A smart insole for monitoring plantar pressure based on the fiber Bragg grating sensing technique

Abstract

The analysis of plantar pressure distribution is essential in the field of biomedical and sports-related applications. In this study, a smart insole was developed for the measurement of plantar pressure distribution and the evaluation of body postures using optical fiber Bragg grating (FBG) sensing technology. Four FBG sensors characterized by four different center Bragg wavelengths, 1528 ± 0.3, 1532 ± 0.3, 1535 ± 0.3 and 1539 ± 0.3 nm, were located at the first metatarsus, third metatarsus, fifth metatarsus and heel position, respectively. The measurement sensitivity of all the FBG sensors was 0.000412 nm/kPa, approximately. Silica gel material of modulus = 10 MPa was selected to incorporate the FBG sensors. All FBG sensors were multiplexed together with one optical fiber cable. The performance and functional properties of all FBG-based pressure sensors were calibrated in the laboratory to evaluate plantar pressure distribution. A male subject was selected for performing four tasks, namely standing in an upright position, leaning forward, squat position and forward fold. During standing tests, plantar pressure observed at the heel position was around 57% higher than that at the first and third metatarsus, while the pressure of the fifth metatarsus position presents minimal pressure, which is only 37% that of the pressure of the heel position. When the subject performs leaning forward, the squat position and forward fold posture, the first and third metatarsi show maximum pressure, while the pressure decreases at the fifth metatarsus position. However, almost zero pressure is observed at the heel position when the subject changes the body postures of leaning forward, squat and forward fold posture. The extreme pressure of the forward fold posture was 1750 kPa acquired at the first metatarsus, which is 52% and 62% higher than those at the fifth and third metatarsi, respectively. Therefore, the smart insole successfully recorded both plantar pressure distribution and body posture changes regarding the wavelength values collected by the FBG sensors.

Keywords

smart insole fiber Bragg grating measurement plantar pressure body posture

Pressure measurement in human physiologic systems is an essential subset. The study of the pressure acting between the plantar surface of the foot and a supporting surface is known as “pedobarography” (pedes referring to the foot, and baros referring to “weight” or “pressure”). This type of study is mainly used for biomechanical gait analysis and body posture. Analysis of foot plantar pressure distribution has received much consideration in the field of biomedical and sports-related applications. Recent advanced applications include footwear design,¹ biometrics,² monitoring posture allocations,³ human identification, rehabilitation activities,⁴ different sports performance analysis, especially athlete performance,⁵ monitoring the plantar pressure of obese people, footwear evaluation, gait analysis^6–8 and medical diagnostics.⁹ It is essential to diagnose foot-related problems at an early stage for maintaining good foot health, injury prevention, risk management and general wellbeing.

Measurement and analysis of foot plantar pressure can be generally categorized into two main types: platform systems and in-shoe systems.

Many platform devices have been used to measure plantar pressure when subjects walk on those platforms; for example, instrumented walkways, piezo dynamic platforms and instrumented floors. In addition, such platforms have also been developed that can function without any subject, primarily by videotape analysis and by using radar. In-shoe techniques are useful compared to the traditional platform system because they permit the most significant interface, that between the foot and shoe, to be monitored and they allow for increased versatility of measurement for the calculation of more robust statistical estimates. To measure plantar pressure distribution, many devices have been developed with significant differences in the data acquisition system and configuration of sensors, such as the number of sensors and their arrangements in the device.¹⁰ Various postures and locomotive activities, such as standing, walking and tilt walking, were performed while measuring foot pressure development.^11–15 Plantar pressure data is influenced by different parameters, such as gender, foot size, body mass index (BMI), changes in foot shape concerning age, the insole surface and the task being performed for testing, so different results can be achieved in each case.^16–20 In clinical gait analysis, plantar pressure measurement is broadly recognized as a critical tool, for example, in the case of gait abnormalities,²¹ diabetes mellitus, peripheral neuropathies and musculoskeletal disorders. Plantar pressure measurements have been used to study hallux valgus, neuropathic ulcers, Morton foot and flat foot.

Regarding previous studies, in 2016 Liang et al.²² propose a novel fiber-optic sensing system based on fiber Bragg grating (FBG) to analyze the foot plantar pressure to classify if the foot is a flat foot, a neutral foot, pronated, supinated or cavus. The primary objective of their work was to examine the functionality and feasibility of the FBG sensor system to measure the plantar pressure of the foot. Recently, in 2017, Zhang et al.²³ presented a sensing platform based on FBG using three-dimensional (3D) printing technology for plantar pressure measurement. The 3D printing technique was implemented for the manufacture of entirely sensing components (pressure sensor) as well as the complete platform. PLA (polylactic acid) material was used in 3D printing. FBG-based pressure sensors were encapsulated at four plantar locations of the foot, namely the first metatarsus, second metatarsus, mid-foot and heel position. It was observed that all of the pressure sensors (without a mid-foot position) present a significant rise in plantar pressure instantly after preliminary loading. The maximum pressure was sensed at the heel position but was negligible in the mid-foot area, while the pressure change of the first and second metatarsi lies between the heel and the mid-foot position.

More recently, in 2017, Domingues et al.²⁴ used cork material to design and insole and embed a six-FBG sensor network for the continuous monitoring of VGRFs (vertical ground reaction forces) during gait. Tavares et al.²⁵ proposed a system for the continuous monitoring of the plantar pressure and shear together during gait. The shear force was prevailing at the completion of the stance phase and the maximum shear stress was detected at the rising of the heel and the toe-off phase, which relates to the backward acceleration force under the metatarsal areas.

To date, several studies have been made on measuring plantar pressure during the gait cycle or using a platform system, but no reports have been presented using a smart insole for plantar pressure measurement during different body postures. The attainment of our research regarding the effects of plantar pressure during different body postures is essential for the field.

This paper presents a novel FBG-based smart insole for monitoring plantar pressure during various body postures. In the second section, the design, fabrication and package methods of this new sensor are presented in detail and, before monitoring tests, the sensor performance was inspected in the laboratory. The third section deals with the results and analysis of the data acquired during the plantar pressure distribution and performing the different body postures by the male subject. Finally, the main conclusions from this work are given in the fourth section.

Fiber Bragg grating-based smart insole for plantar pressure monitoring

Working principle of the FBG sensor

One of the most commonly used optical sensor is the FBG, which has a broad and significant role in optical communication. FBG is a periodic variation of the refractive index in the core of a single-mode optical fiber, which leads to the change in the strain and temperature of different structural components. Figure 1 shows the basic working principle of a single FBG sensor. When broadband light, namely induced light, passes through the Bragg grating section of the optical fiber, it splits into two wavelengths. Only the particular wavelength known as the Bragg wavelength will be reflected back, as given by Kersey et al.,²⁶ while all others will transmit as shown in Figure 1. Numerous techniques can be exploited to analyze the reflected spectrum of a Bragg grating. The most common are the use of optical spectrum analyzers (OSAs), tunable filters and interferometers. Usually, for commercial applications of Bragg grating sensors, OSAs are used due to their low cost and portability with respect to the other options. The principle for using FBGs as sensing devices is that the Bragg grating resonance (the central wavelength of back-reflected light from the grating) depends on the effective refractive index of the core of the optical fiber and the grating period. External mechanical and temperature variations lead to a related shift of central Bragg wavelength inside an optical fiber sensor, as indicated by the reflected light or transmitted light. The wavelength change against strain $Δ ɛ$ and temperature $Δ T$ of a standard single-mode optical fiber can be obtained as²⁷

\frac{Δ λ}{λ} = (1 - p_{eff}) Δ ɛ + (α + ξ) Δ T

(1)

where

Δ λ

and λ are the wavelength change and magnitude of the central Bragg wavelength reflected at the FBG sensing section, respectively, and

p_{eff}

, α and ξ are the photo-elastic parameter, thermal expansion coefficient and thermal-optic coefficient of the optical fiber core, respectively.

Figure 1.

Basic working principle of the fiber Bragg grating sensor.

FBG sensors have various inherent advantages over conventional electrical sensors, such as light weight, small size, frequency modulation, immunity to electromagnetic interference (EMC), non-conductivity, resistance to corrosion, ease of installation, ability to transfer measurement signals over a long distance (about 10 km) without any loss of the signal and their inherent ability to serve as both sensing elements and the signal transmission medium.²⁸ Other sensors when incorporated in insoles may affect the comfort of the subject during the measurement of plantar pressure. These advantages are beneficial for numerous applications, such as foot plantar pressure measuring, structural health monitoring,²⁹ medical applications^30–32 and seismic monitoring,³³ especially for such applications where strong EMC exists or non-conductivity is required, such as nuclear reactors³⁴ and hydrogen tanks.³⁵

Encapsulation of a FBG sensor

The main purpose of this study is to design a smart insole for the measurement of plantar pressure. Synthetic silica gel (rubber) material was used as the base material for pressure measurement. Silica gel is very flexible and elastic and it is easy to incorporate the FBG sensor in silica gel. Synthetic silica gel was purchased from one Chinese company, namely Zhang Hefei Xin Zhong Environmental Protection Technology Co., Ltd. Both the FBG-based pressure sensor and the complete insole were designed by using silica gel material, which also helps to shield the sensing part of the FBG sensor. The reason for choosing the same material (silica gel) for the pressure sensor and insole was that the fixing of the pressure sensor into the insole was made it easier to achieve a smooth surface on the insole even after inserting the pressure sensor, and it gives comfort to the subject during testing. Figure 2 shows the silica gel-based smart insole with four FBG sensors. Four small rectangular (silica gel) sensing components characterized by the dimensions of 3 cm length, 1.5 cm width and 0.4 cm height were fabricated as base elements for pressure measurement. Firstly, a groove was created in the middle of the pressure sensor with length of 1 cm. Only the sensing section of the FBG sensor was incorporated into the groove, but before integrating the FBG, the groove should be opened slightly by hand, as shown in Figure 2. Secondly, The FBG sensor was prestressed by hand from both ends and then placed in the groove, because the encapsulation of the prestressed FBG can give better results. Epoxy resin was used to adhere the FBG sensor into the groove of the silica gel. Thirdly, two different epoxy resins (equal ratio 1:1) were used to fix the FBG sensor into the groove of the silica gel. Epoxy resins are used at that time when the FBG sensor is prestressed and placed into the groove of the silica gel. Lastly, after applying resin into the groove of the pressure sensor, it is dried at room temperature (25℃) for about 5 hours. During testing, there was no change in temperature occurring at FBG positions, because all the tests were performed at room temperature (25 ± 2℃).

Figure 2.

Schematic diagram of the silica gel-based smart insole with four fiber Bragg grating sensors.

A size 42 (right foot) men’s insole was selected for fabricating the smart insole by using synthetic silica gel. Four different positions were marked on the silica gel insole for pressure sensors, as shown in Figure 2. A rectangular-shaped hole (with the same dimensions as a pressure sensor) was cut at the marked positions of the insole to fix the sensing portion of the FBG at the exact positions (first, fifth and third metatarsi and heel position) on the silica gel insole. These marked positions were select by the maximum pressure observed on the insole during standing by the subject.³⁶ In the present study, the four pressure sensors were placed at the first metatarsus, fifth metatarsus, third metatarsus and the heel position, which are characterized by four different initial center Bragg wavelengths, 1528 ± 0.3, 1532 ± 0.3, 1535 ± 0.3 and 1539 ± 0.3 nm, respectively. The reasons for choosing different wavelengths for each FBG are that, firstly, all FBGs were connected with a single optical wire and, secondly, to avoid the overlapping of FBG center wavelengths.

Figure 3 shows the entire FBG-based monitoring system for plantar pressure measurement, including a broadband light source for providing a wide light spectrum, an optical interrogator for receiving and analyzing the reflected light signal coming from the sensing section of the FBG, a wireless router for transferring the data from the interrogator to the computer, an optical fiber cable, a computer for data collection and a smart insole for plantar pressure measurement. The details of each are present in Table 1. The optical interrogator records almost 20 different values of wavelength change each second.

Figure 3.

Fiber Bragg grating (FBG)-based smart insole with the complete plantar pressure monitoring system.

Table 1.

Specifications of the materials used for plantar pressure measurement

1. Fiber Bragg grating (FBG) sensors
Sensors				Wavelength				Sensitivity
FBG 1				1528 ± 0.3 nm				0.000412 nm/kPa
FBG 2				1532 ± 0.3 nm				0.000412 nm/kPa
FBG 3				1535 ± 0.3 nm				0.000412 nm/kPa
FBG 4				1539 ± 0.3 nm				0.000412 nm/kPa
2. Silica gel
Supplier	Modulus		Thickness			Color		Specific gravity		Temperature
Zhang Hefei Xin Zhong Environmental protection Co. Ltd	10 MPa		4.0 mm			Red		1.3 g/cm³		–60 to + 200℃
3. Wi-Fi router
Company	Size	Color			Voltage range		Ethernet		Wi-Fi sock		System memory
Xiaomi (Mi)	Mini	White			100–250 V		2 × LAN, 1 × WAN		Mediatek MT7620A		128 MB DDR2
4. Interrogator
Model no:	Scan frequency	Wavelength range			No: optical channels		FBG sensors capacity		Interfaces		Input voltage
Sm 125	1 Hz	1510–1590 nm			08		60–120		Ethernet		7–36 VDC

Calibration test

Before starting the experiments, a calibration test was carried out to certify the performance of the FBG-based pressure sensors for plantar pressure measurement. The analysis was performed in an isolated laboratory at room temperature. For the calibration test, all the FBG-based pressure sensors were calibrated that were mounted at the first, fifth and third metatarsi and the heel position of the insole.

As the positions of all the FBG sensors on the insole were different, all FBGs sensors cannot be weighted at the same time accurately. This is why each sensor was calibrated separately. Three dead weights of 5 kg each, having dimensions of 7.5 cm length and 3.8 cm width, were used for loading.

The time for the calibration test was set as 1:10 seconds and controlled by a stopwatch. The pressure sensors were placed on a smooth surface, and for a preliminary 10 seconds the sensors carry no weight. Each sensor was tested separately. After 10 seconds, 5 kg dead weight was put on the pressure sensor. The same procedure was repeated three times after every 10 second interval for a simulation of the loading process. Just after the completing of the loading process, the unloading process was continued by removing the dead weights after every 10 second interval. Figure 4 illustrates the relationships between the wavelength change of all the FBG pressure sensors and vertical pressure in the calibration test. An almost linear relationship was achieved between the wavelength change and vertical pressure, and the slope ratio acquired by this relationship can be used as a conversion coefficient to determine the related plantar pressure. The maximum pressure in this relationship approached around 653 kPa. The wavelength changes observed at FBG 1, FBG 2, FBG 3 and FBG 4 were 0.243, 0.259, 0.269 and 0.281 nm, respectively. The other three FBG-based pressure sensors were calibrated using the same experimental process in the laboratory and achieved wavelength–pressure slope ratios. Therefore, the measured pressures of the four plantar positions were finally obtained using the corresponding FBG-based pressure sensors mounted on the insole.

Figure 4.

A typical relationship between the Bragg wavelength shift and applied vertical load in the calibration test. Points and lines correspondent to the experimental data and linear fits, respectively (R²= 0.996–0.999). FBG: fiber Bragg grating.

Plantar pressure monitoring tests

Tested subject and experimental procedures

Considering the availability of FBGs and limited testing time, one male subject was select for the plantar pressure measurement. The subject was 24 years old and 1.6 m tall. The initial weight was 54 kg with a BMI of 33.75. The fabricated insole was placed on the floor. Another layer of silica gel with the same dimensions as the fabricated insole was used to cover the fabricated insole so that the foot of the subject cannot directly touch the optical wires. Two plain layers of silica gel were used for the left foot to balance the subject’s weight. Before starting the tests, the subject was trained and asked to make body balance as much as possible, and the left and right foot should equally distribute body weight. In this study, the subject performed four main tasks using a smart insole. These postures are very common in a human being’s daily activates and so were selected to sense the deformation analyzed by the FBG-based smart insole, described in detail below.

First task “standing in the upright position”: in this test, the subject stands straight keeping the right foot on the fabricated smart insole for 20 seconds, and after 20 seconds, the subject removes the right foot from the fabricated insole. The subject put the foot on the fabricated insole in such a way all the FBG sensors were forced simultaneously and the subject maintained the standing position for 20 seconds. The same practice was done three times by the subject and the output wavelength was recorded.

Second task “leaning forward”: this posture was performed by the subject in such a way that only the upper part of the body (from head to hip) moved forward while the lower part of the body (legs and foot) remained straight. The subject put his hands on a chair and applied very low force to maintain body balance, as shown in Figure 5(a), in which 1, 2 and 3 describe the steps of the leaning forward posture. During this test a chart was prepared at which each centimeter was mentioned; this was pasted on the wall and the subject stood straight beside the wall to perform the leaning forward task. Initially, the body was in the straight position for 10 seconds (first step). The subject was asked to stand straight in such a way that the maximum pressure was observed at the heel area; however, there was very negligible pressure on the first and third metatarsi. The body moves forward up to 9 cm and stops for 10 seconds (second step). After 10 seconds, the body moves further forward, and the same procedure is carried out for the third and fourth steps. The total time for this posture was 40 seconds, which was controlled by the stopwatch.

Figure 5.

The different body postures performed by the subject: (a) leaning forward posture; (b) squat posture; (c) forward fold posture.

Third task “squat position”: in this posture, initially the subject stands straight on the fabricated insole for 10 seconds. In the second step, the subject makes a chair pose for 10 seconds. In the third and final step, the subject sits on the foot by bending both feet for 10 seconds, as shown in Figure 5(b). For this posture, the total time recorded was 30 seconds. The main purpose of using the chair in the forward bend and chair posture is to balance the subject’s weight and to minimize errors during testing.

Fourth task “forward fold”: this task comprises three steps, and the duration of all the steps was maintained at 10 seconds. In the first step, the subject stands straight, the same as in all the cases, with the only difference that the hands of the subject were raised up. In the second step, the upper limb part of the subject folds forward until making a 90° angle, as shown in Figure 5(c). The hands of the subject remain straight in this step. In the third and final step, the subject further folds forward until the hands of the subject touch the ground. This posture also takes 30 seconds to complete. Bear in mind that Figure 5 only shows the way that subject performs the body postures, but in actuality, all the poses were performed using just the smart insole and not the smart shoe.

Results and discussion

To get a clear idea about the effect of postures changes on the plantar pressure distribution of the male subject, all the FBG wavelength data of each posture were transformed into pressure. The conversion of wavelength into pressure was done by using the wavelength-to-pressure conversion coefficient obtained from Figure 4. Figure 6(a) illustrates a variation of plantar pressure of all FBG sensors against elapsed time during standing in an upright position (first task). The four FBG pressure sensors mounted at the first metatarsus, fifth metatarsus, third metatarsus and heel position are characterized by initial pressure values of 3674.05, 3680.90, 3693.21 and 3703.77 kPa, respectively. As introduced in the Tested subject and experimental procedures section, the subject changed postures at the times of 400, 800, 1200 and 1600 seconds. It is evident in Figure 6(a) that all FBG sensors show sudden changes in the wavelength at the times of 400, 800, 1200 and 1600 seconds, indicating that the pressure shows a corresponding change in vertical pressure from the subject. At the first metatarsus position, the pressure seems not to be stable during the first trial, because at that time the subject moved his foot during testing. However, it can also be seen in Figure 6(a) that in second and third trials the subject maintains balance during standing. All FBG sensors can be used to identify the signal change of all sensors subjected to different vertical pressures. The increase in wavelength change of the FBG was dependent on the stress applied at the particular position, supposing that while standing straight on the insole, the maximum pressure is applied on the heel area and the minimum pressure is observed at the third metatarsus position. It also depends on the weight of the subject, as different subjects have different weights. The plantar pressure of the heel position was around 5547 kPa (maximum) when the subject stood straight on the insole. However, for the FBG positioned at the third metatarsus, the pressure value recorded was around 1767 kPa (minimum) as the subject stood onto the insole. The pressure variation of the first metatarsus and the fifth metatarsus lies between third metatarsus and the heel position. When the subject removes the right foot from the fabricated insole, the pressure of all FBG-based pressure sensors reach to zero levels. Almost the same results of all the FBG pressure sensors can be seen when the test is repeated for the second and third times. The measurement sensitivity of all the FBG sensors encapsulated in the silica gel-based insole was 0.00041216 nm/kPa, approximately. This experiment shows that all the FBG-based pressure sensors of the smart insole are reliable and accurate for plantar pressure measurement. A new parameter, that is, the “pressure ratio,” was introduced, which is a ratio of all the measured pressure values divided by the maximum pressure value for the quantitative comparison between all the measured pressures values, so that deep analysis of the plantar pressure measurement can be done. For the standing test, all the measured pressure values were divided by the heel pressure because of the maximum pressure. Figure 6(b) describes the relationships between the pressure ratios of three plantar positions (the first, fifth and third metatarsi) and the elapsed time. The maximum pressure ratios were at the first metatarsus (0.540) then at the fifth metatarsus (0.332) and, finally, the minimum pressure ratio was at the third metatarsus (0.287). These values of the pressure ratios exhibit that when the subject stands straight on the insole, the first metatarsus position of the foot tolerates maximum pressure, after the heel position.

Figure 6.

(a) Pressure change of fiber Bragg grating sensors against time during a standing test of the male subject. (b) Relationship of the pressure ratio against time during the standing test. MP: metatarsus position.

Figure 7(a) shows the plantar pressure change against time when the body was leaning forward (second task). The preliminary pressure values at the first metatarsus, fifth metatarsus, third metatarsus and heel position were recorded as around 21, 3083, 39 and 5048 kPa, respectively, when the subject just stood straight on the insole. When the subject started to move forward up to 9 cm, the pressure value increased at the first metatarsus position. However, when the subject stopped moving for 20 seconds, it can be seen that the pressure becomes stable at 3240 kPa for that duration of time. In the second and third steps of the leaning forward posture, the pressure increases more as the body moves further forward. The highest pressure of the first FBG sensor positioned at the first metatarsus position is achieved in the fourth step, that is, 3807 kPa. The second FBG sensor shows decreasing pressure at every step as the body moves forward. As the body moves for the leaning forward posture, the contact between the foot and the sensor decrease at every step. After the second step of forward movement by the subject, the pressure almost reaches the minimum level, that is, 68 kPa. The third FBG-based pressure sensor positioned at the center of the smart insole means the third metatarsus position, so at each step of the subject’s moving forward the increase in the pressure pragmatic doubles as compared to the previous step. At the third step, the maximum pressure was sensed was around 3091 kPa. Finally, the pressure change of the fourth FBG-based pressure sensor positioned at the heel position of the insole shows decreased pressure because when the subject moves forward to change the posture, the heel of the subject rises at each forwarding step. The substantial pressure reduction at each step of the heel position specifies that the whole body weight transmuted to the forefoot area of the insole during the forwarding moment by the subject. In this posture, the maximum pressure was achieved at the first metatarsus position because of the forwarding moment by the subject. In this case, the pressure ratios were achieved by dividing all the measured pressure values (including the second metatarsus, third metatarsus and heel position) by the pressure values of the first metatarsus position. Figure 7(b) describes the relationship of pressure ratios against time of the forward bend posture. The values of the pressure ratios of the leaning forward posture at the heel position were opposite as compared to the standing test, because when the subject performed the leaning forward posture, at each step the pressure was decreased at the heel position, but at the first metatarsus position, the pressure was increased at each consecutive step.

Figure 7.

(a) Pressure change of FBG sensors against time during the leaning forward posture by a male subject. (b) Relationship of the pressure ratio against time during the forward bend posture. MT: metatarsus.

For the squat posture (third task), the plantar pressure change is as shown in Figure 8(a). In this case, each sensor responds differently. As described in the Tested subject and experimental procedures section, this posture is divided into three main steps, that is, standing in an upright position, squat position and sitting on the feet by the subject. The duration of each step was set to 10 seconds. The first sensor positioned at the first metatarsus shows increased pressure in the squat position (second step), and a further increase in the pressure can be observed when the subject sits on his feet (third step). In this posture, the feet of the subject also start to bend when the subject changes posture from a straight position to the chair pose. The bend angle observed at the third step (when the subject sits on his feet) was greater as compared to the second step (when the subject was in the squat position). Among all the postures performed by the subject, the toughest challenge was to make a squat position, because in this posture controlling body balance was a challenge for the subject. The second FBG-based pressure sensor positioned at the fifth metatarsus shows a decrease in pressure at the second step as well as in the third step, because as the subject changed posture (standing in upright position–squat position–sit on feet) the fifth metatarsus portion of the foot started to leave the fabricated insole. The results of the first pressure sensor and the second pressure sensor are opposite to each other. The first sensor, placed at the first metatarsus, shows maximum pressure increase as compared to all other sensors because, firstly, the position of the first sensor lies at the forefoot area of the insole, as shown in Figure 2. Secondly, when the subject moves for the squat and sit posture, the whole body weight was transferred to the forefoot area. When the subject sits on his feet (third step), the maximum pressure was achieved at around 3367 kPa. The pressure sensor present at the heel position records nearly zero pressure in the second step and third step, because when the subject moves for the squat and sitting postures, the heel position of the foot rises, the same as for the leaning forward posture. So, no pressure was obtained during the posture change. The rise of the first metatarsus and third metatarsus pressure was complemented by the reduction of the fifth metatarsus and heel position, which shows that in order to keep the body balanced by the subject during the second step (squat position), the maximum pressure shifts to the forefoot area (first and third metatarsi). The pressure change in the third step at the first metatarsus was not stable due to poor balance by the subject, while at all others steps of the squat posture the subject maintained very good balance during the performance of the task. We conclude that, in this posture, the maximum pressure appears at the first pressure sensor positioned at the first metatarsus. So, for calculating the pressure ratio for this posture, all the measured pressures (includes the third metatarsus, fifth metatarsus and heel position) were divided by the value of the first pressure sensor. The results of the pressure ratio elapsed time of the chair posture can be seen in Figure 8(b). The highest pressure ratio in the case of the squat posture was 0.619, which was observed at the third metatarsus.

Figure 8.

(a) Pressure change of fiber Bragg grating sensors with elapsed time during the squat posture of the male subject. (b) Relationship of pressure ratio against time during the squat posture. MP: metatarsus position.

The change in the plantar pressure against time of the forward fold posture (fourth task) is shown in Figure 9(a). The initial pressure values during the forward fold posture were almost the same as for the chair and forward bend postures. The pressure increased on the first sensor positioned at the first metatarsus in the second as well as the third steps of the forward fold body posture. The second sensor and the third sensor (positioned at the fifth metatarsus and third metatarsus, respectively) respond quite similar to the pressure increase in the second step when the subject makes a 90° angle and keeps his arms straight, as shown in Figure 5. When the comparison was made among the first FBG sensor and second FBG sensor, a large difference in the pressure was observed in the second step, while a negligible difference can be seen in the third step of the forward fold posture. As the subject moves toward the third step (in which the subject’s hands were touching the ground), the pressure increases more as compared to the second step at all positions except the heel. In this posture, the maximum pressure occurs on the first pressure sensor positioned at the first metatarsus (1750 kPa) during the third step of the posture. When the subject moves toward the third step and folds the body, the pressure was also transformed from the heel position to the forefoot area of the fabricated insole. The sensor placed at the heel position shows a decrease in pressure at the second step, and in the third step, further diminution in the pressure can be observed as the whole body weight shifted toward the forefoot area of the subject. During performance of this posture by the subject, the heel of the foot completely rises only at the third step. As compared to the squat posture, in this posture the feet of the subject were not bent during the change of the body posture. Figure 9(b) shows the relationship between the pressure ratios versus time during the forward fold posture. Like the squat posture, the maximum pressure was observed on the first pressure sensor. So, all the measured pressures are divided by the first pressure sensor value to get the pressure ratio.

Figure 9.

(a) Pressure change of the fiber Bragg grating sensors with elapsed time during the forward fold posture of the male subject. (b) Relationship of pressure ratio against time during the forward fold posture. MP: metatarsus position.

For the comparison, all the peak pressure values of the first metatarsus, fifth metatarsus, third metatarsus and heel position perceived at the final step of standing in the upright position, leaning forward, squat posture and forward fold posture can be seen in Figure 10. The test results show that the maximum pressure was obtained at all the plantar positions during the standing posture, as in this posture the subject stood straight on the fabricated insole. In comparison with the leaning forward and squat posture, about 61% and 72% pressure decreased during the leaning forward posture than in the squat posture at the first metatarsus and third metatarsus, respectively. However, nearly zero pressure was gained at the fifth metatarsus and the heel position during the last step of both postures. The extreme pressure of the forward fold posture was 1750 kPa acquired at the first metatarsus, which is 52% and 62% higher than the fifth and third metatarsi respectively.

Figure 10.

Peak pressures observed at the final step of different body postures.

In the present study, only one male subject was select for the measurement tests; different subjects can be taken into account with different ages, weights and heights as the plantar pressure distribution may vary for each subject. In the future, further body postures can also be performed to explore the relationship between the plantar pressure distribution and body posture.

Conclusions

This paper demonstrates the design and fabrication of a novel FBG-based smart insole for plantar pressure measurement at different body postures of a male subject. The smart insole was instrumented with four FBG networks placed at critical points of the insole. FBG-based pressure sensors were calibrated in the laboratory and assembled into a smart insole for monitoring plantar pressure distribution. Typical findings and the conclusion are drawn as follows.

Calibration test results showed that the new silica gel-based smart insole exhibited a linear wavelength change against the vertical pressure. The results obtained validate the precision and reliability of the proposed system.

The standing test was conducted to evaluate the performance of all the FBG-based pressure sensors encapsulated at first, fifth, third metatarsi and the heel position. The results showed that all the FBG-based pressure sensors are well-founded. Therefore, various body postures regarding plantar pressure distribution can be inspected using this smart insole. The maximum pressure is found on the heel position during the standing test, while the first metatarsus, fifth metatarsus and the third metatarsus show a 43%, 63% and 68% decrease in pressure, respectively, compared with the heel position. Therefore, the safety of the heel area is immensely important.

The postures included (leaning forward, squat position and forward fold) show a systematic increase in pressure at first and third metatarsi after the first step by the subject because of the position these sensors at the forefoot area of the insole. In a comparison of the above three postures, the maximum pressure was achieved during the squat posture. The percentages of the pressure decrease for the leaning forward posture and forward fold posture were 61% and 48%, respectively, compared with the squat posture at the first metatarsus, while at the third metatarsus 71.5% and 68% reduction in pressure was detected during the leaning forward and forward fold postures, respectively. The pressure sensor encapsulated at the fifth metatarsus position exhibited a decrease in pressure, while the heel position senor reached almost to zero levels as the subject moved for the leaning forward, squat position and forward fold posture.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities (Project No: 17D110116) and the Key Laboratory of Functional Textiles of The Education Department of Henan Province.

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