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 28,634 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Poly(L-lactic acid) (PLLA) monofilaments were evaluated for use as intravascular polymeric stents. The PLLA monofilaments were extruded and drawn to different draw ratios. They were then subjected to different thermal treatments and their mechanical properties characterized. Stents constructed with similar monofilaments were tested under hydrostatic pressure, and the results correlated with the properties of the monofilaments. Stent collapse pressure was a decreasing function of stent diameter and filament draw ratio.
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PMID:Evaluation of poly(L-lactic acid) as a material for intravascular polymeric stents. 156 42

Poly(N-isopropylacrylamide) (NIPAAm)/polytetramethylene ether glycol (PTMEG) interpenetrating polymer networks (IPNs) were synthesized and their feasibility as thermosensitive hydrogels for drug release was investigated. The release of indomethacin incorporated into these matrices showed pulsatile patterns in response to temperature changes and was sensitive to a few degrees of temperature fluctuation. The temperature inducing on-off release deviated from the gel collapse temperature of unloaded gel, possibly because of solute effects on network properties. The lag time and release profile of indomethacin in the low-temperature region (on process) of each temperature cycle were affected by the gel composition and applied temperature. The results of this study demonstrate that solute release can be regulated by rapid deswelling of the surface of the gels in response to temperature.
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PMID:"On-off" thermocontrol of solute transport. II. Solute release from thermosensitive hydrogels. 186 77

Poly(dl-lactide) (PLA) microspheres containing quinidine or quinidine sulphate were prepared by the emulsification-solvent evaporation technique. The in vitro release profile of quinidine or quinidine sulphate from the microspheres was characterized by three phases: a lag time, a rapid release phase (burst), and a slow release phase. Drug release was studied as a function of the ionic strength of the dissolution medium, to demonstrate the importance of the water imbition into the microspheres which induced the drug release. The lag time increased with increasing ionic strength. The microspheres stayed intact during the dissolution study as shown by scanning electron microscopy (SEM). Disintegration of microspheres which was initially observed was an artifact introduced during the SEM procedure. The high vacuum applied either during the coating of the microspheres with gold-palladium or during the actual observation in the scanning electron microscope caused the microspheres to collapse or rupture.
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PMID:Polylactic acid microspheres containing quinidine base and quinidine sulphate prepared by the solvent evaporation method. III. Morphology of the microspheres during dissolution studies. 323 52

Poly(oxyethylene)-poly(oxypropylene) (PEO-PPO) co-polymers have been used as surfactants to produce resorbable poly(DL-lactide co-glycolide) (PLG) microspheres in the 500 nm-1 micron size range by an emulsification/solvent evaporation technique based on acetone-dichloromethane mixtures. The high polydispersity of microspheres could be reduced by using low PLG concentrations of 1% (w/v). Surface analysis by static secondary ion mass spectroscopy revealed the presence of PEO-PPO at the microsphere surface after cleaning by centrifuging and resuspension in water, and after further cleaning by dialysis. Physical entrapment of PEO-PPO chains in the particle surface is indicated due to rapid collapse of the solvent swollen PLG network as acetone is extracted from the suspended droplets. Opportunities are presented for simultaneous manufacture and surface modification of microspheres.
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PMID:Resorbable polymeric microspheres for drug delivery--production and simultaneous surface modification using PEO-PPO surfactants. 794 89

Poly(N-isopropyl acrylamide) (PIPAAm) demonstrated a fully expanded chain conformation below 32 degrees C and a collapsed, compact conformation at high temperatures. This unique temperature responsive polymer was grafted onto surfaces of commercial polystyrene dishes and used as temperature switches for creating hydrophilic surfaces below 32 degrees C and hydrophobic surfaces above 32 degrees C. Cell attachment and the growth of bovine endothelial cells and rat hepatocytes on PIPAAm-grafted surfaces at 37 degrees C demonstrated similar behavior to the commercialized culture dishes. Both cell types were observed to detach from the PIPAAm-grafted surface simply by reducing the temperature below the polymer transition temperature (collapse). Cells recovered by this method maintained substrate adhesivity, growth, and secretion activities nearly identical to those found in primary cultured cells in contrast to the compromised function found in cultured cells damaged by trypsinization. These results provide strong evidence that PIPAAm-grafted surfaces, as thermal switches are very effective for reversing cell attachment and detachment without cell damage. Properties of cell culture surfaces can be readily transformed by this technique reversibly into hydrophilic and hydrophobic coatings of PIPAAm-grafted polymers.
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PMID:A novel recovery system for cultured cells using plasma-treated polystyrene dishes grafted with poly(N-isopropylacrylamide). 824 39

Poly(D,L-lactide-co-glycolide) (PLG, 90:10) microspheres containing 20% norethisterone (NET) were prepared by solvent evaporation method. Microspheres in the size ranges of 65 to 100 microns were sterilized by irradiation and used for further study. In vitro release showed fairly constant release of NET from the above microspheres over more than 90 days. However, in vivo drug release determined by residual NET analysis after i.m. injection in rats indicated a faster release rate. About 95% of NET was released in a period of 45 days. At the dose of 80 mg of microspheres, vaginal estrus cycles were inhibited for 45 days compared to 27 days for the same dose of NET crystals in rats. Biodegradation of the microspheres was tested by direct measurement of molecular weight losses and SEM observation of morphological changes of the microspheres, which showed continuous erosion in the internal matrix of microspheres with the decrease of molecular weight of PLG until total collapse of microspheres, and biodegradation was faster in rats than in human serum at 37 degrees C in vitro. Total degradation of 90:10 PLG microspheres was less than 7 months in rats and more than 9 months in vitro.
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PMID:Evaluation of 90:10 poly(D,L-lactide-co-glycolide) microspheres containing norethisterone: drug release and biodegradation. 826 May 76

Poly(dI-dC).poly(dI-dC) was studied using vibrational circular dichroism and IR spectroscopy in both the base deformation C = O and symmetric PO2- stretching regions. VCD spectra of this duplex under low salt conditions are consistent with its having a B-form structure. Addition of 5 M NaCl leads to relatively uniform VCD intensity loss which is consistent with loss of helical structure rather than formation of an intermediate state between the B and Z forms. This duplex polymer under high salt conditions with added NiCl2 shows aggregation effects, but its IR and VCD spectra have characteristic features of the Z-form DNA conformation. The cooperative change of backbone and base pair structure upon thermal denaturation is indicated by the simultaneous collapse of the VCD at 65 degrees C in both the PO2- and C = O stretching regions. This study further demonstrates that the VCD bandshape of a specific localized nucleic acid vibrational transition can be a useful indicator of the helical handedness. The empirical conformational interpretations are supported by simulated VCD spectra, which are in excellent agreement with the experimental results, based on dipole coupling calculations.
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PMID:Helical nature of poly(dI-dC).poly(dI-dC). Vibrational circular dichroism results. 837 87

Poly-glutamic acid, a polyanion, has been shown to induce the all-or-none collapse transition in individual duplex DNAs from an elongated coil into a collapsed compact state, through the observation of single chains by fluorescence microscopy. This experimental finding is discussed in relation to the change in the higher-order structure of DNAs in the cytoplasmic space induced by a change in the concentration of polyanions, such as mRNA, as a possible route for feedback in gene expression.
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PMID:Single chain observation on collapse transition in giant DNA induced by negatively-charged polymer. 944 14

Poly(DL-lactic-co-glycolic acid) (PLGA) foams are an osteoconductive support that holds promise for the development of bone tissue in vitro and implantation into orthopedic defects. Because it is desirable that foams maintain their shape and size, we examined a variety of foams cultured in vitro with osteoblastic cells. Foams were prepared with different porosities and pore sizes by the method of solvent casting/porogen leaching using 80, 85, and 90 wt% NaCl sieved with particle sizes of 150-300 and 300-500 microm and characterized by mercury intrusion porosimetry. Foams seeded with cells were found to have volumes after 7 days in static culture that decreased with increasing porosity: the least porous exhibited no change in volume while the most porous foams decreased by 39 +/- 10%. In addition, a correlation was observed between decreasing foam volume after 7 days in culture and decreasing internal surface area of the foams prior to seeding. Furthermore, foams prepared with the 300-500 microm porogen had lower porosities, greater mean wall thicknesses between adjacent pores, and larger volumes after 7 days in culture than those prepared with the smaller porogen. Two culture conditions for maintaining cells, static and agitated (in a rotary vessel), were found to have similar influences on foam size, cell density, and osteoblastic function for 7 and 14 days in culture. Finally, we examined unseeded foams in aqueous solutions of pH 3.0, 5.0, and 7.4 and found no significant decrease in foam size with degradation. This study demonstrates that adherent osteoblastic cells may collapse very porous PLGA foams prepared by solvent casting/particulate leaching: a potentially undesirable property for repair of orthopedic defects.
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PMID:Effect of osteoblastic culture conditions on the structure of poly(DL-lactic-co-glycolic acid) foam scaffolds. 1058 98

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles dispersed in water have been used as a matrix for the polymerization of a hydrophobic monomer, N-methylpyrrole (MPy). The presence of poly(MPy) (PMPy) within the dried composite particles has been confirmed using electron paramagnetic resonance (EPR) measurements which show a characteristic free-radical signal at g=2.007. Electron microscopy data (TEM) show that the composite PNIPAM-xPMPy particles have a "raspberry-like" morphology. (The value for x represents the volume percent of MPy added during synthesis with respect to the total microgel volume.) PCS data indicate that PMPy occupies the majority of the collapsed composite particle volume. The maximum value of x consistent with colloid stability for PNIPAM-xPMPy dispersions is 4.5%. Higher values of x result in coagulation due to interparticle bridging by PMPy. Variable temperature PCS measurements of the PNIPAM-xPMPy dispersions have been used to study the thermally induced collapse of the composite particles. The extent of collapse becomes less with increasing values for x. The embedded PMPy particles restrict the extent of PNIPAM network contraction. The stability of the PNIPAM-4.5PMPy dispersions was investigated by means of turbidity measurements using aqueous 0.1 M NaCl solution. The upper critical flocculation temperatures (UCFT) for PNIPAM and PNIPAM-4.5PMPy dispersions were identical (38-39 degrees C). The flocculation observed was found to be fully reversible. The composite dispersion stability in the absence of salt was attributed to electrosteric stabilization afforded by the PNIPAM matrix. These results indicate that PNIPAM microgel particles may have application as a matrix for the polymerization of hydrophobic monomers in water. Copyright 2000 Academic Press.
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PMID:Microgel Particles as a Matrix for Polymerization: A Study of Poly(N-isopropylacrylamide)-Poly(N-methylpyrrole) Dispersions. 1065 27


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