MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 353

11.2 INTRODUCTION

In the last few decades, different types of materials have been produced and
further improvements have been made for specifi c medical applications, such as
metals (stainless steel, cobalt-chromium, titanium and alloys), ceramics (alumina,
zirconia, graphite), polymers, and composites [Brunette, 2001; Helsen, 1998;
Williams, 1992 ; Jones, 2001 ; Vallet - Reg í , 2001 ; Kokubo, 1993 ; Ratner, 2004 ].
In particular, ceramic materials designed to be implanted into the human
body have experienced an enormous evolution as a response to the medical needs
of an ageing population. The so-called fi rst generation of bioceramics was devel-
oped to fulfi ll the requirement of bioinertness that is a minimum interaction with
the living tissues. The most representative materials are alumina (Al 2 O 3 ) and zir-
conia (ZrO 2 ). Indeed, today high - density (3.9 g/cc), high - purity ( 99.8%) alumina

is still widely used in load-bearing hip prostheses and dental implants because of
its excellent corrosion resistance, good biocompatibility, low coeffi cient of fric-
tion, high wear resistance, bending strength (550 MPa) and compressive strength
(4500 MPa). Other clinical applications of alumina include knee prostheses, bone
screws, alveolar ridge and maxillofacial reconstruction. Zirconia is also used as
the articulating ball in total hip prostheses and its potential advantages are its
lower modulus of elasticity and higher strength [Ratner, 2004].
In the 1980s, the second generation of ceramics was investigated with the aim
of a favourable interaction with the body. Bioactive and reabsorbable ceramics
are able to form a mechanically strong bond with the living tissues. The most sig-
nifi cant materials are crystalline calcium phosphates, bioactive glasses and glass-
ceramics clinically used for applications such as the bone tissue augmentation,
bone cements or the coating of metallic implants.
Porous ceramics have also been developed due to their potential advantage
to provide good mechanical stability given by the highly convoluted interface that
develops when bone grows into the pores of the ceramic. Although the optimal
type of porosity and degree of interconnectivity of pores is still uncertain, when
pore sizes exceed 100

>

m, bone will grow within the interconnecting pore chan-
nels near the surface and maintain its vascularity and long-term viability. There-
fore, the porous ceramic serves as a structural scaffold for bone formation.
Nevertheless, porous materials like hydroxyapatite are weaker than the equiva-
lent bulk form in proportion to the percentage of porosity, so that as the porosity
increases, the mechanical properties of the material decrease rapidly. This fact
severely restricts the use of these low-strength porous ceramics to non load-
bearing applications.
Today, the concept of tissue replacement is being substituted by tissue regen-
eration and, therefore, more demanding properties are required. The third gen-
eration of advanced ceramics is currently developed to be used as scaffolds and
templates of cells or other biologically active substances (growth factors, hor-
mones, and so on), able to induce regeneration and repair of living tissues. Major
challenges still remain in the implant technology fi eld in terms of development of
light-weight, reliable, and durable ceramic implants with enhanced mechanical

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