Review Reports - A Soft Wearable and Fully-Textile Piezoresistive Sensor for Plantar Pressure Capturing

Round 1

Reviewer 1 Report

The work will benefit from the following comments:

Author should clarify the concept of piezoresistive working principle in introduction in order to make clear the whole perception of the work to the reader.
Piezoresitive sensors are prone to hysteresis, this effect was not discussed.
Author should show a relationship between the change in distance in the conductors and the linearity or drift achieved with the piezoresitive sensors.
Author has analysed the sensor performance such as variation of resistance, response time, recovery time, stability etc. at mostly lower pressure (5kPa, as mention in Figure 3 and explained in page 5, line 165-181), whereas the application environment for shoe insole (as demonstrated by walking) is several times higher in weight. So the question is, how Author’s statements (stability, resistance variation, recover time etc.) are valid for the real application? as it can be seen in Figure 5 (a) the stability/response at elevated weight drastically changed compared to the lower weight..
Page 9, line 287: Figure Movie S1 (the response /pressure distribution of the shoe sensor by pressure) missed the correlation of the sensor-shoe by walking. Such response (video) by the real demonstration e.g; walking would be provided.
While I printed out the paper, I observed, the quality of the graphs in Figure 3 were poor to identify the scale and text/unit etc. Author should provide improved version of graphs.
The manuscript contains text error such as “chang” in page 6, line 198; “success fully” in page 12, line 309 etc. Author should go through whole manuscript and modify the text and improve the language quality.

Author Response

Dear Ms. Amber Zheng

Thank you very much for your kind letter, along with the constructive comments of the reviewers concerning our manuscript (micromachines-1059581). Our manuscript has been revised according to the reviewers’ comments. The manuscript has been carefully revised according to reviewers’ suggestions. All revised sections are highlighted with high light. The detailed responses to reviewer’s comments are seen as follows. Thanks for your time and consideration.

To Reviewer 1

Comment 1：Author should clarify the concept of piezoresistive working principle in introduction in order to make clear the whole perception of the work to the reader.

Response 1: Thank you for your suggestion. The piezoresistive working principle has been clarified in the introduction. The more detailed description has been supplemented in “1. Introduction”.

As a result, a significant number of flexible textile devices with high sensitivity and biocompatibility with skin have been designed for capturing the human movement [8-11]. Particularly piezoresistive pressure sensors possess tremendous potential for wearable device applications due to its excellent sensitivity, durability and biocompatibility with human skin [12-17]. The external pressure stimuli would induce the change of resistance, which can be measured and utilized to feedback the information state of human body. For instance, an ultrasensitive fiber-based piezoresistive sensor that can be used to observe the walking signal was fabricated [18].

Comment 2：Piezoresitive sensors are prone to hysteresis, this effect was not discussed.

Response 2: Thank you for your advice. The more detailed discussion on hysteresis effect of the TPRS has been supplemented in “3. Results”.

Figure 3. (a) The sensitivity performace of TPRS. (b) Real-time response of the sensor under an applied pressure of 5 KPa. (c) Pressure response showing a single cycle, and the corresponding (d) response time and (e) releasing time. (f) The hysteresis curves of TPRS.

Under a larger force, the rGO cotton fabric electrode and Ag fabric circuit electrode are in close contact, and recovery needs a longer time. Hysteresis is mainly caused by the interaction between the rGO cotton fabric electrode and Ag fabric circuit electrode. Small degree of hysteresis (DH) indicates lower hysteresis of the TPRS. And the DH can be calculated by the following formula：

where AL and AUare the area of loading and unloading response curves, respectively. The DH value of TPRS is 28.6%.

Comment 3：Author should show a relationship between the change in distance in the conductors and the linearity or drift achieved with the piezoresitive sensors.

Response 3: Thank you for your advice. When the TPRS is subjected to external pressure, the distance between the two fabric electrodes is very small due to the small thickness of the sensor itself. When the pressure is applied, it is difficult to observe the performance of the sensor with the change of the distance between the two fabric electrodes.

Comment 4：Author has analysed the sensor performance such as variation of resistance, response time, recovery time, stability etc. at mostly lower pressure (5kPa, as mention in Figure 3 and explained in page 5, line 165-181), whereas the application environment for shoe insole (as demonstrated by walking) is several times higher in weight. So the question is, how Author’s statements (stability, resistance variation, recover time etc.) are valid for the real application? as it can be seen in Figure 5 (a) the stability/response at elevated weight drastically changed compared to the lower weight.

Response 4: In this paper, the proposed TPRS has a sensitivity of 3.96kPa^-1 in the range of 0-36kPa. However, at higher pressure (800kPa), the change of sensor resistance is not particularly obvious compared with that at lower pressure, but the TPRS can also respond accordingly, as shown in Fig. 5 (a) and Fig. S2. Besides, the real test was shown in Figure movie S1.

For capturing plantar pressure, the sensor needs to have a high sensitivity and this ability is also tested, as shown in Figure 5a. It can be seen that the TPRS is more sensitive in a smaller pressure range, but less sensitive to pressure in a larger pressure range. This result is in good agreement with the above sensitivity, indicating that the TPRS is particularly sensitive below 50kPa pressure. And the TPRS is capable of operating up to the pressure of 800 kPa before failure (Figure S2). The results show that the TPRS can quickly feedback the pressure without damage, even under the condition of high pressure.

Comment 5：Page 9, line 287: Figure Movie S1 (the response /pressure distribution of the shoe sensor by pressure) missed the correlation of the sensor-shoe by walking. Such response (video) by the real demonstration e.g; walking would be provided.

Response 5: Thank you for your suggestion. The response of each TPRS of the insole during testing has been supplemented to Figure Movie S2.

Comment 6：While I printed out the paper, I observed, the quality of the graphs in Figure 3 were poor to identify the scale and text/unit etc. Author should provide improved version of graphs.

Response 6: Thank you for your advice. The quality of the graphs in Figure 3 has been improved.

Comment 7：The manuscript contains text error such as “chang” in page 6, line 198; “success fully” in page 12, line 309 etc. Author should go through whole manuscript and modify the text and improve the language quality.

Response 7: Thank you for your advice. I have checked the full text carefully and corrected the grammar and expressions mistakes in the article.

Abstract: The trends of health wearable monitoring system have led to growing demands for gait capturing device. The comfortability and durability under repeated stress in the existing sensor-enabled footwear are still problems. Herein, a flexible textile piezoresistive sensor (TPRS) consisting of rGO-cotton fabricelectrode and Ag fabric circuit electrode is prepared. Based on the mechanical and electrical properties of two fabric electrodes, the TPRS exhibits superior sensing performance, which includes high sensitivity of 3.96kPa-¹ in the lower pressure range of 0-36kPa, wide force range (0-100 kPa), fast response time (170 ms), remarkable durability stability (1000 cycles) and detection ability in different pressures. For practical application of capturing plantar pressure, six TPRSs are mounted on a flexible printed circuit board and integrated into an insole. The dynamic plantar pressure distribution is displayed through drawing the pressure maps during walking. The proposed full textile piezoresistive sensor is a strong candidate for next-generation plantar pressure wearables monitoring device.

Wearable plantar pressure capturing system can greatly benefit from the textile piezoresistive pressure sensors. Analysis of dynamic plantar pressure patterns are used to early alarm and prevention of foot deformities or discovery and rehabilitation monitoring in the advanced stages [19, 20].

The sensing mechanism of TPRS is explained with the change in the resistance of the contact between the rGO-cotton electrode and the Ag fabric circuit one upon the application of pressure over the outer sensor surfaces. Increasing pressure leads to the formation of small compressive deformations that enhance the contact between the two conductive fabrics and reduces the interlayer distance between them. Thus, the number of electrical pathways between the two electrodes increases. Also, upon initial contact between the two electrode parts, the resistance of the sensor decreases rapidly, while this decrease becomes gradual upon reaching full contact between the surfaces of the electrodes (Figure 5a). This behavior of change of the sensor resistance is explained with the fact that the graphene sheets are stacked together to form a graphite-like bulk body, which accelerates charge hopping between the overlapping graphene islands [37]. When high pressure is applied to the TPRS, charge hopping occurs between the overlapping graphene islands. After unloading the pressure, the TPRS recovers its initial shape, which results in decreasing the contact area and less electrical pathways. The changes of resistance upon the application of different pressures is also thoroughly tested.

To investigate the performance of TPFS, the relative current changes (ΔI/I0) versus pressure is showed in Figure 3a. And the sensitivity of the TPFS is 3.96kPa-1 in the lower pressure range of 0-36kPa while the sensitivity lowers to 0.49 kPa-1 in higher pressure range.

Interestingly, we observe that the resistance of the TPFS has a jump and then change rapidly as the pressure increases, as shown in yellow area of Figure 4b-d.

The TPRS and the custom sensor appear under the same external pressure exhibit different response. The change of TPRS is more intuitive and obvious after being subjected to external force, which makes the fabric sensor more suitable for the collection of human foot information.

In summary, we design a novel wearable piezoresistive sensor based on excellent mechanical and electrical properties of two kinds of fabric electrodes for capturing plantar pressure. And the smart insole is prepared by mounting TPRS into six different points on each insole. The TPRS demonstrated high sensitivity of 3.96kPa-1 in the lower pressure range of 0-36kPa in a wide pressure range (0-100 kPa), outstanding response to external pressure, stable durability (1000 cycles) and fast response time (170 ms) confirming its potential for real-time detecting human movement. And the develop monitoring insole takes advantage of high sensitivity, high resolution under different pressure and excellent durability, which allows stable plantar signals under different contact pressure.

Author Response File: Author Response.docx

Reviewer 2 Report

This work reports the textile piezoresistive sensors for plantar pressure monitoring purpose. Though this work mainly focuses on the device integration and actual plantar pressure monitoring, the other key part of this work is designing “Full Textile Piezoresistive Sensor” using graphene oxide and Ag paste. However, the material characterizations on each electrode are only three SEM images. It is very difficult to judge whether the electrodes were prepared well or not with such limited information. The material information of graphene oxide such as size, thickness and atomic composition should be provided, and their characterization data such as SEM, XRD and XPS should be provided as well. In addition, the reduced graphene oxide should be properly characterized using SEM, XRD, XPS, and Raman. The photographs of the samples and their optical microscope or low magnification SEM should be also provided to demonstrate the Ag electrode pattern and rGO-cotton sample. Lastly, the electrical properties of each electrode should be measured.

The other issue of this manuscript is there are too many typos from the abstract to the conclusion: rG-cotton, dection, differrent, invetigate, showned, interstingly, chang, and insloe, etc. I am not quite sure whether the authors carefully checked the grammar and expressions before submission. As a result, I do not recommend this manuscript to be published in Micromachines.

Author Response

Comment: This work reports the textile piezoresistive sensors for plantar pressure monitoring purpose. Though this work mainly focuses on the device integration and actual plantar pressure monitoring, the other key part of this work is designing “Full Textile Piezoresistive Sensor” using graphene oxide and Ag paste. However, the material characterizations on each electrode are only three SEM images. It is very difficult to judge whether the electrodes were prepared well or not with such limited information. The material information of graphene oxide such as size, thickness and atomic composition should be provided, and their characterization data such as SEM, XRD and XPS should be provided as well. In addition, the reduced graphene oxide should be properly characterized using SEM, XRD, XPS, and Raman. The photographs of the samples and their optical microscope or low magnification SEM should be also provided to demonstrate the Ag electrode pattern and rGO-cotton sample. Lastly, the electrical properties of each electrode should be measured. The other issue of this manuscript is there are too many typos from the abstract to the conclusion: rG-cotton, dection, differrent, invetigate, showned, interstingly, chang, and insloe, etc. I am not quite sure whether the authors carefully checked the grammar and expressions before submission. As a result, I do not recommend this manuscript to be published in Micromachines.

Response1: Thank you for your suggestion. For your suggestion, the characterization of rGO materials has been added in this paper. The more detailed description has been supplemented in “3. Results”.

Figure 2. The SEM images of the surface morphology of (a)control cooton fabric, (b) the Ag fabric circuit electrode and (c-d) the rGO-cotton electrode. (e) Raman spectrum of rGO cotton. (f) XRD spectrum of cotton and rGO-cotton.

Figure 2 shows the SEM images of morphology of the rGO-cotton electrode and the Ag fabric circuit electrode. Compared with the control cotton fabric (Figure 2a), a uniform coating layer of silver paste is formed on the surface of cotton fabric, as shown in Figure 2b. And the surface resistance of Ag fabric circuit electrode is 0.5Ω/cm (Figure S1a), which can be attributed to the excellent conductivity of silver paste. To be noticed, the reduced graphene oxide (rGO) is wrapped on the surface on the surface of cotton yarns, as shown in Figure 2c-d. The Raman spectroscopy is widely used to exhibit D and G bands from the graphitic structure (Figure 2e). The result shows that the D peak of rGO-cotton appeared at 1350 cm-1, and the G peak appeared at 1593 cm-1. The oxygen-containing groups in GO are effectively removed after reduction. Figure 2(f) compares the XRD patterns of cotton, r GO-cotton fabric. It can be seen that the (001) crystal plane of GO disappears and the characteristic peak of rGO at higher 2θ coincides with that of cotton fabric. And the surface resistance of rGO-cotton electrode is 16.3KΩ/cm (Figure S1b). A uniform rGO is successfully obtained on the surface of cotton fabric.

Response2: The photographs of rGO-cotton fabric electrode and Ag fabric circuit electrode and the measurement of surface resistance have been Supporting Informatin Figure S1.

Figure. S1 Surface resistance of (a) rGO cotton fabric electrode; (b) Ag fabric circuit electrode.

Response3: Thank you for your advice. I have checked the full text carefully and corrected the grammar and expressions mistakes in the article.

Interestingly, we observe that the resistance of the TPFS has a jump and then change rapidly as the pressure increases, as shown in yellow area of Figure 4b-d.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Most of my concerns had been addressed, but the authors should clarify the characteristic peak of graphene is whether (001) or (002) and is where exactly. And the characterization results are recommended to be supported by appropriate references. I would recommend this manuscript to be published after my concerns are addressed.

Author Response

Dear Ms. Amber Zheng

To Reviewer

Comment: Most of my concerns had been addressed, but the authors should clarify the characteristic peak of graphene is whether (001) or (002) and is where exactly. And the characterization results are recommended to be supported by appropriate references. I would recommend this manuscript to be published after my concerns are addressed.

Response: Thank you for your suggestion. The characterization analysis of rGO has been described in the article. The more detailed description has been supplemented in “3. Results”. And we have revised this article in detail according to the requirements of the journal and the template of the journal. At the same time, our article does not contain any previously published graphs / tables.

The oxygen-containing groups in GO are effectively removed after reduction. Figure 2(f) compares the XRD patterns of cotton, rGO-cotton fabric. X-ray diffraction pattern of rGO-cotton electrode shows major peak at 23.3° (002). And the (001) crystal plane of GO disappears and the characteristic peak of rGO at higher 2θ coincides with that of cotton fabric. This can be explained that the the oxygen functional groups disappear after reduction [37]. And the surface resistance of rGO-cotton electrode is 98.3KΩ/cm (Figure S1b). A uniform rGO is successfully obtained on the surface of cotton fabric.

Figure 2. The SEM images of the surface morphology of (a)control cooton fabric, (b) the Ag fabric circuit electrode and (c-d) the rGO-cotton electrode. (e) Raman spectrum of rGO-cotton. (f) XRD spectrum of cotton and rGO-cotton.

Kind regards,

Chaoxia Wang

Author Response File: Author Response.docx