Biomechanical design of lightweight full contacted insole based on structural anisotropy bespoke

Abstract

Purpose

A biomechanical design method of lightweight full contacted insole based on structural anisotropy bespoke (SAB) is proposed, which can better redistribute the stress distribution of SAB designed personalized insole.

Design/methodology/approach

The reconstructed joint biomechanics are simulated using finite element analysis (FEA) to develop a lightweight full contact insole. Innovatively, the anisotropic properties of the triply periodic minimal surface (TPMS) structure, which contribute to reducing insole weight, are considered to optimize stress distribution. Additionally, porosity and manufacturing time are included as design objectives. To validate the lightweight insole design, FEA is employed to simulate the stress distribution of the ergonomic insole, which can be fabricated by additive manufacturing (AM) with TPU.

Findings

With a little 0.924% loss in porosity, the maximum stress of lightweight SAB designed insoles is extremely decreased by 19.2917%.

Originality/value

The biomechanical design of the lightweight full contact insole based on SAB can effectively redistribute stress, avoid stress concentration and improve the mechanical properties of the ergonomic individual insole.

Citation

Tu, Z., Xu, J., Zhang, S. and Tan, J. (2024), "Biomechanical design of lightweight full contacted insole based on structural anisotropy bespoke", Journal of Intelligent Manufacturing and Special Equipment, Vol. 5 No. 2, pp. 265-271. https://doi.org/10.1108/JIMSE-06-2024-0017

1. Introduction

The foot is a vital human sensory organ that plays an indispensable part in daily life (Youssef et al., 2020). The number of people with diabetes is increasing every year. It is expected that by 2030, the number may increase to 578 million, accounting for 10.2% of the global adult population (Saeedi et al., 2019). Diabetic foot is a common severe complication of diabetes, which may cause changes in gait and plantar pressure (Ahmed et al., 2020). Improving plantar pressure distribution through insoles can protect soft tissue to a certain extent.

The insole design focuses on comfort, support, air permeability and durability (Tse et al., 2021; Azam et al., 2024; Sterman et al., 2024; Salles et al., 2014). Shakouri et al. (2020) designed and manufactured new medical insoles with a universal fluid layer.

The contact area of insole can be expanded and local pressure can be more evenly dispersed by reasonably designing the insole's material and construction (Sterman et al., 2024). The lattice structure like triply periodic minimal surface (TPMS) possesses excellent cushioning and lightweight characteristics, which can ensure sufficient strength while reducing pressure (Claybrook et al., 2024). The constraints in additive manufacturing and the anisotropic mechanical behavior of TPMS structure put forward higher requirements for designing a full contacted insole.

Based on the previous studies (Xu et al., 2020, 2021, 2023, 2024), a biomechanical design method of lightweight full contacted insole based on structural anisotropy bespoke (SAB) is proposed. By analyzing the reconstructed joint biomechanics of the foot, a lightweight full contacted insole is designed, which considers the structural anisotropy, porosity and printing time. To verify the effectiveness of the designed lightweight insole, the stress distribution is analyzed by finite element analysis (FEA). Finally, the designed bioengineering insole is fabricated by additive manufacturing (AM) with TPU.

2. Reconstructed joint biomechanics

3D reconstruction, derived from either marching cubes or marching tetrahedra, converts CT or MRI medical images into 3D manifolds, which can be further converted into joint biomechanics through semi-definite computing. Accurately anatomical structures will reproduce and detailed internal tissue information can be provided, on the basis of which the customized design could be carried out, involving human engineering functional structure, joint prosthesis, roboticized exoskeleton and so forth. By selecting an appropriate threshold, the target tissue can be separated from the background in the images.

(1)I(x,y)={1,if I(x,y)>S0,otherwise

where I(x,y) is the gray value of pixel; S is the threshold.

The contours between adjacent slices are interpolated to generate 3D manifolds. Figure 1 depicts the 3D rending reconstructed human joint (right).

B-spline curve is introduced to establish foot joint manifold can accurately simulate the actual structure. The displacement u of the manifold can be obtained.

(2)Ku=F

where K is stiffness matrix; u is displacement vector; F is force vector.

According to the displacement u, the strain ε and stress σ can be calculated.

(3){ε=Buσ=Dε

where B is strain coefficient matrix; D is the elastic matrix of the material.

The biomechanical analysis of the foot joint is carried out by simulating the neutral position of the human body. Each structure is simplified as a homogeneous, isotropic linear elastic material. The contact between joints is defined as frictionless surface contact. The upper end of the fibula and tibia is fixed, and the upward load can be set as 500 N and 300 N. The finite element surrogate method can improve the simulation accuracy and efficiency, thereby laying a foundation for the intelligent customized design through knowledge navigation driven by large model. The stress distributions of the human foot joint (right) under different loads analyzed by FEA are displayed in Figure 2. The maximum stress in Figure 1(a) is 1.4293 MPa, which appeared at (0.2212, 0.5291, 0.4117). The minimum stress is 8.7229e−11 MPa at (0.6454, 0.1046, 0.0563). The maximum stress in Figure 2(b) is 0.95204 MPa.

3. Lightweight full contacted insole design by SAB

By analyzing pressure distribution and changes in various areas of the foot, insoles with appropriate support and damping effects can be tailored to individual needs. Introducing the Primitive structure, as a kind of TPMS structure, innovates insole design. The anisotropic characteristics can provide optimized support and stability in different directions, effectively disperse the pressure and enhance the damping effect. The expression of Primitive structure is:

(4)φ(r)P=cos(2πLaX)+cos(2πLbY)+cos(2πLcZ)=c

where La,Lb,Lc is the unit length of anisotropic Primitive element in x, y, z directions; c is a constant.

According to the ankle joint and its biomechanical properties, the shape of the insole is designed. The goal is to make the insole perfectly fit with the foot, evenly distribute the pressure and use the anisotropic Primitive lattice structure to fill the inside of the full-contact insole.

Porosity D refers to the proportion of pores in the material, which can reduce the overall weight of shoes and improve the portability and comfort of wearing.

(5)D=V0−VV0∗100%,D∈(0,1]

where V0 is the total volume of the structure (mm³); V is the volume of porous structure (mm³).

As one of the methods of additive manufacturing, 3D printing technology provides a new possibility for personalized manufacturing of orthopedic insoles. The printing time T is set as the evaluation index to calculate the production cost.

(6)T=∑kmtk

where m is the total number of printed layers; tk is the printing time of each layer。

In the conceptual design process, the structural anisotropy, porosity D and printing time T of the Primitive structure are design objectives for Latin hypercube experiments. Lightweight full contacted insole (right) designed before and after SAB is summarized in Figure 3. Figure 3(a) is the insole before SAB design. Figure 3(b) is the personalized insole after SAB design.

Figure 4 displays the stress distribution of lightweight full contact insoles (right) before and after SAB design. Quantitative detailed outcomes of the lightweight full contact insole design before and after SAB are listed in Table 1. Compared with the insole before SAB design, the maximum stress of the lightweight full contact insole is reduced by 19.2917% after SAB design. Compared with the insole designed to reduce weight by increasing the porosity, the SAB designed individual insole maintains an appropriate porosity range and provides the necessary foot support, thereby making the foot stress distribution more uniform.

4. Physical manufacturing of designed bespoke insole via AM

A physical case of the lightweight full contacted insole (right) designed by SAB conducted by 3D printing is captured in Figure 5. The forming material is TPU. The effective workspace can reach 220 × 220 × 250 mm while the printing size is 198 × 80 × 18 mm. The temperature of the print nozzle is 230 °C, whereas that of the printing platform is 60 °C. The printing speed is 120 mm/s, with a layer height of 0.2 mm. The maximum volume flow rate during printing is 3.5 mm³/s.

5. Conclusion

• (1)

  A biomechanical design method of lightweight full contacted insole based on structural anisotropy bespoke (SAB) is proposed.

The biomechanical conceptual design of a lightweight full contact insole is bespoken to the foot's geometric structure and joint biomechanics. By combining FEA with Latin hypercube experiments, the structural anisotropy, porosity and printing time of the lightweight full contact insole are SAB design objectives to design the lightweight full contacted insole. Compared with the insole designed to reduce weight by increasing the porosity, the SAB designed personalized insole maintains an appropriate porosity range and provides the necessary foot support, thereby making the foot stress distribution more uniform.

• (2)

  The effect of anisotropy of TPMS structure on stress distribution is considered.

The anisotropy TPMS structural insole, driven by foot joint biomechanics, can enhance the adaptability of the insole surface to the user, which results in improved wearing comfort. Compared with the insole before the SAB design, the maximum stress of the SAB designed lightweight insole structure is reduced by 19.2917%, and the stress distribution is more uniform. The SAB designed bespoke insole effectively avoids stress concentration and improves the mechanical properties of the insole.

• (3)

  The optimized full contacted insole is fabricated and verified via additive manufacturing.

The SAB designed lightweight full contacted insole is conducted by 3D printing with TPU. The experiment proves that the SAB designed insole manufactured by 3D printing can produce the high-precision realization of complex structures. The bioengineering insole reduces stress and significantly improves wearing comfort and durability.

In the future, gait recognition will be employed to analyze dynamic foot pressure distribution and motion trajectory, resulting in a refined insole with personalized and accurate foot support and correction. Additionally, biomechanical analysis and AM will be applied to produce ergonomic prostheses, exoskeletons, and wearable devices that conform to personalized demands. Furthermore, additional soft tissues, including plantar fascia and muscles, will be introduced.

Figures

Figure 1

3D rending reconstructed human joint (right)

Figure 2

Stress distributions of the human foot joint (right) under different loads analyzed by FEA

Figure 3

Lightweight full contacted insole (right) designed before and after SAB

Figure 4

Stress distribution of lightweight full contacted insole (right) designed before and after SAB

Figure 5

A physical case of the lightweight full contacted insole (right) designed by SAB

Table 1

Quantitative detailed outcomes of lightweight full contact insoles design before and after SAB

Quantitative detail	Before SAB design	After SAB design	Ratio
Max stress (MPa)	0.81533	0.6582	19.2917%
Max stress position	(0.2141, 0.8989, 0.0103)	(0.3561, 0.7596, 0.0255)	\
Min stress (MPa)	0.00022213	0.00025696	−15.6800%
Min stress position	(0.1021, 0.3804, 0.6721)	(0.1894, 0.0345, 0.6118)	\
Porosity D	72.52%	71.85%	−0.924%
Printing time T (s)	12,060	12,900	−6.965%

Source(s): Authors' own work