This article describes the importance of the surface/interface characteristics of Biomaterials. The interactions between material surface and living body (protein, cell and tissue) are reviewed from various points of view. Basic principles for protein adsorption, cell adhesion and phagocytosis of foreign materials can be obtained by surface energetic analysis. The surface modifications by grafting of water-soluble polymer, immobilizing biological substances and attaching bone component etc. are useful for improvement of the material's biocompatibility. To develop novel artificial bodysubstituting materials, the improvement of surface/interface biocompatibility is essential.
Chitin, a polymer of 2-acetamido-2-D-glucose, and calcium phosphate composite was prepared by the process of phosphorylation of chitin substrate, Ca(OH)2 treatment and immersion in SBF solution. The C-6 chemical shift positions of 13C MAS NMR in the chitin fibres phosphorylated using urea and H3PO4 obviously indicate that phosphorylation takes place not in the C-1 but in the C-6 region. Chitin samples were phosphorylated using urea and H3PO4 and then soaked in saturated Ca(OH)2 solution at ambient temperature. This leads to the formation of thin coatings by particalhydrolysis of the PO4 functionalities, and stimulated growth of calcium phosphate coating on their surfaces after soaking in SBF solution was observed. The procedure can be applied to chitosan samples. This study demonstrates a useful method for creating faborable lacal conditions that lead to nucleation and growth of calcium phosphate over chitin or chitosan substrates.
Can we synthesize artificial composite materials that have a similar nanostructure to bone in vitro? From the viewpoint of self-organization, a novel apatite/collagen nanocomposite was developed. As a result, apatite nanocrystals aligned along collagen molecules, the nanostructure quite similar to bone. The self-organization in the composite concurred at the almost same condition for bone formation. Origin of self-organization was comprehended on the basis of chemical interaction between organic functional groups and surfaces of inorganic crystals.
In vivo surface properties of the metallic biomaterials used for artificial joints, bone plates, dental implants, etc. are discussed on the basis of empirical data, focusing on the destruction, regeneration, and reconstruction of the surface oxide film in body fluids. Metal ion release from metallic materials occurs through the remaining surface oxide film and with its destruction. The released metal ions are active and immediately combine with water molecules or anions to form oxides, hydroxides, or inorganic salts. Thus, the ions have only a small chance for combination with biomolecules to cause toxicity. Other types of ions can survive in body fluid in an ionic state and eventually combines with biomolecules and cause toxicity. The surface modification techniques for biomaterials to improve corrosion and wear resistance and also for hard tissue compatibility are reviewed. Effect of calcium ion implantation into titanium for improvement of its hard tissue compatibility is given as an example.
Dental implants require mechanical compatibility such as strength and wear resistance as well as surface compatibility with a host tissue in order to reduce their impact as foreign bodies. To satisfy these two requirements, it is preferable to utilize metal surfaces modified with functional ceramics, polymer materials or proteins. Since dental implants are used in contact with various tissues, it is necessary to have optimum surface compatibility with the host bone tissues, subepithelial connective tissues and epithelial tissues. Furthermore, dental implants are required to remain plaque-free at the surface exposed to the oral cavity. Such materials can be created under well-controlled conditions by modifying the surface topography and surface chemistry of metals that are in contact with those tissues.
Some ceramics able to bond to living bone are now widely used as bone-repairing material. Their applications are, however, limited to less loaded regions, because of their low fracture toughnesses and high elastic moduli. It has been revealed that the essential requirement for the material to be bonded to bone is the formation of a bonelike apatite layer on the surface of the material in the living body. For this, the material should have a functional groups effective for the apatite nucleation on its surface and increase in the ionic activity product of the apatite in the fluid near its surface. On the basis of these findings, bioactive metals with high fracture toughnesses have been developed by controlling their surface structure. Preparation of apatite-polymer composites with analogous structure to the natural bone is also being attempted.
Biomedical adhesives play a great role in bonding biomaterials and prosthetics in biomedical and dental fields. These adhesives are classified into two categories according to the different types of adherend, that is, for soft tissues, namely skin, blood vessels and so on, and for hard tissues, namely bone, tooth and so on. In this pater, trends and status of these bioadhesive studies are explained.
Recently, ion implantation has been applied to the surface modification of polymers, for example, ion implantation into proteins (extra-cellular matrix) to develop bio-compatible materials control of cell attachment and platelet in collagen, etc. In this report, ion implantation into the collagen coated inner surface of the polymer tube has been studied to develop hybrid type small diameter vascular grafts. Ion implantation was also applied to alter the surface properties of the catheter, which is used for the treatments of brain aneurysms, to improve their thrombogenicity and to promote wound healing of aneurysms.
The interface between metallic materials, especially titanium, and living cells (osteogenic cell) was investigated using a high-resolution transmission electron microscope, in order to establish the mechanism of the bio-compatibility for metals. At the titanium/cell interface there is an intermediate layer whose thickness is several ten nm involving not only metallic elements but also cell original elements. The intermediate layer does not have long/short range order or is rather complex structure. It is concluded that this intermediate layer is a mixture of titanium oxide and organic materials, which precipitates during cell culture. This precipitation of mixture is one of the advantages for titanium's biocompatibility. It is not cleared that calciumphosphate, one of the main composition of bone, precipitation on the surface on the material has any responsibility for bio-compatibility.
Positron has various unique properties, e.g., positive charge, annihilation with electrons, emission of annihilation gamma ray, positronium formation, etc. These unique properties provide us with new information on surface and subsurface regions. In recent years, generation techniques of intense slow-positron beams have been developed and those beams have been applied to various surface studies. In this article, we briefly introduce slow-positron generation techniques and measurement techniques for surface studies.
Stratigraphy of oxygen isotope anomaly in Ca-Al-rich inclusions (CAIs) in primitive meteorites has been determined by secondary ion mass spectrometry. The results show that the CAIs have experienced multiple heating event in the protoplanetary disk as well as in vacuum. The oxygen isotope anomaly in CAIs was introduced by isotopic exchange reaction between the 16O-rich melt and the surrounding 16O-poor gas during the heating. A proposed energy source for the multiple heating would be flares of active protosun. The proto-CAIs generated near the protosun were launched into the meteorite forming region in the protoplanetary disk by bipolar out flow.