Insight into, and measurements of, muscle contraction during movement may help improve the assessment of muscle function, quantification of athletic performance, and understanding of muscle behavior, prior to and during rehabilitation following neuromusculoskeletal injury. A self-adhesive, elastic fabric, nanocomposite, skin-strain sensor was developed and validated for human movement monitoring. We hypothesized that skin-strain measurements from these wearables would reveal different degrees of muscle engagement during functional movements. To test this hypothesis, the strain sensing properties of the elastic fabric sensors, especially their linearity, stability, repeatability, and sensitivity, were first verified using load frame tests. Human subject tests conducted in parallel with optical motion capture confirmed that they can reliably measure tensile and compressive skin-strains across the calf and tibialis anterior. Then, a pilot study was conducted to assess the correlation of skin-strain measurements with surface electromyography (sEMG) signals. Subjects did biceps curls with different weights, and the responses of the elastic fabric sensors worn over the biceps brachii and flexor carpi radialis (i.e., forearm) were well-correlated with sEMG muscle engagement measures. These nanocomposite fabric sensors were validated for monitoring muscle engagement during functional activities and did not suffer from the motion artifacts typically observed when using sEMGs in free-living community settings.

Strain measurements are vital for monitoring the load-bearing capacity and safety of structures. A common approach is to affix strain gages onto structural surfaces. On the other hand, most aerospace, automotive, civil, and mechanical structures are painted and coated, often with many layers, prior to their deployment. There is an opportunity to design smart and multifunctional paints that can be directly pre-applied onto structural surfaces to serve as a sensing layer among their other layers of functional paints. Therefore, the objective of this study was to design a strain-sensitive paint that can be used for structural monitoring. Carbon nanotubes (CNT) were dispersed in paint by high-speed shear mixing, while paint thinner was employed for adjusting the formulation’s viscosity and nanomaterial concentration. The study started with the design and fabrication of the CNT-based paint. Then, the nanocomposite paint’s electromechanical properties and its sensitivity to applied strains were characterized. Third, the nanocomposite paint was spray-coated onto patterned substrates to form “Sensing Meshes” for distributed strain monitoring. An electrical resistance tomography (ERT) measurement strategy and algorithm were utilized for reconstructing the conductivity distribution of the Sensing Meshes, where the magnitude of conductivity (or resistivity) corresponded to the magnitude of strain, while strain directionality was determined based on the strut direction in the mesh.

Human performance assessment of people of all ages, genders, and professions necessitates the measurement of physical movements and physiological parameters. Wearable skin-like and textile-based sensors, particularly those based on piezoresistive nanocomposites, have been successfully developed for monitoring skin-strains. When using these resistive materials in free living conditions, a common challenge is that their sensing streams can be adversely affected by ambient environmental effects, such as temperature and humidity. This study presents a method for patterning wearable piezoresistive nanocomposites so that strain measurements are automatically compensated and unaffected by ambient conditions. Motion Tape, which is a piezoresistive graphene nanosheet thin film integrated with an elastic fabric substrate, exhibits highly linear and repeatable strain sensing properties but also possesses temperature-dependent electrical characteristics. By designing and fabricating orthogonally oriented, fiber-integrated nanocomposites on the same substrate and then electrically connecting them to yield a materials-based bridge circuit, reliable strain sensing during varying ambient temperatures is successfully validated. The method is promising and potentially broadly applicable to many resistive nanocomposite sensors whose electrical properties are impacted by ambient conditions. Therefore, orthogonally patterned nanocomposite circuits can help mitigate environmental noise signals to improve wearable sensor performance and stability within a compact form factor.

SPIE Digital Library Proceedings