Mismatch In Mechanical Properties Research Articles

Stochastic lattice-based porous implant design for improving the stress transfer in unicompartmental knee arthroplasty

BackgroundUnicompartmental knee arthroplasty (UKA) has been proved to be a successful treatment for osteoarthritis patients. However, the stress shielding caused by mismatch in mechanical properties between human bones and artificial implants remains as a challenging issue. This study aimed to properly design a bionic porous tibial implant and evaluate its biomechanical effect in reconstructing stress transfer pathway after UKA surgery.MethodsVoronoi structures with different strut sizes and porosities were designed and manufactured with Ti6Al4V through additive manufacturing and subjected to quasi-static compression tests. The Gibson-Ashby model was used to relate mechanical properties with design parameters. Subsequently, finite element models were developed for porous UKA, conventional UKA, and native knee to evaluate the biomechanical effect of tibial implant with designed structures during the stance phase.ResultsThe internal stress distribution on the tibia plateau in the medial compartment of the porous UKA knee was found to closely resemble that of the native knee. Furthermore, the mean stress values in the medial regions of the tibial plateau of the porous UKA knee were at least 44.7% higher than that of the conventional UKA knee for all subjects during the most loading conditions. The strain shielding reduction effect of the porous UKA knee model was significant under the implant and near the load contact sites. For subject 1 to 3, the average percentages of nodes in bone preserving and building region (strain values range from 400 to 3000 μm/m) of the porous UKA knee model, ranging from 68.7 to 80.5%, were higher than that of the conventional UKA knee model, ranging from 61.6 to 68.6%.ConclusionsThe comparison results indicated that the tibial implant with designed Voronoi structure offered better biomechanical functionality on the tibial plateau after UKA. Additionally, the model and associated analysis provide a well-defined design process and dependable selection criteria for design parameters of UKA implants with Voronoi structures.

Nonlinear Elasticity of Blood Vessels and Vascular Grafts.

The transplantation of vascular grafts has emerged as a prevailing approach to address vascular disorders. However, the development of small-diameter vascular grafts is still in progress, as they serve in a more complicated mechanical environment than their counterparts with larger diameters. The biocompatibility and functional characteristics of small-diameter vascular grafts have been well developed; however, mismatch in mechanical properties between the vascular grafts and native arteries has not been accomplished, which might facilitate the long-term patency of small-diameter vascular grafts. From a point of view in mechanics, mimicking the nonlinear elastic mechanical behavior exhibited by natural blood vessels might be the state-of-the-art in designing vascular grafts. This review centers on elucidating the nonlinear elastic behavior of natural blood vessels and vascular grafts. The biological functionality and limitations associated with as-reported vascular grafts are meticulously reviewed and the future trajectory for fabricating biomimetic small-diameter grafts is discussed. This review might provide a different insight from the traditional design and fabrication of artificial vascular grafts.

Topology optimization of embracing fixator considering bone remodeling to mitigate stress shielding effect

The embracing fixator is one of the widely used internal fixation implants for bone fracture treatment. However, the stress shielding effect, a stress imbalance between the implant and bone caused by the mismatch in mechanical properties between them, is a significant and critical issue that may lead to treatment failure. Thus, it is of great importance to design the implant with an appropriate stiffness which can mitigate the stress shielding effect and provide the most favorable mechanical environment for bone healing and remodeling. To this end, a time-dependent topology optimization algorithm considering bone remodeling is proposed to optimize an embracing fixator used in the tibia fracture treatment. The change of callus density over time is simulated based on a bone remodeling model, and the callus density after a period of bone remodeling is selected to be the design objective to maximize. The design constraints include volume and the compliance of the whole fixation system. Meanwhile, the influence of the constraints on the regularity of material distribution of the optimized result is also studied. Besides, to test the effectiveness of the consideration of the bone remodeling in the embracing fixator design, a topology optimization concerning the minimization of the compliance of the entire system is also performed to make a comparison. Finally, the safety performance of optimized results considering bone remodeling is also verified by static analysis.

Fixation strength of swelling copolymeric anchors in artificial bone.

Fixation with suture anchors and metallic hardware for osteosynthesis is common in orthopedic surgeries. Most metallic commercial bone anchors achieve their fixation to bone through shear of the bone located between the threads. They have several deficiencies, including stress-shielding due to mechanical properties mismatch, generation of acidic by-products, poor osteointegration, low mechanical strength and catastrophic failure often associated with large bone defects that may be difficult to repair. To overcome these deficiencies, a swelling porous copolymeric material, to be used as bone anchors with osteointegration potential, was introduced. The purpose of this study was to investigate the fixation strength of these porous, swelling copolymeric bone anchors in artificial bone of various densities. The pull-out and subsidence studies indicate an effective fixation mechanism based on friction including re-fixation capabilities, and minimization of damage following complete failure. The study suggests that this swelling porous structure may provide an effective alternative to conventional bone anchors, particularly in low-density bone.

Life prediction and failure analysis in laser powder bed fused TiC/Ti6Al4V titanium matrix composite under high cycle and very high cycle fatigue conditions

This study investigates the failure behavior of a TiC-reinforced titanium matrix composite (TMC) manufactured by laser powder bed fusion (L-PBF) through fully reversed and pulsating tension fatigue tests in both high-cycle and very-high-cycle regimes. Inhomogeneous hardening zone (IHZ) and un-melted TiC zone (UMTZ) resulting from TiC agglomeration, along with pore and lack of fusion (LOF) generated during the L-PBF process, play a crucial role in inducing fatigue failure and influencing the variability of fatigue life. Advanced microscopy techniques, including optical microscopy, scanning electron microscopy, and electron backscatter diffraction, were employed to elucidate four failure modes: LOF-assisted IHZ cracking, IHZ cracking, pore cracking, and UMTZ cracking. IHZ cracking originates from stress concentration caused by the mechanical property mismatch between IHZ and Ti matrix, leading to brittle fracture of α'-grains within the IHZ. This constitutes the predominant failure mode. The presence of LOF near IHZ further exacerbates stress concentration, diminishing the material's load-carrying capacity and significantly reducing fatigue life. Considering the fatigue cracking mechanism, microstructural characteristics and the influence of defect location and stress concentration, a crack nucleation life prediction model is developed. The agreement between experimental and predicted data is satisfactory.

Structure engineering-enabled multi-direction-reconfigurable, soft, rechargeable lithium-ion battery with tissue-compliance low modulus and high performance

The significant demand for wearable and implantable bioelectronics necessitates high-performance power units. However, conventional batteries typically comprise rigid and bulky metals, or soft materials with conductive fillers (e.g., carbon nanotubes). These materials may induce undesired immune responses and battery failures when subjected to violent deformations due to the mismatch in mechanical properties between biological tissues (<100 kPa) and batteries. Additionally, current research on tissue-compliance soft batteries is based on the material-based low modulus and stretchability, where hydrogels are used as the elastic low modulus matrix. In this study, a structure-based low modulus electrode is developed wherein one-dimension (1D) uniaxial and two-dimension (2D) biaxial Kirigami pattern electrodes were carefully designed to provide both tissue-like modulus and multi-directional reconfigurability. The resulting reconfigurable soft battery exhibits excellent compliance and softness with Young's modulus of 64.1 kPa, which, to our knowledge, is the lowest modulus achieved for batteries with decent performance. The electrochemical performance of the reconfigurable lithium-ion soft battery under various deformations, such as stretching, bending, and twisting was investigated, and a high specific capacity of 83.5 mAh/g was achieved at a current density of 0.5 A/g under 100% stretching strain. The multi-directional stretchability, long-term stability, and biocompatibility of the reconfigurable soft battery were demonstrated. This groundbreaking work paves the way for the designing and manufacturing of soft batteries specifically tailored for wearable and implantable bioelectronics.

Multilayer gradient rare earth disilicate coating prepared by in-situ melt infiltration and sintering method with high thermal shock resistance on porous Si3N4 ceramics

Dense multilayer gradient rare earth disilicate (γ-Y2Si2O7 /β-Yb2Si2O7 /β-Lu2Si2O7) coatings were in-situ prepared by melt-infiltration /sintering procedure on porous Si3N4 ceramics for water resistance. Experimental and numerical simulation methods were used to study their thermal shock behavior. As a control, thermal shock behavior of pure γ-Y2Si2O7 coatings were also compared. FEM results showed that the gradient design of modulus in ceramic coating can effectively avoid the mismatch of mechanical properties between coating layer and internal substrate, reducing the transient thermal stress in each layer during thermal shock. All of the pure γ-Y2Si2O7 coatings were failed after thermal shock tests with ΔT ≈ 1200 ℃. However, when sintering temperature of multi-layer disilicate coatings were higher than 1400 ℃, the water absorption rates after thermal shock were all less than 5%, still showing good waterproof performance. The gradient design of modulus could effectively improve the structural stability of ceramic coatings.

Evolution from Bioinert to Bioresorbable: In Vivo Comparative Study of Additively Manufactured Metal Bone Scaffolds.

Additively manufactured scaffolds offer significant potential for treating bone defects, owing to their porous, customizable architecture and functionalization capabilities. Although various biomaterials have been investigated, metals - the most successful orthopedic material - have yet to yield satisfactory results. Conventional bio-inert metals, such as titanium (Ti) and its alloys, are widely used for fixation devices and reconstructive implants, but their non-bioresorbable nature and the mechanical property mismatch with human bones limit their application as porous scaffolds for bone regeneration. Advancements in additive manufacturing have facilitated the use of bioresorbable metals, including magnesium (Mg), zinc (Zn), and their alloys, as porous scaffolds via Laser Powder Bed Fusion (L-PBF) technology. This in vivo study presents a comprehensive, side-by-side comparative analysis of the interactions between bone regeneration and additively manufactured bio-inert/bioresorbable metal scaffolds, as well as their therapeutic outcomes. The research offers an in-depth understanding of the metal scaffold-assisted bone healing process, illustrating that Mg and Zn scaffolds contribute to the bone healing process in distinct ways, but ultimately deliver superior therapeutic outcomes compared to Ti scaffolds. These findings suggest that bioresorbable metal scaffolds hold considerable promise for the clinical treatment of bone defects in the near future.

A Minimalistic, Synthetic Cell‐Inspired Metamaterial for Enabling Reversible Strain‐Stiffening

AbstractStrain‐stiffening, i.e. the nonlinear stiffening of a material in response to a strain, is an intrinsic feature of many biological systems, including skin, blood vessels, and single cells. To avoid a mismatch in mechanical properties, synthetic materials in contact with such biological systems should also be strain‐stiffening. Conventional strain‐stiffening materials are either highly dependent on the applied strain‐rate, or only available for a limited stiffness regime. Both aspects limit the applicability of these materials. In contrast, living cells employ a dynamic strain‐stiffening mechanism that is based on the cross‐linking of cytoskeletal fibers in response to external stress. This strain‐stiffening of the cytoskeleton is mimicked in a mechanical metamaterial by a minimalistic structure consisting of parallel slats connected to backbones. Herein, it is demonstrated experimentally that the structures can be adapted such that the strain required for stiffening, the final stiffness, as well as the degree of stiffening can be tuned, particularly by combining several strain‐stiffening elements. These properties make the structure promising for the development of devices that should resemble the mechanical properties of human soft tissues, e.g., skin‐integrated flexible electronics and blood vessel grafts.

Fine Tuning of the Mechanical Properties of Bio-Based PHB/Nanofibrillated Cellulose Biocomposites to Prevent Implant Failure Due to the Bone/Implant Stress Shielding Effect

A significant mechanical properties mismatch between natural bone and the material forming the orthopedic implant device can lead to its failure due to the inhomogeneous loads distribution, resulting in less dense and more fragile bone tissue (known as the stress shielding effect). The addition of nanofibrillated cellulose (NFC) to biocompatible and bioresorbable poly(3-hydroxybutyrate) (PHB) is proposed in order to tailor the PHB mechanical properties to different bone types. Specifically, the proposed approach offers an effective strategy to develop a supporting material, suitable for bone tissue regeneration, where stiffness, mechanical strength, hardness, and impact resistance can be tuned. The desired homogeneous blend formation and fine-tuning of PHB mechanical properties have been achieved thanks to the specific design and synthesis of a PHB/PEG diblock copolymer that is able to compatibilize the two compounds. Moreover, the typical high hydrophobicity of PHB is significantly reduced when NFC is added in presence of the developed diblock copolymer, thus creating a potential cue for supporting bone tissue growth. Hence, the presented outcomes contribute to the medical community development by translating the research results into clinical practice for designing bio-based materials for prosthetic devices.

A porous swelling copolymeric material for improved implant fixation to bone.

Most metallic commercial bone anchors, such as screws and suture anchors achieve their fixation to bone through shear of the bone located between the threads. They have several deficiencies, potentially leading to failure, which are particularly evident in low-density bone. These include stress-shielding resulting from mechanical properties mismatch; lack of mechanically induced remodeling and osteointegration; and when the pullout force on the anchor, during functional activities, exceeds their pullout strength, catastrophic failure occurs leaving behind large bone defects that may be hard to repair. To overcome these deficiencies, we introduced in this study a porous swelling co-polymeric material and studied its swelling and compressive mechanical characteristics as bone anchor under different configurations. Porosity was achieved by adding a non-dissolvable agent (NaCl) during the process of polymerization, which was later dissolved in water, leaving behind a porous structure with adequate porosity for osteointegration. Three different groups of cylindrical samples of the swelling co-polymer were investigated. Solid, fully porous, and partially porous with a solid core and a porous outer layer. The results of the swelling and simple compression study show that the partially porous swelling co-polymer maintains excellent mechanical properties matching those of cancellous bone, quick swelling response, and an adequate porous outer layer for mechanically induced osteointegration. These suggest that this material may present an effective alternative to conventional bone anchors particularly in low-density bone.

A multifunctional hydrogel dressing with high tensile and adhesive strength for infected skin wound healing in joint regions.

Most hydrogel dressings are designed for skin wounds in flat areas, and few are focused on the joint skin regions which undergo frequent movement. The mismatch of mechanical properties and poor fit between a hydrogel dressing and a wound in joint skin results in hydrogel shedding, bacterial infection and delayed healing. Therefore, it is of great significance to design and prepare a multifunctional hydrogel with high tensile and tissue-adhesive strength as well as other therapeutic effects for the treatment of joint skin wounds. In this work, a multifunctional hydrogel was reasonably prepared by simply mixing polyvinyl alcohol (PVA), borax, tannic acid (TA) and iron(III) chloride in certain proportions, which was further used to treat the skin wounds at the joint of the hind limb. Acting as the physical crosslinkers, borax and TA dynamically bond with PVA and provide the resulting hydrogel with strong tensile, fast shape-adaptive and self-healing properties. The photothermal bacteriostatic activity of the hydrogel is attributed to the formation of a metallic polyphenol network (MPN) between ferric ions and TA. In addition, the hydrogel exhibits high levels of adhesion, hemostatic performance, antioxidant abilities, and biocompatibility, and shows great potential to promote joint skin wound healing.

Additive Manufacturing of Polymer/Mg-Based Composites for Porous Tissue Scaffolds.

Due to their commercial availability, superior processability, and biocompatibility, polymers are frequently used to build three-dimensional (3D) porous scaffolds. The main issues limiting the widespread clinical use of monophasic polymer scaffolds in the bone healing process are their inadequate mechanical strength and inappropriate biodegradation. Due to their mechanical strength and biocompatibility, metal-based scaffolds have been used for various bone regenerative applications. However, due to the mismatch in mechanical properties and nondegradability, they lack integration with the host tissues, resulting in the production of fiber tissue and the release of toxic ions, posing a risk to the durability of scaffolds. Due to their natural degradability in the body, Mg and its alloys increasingly attract attention for orthopedic and cardiovascular applications. Incorporating Mg micro-nano-scale particles into biodegradable polymers dramatically improves scaffolds and implants' strength, biocompatibility, and biodegradability. Polymer biodegradable implants also improve the quality of life, particularly for an aging society, by eliminating the secondary surgery often needed to remove permanent implants and significantly reducing healthcare costs. This paper reviews the suitability of various biodegradable polymer/Mg composites for bone tissue scaffolds and then summarizes the current status and challenges of polymer/magnesium composite scaffolds. In addition, this paper reviews the potential use of 3D printing, which has a unique design capability for developing complex structures with fewer material waste at a faster rate, and with a personalized and on-site fabrication possibility.

Micromechanical property mismatch between pericellular and extracellular matrices regulates stem cell articular and hypertrophic chondrogenesis

<h2>Summary</h2> Within the complex microarchitecture of native cartilage tissue, the micromechanical properties of pericellular and extracellular matrices (PCM and ECM) potentially play important roles in developmental, physiological, and pathological processes. Here, we report a unique biomaterial-based engineering strategy to create cartilage-tissue equivalents possessing PCM-ECM microarchitecture of native cartilage, where human mesenchymal stem cell (hMSC)-laden soft microgels representing PCM are encapsulated in stiff hydrogels representing ECM. Mechanical property mismatches between soft PCM and stiff ECM under cyclic compression regulates hMSC proliferation and chondrogenesis. High PCM-ECM mechanical mismatch (softer PCM) and the presence of PCM degradation under cyclic compression individually or synergistically direct hMSC articular chondrogenesis through the proliferation-associated protein kinase C signaling pathway, whereas low PCM-ECM mechanical mismatch (stiffer PCM) is solely responsible for hMSC hypertrophic chondrogenesis through the yes-associated protein signaling pathway. Our findings highlight PCM-ECM mechanical property mismatch as a critical design cue under dynamic compression for hMSC-based cartilage repair.

Mechanical behaviour of nitric oxide releasing polymers for cardiovascular bypass grafts

The advents of balloon angioplasty and of vascular bypass grafts have resulted in a paradigm shift in the management of cardiovascular diseases, but thrombus formation rising from the progression of intimal hyperplasia due to vessel damage and delayed arterial healing continue to be an intractable clinical failure of synthetic vascular bypass grafts. Nitric oxide (NO) releasing vascular bypass grafts have been developed as alternatives to conventional vascular grafts, but concerns over their short patency owing to the dissimilarity of mechanical properties between bypass grafts and innate arteries have recently been raised. Mismatch in mechanical properties at the endways inosculation of an artery and graft causes shear rate instabilities, which in turn stimulates intimal hyperplasia, thereby leading to graft failure. With the rapid development of manufacturing of innovative bypass grafts, no intervention has hitherto unequivocally proven to be clinically viscoelastic in preventing bypass graft failure. An ideal vascular graft closely mimics the structure and mechanical functions of native blood vessels and the configuration of each layer of the blood vessel. The overarching goal of this perspective is to highlight the importance and effects of mechanical properties of NO-releasing grafts in achieving optimal therapeutic outcomes for the management of cardiovascular diseases.

Maple Leaf Inspired Conductive Fiber with Hierarchical Wrinkles for Highly Stretchable and Integratable Electronics.

Stretchable and durable conductors are significant to the development of wearable devices, robots, human-machine interfaces, and other artificial intelligence products. However, the desirable strain-insensitive conductivity and low hysteresis are restricted by the failure of stretchable structures and mismatch of mechanical properties (rigid conductive layer and elastic core substrate) under large deformation. Here, based on the principles of fractal geometry, a stretchable conductive fiber with hierarchical wrinkles inspired by the unique shape of the maple leaf was fabricated by combining surface modification, interfacial polymerization, and improved prestrain finishing methods to break through this dilemma. The shape and size of wrinkles predicted by buckling analysis via the finite element method fit well with that of actual wrinkles (30-80 μm of macro wrinkles and 4-6 μm of micro wrinkles) on the fabricated fiber. Such hierarchically wrinkled conductive fiber (HWCF) exhibited not only excellent strain-insensitive conductivity denoted by the relative resistance change ΔR/R0 = 0.66 with R0 the original resistance and ΔR the change of resistance after the concrete strain reaching up to 600%, but also low hysteresis (0.04) calculated by the difference in area between stretching and releasing curve of the ΔR/R0 strain under 300% strain and long-term durability (>1000 stretching-releasing cycles). Furthermore, the elastic conductive fiber with such a bionic structure design can be applied as highly stretchable electrical circuits for illumination and monitors for the human motion under large strains through tiny and rapid resistance changes as well. Such a smart biomimetic material holds great prospects in the field of stretchable electronics.

Towards virtually optimized curing cycles for polymeric encapsulations in microelectronics

Surface Mounted Devices (SMDs) are widely used throughout microelectronics and power electronics. They mostly employ epoxy molding compound (EMC) based encapsulations. Thus, enhanced lifetime assessment methods are necessary. To understand the stress situation in SMDs at the end of the production cycle, an improved model approach for the curing of EMC is implemented within Finite Element Analysis (FEA) simulations. During production, e.g., in a Resin Transfer Molding (RTM) process, material properties are spatially varying due to different curing degrees. Hence, a mismatch of mechanical properties is present, which in return leads to internal stresses. The introduced model approach is an extension of the work of Gschwandl et al. (2017) and includes a stress-free deformation before vitrification, changing material properties during curing, as well as plastic deformations and visco-elastic effects. The implementation in numerical FEA simulations allows for a better understanding of arising residual stresses and helps optimize the production cycle of SMDs.

Diamond-like carbon films for tribological modification of rubber

Abstract The service conditions of rubber seals are primarily in the dynamic sliding state, with a high coefficient of friction, which can seriously compromise the equipment’s safety and the services’ lifespan. Diamond-like carbon (DLC) films have been the ideal candidate for tribological modification of rubber surfaces due to their excellent tribological properties over the last two decades. This field can be widely discussed as a hard film on soft substrates, where the challenge is the mismatch of mechanical properties, leading to the exfoliation of DLC films in friction. Early work developed the DLC films with a segment structure to solve this critical issue, illustrating the possibility of wear-resistant rubber. In recent years, researchers have paid attention to further increasing the coated rubber’s lifetime in friction, focusing on adhesion. These research achievements were reviewed on the aspects of structurization, adhesion, and doping in this article. It proposed an alternative direction of understanding the surface wear mechanism for designing wear-resistant DLC films on rubber.

The preparation and lubrication properties of a CrN/Cr multilayer using the double glow plasma surface alloying technique

Metal sealing rings with high hardness can improve wear resistance and reduce internal leakage. In this study, the double glow plasma surface alloying (DGPSA) technique was employed to produce the necessary conditions for a metallurgical bond and lubricant storage. Four primary stages are required in deposition processing: evaporation, destruction, atomization, and ionization. A Cr layer (including a 5 μm interdiffusion layer and a 45 μm pure metallic layer in thickness) and a thin (1–2 μm) CrN layer were prepared using the DGPSA technique. The obtained interdiffusion layer can reduce the mismatch in mechanical properties between the Cr layer and the substrate. When the atomic coverage of N atoms is higher than 1.5 ML, the stable adsorption structure is essentially nonexistent, indicating that improving the N2 ionization rate is more valuable than increasing the nitrogen partial pressure. When the Ar/N2 flow ratio is 2:1, the CrN/Cr multilayers provided microreservoirs for lubricants and improved wear and plastic deformation resistance. For the CrN/Cr multilayer, a higher contact pressure resulted in superior frictional properties.

Solid-state hot forge bonding of aluminium-steel bimetallic gears: Deformation mechanisms, microstructure and mechanical properties

Solid-state dissimilar bi- or multi-metallic bonding is promising for achieving lightweight or multifunctional components in automotive, nuclear power and aerospace industries. To understand how to achieve a high-quality bonding interface between dissimilar materials, aluminium alloy (Al)–steel (Fe) bimetal gears manufactured under hot forge bonding were systematically investigated. In this work, comprehensive analyses of forge bonding mechanics, microstructure features, bonding interface behaviours and resulting mechanical properties were undertaken using ex/in-situ experiments and finite element modelling. The results revealed that the bonding behaviour and microstructure evolution were significantly affected by the mechanical property mismatch between the two dissimilar workpieces (AA6082 and E355). This mismatch could be effectively adjusted by setting different forging temperatures. The interfacial bonding strengths of AA6082 and E355, manufactured at low and high temperatures, were observed to be governed by interdiffusion and oxide particles, respectively. Balancing interdiffusion and oxide breaking appears to be key to achieving optimized interface strength for dissimilar bimetallic forge bonding technology.