A dual-mode fiber-shaped flexible capacitive strain sensor fabricated by direct ink writing technology for wearable and implantable health monitoring applications

The study addresses limitations of existing fiber-shaped strain sensors used in wearable electronics and health monitoring. Conventional sensing mechanisms (piezoelectric and triboelectric) struggle with static measurements and vibration interference, resistive sensors show temperature sensitivity and hysteresis-driven drift, and magnetoelastic sensors are restricted to dynamic pressure sensing. Traditional fabrication routes for fiber sensors (sequential coating and solution extrusion) yield limited stretchability, narrow strain ranges, multiple fragile interfaces prone to delamination, and can require toxic solvents that compromise biocompatibility. There is a pressing need for customizable, biocompatible, and highly stretchable fiber sensors—especially for implantable applications—with adjustable sensitivity and range. The research proposes a direct ink writing (DIW) approach to produce a fiber-shaped flexible capacitive strain sensor (FSFCSS) featuring parallel helical silver electrodes on TPU fibers and a BTO@Ecoflex dielectric/encapsulation layer, enabling dual-mode sensing of axial tensile and radial expansion strains for wearable and implantable health monitoring.

Prior work has developed diverse fiber-shaped strain sensors across mechanisms: resistive, piezoelectric, capacitive, triboelectric, and magnetoelastic. However, piezoelectric and triboelectric sensors lack reliable static sensing and are susceptible to vibration/shock; resistive sensors suffer from temperature sensitivity, nonlinear outputs, and hysteresis; magnetoelastic sensors excel at dynamic but not static pressure sensing. Fabrication via sequential coating can produce hollow or single-fiber sensors (e.g., graphite flakes on silk fibers using dry-Meyer-rod coating), but poor stretchability of conductive/sensitive layers limits overall strain range and multi-interface stacks risk delamination under repeated stretch. Solution extrusion (e.g., coaxial wet spinning of CNT@Ecoflex fibers) integrates materials in a single precursor, yet the composite dictates electromechanical limits, often yielding poor stretchability and requiring potentially toxic solvents that reduce biocompatibility. These drawbacks motivate additive, pattern-customizable methods such as DIW, which can directly print stretchable electrode architectures (snake, horseshoe, 3D helical) onto elastic substrates without degrading their mechanics, improving adhesion, robustness, and accommodating tailored sensitivity/range.

Sensor architecture: a three-layer fiber consisting of an elastic TPU tube fiber core, two parallel helical Ag electrodes printed on its surface, and a high-dielectric BTO@Ecoflex encapsulation layer serving as dielectric and protective coating. Substrate preparation: TPU fibers were plasma and surfactant treated to obtain a hydrophilic surface. Printing setup: a JTO fixture on a synchronous motor held and rotated the TPU fiber while a pneumatic micro-nozzle dispensed Ag ink with linear motion along the fiber; extrusion rate was controlled via pneumatic valve. Parameter control: DIW allowed direct control over electrode line width, thickness, turn density (turns/cm), spacing, and length. Printing pressure was tuned to adjust extruded ink volume and thus feature dimensions. Encapsulation: printed helical electrodes were uniformly overcoated with BTO@Ecoflex to form a tight, conformal dielectric/encapsulation layer improving adhesion and robustness. Characterization: SEM captured surface and cross-sectional morphology, showing smooth, defect-free printed Ag lines with strong adhesion and uniform encapsulation. Mechanical tests: stress–strain characterization of TPU fibers (diameters 0.5, 1.0, 1.5 mm) with and without electrodes established that printing minimally affected the fibers’ intrinsic >400% stretchability. Electrical tests: tensile resistance and stretchability of helical Ag electrodes (with/without encapsulation) were measured across turn densities; encapsulation improved stretchability and integrity. Capacitive sensing theory: the 3D helical electrode pair was modeled as equivalent 2D interdigital electrodes; formulas for initial capacitance and strain-dependent capacitance under axial tensile (ε1) and radial expansion (ε2) modes were derived, relating sensitivity to helical angle, length, and turn count. Axial tensile sensing: systematic variation of helical turn density (0–3 turns/cm) quantified tradeoffs between sensitivity and detection range; dynamic response and hysteresis were measured at a tensile speed of 0.5 mm/s; durability assessed over 1200 cycles at 30% strain. Radial expansion sensing: a syringe pump inflated the sensor while a commercial pressure gauge recorded internal pressure; fibers of 1 mm and 4 mm diameter were evaluated for static/dynamic pressure responsiveness and durability (over 12,000 expansion cycles). Modeling: COMSOL finite element analysis simulated electric field distribution around the helical electrodes.

- Fabrication and structure: DIW produced smooth, continuous, adherent helical Ag electrodes on TPU fibers with uniform BTO@Ecoflex encapsulation; electrode geometry was tunable via printing parameters. TPU fibers retained intrinsic >400% stretchability; non-encapsulated Ag ink exhibited ~35% inherent stretchability. Encapsulation and helical geometry significantly improved overall stretchability and robustness. - Axial tensile sensing performance: detection range up to 178%; sensitivity 0.924; minimum detection limit 0.6%; response and recovery times of 117 ms and 156 ms at 0.5 mm/s; low hysteresis coefficient 1.44% at 100% strain; stable performance over 1200 tensile cycles at 30% strain with no observable degradation. Turn-density tradeoff: increasing helical density from 0 to 3 turns/cm expanded detection range from 59% to 195% while reducing sensitivity from 1.347 to 0.381; breakage strain increased to 186% at 3 turns/cm, confirming strain-buffering by the helical architecture. - Radial expansion sensing performance: linear sensitivities of 0.00171 mmHg^-1 (1 mm diameter) and 0.00086 mmHg^-1 (4 mm diameter); accurate tracking of continuous dynamic and repeated pulse pressures; durability over >12,000 expansion cycles without significant signal degradation. - Applications: integration with a portable data acquisition board enabled wearable, wireless physiological monitoring and human–machine interaction; series integration with a printed RF coil produced a wireless hemodynamic sensor capable of blood pressure and heart rate measurements.

The DIW-fabricated FSFCSS overcomes limitations of traditional coated or solution-extruded fiber sensors by directly patterning helical Ag electrodes onto elastic TPU without compromising substrate mechanics or biocompatibility. The helical geometry and robust BTO@Ecoflex encapsulation mitigate interfacial delamination and distribute strain, enabling large tensile ranges with low hysteresis and high stability. The sensor’s dual-mode operation (axial tensile and radial expansion) broadens functionality: axial mode supports large-deformation motion capture, while radial mode enables pressure-related physiological measurements such as blood pressure waveforms. Sensitivities and response times meet requirements for real-time wearable monitoring, and durability across thousands of cycles supports long-term use. Compared with mechanisms sensitive to temperature or vibrations, the capacitive approach offers low power, dynamic/static responsiveness, and reduced environmental susceptibility, making it suitable for continuous health monitoring and HMI. The demonstrated wireless hemodynamic sensing with an RF coil illustrates potential for implantable or minimally invasive systems and smart textiles.

This work introduces a customizable, dual-mode fiber-shaped capacitive strain sensor fabricated via direct ink writing. Parallel helical silver electrodes on TPU fibers, encapsulated by high-dielectric BTO@Ecoflex, deliver wide tensile strain detection (up to 178%), high sensitivity (0.924), low hysteresis (1.44%), fast response, and excellent durability, while radial expansion mode provides pressure sensitivity suitable for hemodynamic monitoring. DIW enables precise control of electrode geometry (e.g., turn density) to tune sensitivity–range tradeoffs and enhances robustness through strong electrode–substrate integration. The sensor integrates with wireless readout for wearable and potential implantable applications, including physiological signal acquisition and human–machine interaction. Future work could optimize electrode geometries and dielectric materials for even higher sensitivity, explore long-term biocompatibility and in vivo performance, and develop fully integrated textile-based or implantable systems with energy harvesting and advanced wireless telemetry.