Abstract:

Objective Flexible wearable sensors have vast potential applications in the field of healthcare. For example, real-time monitoring of joint torsion in the treatment of elbow and knee arthritis plays a crucial role in the rehabilitation of conditions such as tennis elbow and meniscus injuries. However, current monitoring of joint torsion mainly relies on the traditional optical motion capture methods, which lack real-time capabilities. This research aims to develop flexible wearable torsion sensor devices to meet the joint monitoring needs for treatment of joint-related diseases.

Method Polyacrylonitrile(PAN)/MoS₂ fiber membranes were prepared using electrospinning technology, which were then characterized for their structure, morphology, elemental content, and mechanical properties. A custom-built torsional flexoelectric response test platform were used to evaluate the impact of different MoS₂ mass fractions on the flexoelectric effect of the PAN/MoS₂ fiber membranes. The response of the PAN/MoS₂ torsion sensor to various torsion angles was tested, and its practicality was demonstrated by conducting experiments where the torsion sensor was attached to the joints of a wooden mannequin.

Results The microscopic morphology and physical properties of the fiber membranes showed that MoS₂ was successfully loaded onto PAN, and the membranes exhibited excellent crystallinity. Pure PAN membranes exhibited a weak flexoelectric effect. However, with the addition of MoS₂, the response current and voltage of the fiber membranes was significantly increased with the increase of MoS₂ content. When the MoS₂ mass fraction reached 50%, the flexoelectric response voltage and current peak were 176.41 mV and 102.85 pA, respectively. When the MoS₂ content was further increased to 55%, the response current and voltage drop to 65.61 pA and 97.77 mV. This decline was due to the excessive MoS₂ nanoparticles forming aggregates on the PAN surface, which significantly hindered the orderly arrangement of polymer molecular chains and restricts ion migration. As a result, the potential difference generated by the membrane torsion was lower, reducing the flexoelectric coefficient. For torsion angle sensing tests, the PAN/MoS₂ fiber membrane with 50% MoS₂ was chosen. As the torsion angle increased, the response current and voltage also increased significantly, and the electrical signal waveform remains stable. When the torsion angle was increased from 6° to 30°, the response current and voltage reached a maximum of 183.73 pA and 254.16 mV. This is attributed to the increased shear strain gradient and polarization intensity within the fiber membrane with larger torsion angles. Application experiments with the torsion sensor attached to the joints of a wooden mannequin demonstrated that the PAN/MoS₂ torsion sensor was able to accurately capture joint torsion changes.

Conclusion PAN/MoS₂ fiber membranes were prepared via electrospinning, successfully loading MoS₂ nanoparticles onto the PAN surface. The mass fraction of MoS₂ in the PAN/MoS₂ fiber membranes significantly would affect their flexoelectric effect. Experiments show that when the MoS₂ mass fraction is below 50%, the flexoelectric response current and voltage increase with the MoS₂ content. At 50% MoS₂ content, the flexoelectric current and voltage reach their optimal levels. However, when the mass fraction exceeds 50%, the MoS₂ particles cluster and weaken the flexoelectric effect. Experimental results indicate that the PAN/MoS₂ torsion sensor is highly sensitive to changes in torsion angles. Application experiments with the torsion sensor attached to the joints of a wooden mannequin confirm that the PAN/MoS₂ torsion sensor can accurately detect joint torsion changes.

Key words: shear flexoelectricity effect, torsion sensor, flexible wearable sensor, polyacrylonitrile/MoS₂ fiber membrane, electrospinning, health monitoring