K. G. Neoh

Polyaniline: A polymer with many interesting intrinsic redox states

Abstract Due to its environmental stability, high degree of processability and interesting redox properties associated with its chain heteroatom, polyaniline (PANi) has been one of the most extensively studied electroactive (conductive) polymers during the past ten years. A number of fine reviews on the synthesis, physicochemical and electrochemical properties of the polymer have also appeared during this period. The ability of aniline polymers to exist in a large number of intrinsic redox states makes them a unique and interesting class of polymeric materials. The present review attempts for the first time to summarize the explicit and quantitative dealings with the various intrinsic oxidation states of PANi and its derivatives that have been made during the past decade.

Surface functionalization of titanium with hyaluronic acid/chitosan polyelectrolyte multilayers and RGD for promoting osteoblast functions and inhibiting bacterial adhesion

Titanium (Ti) and its alloys are used extensively in orthopedic implants due to their excellent biocompatibility and mechanical properties. However, titanium-based implant materials have specific complications associated with their applications, such as the loosening of implant-host interface owing to unsatisfactory cell adhesion and the susceptibility of the implants to bacterial infections. Hence, a surface which displays selective biointeractivity, i.e. enhancing beneficial host cell responses but inhibiting pathogenic microbial adhesion, would be highly desirable. This present study aims to improve biocompatibility and confer long-lasting antibacterial properties on Ti via polyelectrolyte multilayers (PEMs) of hyaluronic acid (HA) and chitosan (CH), coupled with surface-immobilized cell-adhesive arginine-glycine-aspartic acid (RGD) peptide. The HA/CH PEM-functionalized Ti is highly effective as an antibacterial surface but the adhesion of bone cells (osteoblasts) is poorer than on pristine Ti. With additional immobilized RGD moieties, the osteoblast adhesion can be significantly improved. The density of the surface-immobilized RGD peptide has a significant effect on osteoblast proliferation and alkaline phosphatase (ALP) activity, and both functions can be increased by 100-200% over that of pristine Ti substrates while retaining high antibacterial efficacy. Such substrates can be expected to have good potential in orthopedic applications.

Plasma-induced immobilization of poly(ethylene glycol) onto poly(vinylidene fluoride) microporous membrane

Abstract Poly(vinylidene fluoride) (PVDF) microporous membranes with surface-immobilized poly(ethylene glycol) (PEG) were prepared by the argon plasma-induced grafting of PEG. The PEG was pre-coated on the membrane surface, including the pore surfaces, by dipping the membrane in a PEG/CHCl3 solution prior to the argon plasma exposure. The microstructure and composition of the PEG-grafted PVDF (PEG-g-PVDF) membranes were characterized by attenuated total reflectance (ATR) FT-IR, X-ray photoelectron spectroscopy (XPS), and thermogravimetric (TG) analysis. A moderate radio-frequency (RF) plasma power and plasma treatment time led to a high concentration of the grafted PEG polymer. The morphology of the modified membranes was studied by scanning electronic microscope (SEM). The pore size and water flux of the modified membranes were also characterized. The flux decreased with increasing surface concentration of the grafted PEG polymer, while the pore size remained almost unchanged. Protein adsorption experiments revealed that the PEG-g-PVDF membranes with a PEG graft concentration, defined as the [CO]/[CF2] ratio above 3.2 exhibited good anti-fouling property.

The effect of VEGF functionalization of titanium on endothelial cells in vitro.

One of the key challenges in bone healing and regeneration is the engineering of an implant with surface properties that can enhance revascularization to meet the metabolic demands of recovery. Successful implant integration into the surrounding tissue is highly dependent on the crucial role of blood supply in driving bone repair and development. Therapeutic application of vascular endothelial growth factor (VEGF) is a promising approach to enhance blood supply and healing through revascularization around an engineered implant in a regulated manner. In this in vitro study, we investigated the effects of immobilized VEGF on titanium alloy substrates coated with thin adherent polydopamine film. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the surfaces at various stages of surface functionalization to verify the successful deposition of polydopamine and VEGF on the metal surface. Surface topography was evaluated from the surface profile determined by atomic force microscopy (AFM). The functionalized surfaces showed a significant increase in human dermal microvascular endothelial cells (HDMECs) attachment, viability and proliferation compared to the pristine substrate. Furthermore the immobilized VEGF was able to induce the differentiation of human mesenchymal stem cells (hMSCs) into endothelial cells. Therefore utilizing the reactivity of polydopamine films to immobilize VEGF onto metal substrates may provide a promising approach for application in situations where revascularization around implants would be beneficial in improving bone healing and implant integration.

Star-Shaped Cationic Polymers by Atom Transfer Radical Polymerization from β-Cyclodextrin Cores for Nonviral Gene Delivery

Cationic polymers with low cytotoxicity and high transfection efficiency have attracted considerable attention as nonviral carriers for gene delivery. Herein, well-defined and star-shaped CDPD consisting of beta-CD cores and P(DMAEMA) arms, and CDPDPE consisting of CDPD and P(PEGEEMA) end blocks (where CD = cyclodextrin, P(DMAEMA) = poly(2-(dimethylamino)ethyl methacrylate), P(PEGEEMA) = poly(poly(ethylene glycol)ethyl ether methacrylate)) for gene delivery were prepared via atom transfer radical polymerization (ATRP) from the bromoisobutyryl-terminated beta-CD core. The CDPD and CDPDPE exhibit good ability to condense plasmid DNA (pDNA) into 100-200 nm size nanoparticles with positive zeta potentials of 25-40 mV at nitrogen/phosphate (N/P) ratios of 10 or higher. CDPD and CDPDPE exhibit much lower cytotoxicity and higher gene transfection efficiency than high molecular weight P(DMAEMA) homopolymers. A comparison of the transfection efficiencies between CDPD and P(DMAEMA) homopolymer indicates that the unique star-shaped architecture involving the CD core can enhance the gene transfection efficiency. In addition to reducing cytotoxicity, the introduction of a biocompatible P(PEGEEMA) end block to the P(DMAEMA) arms in CDPDPE can further enhance the gene transfection efficiency.

Surface modification of stainless steel by grafting of poly(ethylene glycol) for reduction in protein adsorption.

The surface of stainless steel was first modified by the silane coupling agent (SCA), (3-mercaptopropyl)trimethoxysilane. The silanized stainless-steel surface (SCA-SS surface) was subsequently activated by argon plasma and then subjected to UV-induced graft polymerization of poly(ethylene glycol)methacrylate (PEGMA). The chemical structures and composition of the pristine, silane-treated, plasma-treated and PEGMA graft-polymerized stainless-steel coupon surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. The graft polymerization of PEGMA onto the plasma-pretreated SCA-SS surface was studied with different argon plasma pretreatment time, macromonomer concentration, and UV graft polymerization time. In general, a brief plasma pretreatment, high PEGMA concentration, and long UV graft polymerization time readily resulted in a high graft concentration. The PEGMA graft-polymerized stainless-steel coupon (PEGMA-g-SCA-SS) with a high graft concentration, and thus a high PEG content, was found to be very effective in preventing bovine serum albumin and gamma-globulin adsorption.

Lysozyme-coupled poly(poly(ethylene glycol) methacrylate)-stainless steel hybrids and their antifouling and antibacterial surfaces.

An environmentally benign approach to impart stainless steel (SS) surfaces with antifouling and antibacterial functionalities was described. Surface-initiated atom transfer radical polymerization (ATRP) of poly(ethylene glycol) monomethacrylate) (PEGMA) from the SS surface-coupled catecholic L-3,4-dihydroxyphenylalanine (DOPA) with terminal alkyl halide initiator was first carried out, followed by the immobilization of lysozyme at the chain ends of poly(ethylene glycol) branches of the grafted PEGMA polymer brushes. The functionalized SS surfaces were shown to be effective in preventing bovine serum albumin (BSA) adsorption and in reducing bacterial adhesion and biofilm formation. The surfaces also exhibited good bactericidal effects against Escherichia coli and Staphylococcus aureus. The concomitant incorporation of antifouling hydrophilic brushes and antibacterial enzymes or peptides onto metal surfaces via catecholic anchors should be readily adaptable to other metal substrates, and is potentially useful for biomedical and biomaterial applications.

Biomimetic Anchors for Antifouling and Antibacterial Polymer Brushes on Stainless Steel

Barnacle cement (BC) was beneficially applied on stainless steel (SS) to serve as the initiator anchor for surface-initiated polymerization. The amine and hydroxyl moieties of barnacle cement reacted with 2-bromoisobutyryl bromide to provide the alkyl halide initiator for the surface-initiated atom transfer radical polymerization (ATRP) of 2-hydroxyethyl methacrylate (HEMA). The hydroxyl groups of HEMA polymer (PHEMA) were then converted to carboxyl groups for coupling of chitosan (CS) to impart the SS surface with both antifouling and antibacterial properties. The surface-functionalized SS reduced bovine serum albumin adsorption, bacterial adhesion, and exhibited antibacterial efficacy against Escherichia coli (E. coli). The effectiveness of barnacle cement as an initiator anchor was compared to that of dopamine, a marine mussel inspired biomimetic anchor previously used in surface-initiated polymerization. The results indicate that the barnacle cement is a stable and effective anchor for functional surface coatings and polymer brushes.

Electrical Conductance Tuning and Bistable Switching in Poly(N-vinylcarbazole)−Carbon Nanotube Composite Films

By varying the carbon nanotube (CNT) content in poly(N-vinylcarbazole) (PVK) composite thin films, the electrical conductance behavior of an indium-tin oxide/PVK-CNT/aluminum (ITO/PVK-CNT/Al) sandwich structure can be tuned in a controlled manner. Distinctly different electrical conductance behaviors, such as (i) insulator behavior, (ii) bistable electrical conductance switching effects (write-once read-many-times (WORM) memory effect and rewritable memory effect), and (iii) conductor behavior, are discernible from the current density-voltage characteristics of the composite films. The turn-on voltage of the two bistable conductance switching devices decreases and the ON/OFF state current ratio of the WORM device increases with the increase in CNT content of the composite film. Both the WORM and rewritable devices are stable under a constant voltage stress or a continuous pulse voltage stress, with an ON/OFF state current ratio in excess of 10(3). The conductance switching effects of the composite films have been attributed to electron trapping in the CNTs of the electron-donating/hole-transporting PVK matrix.

Surface Functionalization of Titanium with Carboxymethyl Chitosan and Immobilized Bone Morphogenetic Protein-2 for Enhanced Osseointegration

Orthopedic implant failure has been attributed mainly to loosening of the implant from host bone, which may be due to poor bonding of the implant material to bone tissue, as well as to bacterial infection. One promising strategy to enhance tissue integration is to develop a selective biointeractive surface that simultaneously enhances bone cell function while decreasing bacterial adhesion. In this in vitro study, the surfaces of titanium alloy substrates were functionalized by first covalently grafting carboxymethyl chitosan (CMCS), followed by the conjugation of bone morphogenetic protein-2 (BMP-2) to the CMCS-grafted surface. Bacterial adhesion on the substrates was assayed with Staphylococcus aureus and Staphylococcus epidermidis . Cell functions were investigated using osteoblasts and human bone marrow-derived mesenchymal stem cells. The results showed that bacterial adhesion on both the CMCS and CMCS-BMP-2 functionalized surfaces was significantly reduced compared to that on the pristine substrates. In addition, the CMCS-BMP-2 modified substrates significantly promoted attachment, alkaline phosphatase activity, and calcium mineral deposition of both osteoblast and human bone marrow-derived mesenchymal stem cells. The achievement of the dual functions of bacterial adhesion reduction and cell function promotion by the CMCS-BMP-2 modified titanium substrates illustrates the good potential of such surfaces for enhancement of tissue integration and implant longevity.