Andrew Michelmore

Tailoring the surface functionalities of titania nanotube arrays

Nanotubular titanium oxide (TiO(2)) produced by self-ordering processes using electrochemical anodization have been extensively explored in recent years as a new biomaterial for implants, drug delivery systems, cell growth, biosensors, immunoisolations, bioartificial organs and tissue engineering. Chemical inertness is the main weakness of this material when placed in contact with biological systems and surface modification is a possible solution of this problem. The aim of this study is to develop a flexible and facile method for surface modification of TiO(2) nanotubes to tailor new interfacial properties important in many biomedical applications. TiO(2) nanotubes were prepared by electrochemical anodization of titanium foil using ethylene glycol: NH(4)F electrolyte (2% water and 0.3% NH(4)F). Plasma surface modification using allylamine (AA) as a precursor has been applied to generate a thin and chemically reactive polymer (AAPP) film rich in amine groups on top of the TiO(2) nanotube surface. This initial polymer film was used for further surface functionalization by attachment of desired molecules. Two modification techniques were used to demonstrate the flexibility for building of new functionalities on titania nanotube surface: electrostatic adsorption of poly(sodium styrenesulfonate) (PSS) as an example of layer-by-layer assembly (LbL), and covalent coupling of poly(ethylene glycol) (PEG) as an example of creating a protein-resistant surface. These approaches for tailoring the surface chemistry and wettability of TiO(2) nanotubes offer considerable prospects for advancing their interfacial properties to improve existing and develop new functional biomaterials for diverse biomedical applications.

Covalently bonded interfaces for polymer/graphene composites

The interface is well known for taking a critical role in the determination of the functional and mechanical properties of polymer composites. Previous interface research has focused on utilising reduced graphene oxide that is limited by a low structural integrity, which means a high fraction is needed to produce electrically conductive composites. By using 4,4′-diaminophenylsulfone, we in this study chemically modified high-structural integrity graphene platelets (GnPs) of 2–4 nm in thickness, covalently bonded GnPs with an epoxy matrix, and investigated the morphology and functional and mechanical performance of these composites. This covalently bonded interface prevented GnPs stacking in the matrix. In comparison with unmodified composites showing no reduction in electrical volume resistivity, the interface-modified composite at 0.489 vol% GnPs demonstrates an eight-order reduction in the resistivity, a 47.7% further improvement in modulus and 84.6% in fracture energy release rate. Comparison of GnPs with clay and multi-walled carbon nanotubes shows that our GnPs are more advantageous in terms of performance and cost. This study provides a novel method for developing interface-tuned polymer/graphene composites.

Substrate influence on the initial growth phase of plasma-deposited polymer films

In this communication we demonstrate that in the initial stages of deposition of ultrathin plasma polymer films, both the growth rate and the chemical composition of the films are affected by the nature of the substrate which is an important question surprisingly neglected until now.

The interaction of linear polyphosphates with titanium dioxide surfaces

The interaction of linear polyphosphates of the general formula [PnO3n+1](n+2)− with the surface of titanium dioxide and silica has been studied as a function of pH. Both equilibrium and kinetic aspects of the interaction were addressed, using in situ infrared spectroscopy, electrical double layer, ion chromatography, rheological and adsorption investigations. Orthophosphate is adsorbed on to the titania surface over a wide pH range as two kinds of species, a bridging bidentate surface complex and an electrostatically adsorbed orthophosphate ion. The linear polyphosphates behave in a similar fashion, with adsorption driven by combined electrostatic and chemical contributions. There is no detectable interaction between the linear polyphosphates and the surface of silica at any pH investigated. Linear polyphosphates with larger n displace lower molar mass species from the titanium dioxide surface.

The influence of substrate stiffness gradients on primary human dermal fibroblasts.

Materials mechanical properties are known to be an important regulator of cellular processes such as proliferation, differentiation and migration, and have seen increasing attention in recent years. At present, there are only few approaches where the mechanical properties of thin films can be controllably varied across an entire surface. In this work, we present a technique for controlled generation of gradients of surface elastic moduli involving a weak polyelectrolyte multilayer (PEM) system of approximately 100 nm thickness and time dependent immersion in a solution of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) as a crosslinking agent. Uniform surface chemistry across the gradient and wettability was provided by the addition of a 10 nm thick plasma polymer layer deposited from vapour of either allylamine or acrylic acid. We used the resultant stiffness gradients (0.5-110 MPa in hydrated state) to investigate the adhesion, morphology and proliferation on human dermal fibroblasts (HDFs). We show that substrate mechanical properties strongly influence HDF cell fate. We also found that in the experimental range of surface properties used in this study, the surface stiffness was a stronger driving force to cells fate compared to chemistry and wettability.

Processable 3-nm thick graphene platelets of high electrical conductivity and their epoxy composites

Graphene platelets (GnPs) are a class of novel 2D nanomaterials owing to their very small thickness (∼3 nm), high mechanical strength and electric conductivity (1460 S cm(-1)), and good compatibility with most polymers as well as cost-effectiveness. In this paper we present a low-cost processing technique for producing modified GnPs and an investigation of the electrical and mechanical properties of the resulting composites. After dispersing GnPs in solvent N-methyl-2-pyrrolidone, a long-chain surfactant (Jeffamine D 2000, denoted J2000) was added to covalently modify GnPs, yielding J2000-GnPs. By adjusting the ratio of GnPs to the solvent, the modified GnPs show different average thickness and thus electrical conductivity ranging from 694 to 1200 S cm(-1). To promote the exfoliation and dispersion of J2000-GnPs in a polymeric matrix, they were dispersed in the solvent again and further modified using diglycidyl ether of bisphenol A (DGEBA) producing m-GnPs, which were then compounded with an epoxy resin for the development of epoxy/m-GnP composites. A percolation threshold of electrical volume resistivity for the resulting composites was observed at 0.31 vol%. It was found that epoxy/m-GnP composites demonstrated far better mechanical properties than those of unmodified GnPs of the same volume fraction. For example, m-GnPs at 0.25 vol% increased the fracture energy release rate G1c from 0.204 ± 0.03 to 1.422 ± 0.24 kJ m(-2), while the same fraction of unmodified GnPs increased G1c to 1.01 ± 0.24 kJ m(-2). The interface modification also enhanced the glass transition temperature of neat epoxy from 58.9 to 73.8 °C.

Fine-Tuning the Surface of Forward Osmosis Membranes via Grafting Graphene Oxide: Performance Patterns and Biofouling Propensity

Graphene oxide (GO) nanosheets were attached to the polyamide selective layer of thin film composite (TFC) forward osmosis (FO) membranes through a poly L-Lysine (PLL) intermediary using either layer-by-layer or hybrid (H) grafting strategies. Fourier transform infrared spectroscopy, zeta potential, and thermogravimetric analysis confirmed the successful attachment of GO/PLL, the surface modification enhancing both the hydrophilicity and smoothness of the membranes surface demonstrated by water contact angle, atomic force microscopy, and transmission electron microscopy. The biofouling resistance of the FO membranes determined using an adenosine triphosphate bioluminescence test showed a 99% reduction in surviving bacteria for GO/PLL-H modified membranes compared to pristine membrane. This antibiofouling property of the GO/PLL-H modified membrane was reflected in reduced flux decline compared to all other samples when filtering brackish water under biofouling conditions. Further, the high density and tightly bound GO nanosheets using the hybrid modification reduced the reverse solute flux compared to the pristine, which reflects improved membrane selectivity. These results illustrate that the GO/PLL-H modification is a valuable addition to improve the performance of FO TFC membranes.

Nanoscale deposition of chemically functionalised films via plasma polymerisation

Plasma polymerisation is a technologically important surface engineering process capable of depositing ultra-thin functionalised films for a variety of purposes. It has many advantages over other surface engineering processes, including that it is completely dry, can be used for complex geometries, and the physico-chemical properties of the film can be tailored through judicious choice of processing conditions. Despite this, the mechanisms of film growth are largely unknown, and current models are based on purely chemical arguments. Consideration of some basic plasma physics shows that some species can arrive at surfaces with energies greater than 1000 kJ mol−1 (>10 eV), and thus open a range of surface reactions that have not been considered previously. This review aims to close the gap between the physics and chemistry of reactive plasma systems.

pH-tunable gradients of wettability and surface potential

Smart materials that can sense and respond to changes in the environment are of interest in numerous and diverse applications. In this paper, we report gradient surfaces where wettability and surface potential respond to changes in the pH. The gradients are produced by controlling the concentration of amine and carboxyl acid groups across the surface. The response of surface wettability to pH changes was studied by water contact angle measurements. The potential across the surface was determined by atomic force microscopy-based surface force measurements. These studies showed that at low pH the surface potential changes from “no charge” at the acid end to a positive charge at the amine end. At high pH the surface potential changed from negative at the acid end to “no charge” at the amine side. At an intermediate pH the charge across the surface changes from negative at the acid end to positive at the amine end. Potential applications include separation or guidance of charged entities such as particles, proteins or bacteria.

On the effect of monomer chemistry on growth mechanisms of nonfouling PEG-like plasma polymers.

It has been shown that both ions and neutral species may contribute to plasma polymer growth. However, the relative contribution from these mechanisms remains unclear. We present data elucidating the importance of considering monomer structure with respect to which the growth mechanism dominates for nonfouling PEG-like plasma polymers. The deposition rate for saturated monomers is directly linked with ion flux to the substrate. For unsaturated monomers, the neutral flux also plays a role, particularly at low power. Increased fragmentation of the monomer at high power reduces the ability of unsaturated monomers to grow via neutral grafting. Chemical characterization by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirm the role that plasma phase fragmentation plays in determining the deposition rate and surface chemistry of the deposited film. The simple experimental method used here may also be used to determine which mechanisms dominate plasma deposition for other monomers. This knowledge may enable significant improvement in future reactor design and process control.