W.G. Matthews

Tensile Properties of Mineralized and Demineralized Human and Bovine Dentin

The relative contribution of the matrix of dentin to the physical properties of dentin is unknown but thought to be small. The objective of this study was to test the hypothesis that the demineralized matrix of dentin contributes little to the strength of dentin by measuring and comparing the ultimate tensile strength and modulus of elasticity of mineralized and demineralized dentin. Small slabs (4 x 0.5 x 0.5 mm) of bovine and human dentin were tested in a microtensile testing device in vitro. Human coronal mineralized dentin gave a mean ultimate tensile strength (UTS) of 104 MPa. Bovine incisor coronal dentin exhibited a UTS of 91 MPa, and bovine root dentin failed at 129 MPa. The modulus of elasticity of mineralized bovine and human dentin varied from 13 to 15 MPa. When dentin specimens were demineralized in EDTA, the UTS and modulus of elasticity fell to 26-32 MPa and 0.25 GPa, respectively, depending on dentin species. The results indicate that collagen contributes about 30% of the UTS of mineralized dentin, which is higher than was expected.

Controlled manipulation of molecular samples with the nanoManipulator

The nanoManipulator system adds a virtual-reality interface to an atomic-force microscope (AFM), thus providing a tool that can be used by scientists to image and manipulate nanometer-sized molecular structures in a controlled manner. As the AFM tip scans the sample, the tip-sample interaction forces are monitored, which, in turn, can yield information about the frictional, mechanical, material, and topological properties of the sample. Computer graphics are used to reconstruct the surface for the user, with color or contours overlaid to indicate additional data sets. Moreover, a force feedback stylus, which is connected to the tip via software, allows the user to directly interact with the macromolecules. This system is being used to investigate carbon nanotubes, DNA, fibrin, adeno- and tobacco mosaic virus. It is now also possible to insert this system into a scanning electron microscope which provides the user with continuous images of the sample, even while the AFM tip is being used for manipulations.

Bond strengths to superficial, intermediate and deep dentin in vivo with four dentin bonding systems

The shear bond strength of four dentin bonding systems which remove or modify the smear layer were measured in vivo in dog canine and molar teeth as a function of dentin depth. Dentin bond strengths were higher with cuspid teeth compared to molar teeth. Most bonding systems gave higher bonds to superficial dentin and progressively lower bond strengths deeper dentin. The highest bond strengths were obtained with Clearfil Liner Bond, followed by Superbond C&B, Scotchbond 2 and Tenure. The former two bonding systems achieved shear bond strengths to cuspids that were > 10 MPa regardless of dentin depth while the latter two systems produced bond strengths < 10 MPa. In molars, the same ranking of bonding systems was noted but the value that separated the high from the low bonds was 5 MPa.

Determination of the elastic modulus of native collagen fibrils via radial indentation

The authors studied the elastic response of single, native collagen fibrils extracted from tissues of the inner dermis of the sea cucumber, Cucumaria frondosa, via local nanoscale indentation with an atomic force microscope (AFM). AFM imaging of fibrils under ambient conditions are presented, demonstrating a peak-to-peak periodicity, the d band, of dehydrated, unfixed fibrils to be ∼64.5nm. Radial indentation experiments were performed, and the measured value for the reduced modulus is 1–2GPa.

Fluid shifts across human dentine In vitro in response to hydrodynamic stimuli

Most authorities agree that the hydrodynamic theory of dentine sensitivity best explains the stimulus-response relations of most painful stimuli. However, as the usual hydrodynamic stimuli are so different, it has been impossible to compare them. The equivalency of hydrodynamic stimuli can be evaluated from measurements of the fluid movement induced in vitro and relating this to the hydraulic conductance (Lp) of the same dentine specimen. From this determination, a common denominator is obtained which is equivalent to the hydrostatic pressure that would be required to cause the same magnitude of fluid movement. The purpose of this study was to measure the direction and magnitude of fluid shifts across dentine in extracted human crown segments with a flat, dentine occlusal surface in response to the following hydrodynamic stimuli; air blast, 56 degrees C water, 2 degrees C water, tactile and osmotic. In acid-etched superficial dentine, which simulates hypersensitive dentine, the largest to the smallest fluid flows obtained were: hot > cold > air blast > osmotic > tactile. When these were converted to equivalency units, the ranking of stimuli from strongest to weakest was hot > cold > air blast > osmotic > tactile. This new approach to comparing hydrodynamic stimuli should be verified in vivo.

The effects of outward forced convective flow on inward diffusion in human dentine in vitro

In vitro experiments were conducted to evaluate the influence of outward forced convective flow on the inward diffusion of radioactive iodide. When the smear layer was present, application of 15 cmH2O (1.47 kPa) outward-directed filtration pressure reduced the inward flux of iodide by about 10-20% depending upon the hydraulic conductance of each specimen. When the smear layer was removed by acid etching, the same 1.47 kPa pressure lowered the inward iodide flux by as much as 60%, depending on the hydraulic conductance. The results demonstrate the importance of the balance between inward diffusion and outward bulk-fluid movement on the rate of permeation of exogenous solutes.

Investigation and modification of molecular structures with the nanoManipulator

The nanoManipulator system adds a virtual reality interface to an atomic force microscope (AFM), thus providing a tool that enables the user not only to image but also to manipulate nanometer-sized molecular structures. As the AFM tip scans the surface of these structures, the tip-sample interaction forces are monitored, which in turn provide information about the frictional, mechanical, and topological properties of the sample. Computer graphics are used to reconstruct the surface for the user, with color or contours overlaid to indicate additional data sets. Moreover, by means of a force-feedback pen, which is connected to the scanning tip via software, the user can touch the surface under investigation to feel it and to manipulate objects on it. This system has been used to investigate carbon nanotubes, fibrin, DNA, adenovirus, and tobacco mosaic virus. Nanotubes have been bent, translated, and rotated to understand their mechanical properties and to investigate friction on the molecular level. AFM lithography is being combined with the nanoManipulator to investigate the electromechanical properties of carbon nanotubes. The rupture forces of fibrin and DNA have been measured. This article discusses how some of the graphics and interface features of the nanoManipulator made these novel investigations possible. Visitors have used the system to examine chromosomes, bacterial pili fibers, and nanochain aggregates (NCAs). Investigators are invited to apply to use the system as described on the web at http:@www.cs.unc.edu/Research/nano/doc/biovis it.html.

Air blast-induced evaporative water loss from human dentine, in vitro

Short air blasts are commonly used to test for dentine sensitivity but their mechanism of action is poorly understood. In this study, evaporative water loss of teeth in vitro was measured under spontaneous conditions and during air blasts from three-way dental air syringes. Air blasts induced a 15-30-fold increase in evaporative water loss, which varied inversely with distance from the tooth and directly with air temperature. As water evaporation occurs readily across smear layers, which greatly impede bulk fluid movement, the use of air blasts may overestimate dentine sensitivity. Sustained air blasts may remove enough fluid from the pulp-dentine complex to cause disruption of odontoblasts and changes in pulpal blood flow.

Simple and efficient method for carbon nanotube attachment to scanning probes and other substrates

We present a fast, high yield, low cost method for the production of scanning probes with aligned carbon nanotubes protruding from the ends. The procedure is described and images of undercut films are used to demonstrate the improved probe quality for topography measurements. A magnetophoretic model of the attachment and alignment processes is discussed.