Key Engineering Materials Vols. 284-286

Abstract: A study is made of fracture from cyclic loading of WC spheres on the surfaces of brittle layers on compliant substrates, as representative of repetitive occlusal contact on dental crowns. Several damage modes—radial cracks at both top surface and cementation interface, and classical cone cracks as well as deep penetrating cone cracks from the top surface—have been identified and analyzed. The most dangerous fractures are radial cracks that initiate from either the top or bottom surfaces of the brittle layers and spread laterally to failure. In fatigue, these cracks are driven by chemical forces associated with the intrusion of water into the crack. Also dangerous are deep penetrating cone cracks which, unlike their classical cone crack counterparts, are mechanically driven by hydraulic pumping and can evolve rapidly with cyclic loading, threatening the lifetime of a dental crown veneer.

Abstract: In this study, it is shown that it is possible to prepare carboxymethyl-chitosan/Ca-P hybrids using an innovative “auto-catalytic” co-precipitation method, namely by using an acid and an oxidant bath. The X-ray diffraction (XRD) patterns evidenced the formation of crystalline calcium-phosphate precipitates when using an acid bath, while amorphous ones were obtained for those produced in the oxidant bath. The Fourier Transform Infrared spectroscopy (FTIR) and Scanning Electron Microscopy (SEM/EDS) studies revealed that the extent of the polymer precipitation and formation of calcium-phosphates is directly dependent on the pH and composition of the baths. Furthermore, by conducting bioactivity tests in a simulated body fluid (SBF) followed by the SEM/EDS analysis it was possible to detect the formation of an apatite layer with a cauliflower-like morphology on the surface of hybrids prepared by the acid bath, after 7 days of immersion. These results are quite promising because they can allow for the production of bioactive and biodegradable 3D porous scaffolds to be used in bone tissue engineering applications.

Abstract: Sheet nacre is a promising natural bioceramic, which consists on the internal lustrous “mother of pearl” layer of many molluscan shells, e.g. Pinctada, our model. The aim of this work is to study the structure of the flat polygonal tablets of nacre, in order to understand the multi-scale organization of this composite material and the role of the organic template during the growth of the biocrystal. We studied the organic matrix, in situ with techniques such as darkfield transmission electronic microscopy (TEM) on small cross-sections of nacre of Pinctada maxima, or intermittent-contact atomic force microscopy coupled with phase imaging on samples of nacre of Pinctada margaritifera polished parallel to the surface of the tablets. In this study, we demonstrate the continuity of the organic framework and the crystallographic orientation in the biocrystal at 2 relevant levels : nano- and micro-scale.

Abstract: Bovine derived hydroxyapatite was doped with partially stabilized zirconia with 3 mol% yttria. The influence of sintering temperature and the amount of the incorporated zirconia on densification, mechanical properties, crystalline structure and microstructure was investigated. The results indicate that the produced materials exhibit poorer mechanical properties than the relevant composite materials of enamel derived hydroxyapatite. This difference can be attributed to F, since F-content in bone is much higher than in enamel. Hence, in the light of the prospective use of the investigated materials in biomedicine, bovine derived hydroxyapatite doped with zirconia can potentially serve as graft material for load bearing applications if very small amount of F can be incorporated, in order to resemble the composition of enamel derived apatite.

Abstract: Organic-inorganic hybrids involving Ti-O bonds were coated on stainless-steel (SUS316L)substrates. Tetraisopropoxide and titanium methacrylate triisopropoxide were employed as the major starting chemicals to provide TiO2-polydimethylsiloxane (PDMS) layers or organotitanium molecular thin layers, respectively. Fourier transform infrared spectra indicated that each layer contained Ti-O bonds in their structure. The obtained hybrid layers had little effects on the blood-clotting times such as active partial thromboplastin time and prothrombin time. In addition, the number of adhered platelet on the TiO2-PDMS layers depended on the composition, while the organotitanium molecular thin layers suppressed fibrinogen adsorption compared with coating-free SUS 316L substrate.

Abstract: Bioactive poly(e-caprolactone)-siloxane hybrid material was newly developed and its in vitro and in vivo evaluations were made for the potential application as a bone substitute. The polymer precursor, triethoxysilane end capped poly(e-caprolactone) was prepared by the reaction with a,w-hydroxyl poly(e-caprolactone) and 3-isocyanatopropyl triethoxysilane with 1,4-diazabicyclo [2,2,2] octane as a catalyst and toluene as a solvent. The triethoxysilane end capped poly(e-caprolactone) was hydrolyzed and condensed to yield a hybrid sol-gel material. The gelation was carried out for 1 week at ambient condition in a covered Teflon mold with a few pinholes and then dried under vacuum at room temperature for 48 h. Its bioactivity was evaluated by examining the apatite formation on its surface in the SBF and its osteoconductivity was assessed in the tibia of white rabbit. The hybrid material showed apatite-forming ability in the SBF within 1 week soaking. Besides, new bone was formed on the surface of a cylindrical shaped specimen with no histologically demonstrable intervening non-osseous tissue after 6 weeks implantation. There was no evidence of inflammation or foreign body reaction. From the results, it can be concluded that this newly developed hybrid material has osteoconductivity and is likely to be used for the application as a bone graft substitute.

Abstract: Self-assembled nano-materials are currently an area of research with high throughput due to the opportunities it provides to fields ranging from semiconductor engineering to gene delivery. There is also considerable interest in nano-particulate systems that attain a lower energy state by self-assembly through favorable and repeated surface interactions as they mimic those commonly found in natural biological systems. This work presents a simple route to first synthesise a highly stable suspension of nano-hydroxyapatite (~40nm) with chitosan and subsequently self-assemble the suspended nano-hydroxyapatite particulates onto a substrate.

Abstract: Apatite formation in living body is essential condition for artificial materials to exhibit bone-bonding ability, i.e. bioactivity. It has been recently revealed that sulfonic group triggers apatite nucleation in body environment. Organic-inorganic hybrids consisting of organic polymer and the sulfonic group are therefore expected to be useful for novel bone-repairing materials exhibiting flexibility as well as bioactivity. In the present study, organic-inorganic hybrids were prepared from vinylsulfonic acid sodium salt and hydroxyethylmethacrylate (HEMA), a kind of acrylic polymer. Bioactivity of the hybrids was assessed in vitro by examining their acceptance of apatite formation in simulated body fluid (SBF, Kokubo solution). The obtained hybrids showed the apatite deposition after soaking in SBF within 7 d.

Abstract: In tissue engineering, a scaffold helps determine 3-dimensional morphology, increases cell survival, provides initial mechanical stability, supports tissue ingrowth, aids in the formation of tissue structure. Chitosan is the partially deacetylated form of chitin that can be extracted from crustacean. It degrades in the body to non-harmful and non-toxic compounds and has been used in various fields such as nutrition, metal recovery and biomaterials. Hydroxyapatite, a major inorganic component of bone, has been used extensively for biomedical implant applications and bone regeneration due to its bioactive, biodegradable and osteoconductive properties. The application, however, of hydroxyapatite is limited due to own brittleness. Since the natural bone is a composite mainly consisted of organic collagen and inorganic hydroxyapatite, many efforts have been made to modify hydroxyapatite by polymers. In this study, organic/inorganic hybrids were fabricated solid-liquid phase separation and a subsequent freeze-drying process. The microstructure, mechanical properties, and bioactivity of the scaffolds with various contents of hydroxyapatite were studied. The structure of the scaffolds prepared was macroporous and interconnected. The compressive mechanical properties such as compressive modulus and yield strength were improved according to the increase of hydroxyapatite contents mixed with chitosan. After 7 days of sample immersion in a simulated body fluid, for scaffolds containing hydroxyapatite, numerous bonelike apatites were formed on the surfaces of the pore walls. This study suggests that desirable pore structure, mechanical properties, and bioactivity of the hybrid scaffolds might be achieved through controlling the ratio of hydroxyapatite and chitosan.

Abstract: Biomimetic actuators that can produce soft-actuation but large force generation capability are of interest. Nafion, an effective ionomeric material from DuPont, has been shown to produce large deformation under low electric fields (<10V/mm). This response is caused by a direct electro-osmotic effect due to the existence and mobility of cations and subsequent swelling and deswelling of the material. In this effort, multiwalled carbon nanotube (MWNT)/Nafion nanocomposites were prepared by a casting in order to investigate the effect of MWNT loading in the range of 0 to 7 wt% on electromechanical properties of the MWNT/Nafion nanocomposites. The measured elastic modulus and actuation force of the MWNT/Nafion nanocomposites are drastically different, showing larger elastic modulus and improved electromechanical coupling, from the one without MWNT, implying that the effective MWNT loading is crucial to develop high-performance biomimetic actuators.