Surface energy of silk fibroin and mechanical properties of silk cocoon composites

Silkworm cocoons are biological composite structures protecting the silkworms against environmental damage and physical attack by natural predators. In particular, some outdoor reared silk cocoons exhibit outstanding mechanical properties that are relevant to the higher level protection required to enhance the survival chance of silkworms while supporting their metabolic activity. The performance of composite materials strongly depends on the adhesion between the fiber reinforcement and matrix, with the surface properties of the fibers playing a key role in determining the level of adhesion achieved. For this reason it is important to study the surface properties of silk fibroin to further understand the composite properties of the cocoons. In this work, both the mechanical properties of the silk cocoons and silk fibroin were studied. The surface topography was examined using scanning probe microscopy (SPM), which revealed distinct longitudinal ridges and striations along the fiber axis of the four silk fiber types. The fibers were found to exhibit heterogeneity in surface energy as evidenced from inverse gas chromatography (IGC) measurements. The combination of excellent mechanical properties and the more energetically heterogeneous surface nature of the wild A. pernyi silk fibroin fibers correlates well with the excellent mechanical properties of the A. pernyi cocoons. This journal is

Effect of surface functionality of PAN-based carbon fibres on the mechanical performance of carbon/epoxy composites

The performance of composite laminates depends on the adhesion between the fibre reinforcement and matrix, with the surface properties of the fibres playing a key role in determining the level of adhesion achieved. For this reason it is important to develop an in-depth understanding of the surface functionalities on the reinforcement fibres. In this work, multi-scale surface analysis of carbon fibre during the three stages of manufacture; carbonisation, electrolytic oxidation, and epoxy sizing was carried out. The surface topography was examined using scanning electron microscopy (SEM), which revealed longitudinal ridges and striations along the fibre-axis for all fibre types. A small difference in surface roughness was observed by scanning probe microscopy (SPM), while the coefficient of friction measured by an automated single fibre tester showed 51% and 98% increase for the oxidised and sized fibres, respectively. The fibres were found to exhibit heterogeneity in surface energy as evidenced from SPM force measurements. The unsized fibres were much more energetically heterogeneous than the sized fibre. A good correlation was found between fibre properties (both physical and chemical) and interlaminar shear strength (ILSS) of composites made from all three fibre types. © 2014 Elsevier Ltd.

Self-assembled monolayers into the 21st century : recent advances and applications

The modification of an interface on a molecular level with more than one molecular ‘building block' is essentially an example of the ‘bottom–up' fabrication principle of nanotechnology. The fabrication of such integrated molecular systems in electrochemistry has seen rapid progress in recent years via the development of sensing interfaces fabricated using self-assembled monolayers (SAMs). This review outlines recent advances and applications of self-assembled monolayers for modifying electrodes with an emphasis on the development of integrated molecular systems. First, some basic issues regarding fabricating integrated molecular systems, such as the role of the surface topography of the electrode and patterning surfaces, are discussed. Subsequently an overview of recent developments in pH, inorganic and bio sensing involving the use of SAMs is given. Finally emerging trends in using molecular building blocks in the fabrication of integrated molecular systems, such as nanotubes, dendrimers and nanoparticles, are reviewed.

Parameters important in fabricating enzyme electrodes using self-assembled monolayers of alkanethiols

The fabrication of enzyme electrodes using self-assembled monolayers (SAMs) has attracted considerable interest because of the spatial control over the enzyme immobilization. A model system of glucose oxidase covalently bound to a gold electrode modified with a SAM of 3-mercaptopropionic acid was investigated with regard to the effect of fabrication variables such as the surface topography of the underlying gold electrode, the conditions during covalent attachment of the enzyme and the buffer used. The resultant monolayer enzyme electrodes have excellent sensitivity and dynamic range which can easily be adjusted by controlling the amount of enzyme immobilized. The major drawback of such electrodes is the response which is limited by the kinetics of the enzyme rather than mass transport of substrates. Approaches to bringing such enzyme electrodes into the mass transport limiting regime by exploiting direct electron transfer between the enzyme and the electrode are outlined.

Spatial reasoning by active observation

The problem of deriving spatial relationships between objects in general requires high lever' abstract representation, and it would pose difficulties even for human observer. Based on a formalism for spatial layouts proposed earlier, we present methods for deducing spatial relations between objects by an active, sighted agent in a large-scale environment. The deduction of spatial relations is based on simple visual clues, and thus this technique is more feasible than schemes that rely on complex object recognition.

Deduction of high level spatial relations

The problem of deriving spatial relationships between objects in general requires high level abstract representation, and it would pose difficulties even for human observer. Based on a formalism for spatial layouts proposed earlier [KiV92, VeK921, we present methods for deducing high level spatial relations between objects by an active, sighted agent in a large-scale environment. The deduction of spatial relations is based on simple visual clues, and thus this technique is more feasible than schemes that rely on complex object recognition.

A novel approach to functionalise pristine unsized carbon fibre using in situ generated diazonium species to enhance interfacial shear strength

Complex molecules have been successfully grafted onto the surface of unsized carbon fibre, a heterogeneous material which is a challenge to functionalise. The in situ generation of highly reactive phenyldiazo-species from their corresponding anilines was employed to achieve this task. The success of an initial proof-of-concept study (bearing a nitro moiety) supported by X-ray Photoelectron Spectroscopy (XPS) and physical characterisation, led to the design and synthesis of a more complex compound possessing a pendant amine moiety which could theoretically react with an epoxide based resin. After attachment to unsized oxidised fibres, analysis by XPS of the resulting fibres (fluorine used as an XPS tag) indicated a marked difference in functionalisation success which was attributed to steric factors, shown to be critical in influencing the attachment of the phenyldiazo-intermediate to the carbon fibre surface. Analysis of key fibre performance parameters of these fibres showed no change in elastic modulus, strength, surface topography or microscopic roughness when compared to the control unsized oxidised fibres. The functionalised fibres did however show a large increase in coefficient of friction. Single fibre fragmentation tests indicated a marked increase in interfacial shear strength, which was attributed to the pendent amine functionalities interacting with the epoxy resin.

Bacterial attachment on sub-nanometrically smooth titanium substrata

Despite the volume of work that has been conducted on the topic, the role of surface topography in mediating bacterial cell adhesion is not well understood. The primary reason for this lack of understanding is the relatively limited extent of topographical characterisation employed in many studies. In the present study, the topographies of three sub-nanometrically smooth titanium (Ti) surfaces were comprehensively characterised, using nine individual parameters that together describe the height, shape and distribution of their surface features. This topographical analysis was then correlated with the adhesion behaviour of the pathogenic bacteria Staphylococcus aureus and Pseudomonas aeruginosa, in an effort to understand the role played by each aspect of surface architecture in influencing bacterial attachment. While P. aeruginosa was largely unable to adhere to any of the three sub-nanometrically smooth Ti surfaces, the extent of S. aureus cell attachment was found to be greater on surfaces with higher average, RMS and maximum roughness and higher surface areas. The cells also attached in greater numbers to surfaces that had shorter autocorrelation lengths and skewness values that approached zero, indicating a preference for less ordered surfaces with peak heights and valley depths evenly distributed around the mean plane. Across the sub-nanometrically smooth range of surfaces tested, it was shown that S. aureus more easily attached to surfaces with larger features that were evenly distributed between peaks and valleys, with higher levels of randomness. This study demonstrated that the traditionally employed amplitudinal roughness parameters are not the only determinants of bacterial adhesion, and that spatial parameters can also be used to predict the extent of attachment.

Accelerated stem cell attachment to ultrafine grained titanium

Commercial purity titanium with an average grain size in the low sub-micron range was produced by equal channel angular pressing (ECAP). Attachment of human bone marrow-derived mesenchymal stem cells (hMSCs) to the surface of conventional coarse grained and ECAP-modified titanium was studied. It was demonstrated that the attachment and spreading of hMSCs in the initial stages (up to 24h) of culture was enhanced by grain refinement. Surface characterization by a range of techniques showed that the main factor responsible for the observed acceleration of hMSC attachment and spreading on titanium due to grain refinement in the bulk is the attendant changes in surface topography on the nanoscale. These results indicate that, in addition to its superior mechanical properties, ECAP-modified titanium possesses improved biocompatibility, which makes it to a potent candidate for applications in medical implants.

Effect of ultrafine-grained titanium surfaces on adhesion of bacteria

The influence of the ultrafine crystallinity of commercial purity grade 2 (as-received) titanium and titanium modified by equal channel angular pressing (modified titanium) on bacterial attachment was studied. A topographic profile analysis of the surface of the modified titanium revealed a complex morphology of the surface. Its prominent micro- and nano-scale features were 100-200-nm-scale undulations with 10-15 microm spacing. The undulating surfaces were nano-smooth, with height variations not exceeding 5-10 nm. These surface topography characteristics were distinctly different from those of the as-received samples, where broad valleys (up to 40-60 microm) were detected, whose inner surfaces exhibited asperities approximately 100 nm in height spaced at 1-2 microm. It was found that each of the three bacteria strains used in this study as adsorbates, viz. Staphylococcus aureus CIP 68.5, Pseudomonas aeruginosa ATCC 9025 and Escherichia coli K12, responded differently to the two types of titanium surfaces. Extreme grain refinement by ECAP resulted in substantially increased numbers of cells attached to the surface compared to as-received titanium. This enhanced degree of attachment was accompanied with an increased level of extracellular polymeric substances (EPS) production by the bacteria.

The influence of titania-zirconia-zirconium titanate nanotube characteristics on osteoblast cell adhesion

Studies of biomaterial surfaces and their influence on cell behavior provide insights concerning the design of surface physicochemical and topography properties of implant materials. Fabrication of biocompatible metal oxide nanotubes on metallic biomaterials, especially titanium alloys such as Ti50Zr via anodization, alters the surface chemistry as well as surface topography of the alloy. In this study, four groups of TiO2-ZrO2-ZrTiO4 nanotubes that exhibit diverse nanoscale dimensional characteristics (i.e. inner diameter Di, outer diameter Do and wall thicknesses Wt) were fabricated via anodization. The nanotubes were annealed and characterized using scanning electron microscopy and 3-D profilometry. The potential applied during anodization influenced the oxidation rate of titanium and zirconium, thereby resulting in different nanoscale characteristics for the nanotubes. The different oxidation and dissolution rates both led to changes in the surface roughness parameters. The in vitro cell response to the nanotubes with different nanoscale dimensional characteristics was assessed using osteoblast cells (SaOS2). The results of the MTS assay indicated that the nanotubes with inner diameter (Di)≈40nm exhibited the highest percentage of cell adhesion of 41.0%. This result can be compared to (i) 25.9% cell adhesion at Di≈59nm, (ii) 33.1% at Di≈64nm, and (iii) 33.5% at Di≈82nm. The nanotubes with Di≈59nm exhibited the greatest roughness parameter of Sa (mean roughness), leading to the lowest ability to interlock with SaOS2 cells.

Superhydrophobic polyimide films with a hierarchical topography : combined replica molding and layer-by-layer assembly

Artificial superhydrophobic surfaces with a hierarchical topography were fabricated by using layer-by-layer assembly of polyelectrolytes and silica nanoparticles on microsphere-patterned polyimide precursor substrates followed with thermal and fluoroalkylsilane treatment. In this special hierarchical topography, micrometer-scale structures were provided by replica molding of polyamic acid using two-dimensional arrays of polystyrene latex spheres as templates, and nanosized silica particles were then assembled on these microspheres to construct finer structures at the nanoscale. Heat treatment was conducted to induce chemical cross-linking between polyelectrolytes and simultaneously convert polyamic acid to polyimide. After surface modification with fluoroalkylsilane, the as-prepared highly hydrophilic surface was endowed with superhydrophobicity due to the bioinspired combination of low surface energy materials and hierarchical surface structures. A superhydrophobic surface with a static water contact angle of 160 degrees and sliding angle of less than 10 degrees was obtained. Notably, the polyimide microspheres were integrated with the substrate and were mechanically stable. In addition, the chemical and mechanical stability of the polyelectrolyte/silica nanoparticle multilayers could be increased by heat-induced cross-linking between polyelectrolytes to form nylon-like films, as well as the formation of interfacial chemical bonds.

Clinical performance of an innovative back surface multifocal contact lens in correcting presbyopia

Purpose: This prospective study was designed to subjectively and objectively evaluate the performance of an aspheric multifocal back surface rigid gas permeable (RGP) contact lens. The multifocal element of this lens design consisted of an aspheric optical zone that varied according to the patient's ametropia, corneal topography, and required reading addition.

Methods: We fit 28 presbyopic subjects with an aspheric multifocal back surface RGP contact lens (age range: 45 to 68 years). Reading additions ranged from +0.75 D to +2.50 D. Subjects were assessed initially and at 2, 6, and 12 weeks for ocular changes, visual performance, and subjective responses.

Results: We required 116 RGP lenses to achieve an acceptable fit and visual acuity in 28 subjects (55 eyes). At the final visit, the distance logMAR acuity with the multifocal contact lens (+0.12 +/-0.10) was not statistically different (t=-0.623, P= 0.5388) from spectacle acuity at the initial visit (+0.10 +/-0.12). The near logMAR acuity with the multifocal contact lens at the final visit (0.36 +/- 0.12) was not statistically different from that for near acuity with spectacles at the initial visit (0.33 +/- 0.13). No slit lamp signs worsened during the study. A reduction in myopia of 0.67 D was noted by the final visit. Spectacle blur was noted if the acuity at the initial refraction was compared to the acuity with the same refraction at the final visit (t= -3.287, P= 0.0028) but not when the refractive changes were incorporated (t= 1.058, P= 0.3127). All subjects rated the performance of the lenses very highly: comfort, 86%; distance acuity, 83%; near acuity, 73%; and stability of vision, 74%. Twenty-four subjects (86%) chose the multifocal contact lens as their preference.

Conclusion: We demonstrated that a multifocal design is able to provide acceptable distance and near correction for presbyopic patients. The aspheric geometry required can be optimized for a given patient by considering his/her degree of ametropia, as well as the corneal topography.

Cell biological responses of osteoblasts on anodized nanotubular surface of a titanium-zirconium alloy

Anodization of titanium and its alloys, under controlled conditions, generates a nanotubular architecture on the material surface. The biological consequences of such changes are poorly understood, and therefore, we have analyzed the cellular and molecular responses of osteoblasts that were plated on nanotubular anodized surface of a titanium-zirconium (TiZr) alloy. Upon comparing these results with those obtained on acid etched and polished surfaces of the same alloy, we observed a significant increase in adhesion and proliferation of cells on anodized surfaces as compared to acid etched or polished surface. The expression of genes related to cell adhesion was high only on anodized TiZr, but that of genes related to osteoblast differentiation and osteocalcin protein and extracellular matrix secretion were higher on both anodized and acid etched surfaces. Examination of surface morphology, topography, roughness, surface area and wettability using scanning electron microscopy, atomic force microscopy, and contact angle goniometry, showed that higher surface area, hydrophilicity, and nanoscale roughness of nanotubular TiZr surfaces, which were generated specifically by the anodization process, could strongly enhance the adhesion and proliferation of osteoblasts. We propose that biological properties of known bioactive titanium alloys can be further enhanced by generating nanotubular surfaces using anodization.