Noriaki Asada

Composition for hard tissue repair

The materials requirements for composites for biomedical applications include all the usual mechanical property requirements for general engineering composites of stiffness, strength, toughness, and so on. However, there are additional biological requirements, the materials have to be biocompatible, that is, not produce a disadvantageous biological response in the required application [1,2] including in the case of degradable materials the degradation products must also be biocompatible. An additional desirable property is that the material should be bioactive, that is, it should produce a beneficial response in the appropriate application [1,2]. Bioactive means that the body’s cells react beneficially with the material, and in the case of hard tissue applications, the bone cells, instead of producing a layer of fibrous tissue between the patient’s own bone and the implant, produce new bone directly on the implant. This direct bone–implant contact leads to a mechanically stronger interface and thus a stronger and more durable bone–implant structure. From a mechanical point of view, bone has two major functions, to provide a stiff skeleton against which the muscles can contract to provide locomotion and secondly to protect the major organs such as the brain, protected by the skull, and the heart and lungs, protected by the rib cage (Fig. 1). Biologically, bone provides a store of calcium and phosphate ions that are essential for biological function, while the marrow is the source for red and white blood cells [3]. At the macroscopic scale, there are two types of bone. The first type of bone is known as cortical, which is a nearly solid material, making up the outer surfaces of all bones, with a porosity of <3% in normal people [3]. The second type of bone is porous, is called either cancellous or trabecular bone, being a foam of bone made up of small struts called trabeculae. Cancellous bone is found throughout the middle of bones such as the ribs and the skull and at the end of long bones below the joint surface. The porosity ranges from 30% to 90%, depending on the position in the body, age, and activity level of the person. Both cortical and cancellous bones become more porous with age and conditions, such as osteoporosis that particularly affects postmenopausal women. At the structural and microstructural levels, cortical and cancellous bones have different structures in the form of osteons in cortical bone and trabeculae in cancellous bone. At the nanostructural level, all bone material is a composite of collagen, a natural polymer, filled with a calcium phosphate mineral, thus a particulate-reinforced composite. There are three major types of bone cells, osteoblasts that