Sitaraman Krishnan

Advances in polymers for anti-biofouling surfaces

Self-assembling polymers and nanostructured polymer thin films are being actively explored as advanced coatings for marine and biomedical applications. This review highlights recent advances in the design and synthesis of polymers that can resist fouling by biomolecules, cells and organisms. Current understanding of the mechanisms of anti-biofouling activity is also discussed.

Protein adsorption resistance of anti-biofouling block copolymers containing amphiphilic side chains

Surface active block copolymers (SABCs) with amphiphilic side chains containing ethoxylated fluoroalkyl groups have previously demonstrated advantageous properties with regard to marine fouling resistance and release. While it was previously postulated that the ability of the block copolymer surface to undergo an environment-dependent transformation in surface structure aided this behaviour, protein adsorption characteristics of the surface were never explored. This study aims to expand our knowledge of protein interaction with the amphiphilic surface active block copolymer in an aqueous environment through experiments with bovine serum albumin (BSA), a widely utilized test protein. Fluorescence microscopy analysis using BSA labelled with fluorescein isothiocyanate (BSA–FITC) was performed on a SABC test surface to establish the polymers protein adsorption resistance. Additionally, atomic force microscopy (AFM) based chemical force microscopy (CFM) was utilized to examine the force of adhesion of an AFM tip functionalized with strands of BSA protein with the SABC. No measurable force of adhesion was detected for 58% of the measurements of adhesion force taken for a BSA coated AFM tip interacting with the surface of the amphiphilic SABC in a PBS buffer. Furthermore, no measurements of force of adhesion were made in excess of 0.15 nN. This was in contrast to the non-zero mean adhesion force seen for several control surfaces in PBS buffer.

Antimicrobial Behavior of Semifluorinated-Quaternized Triblock Copolymers against Airborne and Marine Microorganisms

Semifluorinated-quaternized triblock copolymers (SQTCs) were synthesized by chemical modification of polystyrene-block-poly(ethylene-ran-butylene)-block-polyisoprene ABC triblock copolymers. Surface characterization of the polymers was performed by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) analysis. The surface of the SQTC showed very high antibacterial activity against the airborne bacterium Staphylococcus aureus with >99 % inhibition of growth. In contrast in marine fouling assays, zoospores of the green alga Ulva settled on the SQTC, which can be attributed to the positively charged surface. The adhesion strength of sporelings (young plants) of Ulva and Navicula diatoms (a unicellular alga) was high. The SQTC did not show marked algicidal activity.

Self-assembled polysaccharide nanostructures for controlled-release applications

Abstract Self-assembling polysaccharide nanostructures have moved to the forefront of many fields due to their wide range of functional properties and unique advantages, including biocompatability and stimulus responsiveness. In particular, the field of controlled release, which involves influencing the location, concentration, and efficacy of active pharmaceutical ingredients (APIs), diagnostics, nutrients, or other bioactive compounds, has benefited from polysaccharide biomaterials. Nanostructure formation, stimulus responsiveness, and controlled-release performance can be engineered through facile chemical functionalization and noncovalent intermolecular interactions. This review discusses polysaccharide nanoparticles, designed for targeted and time-controlled delivery of emerging APIs, with improved in vivo retention, stability, solubility, and permeability characteristics. Topics covered include nanoparticles of cyclodextrin and cyclodextrin-containing polymers, hydrophobically modified polysaccharides, polysaccharide nanoparticles that respond to pH, temperature, or light stimulus, polysaccharide prodrug complexes, polysaccharide complexes with lipids and proteins, and other polysaccharide polyelectrolyte complexes.

Ionic liquids with fluorinated block-oligomer tails: Influence of self-assembly on transport properties

A novel imidazolium iodide ionic liquid with an ω-perfluoroalkyl poly(ethylene glycol) (PEG) tail attached to the imidazolium ring has been synthesized for its potential incorporation as an electrolyte in dye-sensitized solar cells. The ionic liquid molecules, with block oligomer tails, self-assembled to form a solvent-free ionic gel, without the assistance of an external gelator or an immobilizing matrix. The solidification was evidently facilitated by the generation of ionic clusters due to electrostatic interactions, as well as microphase separation of the immiscible perfluoroalkyl and PEG segments of the cation. We report herein the synthesis and electrochemical properties of this block oligomer ionogel, along with the results of self-consistent mean field calculations probing the formation of nanostructures in the ionogel. Although properties such as high viscosity and high ionic conductivity appear incompatible, it is shown that a nano-structured fluid can support high iodide diffusion at low effective fluidity, and that the formation of an organic alloy, by simple blending of two imidazolium iodide salts, can produce significant conductivity enhancements without lowering the viscosity.

Interfacial characteristics of a PEGylated imidazolium bistriflamide ionic liquid electrolyte at a lithium ion battery cathode of LiMn2O4.

Nonvolatile and nonflammable ionic liquids (ILs) have distinct thermal advantages over the traditional organic solvent electrolytes of lithium ion batteries. However, this beneficial feature of ILs is often counterbalanced by their high viscosity (a limiting factor for ionic conductivity) and, sometimes, by their unsuitable electrochemistry for generating protective layers on electrode surfaces. In an effort to alleviate these limiting aspects of ILs, we have synthesized a PEGylated imidazolium bis(trifluoromethylsulfonyl)amide (bistriflamide) IL that exhibited better thermal and electrochemical stability than a conventional electrolyte based on a blend of ethylene carbonate and diethyl carbonate. The electrochemical performance of this IL has been demonstrated using a cathode consisting of ball-milled LiMn2O4 particles. A direct comparison of the ionic liquid electrolyte with the nonionic low-viscosity conventional solvent blend is presented.

Simultaneous electronic and ionic conduction in ionic liquid imbibed polyacetylene-like conjugated polymer films

Polymer films composed of a polyacetylene-like conjugated polymer and 1-propyl-3-methylimidazolium iodide ionic liquid (IL) were synthesized using a ‘one-pot’ hydroiodic acid catalyzed thermal dehydration of poly(vinyl alcohol) (PVA) precursor polymer blended with the IL. The dehydration of the precursor polymer and the formation of polyene segments, in films reacted at temperatures in the range of 60 to 250 °C, were characterized using thermogravimetry and infrared spectroscopy. In reactions catalyzed by HI, infrared, 13C NMR, and Raman spectroscopies confirmed nearly complete elimination of the hydroxyl groups of PVA at 200 °C, which was below the decomposition temperature of the IL in the blend. In contrast, the synthesis of the polyene–IL blend was not feasible in the absence of the HI catalyst, because pure PVA exhibited peak mass loss at about 280 °C, a temperature at which the IL also showed significant thermal degradation. Electrochemical and mechanical properties of the polyene–IL blends synthesized at 200 °C were investigated. The films exhibited both electronic and ionic conductivities. The charge conduction pathways were identified and quantified using electrochemical impedance spectroscopy (EIS). The electronic and ionic conductivities increased by as much as four orders of magnitude over the temperature range of 25 to 115 °C. The storage and loss moduli of the films were measured using dynamic mechanical analysis (DMA), and elongation at break was determined by tensile testing. Addition of IL to the conjugated polymer not only imparted ionic conductivity to the polyene films, but also greatly improved their mechanical properties, including the elongation at break.

Relative Importance of the Effects of Seed and Feed Stage Agitations on Latex Properties in Semibatch Emulsion Copolymerization of n‐Butyl Methacrylate and N‐Methylol Acrylamide

The effects of agitation in a ca. 24% solids semibatch emulsion copolymerization of n‐butyl methacrylate and N‐methylol acrylamide in a 2 dm3 reactor are reported. A Rushton turbine with 8 cm tip‐to‐tip diameter was used as the agitator. The agitation speeds during the seed and feed stages of the semibatch process were varied at two levels. The final latexes obtained from the four experiments were characterized for the size of the polymer particles, viscosity, amount of water‐soluble polymer, and the amount of coagulum at the end of the reaction. A higher agitation speed nucleated a greater number of polymer particles during the in situ seed formation step (seed stage). In the absence of any secondary nucleation during the monomer‐feeding stage, the final latexes had a higher number of particles when the agitation speed during the seed stage was higher. The amount of coagulum increased with an increase in the agitation power‐input. The amount of water‐soluble polymer was influenced mainly by the agitation during the seed stage of the process, through the effect of the latter on the number of polymer particles. However, the pooling of the BMA monomer during the feed stage, because of poor mixing and shear in the reactor, resulted in an increased water‐soluble polymer formation. Latexes prepared using a higher agitation speed during the seed stage had a higher viscosity.

Agitation Effects in Emulsion Copolymerization of n-Butyl Methacrylate and N-Methylol Acrylamide

The effects of agitation in emulsion copolymerization have been studied using a semibatch emulsion copolymerization recipe that gave a latex with ca. 44 wt% solids. The two‐stage polymerization process consisted of an in situ seed‐formation step followed by copolymerization of the continuously added n‐butyl methacrylate and N‐methylol acrylamide monomers under monomer‐starved conditions. A 2‐dm3 glass reactor and Rushton turbine agitators of different diameters (4, 6 and 8 cm) were used in the study. Agitation influenced the particle number at the end of the seed stage. This influenced properties like latex viscosity, the amount of water‐soluble polymer, and the particle size in the final latex. Analysis of the water‐soluble polymer using NMR spectroscopy showed that it was mostly a homopolymer of N‐methylol acrylamide. With the 4 cm diameter agitator, the mixing of the pseudoplastic latex was poor during the feed stage. Formation of a zone of poor mixing could be observed on top of the fluid in the reactor, when the monodisperse particles in the latex formed an iridescent structure. Poor emulsification of the added n‐butyl methacrylate monomer resulted in a greater amount of water‐soluble polymer.

Biofilm Formation on Medical Devices and Infection: Preventive Approaches

Biofilm formation on medical devices is a serious problem associated with deaths resulting from nosocomial (hospital acquired) infections. This chapter reviews strategies to control microbial adhesion to, and colonization of, medical surfaces using cell repellant and nonadhesive coatings, coatings that actively release antimicrobial compounds and biofilm inhibitors, antimicrobial coatings with tethered biocides, and coatings that promote competitive adherence of benign organisms. Antifouling materials such as PEGylated and zwitterionic polymers, silicone hydrogels, fluorinated amphiphilic polymers, natural polysaccharides, glycodendron-functionalized synthetic polymers, polymers tethered with antibiotics such as ciprofloxacin, antimicrobial cationic polymers and peptides, and functionalized polyhydroxyalkanoates are discussed. Controlled release coatings that deliver quorum sensing inhibitors such as 5,6-dimethyl-2-aminobenzimidazole, and antimicrobial species such as ceragenin, nitrofurantoin, nitric oxide, gallium and zinc complexes, silver ions, silver nanoparticles, and selenium nanoparticles are also reviewed.