Biomedical Engineering Reference
In-Depth Information
that the role and purpose of the coatings are not misunderstood. Coatings have specific
functions ranging from improving fixation by establishing strong interfacial bonds, to
shielding the metallic implant from environmental attack or leading effects (Cheang et
al. 1996a). When used in load-bearing such as dental and orthopedic implants, HA coating
has the following advantages: fast bony adaptation, absence of fibrous tissue seams, firm
implant-bone attachment, reduced healing time, increased tolerance of surgical inaccura-
cies, and inhibition of ion release. These advantages are of relatively short-term effect, but
may provide factors that increase long-term implant stability. Generally, the mechanical
properties and phase composition, as well as the operation feasibility, are the most con-
sidered factors as these methods are to be adopted. The HA coatings attained from these
various techniques differ in chemistry and crystallinity, which would in return affect the
biological response of the coatings and ultimately their performance. In addition, compos-
ite coatings have the potential of combining the advanced properties of its components.
Therefore, there are many attempts in recent years of producing HA-based composite coat-
ings for medical applications. One effective way of fabricating such composite coatings is
to deposit precomposite powder so that favorable coatings are anticipated. The criteria for
fabricating such composites include controllable particle sizes, none or minor chemical
reaction between the components, and unique mixing of the components. Pure HA is quite
brittle; therefore, several types of bioinert ceramic, such as alumina (Al
2
O
3
) and partially
stabilized zirconia (PSZ), as well as titanium dioxide (TiO
2
), have been chosen as additives
in HA coatings to improve the implantation duration based on their nonbiochemical influ-
ence on cell differentiation and proliferation (Li et al. 1996; Chang et al. 1997; Champion et
al. 1996). The composite coating made from HA and bioinert ceramic is capable of alleviat-
ing the brittleness and other property limitations of pure HA material, as well as solving
the bioinert instinct of the reinforcements. In order to expand the exploration of the poten-
tial use of composite bioceramic coatings, the behavior of the additives in the composites
and the strengthening mechanism are important factors that need to be clarified.
There have been attempts in recent years toward the development of processing meth-
ods to deposit HA onto metallic substrates while minimizing its inherent mechanical
property limitations. A proliferation of studies has taken place in which various process-
ing techniques have been utilized to fabricate HA coatings. Yankee et al. (1990) divided the
various methods into categories of “thick coatings” and “thin films” techniques. Most of
the processing techniques are listed below.
1.
Sintering method
(Lange et al. 1987; Silva et al. 2001). Thick HA coating (>500 μm)
can be rapidly produced with dense microstructure economically by this method.
However, there are a number of disadvantages that need to be considered; that is,
the elevated temperatures required to sinter dried HA layer tend to degrade the
metallic substrate properties and cause the decomposition of HA.
2
. Electrophoresis method
(Wei et al. 1999; Wei et al. 2001). This method is suitable for
uniform coating formation with complex shapes such as porous-surfaced implant
devices. The process of coating deposition is also relatively rapid. The main short-
comings of this process are porous structure and poor bonding to the substrate
because of the low energy with which particles are deposited on the substrate.
3.
Electrochemical route
(Shirkhanzadeh et al. 1995; Vijayaraghavan et al. 1994). An
important aspect of this process for fabricating bioactive coatings is that the
chemical composition of the electrolyte can be easily altered, and thus coatings
with a desired chemical composition may be tailored for specific applications.
Search WWH ::
Custom Search