POFBG-Embedded Cork Insole for Plantar Pressure Monitoring

Abstract

We propose a novel polymer optical fiber (POF) sensing system based on fiber Bragg gratings (FBGs) to measure foot plantar pressure. The plantar pressure signals are detected by five FBGs, in the same piece of cyclic transparent optical polymer (CYTOP) fiber, which are embedded in a cork insole for the dynamic monitoring of gait. The calibration and measurements performed with the suggested system are presented, and the results obtained demonstrate the accuracy and reliability of the sensing platform to monitor the foot plantar pressure distribution during gait motion and the application of pressure. This architecture does not compromise the patient’s mobility nor interfere in their daily activities. The results using the CYTOP fiber showed a very good response when compared with solutions using silica optical fibers, resulting in a sensitivity almost twice as high, with excellent repeatability and ease of handling. The advantages of POF (e.g., high flexibility and robustness) proved that this is a viable solution for this type of application, since POF’s high fracture toughness enables its application in monitoring patients with higher body mass compared with similar systems based on silica fiber. This study has demonstrated the viability of the proposed system based on POF technology as a useful alternative for plantar pressure detection systems.

Fabrication and Characterization of Bragg Grating in CYTOP POF at 600-nm Wavelength

“…It was measured from 32 º C to 72 º C with 20 º C step and a temperature sensitivity of 11.2±0.5 pm/º C was obtained, providing a red shift performance, as shown in Fig.4. The results show a different behavior when compared with PMMA POF, which temperature sensitivity has been measured around −0.069 ± 0.001 nm/º C [12] at 850 nm wavelength region. The CYTOP FBG fabrication with fs laser also showed a positive wavelength shift with increasing temperature, which can be explained in terms of the positive thermo-optic coefficient, which is close to 7.4×10 −5 /K due to the linear expansion in bulk volume for CYTOPs [5].…”

“…Then, the term (1-Pe) λ B is the strain sensitivity, whereas (α+ξ) λ B is the temperature sensitivity. For this reason, if the Bragg wavelength (λ B ) is in shorter wavelengths, the strain and temperature sensitivities will be lower when comparing with the one of λ B in longer wavelength region, such as 850 nm or 1550 nm [12].…”

“…Also, the refractive index changes introduced by the fs pulses are highly localized. From the first FBG inscription in CYTOP POF with fs laser reported [5], the same group has published a considerable number of works with focus in sensing applications at 1550 nm spectral region [10][11][12][13]. Due to point-by-point or plane-by-plane method limitation, the fs pulses irradiation is limited to typical periods of FBGs, due to the spatial resolution is limited by the diffraction limit [14].…”

Latest Achievements in Polymer Optical Fiber Gratings: Fabrication and Applications

“…Another interesting application is provided by health equipment for the dynamic monitoring of gait. An array of five FBGs inscribed in CYTOP fiber [119] was embedded in a cork insole, as shown in Figure 14. The advantages of POF (e.g., higher flexibility and robustness) enabled monitoring patients with higher body mass compared with similar systems based on silica fiber, with a mean sensitivity of ~8.14 PM/kPa, which is almost four times higher than silica FBGs (~2.51 pm/kPa).…”

Plantar Pressure Changes and Their Relationships with Low Back Pain during Pregnancy Using Instrumented Insoles

“…The pressure sensors were fabricated using a conductive sensing fabric with conductive yarns and a top-and-bottom conversion layer. Moreover, a novel and very recent system was based on optical fiber sensors for plantar pressure measurements [37]. Therefore, with the aim of a better understanding of the relationship of LBP and gait, a longitudinal study was carried out using our developed in-shoe system to analyze plantar pressure distributions and the stance phase time of a group of healthy pregnant women throughout their pregnancy in the first (T1), second (T2), and third (T3) trimesters.…”