Design and biomechanical characteristics of porous meniscal implant structures using triply periodic minimal surfaces

BACKGROUND

Artificial meniscal implants can be used to replace a severely injured meniscus after meniscectomy and restore the normal functionality of a knee joint. The aim of this paper was to design porous meniscal implants and assess their biomechanical properties.

METHODS

Finite element simulations were conducted on eight different cases including intact healthy knees, knee joints with solid meniscal implants, and knee joints with meniscal implants with two types of triply periodic minimal surfaces. Compression stresses, shear stresses, and characteristics of stress concentrated areas were evaluated using an axial compressive load of 1150 N and an anterior load of 350 N.

RESULTS

Compared to the solid meniscal implant, the proposed porous meniscal implant produced lower levels of compression and shear stresses on the cartilage, which facilitated the cartilage to retain a semilunar characteristic similar to the natural meniscus. Moreover, both compression and shear stresses on the artificial cartilage were found to be sensitive to the pore properties of the meniscal implant. The meniscal implants with primitive surfaces (porosity: 41%) showed a better performance in disseminating stresses within the knee joint.

CONCLUSION

The present commercial meniscal implant has the problem of equivalent biomechanical properties compared to natural menisci. The main advantage of the proposed porous structure is that it can be used to prevent excessive compression and shear stresses on the articular cartilages. This structure has advantages both in terms of mechanics and printability, which can be beneficial for future clinical applications.

Bioceramic scaffolds with triply periodic minimal surface architectures guide early-stage bone regeneration

The pore architecture of porous scaffolds is a critical factor in osteogenesis, but it is a challenge to precisely configure strut-based scaffolds because of the inevitable filament corner and pore geometry deformation. This study provides a pore architecture tailoring strategy in which a series of Mg-doped wollastonite scaffolds with fully interconnected pore networks and curved pore architectures called triply periodic minimal surfaces (TPMS), which are similar to cancellous bone, are fabricated by a digital light processing technique. The sheet-TPMS pore geometries (s-Diamond, s-Gyroid) contribute to a 3‒4-fold higher initial compressive strength and 20%-40% faster Mg-ion-release rate compared to the other-TPMS scaffolds, including Diamond, Gyroid, and the Schoen's I-graph-Wrapped Package (IWP) in vitro. However, we found that Gyroid and Diamond pore scaffolds can significantly induce osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). Analyses of rabbit experiments in vivo show that the regeneration of bone tissue in the sheet-TPMS pore geometry is delayed; on the other hand, Diamond and Gyroid pore scaffolds show notable neo-bone tissue in the center pore regions during the early stages (3-5 weeks) and the bone tissue uniformly fills the whole porous network after 7 weeks. Collectively, the design methods in this study provide an important perspective for optimizing the pore architecture design of bioceramic scaffolds to accelerate the rate of osteogenesis and promote the clinical translation of bioceramic scaffolds in the repair of bone defects.

Insights into unit cell size effect on mechanical responses and energy absorption capability of titanium graded porous structures manufactured by laser powder bed fusion

Schwartz diamond graded porous structures (SDGPSs), constructed by a triply-periodic-minimal-surface diamond unit cell topology, were developed with various unit cell sizes and printed by laser powder bed fusion (LPBF) from a commercially pure titanium powder for bone implant applications. The effect of unit cell size on the printability, strut dimensions, stress and strain distributions, mechanical properties and energy absorption capability of SDGPSs was investigated. The results indicate the good printability of SDGPSs via LPBF with multiple unit cell sizes from 3.5 mm to 5.5 mm through the three-dimensional reconstruction from micro-computed tomography. The unit cell size plays a critical role in both strut diameters and specific surface areas of SDGPSs. An increase in the unit cell size leads to a reduction in the experimental Young's modulus from 673.08 MPa to 518.71 MPa and compressive yield strength from 11.43 MPa to 7.73 MPa. The mechanical properties of LPBF-printed SDGPSs are higher than those predicted by the finite element method, which is attributed to the higher volume fractions of the printed SDGPSs than the designed values. Furthermore, a rise in unit cell size leads to the decrease of energy absorption capability from 6.06 MJ/mm³ to 4.32 MJ/mm³ and exhibits little influence on the absorption efficiency. These findings provide a good understanding and guidance to the optimization on the unit cell size of functionally graded porous structures for desirable properties.

Design of a Haversian system-like gradient porous scaffold based on triply periodic minimal surfaces for promoting bone regeneration

INTRODUCTION

The bone ingrowth depth in the porous scaffolds is greatly affected by the structural design, notably the pore size, pore geometry, and the pore distribution. To enhance the bone regeneration capability of scaffolds, the bionic design can be regarded as a potential solution.

OBJECTIVES

We proposed a Haversian system-like gradient structure based on the triply periodic minimal surface architectures with pore size varying from the edge to the center. And its effects in promoting bone regeneration were evaluated in the study.

METHODS

The gradient scaffold was designed using the triply periodic minimal surface architectures. The mechanical properties were analyzed by the finite element simulation and confirmed using the universal machine. The fluid characteristics were calculated by the computational fluid dynamics analysis. The bone regeneration process was simulated using a in silico computational model containing the main biological, physical, and chemical variation during the bone growth process. Finally, the in vitro and in vivo studies were carried out to verify the actual osteogenic effect.

RESULTS

Compared to the uniform scaffold, the biomimetic gradient scaffold demonstrated better performance in stress conduction and reduced stress shielding effects. The fluid features were appropriate for cell migration and flow diffusion, and the permeability was in the same order of magnitude with the natural bone. The bone ingrowth simulation exhibited improved angiogenesis and bone regeneration. Higher expression of the osteogenesis-related genes, higher alkaline phosphatase activity, and increased mineralization could be observed on the gradient scaffold in the in vitro study. The 12-week in vivo study proved that the gradient scaffold had deeper bone inserting depth and a more stable bone-scaffold interface.

CONCLUSION

The Haversian system-like gradient structure can effectively promote the bone regeneration. This structural design can be used as a new solution for the clinical application of prosthesis design.

Corrosion fatigue behavior and anti-fatigue mechanisms of an additively manufactured biodegradable zinc-magnesium gyroid scaffold

Additively manufactured biodegradable zinc (Zn) alloy scaffolds constitute an important branch in orthopedic implants because of their moderate degradation behavior and bone-mimicking mechanical properties. This work investigated the corrosion fatigue response of a zinc-magnesium (Zn-Mg) alloy gyroid scaffold fabricated via laser-powder-bed-fusion additive manufacturing at the first time. The high-cycle compression-compression fatigue testing of the printed Zn-Mg scaffold was conducted in simulated body fluid, showing its favorable fatigue strength, structural reliability, and anti-fatigue capability. The printed Zn-Mg scaffold obtained a 227% higher fatigue strength than that of the printed Zn scaffold but 17% lower strain accumulation at 10⁶ cycles. The accumulative strain of the Zn-Mg scaffold at its fatigue strength was dominant by fatigue ratcheting, since the fatigue damage strain of the scaffold was approximately zero. The corrosion products (ZnO and Zn(OH)₂) were conducive to the inhibition of fatigue ratcheting and fatigue damage. Dislocation pile-up and solid solution phases at the grain boundaries of the Zn-Mg scaffold could retard the spreading of the crack tip and impede excessive grain coarsening, improving its fatigue endurance limit. Notably, the printed Zn-Mg scaffold could dissipate the fatigue energy through moderate grain boundary migration, thus reducing its plastic deformation. These findings illuminated the anti-fatigue mechanisms related to microstructural features and corrosive environments and highlighted the promising prospects of additively manufactured Zn-Mg scaffolds in orthopedic applications. STATEMENT OF SIGNIFICANCE: : Additive manufacturing (AM) of biodegradable metals shows unprecedented prospects for bone tissue regeneration medicine. The corrosion fatigue property is one of the key determinants in the performance of AM biodegradable scaffolds. In this study, a Zn-Mg gyroid scaffold was additively manufactured with admirable fatigue endurance limit and anti-fatigue capability. We reported that the corrosion fatigue performance was highly relevant to the microstructural features, validating that the grain boundary engineering strategy improved fatigue strength and inhibited crack penetration. Notably, moderate grain boundary migration could dissipate fatigue energy and reduce plastic deformation. Furthermore, corrosion products were conducive to impeding fatigue ratcheting and fatigue damage, indicating the promising potential of AM Zn-Mg scaffolds in treating load-bearing bone defects.

A novel artificial vertebral implant with Gyroid porous structures for reducing the subsidence and mechanical failure rate after vertebral body replacement

BACKGROUND

Prosthesis subsidence and mechanical failure were considered significant threats after vertebral body replacement during the long-term follow-up. Therefore, improving and optimizing the structure of vertebral substitutes for exceptional performance has become a pivotal challenge in spinal reconstruction.

METHODS

The study aimed to develop a novel artificial vertebral implant (AVI) with triply periodic minimal surface Gyroid porous structures to enhance the safety and stability of prostheses. The biomechanical performance of AVIs under different loading conditions was analyzed using the finite element method. These implants were fabricated using selective laser melting technology and evaluated through static compression and subsidence experiments.

RESULTS

The results demonstrated that the peak stress in the Gyroid porous AVI was consistently lower than that in the traditional porous AVI under all loading conditions, with a maximum reduction of 73.4%. Additionally, it effectively reduced peak stress at the bone-implant interface of the vertebrae. Static compression experiments demonstrated that the Gyroid porous AVI was about 1.63 times to traditional porous AVI in terms of the maximum compression load, indicating that Gyroid porous AVI could meet the safety requirement. Furthermore, static subsidence experiments revealed that the subsidence tendency of Gyroid porous AVI in polyurethane foam (simulated cancellous bone) was approximately 15.7% lower than that of traditional porous AVI.

CONCLUSIONS

The Gyroid porous AVI exhibited higher compressive strength and lower subsidence tendency than the strut-based traditional porous AVI, indicating it may be a promising substitute for spinal reconstruction.

A Natural Gradient Biological-Enabled Multimodal Triboelectric Nanogenerator for Driving Safety Monitoring

A key element to ensuring driving safety is to provide a sufficient braking distance. Inspired by the nature triply periodic minimal surface (TPMS), a gradient and multimodal triboelectric nanogenerator (GM-TENG) is proposed with high sensitivity and excellent multimodal monitoring. The gradient TPMS structure exhibits the multi-stage stress-strain properties of typical porous metamaterials. Significantly, the multimodal monitoring capability depends on the implicit function of the defined level constant c, which directly contributes to the multimodal driving safety monitoring. The mechanical and electrical responsive behavior of the GM-TENG is analyzed to identify the applied speed, load, and working mode. In addition, optimized peak open-circuit voltage (Voc) is demonstrated for self-awareness of the braking condition. The braking distance factor (L) is conceived to construct the self-aware equation of the friction coefficient based on the integration of Voc with respect to time. Importantly, R-squared up to 94.29 % can be obtained, which improves self-aware accuracy and real-time capabilities. This natural structure and self-aware device provide an effective strategy to improve driving safety, which contributes to the improvement of road safety and presents self-powered sensing with potential applications in an intelligent transportation system.

Water isotope effect on the lipidic cubic phase: Heavy water-Induced interfacial area reduction of monoolein-Water system

Heavy water (D₂O) affects various functions of cells and living things. In order to gain fundamental insight into the molecular mechanism on biological effects of heavy water, D₂O-effects on fully hydrated monoolein (MO) systems were investigated from the structural viewpoints. At room temperature, the MO fully hydrated by pure light water (H₂O) forms a bicontinuous cubic (Pn3m) phase, and then, the Pn3m cubic phase transforms into an inverted hexagonal (H_II) phase at about 90°C. Temperature-scan X-ray diffraction measurements showed that substitution of D₂O for H₂O lowers the Pn3m-to-H_II phase transition temperature and reduces the lattice constants of both phases. The structural analysis of the Pn3m phase using the diffraction intensity data indicated that D₂O reduces the surface occupied area of MO at the interface by 12% in comparison with H₂O. This change is probably due to the difference of the strength of hydrogen bond.

Assessing the effect of manufacturing defects and non-Newtonian blood model on flow behaviors of additively manufactured Gyroid TPMS structures

In the field of medical engineering, Triply Periodic Minimal Surfaces (TPMS) structures have been studied widely owing to their physical attributes similar to those of human bones. Computational Fluid Dynamics (CFD) is often used to reveal the interaction between structural architectures and flow fields. Nevertheless, a comprehensive study on the effect of manufacturing defects and non-Newtonian behavior on the fluid responses in TPMS scaffolds is still lacking. Therefore, the present study fabricated Gyroid TPMS with four relative densities from 0.1 to 0.4. Non-destructive techniques were used to examine surface roughness and geometric deviation. We found that the manufacturing defects had a minor effect on fluid responses. The pressure drop comparison between defect-containing and defect-free models could be differed up to 7%. The same comparison for the average shear stress showed a difference up to 23%, in which greater deviation between both models was observed at higher relative density. On the contrary, the viscosity model played a significant role in flow prediction. By comparing the Newtonian model with Carreau-Yasuda non-Newtonian model, the resulting pressure drop and average wall shear stress from non-Newtonian viscosity could be higher than those of the Newtonian model by more than a factor of two. In addition, we matched the fluid-induced shear stress from both viscosity models with desirable ranges of shear stresses for tissue growth obtained from the literature. Up to 70% from the Newtonian model fell within the desirable range while the matching stress reduced to lower than 8% for the non-Newtonian results. Furthermore, by correlating geometric features with physical outputs, the geometric deviation was seen associated with surface curvature while the local shear stress revealed a strong correlation with inclination angle. Overall, the present work emphasized the importance of the viscosity model for CFD analysis of the scaffolds, especially when resulting fluid-induced wall shear stress is of interest. In addition, the geometric correlation has introduced the alternative consideration of structural architectures from local perspectives, which could assist the further comparison and optimization among different porous scaffolds in the future.

Triply periodic minimal surfaces for thermo-mechanical protection

Triply periodic minimal surface (TPMS) metamaterials show promise for thermal management systems but are challenging to integrate into existing packaging with strict mechanical requirements. Composite TPMS lattices may offer more control over thermal and mechanical properties through material and geometric tuning. Here, we fabricate copper-plated, 3D-printed triply periodic minimal surface primitive lattices and evaluate their suitability for battery thermal management systems. We measure the effects of lattice geometry and copper thickness on pressure drop, mechanical properties, and thermal conductivity. The lattices as internal filling structures in a multichannel cold plate exhibited pressure drops under 6.5 kPa at a 1 LPM flow rate. Pressure drop decreased when the number of channels (width of the cold plate) was increased. With a 0.43% copper volume loading, the lattice more than tripled in thermal conductivity but still retained a polymer-like compliance. A higher lattice relative density did not affect the thermal conductivity but caused a higher elastic modulus and compressive strength, and a stiffer cyclic loading response. The lattice design demonstrates that the structural parameters that control pressure drop, mechanical, and thermal conductivity can be decoupled, which can be used to achieve a wide range of disparate properties in complex multiphysics systems.

Design of 3D printing osteotomy block for foot based on triply periodic minimal surface

The ankle joint, which connects the lower limbs and the sole of the foot, is prone to sprain during walking and sports, which leads to ankle arthritis. Supratroleolar osteotomy is an ankle preserving operation for the treatment of ankle arthritis, in which the osteotomy is an important fixing and supporting part. In order to avoid stress shielding effect as much as possible, the osteotomy block is designed as a porous structure. In this study, the osteotomy block was designed based on three-period minimal surface, and the designed structure was manufactured by 3D printing. The mechanical properties of different structures were studied by mechanical test and finite element simulation. In mechanical tests, the Gyroid structure showed a progressive failure mechanism from bottom to bottom, while the Diamond structure showed a shear failure zone at 45° Angle, which was not conducive to energy absorption and was more prone to brittle fracture than the Gyroid structure. Therefore, the Gyroid structure is valuable for further research in the development of porous osteotomy.

3d-printed sacral reconstruction prosthesis from multiscale topology optimization: A comprehensive numerical assessment of mechanical stability

Sacral chordoma, an invasive tumor, necessitates surgical removal of the tumor and the affected region of the sacrum, disrupting the spinopelvic connection. Conventional reconstruction methods, relying on rod and screw systems, often face challenges such as rod failure, sub-optimal stability, and limited osseointegration. This study proposes a novel design for a porous-based sacral reconstruction prosthesis. The design framework involves a two-step topology optimization (TO) process. The first TO step is utilized to obtain the external shape of a patient-specific prosthesis, while the second TO step determines varied density fields. These fields are later integrated with graded Gyroid structures to generate the porous-based sacral prosthesis. Finite element simulations reveal several benefits of the newly developed device. Firstly, considering only solid-based TO tends to result in a highly rigid spinal movement, which may not be entirely favorable. However, the porous-based technique allows for a wider design space, enabling the sacral device's stiffness to be more comprehensively engineered. Secondly, with porous integration, the prosthesis shows potential for promoting bone integration over time, thereby providing further biological fixation and improving long-term structural stability. Thirdly, the porous-based prosthesis outperforms conventional methods such as four-rod reconstruction (FRR) and four-rod plus anterior column reconstruction (FRACR) by reducing maximum von Mises stress in the instruments by approximately 50-80 %. In summary, this study demonstrates how a two-step TO framework can create a superior sacral prosthesis, enhancing its mechanical performance and impact on spinopelvic stability. This suggests potential improvement for similar orthopedic devices in the future.

Mechanical characterization of 3D printed multi-morphology porous Ti6Al4V scaffolds based on triply periodic minimal surface architectures

Heterogeneous biomaterials that simultaneously mimic the topological and mechanical properties of nature bone tissues are of great interest in recent years. In this study, multi-morphology porous scaffolds based on the triply periodic minimal surfaces (TPMS) were designed and 3D printed with spatially changing pore patterns. Experiments and numerical analyses were carried out to assess the mechanical properties of the multi-morphology graded porous scaffold. As can be seen from the results, the multi-morphology structure showed a combination of relatively low elastic moduli and high yield strength. This combination allows for simultaneously minimizing the bone damage and increasing the stability of bone-implant interface. Thus the 3D printed multi-morphology porous Ti6AlV scaffold had shown significant promise for orthopedic application.

Spatially distributed and interconnected porous architectures for dental implants

PURPOSE

Patients with pre-existing medical conditions that impair bone integrity face challenges in dental implant success due to compromised osseointegration. This study evaluates three titanium interconnected porous architectures: the TPMS solid gyroid, TPMS sheet gyroid, and Voronoi stochastic lattice. We aim to assess manufacturability, design controllability, and cellular interactions to identify an optimal architecture that enhances cellular behavior with the potential to strengthen bone-to-implant contact.

METHODS

Three porous architectures were designed and compared: the two variants of the uniform, periodic triply periodic minimal surface (TPMS) gyroid, and the random, non-uniform Voronoi stochastic lattice. The porous constructs were fabricated using selective laser melting (SLM) and evaluated using microcomputed tomography (microCT) for porosity, manufacturability, and permeability. In vitro experiments used primary bone marrow stromal cells (BMSCs) isolated from 8-week-old wild type C57BL6/J mice. These cells were seeded onto the SLM-fabricated porous architectures and evaluated for adhesion using scanning electron microscopy (SEM) and RNA extraction. Cell trajectory was profiled using fluorescent confocal microscopy.

RESULTS

Selective laser melting (SLM) successfully fabricated all three porous architectures, with the TPMS solid gyroid exhibiting the highest manufacturing resolution, controllability, and the most uniform pore distribution. Computational fluid dynamics (CFD) analysis showed that its permeability outperformed both the TPMS sheet gyroid and stochastic Voronoi architectures. In vitro cell culturing demonstrated superior cell behavior in the TPMS solid gyroid scaffold. RNA quantification after 72 h of culture showed that cells are most adherent to the TPMS solid gyroid, demonstrating a 4-fold increase in RNA quantity compared to the fully dense (control). Additionally, cell trajectory analysis indicated enhanced cell infiltration and cellularization within the pore channels for the TPMS solid gyroid architecture.

CONCLUSION

This research demonstrates that inducing an interconnected porous architecture into a titanium construct enhances cellular behavior compared to a traditional dense implant. The TPMS solid gyroid architecture showed superior manufacturability, making it a promising solution to improve dental implant success in patients with compromised bone integrity.

Evaluating the effect of pore size for 3d-printed bone scaffolds

The present study investigated the influence of pore size of strut-based Diamond and surface-based Gyroid structures for their suitability as medical implants. Samples were made additively from laser powder bed fusion process with a relative density of 0.3 and pore sizes ranging from 300 to 1300 μm. They were subsequently examined for their manufacturability and mechanical properties. In addition, non-Newtonian computational fluid dynamics and discrete phase models were conducted to assess pressure drop and cell seeding efficiency. The results showed that both Diamond and Gyroid had higher as-built densities with smaller pore sizes. However, Gyroid demonstrated better manufacturability as its relative density was closer to the as-designed one. In addition, based on mechanical testing, the elastic modulus was largely unaffected by pore size, but post-yielding behaviors differed, especially in Diamond. High mechanical sensitivity in Diamond could be explained partly by Finite Element simulations, which revealed stress localization in Diamond and more uniform stress distribution in Gyroid. Furthermore, we defined the product of the normalized specific surface, normalized pressure drop, and cell seeding efficiency as the indicator of an optimal pore size, in which this factor identified an optimal pore size of approximately 500 μm for both Diamond and Gyroid. Besides, based on such criterion, Gyroid exhibited greater applicability as bone scaffolds. In summary, this study provides comprehensive assessment of the effect of pore size and demonstrates the efficient estimation of an in-silico framework for evaluating lattice structures as medical implants, which could be applied to other lattice architectures.

Crack propagation in TPMS scaffolds under monotonic axial load: Effect of morphology

In this paper, the mechanical behaviour and failure of porous additively manufactured (AM) polylactide (PLA) scaffolds based on the triply periodic minimal surfaces (TPMS) is investigated using numerical calculations of their unit cells and representative volumes. The strain-amplification factor is chosen as the main parameter, and the most critical locations for failure of different types of scaffold structures are evaluated. The results obtained are presented in comparison with a multiple-crack-growth algorithm using the extended finite element method (XFEM), underpinned by the experimentally obtained fracture properties of PLA. The effect of morphology of TPMS structures on the pre-critical, critical and post-critical behaviours of scaffolds under monotonic loading regimes is assessed. The results provide an understanding of the fracture behaviour and main risk points for crack initiation in structures of AM-PLA scaffolds based on typical commonly used types of TPMS, as well as the influence of structure type and external load on this behaviour.

Stereolithography 3D printing gyroid triply periodic minimal surface vitrified bond diamond grinding wheel

The pores of vitrified bond diamond grinding wheel play a key role in the grinding process. However, uneven pore distribution and low porosity affect the grinding performance of the wheel significantly. Stereolithography based additive manufacturing provides an effective method to fabricate vitrified bond diamond grinding wheels with a uniform distribution and an interconnected pore structure. The key to high-performance grinding wheel via stereolithography 3D printing lies in the preparation of the slurry with high solid loading, low viscosity and uniform stability. In this study, the dispersion and stability of vitrified bond and diamond slurries were investigated systematically. The effects of resin monomers, surface modifiers, and solid loading on the dispersion, rheological behavior and stability of slurries were studied in detail. Finally, an optimal vitrified bond and diamond slurry for stereolithography based additive manufacturing was obtained, and complex-shaped gyroid triply periodic minimal surface grinding wheel were fabricated. By grinding the SiC ceramics, the material removal rate, grinding temperature, and surface roughness were compared to those achieved using a conventional solid structure grinding wheel. The results show that the gyroid porous grinding wheel can achieve better surface roughness and lower the grinding temperature.