Modeling of mechanical properties and structural design of spider web

With a unique combination of strength and toughness among materials, spider silk is the model for engineering materials. This paper presents the stress-strain behavior of Nephila clavipes spider silk under tension, transverse compression, and torsional deformation obtained by a battery of micro testing equipment. The experimental results showed significantly higher toughness than the state-of-the-art fibers in tension and in transverse compression. Higher shear modulus was also observed for the spider silk comparing to other liquid crystalline fibers such as aramid fibers. On the basis of the experimental results finite element analysis is used to simulate static and dynamic properties of spider web and to explore the role of both material properties and architectural design in its structural integrity and mechanical performance.     

Composition of the cement line and its possible mechanical role as a local interface in human compact bone

Human compact bone may be viewed as a fiber reinforced composite material in which the secondary osteons act as the fiber reinforcements. The cement line, which is the interface between the 'fibers' (osteons) and extraosteonal bone matrix, may impart important mechanical properties to compact bone. The nature of these properties is not known partly because the composition of the cement line is unknown. This analysis examines the constituents of the osteon cement line using scanning electron microscopy and X-ray microprobe analysis to address its biomechanical functions as a local interface. The analysis suggests that the cement line is a region of reduced mineralization which may contain sulfated mucosubstances. This composition is consistent with the hypothesis that the cement line provides a relatively ductile interface with surrounding bone matrix, and that it provides the point specific stiffness differences, poor 'fiber'-matrix bonding and energy transfer qualities required to promote crack initiation but slow crack growth in compact bone.     

Preparation and characterization of wheat straw fibers for reinforcing application in injection molded thermoplastic composites

The potential of wheat straw fibers prepared by mechanical and chemical processes as reinforcing additives for thermoplastics was investigated. Fibers prepared by mechanical and chemical processes were characterized with respect to their chemical composition, morphology, and physical, mechanical and thermal properties. Composites of polypropylene filled with 30% wheat straw fibers were prepared and their mechanical properties were also evaluated. The fibers prepared by chemical process exhibited better mechanical, physical and thermal properties. Wheat straw fiber reinforced polypropylene composites exhibited significantly enhanced properties compared to virgin polypropylene. However, the strength properties of the composites were less for chemically prepared fiber filled composites. This was due to the poor dispersion of the fibers under the processing conditions used. These results indicate that wheat straw fibers can be used as potential reinforcing materials for making thermoplastic composites.     

Mechanics of bone/PMMA composite structures: an in vitro study of human vertebrae

The goal of this study was to provide material property data for the cement/bone composite resulting from the introduction of PMMA bone cement into human vertebral bodies. A series of quasistatic tensile and compressive mechanical tests were conducted using cement/bone composite structures machined from cement-infiltrated vertebral bodies. Experiments were performed both at room temperature and at body temperature. We found that the modulus of the composite structures was lower than bulk cement (p<0.0001). For compression at 37( composite function)C: composite =2.3+/-0.5GPa, cement =3.1+/-0.2GPa; at 23( composite function)C: composite =3.0+/-0.3GPa, cement =3.4+/-0.2GPa. Specimens tested at room temperature were stiffer than those tested at body temperature (p=0.0004). Yield and ultimate strength factors for the composite were all diminished (55-87%) when compared to cement properties. In general, computational models have assumed that cement/bone composite had the same modulus as cement. The results of this study suggest that computational models of cement infiltrated vertebrae and cemented arthroplasties could be improved by specifying different material properties for cement and cement/bone composite.     

The effects of bone and pore volume fraction on the mechanical properties of PMMA/bone biopsies extracted from augmented vertebrae

Vertebroplasty forms a porous PMMA/bone composite which was shown to be weaker and less stiff than pure PMMA. It is not known what determines the mechanical properties of such composites in detail. This study investigated the effects of bone volume fraction (BV/TV), cement porosity (PV/(TV-BV), PV…pore volume) and cement stiffness. Nine human vertebral bodies were augmented with either standard or low-modulus PMMA cement and scanned with a HR-pQCT system before and after augmentation. Fourteen cylindrical PMMA/bone biopsies were extracted from the augmented region, scanned with a micro-CT system and tested in compression until failure. Micro-finite element (FE) models of the complete biopsies, of the trabecular bone alone as well as of the porous cement alone were generated from CT images to gain more insight into the role of bone and pores. PV/(TV-BV) and experimental moduli of standard/low-modulus cement (R(2)=0.91/0.98) as well as PV/(TV-BV) and yield stresses (R(2)=0.92/0.83) were highly correlated. No correlation between BV/TV (ranging from 0.057 to 0.138) and elastic moduli was observed (R(2)< 0.05). Interestingly, the micro-FE models of the porous cement alone reproduced the experimental elastic moduli of the standard/low-modulus cement biopsies (R(2)=0.75/0.76) more accurately than the models with bone (R(2)=0.58/0.31). In conclusion, the mechanical properties of the biopsies were mainly determined by the cement porosity and the cement material properties. The study showed that bone tissue inside the biopsies was mechanically "switched off" such that load was carried essentially by the porous PMMA.     

Influence of Nano-HA Coated Bone Collagen to Acrylic (Polymethylmethacrylate) Bone Cement on Mechanical Properties and Bioactivity

Objective

This research investigated the mechanical properties and bioactivity of polymethylmethacrylate (PMMA) bone cement after addition of the nano-hydroxyapatite(HA) coated bone collagen (mineralized collagen, MC).

Materials & Methods

The MC in different proportions were added to the PMMA bone cement to detect the compressive strength, compression modulus, coagulation properties and biosafety. The MC-PMMA was embedded into rabbits and co-cultured with MG 63 cells to exam bone tissue compatibility and gene expression of osteogenesis.

Results

15.0%(wt) impregnated MC-PMMA significantly lowered compressive modulus while little affected compressive strength and solidification. MC-PMMA bone cement was biologically safe and indicated excellent bone tissue compatibility. The bone-cement interface crosslinking was significantly higher in MC-PMMA than control after 6 months implantation in the femur of rabbits. The genes of osteogenesis exhibited significantly higher expression level in MC-PMMA.

Conclusions

MC-PMMA presented perfect mechanical properties, good biosafety and excellent biocompatibility with bone tissues, which has profoundly clinical values.     

Carbon nanotube reinforced Bombyx mori silk nanofibers by the electrospinning process

Nanocomposite fibers of Bombyx mori silk and single wall carbon nanotubes (SWNT) were produced by the electrospinning process. Regenerated silk fibroin dissolved in a dispersion of carbon nanotubes in formic acid was electrospun into nanofibers. The morphology, structure, and mechanical properties of the electrospun nanofibers were examined by field emission environmental scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and microtensile testing. TEM of the reinforced fibers shows that the single wall carbon nanotubes are embedded in the fibers. The mechanical properties of the SWNT reinforced fiber show an increase in Young's modulus up to 460% in comparison with the un-reinforced aligned fiber, but at the expense of the strength and strain to failure.     

Effect of fiber treatments on tensile and thermal properties of starch/ethylene vinyl alcohol copolymers/coir biocomposites

Coir fibers received three treatments, namely washing with water, alkali treatment (mercerization) and bleaching. Treated fibers were incorporated in starch/ethylene vinyl alcohol copolymers (EVOH) blends. Mechanical and thermal properties of starch/EVOH/coir biocomposites were evaluated. Fiber morphology and the fiber/matrix interface were further characterized by scanning electron microscopy (SEM). All treatments produced surface modifications and improved the thermal stability of the fibers and consequently of the composites. The best results were obtained for mercerized fibers where the tensile strength was increased by about 53% as compared to the composites with untreated fibers, and about 33.3% as compared to the composites without fibers. The mercerization improved fiber–matrix adhesion, allowing an efficient stress transfer from the matrix to the fibers. The increased adhesion between fiber and matrix was also observed by SEM. Treatment with water also improved values of Young’s modulus which were increased by about 75% as compared to the blends without the fibers. Thus, starch/EVOH blends reinforced with the treated fibers exhibited superior properties than neat starch/EVOH.     

Collagen fiber orientation and geometry effects on the mechanical properties of secondary osteons.

The effects of collagen fiber orientation and osteon geometry on the mechanical properties of secondary osteons under axial compression/tension and combined loadings (compression, bending and torsion) were investigated using a composite-beam finite-element model. Three cross-sectional shapes of secondary osteons were studied to show the effect of geometry. The results of stiffness are presented using the tension and compression properties for each lamella. The model shows that the mechanical properties of osteons are enhanced in bending and torsion when collagen fibers are oriented within 30 degrees of the loading axis. Osteons with alternating lamellar orientation are not well adapted to resist torsional moments, but alternate collagen fiber orientation has virtually no effect on the bending stiffness of osteons. Fiber orientation affects the mechanical properties less significantly when osteons are non-circular. Collagen fiber orientation and osteon geometry interact to determine the mechanical behavior of the osteon, and may act in a compensatory manner in the adaptive process.     

The mechanical behavior of PMMA/bone specimens extracted from augmented vertebrae: a numerical study of interface properties, PMMA shrinkage and trabecular bone damage

Recently published compression tests on PMMA/bone specimens extracted after vertebral bone augmentation indicated that PMMA/bone composites were not reinforced by the trabecular bone at all. In this study, the reasons for this unexpected behavior should be investigated by using non-linear micro-FE models. Six human vertebral bodies were augmented with either standard or low-modulus PMMA cement and scanned with a HR-pQCT system before and after augmentation. Six cylindrical PMMA/bone specimens were extracted from the augmented region, scanned with a micro-CT system and tested in compression. Four different micro-FE models were generated from these images which showed different bone tissue material behavior (with/without damage), interface behavior (perfect bonding, frictionless contact) and PMMA shrinkage due to polymerization. The non-linear stress-strain curves were compared between the different micro-FE models as well as to the compression tests of the PMMA/bone specimens. Micro-FE models with contact between bone and cement were 20% more compliant compared to those with perfect bonding. PMMA shrinkage damaged the trabecular bone already before mechanical loading, which further reduced the initial stiffness by 24%. Progressing bone damage during compression dominated the non-linear part of the stress-strain curves. The micro-FE models including bone damage and PMMA shrinkage were in good agreement with the compression tests. The results were similar with both cements. In conclusion, the PMMA/bone interface properties as well as the initial bone damage due to PMMA polymerization shrinkage clearly affected the stress-strain behavior of the composite and explained why trabecular bone did not contribute to the stiffness and strength of augmented bone.     

Physico-mechanical properties of chemically treated palm and coir fiber reinforced polypropylene composites

In this work, palm and coir fiber reinforced polypropylene bio-composites were manufactured using a single extruder and injection molding machine. Raw palm and coir were chemically treated with benzene diazonium salt to increase their compatibility with the polypropylene matrix. Both raw and treated palm and coir fiber at five level of fiber loading (15, 20, 25, 30 and 35 wt.%) was utilized during composite manufacturing. Microstructural analysis and mechanical tests were conducted. Comparison has been made between the properties of the palm and coir fiber composites. Treated fiber reinforced specimens yielded better mechanical properties compared to the raw composites, while coir fiber composites had better mechanical properties than palm fiber ones. Based on fiber loading, 30% fiber reinforced composites had the optimum set of mechanical properties.     

Compressive fatigue properties of a commercially available acrylic bone cement for vertebroplasty

Acrylic bone cements are widely used for fixation of joint prostheses as well as for vertebral body augmentation procedures of vertebroplasty and balloon kyphoplasty, with the cement zone(s) being subjected to repeated mechanical loading in each of these applications. Although, in vertebroplasty and balloon kyphoplasty, the cement zone is exposed to mainly cyclical compressive load, the compressive fatigue properties of acrylic bone cements used in these procedures are yet to be determined. The purposes of the present study were to determine the compressive fatigue properties of a commercially available cement brand used in vertebroplasty, including the effect of frequency on these properties; to identify the cement failure modes under compressive cyclical load; and to introduce a screening method that may be used to shorten the lengthy character of the standardized fatigue tests. Osteopal \({^\circledR } \mathrm{V}\) was used as the model cement in this study. The combinations of maximum stress and frequency used were 50.0, 55.0, 60.0, 62.5 and 75.5 MPa at 2 Hz; and of 40.0, 55.0, 60.0, 62.5 or 75.5 MPa at 10 Hz. Through analysis of nominal strain-number of loading cycles results, three cement failure modes were identified. The estimated mean fatigue limit at 2 Hz (55.4 MPa) was significantly higher than that at 10 Hz (41.1 MPa). The estimated fatigue limit at 2 Hz is much higher than stresses commonly found in the spine and also higher than that for other acrylic bone cements tested in a full tension–compression fatigue test, which indicates that tension–compression fatigue testing may substantially underestimate the performance of cements intended for vertebroplasty. A screening method was introduced which may be used to shorten the time spent in performing compressive fatigue tests on specimens of acrylic bone cement for use in vertebral body augmentation procedures.     

Local variations in the micromechanical properties of mouse femur: the involvement of collagen fiber orientation and mineralization

In this study we sought to understand the material level basis for local variations in the uniaxial micromechanical properties of mouse cortical bone. It was hypothesized that the opposing anterior and posterior quadrants will significantly differ in terms of their mechanical function, such that, the anterior portion will be stronger in tension whereas the posterior quadrant will be stronger in compression. Mechanical properties were assessed via microtensile and microcompressive tests of standardized coupon-shaped specimens from femurs of Swiss Webster mice (9 weeks). The mineralization and mineral quality was assessed via Raman spectroscopy and the overall collagen orientation was investigated with quantitative polarized imaging. Micromechanical tests demonstrated that the modulus, yield stress, maximum stress and fracture energy of the posterior quadrant was 66%, 53%, 42% and 31% of anterior quadrant; however, the compressive properties did not differ between the two quadrants. Raman microspectroscopic analysis indicated that the mineral matrix ratio, mineral crystallinity and carbonation did not vary between the quadrants. However, the collagen fibers in the anterior quadrant were significantly (p<0.05) more oriented along the longer axis of the diaphyseal shaft than the collagen fibers of the posterior quadrant. Therefore, we concluded that the orientation of collagen fibers with respect to the anatomical loading axis has a profound effect on the uniaxial mechanical function of murine bone. It will be a matter of further research to reveal the role of local variations in the mode of stress on this material level dichotomy in tissue organization and mechanical function.     

Inducing β-sheets formation in synthetic spider silk fibers by aqueous post-spin stretching

As a promising biomaterial with numerous potential applications, various types of synthetic spider silk fibers have been produced and studied in an effort to produce man-made fibers with mechanical and physical properties comparable to those of native spider silk. In this study, two recombinant proteins based on Nephila clavipes Major ampullate Spidroin 1 (MaSp1) consensus repeat sequence were expressed and spun into fibers. Mechanical test results showed that fiber spun from the higher molecular weight protein had better overall mechanical properties (70 KD versus 46 KD), whereas postspin stretch treatment in water helped increase fiber tensile strength significantly. Carbon-13 solid-state NMR studies of those fibers further revealed that the postspin stretch in water promoted protein molecule rearrangement and the formation of β-sheets in the polyalanine region of the silk. The rearrangement correlated with improved fiber mechanical properties and indicated that postspin stretch is key to helping the spider silk proteins in the fiber form correct secondary structures, leading to better quality fibers.     

Reinforced bioartificial dermis constructed with collagen threads

In this work, a novel type of composite scaffold was designed, which has the suitability of both high biocompatibility and strong mechanical properties, for use in bioartificial dermis applications. The reinforced scaffold consisted of a lyophilized collagen sponge formed around a cross-linked collagen meshwork with an average thread diameter of approximately 55 μm. Fibroblasts were cultured in the reinforced collagen sponge for 7 days, during which time the pores in the sponge became filled with cells that secreted extracellular matrix (ECM) to form a bioartificial dermis. Results of ultimate tensile strength (UTS) measurements and compression tests indicated that the bioartificial dermis formed around the reinforced collagen sponge showed about ten times the strength of the bioartificial dermis formed around a typical collagen sponge (1.5 ± 0.05 vs. 0.15 ± 0.05 and 2.5 ± 0.1 vs. 0.2 ± 0.08 MPa, respectively). As a result, reinforced collagen mesh improved mechanical properties and this technique will be possible to make stronger scaffolds, not only for artificial skin applications but also various artificial tissues, such as synthetic cartilage, bone, and blood vessels.     

Superporous IPN hydrogels having enhanced mechanical properties

The objective of this study was to improve the mechanical properties of superporous hydrogels (SPHs), which were used to develop gastric retention devices for long-term oral drug delivery. The main approach used in this study was to form an interpenetrating polymer network by incorporating a second polymer network inside an SPH structure. Polyacrylonitrile was used as the second network inside an SPH. Mechanical properties including compression strength and elasticity were significantly improved, up to 50 times as compared with the control SPHs. The enhanced mechanical properties were a result of the scaffold-like fiber network structures formed inside the cell walls of SPHs. The fast swelling property of SPHs was not affected by the incorporation of the second polymer network because the interconnected pore structures were maintained. Gastric retention devices based on superporous IPN hydrogels (SPIHs) with the improved mechanical properties are expected to withstand compression pressure and mechanical frictions in the stomach better than the control SPHs.     

X-ray quantitative computed tomography: the relations to physical properties of proximal tibial trabecular bone specimens

Cylindrical bone specimens from the proximal epiphysis of ten normal human proximal tibiae were randomly assigned to a destructive axial compression test-series (N = 94) or to a protocol of standardized mechanical conditioning followed by non-destructive repeated testing to 0.6% strain and a final destructive test (N = 121). Specimen X-ray quantitative computed tomography (QCT) obtained at different scanning energies (100, 120 and 140 kVp) yielded closely related results (r = 1.00). Accordingly, predictions of physically measured densities or mechanical properties were not improved by using more than one scanning energy. QCT and physically measured densities were intimately related (QCT at 140 kVp to apparent density using linear regression: r = 0.94, and to apparent ash density: r = 0.95) and did not differ significantly in their ability to predict the mechanical properties, thus favouring the more easily implemented QCT for routine work. Evaluation of the relation of apparent density to Young's modulus and ultimate strength suggested that a power law regression model is preferable to a linear model, although linear model prediction of mechanical properties does not have significantly worse accuracy within the narrow density range investigated. The effect of conditioning on the behaviour of bone specimens subjected to destructive compression tests was to increase the stiffness and strength by approximately 50 and 20% respectively.     

Quality of chemically modified hemp fibers

Hemp fibers are very interesting natural material for textile and technical applications now. Applying hemp fibers to the apparel sector requires improved quality fibers. In this paper, hemp fibers were modified with sodium hydroxide solutions (5% and 18% w/v), at room and boiling temperature, for different periods of time, and both under tension and slack, in order to partially extract noncellulosic substances, and separate the fiber bundles. The quality of hemp fibers was characterised by determining their chemical composition, fineness, mechanical and sorption properties. The modified hemp fibers were finer, with lower content of lignin, increased flexibility, and in some cases tensile properties were improved. An original method for evaluation of tensile properties of hemp fibers was developed.     

The effect of constraint on the mechanical behaviour of trabecular bone specimens

The effect of the boundary conditions between trabecular bone specimens and the test columns of the testing machine was studied together with the effect of side-constraint on the mechanical behaviour of trabecular bone during axial compression. Cylindrical specimens taken from the upper tibial epiphysis of autopsy knees were tested non-destructively by cyclic compression to 0.8% strain under different conditions. Fixation of the specimens to the test columns by a thin layer of bone cement increased the stiffness by 40% and reduced the energy dissipation to 67% of those measured under unconstrained conditions (p less than 0.001). The thin cement layer alone increased the stiffness 19% and reduced energy dissipation to 86% (n.s.). When the machine was equipped with polished steel columns coated by a film of low-viscous oil, both the stiffness and the energy dissipation were reduced to 93% of those measured under standard conditions (p less than 0.005). Trabecular bone specimens tested side-constrained by the surrounding trabecular bone (in situ) showed a 19% larger stiffness than that measured during later testing of the corresponding machined specimens (p less than 0.005) whereas the energy dissipation was not altered significantly. The same specimens showed a 22% increase of stiffness and a 68% increase of energy dissipation when they were side-constrained by a closely fitting steel cylinder (p less than 0.005).     

Properties and potential applications of natural cellulose fibers from the bark of cotton stalks

Natural cellulose fibers have been obtained from the bark of cotton stalks and the fibers have been used to develop composites. Cotton stalks are rich in cellulose and account for up to 3 times the quantity of cotton fiber produced per acre. Currently, cotton stalks have limited use and are mostly burned on the ground. Natural cellulose fibers obtained from cotton stalks are composed of approximately 79% cellulose and 13.7% lignin. The fibers have breaking tenacity of 2.9 g per denier and breaking elongation of 3% and modulus of 144 g per denier, between that of cotton and linen. Polypropylene composites reinforced with cotton stalk fibers have flexural, tensile and impact resistance properties similar to jute fiber reinforced polypropylene composites. Utilizing cotton stalks as a source for natural cellulose fibers provides an opportunity to increase the income from cotton crops and make cotton crops more competitive to the biofuel crops.