Bone-Like Mineral and Organically Modified Bone-Like Mineral Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 8

although osteogenic precursors are still able to adhere and grow on implants with higher
crystallinity, this does not translate to an osseointegrative response.
Apatite coatings create surface chemistries more analogous to native bone. Different
calcium-phosphate phases present in native bone, such as octacalcium phosphate and car-
bonated apatite, enhance cell adhesion and osteoconduction (Le Guehennec et al. 2007;
Müller et al. 2007; Wang et al. 2004). Altering the phases of the apatite coating can have sig-
nificant effects on the degree of crystallinity, surface roughness, and solubility of the coat-
ing (Barrere et al. 2003a). Apatite coatings therefore afford increased control over surface
chemistry and/or topography while maintaining the elastic characteristics of the biomate-
rial surface. Gaining further control over surface chemistry and topography will enhance
the ability to reconstruct cellular microenvironments, thereby improving osseointegra-
tion. Apatite coatings also increase the stiffness of soft substrates, providing control over
cytoskeletal organization (Murphy et al. 2000b; Leonova et al. 2006). There are several
approaches to depositing apatite coatings with controlled composition, topography, and/
or stiffness for enhancing conduction. The subsequent sections discuss processing and
applications of a biomimetically applied BLM coating precipitated from a supersaturated
salt solution and how controlling biomimetic processing, composition, and structure can
control biological responses in vitro and in vivo.
BiomimeticPrecipitationofMineral
Implants that do not integrate into host tissue become isolated from the surrounding tis-
sue, limiting the efficiency of load transfer (Jacobs, Gilbert, and Urban 1998). Bioactive
materials such as Bioglass 45S5 and A-W glass ceramics form a layer of apatite on the
surface when placed in vivo, which is vital for implant/tissue integration (Ducheyne 1985;
Nakamura et al. 1985). It is possible to simulate this apatite coating in vitro and thus pro-
vide bioactivity to non-bioactive materials using coating techniques such as plasma spray-
ing, electrophoretic deposition, sol-gel deposition, hot isostatic pressing, frit enameling,
ion-assisted deposition, pulsed laser deposition, electrochemical deposition, and sputter
coating (Liu and Hunziker 2009). Each one of these methods has its own advantages and
disadvantages (Table 1.2), and not all techniques can be used with all classes of materials
(Table 1.3). Of these methods, the most widely used technique commercially for metals is
plasma spraying. Plasma spraying, however, is not ideal with small implants and complex
shapes. It requires a coating thickness of 40 to 50 μm to achieve uniform deposition, and
is clinically challenged by delamination issues due to variations in the phases that con-
stitute the coating (Le Guehennec et al. 2007). Other methods such as dynamic mixing
and hot isostatic pressing are limited by the uniformity of coating that they generate as
well (Wie, Hero, and Solheim 1998; Yoshinari, Ohtsuka, and Dérand 1994). However, there
are a number of coating methods that deposit mineral uniformly: sputter coating, pulsed
laser deposition, sol-gel deposition, and electrophoretic deposition are better suited for
uniform coating on complex structures (Wolke et al. 1994; Zeng and Lacefield 2000; Li,
De Groot, and Kokubo 1996). However, the use of high processing temperatures in some
of these methods results in the formation of apatite that differs from the composition of
natural bone apatite, and is also not amenable to soft materials such as polymers (Abe,
Kokubo, and Yamamuro 1990). Among these methods, sol-gel deposition is the only other
method that can achieve uniform mineral coatings at low processing temperatures, but