Control of interfacial adhesion in continuous carbon and kevlar fiber reinforced polymer composites

Carbon fiber surfaces were treated by cold plasmas of oxygen, nitrogen, argon, ammonia, and propylene. A two-component bismaleimide, an epoxy, and a model thermoplastic resin polypropylene were used as the matrices for composites. The effectiveness of various plasmas in improving the interfacial adhesion between carbon fibers and matrix resins was demonstrated. Predominant adhesion promotion mechanisms as influenced by various plasma treatments were determined. Oxygen and argon plasmas were found to promote mechanical keying by increasing the level of fiber surface roughness and porosity. The wettability of carbon fiber surface by the matrix resin was also enhanced by oxygen plasmas and argon plasmas (to a lesser extent), as evidenced by the increased total surface energies and their polar components. These surface energy increases are mainly due to the various oxygen-containing functional groups observed on the oxygen plasma-treated surface. For the cases of ammonia and combined ammonia/argon plasma treatments, possible chemical bonding between bismaleimide and the plasma-deposited amine groups is one important promoter of interfacial bonding. In these cases increased wettability was also observed. Ammonia and ammonia/argon plasmas appear to be the more appropriate treatments for carbon-fiber/thermoset resin composites considering that they generally do not induce any appreciable reduction in fiber strength. In contrast, excessively prolonged exposure of carbon fibers to oxygen, nitrogen or argon plasma could lead to a significant reduction in fiber strength. The plasma-polymerized polypropylene deposited on the fiber surface was capable of improving the compatibility and adhesion between the fiber and the polypropylene matrix.     

Plasma surface modification of polylactic acid to promote interaction with fibroblasts

In this work, medium pressure plasma treatment of polylactic acid (PLA) is investigated. PLA is a biocompatible aliphatic polymer, which can be used for bone fixation devices and tissue engineering scaffolds. Due to inadequate surface properties, cell adhesion and proliferation are far less than optimal and a surface modification is required for most biomedical applications. By using a dielectric barrier discharge (DBD) operating at medium pressure in different atmospheres, the surface properties of a PLA foil are modified. After plasma treatment, water contact angle measurements showed an increased hydrophilic character of the foil surface. X-ray photoelectron spectroscopy (XPS) revealed an increased oxygen content. Cell culture tests showed that plasma modification of PLA films increased the initial cell attachment both quantitatively and qualitatively. After 1 day, cells on plasma-treated PLA showed a superior cell morphology in comparison with unmodified PLA samples. However, after 7 days of culture, no significant differences were observed between untreated and plasma-modified PLA samples. While plasma treatment improves the initial cell attachment, it does not seem to influence cell proliferation. It has also been observed that the difference between the 3 discharge gases is negligible when looking at the improved cell-material interactions. From economical point of view, plasma treatments in air are thus the best choice.     

Plasma treatment as a method for functionalising and improving dispersion of carbon nanotubes in epoxy resins

This study reports on the results of plasma-treated carbon nanotubes (CNTs) in the presence of oxygen and ammonia which can be scaled up for relatively large quantities of nanomaterials. The plasma treatment has been shown to change the surface chemistry and energy as well as the morphology of the carbon nanotubes. X-ray photoelectron spectroscopy analysis shows increases in oxygen and nitrogen groups on the oxygen- and ammonia-treated CNTs, respectively. Titration of the enhanced oxygen plasma-treated CNTs reveals an increased presence of carboxylic acid groups at 2.97 wt% whilst bulk density decreases from 151 kg/m³ for untreated carbon nanotubes to 76 kg/m³ after the enhanced oxygen treatment. The free surface energy has also been shown to increase from 33.70 up to 53.72 mJ/m² determined using a capillary rise technique. The plasma-treated carbon nanotubes have been mixed in epoxy and have shown an improvement in dispersion, which was quantitatively evaluated using an optical coherence tomography (OCT) technique shown to be suitable for nanocomposite characterisation. This research has demonstrated that it is possible to surface functionalise large quantities of carbon nanotubes in a single process, and that this process improves the dispersion of the carbon nanotubes in epoxy.     

Plasma surface modification of advanced organic fibres: Part V Effects on the mechanical properties of aramid/phenolic composites

Aramid fibres have been treated in ammonia and oxygen plasma to enhance adhesion to resole phenolic resins. The plasma treatments resulted in significant improvements in interlaminar shear strength (ILSS) and flexural strength of composites made from these materials. Composites containing aramid fibres with epoxide groups reacted on to the ammonia plasma-treated fibre surface also showed further improvements in ILSS and flexural strength. Scanning electron and optical microscopic observations were used to examine the microscopic basis for these results, which have been compared with those obtained previously for aramid/epoxy and aramid/vinyl ester composites. For composites containing oxygen and ammonia plasma-treated fibres, the enhanced ILSS and flexural strength are attributed to improved wetting of the surface-treated aramid fibres by the phenolic resin. However, for those containing fibres with reacted epoxide groups on the ammonia plasma-treated fibre surfaces, the enhanced composite properties may be due to covalent chemical interfacial bonding between the epoxide groups and the phenolic resin. Effects of catalyst levels and cure cycle on the ILSS of composites laminated with untreated fabric has also been examined and optimum values have been determined. The catalyst concentration has an influence on the phase-separated water domain density in the matrix which in turn, affects the available fibre/matrix bonding area and hence the composite ILSS and flexural strength. This revised version was published online in November 2006 with corrections to the Cover Date.     

Enhanced adhesion of osteoblastic cells on polystyrene films by independent control of surface topography and wettability

We independently controlled surface topography and wettability of polystyrene (PS) films by CF₄ and oxygen plasma treatments, respectively, to evaluate the adhesion and proliferation of human fetal osteoblastic (hFOB) cells on the films. Among the CF₄ plasma-treated PS films with the average surface roughness ranging from 0.9 to 70 nm, the highest adhesion of hFOB cells was observed on a PS film with roughness of ~ 11 nm. When this film was additionally treated by oxygen plasma to provide a hydrophilic surface with a contact angle less than 10°, the proliferation of bone-forming cell was further enhanced. Thus, the plasma-based independent modification of PS film into an optimum nanotexture for human osteoblast cells could be appplied to materials used in bone tissue engineering.     

Polyetheretherketone and titanium surface treatments to modify roughness and wettability-Improvement of bioactivity and antibacterial properties

Among the materials available for implant production,titanium is the most used while polyetherether-ketone (PEEK) is emerging thanks to its stability and to the mechanical properties similar to the ones of the bone tissue.Material surface properties like roughness and wettability play a paramount role in cell adhesion,cell proliferation,osteointegration and implant stability.Moreover,the bacterial adhesion to the biomaterial and the biofilm formation depend on surface smoothness and hydrophobicity.In this work,two different treatments,sandblasting and air plasma,were used to increase respectively roughness and wettability of two materials:titanium and PEEK.Their effects were analyzed with profilometry and contact angle measurements.The biological properties of the material surfaces were also investigated in terms of cell adhesion and proliferation of NIH-3T3 cells,MG63 cells and human Dental Pulp Stem Cells.Moreover,the ability of Staphylococcus aureus to adhere and form a viable biofilm on the samples was evaluated.The biological properties of both treatments and both materials were compared with samples of Synthegra(R) titanium,which underwent laser ablation to obtain a porous micropatterning,character-ized by a smooth surface to discourage bacterial adhesion.All cell types used were able to adhere and proliferate on samples of the tested materials.Cell adhesion was higher on sandblasted PEEK samples for both MG63 and NIH-3T3 cell lines,on the contrary,the highest proliferation rate was observed on sandblasted titanium and was only slightly dependent on wettability;hDPSCs were able to proliferate similarly on sandblasted samples of both tested materials.The highest osteoblast differentiation was ob-served on laser micropatterned titanium samples,but similar effects,even if limited,were also observed on both sandblasted materials and air plasma treated titanium.The lowest bacterial adhesion and biofilm formation was observed on micropatterned titanium samples whereas,the highest biofilm formation was detected on sandblasted PEEK samples,and in particular on samples not treated with air-plasma,which displayed the highest hydrophobicity.The results of this work showed that all the tested materials were able to sustain osteoblast adhesion and promote cell proliferation;moreover,this work highlights the fea-sible PEEK treatments which allow to obtain surface properties similar to those of titanium.The results here reported,clearly show that cell behavior depends on a complex combination of surface properties like wettability and roughness and material nature,and while a rough surface is optimal for cell adhesion,a smooth and less hydrophilic surface is the best choice to limit bacterial adhesion and biofilm formation.     

Influence of atmospheric pressure plasma treatment on various fibrous materials: Performance properties and surface adhesion analysis

Atmospheric pressure plasma treatment using oxygen gas was applied to wool fibrous materials. The plasma-treated fibrous materials were characterised using advanced instrumental techniques including scanning electron microscopy and X-ray photoelectron spectroscopy. They were also tested for performance properties including tensile and tearing strength as well as change in yellowness using international standard testing methods. Wettability analysis was conducted to study the surface area and surface energy of the plasma-treated fibrous materials. Surface modification regarding the enhancement of their adhesion to other substance, i.e. microcapsule treatment, was investigated.     

Influence of atmospheric pressure plasma treatment on various fibrous materials: Performance properties and surface adhesion analysis

Atmospheric pressure plasma treatment using oxygen gas was applied to wool fibrous materials. The plasma-treated fibrous materials were characterised using advanced instrumental techniques including scanning electron microscopy and X-ray photoelectron spectroscopy. They were also tested for performance properties including tensile and tearing strength as well as change in yellowness using international standard testing methods. Wettability analysis was conducted to study the surface area and surface energy of the plasma-treated fibrous materials. Surface modification regarding the enhancement of their adhesion to other substance, i.e. microcapsule treatment, was investigated.     

Plasma processing for surface optical modifications of PET films

The plasma interaction with the surface produces modifications of its chemical structure or morphology. Surface modifications through cold plasma occur, thanks to the high plasma reactivity and ability to affect the surface of materials.The present work shows the surface modification of polyethylene terephthalate (PET) films after the exposure both to low-pressure plasma (film deposition by plasma enhanced chemical vapour deposition) and to an atmospheric pressure dielectric barrier discharge (surface etching). After plasma treatment we have analysed the effect on the PET surface.For the atmospheric pressure plasma-treated samples, contact angle and atomic force microscope analysis enable us to determine roughness changes. For the low-pressure plasma samples, contact angles and Fourier transform infrared absorption spectroscopy analysis are used to estimate the chemical composition of the deposition and focused ion beam analysis to collect the image and calculate the thickness of plasma deposition.Both plasma treatments (film deposition and etching) cause changes in optical properties as indicated by reflectivity measurements.     

Nanophase hydroxyapatite and poly(lactide-co-glycolide) composites promote human mesenchymal stem cell adhesion and osteogenic differentiation in vitro

Human mesenchymal stem cells (hMSCs) typically range in size from 10 to 50 μm and proteins that mediate hMSC adhesion and differentiation usually have a size of a few nanometers. Nanomaterials with a feature size smaller than 100 nm have demonstrated the unique capability of promoting osteoblast (bone forming cell) adhesion and long-term functions, leading to more effective bone tissue regeneration. For new bone deposition, MSCs have to be recruited to the injury or disease sites and then differentiate into osteoblasts. Therefore, designing novel nanomaterials that are capable of attracting MSCs and directing their differentiation is of great interest to many clinical applications. This in vitro study investigated the effects of nanophase hydroxyapatite (nano-HA), nano-HA/poly(lactide-co-glycolide) (PLGA) composites and a bone morphogenetic protein (BMP-7) derived short peptide on osteogenic differentiation of hMSCs. The short peptide was loaded by physical adsorption to nano-HA or by dispersion in nanocomposites and in PLGA to determine their effects on hMSC adhesion and differentiation. The results showed that the nano-HA/PLGA composites promoted hMSC adhesion as compared to the PLGA controls. Moreover, nano-HA/PLGA composites promoted osteogenic differentiation of hMSCs to a similar extent with or without the presence of osteogenic factors in the media. In the MSC growth media without the osteogenic factors, the nanocomposites supported greater calcium-containing bone mineral deposition by hMSC than the BMP-derived short peptide alone. The nanocomposites provided promising alternatives in controlling the adhesion and differentiation of hMSCs without osteogenic factors from the culture media, and, thus, should be further studied for clinical translation and the development of novel nanocomposite-guided stem cell therapies.     

Reduced fibroblast adhesion and proliferation on plasma-modified titanium surfaces

Soft tissue complications are clinically relevant problems after osteosynthesis of fractures. The goal is to develop a method for reduction of fibroblast adhesion and proliferation on titanium implant surfaces by plasma polymerisation of the organo-silicon monomer hexamethyldisiloxane (HMDSO). HMDSO was deposited under continuous wave conditions in excess oxygen (ppHMDSO surface) and selected samples were further modified with an additional oxygen plasma (ppHMDSO + O₂ surface). Surface characterization was performed by scanning electron microscopy, profilometry, water contact angle measurements, infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy. In our experimental setup the mechanical properties, roughness and topography of the titanium were preserved, while surface chemistry was drastically changed. Fibroblast proliferation was assessed by alamarBlue assay, cell morphology by confocal microscopy visualization of eGFP-transducted fibroblasts, and cell viability by Annexine V/propidium iodide assay. Both modified surfaces, non-activated hydrophobic ppHMDSO and activated hydrophilic ppHMDSO + O₂ were able to dramatically reduce fibroblast colonization and proliferation compared to standard titanium. However, this effect was more strongly pronounced on the hydrophobic ppHMDSO surface, which caused reduced cell adhesion and prevented proliferation of fibroblasts. The results demonstrate that plasma modifications of titanium using HMDSO are valuable candidates for future developments in anti-adhesive and anti-proliferative coatings for titanium fracture implants.     

Fluidic Force Microscopy and Atomic Force Microscopy Unveil New Insights into the Interactions of Preosteoblasts with 3D-Printed Submicron Patterns

Physical patterns represent potential surface cues for promoting osteogenic differentiation of stem cells and improving osseointegration of orthopedic implants. Understanding the early cell–surface interactions and their effects on late cellular functions is essential for a rational design of such topographies, yet still elusive. In this work, fluidic force microscopy (FluidFM) and atomic force microscopy (AFM) combined with optical and electron microscopy are used to quantitatively investigate the interaction of preosteoblasts with 3D-printed patterns after 4 and 24 h of culture. The patterns consist of pillars with the same diameter (200 nm) and interspace (700 nm) but distinct heights (500 and 1000 nm) and osteogenic properties. FluidFM reveals a higher cell adhesion strength after 24 h of culture on the taller pillars (32 ± 7 kPa versus 21.5 ± 12.5 kPa). This is associated with attachment of cells partly on the sidewalls of these pillars, thus requiring larger normal forces for detachment. Furthermore, the higher resistance to shear forces observed for these cells indicates an enhanced anchorage and can be related to the persistence and stability of lamellipodia. The study explains the differential cell adhesion behavior induced by different pillar heights, enabling advancements in the rational design of osteogenic patterns.     

Modifications of rigid rod poly(1,4-phenylene-cis-benzobisoxazole) fibers by gas plasma treatments

The surface modifications of poly(1,4-phenylene-cis-benzobisoxazole) fibers by gas plasma treatments have been investigated in this paper. Oxygen, nitrogen, argon, and ammonia were regarded as inlet gas, respectively. After the plasma treatments, the effects on the morphology of the fiber surface, the tensile strength, the surface free energy, and the polymer crystallization behavior in the interface were carefully studied. The results showed that the total surface free energy of the fiber was increased by 35.3%, from 47.6 to 64.4 mJ/m², after the oxygen plasma treatment for 15 min. The increase has been 32.4% for N₂ plasma, 28.8% for argon plasma, and 28.6% for NH₃ plasma. The interfacial crystallization behavior of thermoplastic polyether ether ketone was also presented.     

Improving the NH(3) gas sensitivity of ZnO nanowire sensors by reducing the carrier concentration

We report a method to improve the sensitivity of a zinc oxide (ZnO) nanowire gas sensor towards ammonia (NH(3)) without the use of catalyst nanoparticles on the nanowire surface. This improvement is achieved by lowering the nominal carrier concentration in the as-grown ZnO nanowires. The carrier concentration in the as-grown ZnO nanowires can be tuned by treating these nanowires to either an oxidizing gas plasma or a reducing gas plasma, as observed from the measured current-voltage (I-V) characteristics response. We demonstrate that a ZnO nanowire sensor device that has been subjected to oxygen plasma treatment, thereby having a reduced carrier concentration, exhibits a sensitivity towards 0.75% NH(3) gas that is improved by approximately four times. The origin of this gas sensitivity improvement is discussed based on x-ray photoelectron spectroscopy analysis results of the plasma-treated ZnO nanowires.     

Multiple cracking response of plasma treated polyethylene fiber reinforced cementitious composites under flexural loading

The effects of low frequency cold plasma treatments on the microstructure and chemistry of Polyethylene (PE) have been investigated. PE plates and fibers were exposed to plasmas of argon and oxygen gases. The surface wettabilities of plasma-treated plates were monitored. Possible physical changes on fiber surfaces were observed by a scanning electron microscope (SEM) at micrometer scale and by an atomic force microscope (AFM) at nanometer scale after this process. The effects of plasma treatment on surface chemistry of PE fibers have been analyzed by using an X-ray photoemission spectroscope (XPS). The fibers modified by plasma treatments were used in prismatic cementitious composites. The flexural performance of samples were characterized at two different ages (28 days and 8 months). Results showed that plasma treatment caused significant modifications on fibers’ surface structure and composites’ performance. Proper plasma treatment conditions significantly leads to improvement of multiple cracking behavior of fiber reinforced composites.     

Enhanced osteogenic differentiation of cord blood-derived unrestricted somatic stem cells on electrospun nanofibers

A new stem cell-scaffold construct based on poly-l-lactide (PLLA) nanofibers grafted with collagen (PLLA-COL) and cord blood-derived unrestricted somatic stem cells (USSC) were proposed to hold promising characteristics for bone tissue engineering. Fabricated nanofibers were characterized using SEM, ATR-FTIR, tensile and contact angle measurements. The capacity of PLLA, plasma-treated PLLA (PLLA-pl) and PLLA-COL scaffolds to support proliferation and osteogenic differentiation of USSC was evaluated using MTT assay and common osteogenic markers such as alkaline phosphatase (ALP) activity, calcium mineral deposition and bone-related genes. All three scaffolds showed nanofibrous and porous structure with suitable physical characteristics. Higher proliferation and viability of USSC was observed on PLLA-COL nanofibers compared to control surfaces. In osteogenic medium, ALP activity and calcium deposition exhibited the highest values on PLLA-COL scaffolds on days 7 and 14. These markers were also greater on PLLA and PLLA-pl compared to TCPS. Higher levels of collagen I, osteonectin and bone morphogenetic protein-2 were detected on PLLA-COL compared to PLLA and PLLA-pl. Runx2 and osteocalcin were also expressed continuously on all scaffolds during induction. These observations suggested the enhanced proliferation and osteogenic differentiation of USSC on PLLA-COL nanofiber scaffolds and introduced a new combination of stem cell-scaffold constructs with desired characteristics for application in bone tissue engineering.     

In vitro and in vivo evaluation of bone morphogenetic protein-2 (BMP-2) immobilized collagen-coated polyetheretherketone (PEEK)

Polyetheretherketone (PEEK) is regarded as one of the most potential candidates of biomaterials in spinal implant applications. However, as a bioinert material, PEEK plays a limited role in osteoconduction and osseointegration. In this study, recombinant human bone morphogenetic protein-2 (rhBMP-2) was immobilized onto the surface of collagen-coated PEEK in order to prepare a multi-functional material. After adsorbed onto the PEEK surface by hydrophobic interaction, collagen was cross-linked with N-(3-dimethylaminopropyl)-N'-ethyl carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). EDC/NHS system also contributed to the immobilization of rhBMP-2. Water contact angle tests, XPS and SEM clearly demonstrated the surface changes. ELISA tests quantified the amount of rhBMP-2 immobilized and the release over a period of 30 d. In vitro evaluation proved that the osteogenesis differentiation rate was higher when cells were cultured on modified PEEK discs than on regular ones. In vivo tests were conducted and positive changes of major parameters were presented. This report demonstrates that the rhBMP-2 immobilized method for PEEK modification increase bioactivity in vitro and in vivo, suggesting its practicability in orthopedic and spinal clinical applications.     

Effect of oxygen plasma-treated carbon fibers on the tribological behavior of oil-absorbed carbon/epoxy woven composites

In this study, woven type carbon fibers were plasma-treated using oxygen gas, and the effect of the plasma treatment on tribological behavior of oil-absorbed carbon/epoxy woven composites was investigated. Chemical changes on the surface of the woven carbon fibers due to oxygen plasma treatment were determined by XPS analysis. Ball-on-disk wear tests were performed on untreated and plasma-treated carbon/epoxy woven composites that were fully oil absorbed. It was found that carbonyl functional groups were created on the carbon fibers due to oxygen-plasma treatment. In addition, the friction coefficient and wear rate of the plasma-treated carbon/epoxy composites were lower than that of untreated carbon/epoxy composites. SEM examination of the worn surface showed that the improved wear properties of the plasma-treated carbon/epoxy composites were attributed to enhanced adhesive strength, caused by the carbonyl functional groups between the carbon fibers and epoxy.     

Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

Electrophoretic deposition of polyetheretherketone (PEEK) and PEEK/Bioglass® coatings on NiTi shape memory alloy wires

Polyetheretherketone (PEEK) and PEEK/Bioglass® coatings were produced on shape memory alloy (NiTi, Nitinol®) wires using electrophoretic deposition (EPD). Best results were achieved with suspensions of PEEK powders in ethanol in the range (1–6 wt%), using a deposition time of 5 minutes and applied voltage of 20 Volts. EPD using these parameters led to high quality PEEK coatings with a homogeneous microstructure along the wire length and a uniform thickness of up to 15 μm without development of cracks or the presence of large voids. Suspensions of PEEK powders in ethanol with addition of Bioglass® particles (0.5–2 wt%) (size < 5 μm) were used to produce PEEK/Bioglass® coatings. Sintering was carried out as a post EPD process in order to densify the coatings and to improve the adhesion of the coatings to the substrate. The sintering temperature was 340 °C, sintering time 20 min and heating rate 300 °C/h. Sintering led to uniform and dense PEEK and PEEK/Bioglass® coatings without any cracks. The bioactive behaviour of PEEK/Bioglass® composite coatings was investigated by immersion in acellular simulated body fluid (SBF) for up to two weeks. As expected, hydroxyapatite crystals formed on the surface of the coated wires after 1 week in SBF, confirming the bioactive character of the coatings. The results have demonstrated for the first time that EPD is a very convenient method to obtain homogeneous and uniform bioactive PEEK and PEEK/Bioglass® coatings on Nitinol® wires for biomedical applications.