Porous collagen-apatite nanocomposite foams as bone regeneration scaffolds.

We have created a porous bioresorbable nanocomposite bone scaffold that chemically, structurally and mechanically matched natural bone so that it could be recognized and remodeled by natural bone. Containing collagen fibers and synthetic apatite nanocrystals, our scaffold has high strength for supporting the surrounding tissue. The foam-like scaffold has a similar microstructure as trabecular bone, with nanometer-sized and micron-sized pores. The apatitic phase of the scaffold exhibited similar chemical composition, crystalline phase and grain size as the trabecular bone apatite. The nanocomposite scaffold demonstrated excellent bioactivity for promoting cell attachment and proliferation. It was osteoconductive and successfully healed a non-union fracture in rat femur as well as a critical-sized defect in pig tibia.

Degradation of a collagen-chondroitin-6-sulfate matrix by collagenase and by chondroitinase.

Highly porous, type I collagen-chondroitin-6-sulfate (collagen-GAG) scaffolds, produced by freeze-drying techniques, have proven to be of value as implants to facilitate the regeneration of certain tissues. The objective of this project was to evaluate changes in the microstructure and mechanical properties of selected collagen-GAG scaffolds as they degrade in an in vitro model system. Environmental scanning electron microscopy and video imaging demonstrated that collagenase degradation caused strut erosion through the creation of 1-3 microm diameter micropits within a 2-h period, leading to eventual removal of strut material and strut breakage. Loss of microstructural topography may have been due to gelatinization when collagen was cleaved by collagenase. Chondroitinase degradation of GAG resulted in swelling of the struts, causing the pores to become smaller and rounder. The compressive modulus of the collagen-GAG matrix decreased when degraded by collagenase, but remained unchanged when degraded by chondroitinase. Carbodiimide-cross-linked matrices were found to have a higher cross-link density, a higher compressive stiffness and a greater resistance to collagenase and chondroitinase, compared to non-cross-linked controls and matrices that were cross-linked by the dehydrothermal process. This investigation provides information that can be used to design collagen-GAG scaffolds with desired compressive stiffness and degradation rate to collagenase and chondroitinase.

Physico-mechanical properties of a fast-set highly viscous GIC restorative.

UNLABELLED: The fast-setting reaction of 'fast-set' highly viscous glass-ionomer cements (GIC) may result in superior mechanical properties and good wear resistance as the material can theoretically achieve sufficient strength to resist masticatory loads within a shorter time. The aim of this study was to determine the hardness, strength (compressive and diametral tensile) and wear resistance of a 'fast-set' highly viscous GIC (Fuji IX GP Fast). Its regular set counterpart (Fuji IX GP) was used for comparison. The glass powders of the two cements were also characterized. Hardness testing [Vickers' hardness number (VHN)] was performed with a digital hardness tester (load=50 g; dwell time=30 s) and compressive/diametral tensile strength testing (MPa) was conducted based upon British Standards specification for GICs (BS6039, 1981). Wear testing was conducted using a reciprocal compression-sliding wear instrumentation at 20 MPa contact stress against SS304 counter-bodies with distilled water as lubricant. All specimens were immersed in distilled water at 37 degrees C and tested 24 h after start of mixing. Further mechanical tests (hardness and strength) were conducted with specimens stored for 1 week in distilled water at 37 degrees C. The glass powders were characterized using laser particle sizing, standard electron microscope (SEM) and energy dispressure X-ray (EDX) analysis. Results were analysed using multiple analysis of variance (manova) and independent samples t-test at significance level 0.05. No significant difference in hardness, compressive and diametral tensile strength was observed between Fuji IX GP and Fuji IX GP Fast at 1 day. There was also no significant difference in wear at all cyclic intervals. Although the difference in strength was not significant between the two cements at 1 week, Fuji IX GP Fast was significantly harder than Fuji IX GP. Particle size of both cements ranged from 0.3 to 200 microm. The mean particle sizes were, however, different and were 13.43 and 7.13 microm for Fuji IX GP and Fuji IX GP Fast, respectively. Fuji IX GP Fast offers no other advantage over Fuji IX GP with the exception of improved hardness. CLINICAL RELEVANCE: Besides being harder, the fast-set highly viscous GIC restorative offers no other physico-mechanical advantage over its regular set counterpart.

Experimental studies on a new bioactive material: HAIonomer cements.

The lack of exotherm during setting, absence of monomer and improved release of incorporated therapeutic agents has resulted in the development of glass ionomer cements (GICs) for biomedical applications. In order to improve biocompatibility and biomechanically match GICs to bone, hydroxyapatite-ionomer (HAIonomer) hybrid cements were developed. Ultra-fine hydroxyapatite (HA) powders were produced using a new induction spraying technique that utilizes a radio-frequency source to spheriodize an atomized suspension containing HA crystallites. The spheriodized particulates were then held at 800 degrees C for 4 h in a carbolite furnace using a heating and cooling rate of 25 degrees C/min to obtain almost fully crystalline HA powders. The heat-treated particles were characterized and introduced into a commercial glass ionomer cement. 4 (H4), 12 (H12) and 28 (H28) vol% of fluoroalumino silicate were substituted by crystalline HA particles that were dispersed using a high-speed dispersion technique. The HAIonomer cements were subjected to hardness, compressive and diametral tensile strength testing based upon BS6039:1981. The storage time were extended to one week to investigate the effects of cement maturation on mechanical properties. Commercially available capsulated GIC (GC) and GIC at maximum powder:liquid ratio (GM) served as comparisons. Results were analyzed using factorial ANOVA/Scheffe's post-hoc tests and independent samples t-test at significance level 0.05. The effect of time on hardness was material dependent. With the exception of H12, a significant increase in hardness was observed for all materials at one week. A significant increase in compressive strength was, however, observed for H12 over time. At 1 day and 1 week, the hardness of H28 was significantly lower than for GM, H4, and H12. No significant difference in compression and diametral tensile strengths were observed between materials at both time intervals. Results show that HAIonomers is a promising material, which possess good mechanical properties. Potential uses of this new material include bone cements and performed implants for hard tissue replacement in the field of otological, oral-maxillofacial and orthopedic surgery.

Micromechanics of fibroblast contraction of a collagen-GAG matrix.

The contractile force developed by fibroblasts has been studied by measuring the macroscopic contraction of porous collagen-GAG matrices over time. We have identified the microscopic deformations developed by individual fibroblasts which lead to the observed macroscopic matrix contraction. Observation of live cells attached to the matrix revealed that matrix deformation occurred as a result of cell elongation. The time dependence of the increase in average fibroblast aspect ratio over time corresponded with macroscopic matrix contraction, further linking cell elongation and matrix contraction. The time dependence of average fibroblast aspect ratio and macroscopic matrix contraction was found to be the result of the stochastic nature of cell elongation initiation and of the time required for cells to reach a final morphology (2-4 h). The proposed micromechanics associated with observed buckling or bending of individual struts of the matrix by cells may, in part, explain the observation of a force plateau during macroscopic contraction. These findings indicate that the macroscopic matrix contraction measured immediately following cell attachment is related to the extracellular force necessary to support cell elongation, and that macroscopic time dependence is not directly related to microscopic deformation events.