Composite Materials

Anton Ficai, Ecaterina Andronescu, Georgeta Voicu and Denisa Ficai Politehnica University of Bucharest, Faculty of Applied Chemistry and Materials Science


1. Introduction

The annually necessary human bone grafts are in continuous grown due to the increasing of fractals, congenital and non-congenital diseases. Based on statistical reports (Murugan and Ramakrishna 2005), only in USA about 6.3 million fractures occur every year and about 550.000 of these require bone grafting. The most frequently fractures occur at the level of hip, ankle, tibia and fibula. Due to the higher physical effort, the men are more exposed to fracture than women (2.8% in the case of men comparing to 2.0% in the case of women). The number of fractures increases year by year and consequently many researchers from different research fields co-operate in order to develop new bone graft materials. Also, it is important to mention that, in the present, the bone diseases are overlapped only by hearth diseases.

The history of bone grafting is starting in 1913 when Dr. D.E. Robertson assays a piece of cat's bone and a piece of human bone for bone grafting into dogs (Gallie and Toronto 1914). The microscopic analysis of implanted graft after 20 days shows that the space between graft and living bone is filled with new cancellous bone. These early works made the premises for the development of the bone grafts.

Due to the increasing of the necessarily bone grafts, autografts and allografts can not cover the overall need of bone grafts. For compensate this gap, artificial (synthetic) grafts are necessary and, consequently used. The use of synthetic grafts has some advantages versus allografts, autografts and xenografts: the possibility to obtain unlimited number/quantity of synthetic grafts, more safety use of artificial bone grafts without disease transmission risk, pain limitation by elimination of some secondary surgical intervention.

The need of bone grafts materials lead to the synthesis of many kind of materials with different properties. Function of the nature of these materials and the relation between these grafts and the host tissue, these materials can be divided into 4 generations (Fig. 1). The components of the first generation of bone grafting biomaterials have remarkable mechanical properties but they are neither bioresorbable nor bioactive. More than, the use of these kind of bone grafts have limited lifetime (usually less than 10-15 years) and need to be extracted and replaced surgically. Some of the most representative biomaterials from the first generation of bone grafting biomaterials are: the iron, cobalt, chromium, titan or their alloys: steel (especially 316 L), cobalt or titan based alloys (Corces 2002; Corces and Garcia 2007) etc.

Fig. 1. Biomaterials evolution in the field of bone grafting

The components of the second generation of bone grafting biomaterials are at least bioresorbable or bioactive and they do not require to be replaced in time. The most representative biomaterials from the second generation of bone grafting biomaterials are: calcium phosphates (especially hydroxyapatite and tricalcium phosphate), the bioglasses (Hench et al. 2004), alumina (Sedel et al. 1994); zirconia (Clarke et al. 2003) and the following polymers: poly e-caprolactone (Olah et al. 2007), polyurethanes (Bonzani et al. 2007 ; Guelcher et al. 2004) etc.

The components of the third generation of bone grafting biomaterials are both bioresorbable and bioactive and have superior properties. It has to mention that these biomaterials present higher specific properties than the first two generations of bone graft materials and short time after implantation these materials are resorbed and in time, in the place of the bone graft the new bone is formed. The properties of these (nano)composite materials is strongly influenced by the nature of components, the composition and the morphology. That is why many researchers tried to obtain not only compositional similitude with natural bones from mineralogical and morphological point of view. The most representative biomaterials of the third generation of bone grafting biomaterials are: (nano)hydroxyapatite/collagen (Wahl and Czernuszka 2006), (nano)hydroxyapatite/collagen/hyaluronic acid (Bakos et al. 1999), hydroxyapatite/poly-L-lactic acid (Kesenci et al. 2000), hydroxyapatite/chitosan (Wang and Li 2007).

The fourth generation of bone grafting biomaterials is similar with the third generation materials but improved by the presence of bonny cells, growth factors, bone morphogenetic proteins etc.

One of the most important characteristic of bone grafts materials is the osteointegration. The osteointegration (and also osteoconductivity) of the grafts is related to the degree of porosity and pore size (applicable especially to the last three generation of bone grafting biomaterials) (Develioglu et al. 2005). Based on the literature data and also based on the size of osteoblasts (which vary up to 20-25 ^m) (Chang et al. 2000; Develioglu et al. 2005; Gauthier et al. 1998) the optimum pore size was found to be 50-550^m.

The natural bones contain mainly collagen and hydroxyapatite. That is the reason because many researchers try to understand and obtain (nano)hydroxyapatite/collagen composite for hard tissue repairing. The composition of bones varies function of many factors such as: specie, sex, age, bone type, location etc.. The relative composition of bone is presented in the Table 1, while the most important biomedical properties of the bones are presented in Table2. The in vivo bone biosynthesis is controlled by many factors such as: BMP (bone morphogenetic proteins) (Abe et al. 2000), transforming growth factors (Tashjian et al. 1985), cytokines (de Vernejoul et al. 1993 ), hormones (Bollerslev et al. 1991 ; De Vernejoul et al.

1990; Hock and Gera 1992), transcription factors (Cui et al. 2003; Ogawa et al. 2000), adhesion molecules (Miyake et al. 1991) and so on.


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