Tatsuya Miyajima

Optical indentation microscopy – a new family of instrumented indentation testing

A novel test system/analysis is proposed for determining the in situ true contact area during indentation loading/unloading by the use of an optically transparent indenter. A sapphire planoconvex lens as a spherical indenter or a diamond tetrahedral pyramid indenter is optically coupled with a microscope system equipped with a CCD video camera. This indentation microscope enables us to directly view the in situ contact impression, along with the measurement of the hysteresis curve of the indentation load-penetration depth relation. The in situ contact area can be readily determined from each frame of the video movie using an image analyzer that is synchronized with the indentation load. Accordingly, the mechanical properties including the true indentation stress–strain curve can be evaluated directly from these observable parameters without using any assumptions that have always been required for estimating the contact area in the conventional depth-sensing indentation apparatus.

High-temperature mechanical properties of sinter-forged silicon nitride with ytterbia additive

Anisotropic silicon nitride with ytterbia additive was successfully fabricated by using a sinter-forging technique. The sinter-forged specimen had a strongly anisotropic microstructure where rod-like silicon nitride grains preferentially aligned perpendicular to the forging direction. The specimen exhibited higher strength and higher fracture energy compared to the conventionally hot-pressed specimen. These superior mechanical properties of the sinter-forged silicon nitride were attributed to grain bridging and pullout enhanced by grain alignment. At elevated temperatures, softening of the grain boundary glassy phase and melting of the secondary crystalline phase should lead to degradation of strength and increment of fracture energy.

Elastic/Plastic Surface Deformation of Porous Composites Subjected to Spherical Nanoindentation

The elastic/plastic parameters of Hydroxyapatite/ Poly (D , L-lactide) porous bio-composites are investigated using two distinct spherical indenta tion methods. A conventional constant-rate indentation method focusing only on the initial loading cur ve is used to determine Young’s modulus and the yield stress. For the evaluation of time-depende nt viscoelastic deformation and flow, a novel constant-penetration-depth (stress relax ation) test is conducted, and the stress relaxation time is successfully determined. Introduction Porous composites consisting of biodegradable materials, i.e. hydroxya patite, HAp, and Poly (D, L-lactide), PLA, are candidate materials for numerious applic ations in bio-material engineering because of their potential to exhibit mechano-compatibility and biocom patibility. Material design of the porous biomaterial requires the knowledge of time independent elas tic/p tic properties and time dependent plastic behavior, as well as fracture parameters. Indenta tion tests are technically simpler than other conventional fracture mechanical testing methods for deter mining these parameters with easier to prepare test specimen During the last two decades, numbers of instrumented indentation system equipped with both load and indentation depth sensors have been developed[1-5]. The indentation load ( P) versus depth (h) curve during loading/unloading cycle gives elastic and/or plastic sur face deformation parameters. Although most of the Young’s modulus evaluation study by the use of Ph curve has been addressed in terms of the initial unloading stiffness, dP/dh, it is expected that polymer based bio-composites will exhibit time-dependence other than elastic behavior. The maj or aim of the present work is the experimental determination of the elastic/plastic properties of HAp/PLA porous composites using two different types of spherical indentation methods. Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 927-930 doi:10.4028/www.scientific.net/KEM.240-242.927

Preparation of Porous Poly(Lactic Acid)/Hydroxyapatite Microspheres Intended for Injectable Bone Substitutes

Porous biodegradable microspheres were successfully obtained by an improvement single step and surfactant-free emulsion solvent evaporation method. The organic phase composed of PLA and dichloromethane was stirred in aqueous phase including Ca2+ ions to yield oil in water emulsion. During emulsification, stirring rate was increased so as to produce the W/O/W emulsion that results in microspheres with internal pores. The interface of internal water/oil was stable in W/O/W emulsion, which was explained that the bond between Ca2+ ions and carboxyl group of poly(lactic acid) would be stabilized the internal water/oil interface. Adding PO4 3- aqueous solution prompted to precipitate low crystallized hydroxyapatite on the external oil/water interface, and the precipitated hydroxyapatite would stabilizied microspheres formation. The resulting microspheres were approximately 100-500 µm with internal spherical pores of 10-200 µm in diameter. The porous biodegradable microspheres were expected to be utilized as injectable bone substitutes that allow bone ingrowth and bone regeneration.

Preparation of Porous Composites Consisting of Apatite and Poly(D,L-Lactide)

Porous composites made of microspheres consisting of only biodegradable materials were prepared under the condition at ordinary temperature and pressure. Poly (D,L-lactide) (PLA) and hydroxyapatite (HAp) microspheres were prepared by a surfactant-free emulsion solvent evaporation method using calcium phosphate precipitation as stabilizer. The obtained microspheres were molded with dissolved PLA in solvent and porous materials were formed by solvent evaporation. Introduction Formation of porous materials using biodegradable microspheres would have the possibility to obtain easily three-dimensional designed materials for bone grafts[1]. Biodegradable polymers such as poly (D,L-lactide), PLA are now used as bone implant materials. In general, PLA microspheres are prepared by emulsion method with the aid of surfactants, however it is pointed out that these surfactants remain in the final products[2] and may be affected to human body such as an allergy-like reaction because of their non-biodegradability. Poly(vinyl alcohol), PVA, is most frequently used as a surfactant to stabilize the oil-in-water emulsification for PLA microspheres, but the residual PVA is also suspected of carcinogenicity[3]. In order to solve this problem, we have been studying preparation of PLA and hydroxyapatite (HAp) microspheres without surfactants. In this paper, we report surfactant-free preparation of PLA and HAp microspheres and formation of porous materials consisting of the microspheres Methods An oil-in-water emulsion solvent evaporation method was used for PLA/calcium phosphate microsphere fabrication. The organic phase composed of PLA (Mw = 20,000; WAKO) and dichloromethane was poured in various concentrations of (CH3COOH)2Ca aqueous solution. The mixture was stirred to yield emulsion. (NH4)2HPO4 aqueous solution was added into the emulsion. For the sake of comparison, the organic phase was also stirred in the distilled water without both (CH3COOH)2Ca and (NH4)2HPO4 aqueous solutions. The suspension was stirred for 24 -72 h at room temperature to remove the dichloromethane. The suspension was filtered, washed and dried to obtain microspheres. The resulting microspheres were separated into the following size ranges: <38; 38-75; 75-150; 150-300; and >300 μm. The obtained microspheres of a specific size range were mixed with dissolved PLA and molded by injecting filtration method or centrifugation method to form a disc. The morphology of microspheres and molded discs were observed using SEM (HITACHI, S3000) and microscope (Keyence, VH-6300). Phase of the microspheres was analyzed by XRD (MAC science, MXP3). Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 167-170 doi:10.4028/www.scientific.net/KEM.240-242.167

Double Layered Microshells Composed of Calcium Phosphate and Poly (lactic acid)

Biodegradable microshells were synthesized by an improvement surfactant-free emulsion solvent evaporation method. The oil in water emulsions composed of the organic phase of poly (lactic acid) in dichloromethane and the aqueous phase including Ca 2+ ions and PO4 3ions were firstly cooled down by an ice bath to stabilize emulsification. During the emulsions was returning to room temperature, calcium phosphate was rapidly precipitated at the oil/water interface, which would result in stabilizing the microshell structure on unstable liquid oil droplets. The precipitated hard calcium phosphate shells acted as a framework for poly (lactic acid) deposition by volatilization of dicloromethane. The resulting microshells were approximately 50 300 μm in diameter with the wall thickness of 1 10 μm. The walls of the microshells had a double-layered structure composed of external walls of calcium phosphate and internal walls of poly (lactic acid). The biodegradable double-layered microshells were expected to be utilized as drug delivery carriers for injectable bone substitute.

Fabrication of Hydroxyapatite/Cellulose Fiber Composite with Sheet-Like Structure

Natural bone is a complex material with well-designed architecture. To achieve successful bone integration and regeneration, the constituent and structure of bone-repairing scaffolds need to be flexible and biocompatible. HAp, as the main composition of bone minerals, has excellent biocompatibility, while CMC comprised of a three-dimensional network were high flexibility. Therefore, CMC/HAp composite have been attracted attention due to the development of bone tissue engineering. In this work, carboxymethyl cellulose (CMC)/hydroxyapatite (Ca10(PO4)6(OH)2; HAp) composite have been developed as three-dimensional scaffold for bone tissue engineering. Scanning electron microscopy revealed that the CMC/HAp composite have sheet-like structure. The amount of precipitated HAp of CMC/HAp composite was investigated using Thermogravimetric analysis. The amount of precipitated HAp in products prepared with 100 mg CMC was 49.8 wt%, while the amount of precipitated HAp in products prepared with 1000 mg CMC was 22.3 wt%. These results revealed that the amount of precipitated HAp in CMC/HAp composite was affected by CMC amount as prepared.