Pressure-Reducing Design of 3D-Printed Diabetic Shoe Midsole Utilizing Auxetic Lattice Structure

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I read with great interest the article entitled: Pressure-reducing Design of 3D Printed Diabetic Shoe Midsole Auxetic Lattice Structure. The mechanical properties of auxetic lattice to reduce plantar pressure seems promising. The 20% decreased in metarsal head compared to non-auxetic lattice reach the goal recommended to prevent foot ulcer.

The article is well written and easy to read but some improvement are required in the global structure of the article. The introduction is too long; discussion must not repeat and/or presents results; the conclusion is also too long repeating some points of the discussion.

Methodology to assess insole property is adapted and well-presented.

The main concern is about title and conclusion. The main topic seems to be diabetic foot concern. However, the design of the study is not adapted to this specific population. Most diabetic patients with foot problem are overweight (simulation loaded mass point à 69.8 Kg) and not able to run. Moreover, diabetic patients with foot problem present neuropathy, foot deformities, limited joint mobility, reduced soft tissue plantar thickness leading to high plantar pressure. These patients require custom-made insole to reduced plantar pressure. These point should be discussed and clinical studies available in literature presented.

 

Other points:

-        Images available on line have a too low quality

-        Could we have more information on the print process? What is the fill density? How long is it to print an insole? Do you have any idea of the cost if you want to use it in routine practice? What about the durability of TPU with auxetic lattice?

-        Did you standardized the number of steps for in shoe pressure measurement?

-        A young modulus of 2 mPa seems high for off-loading concern? Even if not perfect, do you test the shore of the material full filled?

Best regards

Author Response

Thank you to the reviewers and editors. I have responded to the questions and suggestions as follows and revised the manuscript (the attached PDF file contains the responds to the comments and revisions of manuscript). The revised parts of the manuscript are also marked with other colors.

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Q1. The introduction is too long.

A1: Thank you for your suggestions. We have already shortened the length of the introduction section.

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Q2. Discussion must not repeat and/or presents results.

A2: Thank you for your suggestions. We have separated the research results that appeared in the discussion section and moved them to subsections 3.3 and 3.4 of the results section, making the content of the discussion section clearer.

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Q3. The conclusion is also too long repeating some points of the discussion.

A3: Thank you for your suggestions. We have removed the unnecessary content from the conclusion section.

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Q4. The design of the study is not adapted to this specific population. Most diabetic patients with foot problem are overweight (simulation loaded mass point à 69.8 Kg) and not able to run. Moreover, diabetic patients with foot problem present neuropathy, foot deformities, limited joint mobility, reduced soft tissue plantar thickness leading to high plantar pressure. These patients require custom-made insole to reduced plantar pressure. These point should be discussed and clinical studies available in literature presented.

A4: Thank you for your questions and suggestions. Existing researches have shown that aerobic exercises such as walking or running can significantly benefit the health and physical condition of diabetic patients (1). Aerobic exercise helps improve patients' body weight, body mass index, and fasting plasma glucose levels (2). In addition, aerobic exercise can also alleviate vascular endothelium-dependent dysfunction in pre-diabetic individuals (3). Under moderate and safe plantar loading conditions, aerobic exercise (running) positively impacts key parameters indicative of the progression of type 1 and type 2 diabetes, including lowering glucose and glycated hemoglobin levels, improving lipid metabolism, reducing inflammation, increasing insulin sensitivity, and enhancing pancreatic β-cell function (4). Therefore, exercise prescriptions that include aerobic exercises such as walking and running are recommended for patients with diabetes (5). Thus, aerobic exercises such as walking and running are essential for most diabetic patients.

Relevant details have been added in the introduction on line 77-87.

Furthermore, the modern therapeutic footwear has a complex, layered, structure including an outersole, midsole and insole. Previous research on diabetic footwear has primarily focused on the customized design of insoles to optimize plantar pressure distribution (6). However, to enhance the cushioning performance of footwear for diabetic patients, several medical and biomaterials science studies have investigated the relationship between midsole design and plantar biomechanics, exploring their applications in foot protection and disease prevention among diabetic patients. For instance, some researchers utilized 3D-printed lattice structures in the shoe midsole design to enhance energy absorption capacity and elasticity (7). Other research analyzed changes in plantar pressure and soft tissue stress in diabetic patients with neuropathy using different midsoles, providing a basis for optimizing the structure of protective diabetic footwear (8). Malki et al. proposed reducing overall plantar pressure in diabetic patients by combining individualized 3D-printed midsoles with self-adjusting insoles (9). These researches show that in addition to customized insoles, an appropriate shoe midsole design is crucial in reducing plantar pressure in diabetic patients.

Relevant details have been added in the introduction on line 35-49.

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Q5. Images available on line have a too low quality.

A5: Thank you for your suggestions. We have updated the images in the article, including increasing the text size in Figures 1, 2, 4, and 6.

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Q6. Could we have more information on the print process? What is the fill density? How long is it to print an insole? Do you have any idea of the cost if you want to use it in routine practice? What about the durability of TPU with auxetic lattice?

A6: Thank you for your question. We have added information in Section 2.1 on line 127-135 regarding the model of the 3D printer, the TPU material supplier, the printing process settings, and the printing time and costs. The added content is as follows:

“These lattice midsoles were fabricated using Fused Deposition Modeling (FDM) 3D printing technology. This study used a UP300 3D printer from Tiertime (China, Beijing) for sample printing, with the pre-designed midsole models imported via the accompanying UP Studio software. The nozzle displacement accuracy of the device is 2, 2, and 0.5 microns on the x, y, and z axes, respectively, and the printing precision reaches 0.1 mm. The layer thickness was set to 0.2 mm, and the infill density was set to the maximum during printing. All samples were printed using TPU material (95A) provided by Tiertime. The material cost for printing a single lattice TPU midsole was approximately $15, but the FDM printing process is time-consuming, taking about 50 hours to print one midsole. As 3D printing technology advances, printing time and costs are expected to decrease (10), facilitating the application of auxetic lattice designs in footwear products.”

Additionally, in Section 2.4 on line 230-233, we have added information regarding the durability of the auxetic lattice TPU midsoles:

“The printed midsoles underwent 60 short-term walking and running tests each without showing any damage or irreversible deformation. This indicates that the printed midsole samples have pressure resistance; however, their durability has yet to be verified through long-term experiments.”

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Q7. Did you standardized the number of steps for in shoe pressure measurement?

A7: Thank you for your question. It is difficult for individuals to consciously count their steps while walking and running. Therefore, the experiment established standards for the speed and distance of movement. Before the pressure measurements began, participants were asked to familiarize themselves with a walking speed of 1.1 m/s and a running speed of 2.8 m/s on a treadmill. After getting accustomed to the walking or running speeds, participants wore shoe covers with integrated 3D-printed midsoles and walked 12 meters and jogged 20 meters on a flat, straight path, with plantar pressure measurements taken for each activity. A pre-test was conducted before the experiment to calibrate the sensors and signals of the plantar pressure testing equipment. These distances were sufficient to record gait cycles and obtain the biomechanical parameters needed for the study.

Relevant details have been added in Section 2.4 on line 222-230.

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Q8. A young modulus of 2 mPa seems high for off-loading concern? Even if not perfect, do you test the shore of the material full filled?

A8: Thank you for your question. According to the study by Li et al.(11), "Technology Construction and Finite Element Analysis of a Coupled Finite Element Model of Foot and Barefoot Running Footwear", the Young's modulus of the insole part in the finite element model constructed in this study was set to 2 MPa. The lattice structure midsoles (A60, A75, and N90) proposed in this study were made using 3D-printed TPU material, which underwent tensile and compression tests. Detailed information on the sample preparation, testing equipment, and testing standards used in the material testing process is described in the first paragraph of Section 2.3. Additionally, the material test results are documented in Section 3.1. The test results indicate that the 3D-printed TPU material used for the lattice structure midsoles exhibits hyperelastic characteristics.

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References in the answers and responses

 

1. Cannata F, Vadalà G, Russo F, Papalia R, Napoli N, Pozzilli P. Beneficial effects of physical activity in diabetic patients. Journal of functional morphology and kinesiology. 2020;5(3):70.
2. Asuako B, Moses MO, Eghan BA, Sarpong PA. Fasting plasma glucose and lipid profiles of diabetic patients improve with aerobic exercise training. Ghana Medical Journal. 2017;51(3):120-7.
3. Wang S, Li J, Zhang C, Xu G, Tang Z, Zhang Z, et al. Effects of aerobic exercise on the expressions and activities of nitric oxide synthases in the blood vessel endothelium in prediabetes mellitus. Experimental and Therapeutic Medicine. 2019;17(5):4205-12.
4. Podvigina T, Yarushkina N, Filaretova L. Effects of Running on the Development of Diabetes and Diabetes-Induced Complications. Journal of Evolutionary Biochemistry and Physiology. 2022;58(1):174-92.
5. Hordern MD, Dunstan DW, Prins JB, Baker MK, Singh MAF, Coombes JS. Exercise prescription for patients with type 2 diabetes and pre-diabetes: a position statement from Exercise and Sport Science Australia. Journal of Science and Medicine in Sport. 2012;15(1):25-31.
6. . !!! INVALID CITATION !!! [7-9].
7. Zolfagharian A, Lakhi M, Ranjbar S, Bodaghi M. Custom shoe sole design and modeling toward 3D printing. International Journal of Bioprinting. 2021;7(4).
8. Cao Z, Wang F, Li X, Li M, He Y, Zhang Y, et al. Plantar pressure at different phases during gait cycle in diabetic patients and midsole structure design. Chinese Journal of Tissue Engineering Research. 2023;27(13):2005.
9. Malki A, Badaya MB, Dekker R, Verkerke G, Hijmans J. Effects of individually optimized rocker midsoles and self-adjusting insoles on plantar pressure in persons with diabetes mellitus and loss of protective sensation. Diabetes Research and Clinical Practice. 2024;207:111077.
10. Kumar R, Kumar S. Trending applications of 3D printing: A study. Asian Journal of Engineering and Applied Technology. 2020;9(1):1-12.
11. Li Y, Leong KF, Gu Y, Technology. Construction and finite element analysis of a coupled finite element model of foot and barefoot running footwear. Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering Technology. 2019;233(1):101-9.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1.      In the introduction section, the author stated :” auxetic structures has demonstrated numerous exceptional characteristics across various fields. Its practical application on the  human body, particularly in the design of shoe midsoles, has yet to be explored.” This is not the case. Many papers on the use of auxetic structures for shoe midsoles have been published. Please do a literature review and mention the novelties of your research.

2.      In 2.1, provide the 3D printer model, printing settings, TPU supplier.

3.      Figures quality is very poor. Please update all of them.

4.      Table 4 and 5: no units.

5.      In this research, healthy adults were recruited to do the wear trial. However, diabetes patients may have a much different foot shape due to nerve damage. Please discuss how your research results can be applied to diabetes patients. 

Comments on the Quality of English Language

Minor editing is needed.

Author Response

Thank you to the reviewers and editors. I have responded to the questions and suggestions as follows and revised the manuscript (the attached PDF file contains the responds to the comments and revisions of manuscript). The revised parts of the manuscript are also marked with other colors.

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Q1. In the introduction section, the author stated “auxetic structures has demonstrated numerous exceptional characteristics across various fields. Its practical application on the human body, particularly in the design of shoe midsoles, has yet to be explored.” This is not the case. Many papers on the use of auxetic structures for shoe midsoles have been published. Please do a literature review and mention the novelties of your research.

A1: Thank you for your suggestion. Shoe midsoles utilizing mass-tunable auxetic geometry can effectively reduce ground reaction forces compared to traditional midsoles, lowering the risk of foot injuries during intense physical activities (1). Auxetic midsoles exhibit higher strain energy and greater energy absorption capacity than traditional midsoles, which helps reduce spinal fatigue during walking (2, 3). Using auxetic midsoles during high-demand activities, such as vertical jumping, can reduce lumbar load and decrease the risk of musculoskeletal injuries (4). Previous research proposed applying auxetic lattice structure midsoles in running shoes to enhance shock absorption, make the shoes lightweight, flexible, and body-conforming, and provide greater tensile strength (5). 

Despite existing research recognizing the superior physical properties and several benefits of auxetic structures in shoe midsoles, there still needs to be more exploration and validation regarding their application in protecting plantar soft tissue and preventing plantar-related diseases. Therefore, the novelty of this study lies in verifying the adaptive capacity of auxetic lattice midsoles to foot morphology and their effectiveness in optimizing plantar pressure distribution through plantar pressure measurements and finite element analysis. This study also analyzes the potential feasibility of auxetic midsoles in preventing or aiding the treatment of diabetic foot ulcers. Moreover, the research employs a different auxetic lattice structure than those used in existing similar studies for midsole applications, providing a new reference paradigm for designing pressure-relief footwear products.

Relevant details have been added in the introduction on line 54-73.

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Q2. In 2.1, provide the 3D printer model, printing settings, TPU supplier.

A2: Thank you for your question. We have added information in Section 2.1 on line 127-136 regarding the model of the 3D printer, the TPU material supplier, the printing process settings, and the printing time and costs. The added content is as follows:

“These lattice midsoles were fabricated using Fused Deposition Modeling (FDM) 3D printing technology. This study used a UP300 3D printer from Tiertime (China, Beijing) for sample printing, with the pre-designed midsole models imported via the accompanying UP Studio software. The nozzle displacement accuracy of the device is 2, 2, and 0.5 microns on the x, y, and z axes, respectively, and the printing precision reaches 0.1 mm. The layer thickness was set to 0.2 mm, and the infill density was set to the maximum during printing. All samples were printed using TPU material (95A) provided by Tiertime. The material cost for printing a single lattice TPU midsole was approximately $15, but the FDM printing process is time-consuming, taking about 50 hours to print one midsole.”

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Q3. Figures quality is very poor. Please update all of them.

A3: Thank you for your suggestion. We have updated the images in the article, including increasing the text size in Figures 1, 2, 4, and 6.

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Q4. Table 4 and 5: no units.

A4: Thank you for your suggestions. We have added units to the titles of Tables 4 and 5.

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Q5. In this research, healthy adults were recruited to do the wear trial. However, diabetes patients may have a much different foot shape due to nerve damage. Please discuss how your research results can be applied to diabetes patients.

A5: Thank you for your question. The purpose of the wear trial and plantar pressure measurement in the study is to validate the finite element simulation results, and thereby compare the conformability and pressure distribution optimization performance of different midsole designs. The proposed auxetic lattice midsole design is primarily aimed at preventing the occurrence of diabetic foot. The target group is mainly early-stage diabetic patients who have not yet developed foot ulcers or deformities, but may have neuropathy (6). There are histomorphological differences between diabetic and non-diabetic patients, with increased stiffness in plantar tissues leading to a decreased ability to dissipate applied pressure (7) and an increase in peak plantar pressure (8). These differences may also contribute to discrepancies between the pressure test results and finite element simulation results. However, comparing the mechanical performance of the midsoles in both experiments showed consistency, ultimately allowing the study to rely on the finite element analysis results. Due to the incomplete design of the shoes in this study, to avoid accidental injuries to diabetic patients during experiment, the plantar pressure measurement experiment did not recruit the diabetic patients. The plantar pressure measurement experiment recruited 20 healthy male adults with similar age and body type (height and weight) and shoe size (EU 42) to minimize other potential sources of error. Based on the finite element analysis results, researchers will further improve the shoe design and conduct more long-term clinical trials.

Relevant details have been added and refined in the introduction on line 92-95, Section 3.2 on line 266-272, and Section 2.4 on line 218-220.

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References in the answers and responses

 

1. Ford RR, Misra M, Mohanty AK, Brandon SC. Effect of Simulated Mass-Tunable Auxetic Midsole on Vertical Ground Reaction Force. Journal of Biomechanical Engineering. 2022;144(11):111007.
2. Nourani A, Daei MD, Honarmand M. Comparison of energy absorption in conventional and auxetic shoes: gait analysis and finite element modeling. Journal of Design Against Fatigue. 2023;1(2).
3. Honarvar S, Nourani A, Yarandi A, Ghehi FF, editors. Three-dimensional finite element modeling of the shoe sole to investigate the impact of various geometries on foot heel stresses and energy absorption. 2022 29th National and 7th International Iranian Conference on Biomedical Engineering (ICBME); 2022: IEEE.
4. Dehaghani MR, Nourani A, Arjmand N. Effects of auxetic shoe on lumbar spine kinematics and kinetics during gait and drop vertical jump by a combined in vivo and modeling investigation. Scientific Reports. 2022;12(1):18326.
5. Emerson R, Rhee J, editors. Analyzing Auxetic Cellular Structures for Personal Protective Gear Designs. International Textile and Apparel Association Annual Conference Proceedings; 2024: Iowa State University Digital Press.
6. Tang UH, Siegenthaler J, Hagberg K, Karlsson J, Tranberg RJFAOJ. Foot anthropometrics in individuals with diabetes compared with the general Swedish population: Implications for shoe design. 2017;10(3):1.
7. Wang Y-N, Lee K, Ledoux WR. Histomorphological evaluation of diabetic and non-diabetic plantar soft tissue. Foot and Ankle International. 2011;32(8):802-10.
8. Gnanasundaram S, Ramalingam P, Das BN, Viswanathan V. Gait changes in persons with diabetes: Early risk marker for diabetic foot ulcer. Foot and Ankle Surgery. 2020;26(2):163-8.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed my comments. It can be accepted for publication now.