Biomedical subjects

Michael F Butler

Publications and source records attributed to Michael F Butler.

6 recordsLinked to original sources

PubMed2006

Effect of block length, polydispersity, and salt concentration on PEO-PDEAMA block copolymer structures in dilute solution.

A series of poly(ethylene oxide)-block-poly(N,N-diethylaminoethyl methacrylate) (PEO-PDEAMA) block copolymers with relatively high polydispersity (1.36 < PDI < 1.96) have been prepared to determine the effect that polydispersity has on the self-assembly of amphiphilic block copolymers in dilute solution. Because monodisperse macroinitiators were used for the ATRP reactions, the polydispersity resides within the hydrophobic block. By adjusting the relative block lengths, spherical micelles, wormlike micelles, vesicles, or a precipitate is formed. Here, we show that relatively high polydispersity in the block copolymer does not preclude efficient self-assembly. We also discuss the effect of increasing the concentration of NaCl in the systems and show that this can result in a shift from one morphology to another. These shifts are reversible in some cases, but for PEO12-PDEAMA39, this method allows access to giant vesicles of between 500 nm and 1 microm in diameter.

Journal Article↗

PubMed2006

Swelling and mechanical properties of biopolymer hydrogels containing chitosan and bovine serum albumin.

Experimental and theoretical investigations of the swelling and mechanical properties of hydrogels formed from chitosan, bovine serum albumin (BSA), and chitosan/BSA mixtures cross-linked with genipin were performed. The properties of cross-linked chitosan hydrogels were explained in terms of its polyelectrolyte behavior, which led to a gradual increase in swelling ratio below the pK value, but whereby its swelling ability was eliminated by the presence of salt that screened the charges. Comparison of theoretical and experimental calculations of the swelling ratio, however, indicated that complications arising from wastage of cross-links, and formation of polymerized genipin cross-links must be considered before quantitative prediction can be achieved. Cross-linked BSA hydrogels swelled even in the presence of salt, and a marked increase in swelling was observed below pH = 3 that was explained as the result of an acid induced denaturation of the protein that led to unfolding of the molecule. Swollen BSA hydrogels were mechanically weak, however. Composite gels made from a cross-linked mixture of chitosan and BSA exhibited the swelling behavior of BSA combined with the mechanical properties of chitosan and were therefore considered most suitable for use in a gastric environment.

Animals↗

PubMed2005

Aligned two- and three-dimensional structures by directional freezing of polymers and nanoparticles.

The preparation of materials with aligned porosity in the micrometre range is of technological importance for a wide range of applications in organic electronics, microfluidics, molecular filtration and biomaterials. Here, we demonstrate a generic method for the preparation of aligned materials using polymers, nanoparticles or mixtures of these components as building blocks. Directional freezing is used to align the structural elements, either in the form of three-dimensional porous structures or as two-dimensional oriented surface patterns. This simple technique can be used to generate a diverse array of complex structures such as polymer-inorganic nanocomposites, aligned gold microwires and microwire networks, porous composite microfibres and biaxially aligned composite networks. The process does not involve any chemical reaction, thus avoiding potential complications associated with by-products or purification procedures.

Anisotropy↗

PubMedDate not supplied

Mechanism and kinetics of phase separation in a gelatin/maltodextrin mixture studied by small-angle light scattering.

Phase separation mechanisms and kinetics were studied using small-angle light scattering in a gelatin/maltodextrin system where phase separation could be studied in both liquid and gelled states. Nucleation and growth or spinodal decomposition occurred, depending on the quench depth. The transition between the two mechanisms occurred relatively sharply. The different mechanisms were distinguishable by the different behavior of the scattering function even though a peak was observed in both cases. Particular differences were the different evolution of the peak intensity and position, the absence of dynamic scaling of the nucleation and growth scattering function, and the final coarsening exponent of 1/3 that was measured when spinodal decomposition occurred but not for nucleation and growth. Gelation severely reduced the coarsening rate and initially placed the phase compositions far from their equilibrium values. Despite the loss of molecular mobility caused by gelation, the gelled systems did continue to evolve, albeit much more slowly than in the liquid case. Multiple coarsening rates were observed for some of the gelled samples, which were ascribed to the gradual movement of these systems toward the equilibrium compositions.

Gelatin↗

PubMedDate not supplied

Phase separation in a sheared gelatin/maltodextrin mixture studied by small-angle light scattering.

The influence of shear on the structure of a gelatin/maltodextrin mixture was investigated using small-angle light scattering both during phase separation and after phase separation was allowed to occur quiescently. In all cases, phase separation occurred via spinodal decomposition to form a droplet morphology, and a characteristic length scale was formed in the structure that was prevalent during shear, as well as in quiescent conditions. Below the critical shear rate for droplet breakup, shear accelerated the coarsening rate of the droplets. A transient regime of rapid hydrodynamic coarsening was present when shear was initiated after phase separation and at late times in all cases once the droplets attained a certain size. At the critical shear rate for droplet breakup (1 s(-1)), the rapid repetition of breakup and coarsening was postulated to occur, which enabled a microstructure consisting of elongated droplets with a narrow size distribution to form. When the shear rate enabled droplets to extend to such an extent that a percolated structure could form (10 s(-1)), then the structure was relatively stable and changed very slowly over time. At very high shear rates (100 s(-1)), droplet breakup was suppressed and a highly fibrillar morphology formed that was stable only while the system was under shear. Cessation of shear at high rates led to fiber breakup and the formation of many small droplets. For a given shear rate, the final microstructure appeared to be independent of the time that shear was started when the structure consisted of discrete droplets or fibers. When a percolated structure could form, however, the shear history appeared to be important.

Gelatin↗

PubMedDate not supplied

"Delayed" phase separation in a gelatin/dextran mixture studied by small-angle light scattering, turbidity, confocal laser scanning microscopy, and polarimetry.

Small-angle light scattering, turbidity, and confocal laser scanning microscopy were used to study microstructure formation and evolution in a gelatin/dextran mixture. There was a time-delay of up to tens of minutes between reaching the quench temperature and the onset of phase separation, because demixing only occurred once a certain amount of ordering of the gelatin molecules, measured by polarimetry, was attained. The accompanying phenomenon of gelation retarded the development of the microstructure to different extents, depending on the quench temperature. At low temperatures, the structure was rapidly trapped in a nonequilibrium state with diffuse interfaces, characteristic of the early and intermediate stages of phase separation. At higher temperatures, coarsening continued for a certain amount of time before the structure was trapped. The duration of the coarsening period increased with increasing temperature and the interface between the phases became sharp, characteristic of the late stages of phase separation. Because the ordering process continued after the target quench temperature was reached, the effective quench depth continued to increase after the initial phase separation. At high quench temperatures, the system was able to respond to the thermodynamic requirements of the increasing effective quench depth by undergoing secondary phase separation to form a droplet morphology within the preexisting bicontinuous one.

Dextrans↗