A wearable shear force transducer based on color spectrum analysis

Abstract

Introduction: The need for miniaturized shear force sensors is expanding, particularly for biomedical applications. Examples include measuring interfacial shear stresses between a human and an external device (e.g., footwear or a prosthesis). However, there are considerable challenges in designing a shear sensor for these applications due to the need for a small package, low power requirements, and resistance to interference from motion artifact and electromagnetic fields. Methods: This paper presents the design, fabrication, and characterization sensor that measures two-axis shear force by detecting displacement between a color panel and a red, green, and blue light-sensing photodiode. The sensor response to applied displacements and forces was characterized under benchtop testing conditions. We also present the design of a prototype wireless version of the sensor for integration into footwear. Results: The sensor exhibited strong agreement with gold standard measurements for two axis shear displacements (R2>0.99, RMSE≤5.0 µm) and forces (R2>0.99, RMSE≤0.94 N). Conclusions: This performance, along with the sensor’s scalability, miniaturized form, and low power requirements make it well-suited a variety of biomedical applications.

An Optical Sensor for Measuring Displacement between Parallel Surfaces

“…Sensors 2024, 24, 3498 2 of 12 A shear force sensor was developed and described by McGeehan and coworkers [12][13][14] that detected the red (R), green (G), blue (B), and white (C, clear/full visible spectrum) spectra of a broad-spectrum LED light that was reflected from a color grid [12][13][14]. In the study, a ninecolor square grid was made up of green in the center, magenta in the four diagonal corners, and blue and red on opposite sides of the center [12]. The LED and sensor were separated from the color grid by a window (a square cutout of an elastomer layer), so at any given time, only 11% of the surface was exposed to the light [12].…”

“…Conversely, the accuracy of piezoelectric sensors in distinguishing shear forces from other compressive forces is still inconsistent despite recent improvements in these technologies [ 10 , 11 ]. Compared to capacitive and resistive-based sensors, optical-based sensors would have certain advantages for medical applications because they are unaffected by electromagnetic fields and are more stable in relation to environmental factors like temperature and humidity [ 12 , 13 ]. Therefore, integrating optical sensors in the human-interfacing layer of medical devices and clothing, such as wearable prosthetic devices or footwear, can help improve the outcomes of medical treatments or the comfort levels of device users [ 12 , 13 , 14 ].…”

“…Compared to capacitive and resistive-based sensors, optical-based sensors would have certain advantages for medical applications because they are unaffected by electromagnetic fields and are more stable in relation to environmental factors like temperature and humidity [ 12 , 13 ]. Therefore, integrating optical sensors in the human-interfacing layer of medical devices and clothing, such as wearable prosthetic devices or footwear, can help improve the outcomes of medical treatments or the comfort levels of device users [ 12 , 13 , 14 ]. Furthermore, integrating shear force measurement in orthotics and prosthetics can reduce complications such as tissue ulcers and infections, as well as deep vein thrombosis in patients with diseases that are characterized by impaired sensations such as diabetic neuropathy/foot and amputated limbs [ 15 , 16 , 17 , 18 ].…”

“…A shear force sensor was developed and described by McGeehan and coworkers [ 12 , 13 , 14 ] that detected the red (R), green (G), blue (B), and white (C, clear/full visible spectrum) spectra of a broad-spectrum LED light that was reflected from a color grid [ 12 , 13 , 14 ]. In the study, a nine-color square grid was made up of green in the center, magenta in the four diagonal corners, and blue and red on opposite sides of the center [ 12 ]. The LED and sensor were separated from the color grid by a window (a square cutout of an elastomer layer), so at any given time, only 11% of the surface was exposed to the light [ 12 ].…”

An Optoelectronics-Based Compressive Force Sensor with Scalable Sensitivity

“…Optical approaches have been successfully implemented for measuring multi-axial shear forces in wearable devices [ 20 , 21 , 22 ]. It was demonstrated that shear force can be accurately measured using an LED and a photoresistor.…”