Directional dependence of hydroxyapatite-collagen interactions on mechanics of collagen

Assembled collagen films modified using polyacrylic acid with improved mechanical properties via mineralization

The imperative task of enforcing collagen materials holds paramount significance in the field of hard tissue repair. We hereby present mineralized collagen fiber films via mineralization with improved mechanical properties. Self-extracted collagen was assembled into an array with an aligned fibrous pattern and then modified with polyacrylic acid (PAA) followed by mineralization in cationic polyacrylamide (CPAM)-SBF. Biomineralization occurred at the inner and outer surface of the assembled collagen fiber films. A tensile strength of up to 40.38 ± 3.08 MPa of mineralized collagen was obtained, for the first time, which may be attributed to the synergistic effect of polyanion and polycation on the mineralization process of assembled intrafibrillar collagen fibers. It was argued that PAA may facilitate the intra-fiber interaction of collagen, which extends the elongation at break of collagen fibers. This study introduces a pioneering approach for the preparation of mineralized collagen materials with superior mechanical properties, which would be beneficial for hard tissue repair.

Study on the Changes of Bone Calcium during the Fermentation of Bone Powders with Different Fermenters

Two fermenters, Lactobacillus acidophilus (LA) and the active dry yellow wine yeast (HY), were utilized to ferment cattle bones in order to release calcium. The influences of fermenters and the fermentation process on the calcium release capacity, particle properties, morphology, and chemical composition of bone powders were assessed, and the underlying mechanism was discussed. The results showed that LA had a better capacity of acid production than yeast, and therefore released more calcium during the fermentation of bone powders. The released calcium in the fermentation broth mainly existed in the forms of free Ca2+ ions, organic acid-bound calcium and a small amount of calcium-peptide chelate. For bone powders, the fermentation induced swollen bone particles, increased particle size, and significant changes of the internal chemical structure. Therefore, fermentation has a great potential in the processing of bone-derived products, particularly to provide new ideas for the development of calcium supplement products.

Molecular mechanics and failure mechanisms in B. mori Silk Fibroin-hydroxyapatite composite interfaces: Effect of crystal thickness and surface characteristics

Bombyx mori Silk Fibroin-hydroxyapatite (B. mori SF-HA) bio-nanocomposite is a prospective biomaterial for tissue engineered graft for bone repair. Here, B. mori SF is primarily a soft and tough organic phase, and HA is a hard and stiff mineral phase. In biomaterial design, an understanding about the nanoscale mechanics of SF-HA interface, such as interfacial interaction and interface debonding mechanisms between the two phases is essential for obtaining required functionality. To investigate such nanoscale behavior, molecular dynamics method is a preferred approach. Present study focuses on understanding of the interface debonding mechanisms at SF-HA interface in B. mori SF-HA bio-nanocomposite at nanometer length scale. For this purpose, nanoscale atomistic models of SF-HA interface are also developed based on the HA crystal size and HA surface type (Ca2+ dominated and OH- dominated) in contact with SF. Mechanical behavior analysis of these SF-HA interface models under pull-out type test were performed using Molecular Dynamics (MD) simulations. Surface pull-off strength values in the range of 0.4-0.8 GPa were obtained for SF-HA interface models, for different HA crystal thicknesses, wherein, the pull-off strength values are found to increase with increase in HA thicknesses. Analyses show that deformation mechanisms in SF-HA interface deformation, is a combination of shear deformation in SF phase followed by disintegration of SF phase from HA block. Furthermore, higher rupture force values were obtained for SF-HA interface with Ca2+ dominated HA surface in contact with SF phase, indicating that SF protein has a higher affinity for Ca2+ dominated surface of HA phase. Current work contributes in developing an understanding of mechanistic interactions between organic and inorganic phases in B. mori SF-HA composite nanostructure.

Functionalization of biomimetic mineralized collagen for bone tissue engineering

Mineralized collagen (MC) is the basic unit of bone structure and function and is the main component of the extracellular matrix (ECM) in bone tissue. In the biomimetic method, MC with different nanostructures of neo-bone have been constructed. Among these, extra-fibrous MC has been approved by regulatory agencies and applied in clinical practice to play an active role in bone defect repair. However, in the complex microenvironment of bone defects, such as in blood supply disorders and infections, MC is unable to effectively perform its pro-osteogenic activities and needs to be functionalized to include osteogenesis and the enhancement of angiogenesis, anti-infection, and immunomodulation. This article aimed to discuss the preparation and biological performance of MC with different nanostructures in detail, and summarize its functionalization strategy. Then we describe the recent advances in the osteo-inductive properties and multifunctional coordination of MC. Finally, the latest research progress of functionalized biomimetic MC, along with the development challenges and future trends, are discussed. This paper provides a theoretical basis and advanced design philosophy for bone tissue engineering in different bone microenvironments.

Biomechanics and mechanobiology of the bone matrix

The bone matrix plays an indispensable role in the human body, and its unique biomechanical and mechanobiological properties have received much attention. The bone matrix has unique mechanical anisotropy and exhibits both strong toughness and high strength. These mechanical properties are closely associated with human life activities and correspond to the function of bone in the human body. None of the mechanical properties exhibited by the bone matrix is independent of its composition and structure. Studies on the biomechanics of the bone matrix can provide a reference for the preparation of more applicable bone substitute implants, bone biomimetic materials and scaffolds for bone tissue repair in humans, as well as for biomimetic applications in other fields. In providing mechanical support to the human body, bone is constantly exposed to mechanical stimuli. Through the study of the mechanobiology of the bone matrix, the response mechanism of the bone matrix to its surrounding mechanical environment can be elucidated and used for the health maintenance of bone tissue and defect regeneration. This paper summarizes the biomechanical properties of the bone matrix and their biological significance, discusses the compositional and structural basis by which the bone matrix is capable of exhibiting these mechanical properties, and studies the effects of mechanical stimuli, especially fluid shear stress, on the components of the bone matrix, cells and their interactions. The problems that occur with regard to the biomechanics and mechanobiology of the bone matrix and the corresponding challenges that may need to be faced in the future are also described.

Bioinspired mineralized collagen scaffolds for bone tissue engineering

Successful regeneration of large segmental bone defects remains a major challenge in clinical orthopedics, thus it is of important significance to fabricate a suitable alternative material to stimulate bone regeneration. Due to their excellent biocompatibility, sufficient mechanical strength, and similar structure and composition of natural bone, the mineralized collagen scaffolds (MCSs) have been increasingly used as bone substitutes via tissue engineering approaches. Herein, we thoroughly summarize the state of the art of MCSs as tissue-engineered scaffolds for acceleration of bone repair, including their fabrication methods, critical factors for osteogenesis regulation, current opportunities and challenges in the future. First, the current fabrication methods for MCSs, mainly including direct mineral composite, in-situ mineralization and 3D printing techniques, have been proposed to improve their biomimetic physical structures in this review. Meanwhile, three aspects of physical (mechanics and morphology), biological (cells and growth factors) and chemical (composition and cross-linking) cues are described as the critical factors for regulating the osteogenic feature of MCSs. Finally, the opportunities and challenges associated with MCSs as bone tissue-engineered scaffolds are also discussed to point out the future directions for building the next generation of MCSs that should be endowed with satisfactorily mimetic structures and appropriately biological characters for bone regeneration.

Phosphorylated chitosan to promote biomimetic mineralization of type I collagen as a strategy for dentin repair and bone tissue engineering

This novel biomimetic mineralization technique provides an efficient method to produce an advanced mineralized matrix.

Shear-mediated orientational mineralization of bone apatite on collagen fibrils

Intrafibrillar mineralization of collagen under a 1.5 Pa FSS environment versus the serious extrafibrillar mineralization of collagen under no FSS.

A perspective on structural and computational work on collagen

Collagen is the single most abundant protein in the extracellular matrix in the animal kingdom, with remarkable structural and functional diversity and regarded one of the most useful biomaterials.

Influence of interfacial interactions on deformation mechanism and interface viscosity in α-chitin-calcite interfaces

The interfaces between organic and inorganic phases in natural materials have a significant effect on their mechanical properties. This work presents a quantification of the interface stress as a function of interface chemical changes (water, organic molecules) in chitin-calcite (CHI-CAL) interfaces using classical non-equilibrium molecular dynamics (NEMD) simulations and steered molecular dynamics (SMD) simulations. NEMD is used to investigate interface stress as a function of applied strain based on the virial stress formulation. SMD is used to understand interface separation mechanism and to calculate interfacial shear stress based on a viscoplastic interfacial sliding model. Analyses indicate that interfacial shear stress combined with shear viscosity can result in variations to the mechanical properties of the examined interfacial material systems. It is further verified with Kelvin-Voigt and Maxwell viscoelastic analytical models representing viscous interfaces and outer matrix. Further analyses show that overall mechanical deformation depends on maximization of interface shear strength in such materials. This work establishes lower and upper bounds of interface strength in the interfaces examined.

Water in hydroxyapatite nanopores: Possible implications for interstitial bone fluid flow

The role of bone water in the activity of this organ is essential in structuring apatite crystals, providing pathways for nutrients and waste involved in the metabolism of bone cells and participating in bone remodelling mechanotransduction. It is commonly accepted that bone presents three levels of porosity, namely the vasculature, the lacuno-canalicular system and the voids of the collagen-apatite matrix. Due to the observation of bound state of water at the latter level, the interstitial nanoscopic flow that may exist within these pores is classically neglected. The aim of this paper is to investigate the possibility to obtain a fluid flow at the nanoscale. That is why a molecular dynamics based analysis of a water-hydroxyapatite system is proposed to analyze the effect of water confinement on transport properties. The main result here is that free water can be observed inside hydroxyapatite pores of a few nanometers. This result would have strong implications in the multiscale treatment of the poromechanical behaviour of bone tissue. In particular, the mechanical properties of the bone matrix may be highly controlled by nanoscopic water diffusion and the classical idea that osteocytic activity is only regulated by bone fluid flow within the lacuno-canalicular system may be discussed again.

Dynamic nanomechanical behaviour of healthy and OI human cortical bone

Viscoelasticity of bone has been of interest for many years because this time-dependent mechanical property relates to the fracture risk of bone under dynamic loading. Several factors have been claimed to contribute to this property including the nature of different constituents of bone and their interactions, as well as moisture content. In the present study, intact normal human cortical bone was demineralised, and molecular structures were identified using infrared spectroscopy. Osteogenesis imperfecta (OI) human cortical bone was also selected for comparison because OI bone has severe defects in collagen molecules, while its mineral phase is almost identical to that of normal bone. The dynamic nanomechanical behaviours of the intact, demineralised and OI human cortical bone specimens were examined using dynamic nanoindentation. Loss tangent, tan δ, was considered as a measure of the degree of the viscoelastic response. Variable dynamic load tests show that the viscoelastic responses of all bone specimens increase with frequency. With demineralisation, bone specimens show greater viscoelastic response than intact specimens. OI bone shows similar viscoelastic response as normal bone. Results suggest that the viscoelasticity of bone is mostly attributable to the mineral phase. The present study adds to the understanding of the viscoelastic response of bone material. In addition, the dynamic mechanical properties of OI bone are firstly reported here.

Predominant role of water in native collagen assembly inside the bone matrix

Bone is one of the most intriguing biomaterials found in nature consisting of bundles of collagen helixes, hydroxyapatite, and water, forming an exceptionally tough, yet lightweight material. We present here an experimental tool to map water-dependent subtle changes in triple helical assembly of collagen protein in its absolute native environment. Collagen being the most abundant animal protein has been subject of several structural studies in last few decades, mostly on an extracted, overexpressed, and synthesized form of collagen protein. Our method is based on a (1)H detected solid-state nuclear magnetic resonance (ssNMR) experiment performed on native collagen protein inside intact bone matrix. Recent development in (1)H homonuclear decoupling sequences has made it possible to observe specific atomic resolution in a large complex system. The method consists of observing a natural-abundance two-dimensional (2D) (1)H/(13)C heteronuclear correlation (HETCOR) and(1)H double quantum-single quantum (DQ-SQ) correlation ssNMR experiment. The 2D NMR experiment maps three-dimensional assembly of native collagen protein and shows that extracted form of collagen protein is significantly different from protein in the native state. The method also captures native collagen subtle changes (of the order of ∼1.0 Å) due to dehydration and H/D exchange, giving an experimental tool to map small changes. The method has the potential to be of wide applicability to other collagen containing biomaterials.

Power-law hereditariness of hierarchical fractal bones

In this paper, the authors introduce a hierarchic fractal model to describe bone hereditariness. Indeed, experimental data of stress relaxation or creep functions obtained by compressive/tensile tests have been proved to be fit by power law with real exponent 0 ⩽ β ⩽1. The rheological behavior of the material has therefore been obtained, using the Boltzmann-Volterra superposition principle, in terms of real order integrals and derivatives (fractional-order calculus). It is shown that the power laws describing creep/relaxation of bone tissue may be obtained by introducing a fractal description of bone cross-section, and the Hausdorff dimension of the fractal geometry is then related to the exponent of the power law.

Photoacoustic FTIR spectroscopic study of undisturbed human cortical bone

Chemical pretreatment has been the prevailing sample preparation procedure for infrared (IR) spectroscopic studies on bone. However, experiments have indicated that chemical pretreatment can potentially affect the interactions between the components. Typically the IR techniques have involved transmission experiments. Here we report experimental studies using photoacoustic Fourier transform infrared spectroscopy (PA-FTIR). As a nondestructive technique, PA-FTIR can detect absorbance spectrum from a sample at controllable sampling depth and with little or no sample preparation. Additionally, the coupling inert gas, helium, which is utilized in the PA-FTIR system, can inhibit bacteria growth of bone by displacing oxygen. Therefore, we used this technique to study the undisturbed human cortical bone. It is found that photoacoustic mode (linear-scan, LS-PA-FTIR) can obtain basically similar spectra of bone as compared to the traditional transmission mode, but it seems more sensitive to amide III and ν(2) carbonate bands. The ν(3) phosphate band is indicative of detailed mineral structure and symmetry of native bone. The PA-FTIR depth profiling experiments on human cortical bone also indicate the influence of water on OH band and the cutting effects on amide I and mineral bands. Our results indicate that phosphate ion geometry appears less symmetric in its undisturbed state as detected by the PA-FTIR as compared to higher symmetry observed using transmission techniques on disturbed samples. Moreover, the PA-FTIR spectra indicate a band at 1747 cm(-1) possibly resulting from CO stretching of lipids, cholesterol esters, and triglycerides from the arteries. Comparison of the spectra in transverse and longitudinal cross-sections demonstrates that, the surface area of the longitudinal section bone appears to have more organic matrix exposed and with higher mineral stoichiometry.

Interactions binding mineral and organic phases in nanocomposites based on bacterial cellulose and calcium phosphates

The interactions responsible for the adhesion of calcium phosphate (CP) nanocrystals and bacterial cellulose (BC) nanofibrils in the composite material obtained by mixing aqueous suspensions of presynthesized CP and BC and the dependence of these interactions on the CP morphology and chemical structure have been elucidated by molecular mechanics calculations of the CP-BC interfacial structures. The interactions between the superficial CP and BC crystal layers have been simulated. Two crystalline CP structures (i.e., hydroxyapatite (HAP) and whitlockite) with two morphologies (plate-shaped and rod-shaped) were considered. Electrostatics has been found to be the major contributor to the adhesion of the CP crystallites and BC nanofibers, and the formation of interfacial hydrogen bonds makes a minor contribution to the interaction energy. It has also been found that, in general, the energy gain resulting from whitlockite-BC binding is greater than that for HAP-BC binding, and the binding of the rod-shaped crystallites of whitlockite with BC is the most profitable. The energy loss and entropy gain upon replacement of the BC-water and CP-water contacts by the BC-CP contacts have been estimated.

Experiments in Nanomechanical Properties of Live Osteoblast Cells and Cell–Biomaterial Interface

Characterizing the mechanical characteristics of living cells and cell–biomaterial composite is an important area of research in bone tissue engineering. In this work, an in situ displacement-controlled nanoindentation technique (using Hysitron Triboscope) is developed to perform nanomechanical characterization of living cells (human osteoblasts) and cell–substrate constructs under physiological conditions (cell culture medium; 37 °C). In situ elastic moduli (E) of adsorbed proteins on tissue culture polystyrene (TCPS) under cell culture media were found to be ∼4 GPa as revealed by modulus mapping experiments. The TCPS substrates soaked in cell culture medium showed significant difference in surface nanomechanical properties (up to depths of ∼12 nm) as compared to properties obtained from deeper indentations. Atomic force microscopy (AFM) revealed the cytoskeleton structures such as actin stress fiber networks on flat cells which are believed to impart the structural integrity to cell structure. Load-deformation response of cell was found to be purely elastic in nature, i.e., cell recovers its shape on unloading as indicated by linear loading and unloading curves obtained at 1000 nm indentation depth. The elastic response of cells is obtained during initial cell adhesion (ECell, 1 h, 1000 nm = 4.4–12.4 MPa), cell division (ECell, 2 days, 1000 nm = 1.3–3.0 MPa), and cell spreading (ECell, 2 days, 1000 nm = 6.9–11.6 MPa). Composite nanomechanical responses of cell–TCPS constructs were obtained by indentation at depths of 2000 nm and 3000 nm on cell-seeded TCPS. Elastic properties of cell–substrate composites were mostly dominated by stiff TCPS (EBulk = 5 GPa) lying underneath the cell.

Thickness of hydroxyapatite nanocrystal controls mechanical properties of the collagen-hydroxyapatite interface

Collagen-hydroxyapatite interfaces compose an important building block of bone structures. While it is known that the nanoscale structure of this elementary building block can affect the mechanical properties of bone, a systematic understanding of the effect of the geometry on the mechanical properties of this interface between protein and mineral is lacking. Here we study the effect of geometry, different crystal surfaces, and hydration on the mechanical properties of collagen-hydroxyapatite interfaces from an atomistic perspective, and discuss underlying deformation mechanisms. We find that the presence of hydroxyapatite significantly enhances the tensile modulus and strength compared with a tropocollagen molecule alone. The stiffening effect is strongly dependent on the thickness of the mineral crystal until a plateau is reached at 2 nm crystal thickness. We observe no significant differences due to the mineral surface (Ca surface vs OH surface) or due to the presence of water. Our result shows that the hydroxyapatite crystal with its thickness confined to the nanometer size efficiently increases the tensile modulus and strength of the collagen-hydroxyapatite composite, agreeing well with experimental observations that consistently show the existence of extremely thin mineral flakes in various types of bones. We also show that the collagen-hydroxyapatite interface can be modeled with an elastic network model which, based on the results of atomistic simulations, provides a good estimate of the surface energy and other mechanical features.

Bone nodules on chitosan-polygalacturonic acid-hydroxyapatite nanocomposite films mimic hierarchy of natural bone

The ultimate goal of bone tissue engineering is to develop bony tissues on tissue engineered constructs that mimic the native bone. Nanoscale characterization of in vitro generated bony tissues on engineered scaffolds is essential to understanding both the physical and mechanical characteristics of the engineered bone. Bone nodule formation, a typical early indicator of bone formation was observed on chitosan-polygalacturonic acid-hydroxyapatite (Chi-PgA-HAP) nanocomposite films without the use of differentiating media. Thus, the Chi-PgA-HAP substrates designed are osteoinductive and provide an appropriate microenvironment for cell organization and tissue regeneration. Imaging using atomic force microscopy revealed several levels of hierarchical structures of bone in the bone nodules, consisting of mineralized collagen fibers, fibrils and mineral deposits in extrafibrillar spaces. The nanoscale elastic properties of the collagen and mineral crystals were found to be in close agreement with the experimental and simulations results on natural bone reported in the literature. Fourier transform infrared spectroscopy experiments indicate the presence of collagen and biological apatite in bone nodules exhibiting the characteristics of newly precipitated, immature bone. Collectively, our structural, chemical, and mechanical analyses support the conclusion that synthetic bone nodules mimic the hierarchy of natural bone.

A density functional theory study of the interaction of collagen peptides with hydroxyapatite surfaces

Density functional theory calculations were applied to investigate the binding of four peptide strands, which are important in the collagen protein, to the bone and tooth mineral hydroxyapatite: amphiphilic PRO-HYP-GLY and HYP-PRO-GLY, and hydrophobic PRO-LYS-GLY and PRO-HYL-GLY. The particular peptide sequences are chosen for their different functional groups, containing (i) hydrophobic; (ii) uncharged polar; and (iii) charged polar side groups, thus allowing direct comparison of the general effect of these carboxylic acid and amine functional groups, as well as hydroxylation and charge, on their interactions with two major hydroxyapatite surfaces, (0001) and (0110). The calculated results are consistent with experiments, confirming that the terminal carboxyl groups and amine groups mainly contribute to the adsorption of the peptides to the hydroxyapatite surfaces and primarily to the (0110) surface rather than the dominant (0001) plane. Of the side groups in the tripeptide motifs representing the collagen protein, the -OH and positively charged -NH(3)(+) groups in particular bind strongly to the surfaces, and their presence should therefore promote hydroxyapatite growth.