Adhesion and motility of fouling diatoms on a silicone elastomer.

Recent demands for non-toxic antifouling technologies have led to increased interest in coatings based on silicone elastomers that 'release' macrofouling organisms when hydrodynamic conditions are sufficiently robust. However, these types of coatings accumulate diatom slimes, which are not released even from vessels operating at high speeds (>30 knots). In this study, adhesion strength and motility of three common fouling diatoms (Amphora coffeaeformis var. perpusilla (Grunow) Cleve, Craspedostauros australis Cox and Navicula perminuta Grunow) were measured on a poly-dimethylsiloxane elastomer (PDMSE) and acid-washed glass. Adhesion of the three species was stronger to PDMSE than to glass but the adhesion strengths varied. The wall shear stress required to remove 50% of cells from PDMSE was 17 Pa for Craspedostauros, 24 Pa for Amphora and >>53 Pa for Navicula; the corresponding values for glass were 3, 10 and 25 Pa. In contrast, the motility of the three species showed little or no correlation between the two surfaces. Craspedostauros moved equally well on glass and PDMSE, Amphora moved more on glass initially before movement ceased and Navicula moved more on PDMSE before movement ceased. The results show that fouling diatoms adhere more strongly to a hydrophobic PDMSE surface, and this feature may contribute to their successful colonization of low surface energy, foul-release coatings. The results also indicate that diatom motility is not related to adhesion strength, and motility does not appear to be a useful indicator of surface preference by diatoms.

Novel sol-gel bioactive fibers.

Bioactive fibers were produced using a sol-gel method. The rheological properties of two different sol compositions prepared from a mixture of TEOS, phosphorous alkoxide and calcium nitrate, or calcium chloride in a water-ethanol solution, are reported. The sols were extruded through a spinneret to produce continuous 10 microm-diameter fibers. Discontinuous fibers and fibrous mats were prepared by air-spraying the multicomponent sols. The sol-gel fibers were converted to the bioactive fibers by three different thermal treatments at either 600 degrees, 700 degrees, or 900 degrees C for 3 h. SEM, BET, EDX, and FTIR were used to characterize the morphology and structure of the fibers. The BET measured surface area of the fibers sintered at 900 degrees C was 0 m(2)/gm compared to a value of 200 m(2)/gm for a typical sol-gel-derived particle of similar composition. Both the continuous and discontinuous fibers exhibited in vitro bioactivity in a simulated body fluid.

A sol-gel derived bioactive fibrous mesh.

Nonwoven sheets of bioactive fibers were produced using a sol-gel process. A high velocity spray process was used to prepare fibers of two compositions in the SiO(2)-CaO-P(2)O(5) ternary system. Both discontinuous fibers and dispersed fibers were evaluated. Viscosity and pH of the sol were the two primary processing variables studied. The formation of hydroxy carbonate apatite (HCA) on the surface of the fibers was used to evaluate the kinetics of the bioactivity in a simulated body fluid (SBF). Diffuse reflection infrared fourier transform spectroscopic (DRIFTS) analysis confirmed the presence of HCA (P-O). A homogenous layer of HCA, as observed with SEM (scanning electron microscopy), typically formed after 3-h immersion in the SBF. The concentration of HCA formed was greater for samples richer in silica. The new bioactive fiber sheets produced by this process are chemically more stable than powders or monoliths prepared from similar precursors. Potential applications are as scaffold for both mineralized and nonmineralized structural tissues.

Cyclic anhydride ring opening reactions: theory and application.

The development of a zero net shrinkage dental restorative material based upon a polymer-bioactive-glass composite requires a second-phase material that expands. This study details the mechanisms of organic cyclic anhydride ring expansion via hydrolysis. Six cyclic anhydrides were used to represent potential side groups, each of which could be an expanding phase or component. Maleic, 4META, tetrahydrophthalic, norbornene, itaconic, and succinic anhydrides were modeled using the Austin method (AM1), a semi-empirical molecular orbital method. The reaction pathways were determined for the anhydride ring opening reaction to form an acid for each case. The activation barriers (Ea) for the ring openings were found from the transition state geometries wherein only one imaginary eigen value in the vibration spectrum existed (a true saddle point). In each case the reaction pathway included the hydrogen bonding of a H2O molecule to the ring, weakening of the C-O bridging bonds of the ring, and, finally, the dissociation of the H2O, forming two carboxyl groups and opening the ring. The activation for the ring openings are +34.3, +36.9, +40.6, +43.1, +45.9, and +47.7 kcal/mol, respectively. The volumetric expansion of the anhydrides was estimated based upon the dilation of C-O-C atomic distances. The dimensional change was found to be 24.0%, 24.0%, 19.1%, 20.3%, 20.8%, and 17.9% for the anhydride rings, respectively. Finally, it was found that a linear correlation exists between the cyclic anhydride C-O asymmetric rocking (as-v) vibration and the activation energy (Ea) for hydrolysis to an acid. This may be used as an experimental indicator of a cyclic anhydride's activity.

Molecular orbital models of ring expansion mechanisms in the silica-carbon monoxide system.

The development of a zero net shrinkage dental restorative material based upon a polymer-bioactive glass composite requires a second-phase material that expands. This study details the mechanisms of silica ring expansion by reaction with carbon monoxide. Carbon monoxide was used as a model adduct to represent potentially active sites on the polymer phase of the dental restorative. Silica rings were used to model the bioactive-glass phase of the composite. The 3-, 4-, 5-, and 6-"member" silica rings have been modeled using the Austin Method (AM1) semi-empirical molecular orbital calculations. The reaction pathways were determined for carbon monoxide (CO) reaction addition to each of the rings. The activation barriers (Ea) for the ring expansions were determined from the transition state geometries wherein only one imaginary eigenvalue in the vibration spectrum existed (a true saddle point). In each case the reaction pathway included the hydrogen bonding of CO with a silicon, exothermic pentacoordinate bonding to silicon by the CO and weakening of the Si-O bridging bonds of the ring, and, finally, the incorporation of CO into the ring, forming a silica-carbonate ring. The activation for the ring expansions are +4.3, +6.1, +7.0, and -2.9 Kcal/mol for 3-, 4-, 5-, and 6-"member" silica rings, respectively. The volumetric expansion of the silica was estimated based upon the dilation of adjacent silicon-silicon atomic distances. The dimensional change was calculated to be 3.9%, 21.3%, 19.4%, and 24.2% for 3-, 4-, 5-, and 6-membered silica-carbonate rings, respectively.