bluuebunny/crossref_metadata_2025 huggingface dataset:
Degradation rates of oral resorbable implants (polylactates and polyglycolates): Rate modification with changes in PLA/PGA copolymer ratios
Authors: Miller Robert A. , Brady John M. , Cutright Duane E.
Published: December 2004

This study determined the difference in rate of degradation between pure polymers of lactic acid (PLA), glycolic acid (PGA), and various ratios of copolymers of these two substances. Fast‐cured and slow‐cured polyglycolide was compared with copolymers of glycolide/lactide intermixed in ratios of 75:25, 50:50, and 25:75, as well as pure polylactide. A total of 420 rats were implanted with carbon‐14 and tritium‐labeled polymers in bone and soft tissue. At intervals of 1, 2, 3, 5, 7, 9, and 11 months, groups of five animals with the implants in bone and five with the implants in the abdominal wall were sacrificed. The implant area as well as tissue from the liver, spleen, kidney, lung and some muscle tissue was analyzed for radioactivity along with the urine and feces collected throughout the experiment. Half‐lives of the different polymers and copolymers were calculated from the radioactivity present in the implant area for each time interval. Half‐life of the polymers and copolymers decreased from 5 months for 100% PGA to 1 week with 50:50 PGA:PLA copolymer and rapidly increased to 6.1 months for 100% PLA. Fast‐cured PGA had a half‐life in tissue of 0.85 months. No difference in rate of degradation was seen in soft tissue or bone. No significant radioactivity was detected in urine, feces, or tissue samples. From this study, it is concluded that control of degradation rate of the implant could best be attained by varying the composition of PLA and PGA between 75% and 100% PLA along with a corresponding 25% to 0% PGA. This would provide a half‐life range of the implant of from 2 weeks to 6 months.
The appearances of wear on polyethylene—a comparison of in vivo and in vitro wear surfaces
Authors: Rostoker W. , Chao E. Y. S. , Galante J. O.
Published: December 2004

An examination has been made of the wear surfaces of 21 ultra‐high‐molecular‐weight polyethylene components of hip and knee joint prostheses removed from patients. Both light optics and scanning electron microscope systems of examination have been used. The appearances are compared with specimens subjected to two different in vitro wear tests. Surface appearances related to the wear experience have been classified into seven modes. At least four of these are evidences of what may be four different wear mechanisms. Entrapment of bone‐cement particles between articulating surfaces is fundamentally extraneous but is seen to generate the most severe wear. In the absence of bone‐cement particles, in vivo, wear seems to be mild. Wear in vitro produces similar surface modes but with different relative emphasis.
Evaluation of sunlight stability of polyurethane elastomers for maxillofacial use. I
Authors: Chu C. C. , Fischer T. E.
Published: December 2004

The evaluation of the efficiency of polymer additives with special emphasis on uv absorbers and antioxidants in polyurethane elastomers has been completed. Aliphatic polyurethanes were chosen for this study because their properties closely relate to the requirements of maxillofacial prosthesis. The polyurethane elastomers were either synthesized by ourselves or formulated according to the manufacturer's recommendation. Eleven different types of uv absorbers, coupled with one antioxidant, were incorporated into the polyurethane systems. The Atlas twin‐lamp carbon arc Weatherometer was used as the source of uv. The samples were periodically withdrawn for examination of yellowing and tackiness. It was found that, although the incorporation of uv stabilizers enhanced the uv resistance of polyurethanes, the problem of tackiness resulting from uv aging was not solved satiafactorily. The phenomenon of yellowing, however, was significantly improved, mainly due to the aliphatic structure of polyurethanes. The most promising uv absorbers are Tinuvin 770 and the combination of Tinuvin 328, ZnO, and an antioxidant. Their effectiveness in other polyurethane systems is not known and further research is underway to explore this field. Hopefully, these findings will greatly assist the successful application of polyurethane elastomers in maxillofacial prosthesis.
Hydraulic permeability of hollow‐fiber membranes
Authors: Stevenson James F. , Parry John S. , Gupta Krishna M.
Published: December 2004

Two experimental methods have been developed to characterize the pressure‐driven flow of a liquid through the walls of single hollow‐fiber membranes. In one method, the permeation rate is measured directly by following the air–water interface in a pressurized pipet connected to a hollow fiber which is sealed at one end. With the second method, the permeation rate is determined by tracking the descent of an air bubble inside a pressurized hollow fiber sealed at one end. Results obtained by the two experimental methods are in good agreement. Darcy's law is used to analyze the experimental data because the Darcy permeability constant, k, is an intrinsic property of the material. The membrane dimensions and the Darcy permeability constant for reconstituted collagen hollow fibers are shown to be quite sensitive to hydrogen ion concentration and ionic strength. A conceptual link between the Darcy permeability constant and the membrane microstructure is obtained by combining the aligned‐rod structural model for reconstituted collagen proposed by Kramer with a hydrodynamic calculation due to Happel and electrical double‐layer theory. On the average, the predictions of this model for volume fraction solids and microstructure dimensions are in reasonable agreement with experimental observations.
Endothelial cell culture on Dacron fabrics of different configurations
Authors: Eskin S. G. , Trevino L. , Chimoskey J. E.
Published: December 2004

The growth of tissue‐cultured aortic endothelial cells from the calf using 12 different configurations of Dacron polyester (U.S. Catheter and Instrument Co.) as substrates was studied. Scanning electron microscopy showed maximum cell coverage on tightly knit configurations, whereas loose knits and velours did not support cell growth.
Histological evaluation of porous titanium cardiac pacemaker electrode tips
Authors: Hirshorn M. S. , Holley L. K. , Money D. K. , Spector M. , Young F. A. , Hales J. R. S.
Published: December 2004

An investigation of the tissue reaction to five porous titanium alloy pacemaker electrode tips inserted in sheep hearts was made. Attachment of the electrode tips to the endocardial wall was facilitated by fibrous tissue ingrowth into the porous material. Vascularized fibrocollagenous tissue was found within the pores of the tips. One tip was found to be infected. The presence of occasional multinucleated foreign‐body giant cells within the porosity was related to the particulate features of the porous tip. The electrode tips had a reduced sensing impedance compared with conventional solid platinum tips. The present findings are consistent with previous observations relating to porous cobalt‐chromium alloy and porous platinum‐iridium alloy tips and suggest that porous titanium alloy may be a suitable pacemaker electrode tip material.
The interface of permucosal dense apatite ceramic implants in humans
Authors: Denissen H. W. , Veldhuis A. A. H. , Jansen H. W. B. , van den Hooff A.
Published: December 2004

Dense apatite ceramic implants inserted into fresh extraction sockets of teeth and submerged under full lower dentures may develop permucosal dehiscences because of vertical alveolar ridge resorption. In a 5‐year clinical follow‐up study, 16 out of 81 of such implants showed this dehiscence, offering the opportunity to study histologically the reaction of the mucosal tissue and the alveolar bone to dense apatite ceramic implants in humans. The histologic findings indicate that there is no bonding between mucosa and the permucosal part of the dense apatite ceramic implant under full lower dentures in humans. On the other hand, a very intimate bonding exists between alveolar bone and the dense apatite ceramic material.
Cell responses to biomaterials II: Endothelial cell adhesion and growth on perfluorosulfonic acid
Authors: McAuslan B. R. , Johnson G. , Hannan G. N. , Norris W. D. , Exner T.
Published: December 2004

We report here the use of perfluorosulfonic acid (Nafion) as a substratum for the growth of bovine aortal endothelial cells. This support which can be generated in a number of forms is at least as efficient in maintaining the growth of endothelial and other cell types as tissue culture grade polystyrene (TCP) and represents and advance in this regard over polytetrafluoroethylene (Teflon). The mechanism underlying the different cell attachment capacities of these three polymers is not readily related to their different protein binding patterns. While Nafion adsorbs more total protein from serum than Teflon or TCP, it adsorbs relatively less of the major cell adhesive proteins, vitronectin and fibronectin, than does Teflon. Both Nafion and Teflon had comparable but low thrombogenic potential by in vitro tests. Teflon or expanded Teflon (Gore‐tex) coated with a thin film of Nafion assumes the cell supportive characteristics of Nafion and hence the modification of these surfaces by the induction of a stable bond between Teflon (in various forms) and Nafion may provide a composite vascular graft material which has all the desirable qualities of both materials.
The effect of phase differences on the time‐dependent variation of the zeta potential of hydroxyapatite
Authors: Ducheyne P. , Kim C. S. , Pollack S. R.
Published: December 2004

The osteoconductive nature of calcium phosphate ceramics (CPC) follows from several proven effects, such as a direct bone attachment and enhanced bone tissue formation. Mechanisms leading to these phenomena are still largely undiscovered. Specifically, little is known about the CPC surface and cell‐driven reactions. These atomic and molecular level events are involved in tissue attachment and enhanced tissue formation. It is hypothesized that the zeta potential of these ceramics is directly related to the surface reactivity governing osteoconductivity. As a first step in our analysis, the zeta potential of stoichiometric and Ca‐deficient hydroxyapatite was determined as a function of immersion time. It is concluded that, under the conditions of the experiment, the observations support the hypothesis in a dual way. First, the absolute values of the zeta potential which were measured are related to electrokinetic potentials known to cause substantial effect on the cellular activities and bone tissue formation when applied exogenously. Second, the magnitude and duration of the changes in zeta potential are related to an ion exchange between the hydrated layer around the ceramic and the ceramic surface, and a net precipitation of new material. If these findings could be confirmed in other solutions, i.e., solutions with a substantially equivalent composition as the fluids in developing bone tissue, a basis would be provided to explain the bridging of the ceramic surface with the surrounding developing tissue.
Differences in ceramic–bone interface between surface‐active ceramics and resorbable ceramics: A study by scanning and transmission electron microscopy
Authors: Neo M. , Kotani S. , Fujita Y. , Nakamura T. , Yamamuro T. , Bando Y. , Ohtsuki C. , Kokubo T.
Published: December 2004

The interface between bioactive ceramics and bone was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The materials were apatite‐wollastonite‐containing glass ceramic (A‐W · GC) as a representative surface‐active ceramic, and calcite and p‐tricalcium phosphate (p‐TCP) as resorbable ceramics. Particles of these materials, ranging between about 100 pm and 300 pm in diameter, were implanted into rat tibiae, and specimens were prepared for observation at 8 weeks after implantation. Both SEM and TEM demonstrated that A‐W · GC was bonded t o bone through a thin Ca‐P‐rich layer consisting of fine apatite crystals apparently different from those of bone in shape, size, and orientation. Collagen fibers of the bone reached the surface of this layer, and chemical bonding between A‐W · GC and the bone was speculated. Calcite and p‐ TCP, on the other hand, made direct contact with the bone, and no apatite layer was present at the interface. The surfaces of the implants became rough due to degradation, and bone grew into the finest surface irregularities. However, we were unable to demonstrate any continuity of crystals between the resorbable implants and bone by high‐resolution TEM. Ac‐ cordingly, the bonding strength was considered to be mainly attributable to mechanical interlocking.
Effect of surface processing on the attachment, orientation, and proliferation of human gingival fibroblasts on titanium
Authors: Köunönen M. , Hormia M. , Kivilahti J. , Hautaniemi J. , Thesleff I.
Published: December 2004

The adhesion, orientation, and proliferation of human gingival fibroblasts was studied on electropolished (elpTi), etched etchTi, and sandblasted (sblTi) titanium surfaces. The texture, chemical state, and composition of the titanium surfaces were analyzed using a surface tracing instrument and electron spectroscopy for chemical analysis. Considerable differences were evident in the surface texture and chemical composition of the differently treated titanium plates. Electropolishing produced the smoothest and cleanest surface. Human gingival fibroblasts attached, spread, and proliferated on all titanium surfaces. However, cells on elpTi exhibited an extremely flat morphology and seemed to form cellular bridges with adjacent cells, whereas the etchTi and sblTi surfaces harbored both round and flat cells with many long processes. Cells on elpTi appeared to grow in thick layers with no specific orientation, whereas on etchTi surfaces they were migrating along the parallel, irregular minor grooves caused by mechanical polishing, and on sblTi surfaces they seemed to grow in clusters. Stress‐fiber type actin bundles and vinculin‐containing focal adhesions were present in cells spreading on elpTi and etchTi surfaces but not in cells spreading on sblTi surfaces. Cell shape, orientation, and proliferation appear to depend on the texture of the titanium surface and probably also on the properties of the oxide layer and adjacent bulk material. Our findings suggest that smooth or finely grooved titanium surfaces could be optimal in implants adjacent to soft tissues as they support the attachment and growth of human gingival fibroblasts. © 1992 John Wiley & Sons, Inc.
Toxic effect of ethylene‐oxide‐sterilized freeze‐dried bone allograft on human gingival fibroblasts
Authors: Kudryk Val L. , Scheidt Michael J. , McQuade Michael J. , Sutherland Donald E. , Vandyke Thomas E. , Hollinger Jeffery O.
Published: December 2004

Freeze‐dried bone allograft (FDBA) which either had or had not been sterilized by exposure to ethylene oxide (EtO) prior to lyophilization was obtained from two commercial sources. EtO‐sterilized FDBA was reexposed to EtO as a positive control. Gas chromatograph assays revealed that three out of four commercially obtained EtO sterilized FDBA had no detectable EtO, with one sample having 0.21 parts per million (PPM). Surprisingly, 0.24 PPM was detected in one sample which had not been sterilized with EtO gas. This was presumed due to contamination from a gas‐sterilized rubber stopper. In the cell toxicity study, FDBA and human gingival fibroblasts (HGF) were added simultaneously, incubated for 72 h, and fixed and stained. Samples of FDBA sterilized with EtO which were free of EtO did not alter HGF growth. However, the positive control FDBA which contained 0.72 PPM EtO had a deleterious effect on HGF. FDBA with EtO residuals caused morphologic change in HGF. © 1992 John Wiley & Sons, Inc.
Alteration of hydroxylapatite coatings exposed to chemicals used in histological fixation
Authors: Kieswetter K. , Bauer T. W. , Brown S. A. , Van Lente F. , Merritt K.
Published: December 2004

Histological evaluation of retrieved devices allows for assessment of the biological response to the implants. Interaction between the biomaterial and the fluids used in the processing may alter the appearance or composition of the device. This may result in artifacts due to processing which are misinterpreted as being caused by the biological response. Alteration by the fluids used in fixation may be particularly important with materials that have a propensity to interact with aqueous environments, as in the case of calcium phosphate ceramics. Therefore, we examined the effects of various standard histological fixation techniques on hydroxylapatite coated metal implants. Of the techniques examined, fixation with ethanol appeared to affect the surface morphology the least, whereas fixation in unbuffered formalin appeared to affect the bulk composition of these samples the least. Fixation in buffered formalin resulted in marked changes in both surface morphology and bulk composition. © 1994 John Wiley & Sons, Inc.
Poloxamer 407 as a bacterial abhesive for hydrogel contact lenses
Authors: Portolés Marta , Refojo Miguel F. , Leong Fee‐Lai
Published: December 2004

Bacterial adherence and colonization of biomedical prosthetic implants is one of the main causes for implant withdrawal. The abhesive (anti‐adhesive) effect of poloxamer 407 on several Gram‐positive and Gramnegative strains and the site of action of its effect have been investigated. Poloxamer 407 significantly inhibited 92–99% of Pseudomonas aeruginosa adherence to hydrophilic contact lenses (P <.05). This adherence inhibition was concentration‐dependent. A reduction of about 50–60% was obtained for Staphylococcus strains, and 50–70% for Gram‐negative strains other than Pseudomonas. The poloxamer seems to act on the bacteria surface, but not on the contact lens surface. Poloxamer 407 could potentially prevent implant‐related infections and keratitis associated with contact lens wear, by inhibiting bacterial adherence onto the implant or contact lens surface. © 1994 John Wiley & Sons, Inc.
Mutagenic potential of root canal sealers: Evaluation through Ames testing
Authors: Stea Susanna , Savarino L. , Ciapetti G. , Cenni E. , Stea Stefano , Trotta F. , Morozzi G. , Pizzoferrato A.
Published: December 2004

The mutagenic potential of 12 commercially available dental cements and of two ‘pure substances’ (zinc oxide and eugenol) used in root canal filling were examined. The cements were prepared according to the manufacturers' indications and set for defined times. Ames tests were performed in their extracts by using Salmonella typhimurium strains TA 98 and TA 100. The results showed that most cements present strong bactericidal activity that disappears or decreases remarkably in time. One of the tested cements showed mutagenicity with both Salmonella strains. Two cements yielded doubtful results. The remaining cements and the two ‘pure substances’ showed no mutagenic potential. The authors conclude that it is convenient to examine endodontic cements with the Ames test and to eliminate those that present mutagenicity in time. © 1994 John Wiley & Sons, Inc.
Mechanisms of biomaterial‐induced superoxide release by neutrophils
Authors: Kaplan S. S. , Basford R. E. , Jeong M. H. , Simmons R. L.
Published: December 2004

Biomaterial‐centered infection is an important cause of the failure of prosthetic implants and organs. Because neutrophils mediate host defense against infection, the effect of biomaterials on neutrophil superoxide release and the mechanism of that effect were investigated using three materials commonly employed in surgical practice. The graft materials were expanded polytetrafluorethylene (PTFE), polyurethane and woven dacron. Polystyrene, a commonly used laboratory support vessel, was also studied. Both polystyrene and polyurethane were activating, but serum inhibitable, whereas PTFE was nonactivating, and woven dacron was not activating unless serum was present. The signaling mechanisms used by these materials demonstrated time and material dependency. Pertussis toxin inhibition of G proteindependent activation had little or no effect on biomaterial induced activation, whereas FMLP‐induced activation of the same biomaterial‐associated cells was inhibited. Protein kinase C inhibition with staurosporine greatly inhibited polystyrene‐induced activation, but had only a partial effect with polyurethane and even less effect with the activation associated with serum‐treated woven dacron. These studies demonstrated that biomaterial contact‐induced neutrophil activation differed from that described for cells in suspension, and showed that activation mechanisms on one material cannot be extrapolated to mechanisms on other materials. © 1994 John Wiley & Sons, Inc.
Activation of human neutrophils by Arg‐Gly‐Asp‐Ser immobilized on microspheres
Authors: Kasuya Yuji , Fujimoto Keiji , Miyamoto Masaki , Kawaguchi Haruma
Published: December 2004

The adhesive interaction of cells with extracellular matrix components is essential for a variety of cellular functions, and is frequently mediated by a tetra peptide, Arg‐Gly‐Asp‐Ser (RGDS), located within fibronectin and other proteins. In this study, the RGDS‐mediated activation of polymorphonuclear leukocytes accompanied by phagocytosis was investigated using monodisperse polymeric microspheres carrying RGDS. The parent and Arg‐Gly‐Glu‐Ser (RGES)‐carrying microspheres, which have no adhesion activity, were employed as controls. The ingestion of microspheres into PMN was not enhanced by immobilizing RGDS. However, PMNs exhibited unique oxygen consumption and enhanced liberation of reactive oxygen when RGDS‐carrying microspheres were phagocytosed. These PMN responses disappeared with the addition of soluble RGDS. Furthermore, cytochalasin D, which inhibits actin polymerization, showed a marked inhibitory effect on oxygen consumption in the RGDS‐carrying microsphere system, as compared with those in other systems. These findings show that RGDS‐carrying microspheres induced the biospecific activation of PMNs by the signal transduction via RGDS‐integrin binding without alteration in the degree of phagocytosis. © 1994 John Wiley & Sons, Inc.
Interactions of plasminogen and fibrinogen with model silica glass surfaces: Adsorption from plasma and enzymatic activity studies
Authors: Woodhouse K. A. , Weitz J. I. , Brash J. L.
Published: December 2004

The adsorption of fibrinogen and plasminogen from plasma to silica glass, sulfonated silica glass, and lysinederivatized silica glass has been investigated. The data indicate that the sulfonated material has a high affinity for both fibrinogen and plasminogen, but that the ratio of plasminogen to fibrinogen is greater on the lysinederivatized surface. The adsorption data also suggest plasminogen as a possible contributor to the fibrinogen Vroman effect, whereby initially adsorbed fibrinogen is displaced from the surface. The plasmin activity of plasminogen adsorbed to the lysine‐derivatized silica glass and its sulfonated precursor was assessed by both a chromogenic substrate assay and a radioimmunoassay for the plasmin cleavage product of fibrinogen, the Bβ 1–42 peptide. The data indicate that (1) the adsorbed plasminogen is not inherently plasmin‐like; (2) the enzymatic activity associated with the bound plasminogen is significantly enhanced on both surfaces in the presence of activator; and (3) in the presence of activator, the plasmin activity per mole of bound plasminogen on the lysinized material is approximately a factor of two greater than on the sulfonated material based on the chromogenic substrate assay, and a factor of four greater based on the Bβ 1–42 radioimmunoassay. The lysinized material thus exhibits several properties that are different from its sulfonated precursor. It adsorbs more plasminogen relative to fibrinogen after the Vroman peak, and this adsorbed plasminogen appears to be in a conformation that is more readily activated to plasmin. Once activated, the surface bound plasmin shows enhanced ability to cleave either a low molecular weight chromogenic substrate or a macromolecular substrate. These properties appear to be directly related to lysine residues on the surface and may be the result of specific conformational changes occurring when plasminogen engages its lysine binding sites. © 1994 John Wiley & Sons, Inc.
Internalization of poly(D,L‐1actic acid) nanoparticles by isolated human leukocytes and analysis of plasma proteins adsorbed onto the particles
Authors: Leroux Jean‐Christophe , Gravel Patricia , Balant Luc , Volet Bernard , Anner Beatrice M. , Allémann Eric , Doelker Eric , Gurny Robert
Published: December 2004

The objective of this work was to investigate the interactions of poly(D,L‐lactic acid) nanoparticles prepared by a recently developed salting‐out process, with lymphocytes and monocytes isolated from healthy human donors. Nanoparticles were labeled with a hydrophobic fluorescent dye and incubated with lymphocytes and monocytes, and their uptake was followed by flow cytometry in the presence and absence of plasma. Plasma protein adsorption increased nanoparticle uptake by monocytes, whereas a decrease of cellular binding of the nanoparticles to lymphocytes was noted. The cellular uptake for both cell types consisted in a passive adsorption and in an energy requiring process, because the cells became 2–3 times more fluorescent when the incubation temperature was increased from 4 to 37°C. When nanoparticles were coated with polyethylene glycol 20,000, uptake by monocytes decreased by 43 and 78% in phosphatebuffered saline and plasma, respectively; a similar decrease in nanoparticle uptake was observed for lymphocytes. Two‐dimensional gel electrophoresis was performed to identify the plasma opsonins adsorbed onto the nanoparticle surface. Protein mappings for uncoated and polyethylene glycol‐coated nanoparticles differed for two spot series. These spots, not yet clearly identified, may represent specific apolipoproteins involved in the metabolism of human lipoproteins, indicating the possible involvement of specific receptors in the uptake of the nanoparticles. © 1994 John Wiley & Sons, Inc.
Adsorption of a novel fluorescent derivative of a poly(ethylene oxide)/poly(butylene oxide) block copolymer on octadecyl glass studied by total internal reflection fluorescence and interferometry
Authors: Gingell D. , Owens N. , Hodge P. , Nicholas C. V. , O'dell R.
Published: December 2004

We have used total internal reflection fluorescence (TIRF) to measure the adsorption kinetics of a newly synthesized fluorescent derivative of a triblock copolymer comprising two poly(ethylene oxide) arms connected by a poly(butylene oxide) segment. The composition is (EO)400 (BO)55 (EO)400, in which EO represents ethylene oxide, BO represents butylene oxide, and one or both of the terminal OH groups of the two (EO)400 arms are labeled with tetramethylrhodamine. The poly(butylene oxide) segment binds to hydrophobic octadecyl glass, used as a substratum. The TIRF signal is shown to be derived almost entirely from surface‐adsobed polymer. This facilitates calculation of adsorption isotherms from 0.1–0.005% bulk polymer solution by means of diffusion kinetics. Information about the effective thickness of the adsorbed polymer, determined by optical interference microscopy, corresponds with what is known about the conformation of similar molecules at interfaces and indicates monolayer adsorption on the glass. © 1994 John Wiley & Sons, Inc.
Mineral apposition rates of human cancellous bone at the interface of porous coated implants
Authors: Bloebaum Roy D. , Bachus Kent N. , Momberger Nate G. , Hofmann Aaron A.
Published: December 2004

Human cancellous bone ingrowth studies were conducted on 19 consenting bilateral total knee arthroplasty (TKA) patients. Titanium porous coated cylinders were implanted into the medial femoral condyle of the contralateral knee during the first of two TKAs. Retrieval was performed at the time of the second TKA (6–131 weeks later), and fluorochrome analysis was conducted. Mean mineral apposition rates (MAR) at the interface measured 1.0 μm/day, whereas 4 mm away, the peripheral bone had a mean MAR of 0.8 μm/day. This represented a 25% acceleration in the interface bone remodeling rate when compared with the periphery (P< .05). This study showed the bone advanced appositionally at the interface at a rate of ≈ 1 μm/day. Analysis showed that when bone was over 50 μm from the porous coating, bone ingrowth did not occur. These results emphasize the need for surgical precision and careful postoperative management to achieve bone ingrowth. © 1994 John Wiley & Sons, Inc.
In vivo tensile testing of fluorapatite and hydroxylapatite plasma‐sprayed coatings
Authors: Kangasniemi I. M. O. , Verheyen C. C. P. M. , van der Velde E. A. , De Groot K.
Published: December 2004

A tensile test has been developed to test bioactive coating materials. Hydroxylapatite (HA) and fluorapatite (FA) coatings with various roughnesses were tested using sandblasted titanium (Ti) as a control material. Twelve goats received 7 implants each for 6 (2 goats), 12 (5), and 24 (4) week implantation periods. After 12 weeks the mean tensile strength values were highest for polished hydroxylapatite (HAP) followed by HA > FAP (polished fluorapatite) > FA > Ti, and hydroxylapatite type coatings were found to exhibit significantly higher values than fluorapatite type coatings or titanium implants. After 24 weeks no statistically significant differences could be found between any of the implant types. The order of the mean strength values was now HAP > HA > FA > FAP > Ti. Fracture always occurred between the coating and titanium if bone contact had been established. In conclusion, it is suggested that results from different types of test methods cannot be used to compare different types of bioactive coatings. © 1994 John Wiley & Sons, Inc.
Ion‐selective membranes with low plasticizer content: Electroanalytical characterization and biocompatibility studies
Authors: Lindner E. , Cosofret V. V. , Ufer S. , Buck R. P. , Kao W. J. , Neuman M. R. , Anderson J. M.
Published: December 2004

High molecular weight poly(vinyl chloride) and aliphatic polyurethane (Tecoflex)‐based ion selective membranes, with normal and reduced amounts of plasticizer, as well as without plasticizer, were tested with respect to their analytical properties, their biocompatibility, and cellular responses. The analytical properties of the membranes did not change significantly within a wide range of polymer to plasticizer rations. However, the membranes with reduced plasticizer content had better adhesive properties, less anion interference, extended life time, and better biocompatibility. Using the cage implant system, the results showed that an increase of plasticizer weight percent in Tecoflex membranes correlated positively with the increase in host inflammatory response up to 14 days of implantation. The results also demonstrated that both PVC and Tecoflex‐based ion‐selective membranes with the most common membrane composition (1:2 polymer to plasticizer ratio) exhibited a similar acute inflammatory response, but the PVC‐based membrane elicited a reduced chronic inflammatory response when compared with the Tecoflex‐based membrane. © 1994 John Wiley & Sons, Inc.
Conformational preferences of 34 valence electron A2X4 molecules: An ab initio Study of B2F4, B2Cl4, N2O4, and C2O
Authors: Clark Timothy , Von Ragué Schleyer Paul
Published: December 2004

Ab initio molecular orbital structures and energies of B2F4, B2Cl4, N2O4, and C2O have been calculated for both perpendicular D2d and planar D2h rotamers. The experimental trend toward greater preference for the D2d forms in going from B2F4 to B2Cl4 is reproduced. N2O4 favors the planar conformation, although the rotation barrier is overestimated at the theoretical levels used. The oxalate dianion is calculated to be more stable in the D2d conformation; the experimental planar arrangement in the solid may be due to crystal packing forces. The preferences for one conformation over another are small; analysis indicates that different effects may predominate in each case: π stabilization for B2F4, hyperconjugation for B2Cl4, lone‐pair interactions for N2O4, and electrostatic repulsions for C2O.
SCF‐Xα‐SW calculation on the molecule N2 using theoretical αta
Authors: Tseng T. J. , Whitehead M. A.
Published: December 2004

The first calculation of a molecule, N2, by the SCF‐Xα‐SW method using the theoretically derived parameter αta is reported.
Simple calculation of electrostatic isopotential maps from bond fragments
Authors: Náaray‐Szabó G. , Grofcsik A. , Kósa K. , Kubinyi M. , Martin A.
Published: December 2004

A simple and very rapid method for the calculation of electrostatic isopotential maps is proposed. The potential is composed of transferable bond fragments. According to chemical evidence, σ‐, lone‐pair, and π‐bond contributions are considered. The computational work is proportional to the first power of the number of bond orbitals; therefore very large systems, such as enzymes or DNA, can be handled also. Transferability of bond orbital characteristics such as polarity, s character, and orientation is discussed. Though the absolute value of the potential is overestimated, trends obtained with STO‐3G calculations are reflected correctly.
On the applicability of the MINDO/3 method in studies of molecular structures in the vapor phase. Geometries, conformational energies, and vibrational amplitudes of alkenes and chloroalkanes
Authors: Van Hemelrijk D. , Versichel W. , Van Alsenoy C. , Geise H. J.
Published: December 2004

The geometries and force fields of the different conformers of five alkenes and five chloroalkanes (17 conformers in total) were calculated in the MINDO/3 approximation and compared with the corresponding experimental values. The direct use of geometrical or vibrational parameters derived from MINDO/3 either as starting values or as a source of constraints in the analysis of gas‐phase electron diffraction data presents great risks. After a scaling procedure in which the calculated values are fitted to the experimental ones for a small number of compounds, scale factors can be obtained which, with due caution, are transferable to other molecules in the same class. Scaled MINDO/3 produces geometries and force fields that fit to experiment considerably better than their unscaled counterparts. However, scaled MINDO/3 does not produce better results than molecular mechanics. Calculated energy differences between rotamers are generally too high.
Electron density redistribution in the stabilization of a molecular stacking complex: The nature and correction of basis set superposition errors
Authors: Osman Roman , Topiol Sid , Weinstein Harel
Published: December 2004

The effect of the basis set superposition error (BSSE) on the calculated electronic structure of a molecular stacking complex is analyzed with the counterpoise correction method. The complex between para‐hydroxyaniline (PHA) and formamidinium cation (FAM) is calculated ab initio with the STO‐3G and Whitman–Hornback minimal bases, and with two split valence basis sets: 4‐31G and STO‐3G(D). When the counterpoise correction is applied, the charge redistribution in the PHA/FAM complex calculated with all four basis sets suggests that the complex is electrostatic in nature and that the main polarization is from the PHA toward FAM. The FAM cation is polarized away from the intermolecular region, thus causing further increase in the electrostatic interaction. This picture is not evident with the STO‐3G related bases if the counterpoise correction is not applied. Thus, the BSSE in the charge redistribution is shown to be particularly large in the STO‐3G basis and in the diffuse, split valence, STO‐3G(D). Both these basis sets suffer from an inappropriate description of the core region. Where there is an improved description of the electron density in the core region, as in the calculations with the energy‐optimized Whitman–Hornback basis and with the 4‐31G basis, the counterpoise correction has only a very small effect on the charge redistribution. After the counterpoise correction is applied, the two minimal basis sets yield nearly identical charge redistribution results, as do the two split valence bases. It is therefore suggested that basis sets used in the calculation of molecular complexes might be classifiable according to criteria such as the degree of contraction and the quality of the description of angular polarization. This would help in the comparative evaluation of results obtained with different bases since minimal basis sets (or split valence bases) would become directly identifiable as groups yielding qualitatively similar results.
Geometry and energy of tetra‐tert‐butylethylene
Authors: Favini G. , Simonetta M. , Todeschini R.
Published: December 2004

The geometry and energy of tetra‐tert‐butylethylene have been determined by the molecular mechanics method. A twisting of the double bond by 45.5° was found. The ground state of the molecule should be a singlet. The calculated strain energy is higher than those of tri‐tert‐butylmethane and tetra‐iso‐propylethylene, but the possibility of synthesis of the compound is not excluded.
AMBER: Assisted model building with energy refinement. A general program for modeling molecules and their interactions
Authors: Weiner Paul K. , Kollman Peter A.
Published: December 2004

We describe a computer program we have been developing to build models of molecules and calculate their interactions using empirical energy approaches. The program is sufficiently flexible and general to allow modeling of small molecules, as well as polymers. As an illustration, we present applications of the program to study the conformation of actinomycin D. In particular, we study the rotational isomerism about the D‐Val‐, L‐Pro, and L‐Pro‐Sar amide bonds as well as comparing the energy and structure of the Sobell model and the x‐ray structure of actinomycin D.
Representation of the molecular electrostatic potential by a net atomic charge model
Authors: Cox S. R. , Williams D. E.
Published: December 2004

Electrostatic potentials and Mulliken net atomic charges were calculated from STO‐3G, 6‐31G, and 6‐31G** SCF‐MO wavefunctions for hydrogen fluoride, water, ammonia, methane, acetylene, ethylene, carbon dioxide, formaldehyde, methanol, formamide, formic acid, acetonitrile, diborane, and carbonate ion. In each case optimized net atomic charges (potential‐derived charges) were also obtained by fitting the electrostatic potentials calculated directly from the wavefunctions in a shell enveloping the molecules outside of their van der Waals surfaces. The electrostatic potentials calculated from the potential‐derived charge distributions were then compared with the defined quantum mechanical electrostatic potentials and with the electrostatic potentials of the Mulliken charge distributions.
Molecular mechanics of organic halides. V. Conformational equilibria in solution
Authors: Meyer A. Y.
Published: December 2004

With a view of using data on solutions and liquids for parameter fitting in molecular mechanical force fields, Abraham's theory of solvation is incorporated in the force field procedure. Geometries and bond moments are estimated internally, partial account being taken of bond–bond induction, and used to calculate the intramolecular electrostatic energy, dipole moment, and the dipole and quadrupole terms in the solvation energy. Three dielectric constants are used, one for the solute in the vapor, one for the solution, and one for the intramolecular space through which dipole–dipole interactions take place. Examples are given, including such where computation differs with measurement, to illustrate the performance of the scheme.
Monte Carlo simulation for inhomogeneous chemical kinetics: Application to the Belousov–Zhabotinskii reaction
Authors: Zaera F. , Rusinek Isak
Published: December 2004

A Monte Carlo algorithm, capable of simulating numerically the time and space dependence of chemical concentrations in a reacting system, is presented. This method is used to study the phenomenon of trigger waves in the Oregonator model of the Belousov–Zhabotinskii reaction, including the diffusion of species X and Y in one dimension. The results show that a small disturbance in a homogeneous mixture can grow into a chemical (trigger) wave propagating in space at constant velocity. The dependence of this velocity on several factors is studied, namely, initial concentrations, the diffusion of Y, and the stoichiometry of the autocatalytic step of the model. A comparison of the Monte Carlo results with a previous simulation also is discussed.
Ab initio studies of structural features not easily amenable to experiment. 18. Conformational analysis and molecular structure of glycine methyl ester
Authors: Schäfer Lother , Van Alsenoy C. , Scarsdale J. N. , Klimkowski V. J. , Ewbank J. D.
Published: December 2004

The structures of four conformations of the methyl ester of glycine were determined by standard single‐determinant molecular orbital (MO) calculations using Pulay's force method and the 4‐21G basis set. The most stable conformation of this compound has a symmetry plane which contains all the heavy atoms; it is stabilized by hydrogen bonds between the NH2 group and the carbonyl oxygen; it corresponds to the most stable, stretched form of free glycine. The structural parameters in the different conformations can vary significantly (bond distance by more than 0.02 Å and bond angles by up to 15°). The structural changes which are caused in glycine by esterification are discussed and some of them are interpreted in terms of hyperconjugative π‐electron delocalization.
Molecular orbital theory of the hydrogen bond. 26. The hydration of uracil
Authors: Bene Janet E. Del
Published: December 2004

Ab initio SCF calculations with the STO‐3G basis set have been performed to determine the structure and stability of a 6:1 water:uracil heptamer in which water molecules are hydrogen bonded to uracil at each of the six hydrogen‐bonding sites in the uracil molecular plane. The structure of the heptamer describes a stable arrangement of these six water molecules, which are the primary solvent molecules in the first solvation shell, and is suggestive of the arrangement of secondary solvent molecules in that shell in the nonpolar region of the uracil molecular plane. The stabilization energy of the heptamer is 49.6 kcal/mol, or 8.3 kcal/mol per water molecule. The hydrogen bonds between uracil and water are the primary factor in the stabilization of the complex, although water–water interactions and nonadditivity effects are also significant.
MNDO calculations for compounds containing aluminum and boron
Authors: Davis L. P. , Guidry R. M. , Williams J. R. , Dewar M. J. S. , Rzepa H. S.
Published: December 2004

Parametrization of MNDO for aluminum and comparisons of calculated molecular properties with experimental values indicate the general usefulness of MNDO to study aluminum‐containing compounds. Although results are not as good as for molecules containing only C, H, N, and O, they are nevertheless accurate enough to be useful, especially when predictive biases noted in this article are considered. Inclusion of the d orbital in the MNDO scheme may improve the aluminum results. Results for boron‐containing molecules not previously reported are also reported in this article.
Selectivity effects and hydrocarbon‐chain growth during Fischer–Tropsch synthesis
Authors: Collis F. P. , Schwarz J. A.
Published: December 2004

It is well known that if the formation of hydrocarbons during Fischer–Tropsch synthesis occurs via the condensation polymerization mechanism, there is little hope for selectivity enhancement in the desired range of products. Recent data on low surface area model catalysts, where readsorption is unlikely, have shown that at low conversions, the product distribution obeys the condensation polymerization mechanism and the distribution of products is shifted to lower‐molecular‐weight hydrocarbons. We have used a computer simulation of the growth of hydrocarbon chains to obtain a picture of the catalyst surface under synthesis conditions. Such an approach could prove useful in distinguishing between various theoretical models. We have applied the simulation to compare the changes in selectivity when readsorption occurs and when it does not. The dynamic behavior of the reacting system which is obtained from the computer results has shown that selectivity to lower‐molecular‐weight hydrocarbons is a stronger function of the extent of reaction than the incorporation of readsorption into the chain growth mechanism.
Thermochemical data from ab initio calculations. II. Total correlation energies, Schrödinger energies, and theoretical heats of formation
Authors: Cremer Dieter
Published: December 2004

Bond correlation energies ϵ(XY) have been derived by partitioning second‐order Rayleigh–Schrödinger–Møller–Plesset (RSMP) correlation energies. Values of ϵ(XY) depend on the type of bonding between atoms X and Y. They can be considered as comprising correlation energies of bond, lone, and inner‐shell electron pairs of the group XY. Once a set of appropriate increments ϵ(XY) has been obtained, it is possible to estimate unknown RSMP energies of larger molecules. This concept also can be used when estimating total correlation energies E(CORR). For this purpose e(XY) values have been derived from known E(CORR) energies of small molecules. It is shown that ∣ϵ(XY)∣ increments increase linearly with the number n of electron pairs of the group XY. The function ϵ(n) becomes zero for n = 1/2 (one uncorrelated electron) and passes through −0.042 for n = 1, which is approximately the correlation energy of the bonding electron pair of H2 or a 1s inner‐shell pair. With the aid of estimated E(CORR) and HF limit energies, Schrödinger energies and theoretical heats of formation of relatively large molecules are obtained.
Configuration interaction (CI): Approximate inclusion of fourfold and threefold excitations, an application of knowledge engineering
Authors: Wenzel Klaus B.
Published: December 2004

It is shown that even the CPMET approximation for the contributions of fourfold excitations can be replaced by a more sophisticated method. Furthermore, an approximation is presented for the contributions of threefold excitations which are neglected in CPMET. Finally, it is described how the advantages of different CEPA methods can be united in a single CEPA scheme (called CEPA‐U). The various approximations are discussed in terms of Slater determinants, and it is shown how the relatively large amount of details can be handled with the help of computer programs, i.e., how knowledge engineering can be successfully applied here.
Optimization of equilibrium geometries and transition structures
Authors: Schlegel H. Bernhard
Published: December 2004

A modified conjugate gradient algorithm for geometry optimization is outlined for use with ab initio MO methods. Since the computation time for analytical energy gradients is approximately the same as for the energy, the optimization algorithm evaluates and utilizes the gradients each time the energy is computed. The second derivative matrix, rather than its inverse, is updated employing the gradients. At each step, a one‐dimensional minimization using a quartic polynomial is carried out, followed by an n‐dimensional search using the second derivative matrix. By suitably controlling the number of negative eigenvalues of the second derivative matrix, the algorithm can also be used to locate transition structures. Representative timing data for optimizations of equilibrium geometries and transition structures are reported for ab initio SCF–MO calculations.