Bioactive Ceramics
Bioactive ceramic is a general term covering bioactive glasses, glass-ceramics and
hydroxyapatite. They exhibit a positive reaction at the interface between the host
tissue and the implant in the biological environment resulting in bonding, each with
varying ability according to its type. the level of bioactivity has been related to the
physiological process of bone formation via an index of bioactivity that relate to the
amount of time it takes for 50 percent of the interface to be bonded:
Bioactivity Index, IB = 100/t0.5bb
Where (t0.5bb) is the time taken for more than 50% of the interface to bond to bone.
According to the Bioactivity Index, bioactive materials can classify into two types as
listed below:
•
Bioactive materials class A: materials exhibiting an IB value greater than 8,
they are actively interacting with tissues and induce their intrinsic repair and
regenerative as well as bond to both soft and hard tissue e.g. the bioactive glass and
glass-ceramic.
•
Bioactive materials class B: materials with an IB less than 8 but greater than 0,
they are considered only as osteoconductive materials, e.g. Synthetic hydroxyapatite
(HA) which has chemical similarity to bone and good biocompatibility, and tricalcium phosphate (β-TCP) .
It is important to know that the nature of biomaterial- tissue interface and the
reactions (e.g. ion exchange) at the ceramic surface and in the tissues dictate the
resulting mechanical, chemical, physical, and biological process that occur. In general
four factors determine the long term effect of bioactive ceramic implants: (1) the site
of the implantation, (2) tissue trauma, (3) the bulk and the surface properties of the
material, and (4) the relative motion at the implant-tissue interface.
Bioactive glass and glass-ceramics
Bioactive glasses are amorphous silica based materials that are biocompatible. The
reaction process between glasses and physiological fluids results in the formation of a
crystallized hydroxycarbonate apatite (HCA) layer at the glass/bone interface. This
HCA layer is similar in composition and structure to the inorganic component of bone
mineral and a strong bond can form without fibrous tissue around it. The degree of
activity and physiological response depend on chemical composition of the glass and
the specific surface area, which are influenced by the particle size (especially when it
is used as a powder) or morphology when it is used as a scaffold.
The reactivity and rate of bond formation can be expressed by the ratio of the network
former to the network modifier: SiO2/(Cao+Na2+K2O), the higher ratio means high
SiO2 and low reactivity.
The oldest bioactive glass composition, namely 45S5 which consists of a silicate
network Na2O-CaO-P2O5-SiO2. A disadvantage of bioactive glasses is that they
generally possess low fracture toughness values and hence poor mechanical strength,
especially in porous form. This leads to limited applications in load-bearing
situations. An early significant modification of the heat treatment to form crystalline
particles in a bioactive glass matrix led to the development of apatite/wollastonite
(A/W), Ceravital and Bioverit glass-ceramics, all of which come under class A
bioactive materials. The A/W glass-ceramic is produced by partially crystallising the
glass matrix that occurs after prolonged heating of the bioactive glass, to promote
nucleation of crystallisation. A/W glass- ceramics show higher bending strength than
bioactive glasses due to the assembly of apatite phases reinforced by ß-wolastonite
(CaSiO3), which is believed to be the reason for the increased fracture toughness that
makes it a better match to bone for load bearing applications.
Calcium- Phosphate Ceramics (CPC)
Calcium phosphate ceramics are ceramics with varying calcium-to-phosphate ratio,
the important properties of calcium phosphate biomaterials are their bioresorption and
bioactivity, the most widely used calcium phosphate based bioceramics are
hydroxyapatite (HAP) and β-tricalcium phosphate (β-TCP).
Synthetic Hydroxyapatite (HA) is a calcium phosphate whose stoichiometric
formula corresponds to Ca10(PO4)6(OH)2 with (Ca/P=1.67) , it is the most stable
phase of various calcium phosphates; it is stable in body fluid and in dry or moist air
up to 1200°C and does not decompose and has shown to be bioactive due to its
resorbable behaviour. It is similar to bone mineral and is widely used as a filler,
spacer and bone graft substitute. Hydroxyapatite can be prepared in either dense or
macroporous forms. Porous HA is osteoconductive (The phenomenon of new bone
formation on the surfaces of bioactive ceramics) and biocompatible; it resorbs with
time but the degradation rate is slow. Non-porous or dense HA is considered to be
non-biodegradable because of its very low degradation rate in body fluids. Thus,
porous hydroxyapatite has now replaced the dense hydroxyapatite form for most
biomaterial applications.
β-tricalcium phosphate (β-TCP) is a resorbable bioceramic that degrade and
dissolve with time after implantation and provide space for new tissue to grow. It
represented by the chemical formula Ca3(PO4)2, the Ca/P ratio is 1.5. The degradation
rate of β-TCP is 3-12 times higher than that of HA. It is thought that a mixture of HA
and β-TCP with different ratios may combine the bioactivity of TCP and the
mechanical integrity of HA in composite materials known as Biphasic calcium
phosphates (BCP). These have been used as filler in composite biomaterials’ systems
and as coating on dense implants and porous surface.
Composite bioceramic
The development of inorganic-organic composite materials offers the possibility of
combining the favourable properties of bioceramics such as the HA, bioactive glass,
alumina or titanium dioxide with the molding capacity of biocompatible polymers.
The properties of composite materials can be engineered to suit the mechanical and
physiologic demands of the host tissue by controlling the volume fraction,
morphology and arrangement of the reinforcing phase. The use of composites also has
advantages in terms of tailoring mechanical properties. While polymers can be
processed into complex shapes and structures, they do not usually provide strong
bonding to bone and exhibit relative low mechanical strength, stiffness and flexible.
They are therefore unable to meet, in several cases, the mechanical demands
associated with applications in bone engineering. On the other hand, bioactive
ceramics and glasses are brittle and exhibit poor fracture resistance, being a
disadvantage when used on their own. However, combination of organic phase
(natural or synthetic polymers) and inorganic phases (HA, TCP, bioactive glass) leads
to composite materials with improved mechanical properties (e.g. stiffness,
compression strength), controlled degradability and stabilise the pH of the
surrounding environment as well as enhance the osteoconductive properties of the
resulting composite material exploiting the “composites approach”, i.e. incorporating
stiffer and strong inorganic particles or fibres in the softer and flexible polymer
matrix.
There are two principal ways for making biocomposite materials. Both of can be used
to fulfil a specific clinical need:
1- Incorporating bioceramic particles or fibres as a dispersed secondary phase into a
polymer matrix through a variety of techniques.
2- Coating polymer, metal and ceramic with a thin layer of bioceramic.
The earliest bioceramic composites were designed to mimic the mechanical properties
of the natural components of bone by replacing natural apatite with a synthetic HA
and collagen with polyethylene (PE). The HA-PE composite has been used to replace
the bones of the middle ear, Carbon coating metals were used in heart surgery, while
HA used for coating porous metal surfaces for fixation of orthopaedic prostheses.
Coating bioinert ceramics, such as alumina, with a thin layer of bioactive
bioceramics(HA) is a reasonable strategy for bone-tissue repair.