The influence of transition metal ions on collagen mineralization

The ions in body fluid play an important role in bone formation besides being a synthesizing material. Transition metal ions Co^2 +, Ni^2 +, Zn^2 +, Fe^3 +, Mn^2 +, Cu^2 +, Cd^2 + and Hg^2 + doped hydroxyapatite (HAP)/collagen composites were synthesized successfully in the presence of collagen traces at mild acidic pH for the first time. However, the amount of doped Hg^2 + and Cd^2 + was relatively low. Meanwhile, through soaking the collagen sponge as a template in simulated body fluid (SBF) which contains different transition metal ions (Mn^2 +, Cu^2 +, Ni^2 +, Co^2 +, Cd^2 +, Hg^2 +), bone-like HAP/collagen composites were synthesized. Hg^2 + had a certain inhibitory effect on the formation of HAP crystals on the surface of the collagen sponge while Co^2 + can promote the formation of HAP on the collagen sponge. For both HAP/collagen composites and HAP/collagen sponge, it was found that transition metal ions Mn^2 + had a significant effect on the morphology of HAP particles and could induce to form floc-like HAP particle aggregates.     

Preparation of zeolite-A/chitosan hybrid composites and their bioactivities and antimicrobial activities

Zeolite-A/chitosan hybrid composites with zeolite contents of 20–55 wt.% were prepared by in situ transformation of silica/chitosan mixtures in a sodium aluminate alkaline solution through impregnation–gelation–hydrothermal synthesis. The products were characterized by X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, scanning electron microscopy, thermogravimetric analysis, and mercury penetration porosimetry. Their in vitro bioactivities were examined using as-synthesized and Ca^2 +-exchanged hybrid composites in simulated body fluid (SBF) for hydroxyapatite (HAP) growth. Their antimicrobial activities for Escherichia coli (E. coli) in trypticase soy broth (TSB) were evaluated using Ag⁺-exchanged hybrid composites. The zeolite-A/chitosan hybrid composites could be prepared as various shapes, including cylinders, plates and thin films. They possessed macropores with pore sizes ranging from 100 to 300 μm and showed compressive mechanical strength as high as 3.2 MPa when the zeolite content was 35 wt.%. Fast growth on the Ca^2 +-exchanged hybrid composites was observed with the highest weight gain of 51.4% in 30 days. The 35 wt.% Ag⁺-exchanged hybrid composite showed the highest antimicrobial activity, which could reduce the 9 × 10⁶ CFU mL^? 1 E. coli concentration to zero within 4 h of incubation time with the Ag⁺-exchanged hybrid composite amount of 0.4 g L^? 1. The bioactivity and antimicrobial activity could be combined by ion-exchanging the composites first with Ca^2 + and then with Ag⁺. These zeolite-A/chitosan hybrid composites have potential applications on tissue engineering and antimicrobial food packaging.     

Fabrication and mechanical properties of PLA/HA composites: A study of in vitro degradation

The adverse effects of stress shielding from the use of high-modulus metallic alloy bio-implant materials has led to increased research into developing polymer–ceramic composite materials that match the elastic modulus of human bone. Of particular interest are poly-l-lactic acid–hydroxyapatite (PLA/HA)-based composites which are fully resorbable in vivo. However, their bioresorbability has a deleterious effect on the mechanical properties of the implant. The purpose of this study is to investigate, from a micromechanistic perspective, the in vitro degradation behavior of such composites manufactured using a simple hot-pressing route for two different hydroxyapatite particles: a fine-grained (average particle size ∼5 μm) commercial powder or coarser whiskers (∼ 25–30 μm long, ∼ 5 μm in diameter). We observed that composites with ceramic contents ranging between 70 and 85 wt.% have mechanical properties that match reasonably those of human cortical bone. However, the properties deteriorate with immersion in Hanks' Balanced Salt Solution due to the degradation of the polymer phase. The degradation is more pronounced in samples with larger ceramic content due to the dissolution of the smaller amount of polymer between the ceramic particles.     

Size-scale effects of silica on bis-GMA/TEGDMA based nanohybrid dental restorative composites

The present study focuses on the effect of size-scale combination of silica on the mechanical and dynamic mechanical properties of acrylate based (50% Bis-GMA and 50% TEGDMA by weight) composites with an aim to overcome the conventional problem of high-volume fraction filling of acrylate based composites, typically used in restorative dentistry. Two classes of light-cured composites based on the size-scale combination of silica (7 nm + 2 μm; 14 nm + 2 μm) as the filler were prepared. FTIR spectroscopy revealed functionality and interactions whereas morphological investigations concerning the state of distribution and dispersion of nano- and micro-silica has been carried out by SEM–EDX Si-dot mapping. The dynamic mechanical properties, compressive, flexural and diametral tensile strengths were characterized. Micromechanical analysis of viscoelastic storage moduli following Kerner composite model has revealed an enhancement in the reinforcement efficiency of the nanohybrid composites based on the filler size-scale combination of 14 nm + 2 μm with 10 wt.% nanofiller loading. The compressive strength of the micro-filled composite (with 2 μm silica only) was found to remain comparable to that of the nanohybrid with 5 wt.% of 7 nm silica and 10 wt.% of 14 nm silica filled composites. Diametral tensile strength has been observed to be influenced by the size-scale combination and extent of nanofiller loading. The effective volume fractions in the composites validating the experimentally determined DTS were calculated following Nicolais–Narkis model. Our study demonstrates the conceptual feasibility of exploring the optimization of size-scale combinations of filler for enhancement in reinforcement efficiency by manipulating the volume fraction of filler induced immobilized polymer chains by resorting to the principle of micromechanics.     

Improvement of the bonding interface of a sintered Al 2014–(Ti5Si3)p composite by the copper coating of the reinforcement

AA 2014 aluminium-based composites reinforced with (5–20 wt.%) Ti₅Si₃ intermetallic particles, with and without Cu coating, were obtained in a Turbula powder mixer from commercially-available prealloyed powders. Mechanical alloying was used for the deposition of Cu on the surface of the Ti₅Si₃ particles. Compaction of the specimens was performed using a hydraulic press and a floating die. The results show that the liquid formation and phase distribution are modified by the copper coating of the ceramic reinforcement, resulting in changes in the materials microstructure and the mechanical properties. The presence of the reinforcement particles improves densification of the composites. Improved densification was found for the 2014 + Ti₅Si₃ composites. 2014 + Ti₅Si₃–Cu composites exhibit superior mechanical properties compared to the 2014 + Ti₅Si₃ composites.     

Microstructure,mechanical property and corrosion behavior of interpenetrating (HA + β-TCP)/MgCa composite fabricated by suction casting

The novel interpenetrating (HA + β-TCP)/MgCa composites were fabricated by infiltrating MgCa alloy into porous HA + β-TCP using suction casting technique. The microstructure, mechanical properties and corrosion behaviors of the composites have been evaluated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), mechanical testing, electrochemical and immersion tests. It was shown that the composites had compact structure and the interfacial bonding between MgCa alloy and HA + β-TCP scaffolds was very well. The ultimate compressive strength of the composites was about 500–1000 fold higher than that of the original porous scaffolds, and it still retained quarter-half of the strength of the bulk MgCa alloy. The electrochemical and immersion tests indicated that the corrosion resistance of the composites was better than that of the MgCa matrix alloy, and the corrosion products of the composite surface were mainly Mg(OH)₂, HA and Ca₃(PO₄)₂. Meanwhile, the mechanical and corrosive properties of the (HA + β-TCP)/MgCa composites were adjustable by the choice of HA content.     

The effect of alginate addition on the structure and morphology of hydroxyapatite/gelatin nanocomposites

A series of hydroxyapatite/gelatin/alginate nanocomposites with different amount of alginate were synthesized by a co-precipitation method. With the increase of alginate amount, a cross-linked alginate/gelatin polymer network formed, which induced a gradual red shift of organic absorption peaks in FT-IR analysis. TEM images indicated that the development of HAP nanocrystals in an aqueous gelatin/alginate mixture was highly influenced by the alginate content. On increasing alginate content, the dimensions of the crystals increased and their morphology changed from needle-like to long fiber-like, and at high alginate content, the crystals tended to aggregate in separate clusters. The results of the electron diffraction strongly indicated alginate promoted the preferential alignment in c direction of HAP nanocrystals. SEM results showed that high amount of alginate led to regular shape and large size of HAP crystals after the composites were calcined for 4 h at 600 °C.     

Mechanical and thermal properties of natural fibers reinforced polymer composites: Doum/low density polyethylene

Polymer composite materials with vegetable fibers were an attractive field for many industries and researchers, however, these materials required the issues of compatibility between the fibers and the polymeric matrix. This work evaluates the thermal and mechanical properties of Doum-fibers reinforcing a low density polyethylene (LDPE) composite to follow the effect of adding fibers into polymer matrix. Doum-fibers were Alkali treated to clean the fiber surface and improve the polymer/fibers adhesion. The Doum-fibers were compounded in LDPE matrix at various contents and extruded as continuous strands. An enhance on mechanical properties of composites was found, a gain of 145% compared to neat polymer at 30 wt.% fiber loading in Young’s modulus, a gain of 135% in flexural modulus at 20 wt.% fiber loading and a gain of 97% in torsional modulus at 0.1 Hz. Thermal properties were evaluated and the results show a slight decrease with increase of added Doum.     

Synthesis and characterization of calcium phosphate/collagen biocomposites doped with Zn2+

Composites were developed using calcium phosphate (CaP)/collagen (COL) doped with Zn⁺² to attempt the materials association with adequate properties for biological applications in the recovery of the bone tissue by trauma or pathogenies. Hydroxyapatite (HAP) and hydroxyapatite-βtricalcium phosphate (HAPβTCP) were synthesized and doped with zinc nitrate. High purity grade type I collagen was extracted and purified from bovine pericardium. CaP doped and undoped with Zn⁺² were produced with COL and the composites were developed using a simple mixture process. All samples were characterized by Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction analysis (XRD. In addition, biocompatibility and cell viability were assessed by MTT assay (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) using osteoblast cell culture. The results have indicated that both morphological and structural features and chemical composition of the composites were very similar to their precursors, collagen and calcium phosphate components. Also, the biocomposites presented a homogeneous aspect with the calcium phosphate particles aggregated to the collagen fibers. The biological evaluation of the composites in vitro showed cellular viability, presenting proliferation of the osteoblasts compared to the control cells (P < 0.05). The composites showed appropriate physical and biological properties creating more biologically active scaffolds that may support bone growth. Therefore, the novel developed biocomposites have high potential to be used for rebuilding small lesions in bone tissue engineering.     

Antibacterial effects of silver-doped hydroxyapatite thin films sputter deposited on titanium

Since many orthopedic implants fail as a result of loosening, wear, and inflammation caused by repeated loading on the joints, coatings such as hydroxyapatite (HAp) on titanium with a unique topography have been shown to improve the interface between the implant and the natural tissue. Another serious problem with long-term or ideally permanent implants is infection. It is important to prevent initial bacterial colonization as existing colonies have the potential to become encased in an extracellular matrix polymer (biofilm) that is resistant to antibacterial agents. In this study, plasma-based ion implantation was used to examine the effects of pre-etching on plain titanium. Topographical changes to the titanium samples were examined and compared via scanning electron microscopy. Hydroxyapatite and silver-doped hydroxyapatite thin films were then sputter deposited on titanium substrates etched at ? 700 eV. For silver-doped films, two concentrations of silver (~ 0.5 wt.% and ~ 1.5 wt.%) were used. Silver concentrations in the film were determined using energy dispersive X-ray spectroscopy. Hydroxyapatite film thicknesses were determined by measuring the surface profile using contact profilometry. Staphylococcus epidermidis and Pseudomonas aeruginosa adhesion studies were performed on plain titanium, titanium coated with hydroxyapatite, titanium coated with ~ 0.5 wt.% silver-doped hydroxyapatite, and titanium coated with ~ 1.5 wt.% silver-doped hydroxyapatite. Results indicate that less bacteria adhered to surfaces containing hydroxyapatite and silver; further, as the hydroxyapatite films delaminated, silver ions were released which killed bacteria in suspension.     

Sol–gel derived nanoscale bioactive glass (NBG) particles reinforced poly(ε-caprolactone) composites for bone tissue engineering

This study investigated the effect of the addition of sol–gel derived nanoscale bioactive glass (NBG) particles on the mechanical properties and biological performances of PCL polymer, in order to evaluate the potential applications of PCL/NBG composites for bone tissue regeneration. Regardless of the NBG contents (10, 20, and 30 wt.%), the NBG particles, which were synthesized through the sol–gel process using polyethylene glycol (PEG) polymer as a template, could be uniformly dispersed in the PCL matrix, while generating pores in the PCL/NBG composites. The elastic modulus of the PCL/NBG composites increased remarkably from 89 ± 11 MPa to 383 ± 50 MPa with increasing NBG content from 0 to 30 wt.%, while still showing good ultimate tensile strength in the range of 15–19 MPa. The hydrophilicity, water absorption and degradation behavior of the PCL/NBG composites were also enhanced by the addition of the NBG particles. Furthermore, the PCL/NBG composite with a NBG content of 30 wt.% showed significantly enhanced in vitro bioactivity and cellular response compared to those of the pure PCL.     

Enhanced sintering ability of biphasic calcium phosphate by polymers used for bone scaffold fabrication

Biphasic calcium phosphate (BCP), which is composed of hydroxyapatite [HAP, Ca₁₀(PO₄)₆(OH)₂] and β-tricalcium phosphate [β-TCP, β-Ca₃(PO₄)₂], is usually difficult to densify into a solid state with selective laser sintering (SLS) due to the short sintering time. In this study, the sintering ability of BCP ceramics was significantly improved by adding a small amount of polymers, by which a liquid phase was introduced during the sintering process. The effects of the polymer content, laser power and HAP/β-TCP ratios on the microstructure, chemical composition and mechanical properties of the BCP scaffolds were investigated. The results showed that the BCP scaffolds became increasingly more compact with the increase of the poly(l-lactic acid) (PLLA) content (0–1 wt.%) and laser power (6–10 W). The fracture toughness and micro-hardness of the sintered scaffolds were also improved. Moreover, PLLA could be gradually decomposed in the late sintering stages and eliminated from the final BCP scaffolds if the PLLA content was below a certain value (approximately 1 wt.% in this case). The added PLLA could not be completely eliminated when its content was further increased to 1.5 wt.% or higher because an unexpected carbon phase was detected in the sintered scaffolds. Furthermore, many pores were observed due to the removal of PLLA. Micro-cracks and micro-pores occurred when the laser power was too high (12 W). These defects resulted in a deterioration of the mechanical properties. The hardness and fracture toughness reached maximum values of 490.3 ± 10 HV and 1.72 ± 0.10 MPa m^1/2, respectively, with a PLLA content of approximately 1 wt.% and laser power of approximately 10 W. Poly(l-lactic-co-glycolic acid) (PLGA) showed similar effects on the sintering process of BCP ceramics. Rectangular, porous BCP scaffolds were fabricated based on the optimum values of the polymer content and laser power. This work may provide an experimental basis for improving the mechanical properties of BCP bone scaffolds fabricated with SLS.     

Effect of highly dispersed yttria addition on thermal stability of hydroxyapatite

The capability of the colloidal method to produce yttria (Y₂O₃) dispersed hydroxyapatite (HA) has been investigated as an alternative method to the conventional method of mechanical mixing and sintering for developing HA-based materials that could exhibit controllable and enhanced functional properties. A water based colloidal route to produce HA materials with highly dispersed Y₂O₃ has been applied, and the effect of 10 wt.% Y₂O₃ addition to HA investigated by thermal analysis, X-ray diffraction and Fourier transform infrared spectroscopy. These measurements evidence a remarkable effect of this Y₂O₃ addition on decomposition mechanisms of synthetic HA. Results show that incorporation of Y₂O₃ as dispersed second phase is beneficial because it hinders the decomposition mechanisms of HA into calcium phosphates. This retardation will allow the control of the sintering conditions for developing HA implants with improved properties. Besides, substitution of Ca^2 + with Y^3 + ions appears to promote the formation of OH^? vacancies, which could improve the conductive properties of HA favorable to osseointegration.     

High-performance silica nanoparticle reinforced poly (vinyl alcohol) as templates for bioactive nanocomposites

Silica nanoparticle reinforced poly (vinyl alcohol) cast sheets 40 μm thick were tested for mechanical and biological properties. The films were characterized using X-ray diffraction, scanning electron microscopy, and infrared spectroscopy. The crystallinity decreased with increased silica content. Changes in the morphology and structure upon the addition of silica suggest the formation of cross-linking. The modulus increased from 300 MPa for PVA to 7.2 GPa for 120 wt.% silica nanoparticle in the blend and the tensile strength increased from 3.5 MPa to 35 MPa. The modulus estimated using dynamic tests, tensile tests, and nanoindentation was comparable and was predicted well using the Halpin-Tsai's equation. The nanocomposites were an order of magnitude tougher than the pure polymer. Silica based nanocomposite was also found to be an excellent template for the deposition of calcium hydroxyapatite when immersed in simulated body fluid. The modulus and tensile strength of apatite coated silica nanoparticle (120 wt.%)–PVA composite increased to 11 GPa and 65 MPa respectively, close to that of cortical bone. The results represent one of the largest increases in mechanical properties of nanocomposite mimicking the properties of human bone. The addition of silica can also aid in osseointegration.     

Calcium silicate ceramic scaffolds toughened with hydroxyapatite whiskers for bone tissue engineering

Calcium silicate possessed excellent biocompatibility, bioactivity and degradability, while the high brittleness limited its application in load-bearing sites. Hydroxyapatite whiskers ranging from 0 to 30 wt.% were incorporated into the calcium silicate matrix to improve the strength and fracture resistance. Porous scaffolds were fabricated by selective laser sintering. The effects of hydroxyapatite whiskers on the mechanical properties and toughening mechanisms were investigated. The results showed that the scaffolds had a uniform and continuous inner network with the pore size ranging between 0.5 mm and 0.8 mm. The mechanical properties were enhanced with increasing hydroxyapatite whiskers, reached a maximum at 20 wt.% (compressive strength: 27.28 MPa, compressive Young's modulus: 156.2 MPa, flexural strength: 15.64 MPa and fracture toughness: 1.43 MPa·m^1/2) and then decreased by addition of more hydroxyapatite whiskers. The improvement of mechanical properties was due to whisker pull-out, crack deflection and crack bridging. Moreover, the degradation rate decreased with the increase of hydroxyapatite whisker content. A layer of bone-like apatite was formed on the scaffold surfaces after being soaked in simulated body fluid. Human osteoblast-like MG-63 cells spread well on the scaffolds and proliferated with increasing culture time. These findings suggested that the calcium silicate scaffolds reinforced with hydroxyapatite whiskers showed great potential for bone regeneration and tissue engineering applications.     

Enhanced properties of alumina–aluminium titanate composites obtained by spark plasma reaction-sintering of slip cast green bodies

Full dense alumina + 40 vol.% aluminium titanate composites were obtained by colloidal filtration and fast reaction-sintering of alumina/titania green bodies by spark plasma sintering at low temperatures (1250–1400 °C). The composites obtained had near-to-theoretical density (>99%) with a bimodal grain size distribution. Phase development analysis demonstrated that aluminium titanate has already formed at 1300 °C. The mechanical properties such as Vickers hardness, flexural strength and fracture toughness of bulk composites are significantly higher than those reported elsewhere, e.g. the composite sintered at 1350 °C show values of about 24 GPa, 424 MPa and 5.4 MPa m^1/2, respectively. The improved mechanical properties of these composites are attributed to the enhanced densification and the finer and more uniform nanostructure achieved by non-conventional fast sintering of slip-cast dense green compacts.     

Fabrication and technological properties of nanoporous spinel/forsterite/zirconia ceramic composites

In this work, nanoporous spinel/forsterite/zirconia ceramic composites were fabricated at 1600 °C for 2 h. The influence of zirconia content (up to 10 mass%) on the technological properties, nanopores formation, phase compositions, microstructure and thermal diffusivity of nanoporous ceramic composites was investigated. Nanospinel and nanoforsterite powders were synthesized via a modified co-precipitation and sol–gel techniques, respectively. Results indicated that apparent porosity of the fired nanoporous ceramic composites is mostly in the range 14.26–56.14% with the average pores diameter 35.8 nm. Using of nanopowders (spinel and forsterite) as the staring materials were achieved high mechanical (cold crushing strength ∼ 235–164 MPa) and elastic (Young’s modulus ∼ 123.6–4.5 GPa) properties of the prepared nanoporous ceramic composites. Microstructure analysis exhibited all of the crystalline phases and pores of the nanoporous ceramic composites are in the nanosize (35–40 nm). These nanoporous ceramic composites are promising porous ceramic materials for using in advanced applications due to their excellent combination properties.     

Bioactive glass-based composites for the production of dense sintered bodies and porous scaffolds

Recently several attempts have been made to combine calcium phosphates, such as β-tricalcium phosphate (β-TCP) and, most of all, hydroxyapatite (HA), with bioactive glasses of different composition, in order to develop composites with improved biological and mechanical performance. Unfortunately, the production of such systems usually implies a high-temperature treatment (up to 1300 °C), which may result in several drawbacks, including crystallization of the original glass, decomposition of the calcium phosphate phase and/or reactions between the constituent phases, with non-trivial consequences in terms of microstructure, bioactivity and mechanical properties of the final samples. In the present contribution, novel binary composites have been obtained by sintering a bioactive glass, characterized by a low tendency to crystallize, with the addition of HA or β-TCP as the second phase. In particular, the composites have been treated at a relatively low temperature (818 °C and 830 °C, depending on the sample), thus preserving the amorphous structure of the glass and minimizing the interaction between the constituent phases. The effects of the glass composition, calcium phosphate nature and processing conditions on the composite microstructure, mechanical properties and in vitro bioactivity have been systematically discussed. To conclude, a feasibility study to obtain scaffolds for bone tissue regeneration has been proposed.     

Research into mechanical properties of reaction-bonded SiC composites at cryogenic temperatures

The mechanical properties of reaction-bonded silicon carbide (RBSC) composites at cryogenic temperatures have been reported for the first time. The results show that the flexural strength and fracture toughness increase from 277.93 ± 23.21 MPa to 396.74 ± 52.74 MPa and from 3.69 ± 0.45 MPa·m^1/2 to 4.98 ± 0.53 MPa·m^1/2 as the temperature decreases from 293 K to 77 K, respectively. The XRD analysis of the phase composition reveals that there is no phase transformation in the composites at cryogenic temperatures, indicating cryogenic mechanical properties are independent of phase composition. The enhancement of mechanical properties at 77 K over room temperature could be explained by the transition of fracture mode from predominant transgranular fracture to intergranular fracture and stronger resistance to crack propagation resulting from higher residual stress at 77 K. The above results demonstrate that such composites do not undergo similar deteriorations in the fracture toughness as other materials (some kinds of metals and polymers), so it is believed that such composites could be a potential material applied in cryogenic field.     

Preparation and characterization of dense nanohydroxyapatite/PLLA composites

Synthetic bone graft substitutes based on PLLA have been largely studied during the past decade. PLLA/hydroxyapatite composites appear as promising materials for large bone defect healing. In this study dense PLLA/nano-hydroxyapatite composites were prepared by hot pressing. Dense samples were investigated rather than porous scaffolds, in order to shed light on possible correlations between intrinsic mechanical properties and nano-hydroxyapatite concentration. Hydroxyapatite deagglomerated by wet attrition milling, and further dispersed into chloroform was used (median diameter = 80 nm). Particle size distribution measurements and transmission electron microscopy show evidence that particle size and dispersion are maintained throughout the successive steps of composite processing. Mechanical properties were tested (uni-axial and diametral compression tests) as a function of nano-hydroxyapatite content. Increasing concentrations of nano-hydroxyapatite (0, 25 and 50 wt.%) increase the Young's modulus and the mechanical strength of the composite; at the same time, the failure mechanism of the material changes from plastic to brittle. Young's modulus over 6 GPa and uniaxial compressive strength over 100 MPa have been achieved. These values expressed in terms of intrinsic tensile and shear strengths indicate that 50 wt.% nano-hydroxyapatite containing samples develop properties comparable to those of cortical bone. PLLA/nano-hydroxyapatite composites are thus promising candidates to develop bioresorbable porous bone substitutes showing superior mechanical performance.