Neil Ayres

Polymer brushes: Applications in biomaterials and nanotechnology

Surface-confined macromolecules known as polymer brushes are being increasing applied to a variety of areas. As more information is gained on the molecular structure of polymer brushes and how they respond to environmental stimuli, these applications are becoming wider ranging and better defined. This review seeks to highlight recent contributions in two broad areas: biotechnology and nanotechnology. These are positions in which polymer brushes are well-suited to offer performance gains. Examples are given that describe the benefits of using a well defined, covalently bound, and densely grafted polymer including in areas such as prevention of bacterial adherence, cell attachment, electrochemistry, and formation of colloidal crystals.

Atom Transfer Radical Polymerization: A Robust and Versatile Route for Polymer Synthesis

Atom transfer radical polymerization (ATRP) has become an established synthetic methodology to prepare polymers of high molecular weights with narrow molecular weight distributions and living polymerization characteristics. This review is a conceptual overview of ATRP and the components of an ATRP system. Current approaches to reduce the transition metal concentration are then described including activators regenerated by electron transfer (ARGET) ATRP. Finally, a recent polymerization approach, single electron transfer living radical polymerization (SET-LRP), is described.

Synthesis and anticoagulant activity of polyureas containing sulfated carbohydrates.

Polyurea-based synthetic glycopolymers containing sulfated glucose, mannose, glucosamine, or lactose as pendant groups have been synthesized by step-growth polymerization of hexamethylene diisocyanate and corresponding secondary diamines. The obtained polymers were characterized by gel permeation chromatography, nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy. The nonsulfated polymers showed similar results to the commercially available biomaterial polyurethane TECOFLEX in a platelet adhesion assay. The average degree of sulfation after reaction with SO3 was calculated from elemental analysis and found to be between three and four −OSO3 groups per saccharide. The blood-compatibility of the synthetic polymers was measured using activated partial thromboplastin time, prothrombin time, thrombin time, anti-IIa, and anti-Xa assays. Activated partial thromboplastin time, prothrombin time, and thrombin time results indicated that the mannose and lactose based polymers had the highest anticoagulant activities among all the sulfated polymers. The mechanism of action of the polymers appears to be mediated via an anti-IIa pathway rather than an anti-Xa pathway.

pH-Degradable Mannosylated Nanogels for Dendritic Cell Targeting

We report on the design of glycosylated nanogels via core-cross-linking of amphiphilic non-water-soluble block copolymers composed of an acetylated glycosylated block and a pentafluorophenyl (PFP) activated ester block prepared by reversible addition-fragmentation (RAFT) polymerization. Self-assembly, pH-sensitive core-cross-linking, and removal of remaining PFP esters and protecting groups are achieved in one pot and yield fully hydrated sub-100 nm nanogels. Using cell subsets that exhibit high and low expression of the mannose receptor (MR) under conditions that suppress active endocytosis, we show that mannosylated but not galactosylated nanogels can efficiently target the MR that is expressed on the cell surface of primary dendritic cells (DCs). These nanogels hold promise for immunological applications involving DCs and macrophage subsets.

The poor solubility of ureidopyrimidone can be used to form gels of low molecular weight N-alkyl urea oligomers in organic solvents

A synthesis strategy for low molecular weight organogelators using the ureidopyrimidinone (UPy) group is reported. The prepared gelators showed robust thermal reversible gelation abilities in various solvents, including dimethyl sulfoxide. The morphology of the dried gels was determined using scanning electron microscopy, revealing a macroscopic porous structure of the gels. Rheology was performed to determine storage (G′) and loss modulus (G″) confirming network gel structures.

Investigation into fiber formation in N-alkyl urea peptoid oligomers and the synthesis of a water-soluble PEG/N-alkyl urea peptoid oligomer conjugate

The synthesis and self assembly of an N-alkyl urea peptoid oligomer/polymer conjugate that self-assembles into ribbon-like structures is reported. The aggregation of the N-alkyl urea peptoid oligomers has been investigated using NMR spectroscopy, with long and highly uniform fibers with micron dimensions observed in scanning electron microscopy images from both organo-soluble and water-soluble versions. The results of these studies indicate that hydrogen bonding likely is the major driving force for the self assembly. The oligomer was conjugated to poly(ethylene glycol) (PEG) producing a water soluble polymer/N-alkyl urea peptoid conjugate where the self-assembly of the N-alkyl urea peptoid was retained. These polymer conjugates represent a new approach to polymer biomimetic conjugates.

Synthesis of polymer organogelators using hydrogen bonding as physical cross-links

The synthesis of a monomer containing fourfold hydrogen bonding groups 2-(((6-(6-methyl-4[1H]pyrimidionylureido)hexyl)carbamoyl)oxy)ethyl methacrylate (UPyEMA) and its copolymerization with either n-butyl acrylate, tert-butyl acrylate, or styrene using reversible addition-fragmentation chain transfer (RAFT) polymerization is reported. The copolymers possessed high molecular weight and narrow molecular weight distributions and formed stable organogels in both chloroform and 1,2-dichlorobezene. Critical gelation concentrations were determined and the rheology of the organogels characterized. A novel monomer containing pyrene was prepared, and its polymerization under RAFT control was demonstrated. The pyrene-containing monomer was copolymerized with the polymer organogelators forming fluorescent organogels. It is proposed that these gels are suitable for two-photon upconversion applications.

Stimuli-Responsive Self-Immolative Polymer Nanofiber Membranes Formed by Coaxial Electrospinning

The first self-immolative polymer (SIP) nanofiber membrane is demonstrated in this report, in which the immolation can be triggered by external stimulus. Electrospun SIP/polyacrylonitrile (PAN) fibers provide depolymerization that is ∼25 times quicker and more responsive (i.e., immolation) than that of a cast film in the triggering condition. Depolymerization of SIP in the SIP/PAN blended fiber membrane results in the transition of the surface properties from hydrophobic (∼110°) to hygroscopic (∼0°). Triggered release of encapsulated functional molecules was demonstrated using coaxially electrospun fiber membrane made of a SIP/PAN blend sheath and polyvinylpyrrolidone/dye core. Coaxial fibers with the SIP/PAN sheath provide minimal release of the encapsulated material in nontriggering solution, while it releases the encapsulated material instantly when the triggering condition is met. Its versatility has been strengthened compared to that of non-SIP coaxial fibers that provide no triggering reaction by external stimulus.

Hybrid photosensitizer based on amphiphilic block copolymer stabilized silver nanoparticles for highly efficient photodynamic inactivation of bacteria

We report the development of a type of novel hybrid photosensitizers for photodynamic inactivation of broad-spectrum bacteria. A thiol-modified amphiphilic block copolymer, poly(N-isopropylacrylamide-block-styrene), was synthesized and characterized. Subsequently, silver nanoparticles stabilized by poly(N-isopropylacrylamide-block-styrene) were synthesized and used to entrap hydrophobic photosensitizing molecules (such as hematoporphyrin). The resulting water-dispersible hybrid photosensitizers demonstrated enhanced singlet oxygen generation with a broadened excitation profile, as compared to the pristine hematoporphyrin molecules. Photodynamic inactivation of Staphylococcus epidermidis and Escherichia coli by the hybrid photosensitizer showed significantly enhanced killing efficacy, up to ∼5 orders of magnitude, under both white light and red/near-infrared light illuminations. The hybrid photosensitizers at the concentration used in the photodynamic inactivation assays displayed low cytotoxicity to Hela cells under ambient light conditions. These results demonstrate the great potential of such hybrid photosensitizers for photodynamic inactivation and photodynamic therapy applications.

Development of Non-Viral, Trophoblast-Specific Gene Delivery for Placental Therapy

Low birth weight is associated with both short term problems and the fetal programming of adult onset diseases, including an increased risk of obesity, diabetes and cardiovascular disease. Placental insufficiency leading to intrauterine growth restriction (IUGR) contributes to the prevalence of diseases with developmental origins. Currently there are no therapies for IUGR or placental insufficiency. To address this and move towards development of an in utero therapy, we employ a nanostructure delivery system complexed with the IGF-1 gene to treat the placenta. IGF-1 is a growth factor critical to achieving appropriate placental and fetal growth. Delivery of genes to a model of human trophoblast and mouse placenta was achieved using a diblock copolymer (pHPMA-b-pDMAEMA) complexed to hIGF-1 plasmid DNA under the control of trophoblast-specific promoters (Cyp19a or PLAC1). Transfection efficiency of pEGFP-C1-containing nanocarriers in BeWo cells and non-trophoblast cells was visually assessed via fluorescence microscopy. In vivo transfection and functionality was assessed by direct placental-injection into a mouse model of IUGR. Complexes formed using pHPMA-b-pDMAEMA and CYP19a-923 or PLAC1-modified plasmids induce trophoblast-selective transgene expression in vitro, and placental injection of PLAC1-hIGF-1 produces measurable RNA expression and alleviates IUGR in our mouse model, consequently representing innovative building blocks towards human placental gene therapies.