile device running a speech assistant (Galaxy S10 5G, Samsung Electronics). e) Photograph of the high-fidelity sensor attached to a human hand, illustrating potential auditory electronic skin applications. Electrical connection details between the high-fidelity sensor and a smartphone are shown in supporting information. f,g) Mobile device screen showing the user's command "search electronic skin" recognized by the speech assistant (f) and the assistant returning web search results for the query (g).
Conclusion
This work presents an ultra-compact wearable acoustic sensor that uses polymer materials and a complex diaphragm structure to achieve commercial-grade, high-fidelity sound detection. The sensor accommodates the imperfect characteristics of human hearing and can be used on curved surfaces and human skin. Paired with a readout circuit, the sensor is compatible with modern speech recognition systems and offers high practicality. Although SU-8 polymer was used here, other polymer materials may be more suitable for this sensor architecture. Further investigation of various polymer materials could improve fidelity for acoustic sensors and auditory electronic skin.
