Fiber-shaped strain sensors are gaining significant attention for their potential in wearable health monitoring and human-machine interfaces due to their comfort and ease of integration. Existing fabrication methods, such as sequential coating and solution extrusion, often result in limited stretchability, impacting the sensor's range and stability. This paper addresses these limitations by employing DIW technology to fabricate a novel FSFCSS. DIW offers advantages such as pattern customization, material versatility, and cost-effectiveness. The chosen materials—TPU for its elasticity, Ag ink for conductivity, and BTO@Ecoflex for its dielectric properties and biocompatibility—are key to the sensor's performance. The authors hypothesize that the DIW-fabricated FSFCSS will exhibit superior stretchability, sensitivity, and stability compared to sensors made using traditional methods, enabling its use in both wearable and implantable applications.

The paper reviews various types of fiber-shaped strain sensors, including resistive, piezoelectric, capacitive, triboelectric, and magnetoelastic sensors. It highlights the limitations of each type: piezoelectric and triboelectric sensors are susceptible to external interference; resistive sensors suffer from temperature sensitivity and hysteresis; and magnetoelastic sensors are limited to dynamic pressure sensing. Capacitive sensors are presented as a superior alternative due to their high sensitivity, low temperature dependence, low power consumption, and ability to respond to both dynamic and static stimuli. The authors discuss the limitations of traditional fabrication methods, sequential coating and solution extrusion, noting their poor stretchability, interface delamination issues, and potential use of toxic solvents. These limitations motivate the use of DIW as a more versatile and biocompatible fabrication technique.

The FSFCSS was fabricated using DIW technology. The process involved plasma and surfactant treatment of the TPU fiber to improve surface hydrophilicity. A JTO fixture fixed to a synchronous motor held and rotated the fiber while a pneumatic nozzle printed the helical Ag electrodes. The ink extrusion speed was controlled by a pneumatic valve. The BTO@Ecoflex dielectric layer was then applied as an encapsulation layer. The researchers used scanning electron microscopy (SEM) to characterize the morphology of the fabricated sensor. Mechanical testing was conducted to determine the stress-strain behavior of the TPU fiber with and without the printed electrodes. Electrical characterization involved measuring the resistance of the electrodes under different strains and turn densities. The sensor's dual-mode sensing capabilities were evaluated by applying both axial tensile strain and radial expansion strain. The axial tensile strain sensing performance was characterized by measuring the capacitance change under various levels of strain, while radial expansion strain sensing was evaluated by inflating the FSFCSS with a syringe pump and measuring the capacitance change with a commercial pressure gauge. The response time and recovery time were recorded, and a cyclic test was performed to assess the sensor's long-term stability. Finally, the FSFCSS was integrated into a wearable wireless sensing system for physiological signal monitoring and a conceptual implantable wireless blood hemodynamic sensor.

The DIW-fabricated FSFCSS exhibited excellent performance in both axial tensile strain and radial expansion strain sensing modes. For axial tensile strain sensing, the sensor showed a wide detection range of 178%, a high sensitivity of 0.924, a low detection limit of 0.6%, and a low hysteresis coefficient of 1.44%. The response and recovery times were 117 ms and 156 ms, respectively. The sensor demonstrated excellent long-term stability, with no observable degradation after 1200 cycles at 30% strain. For radial expansion strain sensing, the FSFCSS exhibited a sensitivity of 0.00171 mmHg⁻¹ (1 mm diameter) and 0.00086 mmHg⁻¹ (4 mm diameter). The sensor showed excellent responsiveness to both static and dynamic expansion pressures, maintaining consistent performance even after over 12,000 cycles. The optimal helical turn density for balancing sensitivity and detection range was determined to be 1 turn/cm. The study successfully demonstrated the integration of the FSFCSS into wearable and implantable applications, showcasing its use in physiological signal monitoring (smart gloves) and a conceptual implantable wireless blood hemodynamic sensor.

The results demonstrate the successful fabrication of a high-performance, dual-mode FSFCSS using DIW technology. The sensor's superior performance compared to sensors made using traditional methods is attributed to the advantages of DIW: improved stretchability due to the absence of layer-by-layer interfaces, customizable electrode structures, and the ability to utilize the excellent mechanical properties of the substrate material. The dual-mode sensing capability expands the sensor's applications to a wider range of physiological measurements and human-machine interactions. The successful integration of the sensor into wearable and implantable prototypes highlights its potential for practical applications in health monitoring. These findings contribute to the advancement of smart textiles and the development of more comfortable and versatile wearable and implantable health monitoring devices.

This study successfully demonstrated the fabrication of a high-performance dual-mode FSFCSS using DIW technology. The sensor exhibits superior characteristics compared to traditional methods, enabling applications in wearable and implantable health monitoring. Future research could focus on exploring different materials for further improving sensitivity and biocompatibility, as well as miniaturizing the sensor for even more diverse applications. Investigating the sensor's performance in vivo would further validate its potential for real-world applications.

The study primarily focused on in vitro characterization. Further in vivo testing is needed to validate the sensor's performance in a real-world environment and assess its long-term stability and biocompatibility. The current design and fabrication method may need optimization for mass production and cost reduction. The sensitivity of the radial expansion strain sensing mode could potentially be improved through further optimization of the sensor design and material selection.