Shoe Midsole Research Articles

Design method for individualised 3D printed lattice shoe midsoles

Additive manufacturing has enabled the production of individualised 3D printed shoe soles with improved properties. A promising approach is to use lattice structures that have high energy absorption properties and low weight. A design method of a cellular shoe midsole with optimised strut diameters of lattice structures is proposed. The shoe sole model is obtained by re-modelling a 3D scan of a foot to ensure a customised fit. The optimisation of strut thicknesses is based on a simplified stress distribution acting on the shoe sole during walking or running, using finite element simulations. Therefore, the optimal strut thickness for each region of the sole can be determined and adjusted. Two types of lattice structures with different topology and thus significant variations in stiffness are chosen, resulting in a wide range of required strut thicknesses. The developed design process allowed for the creation of a 3D printed shoe sole with improved strut thicknesses and a customised fit. The resulting cellular shoe soles are additively manufactured and experimental compressive tests are conducted to investigate the mechanical behaviour and differences between the shoe soles with the corresponding lattice types. The results show that both shoe soles have a similar behaviour under compression. The design tool developed has the potential to improve foot health and comfort, especially for people with foot problems, as all parameters affecting the performance of a shoe sole can be adjusted. However, more research is needed to fully understand the durability and performance of these shoe soles in real-world conditions.

In-plane density gradation of shoe midsoles for optimal energy absorption performance

Midsoles are important components in footwear as they provide shock absorption and stability, thereby improving comfort and effectively preventing certain foot injuries. A strategically engineered midsole designed to mitigate plantar pressure can enhance athletic performance and comfort levels. Despite the importance of midsole design, the potential of using in-plane density gradation (deliberate variation of material density across the horizontal plane) in midsoles has been rarely explored. The present work investigated the effectiveness of in-plane density gradation in shoe midsoles using novel polyurea foams as the material candidate. Different polyurea foam densities, ranging from 95 to 350 kg/m2 were examined and tested to construct density-dependent correlative mathematical relations required for optimizing the midsole design for enhanced cushioning and reduced weight. This study combined mechanical testing and plantar pressure measurements to validate the efficacy of density-graded midsoles. The methodology introduced here is relevant to realistic walking conditions, ensured by biomechanical tests supplemented by digital image correlation analyses. An optimization framework was then created to allocate foam densities at certain plantar zones based on the required cushioning performance constrained by the local pressure. The optimization algorithm was specifically tailored to accommodate varying local pressures experienced by different areas of the foot. The optimization strategy in this study aimed at reducing the overall weight of the midsole while ensuring there were no compromises in cushioning efficacy or distribution of plantar pressure. The approach presented herein has the potential to be applied to a wide range of gait speeds and user-specific plantar pressure patterns.

Custom Shoe Sole Design and Modeling Toward 3D Printing.

This study introduces a design procedure for improving an individual’s footwear comfort with body weight index and activity requirements by customized three-dimensional (3D)-printed shoe midsole lattice structure. This method guides the selection of customized 3D-printed fabrications incorporating both physical and geometrical properties that meet user demands. The analysis of the lattice effects on minimizing the stress on plantar pressure was performed by initially creating various shoe midsole lattice structures designed. An appropriate common 3D printable material was selected along with validating its viscoelastic properties using finite element analysis. The lattice structure designs were analyzed under various loading conditions to investigate the suitability of the method in fabricating a customized 3D-printed shoe midsole based on the individual’s specifications using a single material with minimum cost, time, and material use.

Multidisciplinary optimization of shoe midsole structures using swarm intelligence

Multidisciplinary optimization of shoe midsole structures using swarm intelligence

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

With the global rise in the prevalence of diabetes, diabetic patients need innovative footwear designs to reduce the risk of foot ulcers. This study examined the mechanical properties of diabetic shoe midsoles featuring auxetic lattice structures. Through the construction of finite element models and simulation, this research compared the biomechanical parameter differences in the plantar regions of the metatarsal head, midfoot, and hindfoot when wearing two types of auxetic midsoles with internal angles of 60° and 75° and a non-auxetic midsole with an internal angle of 90° under both walking and running conditions. Compared to the non-auxetic midsole, the auxetic midsoles significantly reduced the peak plantar pressure and optimized the pressure distribution across various plantar regions. Notably, the auxetic 60° midsole reduced the peak plantar pressure by 19.68–55.25% and 16.19–54.39% compared to the non-auxetic 90° midsole during walking and running, respectively. This study also verified that the auxetic midsoles exhibited greater adaptability and compliance to the plantar foot shape, contributing to reductions in plantar pressure in comparisons of deformation values and plantar contact areas across the different midsoles. Auxetic midsoles manufactured using 3D printing technology have significant potential to prevent diabetic foot ulcers and maintain human foot health. This research integrates insights and techniques from materials science and ergonomics, offering a new direction for footwear design.

Natural Fiber Reinforced Shoe Midsoles with Balanced Stiffness/Damping Behavior.

The comfort of walking depends heavily on the shoes used. Consequently, the midsole of shoes is designed in such a way that it can dampen force peaks during walking. This significantly increases the overall wellness during walking. Therefore, the midsole usually consists of rubber-like polymers, such as polyurethane and ethylene-vinyl acetate copolymer. Furthermore, the manufacturing process of these polymers results in a foam-like structure. This further enhances the damping behavior of the material. Nevertheless, it would be desirable to find a cheap and sustainable method to enhance the damping behavior of the shoe midsole. The purpose of this work is to see if hemp fibers, which are part of the polymer matrix material, could improve the stiffness without losing the damping behavior. The mechanical properties of such prepared fiber-reinforced composites were characterized by quasi-static tensile testing and dynamic mechanical analysis. The mechanical properties were examined in relation to the fiber type, weight fraction, and type of polyurethane used. Furthermore, the investigation of the embedding of these fibers in the polymer matrix was conducted through the utilization of optical and electron microscopy.

Valorization of shoe sole waste into high-performance cationic dye sorbents via sulfonation

This work demonstrates a straightforward method to convert real-world shoe waste midsoles into water remediation sorbents for the removal of cationic organic pollutants.

Enhancing dynamic energy return and performance of running shoes: Replacing talc with multi-walled carbon nanotubes derived from plastic wastes in midsole foam

Boosting both the lightweight and rebound of a shoe's midsole without compromising its durability is regarded as a challenging aspect of developing excellent running shoes. This study explores the replacement of talc, a conventional reinforcing and nucleating agent for polymers, with multi-walled carbon nanotubes (MWCNTs) derived from plastics in the midsole foam of running shoes to enhance lightweight, rebound, and durability. Two types of MWCNTs, non-functionalized and oxygen-functionalized, derived from upcycling mixed plastics were processed with copolymer of ethyl-vinyl acetate (EVA) to create nanocomposite foams. The foam reinforced with non-functionalized MWCNTs exhibited higher dynamic stiffness and similar energy return to oxygen-functionalized MWCNTs. The running shoe prototypes with EVA midsole foam containing 0.5 wt% MWCNTs was 13 % lighter and returned more than 10 % higher energy than the conventional EVA midsole foam with mineral fillers. Additionally, the midsole foam produced from EVA/MWCNTs demonstrated greater flexibility, and durability after 500 km of dynamic impact cycles. The cost difference per pair of running shoe midsole is merely 0.08 USD, considering the exceptional performance of the EVA/MWCNTs midsole as compared to conventional mineral filled EVA midsole. These findings indicate the potential for commercializing EVA/MWCNTs nanocomposite foam as a viable option for high-performance running shoe midsoles, offering athletes improved running performance.

Numerical Simulation of the Effect of Different Footwear Midsole Structures on Plantar Pressure Distribution and Bone Stress in Obese and Healthy Children.

The foot, as the foundation of the human body, bears the vast majority of the body's weight. Obese children bear more weight than healthy children in the process of walking and running. This study compared three footwear midsole structures (solid, lattice, and chiral) based on plantar pressure distribution and bone stress in obese and healthy children through numerical simulation. The preparation for the study included obtaining a thin-slice CT scan of a healthy 9-year-old boy's right foot, and this study distinguished between a healthy and an obese child by applying external loadings of 25 kg and 50 kg in the finite element models. The simulation results showed that the plantar pressure was mainly concentrated in the forefoot and heel due to the distribution of gravity (first metatarsal, fourth metatarsal, and heel bone, corresponding to plantar regions M1, M4, and HM and HL) on the foot in normal standing. Compared with the lattice and solid EVA structures, in both healthy and obese children, the percentage reduction in plantar pressure due to the chiral structure in the areas M1, M4, HM, and HL was the largest with values of 38.69%, 34.25%, 64.24%, and 54.03% for an obese child and 33.99%, 28.25%, 56.08%, and 56.96% for a healthy child. On the other hand, higher pressures (15.19 kPa for an obese child and 5.42 kPa for a healthy child) were observed in the MF area when using the chiral structure than when using the other two structures, which means that this structure can transfer an amount of pressure from the heel to the arch, resulting in a release in the pressure at the heel region and providing support at the arch. In addition, the study found that the chiral structure was not highly sensitive to the external application of body weight. This indicates that the chiral structure is more stable than the other two structures and is minimally affected by changes in external conditions. The findings in this research lay the groundwork for clinical prevention and intervention in foot disorders in obese children and provide new research ideas for shoe midsole manufacturers.

Influence of structural arrangements on static and dynamic properties of additively manufactured polyester elastomer lattice metamaterials

In the current study, we present novel carbon organic framework-inspired lattice metamaterials (CFLM) with thermoplastic polyester elastomer (TPEE) to analyze the structural effects. The CFLMs (S1, S2, S3, and SS) from TPEE were successfully produced by material extrusion (MEX) via fused-filament fabrication and had never been reported previously. In the beginning stage, static compression tests were utilized to evaluate mechanical properties, densification strain, and energy absorption, which were validated using the finite element method (FEM). According to static compression results, S2 had the largest energy absorption, followed by the SS structure. In the second stage based on the static test, dynamic sinusoidal compression was performed to calculate the dynamic elastic recovery (DER), hysteresis work absorption, and Tan δ. The results showed that the proposed lattice metamaterials showed different the hysteresis energy absorption, DER, and viscoelasticity nature from Tan δ. To apply static and dynamic compression results to shoe midsoles, they were subjected to a dynamic drop weight impact test. Based on static and dynamic compression testing, including the results of dynamic drop weight impact tests, S1 was the most suitable for the toe part of the shoe midsole, whereas SS was the best for the heel part of the shoe midsole. From the overall results of all characterizations, it was concluded that the structural effect of CFLMs successfully provided different mechanical properties in the static and dynamic tests from the signal TPEE material.

Multi-objective design and optimization of high cushioning bionic shoe midsole under limited thickness of forefoot

To enhance the shock-absorbing performance of the midsole in conditions where thickness is limited, particularly in the forefoot region, this paper presents a novel design inspired by the structure and material assembly characteristics of ostrich metatarsophalangeal joints and toe pads. The shock-absorbing performance of the bionic unit was validated through finite element simulations and experimental testing using 3D-printed samples. In order to unravel the underlying mechanisms that enhance the cushioning performance, optimization factors for the forefoot bionic cushioning unit were determined based on the cushioning mechanism. The influence of the cantilever angle (La), cantilever taper (Lo), cantilever spacing (L3), thickness of the elastic energy-absorbing component (L4), and selected materials on the cushioning performance was discussed. The results demonstrate that with the configuration of La:Lo:L3:L4 = 60°:0.6:9 mm:3 mm and the outer frame material to elastic energy-absorbing component material ratio of T-4:T-2, compared to the non-optimized unit, the peak negative acceleration of the forefoot bionic unit has been reduced by 32.64%.Based on the optimization results, the production of the bionic cushioning sports shoes and the control shoes was completed. Finally, human wear experiments have shown that the bionic shoes exhibit a 23.7% to 29.8% increase in impact absorption compared to control shoes.

Interaction of running shoe midsole hardness and bending stiffness on the lower-limb joints mechanics

Interaction of running shoe midsole hardness and bending stiffness on the lower-limb joints mechanics

The Impacts of the Airflow System in the Shoe on the Foot Environment

Army soldiers stationed in frigid regions are highly susceptible to foot health problems, partly due to the constant sweating and moisture inside closed military boots. Winter military boots on the market lack attention to dehumidification and ventilation. To address these issues, we propose a boot system that includes a dynamic airflow system integrated into the core of the shoe midsole. This system works on the principle that the user's heel naturally squeezes the airbag under the heel during movement, thus creating an air pressure difference that drives the air movement inside the shoe. Through the experiments on the product prototype, it was found that this system allows the temperature and humidity of the foot to drop significantly within the shoe and boot environment, improving foot comfort and reducing the generation of sweat on foot. What can be expected is that after soldiers wear this shoe and boot product, the moist air inside the shoe can be continuously exported out of the shoe during dynamic training while also avoiding the consumption of external energy, keeping the foot dry and comfortable, and reducing foot injuries. And this system has the potential to be extended to a broader civilian market.

Effect of Simulated Mass-Tunable Auxetic Midsole on Vertical Ground Reaction Force.

When runners impact the ground, they experience a sudden peak ground reaction force (GRF), which may be up to 4× greater than their bodyweight. Increased GRF impact peak magnitude has been associated with lower limb injuries in runners. Yet, shoe midsoles are capable of cushioning the impact between the runner and the ground to reduce GRF. It has been proposed that midsoles should be tunable with subject mass to minimize GRF and reduce risk of injury. Auxetic metamaterials, structures designed to achieve negative Poisson's ratios, demonstrate superior impact properties and are highly tunable. Recently, auxetic structures have been introduced in footwear, but their effects on GRF are not documented in literature. This work investigates the viability of a three-dimensional auxetic impact structure with a tunable force plateau as a midsole through mass-spring-damper simulation. An mass-spring-damper model was used to perform 315 simulations considering combinations of seven subject masses (45-90 kg), 15 auxetic plateau forces (72-1080 N), and three auxetic damping conditions (450, 725, and 1000 Ns/m) and regression analysis was used to determine their influence on GRF impact peak, energy, instantaneous, and average loading rate. Simulations showed that tuning auxetic plateau force and damping based on subject mass may reduce GRF impact and loading rate versus simulated conventional midsoles. Auxetic plateau force and damping conditions of 450 Ns/m and ∼1 bodyweight (BW), respectively, minimized peak impact GRF. This work demonstrates the need for tunable auxetic midsoles and may inform future work involving midsole testing.

Multi-material additive manufacturing with lightweight closed-cell foam-filled lattice structures for enhanced mechanical and functional properties

This work studies the novel concept of multi-material additive manufacturing by filling closed-cell lattice structures with secondary material. Filling closed cells incorporate new functional properties that unfilled or open-cell lattice structures cannot otherwise achieve. Filled closed cells also prevent materials from escaping the cellular cavity that can prove advantageous while combining dissimilar materials. For this, a hybrid 3D printing and foaming process is developed, which involves simultaneous 3D printing and foam-filling of closed-cell lattice structures on an open-source fused filament fabrication (FFF) 3D printer. This hybrid system targets direct digital manufacturing (DDM) by combining two separate processes into a single process, eliminating post-process operations. Here, the global closed-cell sea-urchin (SU) lattice structure is 3D printed with thermoplastic polyurethane (TPU), and the secondary functional material filled in the lattice structures is polyurethane (PU) foam. The load vs. deformation responses of the composite of PU foam and TPU lattice structures has shown higher stiffness, energy dissipation, and damping characteristics which otherwise could not have been achieved by the lattice structure alone. Possible applications for these could be protective equipment, shoe midsoles, and other energy absorbing and damping devices.

Using Gas Counter Pressure and Combined Technologies for Microcellular Injection Molding of Thermoplastic Polyurethane to Achieve High Foaming Qualities and Weight Reduction.

Microcellular injection molding technology (MuCell®) using supercritical fluid (SCF) as a foaming agent offers many advantages, such as material and energy savings, low cycle time, cost-effectiveness, and the dimensional stability of products. MuCell® has attracted great attention for applications in the automotive, packaging, sporting goods, and electrical parts industries. In view of the environmental issues, the shoe industry, particularly for midsole parts, is also seriously considering using physical foaming to replace the chemical foaming process. MuCell® is thus becoming one potential processing candidate. Thermoplastic polyurethane (TPU) is a common material for molding the outsole of shoes because of its outstanding properties such as hardness, abrasion resistance, and elasticity. Although many shoe manufacturers have tried applying Mucell® processes to TPU midsoles, the main problem remaining to be overcome is the non-uniformity of the foaming cell size in the molded midsole. In this study, the MuCell® process combined with gas counter pressure (GCP) technology and dynamic mold temperature control (DMTC) were carried out for TPU molding. The influence of various molding parameters including SCF dosage, injection speed, mold temperature, gas counter pressure, and gas holding time on the foaming cell size and the associated size distribution under a target weight reduction of 60% were investigated in detail. Compared with the conventional MuCell® process, the implementation of GCP technology or DMTC led to significant improvement in foaming cell size reduction and size uniformity. Further improvement could be achieved by the simultaneous combination of GCP with DMT, and the resulting cell density was about fifty times higher. The successful possibility for the microcellular injection molding of TPU shoe midsoles is greatly enhanced.

A Re-examination of the Measurement of Foot Strike Mechanics During Running: The Immediate Effect of Footwear Midsole Thickness.

PurposeMidsole cushioning thickness (MT) is a key component of running footwear that may influence the stiffness setting of the joints, performance enhancement, and injury prevention. Most studies that have investigated the influence of manipulating shoe midsole characteristics on foot strike patterns and vertical force loading rates have not considered the dynamic conditions of initial landing and the associated initial lower limb joint stiffness. In this study, we examined the effect of running in shoes with large changes in MT on both the posture and dynamics associated with foot strike.Methods12 injury-free runners with habitual rearfoot strike patterns ran at 4.5 m/s along a 40-m runway in shoe conditions with MT of 30, 42, and 54 mm, respectively. Ground reaction force and the right leg kinematic data were collected. One-way repeated measures ANOVA was conducted to statistically analyze the effect of MT on key variables linked to foot strike.ResultsIncreased midsole thickness resulted in a slightly flatter foot strike posture (p < 0.05), a decreased shank retraction velocity (p < 0.05), and an increase in forward horizontal foot velocity (p < 0.05), all at initial ground contact. Vertical force loading rates were reduced with increasing MT (p < 0.05), but this was associated with large increases in the initial ankle and knee joint stiffness (p < 0.05).ConclusionAdjustments in the initial conditions of contact with the ground during running were seen in both the posture and dynamics of the lower limbs. To help to mitigate the impact severity from foot-ground collision with the thinnest shoe condition, there was an increased shank retraction velocity and decreased forward velocity of the foot at landing. These active impact-moderating adaptations likely served to reduce the changes in impact severity expected due to midsole material properties alone and should be considered in relation to altering the risk of running-related injuries.

Design and performance evaluation of multifunctional midsole using functionally gradient wave springs produced using multijet fusion additive manufacturing process

Additive manufacturing (AM) is getting more attention and is considered a better choice of fabrication in almost every industry due to its unlimited design freedom, optimization, light-weight and customization. Currently, manufacturing constraints, limited design freedom and customization issues restrict manufacturers from designing specific shoes for each particular application e.g., walking, running, hiking, sports, etc. However, AM can design and fabricate a multifunctional, usable single shoe-midsole for all aforementioned applications. This study aims to design a multifunctional shoe midsole incorporated with functionally gradient wave springs (FGWS) at the critical areas of foot pressure (heel, forefoot and toe) measured using the F-scan system (insole plantar pressure measurement system). The non-critical areas were designed by a gradient cellular structure with graded unit cells as per load requirement. The designed midsole with FGWS was printed by an HP MultiJet Fusion printer using PA 12 (polyamide). A LISA printer (selective laser sintering) using TPU (thermoplastic polyurethane) material was also used to check its manufacturability. Compression testing, each spring up to 80% of its compressible distance studied the load-bearing capacity, energy absorption, stiffness and cushioning properties. The load-bearing capacity of FGWS was decreased and energy absorption was enhanced by compressing each spring with EVA (Ethylene-vinyl acetate) foam while traditionally manufactured non-contact metal wave springs showed a lower load-bearing capacity than AM (contact) wave springs. Moreover, the fatigue properties of both were compared after 100 cycles of compression, which found a good response of AM wave springs. Experimental compression results revealed a gradual smooth response of load against compression, i.e., cushioning and more energy absorption, validated by finite element analysis (FEA) with very little deviation.

Effects of Shoe Midsole Hardness on Lower Extremity Biomechanics during Jump Rope in Healthy Males.

This study investigated differences in lower extremity muscle activations and vertical stiffness during a 2.2 Hz jump rope exercise with different midsole hardnesses (45, 50, 55, and 60 Shores C). Twelve healthy male participants wore customized shoes with different hardness midsoles and performed jump rope exercises in a random order. A nine-camera motion analysis system (150 Hz), a force platform (1500 Hz), and a wireless electromyography (EMG) system (Noraxon, 1500 Hz) were used to measure the biomechanical parameters during the jump rope exercise. The biceps femoris %MVC of barefoot participants was significantly greater than that of those wearing the 45 Shores C (p = 0.048) and 55 Shores C (p = 0.009) midsole 100 ms before landing. The vastus medialis %MVC of barefoot participants was significantly greater than that of those wearing the 55 C midsole (p = 0.005). Nonsignificant differences in vertical stiffness were found between midsole hardnesses and barefoot. Lower extremity muscle activation differed between conditions. The results of this study indicate that for repetitive activities that entail multiple impacts, sports shoes with a low midsole hardness (e.g., 50 Shores C or 45 Shores C) may be appropriate. It is important to provide customers with information regarding midsole hardness in shoe product labeling so that they properly consider the function of the shoes.

Performance of ethylene vinyl acetate waste (EVA-w) when incorporated into expanded EVA foam for footwear

Ethylene vinyl acetate copolymer blends (EVA-B), with vinyl acetate set at 19 % and 28 % by weight (wt%), were mixed with EVA waste (EVA-w), (a reticulated and micronized ethylene vinyl acetate copolymer) from injection branches and defective midsoles. The EVA-B/EVA-w composites were prepared with 15, 25, and 35 per hundred rubber (phr) EVA-w using a Banbury mixer to mix all components. Afterwards, the mixture was submitted to compression molding to obtain expanded EVA-B/EVA-w composite plates. A standard compound, provided by a Brazilian footwear industry (EVA-ref), was used as reference for thermal and mechanical property comparisons. During the thermal analysis, when increasing the EVA-w content, a delay in the deacetylation of the EVA-B composites in relation to the EVA-ref was observed. There was also an increase in the maximum degradation rate temperature and a reduction in loss of mass in the first decomposition event. In the second event, the maximum temperature did not vary in the composites with EVA-w added as compared to the EVA-ref. The residue content increased at 600 °C, indicating that EVA-w acts as a filler for EVA-B. The mechanical properties evaluated were hardness, tension at break, elongation at break, tear strength, compression set, and shrinkage. Addition of EVA-w to the composites (comparing 15 phr–35 phr) increased tear strength properties by 4 %, (EVA-w acts as a nucleation cell agent, increasing the EVA foam cell density). However, shrinkage increased by roughly 35 % above the specifications for footwear applications. Hardness increased by 5 %, tensile strength increased by 10 %, and elongation at break decreased by roughly 4 %. The compression set property increased by nearly 5 % with the EVA-w content (due to decreasing matrix elongation). Thus, EVA-w acts as a reinforcement filler for EVA-B. A statistical study based on composite performance indices for the mechanical properties, and a cost analysis was performed. Except for the average linear shrinkage (35 phr EVA-w composite), all other studied composites met the mechanical property specifications for sports shoe midsoles. The best combination for process performance and cost analysis was for the EVA-w composite at 25 phr.