Development of a Flexible Piezoelectric Biosensor That Integrates BaTiO₃–Poly(Dimethylsiloxane) for Posture Correction Applications ^†

Abstract

The prolonged issue of poor posture due to desk work has led to innovative technological remedies. This study shows the development of a flexible piezoelectric biosensor integrating BaTiO₃ nanoparticles within a Polydimethylsiloxane (PDMS) matrix for practical posture correction. The biosensor is capable of real-time posture monitoring and correction by leveraging the piezoelectric properties of BaTiO₃. Comprehensive synthesis and characterization using X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) validated the ideal particle size and crystalline structure of the composite. COMSOL Multiphysics simulations showed a peak potential of 0.87 volts under mechanical stress, which further confirmed the sensor’s efficiency. Electrical testing revealed that the sensor with 35 wt.% BaTiO₃ exhibited a higher output voltage of 0.87 V compared to 0.34 V for the sensor with 30 wt.% BaTiO₃, emphasizing its exceptional potential for addressing posture-related issues.

1. Introduction

Desk work, which is often required in modern offices, can sometimes play a factor in the development of an abnormal body position in the workplace, which includes poor posture and negative effects like musculoskeletal diseases and chronic pain. Traditional methods for posture correction, such as ergonomic chairs and posture braces, offer limited and often passive solutions [1,2,3]. Therefore, there is a pressing need for innovative real-time and active posture monitoring and correction strategies [4]. Piezoelectric sensors are vital for real-time monitoring and quality control in additive manufacturing [5]. The piezoelectric effect involves generating an electric charge in certain materials under mechanical stress, converting mechanical energy into electrical energy through the reorientation of electric dipoles within the material’s structure [6]. Biosensors are miniaturized devices composed from the sensor part, also known as physicochemical transducers, and are partly of a biological origin like antibodies, enzymes, nucleic acid sequences, organelles, viable cells, or slices of a tissue [7,8].

Piezoelectric sensors and biosensors are versatile tools, known for their sensitivity and quick response times [9]. They play crucial roles across various sectors: in medical diagnostics, they detect biomarkers in bodily fluids for diseases like cancer and diabetes [10]. In industry, these sensors monitor machinery vibrations, accurately predicting maintenance needs. Environmental applications involve detecting pollutants in the air and water, supporting environmental conservation efforts. Moreover, they enhance consumer electronics like microphones and accelerometers, improving device functionality. Posture corrector devices often face issues related to cost and practicality. They are typically expensive, making them inaccessible to individuals from lower socioeconomic backgrounds who might benefit the most. Additionally, users must remember to wear and charge them regularly, leading to decreased usage; surveys show that half of the users stop using these devices within six months. Moreover, the absence of standardized measurements and limited research on postural data reduce their effectiveness and reliability in clinical settings [11].

Posture correction sensors often use capacitive textile sensors that are integrated into chairs to detect changes in pressure and capacitance, providing real-time feedback for maintaining a correct posture [12]. Additionally, systems incorporating force-sensing resistors (FSRs) and load cells analyze the pressure distribution through machine learning algorithms, while accelerometers, gyroscopes, and magnetometers are used for precise spine posture monitoring [13]. Advanced materials like these enhance the effectiveness of posture correction devices, with innovations like permanent magnets improving posture classification accuracy [14]. BaTiO₃/PDMS biosensors have been employed in various fields due to their versatile properties. These sensors are used in energy harvesting, pressure sensing applications, high-frequency applications, tactile sensing, and hand-gesture sensors [15,16,17,18,19,20]. They are used in these applications for their high sensitivity, flexibility, and ability to efficiently convert mechanical energy into electrical energy. The BaTiO₃/PDMS composite has yet to be fabricated for use in posture correction applications.

In this study, BaTiO₃ and PDMS were combined to create a flexible piezoelectric biosensor. BaTiO₃’s high piezoelectric coefficient and dielectric constant provide superior sensitivity and performance in converting mechanical stress to electrical signals, surpassing materials like ZnO and PVDF, which is crucial for real-time posture monitoring and correction. The lead-free nature of BaTiO₃ also enhances the sustainability of wearable devices. PDMS adds necessary flexibility, making the biosensor suitable for wearable applications and contributing to its mechanical reliability, like ZnO nanowires, while also offering ease of processing and cost benefits. Although recent advancements in materials like α-In₂Se₃ show promise for self-powered systems, BaTiO₃’s proven performance in bulk applications continues to support its role in this biosensor. This research highlights the integration of BaTiO₃/PDMS in a biosensor that offers a rapid response and high sensitivity for effective biomechanical sensing [18,21,22,23,24,25].

2. Materials and Methods

2.1. Synthesis of BaTiO₃ Nanoparticles

BaTiO₃ nanoparticles were prepared via the sol–gel method using a 1:1 mole ratio of barium and titanium precursors [26]. Barium acetate (Ba(C₂H₃O₂)₂) (Carl Roth 2553, Karlsruhe, Germany) was dissolved in a mixture of acetic acid (CH₃COOH) (Merck 100056, Darmstadt, Germany) and n-butanol (C₄H₁₀O) (Merck 101990, Darmstadt, Germany). Then, the mixture was stirred using a heating magnetic stirrer (Dlab Ms-H-Pro+, Beijing, China) at 1500 rpm and 60 °C for 45 min until completely dissolved, followed by being kept at room temperature for a day (Figure 1a,b). Under a nitrogen (N₂) atmosphere, Ti (IV) isopropoxide (Ti(OCH(CH₃)₂)₄) (Sigma-Aldrich 205273, Burlington, MA, USA) was slowly added to the solution. The resulting mixture underwent washing with n-butanol and ethanol (Figure 1c) to remove impurities. Subsequently, purification was conducted using an ultrasonic cleaner (Bandelin Sonorex Super RK 100 H, Berlin, Germany) (Figure 1d) to enhance separation, followed by centrifugation in a centrifuge (Hettich EBA 20S, Tuttlingen, Germany) (Figure 1e) to further purify and isolate the desired components. Finally, the purified precipitates were meticulously dried at 70 °C for 90 min to ensure complete removal of residual solvents. Finally, BaTiO₃ was calcined at approximately 850 °C in a furnace to form crystalline nanoparticles (Figure 1f). This process enhances the material’s dielectric and piezoelectric properties, optimizing its electrical characteristics for a more sensitive and effective posture-monitoring and correction device.

2.2. Fabrication of BaTiO₃/PDMS Composite

The fabrication of the flexible composite material, integrating BaTiO₃ nanoparticles into a Polydimethylsiloxane (PDMS) matrix, involves precise steps, as shown in Figure 2. Initially, the PDMS base polymer and curing agent (Dow Sylgard-184, Midland, MI, USA) are mixed in a 10:1 ratio by weight. To study the effect of the filler content, BaTiO₃ nanoparticles are incrementally added to the PDMS mixture at 30 wt.% and 35 wt.%, ensuring thorough hand-mixing for uniform dispersion. The homogeneous mixture is then poured into molds, where vacuum desiccation removes air bubbles to prevent defects. Curing at 80 °C for 2 h solidifies the BaTiO₃/PDMS composite into a flexible structure. To enhance its electrical properties, a conductive layer is applied via thermal evaporation using an e-Beam evaporation system (Nanovak NVTS-400, Ankara, Turkey). This process involves heating copper in a high-vacuum chamber until it evaporates and then condensing it onto the composite surface. This conductive coating is crucial for efficient electrical signal transduction from the piezoelectric BaTiO₃ nanoparticles, significantly boosting the composite’s performance.

3. Results and Discussion

3.1. Structural Characterization

3.1.1. X-ray Diffraction (XRD) Analysis

The X-ray diffraction (XRD) analysis of the synthesized BaTiO₃ nanoparticles revealed distinct peaks at 2θ values, which are characteristic of the tetragonal phase of BaTiO₃, confirming their crystalline nature and high phase purity [27]. The BaTiO₃ nanostructures are characterized by peaks corresponding to (0 0 1), (1 0 1), (1 1 1), (2 0 0), (2 0 1), (2 1 1), (2 0 2), and (3 0 1) planes located at angle values of 22,2°, 31.6°, 38.9°, 45.2°, 50.9°, 56.2°, 65.9°, and 74.9° (Figure 3). These results not only validate the successful synthesis but also establish a strong basis for leveraging BaTiO₃’s piezoelectric capabilities in sensor technologies. Integrating BaTiO₃ nanoparticles into a flexible Polydimethylsiloxane (PDMS) matrix demonstrates the effectiveness of this approach, enhancing the composite material’s flexibility and sensitivity to pressure and movement [28].

3.1.2. Transmission Electron Microscopy (TEM) Analysis

The transmission electron microscopy (TEM) images revealed the structural attributes of the BaTiO₃ nanoparticles, demonstrating their uniform distribution and spherical morphology, averaging around 100 nm in size (Figure 4a). This uniformity indicates effective synthesis and purification processes, essential for ensuring a consistent piezoelectric response in the composite material. Additionally, detailed particle measurements, including mean size and standard deviation calculations, provide quantitative insights into the nanoparticles’ size distribution and homogeneity. The average standard deviation of BaTiO₃, determined from TEM analysis using ImageJ with measurements from ten samples, was found to be 33.121, which confirms its uniformity and quality (Figure 4b). These combined qualitative and quantitative analyses validate the potential of BaTiO₃ nanoparticles for integration into piezoelectric composites, underscoring their suitability for various sensor applications.

3.2. Electrical Characterization

3.2.1. COMSOL Multiphysics Simulations

COMSOL Multiphysics simulations using finite element analysis demonstrated the BaTiO₃/Polydimethylsiloxane (PDMS) composite’s capability to convert mechanical energy into electrical signals, yielding a peak potential of 0.87 volts under mechanical stress and affirming its efficiency. Firstly, the image shows a BaTiO₃/PDMS composite sensor with a BaTiO₃ layer between two PDMS layers, designed to convert mechanical stress into electrical signals (Figure 5a). Afterwards, the provided graph illustrates the relationship between floating potential (V) and force (N) over time (ms). It shows that the floating potential and force both increase linearly from 0 V to approximately 0.35 V and 0.03 N, respectively, within one millisecond (Figure 5b). The floating potential slightly surpasses the force throughout the duration, suggesting a direct correlation between the applied force and generated voltage. The uniform electric field distribution confirmed the even dispersion of nanoparticles within the PDMS matrix, enhancing the composite’s effectiveness. The analysis of the Von Mises stress distribution verified its resilience to mechanical deformations, which is crucial for wearable biosensors requiring durability. At t = 0.5 s, the output voltage reached approximately 0.3 V, and at t = 1 s, it increased to 0.7 V (Figure 5c), highlighting its robustness and reliability for continuous posture monitoring in wearable biosensors. The voltage values of the sensors increase as the load site moves away from the structure’s fixed edge, indicating a correlation between the load position and sensor output [29].

3.2.2. Electrical Response Testing and Final Product

The piezoelectric performance of the biosensor was evaluated by measuring the output voltage in response to the mechanical deformation, a critical functionality. The data were visualized using an oscilloscope’s output graph (Figure 6a). The graph shows a mean voltage of approximately 29.73 mV, with a minimum of 2.891 mV and a maximum of 866.2 mV, and a standard deviation of 141.8 mV. Distinct voltage signals were observed for composites with different BaTiO₃ weight percentages. Notably, the sensor with 35 wt.% BaTiO₃ demonstrated a peak voltage of 0.87 V, which was higher than the 0.34 V recorded for 30 wt.% BaTiO₃. This highlights the significant impact of the BaTiO₃ content on enhancing the composite’s piezoelectric response. These measurements confirm the sensor’s sensitivity to BaTiO₃ concentration variations and its potential effectiveness in posture correction applications.

The image shown in Figure 6b illustrates the BaTiO₃/Polydimethylsiloxane (PDMS) piezoelectric biosensor designed to address posture issues during prolonged desk work. With a sophisticated design and precise thermal evaporation coating, it effectively corrects an individual’s posture. This device integrates advanced materials and manufacturing techniques, marking a new frontier in ergonomic support and musculoskeletal health maintenance. Leveraging BaTiO₃ nanoparticles and PDMS flexibility, it responds to posture changes sensitively. Its ability to detect even slight posture variations, as shown in the oscilloscope plot, underscores its potential for real-time feedback, preventing musculoskeletal problems.

The comparison of various materials for sensor applications is summarized in

Table 1. Comparison of the output performance of the BaTiO₃/PDMS piezoelectric biosensor with other types of piezoelectric sensors.

Material	Synthesis Technique	Electrode	Voltage Output	Applications	References
BaTiO3/PDMS	Simple dispersion	Silver	35 mV	High-Frequency Nanogenerators, Energy Harvesting	[18]
2D α-In2Se3	PVD	Silver	35.7 mV	Body Motion Sensor	[21]
Kevlar Fiber/ZnO Nanowires	PVD	Palladium	1.8 mV	UV Detector	[22]
ZnO Nanowire	PVD	Platinum	0.243 mV	pH Sensor, UV Sensor	[23]
ZnO Nanowire Arrays	PVD	Aluminum	50 mV	Skin Motion Sensor	[24]
ZnO/PVDF	Spin coating	Gold/Chromium	410 mV	-	[25]
BaTiO3/PDMS	Sol–gel	Copper	870 mV	Posture Correction	This work

. The PDMS/BaTiO₃ composites prepared in this work, particularly using the sol–gel method, exhibit a significantly higher voltage output (0.87 V) compared to other materials such as ZnO nanowire arrays (50 mV) and Kevlar Fiber/ZnO nanowires (1.8 mV). This indicates a notable improvement in performance for posture correction applications. Additionally, the PDMS/BaTiO₃ composite synthesized through simple dispersion demonstrates a voltage output of 35 mV, which is comparable to that of other high-frequency nanogenerators and energy-harvesting materials. These results highlight the potential of the developed PDMS/BaTiO₃ composites in advancing sensor technologies, particularly for energy harvesting and motion-sensing applications.

4. Conclusions

This study has successfully developed a flexible BaTiO₃/Polydimethylsiloxane (PDMS) piezoelectric biosensor for posture correction, demonstrating the efficacy of integrating BaTiO₃ nanoparticles into a PDMS matrix. Our comprehensive characterization through X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) confirmed the optimal crystalline structure and particle size of the synthesized BaTiO₃. The electrical testing revealed that the sensor with 35 wt.% BaTiO₃ exhibited superior performance, with a peak output voltage of 0.87 V, compared to 0.34 V for the sensor with 30 wt.% BaTiO₃. This enhanced piezoelectric response is critical for effective real-time posture monitoring and correction. COMSOL Multiphysics simulations corroborated these findings, showing a robust potential of the sensor under mechanical stress. The oscilloscope measurements further validated the distinct voltage signals, underscoring the role of BaTiO₃ content in enhancing the sensor’s performance. The successful fabrication and testing of this biosensor marks a significant step forward in addressing the prevalent issue of poor posture due to prolonged desk work, providing a reliable and efficient solution for posture correction.

Future Works

The biosensor is designed to detect mechanical stress, and while these initial results are promising, the integration of additional components to enhance the sensor’s practical application is ongoing. It aims to monitor pressure exceeding the average forward head posture of approximately 64 lbs. on the spine, which is indicative of poor posture [30]. Specifically, a vibration circuit and a high-pass filter circuit will be developed to amplify the voltage output and provide immediate feedback. High-pass filters allow signals with frequencies higher than a certain cutoff frequency to pass through, attenuating lower frequencies to isolate relevant piezoelectric signals [31]. The vibration circuit will convert the electrical signal into a mechanical vibration to alert the user. At this stage, our focus has primarily been on the fabrication and initial testing of the biosensor itself. The design and integration of the vibration and high-pass filter circuits are still in progress. Future work will involve rigorous testing and optimization of these circuits to ensure reliable performance and user compliance.