Exploring the hierarchical structure of lamellar bone and its impact on fracture behaviour: A computational study using a phase field damage model

Bone is a naturally occurring composite material composed of a stiff mineral phase and a compliant organic matrix of collagen and non-collagenous proteins (NCP). While diverse mineral morphologies such as platelets and grains have been documented, the precise role of individual constituents, and their morphology, remains poorly understood. To understand the role of constituent morphology on the fracture behaviour of lamellar bone, a damage based representative volume element (RVE) was developed, which considered various mineral morphologies and mineralised collagen fibril (MCF) configurations. This model framework incorporated a novel phase-field damage model to predict the onset and evolution of damage at mineral-mineral and mineral-MCF interfaces. It was found that platelet-based mineral morphologies had superior mechanical performance over their granular counterparts, owing to their higher load-bearing capacity, resulting from a higher aspect ratio. It was also found that MCFs had a remarkable capacity for energy dissipation under axial loading, with these fibrillar structures acting as barriers to crack propagation, thereby enhancing overall elongation and toughness. Interestingly, the presence of extrafibrillar platelet-based minerals also provided an additional toughening through a similar mechanism, whereby these structures also inhibited crack propagation. These findings demonstrate that the two primary constituent materials of lamellar bone play a key role in its toughening behaviour, with combined effect by both mineral and MCFs to inhibit crack propagation at this scale. These results have provided novel insight into the fracture behaviour of lamellar bone, enhancing our understanding of microstructure-property relationships at the sub-tissue level.

Micromechanical modelling of transverse fracture behaviour of lamellar bone using a phase-field damage model: The role of non-collagenous proteins and mineralised collagen fibrils

At the tissue-scale and above, there are now well-established structure-property relationships that provide good approximations of the biomechanical performance of bone through, for example, power-law relationships that relate tissue mineral density to elastic properties. However, below the tissue-level, the individual role of the constituents becomes prominent and these simple relationships tend to break down, with more detailed theoretical and computational models are required to describe the mechanical response. In this study, a two-dimensional micromechanics damage-based representative volume element (RVE) of lamellar bone was developed, which included a novel implementation of a phase-field damage model to describe the behaviour of non-collagenous proteins at mineral-mineral and mineral-fibril interface regions. It was found that, while the stiffness of the tissue was governed by the relative proportion of extra-fibrillar mineral and mineralised collagen fibrils, the strength and toughness of the tissue in transverse direction relied on the interactions occurring at mineral-mineral and mineral-fibril interfaces, highlighting the prominence of non-collagenous proteins in determine fracture-based processes at this scale. While fractures tended to initiate in mineral rich areas of the extra-fibrillar mineral matrix, it was found that the presence of mineralised collagen fibrils at low density did not provide a substantial contribution to crack propagation behaviour under transverse loading. However, at physiological volume fraction (VfMCF=50%), different scenarios could arise depending on the relative strength value of the interphase around the MCFs ( [Formula: see text] ) to the interphase between individual minerals ( [Formula: see text] ): (i) When [Formula: see text] , MCFs appear to facilitate crack propagation with MCF-mineral debonding being the dominant failure mode; (ii) once γ>1, the MCFs hinder the microcracks, leading to inhibition of crack propagation, which can be regarded as an energy dissipation mechanism. The effective fracture properties of the tissue also experience a sudden increase in fracture work density (J-integral) once the crack is arrested by MCFs or severely deflected. Collectively, the predicted behaviour of the model compared well to those reported through experimental and computational methods, highlighting its potential to provide further understanding into the mechanistic response of bone ultrastructure alterations related to the structural and compositional changes resulting from disease and aging.

An integrated mechanical degradation model to explore the mechanical response of a bioresorbable polymeric scaffold

Simulation of bioresorbable medical devices is hindered by the limitations of current material models. Useful simulations require that both the short- and long-term response must be considered; existing models are not physically-based and provide limited insight to guide performance improvements. This study presents an integrated degradation framework which couples a physically-based degradation model, which predicts changes in both crystallinity (Xc) and molecular weight (Mn), with the results of a micromechanical model, which predicts the effective properties of the semicrystalline polymer. This degradation framework is used to simulate the deployment of a bioresorbable PLLA (Poly (L-lactide) stent into a mock vessel and the subsequent mechanical response during degradation under different diffusion boundary conditions representing neointimal growth. A workflow is established in a commercial finite element code that couples both the immediate and long-term responses. Clinically relevant lumen loss is reported and used to compare different responses and the effect of neo-intimal tissue regrowth post-implantation on degradation and on the mechanical response is assessed. In addition, the effects of possible changes in Xc, which could occur during processing and stent deployment, are explored.

Sustainable Bombyx mori's silk fibroin for biomedical applications as a molecular biotechnology challenge: A review

Silk is a natural engineering material with a unique set of properties. The major constituent of silk is fibroin, a protein widely used in the biomedical field because of its mechanical strength, toughness and elasticity, as well as its biocompatibility and biodegradability. The domestication of silkworms allows large amounts of fibroin to be extracted inexpensively from silk cocoons. However, the industrial extraction process has drawbacks in terms of sustainability and the quality of the final medical product. The heterologous production of fibroin using recombinant DNA technology is a promising approach to address these issues, but the production of such recombinant proteins is challenging and further optimization is required due to the large size and repetitive structure of fibroin's DNA and amino acid sequence. In this review, we describe the structure-function relationship of fibroin, the current extraction process, and some insights into the sustainability of silk production for biomedical applications. We focus on recent advances in molecular biotechnology underpinning the production of recombinant fibroin, working toward a standardized, successful and sustainable process.

Experimental and computational analysis of energy absorption characteristics of three biomimetic lattice structures under compression

The objective of this study is to evaluate the mechanical properties and energy absorption characteristics of the gyroid, dual-lattice and spinodoid structures, as biomimetic lattices, through finite element analysis and experimental characterisation. As part of the study, gyroid and dual-lattice structures at 10% volume fraction were 3D-printed using an elastic resin, and mechanically tested under uniaxial compression. Computational models were calibrated to the observed experimental data and the response of higher volume fraction structures were simulated in an explicit finite element solver. Stress-strain data of groups of lattices at different volume fractions were studied and energy absorption parameters including total energy absorbed per unit volume, energy absorption efficiency and onset of densification strain were calculated. Also, the structures were characterized into bending-dominant and stretch-dominant structures, according to their nodal connectivity and Gibson-and-Ashby's law. The results of the study showed that the dual-lattice is capable of absorbing more energy at each volume fraction cohort. However, gyroid structures showed higher energy absorption efficiency and the onset of densification at higher strains. The spinodoid structure was found to be the poorest structure in terms of energy absorption, specifically at low volume fractions. Also, the results showed that the dual-lattice was a stretch dominated structure, while the gyroid structure was a bending dominated structure, which may be a reason that it is a better candidate for energy absorption applications.

Elucidating the role of diverse mineralisation paradigms on bone biomechanics - a coarse-grained molecular dynamics investigation

Bone as a hierarchical composite structure plays a myriad of roles in vertebrate skeletons including providing the structural stability of the body. Despite this critical role, the mechanical behaviour at the sub-micron levels of bone's hierarchy remains poorly understood. At this scale, bone is composed of Mineralised Collagen Fibrils (MCF) embedded within an extra-fibrillar matrix that consists of hydroxyapatite minerals and non-collagenous proteins. Recent experimental studies hint at the significance of the extra-fibrillar matrix in providing the bone with the stiffness and ductility needed to serve its structural roles. However, due to limited resolution of experimental tools, it is not clear how the arrangement of minerals, and in particular their relative distribution between the intra- and extra-fibrillar space contribute to bone's remarkable mechanical properties. In this study, a Coarse Grained Molecular Dynamics (CGMD) framework was developed to study the mechanical properties of MCFs embedded within an extra-fibrillar mineral matrix and the precise roles extra- and intra-fibrillar mineralisation on the load-deformation response was investigated. It was found that the presence of extra-fibrillar mineral resulted in the development of substantial residual stress in the system, by limiting MCF shortening that took place during intra-fibrillar mineralisation, resulting in substantial compressive residual stresses in the extra-fibrillar mineral phase. The simulation results also revealed the crucial role of extra-fibrillar mineralisation in determining the elastic response of the Extrafibrillar mineralised MCF (EFM-MCF) system up to the yield point, while the fibrillar collagen affected the post-yield behaviour. When physiological levels of mineralisation were considered, the mechanical response of the EFM-MCF systems was characterised by high ductility and toughness, with micro-cracks being distributed across the extra-fibrillar matrix, and MCFs effectively bridging these cracks leading to an excellent combination of strength and toughness. Together, these results provide novel insight into the deformation mechanisms of an EFM-MCF system and highlight that this universal building block, which forms the basis for lamellar bone, can provide an excellent balance of stiffness, strength and toughness, achieving mechanical properties that are far beyond the capabilities of the individual constituents acting alone.

Shape-Setting of Self-Expanding Nickel-Titanium Laser-Cut and Wire-Braided Stents to Introduce a Helical Ridge

PURPOSE

Altered hemodynamics caused by the presence of an endovascular device may undermine the success of peripheral stenting procedures. Flow-enhanced stent designs are under investigation to recover physiological blood flow patterns in the treated artery and reduce long-term complications. However, flow-enhanced designs require the development of customised manufacturing processes that consider the complex behaviour of Nickel-Titanium (Ni-Ti). While the manufacturing routes of traditional self-expanding Ni-Ti stents are well-established, the process to introduce alternative stent designs is rarely reported in the literature, with much of this information (especially related to shape-setting step) being commercially sensitive and not reaching the public domain, as yet.

METHODS

A reliable manufacturing method was developed and improved to induce a helical ridge onto laser-cut and wire-braided Nickel-Titanium self-expanding stents. The process consisted of fastening the stent into a custom-built fixture that provided the helical shape, which was followed by a shape-setting in air furnace and rapid quenching in cold water. The parameters employed for the shape-setting in air furnace were thoroughly explored, and their effects assessed in terms of the mechanical performance of the device, material transformation temperatures and surface finishing.

RESULTS

Both stents were successfully imparted with a helical ridge and the optimal heat treatment parameters combination was found. The settings of 500 °C/30 min provided mechanical properties comparable with the original design, and transformation temperatures suitable for stenting applications (Af = 23.5 °C). Microscopy analysis confirmed that the manufacturing process did not alter the surface finishing. Deliverability testing showed the helical device could be loaded onto a catheter delivery system and deployed with full recovery of the expanded helical configuration.

CONCLUSION

This demonstrates the feasibility of an additional heat treatment regime to allow for helical shape-setting of laser-cut and wire-braided devices that may be applied to further designs.

Multi-objective design optimization of bioresorbable braided stents

BACKGROUND AND OBJECTIVES

Bioresorbable braided stents, typically made of bioresorbable polymers such as poly-l-lactide (PLLA), have great potential in the treatment of critical limb ischemia, particularly in cases of long-segment occlusions and lesions with high angulation. However, the successful adoption of these devices is limited by their low radial stiffness and reduced elastic modulus of bioresorbable polymers. This study proposes a computational optimization procedure to enhance the mechanical performance of bioresorbable braided stents and consequently improve the treatment of critical limb ischemia.

METHODS

Finite element analyses were performed to replicate the radial crimping test and investigate the implantation procedure of PLLA braided stents. The stent geometry was characterized by four design parameters: number of wires, wire diameter, initial stent diameter, and braiding angle. Manufacturing constraints were considered to establish the design space. The mechanical performance of the stent was evaluated by defining the radial force, foreshortening, and peak maximum principal stress of the stent as objectives and constraint functions in the optimization problem. An approximate relationship between the objectives, constraint, and the design parameters was defined using design of experiment coupled with surrogate modelling. Surrogate models were then interrogated within the design space, and a multi-objective design optimization was conducted.

RESULTS

The simulation of radial crimping was successfully validated against experimental data. The radial force was found to be primarily influenced by the number of wires, wire diameter, and braiding angle, with the wire diameter having the most significant impact. Foreshortening was predominantly affected by the braiding angle. The peak maximum principal stress exhibited contrasting behaviour compared to the radial force for all parameters, with the exception of the number of wires. Among the Pareto-optimal design candidates, feasible peak maximum principal stress values were observed, with the braiding angle identified as the differentiating factor among these candidates.

CONCLUSIONS

The exploration of the design space enabled both the understanding of the impact of design parameters on the mechanical performance of bioresorbable braided stents and the successful identification of optimal design candidates. The optimization framework contributes to the advancement of innovative bioresorbable braided stents for the effective treatment of critical limb ischemia.

Recommendations for finite element modelling of nickel-titanium stents-Verification and validation activities

The objective of this study is to present a credibility assessment of finite element modelling of self-expanding nickel-titanium (Ni-Ti) stents through verification and validation (VV) activities, as set out in the ASME VV-40 standard. As part of the study, the role of calculation verification, model input sensitivity, and model validation is examined across three different application contexts (radial compression, stent deployment in a vessel, fatigue estimation). A commercially available self-expanding Ni-Ti stent was modelled, and calculation verification activities addressed the effects of mesh density, element integration and stable time increment on different quantities of interests, for each context of use considered. Sensitivity analysis of the geometrical and material input parameters and validation of deployment configuration with in vitro comparators were investigated. Results showed similar trends for global and local outputs across the contexts of use in response to the selection of discretization parameters, although with varying sensitivities. Mesh discretisation showed substantial variability for less than 4 × 4 element density across the strut cross-section in radial compression and deployment cases, while a finer grid was deemed necessary in fatigue estimation for reliable predictions of strain/stress. Element formulation also led to substantial variation depending on the chosen integration options. Furthermore, for explicit analyses, model results were highly sensitive to the chosen target time increment (e.g., mass scaling parameters), irrespective of whether quasistatic conditions were ensured (ratios of kinetic and internal energies below 5%). The higher variability was found for fatigue life simulation, with the estimation of fatigue safety factor varying up to an order of magnitude depending on the selection of discretization parameters. Model input sensitivity analysis highlighted that the predictions of outputs such as radial force and stresses showed relatively low sensitivity to Ni-Ti material parameters, which suggests that the calibration approaches used in the literature to date appear reasonable, but a higher sensitivity to stent geometry, namely strut thickness and width, was found. In contrast, the prediction of vessel diameter following deployment was least sensitive to numerical parameters, and its validation with in vitro comparators offered a simple and accurate (error ~ 1-2%) method when predicting diameter gain, and lumen area, provided that the material of the vessel is appropriately characterized and modelled.

Longitudinal alterations in bone morphometry, mechanical integrity and composition in Type-2 diabetes in a Zucker diabetic fatty (ZDF) rat

Individuals with Type-2 Diabetes (T2D) have an increased risk of bone fracture, without a reduction in bone mineral density. It is hypothesised that the hyperglycaemic state caused by T2D forms an excess of Advanced Glycated End-products (AGEs) in the organic matrix of bone, which are thought to stiffen the collagen network and lead to impaired mechanical properties. However, the mechanisms are not well understood. This study aimed to investigate the geometrical, structural and material properties of diabetic cortical bone during the development and progression of T2D in ZDF (fa/fa) rats at 12-, 26- and 46-weeks of age. Longitudinal bone growth was impaired as early as 12-weeks of age and by 46-weeks bone size was significantly reduced in ZDF (fa/fa) rats versus controls (fa/+). Diabetic rats had significant structural deficits, such as bending rigidity, ultimate moment and energy-to-failure measured via three-point bend testing. Tissue material properties, measured by taking bone geometry into account, were altered as the disease progressed, with significant reductions in yield and ultimate strength for ZDF (fa/fa) rats at 46-weeks. FTIR analysis on cortical bone powder demonstrated that the tissue material deficits coincided with changes in tissue composition, in ZDF (fa/fa) rats with long-term diabetes having a reduced carbonate:phosphate ratio and increased acid phosphate content when compared to age-matched controls, indicative of an altered bone turnover process. AGE accumulation, measured via fluorescent assays, was higher in the skin of ZDF (fa/fa) rats with long-term T2D, bone AGEs did not differ between strains and neither AGEs correlated with bone strength. In conclusion, bone fragility in the diabetic ZDF (fa/fa) rats likely occurs through a multifactorial mechanism influenced initially by impaired bone growth and development and proceeding to an altered bone turnover process that reduces bone quality and impairs biomechanical properties as the disease progresses.

Predicting localised corrosion and mechanical performance of a PEO surface modified rare earth magnesium alloy for implant use through in-silico modelling

In this study, the influence of a plasma electrolytic oxidation (PEO) surface treatment on a medical-grade WE43-based magnesium alloy is examined through an experimental and computational framework that considers the effects of localised corrosion features and mechanical properties throughout the corrosion process. First, a comprehensive in-vitro immersion study was performed on WE43-based tensile specimens with and without PEO surface modification, which included fully automated spatial reconstruction of the phenomenological features of corrosion through micro-CT scanning, followed by uniaxial tensile testing. Then the experimental data of both unmodified and PEO-modified groups were used to calibrate parameters of a finite element-based surface corrosion model. In-vitro, it was found that the WE43-PEO modified group had a significantly lower corrosion rate and maintained significantly higher mechanical properties than the unmodified. While corrosion rates were ∼50% lower in the WE43-PEO modified specimens, the local geometric features of corroding surfaces remained similar to the unmodified WE43 group, however evolving after almost the double amount of time. We were also able to quantitatively demonstrate that the PEO surface treatment on magnesium continued to protect samples from corrosion throughout the entire period tested, and not just in the early stages of corrosion. Using the results from the testing framework, the model parameters of the surface-based corrosion model were identified for both groups. This enabled, for the first time, in-silico prediction of the physical features of corrosion and the mechanical performance of both unmodified and PEO modified magnesium specimens. This simulation framework can enable future in-silico design and optimisation of bioabsorbable magnesium devices for load-bearing medical applications.

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Examination of corrosion morphology and mechanics of PEO modified WE43.

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Automated phenomenological tracking of corrosion features by PitScan.

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Corrosion model of unmodified WE43 and WE43 PEO modified.

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Calibration through geometrical features and mechanical parameters followed.

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PEO treatment does not influence the severity of localised corrosion.

Linking the effect of localised pitting corrosion with mechanical integrity of a rare earth magnesium alloy for implant use

This study presents a computational framework that investigates the effect of localised surface-based corrosion on the mechanical performance of a magnesium-based alloy. A finite element-based phenomenological corrosion model was used to generate a wide range of corrosion profiles, with subsequent uniaxial tensile test simulations to predict the mechanical response to failure. The python-based detection framework PitScan provides detailed quantification of the spatial phenomenological features of corrosion, including a full geometric tracking of corroding surface. Through this approach, this study is the first to quantitatively demonstrate that a surface-based non-uniform corrosion model can capture both the geometrical and mechanical features of a magnesium alloy undergoing corrosion by comparing to experimental data. Using this verified corrosion modelling approach, a wide range of corrosion scenarios was evaluated and enabled quantitative relationships to be established between the mechanical integrity and key phenomenological corrosion features. In particular, we demonstrated that the minimal cross-sectional area parameter was the strongest predictor of the remaining mechanical strength (R² = 0.98), with this relationship being independent of the severity or spatial features of localised surface corrosion. Interestingly, our analysis demonstrated that parameters described in ASTM G46-94 showed weaker correlations to the mechanical integrity of corroding specimens, compared to parameters determined by Pitscan. This study establishes new mechanistic insight into the performance of the magnesium-based materials undergoing corrosion.

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Corrosion profile (uniform/localised) generation with phenomenological degradation model.

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Automated phenomenological tracking of corrosion features by PitScan.

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Linking mechanical key parameters and geometrical corrosion features including in-vitro data.

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Identification of profile independent corrosion features and model fitting.

A morphological, topological and mechanical investigation of gyroid, spinodoid and dual-lattice algorithms as structural models of trabecular bone

In this study, we evaluate the performance of three algorithms as computational models of trabecular bone architecture, through systematic evaluation of morphometric, topological, and mechanical properties. Here, we consider the widely-used gyroid lattice structure, the recently-developed spinodoid structure and a structure similar to Voronoi lattices introduced here as the dual-lattice. While all computational models were calibrated to recreate the trabecular tissue volume (e.g. BV/TV), it was found that both the gyroid- and spinodoid-based structures showed substantial differences in many other morphometric and topological parameters and, in turn, showed lower effective mechanical properties compared to trabecular bone. The newly-developed dual-lattice structures better captured both morphometric parameters and mechanical properties, despite certain differences being evident their topological configuration compared to trabecular bone. Still, these computational algorithms provide useful platforms to investigate trabecular bone mechanics and for designing biomimetic structures, which could be produced through additive manufacturing for applications that include bone substitutes, scaffolds and porous implants. Furthermore, the software for the creation of the structures has been added to the open source toolbox GIBBON and is therefore freely available to the community.

An enhanced phenomenological model to predict surface-based localised corrosion of magnesium alloys for medical use

This study developed an enhanced phenomenological model for the predictions of surface-based localised corrosion of magnesium alloys for use in medical applications. The modelling framework extended previous surface-based approaches by considering the role of β-phase components throughout the material volume to better predict spatial and temporal aspects of surface-based corrosion in magnesium alloys. This enhanced surface-based corrosion model offers many advantages as it (i) captures multi-directional pitting, (ii) captures various pit morphologies, (iii) eliminates mesh sizing effects, (iv) reduces computational cost through custom time controls (v) offers control of pit sizing and (vi) produces corrosion rates that are independent of pitting parameter values. The model was fully implemented in three dimensions within the finite element framework and shows excellent potential to enable robust predictions of the long-term performance of magnesium-based implants undergoing corrosion.

An experimental investigation of the mechanical performance of PLLA wire-braided stents

Much of our current understanding of the performance of self-expanding wire-braided stents is based on mechanical testing of Nitinol-based or polymeric non-bioresorbable (e.g. PET, PP etc.) devices. The small amount of data present for bioresorbable devices characterizes stents with big nominal diameters (D>6mm), with a distinct lack of data describing the mechanical performance of small-diameter wire-braided bioresorbable devices (D≤5mm). This study presents a systematic investigation of the mechanical performance of wire-braided bioresorbable Poly-L-Lactic Acid (PLLA) stents having different braiding angles (α=45° , α=30°, and α=20°), wire diameters (d=100μm, and d=150μm), wire count (n=24 and n=48), braiding patterns (1:1-1, 2:2-1 and 1:1-2) and stent diameters (D=5mm, D=4mm, and D=2.5mm). Mechanical characterisation was carried out by evaluating the radial, longitudinal and bending response of the devices. Our results showed that smaller braid angles, larger wire diameters, higher number of wires and smaller stent diameter led to an increase in the stent mechanical properties across each of the three mechanical tests performed. It was found that geometrical features of a polymeric braided stent could be adapted to achieve a similar performance to the one of a metallic device. In particular, substantial increases in stent mechanical properties were found for a low braiding angle and when the braiding pattern followed a one-over-one-under configuration with two wires in parallel (1:1-2). Finally, it was shown that a mathematical model proposed in literature for metal braided stents can provide reasonable predictions also of polymeric stent performance but just in circumstances where wire friction does not have a dominant role. This study presents a wide range of experimental data that can provide an important reference for further development of wire-braided bioresorbable devices.

An investigation on the effects of in vitro induced advanced glycation end-products on cortical bone fracture mechanics at fall-related loading rates

It has been suggested that adverse changes in bone quality due to the accumulation of advanced glycation end-products (AGEs) may play a role in the increased skeletal fragility. These non-enzymatic glycation mediated crosslinks are caused due to the presence of sugars in the extracellular space and can be induced in-vitro. AGEs exist naturally in bone, but with diseases such as type-2 diabetes, they are found at higher levels. While previous studies have examined the relationships between AGE accumulation and some mechanical properties, there is a lack of understanding of how AGE accumulation affects the fracture mechanics behaviour of bone tissue at fall-related loading rates. The objective of this study was to investigate the relationship between AGE accumulation and the fracture mechanics of cortical bone tissue. An in vitro glycation model was used to simulate diabetic conditions in twenty anatomically adjacent pairs of bone from a single bovine femur, which reduced the possibility of inter-specimen variability. Mechanical characterisation was carried out using 3-point bend, fracture toughness and nanoindentation testing, while bone composition was analysed by quantifying the accumulation of fluorescent AGEs. Under three-point bend testing, it was found that the yield stress, ultimate flexural strength, and secant modulus of the glycated samples were significantly higher than the controls. Furthermore, fracture toughness testing showed that the critical fracture toughness was increased by 16% in glycated samples compared to controls. These results provide no evidence that AGEs alone play a role in bone fragility at fall-related loading rates, with AGE accumulation actually found to enhance several pre- and post-yield properties of the tissue.

A coarse-grained molecular dynamics investigation of the role of mineral arrangement on the mechanical properties of mineralized collagen fibrils

Mineralized collagen fibrils (MCFs) comprise collagen molecules and hydroxyapatite (HAp) crystals and are considered universal building blocks of bone tissue, across different bone types and species. In this study, we developed a coarse-grained molecular dynamics (CGMD) framework to investigate the role of mineral arrangement on the load-deformation behaviour of MCFs. Despite the common belief that the collagen molecules are responsible for flexibility and HAp minerals are responsible for stiffness, our results showed that the mineral phase was responsible for limiting collagen sliding in the large deformation regime, which helped the collagen molecules themselves undergo high tensile loading, providing a substantial contribution to the ultimate tensile strength of MCFs. This study also highlights different roles for the mineralized and non-mineralized protofibrils within the MCF, with the mineralized groups being primarily responsible for load carrying due to the presence of the mineral phase, while the non-mineralized groups are responsible for crack deflection. These results provide novel insight into the load-deformation behaviour of MCFs and highlight the intricate role that both collagen and mineral components have in dictating higher scale bone biomechanics.

Hybrid Mineral/Organic Material Induces Bone Bridging and Bone Volume Augmentation in Rat Calvarial Critical Size Defects

In craniofacial bone defects, the promotion of bone volume augmentation remains a challenge. Finding strategies for bone regeneration such as combining resorbable minerals with organic polymers would contribute to solving the bone volume roadblock. Here, dicalcium phosphate dihydrate, chitosan and hyaluronic acid were used to functionalize a bone-side collagen membrane. Despite an increase in the release of inflammatory mediators by human circulating monocytes, the in vivo implantation of the functionalized membrane allowed the repair of a critical-sized defect in a calvaria rat model with de novo bone exhibiting physiological matrix composition and structural organization. Microtomography, histological and Raman analysis combined with nanoindentation testing revealed an increase in bone volume in the presence of the functionalized membrane and the formation of woven bone after eight weeks of implantation; these data showed the potential of dicalcium phosphate dihydrate, chitosan and hyaluronic acid to induce an efficient repair of critical-sized bone defects and establish the importance of thorough multi-scale characterization in assessing biomaterial outcomes in animal models.

A coupled computational framework for bone fracture healing and long-term remodelling: Investigating the role of internal fixation on bone fractures

In this study, a coupled computational modelling framework for bone fracture repair is presented that enables predictions of both healing and remodelling phases of the fracture region and is used to investigate the role of an internal fixation plate on the long-term healing performance of a fracture tibia under a range of different conditions. It was found that introduction of a titanium plate allowed the tibia to undergo successful healing at higher loading conditions and fracture gaps, compared with the non-plated versions. While these plated cases showed faster rates of repair in the healing phase, their performance was substantially different once they entered the remodelling phase, with substantial regions of stress shielding predicted. This framework is one of the few implementations of both fracture healing and remodelling phases of bone repair and includes several innovative approaches to smoothing, time-averaging and time incrementation in its implementation, thereby avoiding any unwanted abrupt changes between tissue phenotypes. This provides a better representation of tissue development in the fracture site when compared with fracture healing models alone and provides a suitable platform to investigate the long-term performance of orthopaedic fixation devices. This would enable the more effective design of permanent fixation devices and optimisation of the spatial and temporal performance of bioabsorbable implants.

Design and Verification of a Novel Perfusion Bioreactor to Evaluate the Performance of a Self-Expanding Stent for Peripheral Artery Applications

Endovascular stenting presents a promising approach to treat peripheral artery stenosis. However, a significant proportion of patients require secondary interventions due to complications such as in-stent restenosis and late stent thrombosis. Clinical failure of stents is not only attributed to patient factors but also on endothelial cell (EC) injury response, stent deployment techniques, and stent design. Three-dimensional in vitro bioreactor systems provide a valuable testbed for endovascular device assessment in a controlled environment replicating hemodynamic flow conditions found in vivo. To date, very few studies have verified the design of bioreactors based on applied flow conditions and their impact on wall shear stress, which plays a key role in the development of vascular pathologies. In this study, we develop a computationally informed bioreactor capable of capturing responses of human umbilical vein endothelial cells seeded on silicone tubes subjected to hemodynamic flow conditions and deployment of a self-expanding nitinol stents. Verification of bioreactor design through computational fluid dynamics analysis confirmed the application of pulsatile flow with minimum oscillations. EC responses based on morphology, nitric oxide (NO) release, metabolic activity, and cell count on day 1 and day 4 verified the presence of hemodynamic flow conditions. For the first time, it is also demonstrated that the designed bioreactor is capable of capturing EC responses to stent deployment beyond a 24-hour period with this testbed. A temporal investigation of EC responses to stent implantation from day 1 to day 4 showed significantly lower metabolic activity, EC proliferation, no significant changes to NO levels and EC's aligning locally to edges of stent struts, and random orientation in between the struts. These EC responses were indicative of stent-induced disturbances to local hemodynamics and sustained EC injury response contributing to neointimal growth and development of in-stent restenosis. This study presents a novel computationally informed 3D in vitro testbed to evaluate stent performance in presence of hemodynamic flow conditions found in native peripheral arteries and could help to bridge the gap between the current capabilities of 2D in vitro cell culture models and expensive pre-clinical in vivo models.

In silico modelling of aortic valve implants - predicting in vitro performance using finite element analysis

The competing structural and hemodynamic considerations in valve design generally require a large amount of in vitro hydrodynamic and durability testing during development, often resulting in inefficient "trial-and-error" prototyping. While in silico modelling through finite element analysis (FEA) has been widely used to inform valve design by optimising structural performance, few studies have exploited the potential insight FEA could provide into critical hemodynamic performance characteristics of the valve. The objective of this study is to demonstrate the potential of FEA to predict the hydrodynamic performance of tri-leaflet aortic valve implants obtained during development through in vitro testing. Several variations of tri-leaflet aortic valves were designed and manufactured using a synthetic polymer and hydrodynamic testing carried out using a pulsatile flow rig according to ISO 5840, with bulk hydrodynamic parameters measured. In silico models were developed in tandem and suitable surrogate measures were investigated as predictors of the hydrodynamic parameters. Through regression analysis, the in silico parameters of leaflet coaptation area, geometric orifice area and opening pressure were found to be suitable indicators of experimental in vitro hydrodynamic parameters: regurgitant fraction, effective orifice area and transvalvular pressure drop performance, respectively.

How to Validate in silico Deployment of Coronary Stents: Strategies and Limitations in the Choice of Comparator

This study aims at proposing and discussing useful indications to all those who need to validate a numerical model of coronary stent deployment. The proof of the reliability of a numerical model is becoming of paramount importance in the era of in silico trials. Recently, the ASME V&V Standard Committee for medical devices prepared the V&V 40 standard document that provides a framework that guides users in establishing and assessing the relevance and adequacy of verification and validation activities performed for proving the credibility of models. To the knowledge of the authors, only a few examples of the application of the V&V 40 framework to medical devices are available in the literature, but none about stents. Specifically, in this study, the authors wish to emphasize the choice of a relevant set of experimental activities to provide data for the validation of computational models aiming to predict coronary stent deployment. Attention is focused on the use of ad hoc 3D-printed mock vessels in the validation plan, which could allow evaluating aspects of clinical relevance in a representative but controlled environment.

Energy dissipation of osteopontin at a HAp mineral interface: Implications for bone biomechanics

Osteopontin (OPN) is a one of the most abundant non-collagenous proteins in the bone's organic matrix. OPN is responsible for mediating bonding at mineral interfaces in the extrafibrillar space and recent evidence shows that it is a major contributor to bone's fracture resistance. While several experimental studies have identified an important role for calcium ions in mediating energy dissipation in OPN protein networks, the underlying molecular mechanisms remain largely unknown. In the current study, the role of calcium ions on energy dissipation at OPN interface with hydroxyapatite (HAp) as the main bone mineral was investigated. For the first time, the three-dimensional structure of OPN proteins were predicted, and it was found that calcium ions greatly influenced the final protein configuration and energy dissipation performance. Under small deformation, the compact cOPN structure, resulting from calcium ions presence, facilitated greater energy dissipation through sacrificial bond breaking and mechanisms mediated by the surface-bound calcium. At larger deformation, the compact structure also enabled cOPN to dissipate higher energy. Moreover, it was found that phosphorylation of OPN played an important role in energy dissipation. While previous studies have shown that OPN dissipated energy by forming aggregate networks, this study also showed that network formation is not necessary and that individual OPN proteins can dissipate large amounts of energy at HAp interfaces.

An experimental investigation into the physical, thermal and mechanical degradation of a polymeric bioresorbable scaffold

This study presents a comprehensive evaluation of the mechanical, micro-mechanical and physical properties of Reva Medical Fantom Encore Bioresorbable Scaffolds (BRS) subjected to a thermally-accelerated degradation protocol. The Fantom Encore BRS were immersed in phosphate buffered saline solution at 50 °C for 112 days with radial compression testing, nanoindentation, differential scanning calorimetry, gel permeation chromatography and mass loss characterisation performed at consecutive time points. In the initial stages of degradation (Days 0-21), the Fantom Encore BRS showed increases in radial strength and stiffness, despite a substantial reduction in in molecular weight, with a slight increase in the melt temperature also observed. In the second phase (Days 35-54), the radial strength of the BRS samples were maintained despite a continued loss in molecular weight. However, during this phase, the ductility of the stent showed a reduction, with stent fracture occurring earlier in the crimp process and with lower amounts of plastic deformation evident under visual examination post-fracture. In the final phase (Days 63-112), the load-bearing capacity of the Fantom Encore BRS showed continued reduction, with decreases in radial stiffness and strength, and drastic reduction in the work-to-fracture of the devices. Throughout each phase, there was a steady increase in the relative crystallinity, with limited mass loss until day 112 and only minor changes in glass transition and melt temperatures. Limited changes were observed in nano-mechanical properties, with measured local elastic moduli and hardness values remaining largely similar throughout degradation. Given that the thermally-accelerated in vitro conditions represented a four-fold acceleration of physiological conditions, these results suggest that the BRS scaffolds could exhibit substantially brittle behaviour after ∼ one year of implantation.

Automated ex-situ detection of pitting corrosion and its effect on the mechanical integrity of rare earth magnesium alloy - WE43

This study develops a three-dimensional automated detection framework (PitScan) that systematically evaluates the severity and phenomenology of pitting corrosion. This framework uses a python-based algorithm to analyse microcomputer-tomography scans (μCT) of cylindrical specimens undergoing corrosion. The approach systematically identifies several surface-based corrosion features, enabling full spatial characterisation of pitting parameters, including pit density, pit size, pit depth as well as pitting factor according to ASTM G46-94. Furthermore, it is used to evaluate pitting formation in tensile specimens of a Rare Earth Magnesium alloy undergoing corrosion, and relationships between key pitting parameters and mechanical performance are established. Results demonstrated that several of the parameters described in ASTM G46-94, including pit number, pit density and pitting factor, showed little correlation to mechanical performance. However, this study did identify that other parameters showed strong correlations with the ultimate tensile strength and these tended to be directly linked to the reduction of the cross-sectional area of the specimen. Specifically, our results indicate, that parameters directly linked to the loss of the cross-sectional area (e.g. minimum material width), are parameters that are most suited to provide an indication of a specimen's mechanical performance. The automated detection framework developed in this study has the potential to provide a basis to standardise measurements of pitting corrosion across a range of metals and future prediction of mechanical strength over degradation time.

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In-vitro immersion study of dog bones manufactured from a WE43 Magnesium alloy.

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Novel approach characterizing spatial pit formation using micro-CT scans.

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Comparison of mass loss by hydrogen gas measurement and volume loss by μCT scans.

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Correlation between mechanical strength and geometrical pit formation features.

Physical and mechanical degradation behaviour of semi-crystalline PLLA for bioresorbable stent applications

This study presents a systematic evaluation of the physical, thermal and mechanical performance of medical-grade semi-crystalline PLLA undergoing thermally-accelerated degradation. Samples were immersed in phosphate-buffered saline solution at 50 °C for 112 days and mass loss, molecular weight, thermal properties, degree of crystallinity, FTIR and Raman spectra, tensile elastic modulus, yield stress and failure stress/strain were evaluated at consecutive time points. Samples showed a consistent reduction in molecular weight and melting temperature, a consistent increase in percent crystallinity and limited changes in glass transition temperature and mass loss. At day 49, a drastic reduction in tensile failure strain was observed, despite the fact that elastic modulus, yield and tensile strength of samples were maintained. Brittleness increase was followed by rapid increase in degradation rate. Beyond day 70, samples became too brittle to test indicating substantial deterioration of their load-bearing capacity. This study also presents a computational micromechanics framework that demonstrates that the elastic modulus of a semi-crystalline polymer undergoing degradation can be maintained, despite a reducing molecular weight through compensatory increases in percent crystallinity. This study presents novel insight into the relationship between physical properties and mechanical performance of medical-grade PLLA during degradation and could have important implications for design and development of bioresorbable stents for vascular applications.

A finite element investigation on design parameters of bare and polymer-covered self-expanding wire braided stents

Self-expanding covered braided stents are routinely used across a diverse range of clinical applications, but few computational studies have attempted to replicate their complex behaviour. In this study, a computational framework was developed to predict the functional performance of bare and covered self-expanding wire braided stents, with a systematic evaluation on the effect of various braid and cover parameters presented. Simulated radial force and kink deformation tests show good agreement to experimental data for covered braided stents across a range of braid angles and cover thicknesses. Our results demonstrate that braid angle is a key governing parameter that dictates the radial and kink performance of both bare-metal and covered wire braided stents. It was also demonstrated that addition of a polymeric cover to a wire braided stent causes a stiffer radial response across all braid angles, particularly when thicker and/or stiffer covering systems were considered. This study represents the first experimentally-validated computational model for covered wire braided stent systems and has excellent potential to be used in future design of these devices for a range of applications.

The structural role of osteocalcin in bone biomechanics and its alteration in Type-2 Diabetes

This study presents an investigation into the role of Osteocalcin (OC) on bone biomechanics, with the results demonstrating that the protein's α-helix structures play a critical role in energy dissipation behavior in healthy conditions. In the first instance, α-helix structures have high affinity with the Hydroxyapatite (HAp) mineral surface and provide favorable conditions for adsorption of OC proteins onto the mineral surface. Using steered molecular dynamics simulation, several key energy dissipation mechanisms associated with α-helix structures were observed, which included stick-slip behavior, a sacrificial bond mechanism and a favorable binding feature provided by the Ca2+ motif on the OC protein. In the case of Type-2 Diabetes, this study demonstrated that possible glycation of the OC protein can occur through covalent crosslinking between Arginine and N-terminus regions, causing disruption of α-helices leading to a lower protein affinity to the HAp surface. Furthermore, the loss of α-helix structures allowed protein deformation to occur more easily during pulling and key energy dissipation mechanisms observed in the healthy configuration were no longer present. This study has significant implications for our understanding of bone biomechanics, revealing several novel mechanisms in OC's involvement in energy dissipation. Furthermore, these mechanisms can be disrupted following the onset of Type-2 Diabetes, implying that glycation of OC could have a substantial contribution to the increased bone fragility observed during this disease state.

Bone Mineral Is More Heterogeneously Distributed in the Femoral Heads of Osteoporotic and Diabetic Patients: A Pilot Study

Osteoporosis is associated with systemic bone loss, leading to a significant deterioration of bone microarchitecture and an increased fracture risk. Although recent studies have shown that the distribution of bone mineral becomes more heterogeneous because of estrogen deficiency in animal models of osteoporosis, it is not known whether osteoporosis alters mineral distribution in human bone. Type 2 diabetes mellitus (T2DM) can also increase bone fracture risk and is associated with impaired bone cell function, compromised collagen structure, and reduced mechanical properties. However, it is not known whether alterations in mineral distribution arise in diabetic (DB) patients’ bone. In this study, we quantify mineral content distribution and tissue microarchitecture (by μCT) and mechanical properties (by compression testing) of cancellous bone from femoral heads of osteoporotic (OP; n = 10), DB (n = 7), and osteoarthritic (OA; n = 7) patients. We report that though OP cancellous bone has significantly deteriorated compressive mechanical properties and significantly compromised microarchitecture compared with OA controls, there is also a significant increase in the mean mineral content. Moreover, the heterogeneity of the mineral content in OP bone is significantly higher than controls (+25%) and is explained by a significant increase in bone volume at high mineral levels. We propose that these mineral alterations act to exacerbate the already reduced bone quality caused by reduced cancellous bone volume during osteoporosis. We show for the first time that cancellous bone mineralization is significantly more heterogeneous (+26%) in patients presenting with T2DM compared with OA (non‐DB) controls, and that this heterogeneity is characterized by a significant increase in bone volume at low mineral levels. Despite these mineralization changes, bone microarchitecture and mechanical properties are not significantly different between OA groups with and without T2DM. Nonetheless, the observed alterations in mineral heterogeneity may play an important tissue‐level role in bone fragility associated with OP and DB bone. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Investigating the interplay between substrate stiffness and ligand chemistry in directing mesenchymal stem cell differentiation within 3D macro-porous substrates

Dimensionality can have a profound impact on stiffness-mediated differentiation of mesenchymal stem cells (MSCs). However, while we have begun to understand cellular response when encapsulated within 3D substrates, the behavior of cells within macro-porous substrates is relatively underexplored. The goal of this study was to determine the influence of macro-porous topographies on stiffness-mediated differentiation of MSCs. We developed macro-porous recombinant elastin-like protein (ELP) substrates that allow independent control of mechanical properties and ligand chemistry. We then used computational modeling to probe the impact of pore topography on the mechanical stimulus that cells are exposed to within these substrates, and finally we investigated stiffness induced biases towards adipogenic and osteogenic differentiation of MSCs within macro-porous substrates. Computational modeling revealed that there is significant heterogeneity in the mechanical stimuli that cells are exposed to within porous substrates and that this heterogeneity is predominantly due to the wide range of possible cellular orientations within the pores. Surprisingly, MSCs grown within 3D porous substrates respond to increasing substrate stiffness by up-regulating both osteogenesis and adipogenesis. These results demonstrate that within porous substrates the behavior of MSCs diverges from previously observed responses to substrate stiffness, emphasizing the importance of topography as a determinant of cellular behavior.

In silico study of bone tissue regeneration in an idealised porous hydrogel scaffold using a mechano-regulation algorithm

Mechanical stimulation, in the form of fluid perfusion or mechanical strain, enhances osteogenic differentiation and overall bone tissue formation by mesenchymal stems cells cultured in biomaterial scaffolds for tissue engineering applications. In silico techniques can be used to predict the mechanical environment within biomaterial scaffolds, and also the relationship between bone tissue regeneration and mechanical stimulation, and thereby inform conditions for bone tissue engineering experiments. In this study, we investigated bone tissue regeneration in an idealised hydrogel scaffold using a mechano-regulation model capable of predicting tissue differentiation, and specifically compared five loading cases, based on known experimental bioreactor regimes. These models predicted that low levels of mechanical loading, i.e. compression (0.5% strain), pore pressure of 10 kPa and a combination of compression (0.5%) and pore pressure (10 kPa), could induce more osteogenic differentiation and lead to the formation of a higher bone tissue fraction. In contrast greater volumes of cartilage and fibrous tissue fractions were predicted under higher levels of mechanical loading (i.e. compression strain of 5.0% and pore pressure of 100 kPa). The findings in this study may provide important information regarding the appropriate mechanical stimulation for in vitro bone tissue engineering experiments.

The effect of equiaxial stretching on the osteogenic differentiation and mechanical properties of human adipose stem cells

Although mechanical cues are known to affect stem cell fate and mechanobiology, the significance of such stimuli on the osteogenic differentiation of human adipose stem cells (hASCs) remains unclear. In this study, we investigated the effect of long-term mechanical stimulation on the attachment, osteogenic differentiation and mechanical properties of hASCs. Tailor-made, pneumatic cell stretching devices were used to expose hASCs to cyclic equiaxial stretching in osteogenic medium. Cell attachment and focal adhesions were visualised using immunocytochemical vinculin staining on days 3 and 6, and the proliferation and alkaline phosphatase activity, as a sign of early osteogenic differentiation, were analysed on days 0, 6 and 10. Furthermore, the mechanical properties of hASCs, in terms of apparent Young's modulus and normalised contractility, were obtained using a combination of atomic force microscopy based indentation and computational approaches. Our results indicated that cyclic equiaxial stretching delayed proliferation and promoted osteogenic differentiation of hASCs. Stretching also reduced cell size and intensified focal adhesions and actin cytoskeleton. Moreover, cell stiffening was observed during osteogenic differentiation and especially under mechanical stimulation. These results suggest that cyclic equiaxial stretching modifies cell morphology, focal adhesion formation and mechanical properties of hASCs. This could be exploited to enhance osteogenic differentiation.

Mechanisms of osteocyte stimulation in osteoporosis

Experimental studies have shown that primary osteoporosis caused by oestrogen-deficiency results in localised alterations in bone tissue properties and mineral composition. Additionally, changes to the lacunar-canalicular architecture surrounding the mechanosensitive osteocyte have been observed in animal models of the disease. Recently, it has also been demonstrated that the mechanical stimulation sensed by osteocytes changes significantly during osteoporosis. Specifically, it was shown that osteoporotic bone cells experience higher maximum strains than healthy bone cells after short durations of oestrogen deficiency. However, in long-term oestrogen deficiency there was no significant difference between bone cells in healthy and normal bone. The mechanisms by which these changes arise are unknown. In this study, we test the hypothesis that complex changes in tissue composition and lacunar-canalicular architecture during osteoporosis alter the mechanical stimulation of the osteocyte. The objective of this research is to employ computational methods to investigate the relationship between changes in bone tissue composition and microstructure and the mechanical stimulation of osteocytes during osteoporosis. By simulating physiological loading, it was observed that an initial decrease in tissue stiffness (of 0.425GPa) and mineral content (of 0.66wt% Ca) relative to controls could explain the mechanical stimulation observed at the early stages of oestrogen deficiency (5 weeks post-OVX) during in situ bone cell loading in an oestrogen-deficient rat model of post-menopausal osteoporosis (Verbruggen et al., 2015). Moreover, it was found that a later increase in stiffness (of 1.175GPa) and mineral content (of 1.64wt% Ca) during long-term osteoporosis (34 weeks post-OVX), could explain the mechanical stimuli previously observed at a later time point due to the progression of osteoporosis. Furthermore, changes in canalicular tortuosity arising during osteoporosis were shown to result in increased osteogenic strain stimulation, though to a lesser extent than has been observed experimentally. The findings of this study indicate that changes in the extracellular environment during osteoporosis, arising from altered mineralisation and lacunar-canalicular architecture, lead to altered mechanical stimulation of osteocytes, and provide an enhanced understanding of changes in bone mechanobiology during osteoporosis.

The effect of substrate stiffness, thickness, and cross-linking density on osteogenic cell behavior

Osteogenic cells respond to mechanical changes in their environment by altering their spread area, morphology, and gene expression profile. In particular, the bulk modulus of the substrate, as well as its microstructure and thickness, can substantially alter the local stiffness experienced by the cell. Although bone tissue regeneration strategies involve culture of bone cells on various biomaterial scaffolds, which are often cross-linked to enhance their physical integrity, it is difficult to ascertain and compare the local stiffness experienced by cells cultured on different biomaterials. In this study, we seek to characterize the local stiffness at the cellular level for MC3T3-E1 cells plated on biomaterial substrates of varying modulus, thickness, and cross-linking concentration. Cells were cultured on flat and wedge-shaped gels made from polyacrylamide or cross-linked collagen. The cross-linking density of the collagen gels was varied to investigate the effect of fiber cross-linking in conjunction with substrate thickness. Cell spread area was used as a measure of osteogenic differentiation. Finite element simulations were used to examine the effects of fiber cross-linking and substrate thickness on the resistance of the gel to cellular forces, corresponding to the equivalent shear stiffness for the gel structure in the region directly surrounding the cell. The results of this study show that MC3T3 cells cultured on a soft fibrous substrate attain the same spread cell area as those cultured on a much higher modulus, but nonfibrous substrate. Finite element simulations predict that a dramatic increase in the equivalent shear stiffness of fibrous collagen gels occurs as cross-linking density is increased, with equivalent stiffness also increasing as gel thickness is decreased. These results provide an insight into the response of osteogenic cells to individual substrate parameters and have the potential to inform future bone tissue regeneration strategies that can optimize the equivalent stiffness experienced by a cell.

The in situ mechanics of trabecular bone marrow: the potential for mechanobiological response

Bone adapts to habitual loading through mechanobiological signaling. Osteocytes are the primary mechanical sensors in bone, upregulating osteogenic factors and downregulating osteoinhibitors, and recruiting osteoclasts to resorb bone in response to microdamage accumulation. However, most of the cell populations of the bone marrow niche,which are intimately involved with bone remodeling as the source of bone osteoblast and osteoclast progenitors, are also mechanosensitive. We hypothesized that the deformation of trabecular bone would impart mechanical stress within the entrapped bone marrow consistent with mechanostimulation of the constituent cells. Detailed fluid-structure interaction models of porcine femoral trabecular bone and bone marrow were created using tetrahedral finite element meshes. The marrow was allowed to flow freely within the bone pores, while the bone was compressed to 2000 or 3000 microstrain at the apparent level.Marrow properties were parametrically varied from a constant 400 mPas to a power law rule exceeding 85 Pas. Deformation generated almost no shear stress or pressure in the marrow for the low viscosity fluid, but exceeded 5 Pa when the higher viscosity models were used. The shear stress was higher when the strain rate increased and in higher volume fraction bone. The results demonstrate that cells within the trabecular bone marrow could be mechanically stimulated by bone deformation, depending on deformation rate, bone porosity, and bone marrow properties. Since the marrow contains many mechanosensitive cells, changes in the stimulatory levels may explain the alterations in bone marrow morphology with aging and disease, which may in turn affect the trabecular bone mechanobiology and adaptation.

How bone tissue and cells experience elevated temperatures during orthopaedic cutting: an experimental and computational investigation

During orthopaedic surgery elevated temperatures due to cutting can result in bone injury, contributing to implant failure or delayed healing. However, how resulting temperatures are experienced throughout bone tissue and cells is unknown. This study uses a combination of experiments (forward-looking infrared (FLIR)) and multiscale computational models to predict thermal elevations in bone tissue and cells. Using multiple regression analysis, analytical expressions are derived allowing a priori prediction of temperature distribution throughout bone with respect to blade geometry, feed-rate, distance from surface, and cooling time. This study offers an insight into bone thermal behavior, informing innovative cutting techniques that reduce cellular thermal damage.

Cell morphology and focal adhesion location alters internal cell stress

Extracellular mechanical cues have been shown to have a profound effect on osteogenic cell behaviour. However, it is not known precisely how these cues alter intracellular mechanics to initiate changes in cell behaviour. In this study, a combination of in vitro culture of MC3T3-E1 cells and finite-element modelling was used to investigate the effects of passive differences in substrate stiffness on intracellular mechanics. Cells on collagen-based substrates were classified based on the presence of cell processes and the dimensions of various cellular features were quantified. Focal adhesion (FA) density was quantified from immunohistochemical staining, while cell and substrate stiffnesses were measured using a live-cell atomic force microscope. Computational models of cell morphologies were developed using an applied contraction of the cell body to simulate active cell contraction. The results showed that FA density is directly related to cell morphology, while the effect of substrate stiffness on internal cell tension was modulated by both cell morphology and FA density, as investigated by varying the number of adhesion sites present in each morphological model. We propose that the cells desire to achieve a homeostatic stress state may play a role in osteogenic cell differentiation in response to extracellular mechanical cues.

The role of integrin α(V)β(3) in osteocyte mechanotransduction

Recent in vivo studies have proposed that integrin αvβ3 attachments between osteocyte cell processes and the extracellular matrix may facilitate mechanosensation in bone. However the role of these attachments in osteocyte biochemical response to mechanical stimulus has yet to be investigated. With this in mind, the objective of this study was to determine the effect of blocking integrin αvβ3 function on the biochemical response of osteocytes to mechanical stimulus. Antagonists specific to integrin subunit β3 were used to block integrin αvβ3 on MLO-Y4 mouse osteocytes. After treatment, cells were subjected to laminar oscillatory fluid flow stimulus (1 Pa, 1 Hz) for one hour. Fluorescent staining was performed to visualise cell morphology. Prostaglandin E2 (PGE2) release was assayed using an enzyme immunoassay and qRT-PCR was used to analyse the relative expression of cyclooxygenase-2 (COX-2), receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG). Our results show that blocking integrin αvβ3 disrupts osteocyte morphology, causing a reduction in spread area and process retraction. Integrin αvβ3 blocking also disrupted COX-2 expression and PGE2 release in response to fluid shear stress. Taken together, the results of this study indicate that integrin αvβ3 is essential for the maintenance of osteocyte cell processes and also for mechanosensation and mechanotransduction by osteocytes. A better understanding of this process may lead to the development of novel treatments for bone pathologies where mechanosensitivity is thought to be compromised.

Are all osteocytes equal? Multiscale modelling of cortical bone to characterise the mechanical stimulation of osteocytes

Bone continuously adapts its internal structure to accommodate the functional demands of its mechanical environment. This process is orchestrated by a network of mechanosensitive osteocytes that respond to external mechanical signals and recruit osteoblasts and osteoclasts to alter bone mass to meet loading demands. Because of the irregular hierarchical microarchitecture of bone tissue, the precise mechanical stimuli experienced by osteocytes located in different regions of the tissue is not well-understood. The objective of this study is to characterise the local stimulus experienced by osteocytes distributed throughout the tissue structure. Our models predict that an inhomogeneous microstructural strain field contributes to osteocytes receiving vastly different stimuli at the cellular level, depending on their location within the microstructure. In particular, osteocytes located directly adjacent to micropores experienced strain amplifications in their processes of up to nine times the applied global strain. Furthermore, it was found that the principal orientation of lamellar regions was found to contribute significantly to the magnitude of the stimulus being received at the cellular level. These findings indicate that osteocytes are not equal in terms of the mechanical stimulus being received, and we propose that only a subset of osteocytes may be sufficiently stimulated to function as mechanoreceptors.

A fluid-structure interaction model to characterize bone cell stimulation in parallel-plate flow chamber systems

Bone continuously adapts its internal structure to accommodate the functional demands of its mechanical environment and strain-induced flow of interstitial fluid is believed to be the primary mediator of mechanical stimuli to bone cells in vivo. In vitro investigations have shown that bone cells produce important biochemical signals in response to fluid flow applied using parallel-plate flow chamber (PPFC) systems. However, the exact mechanical stimulus experienced by the cells within these systems remains unclear. To fully understand this behaviour represents a most challenging multi-physics problem involving the interaction between deformable cellular structures and adjacent fluid flows. In this study, we use a fluid-structure interaction computational approach to investigate the nature of the mechanical stimulus being applied to a single osteoblast cell under fluid flow within a PPFC system. The analysis decouples the contribution of pressure and shear stress on cellular deformation and for the first time highlights that cell strain under flow is dominated by the pressure in the PPFC system rather than the applied shear stress. Furthermore, it was found that strains imparted on the cell membrane were relatively low whereas significant strain amplification occurred at the cell-substrate interface. These results suggest that strain transfer through focal attachments at the base of the cell are the primary mediators of mechanical signals to the cell under flow in a PPFC system. Such information is vital in order to correctly interpret biological responses of bone cells under in vitro stimulation and elucidate the mechanisms associated with mechanotransduction in vivo.

Strain amplification in bone mechanobiology: a computational investigation of the in vivo mechanics of osteocytes

The osteocyte is believed to act as the main sensor of mechanical stimulus in bone, controlling signalling for bone growth and resorption in response to changes in the mechanical demands placed on our bones throughout life. However, the precise mechanical stimuli that bone cells experience in vivo are not yet fully understood. The objective of this study is to use computational methods to predict the loading conditions experienced by osteocytes during normal physiological activities. Confocal imaging of the lacunar-canalicular network was used to develop three-dimensional finite element models of osteocytes, including their cell body, and the surrounding pericellular matrix (PCM) and extracellular matrix (ECM). We investigated the role of the PCM and ECM projections for amplifying mechanical stimulation to the cells. At loading levels, representing vigorous physiological activity (3000 µε), our results provide direct evidence that (i) confocal image-derived models predict 350-400% greater strain amplification experienced by osteocytes compared with an idealized cell, (ii) the PCM increases the cell volume stimulated more than 3500 µε by 4-10% and (iii) ECM projections amplify strain to the cell by approximately 50-420%. These are the first confocal image-derived computational models to predict osteocyte strain in vivo and provide an insight into the mechanobiology of the osteocyte.

A three-scale finite element investigation into the effects of tissue mineralisation and lamellar organisation in human cortical and trabecular bone

Bone is an exceptional material that is lightweight for efficient movement but also exhibits excellent strength and stiffness imparted by a composite material of organic proteins and mineral crystals that are intricately organised on many scales. Experimental and computational studies have sought to understand the role of bone composition and organisation in regulating the biomechanical behaviour of bone. However, due to the complex hierarchical arrangement of the constituent materials, the reported experimental values for the elastic modulus of trabecular and cortical tissue have conflicted greatly. Furthermore, finite element studies of bone have largely made the simplifying assumption that material behaviour was homogeneous or that tissue variability only occurred at the microscale, based on grey values from micro-CT scans. Thus, it remains that the precise role of nanoscale tissue constituents and microscale tissue organisation is not fully understood and more importantly that these have never been incorporated together to predict bone fracture or implant outcome in a multiscale finite element framework. In this paper, a three-scale finite element homogenisation scheme is presented which enables the prediction of homogenised effective properties of tissue level bone from its fundamental nanoscale constituents of hydroxyapatite mineral crystals and organic collagen proteins. Two independent homogenisation steps are performed on representative volume elements which describe the local morphological arrangement of both the nanostructural and microstructural levels. This three-scale homogenisation scheme predicts differences in the tissue level properties of bone as a function of mineral volume fraction, mineral aspect ratio and lamellar orientation. These parameters were chosen to lie within normal tissue ranges derived from experimental studies, and it was found that the predicted stiffness properties at the lamellar level correlate well with experimental nanoindentation results from cortical and trabecular bone. Furthermore, these studies show variations in mineral volume fraction, mineral crystal size and lamellar orientation could be responsible for previous discrepancies in experimental reports of tissue moduli. We propose that this novel multiscale modelling approach can provide a more accurate description of bone tissue properties in continuum/organ level finite element models by incorporating information regarding tissue structure and composition from advanced imaging techniques. This approach could thereby provide a preclinical tool to predict bone mechanics following prosthetic implantation or bone fracture during disease.

Validation of glutathione quantitation from STEAM spectra against edited 1H NMR spectroscopy at 4T: application to schizophrenia

OBJECTIVE

Quantitation of glutathione (GSH) in the human brain in vivo using short echo time 1H NMR spectroscopy is challenging because GSH resonances are not easily resolved. The main objective of this study was to validate such quantitation in a clinically relevant population using the resolved GSH resonances provided by edited spectroscopy. A secondary objective was to compare several of the neurochemical concentrations quantified along with GSH using LCModel analysis of short echo time spectra in schizophrenia versus control.

MATERIALS AND METHODS

GSH was quantified at 4T from short echo STEAM spectra and MEGA-PRESS edited spectra from identical volumes of interest (anterior cingulate) in ten volunteers. Neurochemical profiles were quantified in nine controls and 13 medicated schizophrenic patients.

RESULTS

GSH concentrations as quantified using STEAM, 1.6 +/- 0.4 micromol/g (mean +/- SD, n = 10), were within error of those quantified using edited spectra, 1.4 +/- 0.4 micromol/g, and were not different (p = 0.4). None of the neurochemical measurements reached sufficient statistical power to detect differences smaller than 10% in schizophrenia versus control. As such, no differences were observed.

CONCLUSIONS

Human brain GSH concentrations can be quantified in a clinical setting using short-echo time STEAM spectra at 4T.

Isolation and tissue profiles of a large panel of phage antibodies binding to the human adipocyte cell surface

Phage display is a powerful technique for the rapid selection and isolation of antibodies to any given target antigen. We have applied this technology to isolate over 100 different human antibodies that bind to antigens expressed in situ on the human adipocyte cell surface. This is a diverse panel of antibodies, as indicated by the V-region sequences. The binding profile of each anti-adipocyte antibody has been characterised using phage antibody immunocytochemistry against a panel of normal human tissues. Although there was some variation in the intensity of the adipocyte staining, each antibody consistently recognised adipocytes, where present, irrespective of the tissue source. In addition, all of the antibodies recognised at least one other cell type other than the adipocyte cell surface. In total, over 50 different tissue-binding profiles were recorded, with the most frequently recognised tissues identified as capillaries or smooth muscle. Extensive tissue binding profiles were generated for some antibodies using a panel of 37 different human tissues. This identified anti-adipocyte antibodies with unexpected profiles, such as FAT.13, which binds only to adipocytes and capillaries in the entire tissue panel. We believe this is the most extensive survey ever undertaken of the human adipocyte cell surface. Moreover, similar methodology could be used to derive complete tissue-binding profiles of antibodies against cell-surface antigens of any cell type. Indeed, by screening antibodies on both normal and diseased tissues, it may be possible to identify antigenic associations between different cell types and the pathologies of many diseases.

Tagging bloodmeals with phagemids allows feeding of multiple-sample arrays to single cages of mosquitoes (Diptera: Culicidae) and the recovery of single recombinant antibody fragment genes from individual insects

A recombinant single-chain variable-region human antibody fragment (scFv) was expressed in Escherichia coli, extracted in hypertonic sucrose, mixed directly with blood and fed to Anopheles gambiae Giles mosquitoes. When E. coli containing the phagemids that encode these scFv were included in bloodmeals, phagemids could be recovered from the mosquito midgut for up to 3 d after feeding. Furthermore, large arrays of such gene-tagged scFv-containing bloodmeals could be fed to cages of mosquitoes using microtiter plates. Arrays of phagemids with and without an antibody insert were fed to single cages of mosquitoes to test whether individual mosquitoes fed from single wells of such arrays. Phagemids were recovered from 95% of blood-fed females and > 80% of these phagemids were monoclonal. Therefore, it is possible to feed multiple sample arrays of recombinant proteins to single cages of mosquitoes and to recover the genetic material that encodes for only one of the array elements from individual mosquitoes. This demonstration indicates that multiple-sample feeding and recovery strategies are feasible and may represent a viable strategy for future rapid screening of biologically active genes, gene products or microorganisms in live arthropods.