Naohide Tomita

Effects of static magnetic field on bone formation of rat femurs

Effects of static magnetic fields (SMF) on bone formation of rat femurs, were evaluated using tapered rods made of magnetized and unmagnetized samarium cobalt of the same size. They were implanted transcortically into the middle diaphysis of rat femurs under press-fit loading. The bone mineral density (BMD) and bone calcium content were measured 12 weeks after implantation by dual-energy X-ray absorptiometry and chemical analysis with o-cresolphthalein complexon, respectively. The result revealed that the femurs adjacent to magnetized specimens had significantly higher BMD and calcium content than those adjacent to the unmagnetized specimen (p < 0.01). However, the value of BMD and calcium content of rats with magnetized specimens was similar to that of non-operated rats. No specific change was found in the body weight, serum Ca, activity of alkaline phosphatase, hemogram, and BMD of the tibia and humerus among the magnetized and unmagnetized. These results suggest that the long-term local SMF stimulation on the bone has a local effect to prevent the decrease in BMD caused by surgical invasion or implantation.

Locomotion of medical micro robot with spiral ribs using mucus

An intracorporeal microrobot will not become practical unless an efficient and low invasive driving device is developed. This paper presents a new technology to drive medical microrobots and experimental results which confirm the new principle. Thrust force up to 1 N was measured when gap width was small and sliding speed was high. While it is pointed out that rupture of fluid film may limit the magnitude of thrust force. The experimental study also confirms that as thrust force arises only due to hydrodynamic action and direct contact does not arise, low invasive operation is possible if good lubrication conditions are kept.

Study on the possibility of silicon nitride—silicon nitride as a material for hip prostheses

Abstract In order to study the possibility of silicon nitride-silicon nitride as a material for hip prostheses, the running-in property, the start-up and steady-state friction of silicon nitride against itself in distilled water before and after the running-in were investigated. The experimental results indicate that after the running-in the surfaces of silicon nitride can be polished to be ultra smooth, and the friction coefficient of silicon nitride against itself in water can become very small under some test conditions. From the tribological point of view, it seems that silicon nitride-silicon nitride is a very good material for the hip prostheses.

Low invasive propulsion of medical devices by traction using mucus

This paper presents a new low invasive method to drive a medical device in an organic tube. The device has a spiral ribbed impeller which converts rotation to axial motion hydrodynamically using mucus. Generated thrust and drag were estimated according to linearized hydrodynamic lubrication theory. The optimum rib shape which makes thrust force maximum is predicted theoretically. As the proposed method of driving does not require direct contact against living tissue, injury may be prevented by keeping good lubrication conditions. Principles of designing a medical device are discussed such that reliable driving is compatible with low invasive operations.

Friction control with a graft layer of a thermo-sensing polymer

The lubricating mechanism of surfaces grafted with poly-N-isopropylacrylamide (PNIPAAM) was investigated with a thrust collar apparatus. The coefficient of friction increases with temperature rise, because PNIPAAM changes its surface property from hydrophilic to hydrophobic. Conversely the friction coefficient decreases with temperature fall. The role of water in lubrication is discussed based on the experimental result that the thickness of the graft layer decreases with temperature rise. A new technology is presented to control friction in water environments.

Marrow cell culture on poly-L-lactic acid fabrics

Bone marrow cells from rat femurs were cultured in Eagle Minimum Essential Medium containing 15% fetal calf serum until confluence. After the cells were trypsinized, they were subcultured on fabrics made of biodegradable poly-L-lactic acid for 2 weeks in the medium containing fetal calf serum, ascorbic acid phosphate, beta-glycerophosphate, and with and without dexamethasone. In the presence of dexamethasone, the fabrics showed many mineralized nodules together with cuboidal shaped cells that had osteoblastic activity, as evidenced by high alkaline phosphatase activity and the appearance of osteocalcin messenger ribonucleic acid. However, in the absence of dexamethasone, nodules did not form and many fibroblastic cells appeared with no evidence of osteoblastic activity. These results indicate the possibility of making a hybrid ligament substitute having an in vitro prefabricated bone anchor.

Collagen immobilization onto the surface of artificial hair for improving the tissue adhesion

To improve the bonding ability of artificial hair towards soft tissue, type I atelocollagen was immobilized onto the hair surface. The artificial hair used was made of a poly(ethylene terephthalate) monofilament. Following photo-induced graft polymerization of a hydrophilic monomer onto the surface of artificial hair, collagen was complexed with the graft chains. Poly(acrylic acid) was selected as the polymer to be grafted onto the artificial hair because this synthetic polymer exhibited the greatest ability to form an interpolymer complex in solution with collagen among the three anionic polymers poly(acrylic acid), poly(2-acrylamido methylpropane sulfonic acid), and sodium poly(styrene sulfonic acid). When the surface of the poly(ethylene terephthalate) film used as a model substrate was grafted with poly(acrylic acid), the surface density of the collagen immobilized by interpolymer complexation was found to increase with increasing surface density of the graft chains. Immobilization of collagen onto the filament surface was confirmed by surface analysis with X-ray photoelectron spectroscopy and transmission electron microscopy. It was shown that in vitro degradation of the collagen immobilized onto poly(ethylene terephthalate) was suppressed by crosslinking the collagen molecules with glutaraldehyde. Cell culture tests revealed that L-cells were attached well to the surface of collagen-immobilized artificial hair. The surface-modified hairs were implanted percutaneously in the scalp of a human volunteer. Neither infection nor rejection of the hair filaments was observed after 1 year of implantation. It was found that the number of collagenimmobilized filaments remaining fixed in the scalp after 3 years of implantation was significantly larger than that of untreated filaments. These results indicate that surface-modified artificial hair is highly biosafe and shows excellent tissue adhesion.

An observation on subsurface defects of ultra high molecular weight polyethylene due to rolling contact.

Ultra high molecular weight polyethylene (UHMWPE) has been used in artificial joints for a few decades, and wear of UHMWPE has been one of the main problems. Though many other materials have been tested over the years, the best clinical results are still achieved with UHMWPE. This makes the study of UHMWPE, especially in relation to artificial joints, very important. Frequently, more severe wear can be observed in artificial knee joints than in artificial hip joints especially when the flaking-like wear occurs. This flaking-like wear can lead to significant destruction of the artificial knee joint. Macroscopically, artificial knee joints have combinational movements of rolling and sliding in order to simulate the motion of the normal knee joint. The components of motion are separated to make study easier. Fatigue tests of UHMWPE under the rolling contact condition were performed in this study. Three ceramic spheres were rolled over the UHMWPE specimen using 37 degrees C distilled water as a lubricant. The UHMWPE specimen was observed by the scanning acoustic tomography, microscopy, and SEM. Some subsurface defects could be observed by SAT even before experiments. Although the apparent wear is not observed on the surface, there was an increase in the number of observable subsurface cracks in the UHMWPE specimen. This shows that cracks occur under the surface after a 10(7) rolling contact loading, which is very close to the cyclic loading and unloading with very little friction compared to the sliding contact.

Design Concept of Artificial Knee Joint for High Durability

Flaking-type wear, so-called delamination, is thought to occur when an applied load to a polyethylene component is increased. This type of wear is caused by fatigue of the polyethylene accelerated by some physical and chemical factors. In the study reported here, the flaking-type wear was reproduced using a rolling fatigue testing machine, and an influence of loading condition on crack formation was examined using a joint-simulated fatigue testing machine. Subsurface cracks were observed by scanning acoustic tomography (SAT). The results showed the fatigue to develop slowly during the early and middle periods of loading, followed by sudden explosive destruction (flaking), accelerated by a complicated sliding motion. A new design concept for an artificial knee joint was proposed based on these results.

A new method for accurate measurement of displacement of the knee menisci

Abstract It is difficult to measure the displacement of the menisci under external forces by the radiographic method using markers because the movement may be smaller than the detecting limit. A device with a flexible needle attached to a journal of a miniature linear bearing is developed to investigate the meniscal functions which depend on the mobility. The needle tip is fixed to the target point in the meniscus and the axial displacement is measured with a laser sensor. According to the test, the lower limit for detection was 10 μm, the error due to non-linearity was less than 5 per cent and the reactive force was less than 0.15 N in the operating range of ± mm. Displacement vectors were determined at four points in a pair of menisci in a porcine knee subjected to external forces. This accurate method will lead to further investigation into the mechanical function of the menisci.