Cameron Alexander

Stimuli responsive polymers for biomedical applications

Polymers that can respond to external stimuli are of great interest in medicine, especially as controlled drug release vehicles. In this critical review, we consider the types of stimulus response used in therapeutic applications and the main classes of responsive materials developed to date. Particular emphasis is placed on the wide-ranging possibilities for the biomedical use of these polymers, ranging from drug delivery systems and cell adhesion mediators to controllers of enzyme function and gene expression (134 references).

Variable Adhesion of Micropatterned Thermoresponsive Polymer Brushes: AFM Investigations of Poly(N‐isopropylacrylamide) Brushes Prepared by Surface‐Initiated Polymerizations

Thermoresponsive polymer brushes have been grown via surface-initiated atom-transfer radical polymerization from micropatterned substrates (see Figure). Atomic force microscopy both in topography and adhesion force mode shows that the brush domains undergo reversible phase changes in water around the polymer’s lower critical solution temperature. These switchable patterned surfaces may form a new class of microdevices.

Directed nucleation of calcite at a crystal-imprinted polymer surface

The finely tuned properties of natural biominerals and composites reflect the remarkable level of control that is exercised over the size, shape and organization of the constituent crystals. Achieving this degree of control over synthetic materials might therefore lead to superior material properties. Organic small molecules, polymers or surfactant mesophases have been used to guide the growth and morphology of inorganic materials via steric constraints or structure-directing interactions. Here we show that synthetic polymers can be imprinted with motifs of crystal surfaces so as to template the growth of specific crystal phases. Our polymers, imprinted with calcite, are able to induce the nucleation of calcite under conditions favouring the growth of aragonite (another polymorph of calcium carbonate). The synthesis of the polymers, based on the principles of molecular imprinting, involves the adsorption of functional monomers to a calcite surface, followed by co-polymerization with a crosslinker to create an imprint of the crystal surface. Subsequent removal of the calcite template yields a polymer matrix with a surface functionality mirroring the crystal face and able to promote the nucleation of calcite. We expect that the molecular-imprinting approach to directed nucleation can be applied to crystals other than calcite.

Sweet Talking Double Hydrophilic Block Copolymer Vesicles

The language of cellular interactions in nature includes a wealth of code based on sugar molecules and macromolecular frameworks. The complexity in carbohydrate structure is matched by a subtlety in function that leads to a huge variety of roles for these polymeric sugars in biology. It is not surprising therefore that chemists seek to “talk” to biological entities through the language of the glycocode, for this would enable intervention in events such as cell-surface recognition, detection, and signaling, leading in turn to prevention of infectivity, treatment of disease, and even to control of new tissue formation. As our comprehension of the glycocode advances, the use of synthetic sugar-containing polymers to address cells is becoming an area rich with potential. Herein we describe an approach towards controlling cell-surface interactions and molecular transport at biointerfaces, by using self-assembling polymer vesicles with multiple copies of a simple glycoligand, glucose, as a first step towards a “conversation” between artificial cell mimics and real cells. The key to this approach is the preparation of synthetic polymers capable of assembling into capsule-like structures with sugar functionality presented into solution. The aim is to produce a simple mimic of eukaryotic cell surfaces, which might in turn be recognized by biological species that normally bind to glycosylated residues on the cell. To do this, we prepared block copolymers with highly hydrophilic poly(2-glucosyloxyethyl methacrylate) (pGEMA) as one block and more sparingly water-soluble poly(diethyleneglycol methacrylate) (pDEGMA) as the second block by using controlled free-radical techniques (Scheme 1 and Table 1). We chose glucose as the recognition element, as it exhibits generally weak individual interactions with receptors, but when multiple copies are present on a polymer backbone strong binding can occur through polyvalency. Initial polymer synthesis involved growing a protected precursor polymer by atom transfer radical polymerization (ATRP) or reversible addition–fragmentation chain transfer (RAFT) which was subsequently deprotected to render the block active and hydrophilic. This polymer was used as a macroinitiator to grow the pDEGMA block. Dynamic light scattering (DLS) showed that below 15 8C, the polymers existed in solution as separate chains, but, at 20 8C, P1 and P2 assembled into vesicles with mean diameters

Functional hyperbranched polymers: toward targeted in vivo 19F magnetic resonance imaging using designed macromolecules.

We have demonstrated the design and synthesis of hyperbranched molecules that can be successfully imaged in vivo using (19)F MRI in under 10 min. These polymers are cytocompatible following chain extension with PEGMA. In addition, functionalization of these macromolecules can be achieved in a facile manner and with accessible and correct ligand presentation. Such hyperbranched polymers hold promise as new generation tracking and targeting MRI contrast agents.

Multimodal Polymer Nanoparticles with Combined 19F Magnetic Resonance and Optical Detection for Tunable, Targeted, Multimodal Imaging in Vivo

Understanding the complex nature of diseased tissue in vivo requires development of more advanced nanomedicines, where synthesis of multifunctional polymers combines imaging multimodality with a biocompatible, tunable, and functional nanomaterial carrier. Here we describe the development of polymeric nanoparticles for multimodal imaging of disease states in vivo. The nanoparticle design utilizes the abundant functionality and tunable physicochemical properties of synthetically robust polymeric systems to facilitate targeted imaging of tumors in mice. For the first time, high-resolution (19)F/(1)H magnetic resonance imaging is combined with sensitive and versatile fluorescence imaging in a polymeric material for in vivo detection of tumors. We highlight how control over the chemistry during synthesis allows manipulation of nanoparticle size and function and can lead to very high targeting efficiency to B16 melanoma cells, both in vitro and in vivo. Importantly, the combination of imaging modalities within a polymeric nanoparticle provides information on the tumor mass across various size scales in vivo, from millimeters down to tens of micrometers.

In Situ Growth of Side-Chain PEG Polymers from Functionalized Human Growth Hormone—A New Technique for Preparation of Enhanced Protein−Polymer Conjugates

The application of atom transfer radical polymerization (ATRP) for preparation of a novel class of protein-polymer bioconjugates is described, exemplified by the synthesis of a recombinant human growth hormone (rh-GH) poly(ethylene glycol) methyl ether methacrylate (PEGMA) hybrid. The rh-GH protein was activated via a bromo-ester functionalized linker and used as a macroinitiator to polymerize the hydrophilic monomer PEGMA under solely aqueous conditions at 4 degrees C. ATRP conditions resulted in controlled polymer growth from rh-GH with low-polydispersity polyPEGMA chains. The rh-GH PEGMA product exhibited properties consistent with the presence of attached hydrophilic polymer chains, namely, high stability to denaturation and proteolysis. The polymerization conditions and conjugation proceeded with retention of the biological activity of the hormone. The rh-GH PEGMA was administered subcutaneously to rats and the activity compared to native rh-GH. The rh-GH PEGMA exhibited similar activity as the native rh-GH in vivo when a daily dose of 40 microg was administered. However, when a higher dose of 120 microg was administered with 3 days between injections the bioavailability of the rh-GH PEGMA was significantly better than that of the native. The results therefore demonstrate that ATRP can be successfully used as a general alternative approach to direct polymer conjugation, namely, PEGylation, to produce PEG-like protein conjugates. This technique can be exploited to design and synthesize protein-polymer derivatives with tailored therapeutic properties.

Bioadhesion at micro-patterned stimuli-responsive polymer brushes

The synthesis of poly(N-isopropylacrylamide) brushes within micropatterned domains at surfaces and the performance of these functionalised surfaces in short-term bioadhesion assays under varying conditions are described. The polymer brushes show temperature dependent behaviour at surfaces as demonstrated by changes in contact angle, surface energy components and aqueous phase AFM. The responses in the polymer brush domains result in spatially defined, and temperature mediated, attachment of a model protein, BSA, and the common oral bacteria Streptococcus mutans.

Cell up-take control of gold nanoparticles functionalized with a thermoresponsive polymer

Surface decoration of gold nanoparticles with thermoresponsive polymers endows a temperature tunable colloidal system switchable for enhanced intracellular up-take. Gold nanoparticles (AuNP, 18 ± 11 nm-diameter) produced by laser ablation synthesis in liquid solution were surface coated with thermoresponsive thiol terminated poly-N-isopropylacrylamide-co-acrylamide co-polymer possessing a lower critical solution temperature (LCST) at 37 °C. Under selected conditions about 3800 polymer chains were conjugated per particle. The polymer coated nanoparticles were found to display thermosensitive properties, as in solution they exhibited reversible aggregation/deaggregation above and below the LCST, respectively. Cell culture studies showed that the polymer decorated AuNP were located into human breast adenocarcinoma MCF7 cells treated at 40 °C (12000 AuNP/cell) with more than 80-fold greater up-take compared to cells treated at 34 °C with the same particles (140 AuN/cell). This difference is attributable to a ‘switching’ of the polymer coating to a globule state at 37 °C and an increased hydrophobicity of the particles with a simultaneous loss of the ‘stealth’ properties of the polymer coating. By contrast, cell up-take of uncoated AuNP (about 6000 AuNP/cell) did not depend on the incubation temperature. These data show that good control of the AuNP cell up-take can be obtained with the new polymer-gold nanoconjugates, and suggest that these systems might find use for targeting cellsin vitro by a small temperature change or in vivo in body sites, such as inflamed or tumour tissues, where a temperature variation is already present.

A highly effective gene delivery vector – hyperbranched poly(2-(dimethylamino)ethyl methacrylate) from in situ deactivation enhanced ATRP

A hyperbranched 2-(dimethylamino)ethyl methacrylate (DMAEMA) based polymer has been synthesised by a one-pot in situ deactivation enhanced atom transfer radical polymerisation (DE-ATRP); it exhibits much higher transfection ability than linear poly(DMAEMA) and is comparable to the well known branched poly(ethylene imine) (PEI) and the SuperFect dendrimer but with lower cytotoxicity.