Black phosphorous-based human-machine communication interface

The study addresses the need for assistive auditory human-machine interfaces (HMI) that can convert tactile inputs into audio for users who are visually impaired or have speech/language difficulties. While tactile sensors are widely developed for applications such as electronic skin, motion detection, and HMIs, most work focuses on touch sensing without integrated auditory feedback. Existing active materials (e.g., carbon nanotubes, graphene, metals, polymers) can be costly and low-yield, and although MXene-based sensors show promise, they often face stability issues. Black phosphorus (BP), with its puckered honeycomb lattice and high carrier mobility, is an attractive but underexplored material for piezoresistive tactile sensors aimed at real-world applications like physiological signal monitoring and HMIs. The purpose of this study is to develop a low-cost, stable, and sensitive BP@PANI-based piezoresistive tactile sensor and to demonstrate a practical touch-to-audio braille communication interface.

The paper reviews tactile sensing technologies based on piezoelectric, piezocapacitive, and piezoresistive mechanisms, noting the advantages of piezoresistive sensors for simple fabrication and low power consumption. Prior active materials include carbon-based materials (CNTs, graphene), metal wires, and conducting polymers and their composites; however, these approaches can be costly and suffer from low yields. MXenes (e.g., Ti3C2) have been used to construct high-sensitivity piezoresistive sensors due to adjustable interlayer spacing, high surface area, and metallic conductivity, but they have reported limitations in stability. HMI-related prior work includes Ti3C2-based tactile sensors for braille recognition where outputs are visual rather than auditory, dielectric elastomer-based refreshable braille displays with stability and cost issues, and other braille display devices that require high voltage, potentially unsafe for visually impaired users. BP-based sensors have shown promise in structural/electronic properties studies, but real-world HMI applications are limited, highlighting the gap this work aims to fill.

Materials: Black phosphorus, aniline, dimethylformamide (DMF), ammonium persulfate (APS), and HCl (Sigma-Aldrich); PDMS (Biesterfeld AG, Germany); polyester/cellulose fabric (VWR International). Circuit components were sourced locally (Czech Republic). Preparation of BP@PANI-coated fabric: A BP dispersion (0.36 mg/mL) was prepared by ultrasonication for 2 h. Polyester/cellulose fabric pieces were immersed in the BP dispersion for 30 s, removed, and dried at 55 °C for 3 h. Aniline (100 mM) was dissolved in 10 mL of 1 M HCl and cooled in an ice-water bath; APS was dissolved in 5 mL of 1 M HCl and cooled similarly. The BP-coated fabric was soaked in the aniline solution for 30 min, then APS solution was added to initiate in situ oxidative polymerization. After 24 h, the fabric was removed, rinsed with deionized water, and dried to obtain BP@PANI-coated fabric. Device fabrication: Single devices were assembled by stacking BP@PANI-coated fabric layers (sizes 1.5 cm × 1.5 cm). Copper electrodes were applied to the top and bottom layers. The stack was encapsulated in a transparent plastic sheet to maintain conformal contact. Devices with 1, 3, and 5 active layers were fabricated to optimize performance; the optimized sensor had five layers. A wearable six-pixel tactile sensor array was built by mounting six BP@PANI sensors on a PDMS substrate with 1.5 cm spacing. Braille dots were formed from PDMS and placed atop the six sensors. A rigid acrylic (plexiglass) layer was inserted between the sensor and PDMS to keep the array straight during wear or bending. Characterization: Morphologies were examined via SEM, STEM, and EDS mapping to confirm BP lamellar structures and uniform BP@PANI distribution (elements P, N, C). Chemical structures were analyzed by ATR-FTIR, XRD (BP peaks at (020), (040), (060), (041)), and Raman (BP modes near 360, 436, 464 cm−1; PANI features near 1134, 1167, 1515, 1600 cm−1). Electrical measurements used an electrochemical workstation (PGSTAT204 Autolab) with Nova 1.1 software. I–V characteristics were measured from −0.5 to 0.5 V; standard tests used 0.1 V DC bias. Pressure-dependent current responses and cycling tests were performed; sensitivity S defined as S = (ΔI/I0)/ΔP. System integration for auditory HMI: A microcontroller unit (with ADC) acquired and digitized voltage signals from the six-sensor array, mapped pressed/unpressed states to preassigned audio clips via an audio amplifier and speaker. Each sensor’s baseline and pressed-state voltages were characterized to set thresholds (Supplementary Tables). The firmware recognized braille patterns (e.g., letters A, B, D, G) and words (e.g., “nanomaterials,” “hello,” “good,” “no,” “yes,” “ok”) to trigger corresponding audio output. Sensing mechanism: Under applied pressure, BP@PANI-coated fabric layers increase contact area and form additional conductive pathways, reducing resistance and increasing current. Total device resistance includes BP@PANI-coated fabric resistance and interface resistance with copper electrodes. Sensitivity improvements with additional layers are attributed to porous fabric structure and air gaps that compress under low pressure; tunneling/contact effects between multiple layers are described in Supplementary Note 1.

• BP@PANI tactile sensor shows superior sensitivity compared to pristine BP or PANI alone under identical test conditions (0.1 V bias).
• Layer dependence: Five-layer device achieved highest low-pressure sensitivity: 5.57 kPa−1 with high linearity (R2 = 0.98) over 0.5–20 kPa. Sensitivities for single- and three-layer devices were 1.73 and 2.95 kPa−1, respectively (0.5–20 kPa).
• High-pressure regime: Sensitivity decreases to 0.154 kPa−1 between 30–100 kPa; response tends to saturate above ~20 kPa, limiting quantitative pressure determination at higher loads.
• Hysteresis: Low hysteresis of 3.43% calculated from loading/unloading ΔI/I0 vs pressure curves, lower than many reported tactile sensors.
• Dynamic response: Response and recovery times of ~0.20 s and ~0.21 s at 1 kPa.
• Stability: Maintains consistent current difference over 350 loading/unloading cycles at 12 kPa, with some waveform distortion but stable amplitude.
• Benchmarking: Sensing response closely matches a commercial FSR400 sensor; BP@PANI device outperforms FSR400 in lower pressure detection range and lower hysteresis (FSR400 reported low-pressure detection ~−1 kPa and hysteresis ~+10%).
• I–V behavior: Linear I–V over −0.5 to 0.5 V indicates good ohmic contact across a wide pressure range.
• Practical sensing demos: Detects finger tapping with pronounced current peaks; records finger bending/straightening with clear on/off current changes; detects impacts of single water drops and varying numbers of drops; monitors carotid pulse (~78 bpm) when mounted on the neck.
• Auditory HMI: A six-pixel BP@PANI sensor array mapped braille patterns to audio, correctly pronouncing letters (e.g., A, B, D, G) and words (“nanomaterials,” and dialog words: “hello,” “good,” “no,” “yes,” “ok”).
• Device attributes: Thin, lightweight, robust, low-voltage operation, cost-effective fabrication, and ease of integration on flexible substrates.

The work demonstrates that integrating BP with PANI on a textile substrate yields a piezoresistive tactile sensor with high sensitivity in the low-pressure range, low hysteresis, and reasonable response times suitable for dynamic touch sensing. These characteristics address the need for reliable, low-power tactile interfaces capable of real-time interaction. By implementing a six-pixel array aligned with the standard braille cell and interfacing it with a microcontroller and audio amplifier, the study closes the gap between tactile sensing and auditory feedback, enabling direct press-to-audio communication. The results confirm that BP@PANI-based sensors can discern fine pressure variations, sustain repeated operation, and function in wearable contexts, validating their relevance for assistive technologies. The successful translation of braille inputs into spoken output illustrates practical utility for individuals with visual or speech impairments and highlights broader potential for HMIs and portable electronic reading devices. Compared with commercial and literature-reported sensors, the proposed device offers an advantageous combination of sensitivity, linear range in the low-kPa regime, and low hysteresis, alongside a simple, scalable fabrication route.

The study presents a scalable, low-cost BP@PANI textile-based piezoresistive tactile sensor and a prototype six-pixel braille-to-audio HMI. Key contributions include: (i) a sensitive, low-hysteresis, and reasonably fast sensor leveraging BP’s electrical properties and a porous textile architecture; (ii) demonstration of practical, wearable sensing (finger motion, water drops, carotid pulse) and (iii) an integrated system that converts braille patterns into spoken audio, enabling assistive communication. The approach can facilitate affordable, wearable tactile interfaces, potentially enabling portable electronic books and broader HMI applications. Future work could focus on enhancing high-pressure linearity, extending cycling durability, scaling arrays to larger pixel counts while mitigating crosstalk, and further optimizing stability and encapsulation for long-term use in diverse environments.

• Sensitivity reduction and near-saturation above ~20 kPa, with low sensitivity (0.154 kPa−1) in the 30–100 kPa range, limiting quantitative measurement at higher pressures.
• Cycling tests demonstrated stability over 350 cycles with some signal distortion; longer-term endurance and environmental robustness (e.g., humidity, temperature, sweat) were not extensively reported.
• Finger tapping tests showed variability in peak intensity due to uneven applied pressure, indicating potential susceptibility to user-dependent variability.
• The demonstrated array is limited to six pixels (one braille cell); performance and potential signal crosstalk in larger arrays or higher-density configurations were not characterized in detail.
• While BP materials can be sensitive to ambient conditions, explicit long-term air stability or encapsulation strategies for BP@PANI in the presented device were not detailed.