Superstructured fiber-optic contact force sensor with minimal cosensitivity to temperature and axial strain.

In this work a new superstructured, in-fiber Bragg grating (FBG)-based, contact force sensor is presented that is based on birefringent D-shape optical fiber. The sensor superstructure comprises a polyimide sheath, a stress-concentrating feature, and an alignment feature that repeatably orients the sensor with respect to contact forces. A combination of plane elasticity and strain-optic models is used to predict sensor performance in terms of sensitivity to contact force and axial strain. Model predictions are validated through experimental calibration and indicate contact force, axial strain, and temperature sensitivities of 169.6 pm/(N/mm), 0.01 pm/µÎµ, and -1.12 pm/°C in terms of spectral separation. The sensor addresses challenges associated with contact force sensors that are based on FBGs in birefringent fiber, FBGs in conventional optical fiber, and tilted FBGs. Relative to other birefringent fiber sensors, the sensor has contact force sensitivity comparable to the highest sensitivity of commercially available birefringent fibers and, unlike other birefringent fiber sensors, is self-aligning with respect to contact forces. Unlike sensors based on Bragg gratings in conventional fiber and tilted Bragg gratings, the sensor has minimal cosensitivity to both axial strain and changes in temperature.

Sensitivity of Bragg gratings in birefringent optical fiber to transverse compression between conforming materials.

A theoretical and experimental investigation of the transverse load sensitivity of Bragg gratings in birefringent fibers to conforming contact is presented. A plane elasticity model is used to predict the contact dimensions between a conforming material and optical fiber and the principal stresses, indicating birefringence, created as a result of this contact. The transverse load sensitivity of commercially available birefringent fiber is experimentally measured for two cases of conforming contact. Theoretical and experimental results show that birefringent optical fiber can be used to make modulus-independent measurements of contact load. Therefore, Bragg gratings could be applied to conforming contact load measurements while avoiding some of the complications associated with existing contact sensors: specifically, the necessity to precalibrate by using materials with mechanical properties identical to those found in situ.

Validation of a novel minimally invasive intervertebral disc pressure sensor utilizing in-fiber Bragg gratings in a porcine model: an ex vivo study.

STUDY DESIGN: Nucleus pressure was measured within porcine intervertebral discs (IVDs) with a novel in-fiber Bragg grating (FBG) sensor (0.4 mm diameter) and a strain gauge (SG) sensor (2.45 mm). OBJECTIVE: To validate the accuracy of a new FBG pressure sensor designed for minimally invasive measurements of nucleus pressure. SUMMARY OF BACKGROUND DATA: Although its clinical utility is controversial, it is possible that the predictive accuracy of discography can be improved with IVD pressure measurements. These measurements are typically obtained using needle-mounted SG sensors inserted into the nucleus. However, by virtue of their size, SG sensors alter disc mechanics, injure anulus fibers, and can potentially initiate or accelerate degenerative changes thereby limiting their utility particularly clinically. METHODS: Six functional spinal units were loaded in compression from 0 N to 500 N and back to 0 N; nucleus pressure was measured using the FBG and SG sensors at various locations along anterior and anterolateral axes. RESULTS: On average maximum IVD pressures measured using the FBG and SG sensors were within 9.39% of each other. However, differences between maximum measured pressures from the FBG and SG sensors were larger (22.2%) when the SG sensor interfered with vertebral endplates (P < 0.05). The insertion of the FBG sensor did not result in visible damage to the anulus, whereas insertion of the SG sensor resulted in large perforations in the anulus through which nucleus material was visible. CONCLUSION: The new FBG sensor is smaller and less invasive than any previously reported disc pressure sensor and gave results consistent with previous disc pressure studies and the SG sensor. There is significant potential to use this sensor during discography while avoiding the controversy associated with disc injury as a result of sensor insertion.

Ex vivo measurement of lumbar intervertebral disc pressure using fibre-Bragg gratings.

Methods were developed to measure intervertebral disc pressure using optical fibre-Bragg gratings (FBGs). The FBG sensor was calibrated for hydrostatic pressure in a purpose-built apparatus and the average sensitivity was determined to be -5.7 +/- 0.085 pm/MPa (mean +/- SD). The average coefficient of determination (r(2)) for the calibration data was 0.99, and the average hysteresis of the sensor was 2.13% of full scale. The FBG was used to measure intradiscal pressure response to compressive load in five lumbar functional spine units. The pressure measured by the FBG sensor varied linearly with applied compressive load with coefficients of determination ranging from 0.84 to 0.97. The FBG sensor's sensitivity to compressive load ranged from 0.702 +/- 0.043 kPa/N (mean +/- SD) in a L1-L2 specimen, to 1.07 +/- 0.069 kPa/N in a L4-L5 specimen. These measurements agree with those of previous studies in lumbar spines. Two strain gauge pressure sensors were also used to measure intradiscal pressure response to compressive load. The measured pressure sensitivity to load ranged from 0.251 kPa/N (L4-L5) to 0.850 kPa/N (L2-L3). The average difference in pressure sensitivity to load between Sensors 1 and 2 was 12.9% of the value for Sensor 1, with a range from 1.1% to 20.4%, which suggests that disc pressure was not purely hydrostatic. This may have contributed to the difference between the responses of the FBG and strain gauge sensors.