Biomedical subjects

Alamgir Karim

Publications and source records attributed to Alamgir Karim.

7 recordsLinked to original sources

PubMed2006

Spontaneous formation of vesicles of diblock copolymer EO6BO11 in water: a SANS study.

Small angle neutron scattering (SANS) is used to study the structures formed in water by a diblock copolymer EO6BO11 (having 6 ethylene oxide, EO, and 11 butylene oxide, BO, units). The data show that polymer solutions over a broad concentration range (0.05-20 wt %) contain vesicular structures at room temperature. Interestingly, these vesicles could be formed without any external energy input, such as extrusion, which is commonly required for the formation of other block copolymer or lipid vesicles. The EO6BO11 vesicles are predominantly unilamellar at low polymer concentrations, whereas at higher polymer concentrations or temperatures there is a coexisting population of unilamellar and multilamellar vesicles. At a critical concentration and temperature, the vesicular structures fuse into lyotropic arrays of planar lamellar sheets. The findings from this study are in broad agreement with the work of Harris et al. (Langmuir, 2002, 18, 5337), who used electron microscopy to identify the vesicle phase in the same system.

Epoxy Compounds↗

PubMed2005

Phase behavior of block co-poly(ethylene oxide-butylene oxide), E18B9 in water, by small angle neutron scattering.

We present a small angle neutron scattering (SANS) study into the micellar structures of diblock copolymer E18B9 (where E denotes a ethylene oxide unit and B denotes a butylene oxide unit, 18 and 9 being the number of repeat units respectively) in aqueous solution over a range of five different concentrations (0.2, 1.0, 10.0, 20.0, and 40.0% (by mass fraction)) and eight temperatures (10 to 90 degrees C). The NG7 30 m SANS instrument provides a q range of 0.0009 to 0.5548 A(-1), thus probing the structure over a very broad length scale. At low temperature and low concentration, spherical micelles exist, elongating into worm-like structures at higher temperatures. This transition is observed by the scaling of the scattered intensity at low q and confirmed upon fitting to an appropriate model. Upon increasing concentration, the micelles pack into ordered arrays of either hexagonally packed rod-like micelles or lamellar sheets, again dependent on temperature. Both concentration and temperature effects of this block copolymer have been discussed.

Journal Article↗

PubMed2004

A buckling-based metrology for measuring the elastic moduli of polymeric thin films.

As technology continues towards smaller, thinner and lighter devices, more stringent demands are placed on thin polymer films as diffusion barriers, dielectric coatings, electronic packaging and so on. Therefore, there is a growing need for testing platforms to rapidly determine the mechanical properties of thin polymer films and coatings. We introduce here an elegant, efficient measurement method that yields the elastic moduli of nanoscale polymer films in a rapid and quantitative manner without the need for expensive equipment or material-specific modelling. The technique exploits a buckling instability that occurs in bilayers consisting of a stiff, thin film coated onto a relatively soft, thick substrate. Using the spacing of these highly periodic wrinkles, we calculate the film's elastic modulus by applying well-established buckling mechanics. We successfully apply this new measurement platform to several systems displaying a wide range of thicknessess (nanometre to micrometre) and moduli (MPa to GPa).

Crystallography↗

PubMed2003

Combinatorial characterization of cell interactions with polymer surfaces.

We report a novel combinatorial methodology for characterizing the effects of polymer surface features on cell function. Libraries containing hundreds to thousands of distinct chemistries, microstructures, and roughnesses are prepared using composition spread and temperature gradient techniques. The method enables orders of magnitude increases in discovery rate, decreases variance, and allows for the first time high-throughput assays of cell response to physical and chemical surface features. The technique overcomes complex variable spaces that limit development of biomaterial surfaces for control of cell function. This report demonstrates these advantages by investigating the sensitivity of osteoblasts to the chemistry, microstructure, and roughness of poly(D,L-lactide) and poly(epsilon-caprolactone) blends. In particular, we use the phenomenon of heat-induced phase separation in these polymer mixtures to generate libraries with diverse surface features, followed by culture of UMR-106 and MC3T3-E1 osteoblasts on the libraries. Surface features produced at a specific composition and process temperature range were discovered to enhance dramatically alkaline phosphatase expression in both cell lines, not previously observed for osteoblasts on polymer blends.

3T3 Cells↗

PubMed2002

Growth pulsations in symmetric dendritic crystallization in thin polymer blend films.

The crystallization of polymeric and metallic materials normally occurs under conditions far from equilibrium, leading to patterns that grow as propagating waves into the surrounding unstable fluid medium. The Mullins-Sekerka instability causes these wave fronts to break up into dendritic arms, and we anticipate that the normal modes of the dendrite tips have a significant influence on pattern growth. To check this possibility, we focus on the dendritic growth of polyethylene oxide in a thin-film geometry. This crystalline polymer is mixed with an amorphous polymer (polymethyl-methacrylate) to "tune" the morphology and clay was added to nucleate the crystallization. The tips of the main dendrite trunks pulsate during growth and the sidebranches, which grow orthogonally to the trunk, pulsate out of phase so that the tip dynamics is governed by a limit cycle. The pulsation period P increases sharply with decreasing film thickness L and then vanishes below a critical value L(c) approximately 80 nm. A change of dendrite morphology accompanies this transition.

Journal Article↗

PubMed2002

Nonequilibrium pattern formation in the crystallization of polymer blend films.

We show that the morphology of polyethylene oxide crystallization in thin films can be tuned to obtain circular spherulites, seaweed and symmetric dendrites, and fractal aggregation forms through the addition of clay particles and the amorphous polymer, polymethyl methacrylate. The thin-film polymer crystallization patterns are compared to a two-dimensional phase field model of dendritic growth in Ni/Cu alloys with a variable surface tension anisotropy epsilon. Some aspects of polymer crystallization patterns can be understood from the phase field calculations, but a more general model is required to describe the full range of observed patterns.

Anisotropy↗

PubMed2002

Co-extrusion of biocompatible polymers for scaffolds with co-continuous morphology.

A methodology for the preparation of porous scaffolds for tissue engineering using co-extrusion is presented. Poly(epsilon-caprolactone) is blended with poly(ethylene oxide) in a twinscrew extruder to form a two-phase material with micron-sized domains. Selective dissolution of the poly(ethylene oxide) with water results in a porous material. A range of blend volume fractions results in co-continuous networks of polymer and void spaces. Annealing studies demonstrate that the characteristic pore size may be increased to larger than 100 microm. The mechanical properties of the scaffolds are characterized by a compressive modulus on the order of 1 MPa at low strains but displaying a marked strain-dependence. The results of osteoblast seeding suggest it is possible to use co-extrusion to prepare polymer scaffolds without the introduction of toxic contaminants. Polymer co-extrusion is amenable to both laboratory- and industrial-scale production of scaffolds for tissue engineering and only requires rheological characterization of the blend components. This method leads to scaffolds that have continuous void space and controlled characteristic length scales without the use of potentially toxic organic solvents.

Algorithms↗