**2. Natural bone and bone implants**

#### **2.1. Natural bone**

Bone is a complex living tissue that harnesses the synergies of osseous tissue, cartilage, dense connective tissues, epithelium, adipose tissue and nervous tissue. Bone as a functional organ in the human body has various roles, such as supporting soft tissues, protecting many internal organs, enabling movements in human activity and facilitating mineral homeosta‐ sis, *i.e.,* storage of osseous tissue minerals such as calcium and phosphate, providing blood cell production sites and acting as a location for triglyceride storage [1].

Bone consists of both organic and inorganic materials that are distributed within an extracel‐ lular matrix. Organic material, called fibrous protein collagen, is predominant in bone struc‐ ture and this collagen contributes to the tensile strength of bone. The inorganic material impregnated inside bone is mainly hydroxyapatite, *i.e.,* minerals of calcium phosphate and calcium carbonate. Usually, the calcium to phosphorus ratio of natural bone ranges between 1.50-1.65 depending on its location. Around 25 wt.% of bone consists of water that is present in bone pores, thereby ensuring nutrient diffusion and contributing to the viscoelastic prop‐ erties of the material. Calcification is a process of crystallisation of mineral salts *i*.*e*., calcium phosphate, which occurs in the biological framework formed by the collagen fibres [2].

There are four types of cells in osseous tissues: osteogenic cells, osteoblasts, osteocytes and osteoclasts. Osteogenic cells undergo cell division and develop into osteoblasts. Osteoblasts play a role in bone formation and collagen secretion. As osteoblasts secrete extracellular ma‐ trix, then osteoblasts evolve into osteocytes. Osteocytes, also known as mature bone cells, are responsible for nutrients and waste exchange with the blood. Osteoclasts are bone de‐ stroying cells and responsible for bone resorption. Bone consists of bone lining cells, fibro‐ blasts, and fibrocytes. Bone lining cells control the movement of ions between bone and the surrounding tissue. The role of fibroblasts and fibrocytes is, in brief, to form collagen [1].

Bone can be categorized into five types on the basis of its shape, namely long, short, flat, ir‐ regular, and sesamoid. In addition to the dense structures present, osseous tissue has many small spaces between its cellular and extracellular matrix. There are two types of osseous tis‐ sue on the basis of the size and distribution of these spaces: compact bone tissue and spongy bone tissue. About 80 wt.% of the human skeleton is compact bone tissue. Compact bone consists of a packed osteon within the Haversian architecture. Each osteon consists of a cen‐ tral Haversian canal, concentric lamellae, lacunae, osteocytes, and canaliculi. Spongy bone, also termed as trabecular bone, exhibits a porous structure with porosity ranging from 50-90 wt.% and consists of an integrate lamellae network. The role of trabeculae is to support and protect the red bone marrow [2].

Bone structure contains macro, micro and nanoscale pores with different functions and char‐ acteristics. Macro-scale porosity gives rise to mechanical anisotropy. Micro-scale porosity provides sufficient vascularisation and cell migration, while nanoscale features act as a framework for cell and mineral binding [2].

Bone mechanics is determined mainly by the bone structure. Compact bone is stiffer and stronger than cancellous bone. The mechanical properties of human bone are listed in Table 1 [2]. The elastic modulus of human bone is approximately 0.05-2 GPa for cancellous bone and 7-30 GPa for compact bone [2]. It should be kept in mind that "elastic modulus" is not an exact description for bone properties since they are anisotropic and viscoelastic.


**Table 1.** Mechanical properties of human haversian [2]

#### **2.2. Bone implant**

scope (SEM) and elemental analysis was performed by the energy dispersive X-ray spectro‐ scopy method (EDS). The crystal structure was examined using X-ray diffractometer (XRD) to identify the phase components, while nanocoating thickness was measured using profil‐

This chapter is divided into five major parts. First is an overview of bone and bone implants, including their structure and mechanical properties. The second part highlights the impor‐ tance of nanocoatings for bone implants longevity. Various coatings and surface modifica‐ tion techniques of titanium and its alloys are also elucidated. The advantages and drawbacks of each technique are reviewed. The last part focuses on the study of sputtered hydroxyapatite and SiO2 nanocoatings on titanium. A thorough discussion of the results is

Bone is a complex living tissue that harnesses the synergies of osseous tissue, cartilage, dense connective tissues, epithelium, adipose tissue and nervous tissue. Bone as a functional organ in the human body has various roles, such as supporting soft tissues, protecting many internal organs, enabling movements in human activity and facilitating mineral homeosta‐ sis, *i.e.,* storage of osseous tissue minerals such as calcium and phosphate, providing blood

Bone consists of both organic and inorganic materials that are distributed within an extracel‐ lular matrix. Organic material, called fibrous protein collagen, is predominant in bone struc‐ ture and this collagen contributes to the tensile strength of bone. The inorganic material impregnated inside bone is mainly hydroxyapatite, *i.e.,* minerals of calcium phosphate and calcium carbonate. Usually, the calcium to phosphorus ratio of natural bone ranges between 1.50-1.65 depending on its location. Around 25 wt.% of bone consists of water that is present in bone pores, thereby ensuring nutrient diffusion and contributing to the viscoelastic prop‐ erties of the material. Calcification is a process of crystallisation of mineral salts *i*.*e*., calcium phosphate, which occurs in the biological framework formed by the collagen fibres [2].

There are four types of cells in osseous tissues: osteogenic cells, osteoblasts, osteocytes and osteoclasts. Osteogenic cells undergo cell division and develop into osteoblasts. Osteoblasts play a role in bone formation and collagen secretion. As osteoblasts secrete extracellular ma‐ trix, then osteoblasts evolve into osteocytes. Osteocytes, also known as mature bone cells, are responsible for nutrients and waste exchange with the blood. Osteoclasts are bone de‐ stroying cells and responsible for bone resorption. Bone consists of bone lining cells, fibro‐ blasts, and fibrocytes. Bone lining cells control the movement of ions between bone and the surrounding tissue. The role of fibroblasts and fibrocytes is, in brief, to form collagen [1].

Bone can be categorized into five types on the basis of its shape, namely long, short, flat, ir‐ regular, and sesamoid. In addition to the dense structures present, osseous tissue has many

cell production sites and acting as a location for triglyceride storage [1].

ometer.

presented.

**2.1. Natural bone**

**2. Natural bone and bone implants**

22 Titanium Alloys - Advances in Properties Control

The history of implants started with the applications of autograph, allograph, and artificial de‐ vice techniques [3]. Autographs utilized tissues from other parts of the patient's body, whilst allograft techniques used tissue from a donor. However, both techniques had drawbacks in application. The autograph method was limited only to nose bone and finger junctions [3]. Moreover, there were adverse side effects, such as infections and pain at the implant area. The allograft technique required a compatible donor that matched the patient's body system, which was usually difficult to find. There was always the potential risk of infections and dis‐ ease transmission from the donor to the recipient's body. Artificial grafts employed artificial materials, now known as biomaterials. The advantages of using artificial device grafts include (i) lower risk for any transmission of disease, (ii) a reduced risk of infections, and (iii) the avail‐ ability of many biomaterials for potential use as scaffolds. Therefore, ongoing studies aim to develop a new generation of biomaterials for bone implants.

### **2.3. Criteria of ideal bone implant**

An ideal bone implant material should be osteoconductive, osteoinductive and should have osseointegration ability [3]. Furthermore, other key criteria for excellent implant perform‐ ance include biocompatibility and mechanical compatibility. In addition, any implant waste after degradation should not cause harmful effects to the body. Recent trends in bone tissue engineering studies have revealed that bone implants may also serve as a drug delivery sys‐ tem if they are appropriately designed.

years of implantation [9]. The development of porous titanium may potentially overcome

Sputtered Hydroxyapatite Nanocoatings on Novel Titanium Alloys for Biomedical Applications

http://dx.doi.org/10.5772/54263

25

The development of new titanium alloys has been extensively explored. Usually Al, Sn, O, C, N, Ga, and Zr are used as α stabilizers, while V, Mo, Ta, Nb, and Cr are used as β stabiliz‐ ers [10]. Titanium alloys such as Ti6Al4V with aluminium and vanadium as α and β stabiliz‐ ing elements have been widely used as implant materials. These first generation biomedical titanium alloys, however, have revealed that the release of Al and V metal ions is harmful to the human body [11]. The decisive requirement of a biomedical implant is its biocompatibili‐ ty in the human body. Thus, alloying elements must be carefully chosen to reduce any bio‐ logically adverse impacts. Alloying elements that attract biomedical applications are Ta, Nb, and Zr due to their non-cytotoxicity, good biocompatibility, high corrosion resistance and

Beta alloys that have higher β stabilizers content are attracting great interest for bone im‐ plant applications due to their low elastic modulus. Beta alloys that have been studied for bone implant applications include Ti50Ta20Zr, Ti64Ta, Ti13Nb13Zr, Ti42Nb, and Ti30Zr10Nb10Ta. Studies conducted by Obbard *et al.* [12] showed that by adjusting the concentration of β stabilizer Ta, the elastic modulus could be reduced. In this fashion the compliance mismatch between the implant and bone would be reduced, leading to lesser

Alpha-beta alloys may have some advantages over β alloys, namely lower density and high‐ er tensile ductility. Some studies have succeeded in the production of alpha-beta alloys with a porous structure. The porous structure serves as an anchorage for bone in-growth and ex‐ hibits a lower elastic modulus, while the α and β phases provide sufficient mechanical

The development of porous titanium alloys with a variety of alloy components has brought about many improvements in bio-mechanical properties. For example, porous Ti10Nb10Zr with 69% porosity exhibited a tensile strength of 67 MPa, while pure Ti and pure Ta scaf‐ folds with the same porosity demonstrated lower strengths of 53 MPa and 35.2 MPa, respec‐ tively [13]. Xiong *et al.* [14] reported that the elastic moduli of porous Ti-26Nb alloys with porosity of 50, 60, 70, and 80% were 25.4, 11.0, 5.2 and 2.0 GPa, respectively, while the pla‐

Surface modification is a process that changes the composition, microstructure and mor‐ phology of a surface layer while maintaining the mechanical properties of the material. The aim of surface modification is to improve the bioactivity of the biomaterials so that the bio‐ materials demonstrate a higher apatite inducing ability that, in turn, leads to rapid osseoin‐ tegration. After surface treatment, it is expected that the implant's surface will form an active apatite layer. The role of the thin apatite layer is to be a bonding interface to stimulate

**4. The importance of nano-coatings for bone implant materials**

problems of this nature.

stress shielding.

their complete solid solubility in titanium [10].

strength for load bearing applications.

teau strength ranged from 180 MPa to 11 MPa.

Osteoconduction is a process by which bone is directed to conform to a material's surface, while osteoinduction is the ability of an implant to induce osteogenesis. An inductive agent will stimulate undifferentiated cells to form preosteoblasts [3]. According to Branemark *et al*. [4], osseointegration could be defined as the "continuing structural and functional co-exis‐ tence, possibly in a symbiotic manner, between differentiated, adequately remodelled, bio‐ logic tissues, and strictly defined and controlled synthetic components, providing lasting, specific clinical functions without initiating rejection mechanisms".

In the context of orthopaedic implants, the development of a drug delivery system is still at an early developing stage. The promising concept of using an implant as part of a drug de‐ livery system could be described as the integration of therapeutic agents and devices.

In addition to high mechanical strength, the Young's modulus is a critical mechanical prop‐ erty in an artificial device when designing materials for bone implants. Other fundamental requirements for an ideal orthopaedic biomedical implant include high wear resistance, good fatigue properties if used under cyclic loading, no adverse tissue reactions, and high corrosion resistance.
