Bioactive Glass-Based Coatings and Modified Surfaces: Strategies for the Manufacture, Testing, and Clinical Applications for Regenerative Medicine - Biological and Biomedical Coatings: Processing and Characterization - page 409

Recently, Hill and coworkers stated the following material requirements for the ideal
thick bioactive glass coating [31]:
1. Matching thermal expansion coefficient (TEC) between the bioactive glass and
that of the alloy.
2. The coating can sinter at 750°C or lower (to prevent crystallization and inhibit
oxidation of the alloy at the surface).
3. The glass composition has a network connectivity (NC) value close to 2.0, in order
to maintain bioactivity, where NC is a measure of the average number of bridging
bonds per network forming element in the glass structure. NC determines glass
properties such as viscosity, crystallization rate, and degradability.
Thin Coatings (<100 μ m)
Under fatigue tensile loading conditions, thick coatings fail due to delamination and frag-
mentation. Compared with thick coatings, thinner coatings (<100 μm) have been found to
enhance interfacial stability and in some cases exhibit stronger biological fixation.
Pulsed Laser Deposition
Pulsed laser deposition (PLD) is a PVD method where a high-power pulsed laser is
focused, inside a vacuum chamber, on a target of the material that is to be deposited [32].
Referring to Figure 9.12, a plasma plume, the vaporized material is deposited as a thin film
on a substrate. PLD usually occurs in ultrahigh vacuum or in the presence of a background
gas such as oxygen that is commonly used when depositing oxides to fully oxygenate
the deposited films. The advantage of PLD method is that a thin coating can be precisely
deposited at rates of 1 μm cm −2 s −1 . For further detail, the reader is referred to the text by
Eason [33].
Figure 9.12 shows a bioactive glass (BG) thin film coating that has been deposited on
titanium substrates using the PLD method [35]. A UV laser KrF (λ = 248 nm, τ = 25 ns) was
used for the multipulse irradiation of the BG targets with 57 wt.% or 61 wt.% SiO 2 con-
tent (and Na 2 O-K 2 O-CaO-MgO-P 2 O 5 oxides). The depositions were performed in oxygen
atmosphere at 13 Pa and for substrates temperature of 400°C. The PLD films displayed
typical BG of 2 to 5 μm particulates nucleated on the film surface or embedded in. The
PLD films stoichiometry was found to be the same as the targets. After 3 to 7 days immer-
sion in SBF, the Si content substantially decreases in the coatings and PO 4 3− maxima start
to increase in FTIR spectra. After 14 to 21 days the XRD peaks show a crystallized fraction
of the carbonated hydroxyapatite. The SEM micrographs show also significant changes in
the films' surface morphology.
Laser Cladding
Laser cladding is a method of depositing material onto a substrate via powered injection
where the material is melted and consolidated by use of a laser in order to coat a substrate
(see Figure 9.13). The application of this technique to create bioactive coatings was first
presented in the literature by Lusquiños and coworkers [36]. They used laser cladding to
apply a CaP coating on a Ti alloy substrate.
Recently, Comesañaa and coworkers [37] used this method to clad bioactive glass onto
titanium alloy (Ti 6 Al 4 V) substrates. A homogeneous deposition of the coating with limited
Ti migration over the substrate was achieved using 45S5 composition glass. In addition,