Biomedical subjects

Jeremy L Gilbert

Publications and source records attributed to Jeremy L Gilbert.

17 recordsLinked to original sources

PubMed2007

Quantification of fibrinogen adsorption onto 316L stainless steel.

Adsorption of the plasma protein fibrinogen (Fb) onto 316L stainless steel (316L SS) was observed and quantified using both in situ and ex situ atomic force microscopy techniques. Industry standard mechanical and electrochemical polishing techniques were used to prepare bulk alloy 316L SS samples, rendering the surfaces flat enough to directly observe and measure Fb adsorption. The data were analyzed kinetically using a Langmuir model. Largely irreversible adsorption was found on the 316L SS surface with an adsorption rate constant (k(o)) of 1.9 x 10(-4) mL microg(-1) s(-1) using the ex situ method and 1.7 x 10(-4) mL microg(-1) s(-1) using the in situ method. Additionally, protein conformation and assembly orientation on these surfaces were documented, where the adsorption pattern appeared random. Complete area coverage was never obtained. That is, after adsorption for over 5 time constants (5tau), voids in the structure were always observed.

Adsorption↗

PubMed2006

Complexity in modeling of residual stresses and strains during polymerization of bone cement: effects of conversion, constraint, heat transfer, and viscoelastic property changes.

Aseptic loosening of cemented joint prostheses remains a significant concern in orthopedic biomaterials. One possible contributor to cement loosening is the development of porosity, residual stresses, and local fracture of the cement that may arise from the in-situ polymerization of the cement. In-situ polymerization of acrylic bone cement is a complex set of interacting processes that involve polymerization reactions, heat generation and transfer, full or partial mechanical constraint, evolution of conversion- and temperature-dependent viscoelastic material properties, and thermal and conversion-driven changes in the density of the cement. Interactions between heat transfer and polymerization can lead to polymerization fronts moving through the material. Density changes during polymerization can, in the presence of mechanical constraint, lead to the development of locally high residual strain energy and residual stresses. This study models the interactions during bone cement polymerization and determines how residual stresses develop in cement and incorporates temperature and conversion-dependent viscoelastic behavior. The results show that the presence of polymerization fronts in bone cement result in locally high residual strain energies. A novel heredity integral approach is presented to track residual stresses incorporating conversion and temperature dependent material property changes. Finally, the relative contribution of thermal- and conversion-dependent strains to residual stresses is evaluated and it is found that the conversion-based strains are the major contributor to the overall behavior. This framework provides the basis for understanding the complex development of residual stresses and can be used as the basis for developing more complex models of cement behavior.

Bone Cements↗

PubMed2006

Tissue response to in situ polymerization of a new two-solution bone cement: evaluation in a sheep model.

A two-solution bone cement (2-SC) was evaluated in a non-load bearing sheep model that simulated insertion of a cemented total joint replacement. A commercial powder-liquid bone cement formulation (Palacos R) was used as the control. The systemic response to the two cements was determined by monitoring changes in arterial blood pressure (ABP) and serum concentrations of methyl methacrylate monomer at intervals after insertion of the cement. The short-term tissue response to the two cements was assessed by quantifying histomorphometric parameters of new bone formation at 2, 4, and 12 weeks postsurgery. Intraoperatively, injection and pressurization of bone cement were well tolerated, with no significant changes in ABP in either group and no detectable circulating monomer in any animal. Several interesting trends were identified in the histomorphometry data. In the trabecular specimens, new bone formation immediately adjacent to the cement mantle was apparently suppressed in the first 2 weeks postsurgery, increased dramatically at 4 weeks, and then returned to baseline values by 12 weeks. This pattern was seen with both Palacos and 2-SC. In the cortical specimens, new bone formation was reduced on the endosteal surface when compared with the periosteal surface, with this effect being more noticeable at 2 and 4 weeks than at 12 weeks. There were no significant histopathological findings in either the bone or the draining lymph nodes. These data indicate that the biological response to 2-SC is substantially equivalent to that of Palacos R. Additional testing in a functional, load-bearing animal model is now recommended to more fully characterize the long-term biological response to 2-SC and to determine the mechanical performance of this new cement in vivo.

Animals↗

PubMed2005

A study of biologically active peptide sequences (P-15) on the surface of an ABM scaffold (PepGen P-15) using AFM and FTIR.

Cellular response to any biomaterial surface is governed by a number of factors including topography, surface chemistry, surface charge, structural heterogeneity, and physiological conditions. Understanding these factors at the nanoscale level is crucial to develop improved biomaterials. Any changes in these properties due to surface modifications need to be addressed properly, as they could have significant impact on the cellular interaction with biomaterials. In this study, the topography and surface chemistry of commercially available tissue engineered xenograft, PepGen P-15 [comprised of a synthetic peptide P-15 irreversibly attached to anorganic bovine bone mineral (OsteoGraf/-N)] was studied using Atomic Force Microscopy (AFM), and Fourier Transform Infrared Spectroscopy (FTIR). FTIR confirmed the presence of the peptide on the surface of PepGen P-15. Changes in the peptide conformation, which includes a decrease in the beta-strand accompanied by an increase in unordered structures/random coil structures after attachment on OsteoGraf/-N is observed. Specific functional groups, which are involved in the binding mechanism, are identified. The results suggest that the attachment of the peptide on OsteoGraf/-N occurs via a specific surface docking ionic interaction involving the C-terminal carboxylic group on the peptide with positive domains generated by hydroxyl vacancies on the apatite surface.

Animals↗

PubMed2005

Quantification of the kinetics and thermodynamics of protein adsorption using atomic force microscopy.

Both in situ and ex situ methods for quantifying area fraction coverage of protein on a surface using atomic force microscopy were developed. The in situ method used a continuous fluid flow system to observe the kinetics of adsorption in real time. The ex situ method required immersing the sample in solution, drying the sample, and imaging in an ambient environment to obtain kinetic and isothermal data. These methods were developed using the plasma protein fibrinogen in a phosphate-buffered saline solution on grade IV muscovite mica and highly ordered pyrolytic graphite (HOPG) substrates. Kinetic and quasiisothermal data were obtained and a Langmuir model was fit to the data. An adsorption rate constant of 2.2 x 10(-4) mL . microg(-1)s(-1) and a desorption rate constant of 8.3 x 10(-5) s(-1) were found on an HOPG surface. Completely irreversible adsorption was found on the mica surface with an adsorption rate constant of 2.7 x 10(-4) mL . microg(-1)s(-1). Additionally, protein conformation and assembly orientation on these surfaces were documented where fibrinogen on HOPG formed a network-like structure, whereas fibrinogen on mica was more random. Also, nano-topographical factors (ledges) were seen as sites of preferential adsorption.

Aluminum Silicates↗

PubMed2005

Quantitative analysis of monomer vapor release from two-solution bone cement by using a novel FTIR technique.

Methyl methacrylate monomer can evaporate from bone cement to reach cytotoxic levels of concentrations in the implant bed of total joint prosthesis. Therefore, this study was performed by using a novel Fourier transform infrared spectroscopy method to quantify the release of monomer vapor from experimental two-solution bone cement in vitro during polymerization, to examine the effect of surface area versus cement mass, and to explore the effect of initiation chemistry. The results revealed that monomer vapor release is a surface phenomenon. In addition, initiation chemistry plays a major role in controlling the reaction time, and therefore heat concentration and dissipation, which resulted in a higher absorbance peak as initiation chemicals concentration increased. It was concluded that using the FTIR to monitor MMA vapors is an effective technique to obtain quantitative information about monomer vapor release from bone cements during polymerization and provides insight on the polymerization kinetics of two-solution acrylic bone cement.

Biocompatible Materials↗

PubMed2005

Micromechanics of shelf-aged and retrieved UHMWPE tibial inserts: indentation testing, oxidative profiling, and thickness effects.

Understanding the surface micromechanical properties of ultra-high-molecular-weight polyethylene (UHMWPE) may allow for improvement in its wear characteristics. Microtomed sections of two UHMWPE tibial bearings, one that had been irradiation sterilized and shelf-aged, and the other irradiation sterilized and used in a patient, were subjected to depth sensing indentation testing. The microtomed sections exhibited a white band in the subsurface region that is characteristic of oxidation, and the indentation testing, followed by FTIR analysis at the same testing locations, was performed across this region and into the bulk of the material. Indentation testing yielded data leading to hardness, modulus, and energy dissipation factor (EDF). FTIR profiling generated information about oxidation; oxidation indices were calculated by taking a ratio of peak heights at 1,716 cm(-1) (ketone) to 2,022 cm(-1) (methylene). The mechanical properties showed a strong linear correlation with oxidation index above a minimum thickness. Modulus, hardness, and EDF all increased with increasing oxidation. The appropriate thickness-to-indentation-depth ratio was determined by two methods and was found to be approximately 20:1. The mechanical properties through the oxidized region were seen to vary with depth into the sample in a profile similar to the oxidation profile. The differing aging environments of the tibial bearings are hypothesized to have had an effect on both the mechanical and oxidation profiles. The retrieved bearing exhibited a narrower oxidation profile with peaks closer to the surface, and oxidation indices of lower magnitude. The mechanical properties proved similar, with less intense and narrower readings for the retrieved sample. This research is consistent with much of the literature.

Humans↗

PubMed2004

Rate effects on the microindentation-based mechanical properties of oxidized, crosslinked, and highly crystalline ultrahigh-molecular-weight polyethylene.

The surface micromechanical properties of ultrahigh-molecular-weight polyethylene (UHMWPE) are critical in determining the wear, deformation, and fracture in the surface region. These properties have not been accessible to simple mechanical testing on a spatial scale relevant to these mechanical processes until recently. The structural factors associated with surface mechanical properties (crosslinking, oxidation state, local orientation of polymer, crystallinity, etc.) can be highly variable and localized and may vary on micron spatial scales or smaller. Furthermore, time/frequency-dependent behavior of the surface may have an important role in the overall surface mechanical behavior. Recent work has shown the utility of depth sensing microindentation/nanoindentation testing to interrogate local surface mechanical properties. The goal of the present study was to measure the effect of loading rate on the depth-sensing microindentation testing of UHMWPE. Three different UHMWPE materials (Hylamer, a large crystal material; GUR 1020, a standard medical-grade material; and Marathon, a crosslinked material) were tested using a microindentation method at loading rates ranging from 0.01 to 1 Hz. Similarly, a gamma-irradiated in air and 15-year shelf-aged tibial component was tested through its cross-section to assess the variations in mechanical properties with location and to compare the micromechanical profile with the oxidation profile. It was found that rate of testing affected the microhardness of each material, however, only GUR 1020 and Hylamer showed rate-dependent behavior for modulus and energy dissipation factor. Micromechanical profiles through oxidized regions of the tibial component showed a high correlation with the oxidation profile. Increases in modulus, hardness, and energy dissipation factor were seen with increasing oxidation and each property was loading-rate dependent. These results show that depth-sensing microindentation/nanoindentation testing on the micron scale provides highly consistent and reproducible measurements of surface mechanical properties. This scale of testing minimizes the potential variations caused by local heterogeneity in crystallinity, surface orientation, and other submicron structural features.

Cross-Linking Reagents↗

PubMed2004

Gentamicin release from two-solution and powder-liquid poly(methyl methacrylate)-based bone cements by using novel pH method.

The release of gentamicin as a function of time was measured for Palacos and two-solution bone cements by using a novel pH technique. The pH of an aqueous solution of gentamicin is a function of the gentamicin concentration and it decreases linearly over concentrations of 0.0-0.1 wt %. Therefore, a new, direct, and inexpensive in vitro technique was developed based on continuous readings of the pH in phosphate-buffered saline (PBS) at 37 degrees C to determine the release kinetics of gentamicin from poly(methyl methacrylate) (PMMA)-based bone cement. In addition, this method was used to compare the release profiles of Palacos R-40 bone cement with a two-solution bone cement developed in our laboratory and loaded with two different concentrations of gentamicin sulfate. Finally, the pH-based method was used to track the elution of gentamicin in both mixed and static conditions to determine the effect of mixing on the diffusion of gentamicin out of the cement. It was found that Palacos R-40 released 4.95 +/- 0.22 wt % of its gentamicin after 24 h in PBS solution. This data compares favorably with previously reported values of gentamicin elution from Palacos R-40, which ranged from 3 to 8 wt % of the total amount of incorporated gentamicin, depending on the size and the surface area of the samples. The results show that Palacos samples released 4.84 +/- 0.27 mg after 24 h, a two-solution cement loaded with an equivalent concentration of gentamicin sulfate released 3.81 +/- 0.52 mg, and two-solution cement loaded with twice the concentration of Palacos released 5.53 +/- 0.26 mg of gentamicin. A higher percentage of release was recorded from Palacos than from the two-solution bone cement, and the effect of PBS mixing conditions on the release kinetics was only significant in the early stages of release and not at 24 h. It was concluded that monitoring the pH is an effective technique to measure gentamicin release from PMMA-based bone cements in PBS solution.

Anti-Bacterial Agents↗

PubMed2004

The electrochemical and mechanical behavior of passivated and TiN/AlN-coated CoCrMo and Ti6Al4V alloys.

The mechanical and electrochemical behavior of the surface oxides of CoCrMo and Ti6Al4V alloys during fracture and repassivation play an important role in the corrosion of the taper interfaces of modular hip implants. This behavior was investigated in one group of CoCrMo and Ti6Al4V alloy samples passivated with nitric acid and another group coated with a novel TiN/AlN coating. The effects of mechanical load and sample potential on peak currents and time constants resulting from fracture of the surface oxide or coating, and the effects of mechanical load on scratch depth were investigated to determine the mechanical and electrochemical properties of the oxides or coating. The polarization behavior of the samples after fracture of the oxide or coating was also investigated. CoCrMo had a stronger surface oxide and higher interfacial adhesion strength, making it more resistant to fracture than Ti6Al4V. If undisturbed, the oxide on the surface of Ti6Al4V significantly reduced dissolution currents at a wider range of potentials than CoCrMo, making Ti6Al4V more resistant to corrosion. The TiN/AlN coating had a higher hardness and modulus of elasticity than CoCrMo and Ti6Al4V. It was much less susceptible to fracture, had a higher interfacial adhesion strength, and was a better barrier to ionic diffusion than the surface oxides on CoCrMo and Ti6Al4V. The coating provided increased corrosion and fretting resistance to the substrate alloys.

Alloys↗

PubMed2003

Effect of hydrogen peroxide on titanium surfaces: in situ imaging and step-polarization impedance spectroscopy of commercially pure titanium and titanium, 6-aluminum, 4-vanadium.

To analyze titanium's response to representative surgical wound environments, a study was conducted on commercially pure titanium (CPTi) and titanium, 6-aluminum, 4-vanadium (Ti-6Al-4V) exposed to phosphate-buffered saline (PBS) with 30 mM of hydrogen peroxide (H(2)O(2)) added. The study was characterized by simultaneous electrochemical atomic force microscopy (EC AFM) and step-polarization impedance spectroscopy (SPIS). Surfaces were covered with protective oxide domes that indicated topography changes with potential and time of immersion. Less oxide dome coarsening was noted on surfaces treated with PBS containing H(2)O(2) than on surfaces exposed to pure PBS. Electrical data deduced from current transients collected while stepping voltage between 0 V and 1 V indicated that charge transfer in hydrogen peroxide solutions was an order of magnitude larger than it was in pure PBS. Oxide (early) resistances of CPTi samples were higher than were Ti-6Al-4V oxide resistances in both types of solutions, but CPTi oxide resistance was lower in the hydrogen peroxide solution compared to pure PBS. Capacitance data suggest that CPTi oxide films thicken in hydrogen peroxide solution more than they do in pure PBS. Differences in electrical properties between CPTi and Ti-6Al-4V surfaces suggest that CPTi, but not Ti-6Al-4V, has catalytic activity on H(2)O(2) and that the catalytic activity of CPTi oxide affects its ability to grow TiO(2). Differences in electrical properties are related to catalytic and oxidative mechanisms that take place directly on the titanium oxide surface and in wound environments. The study provides a foundation and theoretic basis for the porous oxide model on commercially pure titanium exposed to hydrogen peroxide.

Aluminum↗

PubMed2003

In vitro corrosion testing of modular hip tapers.

The in vivo fretting behavior of modular hip prostheses was simulated to determine the effects of material combination and a unique TiN/AlN coating on fretting and corrosion at the taper interface. Fretting current, open-circuit potential (OCP), and quantities of soluble debris were measured to determine the role of mechanically assisted crevice corrosion on fretting and corrosion of modular hip tapers. Test groups consisting of similar-alloy (Co-Cr-Mo head/Co-Cr-Mo neck), mixed-alloy (Co-Cr-Mo head/Ti-6Al-4V neck), and TiN/AlN-coated mixed-alloy modular hip taper couples were used. Loads required to initiate fretting were similar for all test groups and were well below loads produced by walking and other physical activities. Decreases in OCP and increases in fretting current observed during long-term cyclic loading were indicative of fretting and corrosion. Current measured after cessation of cyclic loading suggests that once the conditions for crevice corrosion are established, corrosion can continue in the absence of loading. The chemical, mechanical, and electrochemical measurements, along with microscopic inspections of the taper surfaces indicate that the fretting and corrosion behavior of similar- and mixed-alloy taper couples are similar and that the coated samples are more resistant to fretting and corrosion. The results of this study clearly indicate the role of mechanical loading in the corrosion process, and support the hypothesis of mechanically assisted crevice corrosion.

Alloys↗

PubMed2003

In situ imaging and impedance measurements of titanium surfaces using AFM and SPIS.

Surfaces of commercially pure titanium and titanium, 6-aluminum, 4-vanadium were subjected to simultaneous polarization/impedance testing and in situ electrochemical atomic force microscopy imaging to evaluate how the structure and properties of the passive oxide film is affected by varying potential and hydration. Current transients were acquired via a step polarization impedance spectroscopy technique: the voltage was stepped between -1 and 1 V in 50 mV increments, while current transients and surface morphology were digitally recorded. Numerical Laplace transformation applied to the current transient data provided frequency-dependent admittance (impedance(-1)). Simultaneous AFM imaging of dry surfaces, initially hydrated surfaces, and surfaces immersed and changing with potential revealed that all sample surfaces were covered with protective titanium oxide domes that grew in area and coalesced due to hydration and as a function of increasing applied voltage and time. Reversal of dome growth did not occur upon voltage reduction, while impedance behavior was quasi-reversible, suggesting independence between structural and electrical properties. Oxide growth appeared to occur in part by lateral spreading and overgrowth of domes at the oxide-solution interface. Interfacial impedance data reflect oxide passivity and n-type semiconductor behavior. Non-linear Mott-Schottky fits specified multi-layer donor concentrations between 10(18) and 10(19)cm(-3), depending on the surface.

Alloys↗

PubMed2002

Effect of initiation chemistry on the fracture toughness, fatigue strength, and residual monomer content of a novel high-viscosity, two-solution acrylic bone cement.

Porous-free, two-solution bone cements have been developed in our laboratory as an alternative to commercial powder/liquid formulations. Each pair of solutions consist of poly(methyl methacrylate) (PMMA) powder dissolved in methyl methacrylate (MMA) monomer, with benzoyl peroxide (BPO) added to one solution as the initiator and N,N-dimethyl-p-toluidine (DMPT) added to the other as the activator. When mixed, the solutions polymerize via a free radical reaction, which is governed by the concentrations of initiator and activator and their molar stoichiometry. Previous work by the authors has demonstrated that these two-solution cement compositions are comparable to Simplex P bone cement in polymerization exotherm, setting time, and flexural mechanical properties. This study was designed to evaluate the effect of BPO and DMPT concentrations, along with their molar ratio, on the fracture toughness, fatigue strength, and residual monomer content of the experimental compositions. The results showed that fracture toughness and fatigue strength for the solution cements were comparable to Simplex P and were not significantly affected by the BPO concentration or the BPO:DMPT molar ratio; however, the highest DMPT concentration yielded significantly lower values for both variables. Residual monomer content was significantly affected by both the individual concentrations of BPO and DMPT and their molar ratios. The two-solution cements had significantly higher residual monomer contents versus Simplex P; however, this can be attributed to their higher initial monomer concentration rather than a lower degree of conversion.

Benzoyl Peroxide↗

PubMed2002

The effect of processing conditions on the properties of poly(methyl methacrylate) fibers.

Brittle failure of bone cement remains a problem for the long-term stability of hip prostheses. Fibers have been developed from poly(methyl methacrylate) (PMMA) that retain the chemistry of bone cement, but improve the mechanical properties greatly. To fabricate the fibers, a polymer melt is extruded out of a small die (spinneret) and pulled onto a take-up wheel. Varying the speed of extrusion and temperature of the melt can control the viscosity of the molten polymer. This study examines the effect of melt viscosity on the resultant properties of fibers fabricated from PMMA. The goals are to optimize fiber processing and determine processing-structure--property relationships. Resultant fibers had moderate to high levels of retained molecular orientation, and ultimate tensile strengths (UTS) ranging from 60 to 225 MPa, moduli from 1.5 to 3.5 GPa, and strain to failure from 10 to 40%. Fibers fabricated at a constant viscosity and draw velocity had identical properties, whereas decreasing the viscosity generally increased the mechanical properties and retained orientation. Linear regression models were constructed to predict how the processing variables affect the structure (orientation) of the fiber and how the structure affects the UTS. This can be used to design efficient processing methods for PMMA fibers.

Anisotropy↗

PubMed2002

Surface micromechanics of ultrahigh molecular weight polyethylene: Microindentation testing, crosslinking, and material behavior.

The wear behavior of ultrahigh molecular weight polyethylene (UHMWPE) is critical to the success of total joint replacements. Recent attempts to modify the wear behavior of UHMWPE by processing, in particular, crosslinking UHMWPE have shown promise to increase wear resistance, but concerns persist regarding other mechanical properties. It is also unclear what specific surface mechanical properties govern the wear resistance seen in these materials. The goal of this study was to demonstrate a custom-built surface mechanical test system and method that measures the micromechanical response of microtomed UHMWPE surfaces to depth-sensing microindentation tests. The surface structure of these UHMWPE materials was also examined using scanning electron microscopy and atomic force microscopy. A custom designed microindentation test system assessed the microindentation behavior of three UHMWPE resins: 1. Hylamertrade mark, 2. GUR-1020 CMS, and 3. Marathontrade mark-a lightly crosslinked material. The effects of material and indentation depth were studied. Microindentation tests were performed with indentation depths ranging from 2 to 45 microm. Four different measurements of surface micromechanical behavior were obtained including the surface modulus, microhardness, hysteresis energy (irreversible work done to the sample per unit cycle) and its associated energy dissipation factor, and loading slope. Statistically significant differences in each of these parameters were found for each material. Generally, Hylamer had the largest values for these parameters, followed by the GUR resin and then the Marathon. Surface modulus was independent of depth of testing and found to be 651 MPa for Marathon, 738 MPa for GUR, and 1015 MPa for Hylamer (Modulus for bulk UHMWPE is 540 MPa for Hylamer, 620 for GUR, and 1380 for Hylamer). The microhardness varied between 67 and 162 MPa depending on material and depth of testing. Surface structural characterization shows that the microtoming process for surface preparation generated distinct surface features that varied between materials. Intermittent drawn ribbons of polymer with oriented crystals were observed in both scanning electron microscopy and atomic force microscopy. The surface density and size of these features were characteristic of the materials with the Hylamer having the fewest, but largest ribbons, followed by GUR and then Marathon.

Biocompatible Materials↗

PubMed2002

A multicenter retrieval study of the taper interfaces of modular hip prostheses.

A multicenter retrieval analysis of 231 modular hip implants was done to investigate the effects of material combination, metallurgic condition, flexural rigidity, head and neck moment arm, neck length, and implantation time on corrosion and fretting of modular taper surfaces. Scores for corrosion and fretting were assigned to medial, lateral, anterior, and posterior quadrants of the necks, and proximal and distal regions of the heads. Neck and head corrosion and fretting scores were found to be significantly higher for mixed alloy versus similar alloy couples. Moderate to severe corrosion was observed in 28% of the heads of similar alloy couples and 42% of the heads of mixed alloy couples. Differences in corrosion scores were observed between components made from the same base alloy, but of different metallurgic conditions. Corrosion and fretting scores tended to be higher for heads than necks. Implantation time and flexural rigidity of the neck were predictors of head and neck corrosion and head fretting. The results of this study suggest that in vivo corrosion of modular hip taper interfaces is attributable to a mechanically-assisted crevice corrosion process. Larger diameter necks will increase neck stiffness and may reduce fretting and subsequent corrosion of the taper interface regardless of the alloy used. Increasing neck diameter must be balanced, however, with the resulting loss of range of motion and joint stability.

Adult↗