Interconnected porosity analysis by 3D X-ray microtomography and mechanical behavior of biomimetic organic-inorganic composite materials.

Hydroxyapatite-based materials have been used for dental and biomedical applications. They are commonly studied due to their favorable response presented when used for replacement of bone tissue. Those materials should be porous enough to allow cell penetration, internal tissue growth, vascular incursion and nutrient supply. Furthermore, their morphology should be designed to guide the growth of new bone tissue in anatomically applicable ways. In this work, the mechanical performance and 3D X-ray microtomography (X-ray µCT) study of a biomimetic, organic-inorganic composite material, based on hydroxyapatite, with physicochemical, structural, morphological and mechanical properties very similar to those of natural bone tissue is reported. Ceramic pieces in different shapes and several porous sizes were produced using a Modified Gel Casting Method. Pieces with a controlled and 3D hierarchical interconnected porous structure were molded by adding polymethylmethacrylate microspheres. Subsequently, they were subject to a thermal treatment to remove polymers and to promote a sinterization of the ceramic particles, obtaining a HAp scaffold with controlled porosity. Then, two different organic phases were used to generate an organic-inorganic composite material, so gelatin and collagen, which was extracted from bovine tail, were used. The biomimetic organic-inorganic composite material was characterized by Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, X-ray Diffraction, Fourier Transform Infrared Spectroscopy and 3D X-ray microtomography techniques. Mechanical properties were characterized in compression tests, obtaining a dramatic and synergic increment in the mechanical properties due to the chemical and physical interactions between the two phases and to the open-cell cellular behavior of the final composite material; the maximum compressive strength obtained corresponds to about 3 times higher than that reported for natural cancellous bone. The pore size distribution obtained could be capable to allow cell penetration, internal tissue in-growth, vascular incursion and nutrient supply and this material has tremendous potential for use as a replacement of bone tissue or in the manufacture and molding of prosthesis with desired shapes.

A Two-Dimensional Model for Equilibrium Partitioning of a Fluid Mixture through a Microporous Semipermeable Crystalline Membrane. A Monte Carlo Study.

We have considered a simple two-dimensional model for a system consisting of a two-component mixture of hard discs on one side of a microporous slit-like semipermeable membrane and one-component fluid of discs on the other side. The particles of a slit-like membrane are fixed according to either (11) or (10) crystal symmetry. The distance between these particles is chosen such that only one fluid component can permeate the membrane. Osmotic equilibrium in the system is then established. The entire system is confined, for technical convenience, to a wide slit-like pore with the membrane in the center. The walls of the wide pore are distanced from the external surfaces of the membrane to provide the bulk region where the density profiles appear to be constant. Monte Carlo canonical simulation results are presented for the density distributions of the fluid particles in the entire wide pore. We have observed that partitioning of the smaller particles essentially depends on the concentration of the larger particles on one side of the membrane. The osmotic pressure is calculated from the contact values of the density profiles on the walls of a wide pore using the contact theorem. The pressure also has been obtained via Boublik's equation of state for a mixture of hard discs using the bulk densities of species obtained from simulations. The values for the partition coefficients on the osmotic pressure are discussed. Copyright 1998 Academic Press.

Liver blood flow in chronic hemodialysis patients.

The purpose of this study was to assess liver blood flow and cardiac output in chronic hemodialysis patients (n = 7) before and after a hemodialysis session, and to compare it to normal volunteers (n = 11). We used the hepatic clearance of sorbitol to calculate liver blood flow and echocardiograms to evaluate cardiac output. The latter was higher in hemodialysis patients (predialysis 4.7 +/- 1.8 liters/min, postdialysis 4.5 +/- 0.9 liters/min) compared to normal subjects (3.8 +/- 0.9 liters/min, p = 0.09 and p = 0.05). Hepatic blood clearance of sorbitol was similar in hemodialysis patients before and after dialysis (1,610 +/- 266 and 1,541 +/- 415 ml/min) as well as in normal volunteers (1,565 +/- 313 ml/min). The hepatic extraction ratio of sorbitol is slightly decreased in hemodialysis patients (n = 3) (0.87 +/- 0.03) compared to values reported in the literature for healthy subjects. We conclude that liver blood flow is not significantly altered in hemodialysis patients.

Variable-spectral-=response optical waveguide Bragg grating filters for optical signal processing.

A simple method is described that permits the spectral response (spectral width, shape, and center resonant wavelength) of an optical waveguide Bragg grating to be controlled accurately in a prescribed manner. The control methodology consists of bonding the Bragg grating along the length of a mechanical support structure, which is then loaded with an appropriate force distribution. The function of the support structure is to transfer the strain induced by loading to the grating, thus modifying the grating's spectral response in accordance with the variation in effective optical pitch induced by the strain transfer. We design and demonstrate a support structure that provides independent control over the spectral width and center wavelength of a Bragg grating.

Photosensitivity in Ge-doped silica optical waveguides and fibers with 193-nm light from an ArF excimer laser.

Photosensitivity in optical fibers and waveguides has been associated with the bleaching of an absorption band located near 5.0 eV (or 242 nm). We present new results for Bragg grating formation and UV bleaching experiments carried out using 193-nm light from an ArF excimer laser instead of the usual laser sources operating near 242 or 248 nm.

Chirped in-fiber Bragg gratings for compensation of optical-fiber dispersion.

A linearly chirped in-fiber Bragg grating is reported that can compensate at 1549 nm for the dispersion [ approximately -19 ps/(nmkm)] of standard telecommunications optical fiber optimized for 1300-nm operation.

Photosensitization of optical fiber and silica-on-silicon/silica waveguides.

Localized heating with a flame is shown to be a simple and effective method for substantially augmenting the photosensitivity of high-silica optical waveguides to (UV) light. The method increases the photosensitivity of standard (Ge-doped core) telecommunications fiber by a factor greater than 10 (photoinduced Deltan(uv) > 10(-3)) and renders strongly photosensitive the cores of high-quality Ge:SiO(2)-on-Si and Ge:SiO(2)-on-SiO(2) planar waveguides that were negligibly photosensitive before treatment. We have written large-modulation-depth Bragg gratings, in both fiber and planar optical waveguides photosensitized by our method, using KrF (249-nm) radiation incident upon the waveguides through a zero-order-nulled phase mask. It is noteworthy that photosensitization by our method is achieved with a negligible increase in loss at the three principal optical communication windows.

Single-excimer-pulse writing of fiber gratings by use of a zero-order nulled phase mask: grating spectral response and visualization of index perturbations.

Optical fiber Bragg reflectors have been written by irradiating the fiber from the side through a phase mask with a single pulse of high-power 249-nm excimer-laser light. Efficient tapping of light to the radiation modes has been achieved for light at wavelengths shorter than the Bragg wavelength. The photoinduced periodic refractive-index perturbations have been observed directly with an optical microscope and are shown to have the same period as the phase mask and to be highly localized on one side, the irradiated side of the fiber core-cladding boundary.

Long-length, long-period rocking filters fabricated from conventional monomode telecommunications optical fiber.

Using fiber photosensitivity, we have made efficient, application-specific rocking filters from conventional monomode telecommunications optical fiber. Filters with a length of 10.89 m, a bandwidth of 58 nm, and a transmission of 99% at the resonant wavelength of switching 200-fs solitons.

Refractive-index changes in fused silica produced by heavy-ion implantation followed by photobleaching.

The changes in refractive index, optical absorption, and volume of synthetic fused silica resulting from the implantation of germanium and silicon ions at energies of 3 and 5 MeV are reported. Implantation changes the density and generates ultraviolet color centers in the silica, which increases the refractive index at visible wavelengths by ~1%. Irradiation of the implanted samples with 249-nm light from a KrF excimer laser photobleaches the color centers and reduces the index by more than 0.1%. Photobleaching is used to write a 4.3-microm pitch diffraction grating in the implanted silica.

Ultraviolet light photosensitivity in Ge-doped silica fibers: wavelength dependence of the light-induced index change.

A novel technique is reported for detecting permanent and transient light-induced refractive-index changes (photosensitivity) in optical fibers. The index change is detected by irradiating one arm of an unbalanced Mach-Zehnder fiber interferometer with UV light, thereby changing its optical path length. From a measurement of the change in the spectral response of the Mach-Zehnder interferometer, the change in the fiber core index as a function of wavelength can be determined. The equilibrium change in the core index is found to have an almost constant value of approximately 2.3 x 10(-5) over the measured wavelength range of 700 to 1400 nm.

Ultraviolet-light photosensitivity in Er(3+)-Ge-doped optical fiber.

Light-induced refractive-index change in Er(3+)-Ge-doped optical fiber is reported for the first time to the authors' knowledge. Evidence of the change is observed when UV light (lambda = 249 nm) irradiates one arm of an unbalanced Mach-Zehnder interferometer made from Er(3+)-Ge-doped fiber. From a measurement of the change in the spectral response of the interferometer with UV exposure, the change in fiber core index as a function of wavelength is determined. The equilibrium change in core index is found to vary between 2.3 x 10(-5) and 3.7 x 10(-5) over the measured wavelength region of 800 to 1700 nm. Also for the first time to our knowledge, fused couplers made of Er(3+)-doped fiber are reported. These identical fiber couplers are used in a novel all-fiber unbalanced Mach-Zehnder interferometer.

Atom confinement in a laser open atomizer.

We have investigated the effects of a buffer gas (air) on the vapor plume produced in a laser open atomizer using atomic absorption spectroscopy. The absorption signal is related to the concentration of absorbing species by a modified Beer-Lambert equation in which the buffer gas pressure and temperature and the diameter of the diffusing species appear explicitly. Our experimental results show that the observed maximum of absoprtion as a function of gas pressure is well explained by the classical gas diffusion theory. Furthermore, our measurements indicate that the plume created by the laser has a conical shape.