Microstructure, mechanical property and corrosion behavior of porous Ti-Ta-Nb-Zr.

In this paper, biomedical porous Ti-Nb-Ta-Zr with 40% porosity and 166 ± 21 µm macro-pore size was successfully fabricated by space holder method. The microstructure, Vickers hardness, compressive and electrochemistry behavior were studied. It results that a few second phases exist in ß matrix of the porous Ti-Nb-Ta-Zr. Its Young's modulus is 0.8 GPa, close to 0.01-3 GPa for trabecular bone. The total recovery strain ratio and pseudoelastic strain ratio are 8.8% and 2.7%, respectively. It fails mainly by brittle cleavage with the fan-shaped and smooth cleaved facets. Although, local ductile fracture by a few dimples and a small amount of transcrystalline fracture with the cleavage of similarly oriented laths in a colony are observed on the fracture surface. The impedance spectrum of porous Ti-Nb-Ta-Zr has the characteristics of half capacitive arc resistance, showing good corrosion resistance in SBF solution.

Nutritional requirements of the corneal epithelium and anterior stroma: clinical findings.

PURPOSE: To monitor the health of the epithelium and the anterior stroma when porous membranes are implanted into the feline cornea and to determine membrane diffusivity characteristics needed to maintain corneal integrity. METHODS: Filtration membranes in a range of effective pore sizes of less than 15 nm (groups 1 and 2, n = 11), 25 nm (group 3, n = 8), 50 nm (group 4, n = 16), and 100 nm (group 5, n = 15) were implanted into an interlamellar corneal pocket of the stroma. The implanted membranes ranged in thickness from 6 nm to 15 nm and were between 8 mm and 12 mm in diameter. Animals were monitored for clinical signs of intolerance to the implants. RESULTS: At 1 month, thinning and ulceration had occurred in the epithelium and the anterior stroma of all animals in groups 1 and 2; epithelial changes, anterior stromal thinning, and ulceration had developed in 75% of animals of group 3; 50% of animals showed vascularization and only 7% showed epithelial degeneration in group 4; and local anterior stromal thinning was observed in 7% of animals in group 5, indicating clinical acceptance of the implanted membrane. In the long term (greater than 50 days), 30% and 73% of the group 4 and 5 corneas, respectively, were clinically quiet. Analysis of the failure times indicated an inverse relation between failure rate and pore size: less than 15 nm > 25 nm > 50 nm > 100 nm. The difference between the 100-nm and 50-nm membranes was significant (P = 0.03). CONCLUSIONS: A corneal implant must have a porosity greater than that provided by 50-nm membranes. The 100-nm membranes used in this study establish the porosity needed to satisfy the nutritional requirements of the cornea.

Epithelialization of a synthetic polymer in the feline cornea: a preliminary study.

PURPOSE: This study examined the potential of a synthetic polymer to support stable epithelial growth when implanted in the feline cornea. METHODS: A perfluoropolyether-based polymer was cast into lenticules that were coated with collagen I and implanted in four feline corneas. Epithelial growth onto the lenticules was monitored clinically for 6 weeks, after which time the animals were killed, and three corneas were evaluated histologically. Immunohistochemistry was used to identify proteins associated with the formation of a basement membrane (laminin) and adhesion complexes (bullous pemphigoid antigen and collagen VII). Electron microscopy was used to examine the tissue-polymer interface for evidence of the assembly of these adhesive structures. RESULTS: Postoperative epithelial growth began on days 2 to 3, and lenticules were fully epithelialized by days 5 to 9. Lenticules were clinically well tolerated and histology showed epithelium consisting of multiple layers adherent to the lenticule's surface. Laminin, bullous pemphigoid antigen and collagen VII were identified at the tissue-polymer interface using immunohistochemistry. Ultrastructural examination showed evidence of assembly of these proteins into a recognizable basement membrane and hemidesmosomal plaques. CONCLUSIONS: A perfluoropolyether-based polymer coated with collagen I was implanted in the feline cornea and supported epithelial growth that showed signs of persistent adhesion, both clinically and histologically. This polymer shows potential for ophthalmic applications that require sustained epithelialization.

Artificial cornea: towards a synthetic onlay for correction of refractive error.

Synthetic onlays that are implanted onto the surface of the cornea have the potential to become an alternative to spectacles and contact lenses for the correction of refractive error. A successful corneal onlay is dependent on development of a biocompatible polymer material that will maintain a healthy cornea after implantation and that will promote growth of corneal epithelial cells over the onlay, and development of a method for attachment of the onlay with minimal surgical invasiveness. The ideal onlay should be made of a material that is highly permeable yet has sufficient surface characteristics to stimulate stable and firm attachment of the corneal epithelium over the onlay. Recent research indicates that collagen I coated polymer materials that mimic the basement membrane of the corneal epithelium promote the most favorable growth of epithelial cells in vivo in comparison to wholly biological or synthetic materials.

A thin glycoprotein coating of a synthetic lenticule does not cause nutritional deficiency of the anterior cornea.

PURPOSE: This study investigated whether a glycoprotein coating that will be used to enhance corneal epithelialization affects in situ nutritional passage through a permeable membrane. METHODS: Sixteen adult cats were equally divided into two groups. Polycarbonate membranes with pore size of 0.1 microm and total pore area (porosity) of 3.1% were used as implant materials. The membranes for Group 1 were coated with a thin layer of Collagen I, while the membranes for Group 2 were uncoated. Each membrane with 8-mm diameter was implanted into an interlamellar pocket of the cornea. The eyes were observed for approximately 35 days to monitor clinical signs of nutritional deficiency of the cornea, and then 7 membranes were removed from the eyes. The permeability of the explanted membranes to glucose, inulin and albumin was used to predict the in situ difference between the coated and uncoated groups in regard to nutritional passage through the membranes. To investigate the long-term effect of the surface coating on corneal health, two animals from Group 1 were followed for up to two years and then both eyes of each animal underwent histological examination. RESULTS: Clinically, no post-surgical complications associated with nutritional deficiency were observed in any of the eyes. Nutritional permeability tests showed no significant differences between the coated and uncoated membranes. Histologically, the long-term animals showed no abnormal morphology associated with nutritional deficiency in the cornea anterior or posterior to the membranes. CONCLUSIONS: A thin glycoprotein coating on a permeable membrane does not appear to affect the nutritional supply of the anterior cornea and therefore can be used to enhance epithelialization of synthetic corneal onlays in vivo.

Changes in eye growth produced by drugs which affect retinal ON or OFF responses to light.

The interaction between drugs which affect ON and OFF responses to light and ocular growth was investigated in 154 eyes of newly-hatched chicks raised with normal vision or monocular occlusion with regular intravitreal injections of APB (2-amino-4 phosphonobutyric acid) or PDA (cis 2,3-piperidine-dicarboxylic acid). The experimental results indicate that APB, at a dose sufficient to abolish the b-wave of the electroretinogram (but not to show extensive damage to the retinal neurons at the light microscopic level), caused a significant decrease in the axial growth of the eyes when compared with normal eyes. APB did not result in a significant difference in the growth rates of the occluded eyes compared with occluded controls. By contrast, injection of PDA into occluded eyes at a dose which reduced the response at light offset in the electroretinogram (and also affected the ON-response), caused a dramatic reduction in elongation of those eyes compared with occluded controls. Injection of PDA into eyes raised in a normal visual environment did not induce a significant difference from the growth rate of normally reared control eyes. These growth changes support the hypothesis that drugs which affect the physiological function of the retina, in particular, the strength of the ON and OFF responses, interact with the visual rearing environment to cause a consistent pattern of changes to eye growth and refractive error.

Effects of surface topography on corneal epithelialization in vivo: a preliminary study.

PURPOSE: In this study we investigated the relationship between surface topography and initial epithelialization of synthetic lenticules in vivo. METHODS: Millipore ultrafiltration membranes of three different pore sizes were used as model lenticule materials. The nominal membrane pore sizes were 0.1 microm, 0.45 microm, and 3 microm; the surface roughness increased in the same order The membranes were coated with a thin layer of collagen I and implanted in a circular pocket of the anterior cornea of adult cats, and were clinically evaluated for the extent of epithelialization and the persistence of epithelial attachment. RESULTS/CONCLUSIONS: It was demonstrated that a smooth surface appears more attractive for initial epithelialization than a rough surface in vivo.

Effects of biologically modified surfaces of synthetic lenticules on corneal epithelialization in vivo.

PURPOSE: The present study investigated the association of extracellular matrix (ECM) glycoprotein coatings with initial epithelialization of artificial lenticules in vivo. METHODS: Collagen I, collagen IV, ECM and fibronectin were individually coated onto the surface of polycarbonate membranes. The membranes were then implanted in the anterior stroma of adult cats and were clinically assessed for rapidity and extent of epithelialization and the persistence of epithelial attachment. RESULTS/CONCLUSIONS: It was demonstrated that membrane surfaces modified by collagen I, collagen IV and ECM consistently supported initial migration and attachment of corneal epithelial cells and that the surface modified with collagen I performed best.

Persistent adhesion of corneal epithelial tissue on synthetic lenticules in vivo.

PURPOSE: In intact cornea, firm anchorage of the epithelium to its underlying stroma is provided by a system of adhesive structures, which include the epithelial basement membrane, adhesion complexes and associated alpha6beta4 integrin receptors. In the present study we sought to determine whether the sustained epithelialization of synthetic lenticules observed in vivo involved the formation of adhesive structures at the epithelial-lenticule interface similar to those present in intact tissue. METHODS: Collagen I-coated microporous polycarbonate membranes were implanted in the anterior stroma of adult cats. Successfully epithelialized lenticules were maintained on eye for 9-12 weeks, after which time the epithelial-lenticule interface was examined for evidence of adhesive structures using immunohistochemistry and electron microscopy. RESULTS: Immunohistochemistry identified laminin, hemidesmosomal plaque, collagen VII and alpha6 integrin at the tissue lenticule interface. Ultrastructural examination showed evidence of assembly of these components into a basement membrane and adhesion complexes. CONCLUSIONS: The formation of these adhesive structures is likely to have contributed to the sustained epithelialization observed clinically on the collagen-coated synthetic lenticules.