C Liu

Gradient collagen/nanohydroxyapatite composite scaffold: Development and characterization

This paper reports an in situ diffusion method for the fabrication of compositionally graded collagen/nanohydroxyapatite (HA) composite scaffold. The method is diffusion based and causes the precipitation of nano-HA crystallites in situ. A collagen matrix acts as a template through which calcium ions (Ca(2+)) and phosphate ions (PO4(3-)) diffuse and precipitate a non-stoichiometric HA. It was observed that needle-like prismatic nano-HA crystallites (about 2 x 2 x 20 nm) precipitated in the interior of the collagen template onto the collagen fibrils. Chemical and microstructural analysis revealed a gradient of the Ca to P ratio across the width of the scaffold template, resulting in the formation of a Ca-rich side and a Ca-depleted side of scaffold. The Ca-rich side featured low porosity and agglomerates of the nano-HA crystallites, while the Ca-depleted side featured higher porosity and nano-HA crystallites integrated with collagen fibrils to form a porous network structure.

Abrasive wear behavior of particle reinforced ultrahigh molecular weight polyethylene composites

This paper, based on orthogonal experimental design method, reports the results of abrasive wear investigations of various composites of ultrahigh molecular weight polyethylene (UHMWPE) reinforced with quartz powder rubbed against abrasive papers under dry conditions. The main purpose was to study the influence of such parameters such as filler particle size, load, sliding speed and abrading particle size on the abrasive wear performance of UHMWPE matrix composites. Statistical analysis was carried out to develop an equation in which the wear volume of the specimen was expressed in terms of load, abrading particle size, sliding speed, and their interactions. It was observed that wear rate was lower for reinforced composites than for the unfilled polymer. Load is the most important factor in the wear of unfilled UHMWPE specimens. However, for the wear of filler reinforced UHMWPE composites, the role of the load abates and the role of abrasive particle size increases with the increase in filler particle size. The main controlling parameter shifted from load to abrasive particle size when the filler size shifted from 0.18 to 0.35 mm in composites. Sliding speed seems to have little effect on the total wear volume. Worn surfaces were studied using a scanning electron microscope (SEM) to give an insight into the wear mechanisms. The results show that the hardness and plowing resistance increased with the addition of hard powder, which leads to an enhancement of abrasive resistance.

Development of biodegradable scaffolds for tissue engineering : a perspective on emerging technology

Abstract The scaffold, a three-dimensional (3D) substrate that serves as a template for tissue regeneration, plays an essential role in tissue engineering. The ideal scaffold should have surface chemistry and microstructure tailored to facilitate cellular attachment, proliferation and differentiation; adequate mechanical strength for handling; and an appropriate biodegradation rate without undesirable byproducts. Research on biopolymer formulation and scaffold fabrication has been intense over the past 10 years. A perspective is provided of important issues related to scaffold development from biodegradable polymers. The mechanical properties and biocompatibility (including biodegradability and bioresorptability) of commonly used biopolymers are reviewed. Scaffold design and fabrication techniques are assessed and compared. Scaffold architecture, including pore size and size distribution, and its effects on cell growth are discussed. The importance of structural hierarchy over a range of length scales is highlighted. Unfortunately, conventional processing techniques cannot yet control both scaffold architecture and surface chemistry. An emerging scaffold fabricating technique using solid free form fabrication (SFF), although currently restricted to relatively symmetrical structures, has been shown to be highly effective in integrating structural architecture with changes in surface chemistry of the scaffolds, and integration of growth factors. Several examples of the application of SFF are presented.

Statistical wear analysis of PA-6/UHMWPE alloy, UHMWPE and PA-6

This paper, based on an orthogonal experimental design and analysis method, reports the wear of a blend of polyamide-6/ultrahigh molecular weight polyethylene (PA-6/UHMWPE), using a pin-on-disc test, and rubbing against a stainless steel counterface. The wear behavior of PA-6 and UHMWPE was also investigated for the purpose of comparison. The main purpose was to study the influence of the parameters: sliding distance, contact pressure and sliding speed on the wear performance of the investigated materials. Statistical analysis was carried out to develop an equation in which the wear volume of the specimen was expressed in terms of sliding distance, contact pressure and sliding speed. It was observed that the wear rate was lower for PA-6 than the other two materials. UHMWPE exhibited the lowest wear resistance. Contact pressure was found to be the most important factor in the wear of materials, followed by sliding distance and sliding speed. Sliding distance had the highest relative effect on the wear of PA-6/UHMWPE. Sliding speed seemed to have the least effect on the wear volume of the investigated materials. The results show that the wear behavior of these materials, and the effects of factors on the wear, depend on the physical and mechanical properties of the materials.

Effects of operating parameters on the lubricated wear behavior of a PA-6/UHMWPE blend: a statistical analysis

In this paper, based on an orthogonal test design and analysis method, the lubricated wear performance of a ultra-high molecular weight polyethylene and polyamide (PA-6/UHMWPE) alloys were studied using a pin-on-disc method. The effects of several parameters on the wear of the PA-6/UHMWPE alloy, rubbing against a stainless steel counterface, are reported. The main purpose was to study the influence of parameters such as sliding distance, counterface surface roughness, load and sliding speed, as well as their interactions on the wear performance. Statistical analysis was carried out to develop an equation, in which the wear volume of the polymeric specimen was expressed in terms of the investigated parameters. It was observed that the pressure and surface roughness are the two important and controlling factors; sliding distance and sliding speed have a minor effects on the wear of the specimens. Although the two-factor and three-factor interactions have little effect, the four-factor interaction has a strong effect on the wear of specimens. The results give a comprehensive insight into the wear of the PA-6/UHMWPE alloy.

Some failure modes of four clinical bone cements

Abstract The fracture or failure behaviours of four commercial acrylic-based bone cements have been examined in tensile, bending and compression modes, and their mechanical properties are reviewed. It was found that Palacos R-40 bone cement had high radiopaque agent concentration, with high surface hardness. It exhibited a much lower bending strength and bending modulus compared with the other three bone cements (CMW1, CMW2000 and Simplex P). The textures of tensile fracture surfaces produced were similar for the four bone cements studied. The fracture surface was fragmented by crevices, which developed through the matrix and around large undissolved polyme-thylmethacrylate (PMMA) beads. Three bands with different features existed on the bending fracture surfaces, with an abrupt transition between them. It appears that the agglomerates of zirconium dioxide particles are implicated in Palacos R-40 bone cement fracture surface. The examination of compressive failed specimens revealed that a ‘yielded crack band’ existed across the transverse section. Plastic deformation resulted in the PMMA beads being squashed in the longitudinal direction and dilated in the transverse direction.

Biomimetic Collagen/Hydroxyapatite Composite Scaffolds: Fabrication and Characterizations

Biomimetic collagen/hydroxyapatite scaffolds have been prepared by microwave assisted co-titration of phosphorous acid-containing collagen solution and calcium hydroxide-containing solution. The resultant scaffolds have been characterised with respect to their mechanical properties, composition and microstructures. It was observed that the in situ precipitation process could combine collagen fibril formation and hydroxyapatite (HAp) formation in one process step. Collagen fibrils guided hydroxyapatite precipitation to form bone-mimic collagen/hydroxyapatite composite containing both intrafibrillar and interfibrillar hydroxyapatites. The mineral phase was determined as low crystalline calcium-deficient hydroxyapatite with calcium to phosphorus ratio (Ca/P) of 1.4. The obtained 1% (collagen/HAp = 75/25) scaffold has a porosity of 72% and a mean pore size of 69.4 μm. The incorporation of hydroxyapatite into collagen matrix improved the mechanical modulus of the scaffold significantly. This could be attributed to hydroxyapatite crystallites in collagen fibrils which restricted the deformation of the collagen fibril network, and the load transfer of the collagen to the higher modulus mineral component of the composite.

Effect of Air Plasma Processing on the Adsorption Behaviour of Bovine Serum Albumin on Spin-Coated PMMA Surfaces

This paper reports the adsorption of Bovine Serum Albumin (BSA) onto Dielectric Barrier Discharge (DBD) processed Poly(methyl methacrylate) (PMMA) surfaces by a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) technique. The purpose is to study the influence of DBD processing on the nature and scale of BSA adsorption on PMMA surface in vitro. It was observed that DBD processing improves the surface wettability of PMMA film, a fact attributable to the changes in surface chemistry and topography. Exposure of the PMMA to Phosphate Buffed Saline (PBS) solution in the QCM-D system resulted in surface adsorption which reaches an equilibrium after about 30 minutes for pristine PMMA, and 90 minutes for processed PMMA surface. Subsequent injection of BSA in PBS indicated that the protein is immediately adsorbed onto the PMMA surface. It was revealed that adsorption behaviour of BSA on pristine PMMA differs from that on processed PMMA surface. A slower adsorption kinetics was observed for pristine PMMA surface, whilst a quick adsorption kinetics for processed PMMA. Moreover, the dissipation shift of protein adsorption suggested that BSA forms a more rigid structure on pristine PMMA surface that on processed surface. These data suggest that changes in wettability and attendant chemical properties and surface texture of the PMMA surface may play a significant role in BSA adsorption process.

Co-deposition of titanium/polytetrafluoroethylene films by unbalanced magnetron sputtering

Abstract Graded nanocomposite coatings, consisting of a polytetrafluoroethylene (PTFE)-rich surface layer and functionally graded titanium–titanium carbide–PTFE mixed sublayer, were deposited onto stainless steel substrates using a radio frequence (RF) unbalanced magnetron sputter system with the purpose of improving the tribological performance of the substrate and endowing the surface with hydrophobicity. The results show that decomposition of PTFE during RF plasma sputtering results mainly in the evolution of fluoropolymer species. High incident power results in low F/C ratio in the resulting films, and low incident power results in high F/C ratio films. The tribological performance of the coatings depends on the fluoropolymer content in the film bulk, as well as on the film growth process. The films with high fluoropolymer content, with a gradient multilayer structure, possess good tribological performance especially at the initial stage of testing. During the co-deposition process, the segments were inlaid into the titanium matrix and became strongly mechanically bonded. It is speculated that some of the carbon atoms may react with titanium to form titanium carbide in the coatings. Multilayer structure attributed to the decrease in the stress developed between layers. All these factors attributed to the improvement of the tribological performance of the stainless steel substrate.

Creep behavior comparison of CMW1 and palacos R-40 clinical bone cements

The restrained dynamic creep behaviors of two clinical bone cements, Palacos R-40 and CMW1 have been investigated at room temperature and body temperature. It was found that the two cements demonstrated significantly different creep deformations, with Palacos R-40 bone cement demonstrating higher creep strain than CMW1 bone cement at each loading cycle. For both cements, two stages of creep were identified with a higher creep rate during early cycling followed by a steady-state creep rate. The test temperature had a strong effect on the creep performance of the bone cements with higher creep rate observed at body temperature. The relationship between creep deformation and loading cycles can be expressed by single logarithmic model. The SEM examinations revealed that CMW1 bone cement is more sensitive to defects within the specimen especially to the defects at the edges of the specimen than Palacos R-40 bone cement. However, in the absence of micro-cracks or defects within the inner surface layer, the dynamic loading (at less than 10.6 MPa) is unlikely to produce micro-cracks in the CMW1 bone cement. The different behaviors between the two bone cements may be attributed to differences in chemical compositions and molecular weight distributions.