Designing Bio-Inspired Composite Materials for Medical Applications

Composite materials are multi-phased combinations of two or several components, which acquire new characteristic properties that the individual constituents, by themselves, cannot obtain. A composite material typically consists of a certain matrix containing one or more fillers which can be made up of particles, sheets or fibers. When at least one of these phases has dimensions less than 100 nm, the material is named a nanocomposite and offers in addition a higher surface to volume ratio. There are natural composite materials, like wood and plant leaves, in the vegetal kingdom and bird feathers, silky threads spun by the spider and shells, in the animal kingdom. Also, connective tissues from animal and human body are natural composite materials due to their composition and structure. Connective tissues are the major supporting tissues of the body. They are named after their main function, i.e. packing and binding other structures together, and also providing a framework for the body. Connective tissues are generally soft tissues (e.g., skin, cartilage, cornea, etc), excepting bone which is a dense connective tissue. Similar to other natural composite materials, bone consists of an organic part that forms the matrix and an inorganic part representing the filler. Bone matrix is a framework mainly composed of collagen fibers which together with small quantities of other non-collagenous proteins, proteoglycans, lipids, peptides and water form a hydrogel (~ 30 % from bone dry weight). The filler, that reinforces bone matrix, is formed of nano-sized crystals of carbonated calcium phosphate apatite (~ 70 % from bone dry weight). This natural nanocomposite material has superior strength and toughness than its individual components. Bone matrix is a source for nourishing bone cells, such as osteoclasts, osteoblasts and osteocytes, which grow inside it. It also increases cell biological activities like adhesion, proliferation and differentiation. Bone has key functions as skeletal tissue of the body, including support of softer tissues, mechanical protection for many internal organs and storage of minerals. In the development phase, bone attains the most suited structure to resist the forces acting upon it. During life time, bone is subjected to various diseases that are inherited (osteogenesis imperfecta) or caused by metabolism disturbances, such as osteoporosis, osteosarcoma, osteoarthritis. Bone is also affected by traumas, i.e. fractures, micro-fractures. Each year, millions of people are treated in hospitals for fractures presenting risk of developing into delayed union or nonunion. Also, progressive aging of