Biomedical Engineering Reference
In-Depth Information
Matrix/drug
316 Stainless steel stent, express (132 µm)
Core
FIGURE 6.21
Micro- and nanoporous ceramic coating loaded with drug.
Surface-Built Micro- and Nanoporous Membrane
Combining physical vapor deposition of aluminum with oxalic acid oxidation, a nanopo-
rous ceramic (alumina/aluminum oxide/Al
2
O
3
) membrane has been produced for loading
tacrolimus (Wieneke et al., 2003). The ceramic coating is relatively thin (1 µm) with tacroli-
mus loaded in a dip coating process in methanol (Figure 6.21).
Top Coat
The reason for applying a layer of top coat is to minimize the burst release of the active
ingredient in a fast delivery system such as the CYPHER Sirolimus-eluting stent. It is sim-
ply a drug-free layer of matrix. In view of the problems caused by polymeric matrices,
new DES seldom use top coats (Deconinck et al., 2008; Kukreja et al., 2008; Nakazawa et
al., 2009).
Coating Techniques
The active ingredients, dominantly drugs, are applied onto stent surfaces using two major
techniques: dip coating and spray coating. Dip coating provides a nonpreferential layer
to any part of the open stent surface. It is considered an inferior technique for a matrix
that is polymer-based because of bridging and imperfect coating of tight spaces. Spray
coating produces a thinner layer with better surface quality. It is considered as a superior
technique for manufacturing DES. Embracing the technical advances in micro- and nano-
fabrication, dip and spray coating techniques have their own advantages in preparing new
generation of DES. As DES are moving in the direction of polymer-free devices, dip coating
is probably the simplest and cheapest forming process, requiring low technical input in
a preclinical treatment. When DES require preferential delivery of the active ingredients,
spray coating is advantageous in abluminal coating of functionalized devices. In addition,
ink-jet technology is an emerging process for acquiring a layer of a preferential coating.
Dip Coating
Dip coating is a simple process used for coating bare metal stents, both surface-modified
or not. The matrix/drug mixture is mixed into a dilute solution and then the bare metal
stent dipped into it, then often allowed to air-dry. The thickness of the coating layer is
controlled mixture concentration and chemical cross-linking (Juan et al., 2009). Cordis coat
the combination of PEVA, PBMA, and sirolimus by successive dipping and air-drying of
the stent in a solution of THF a controlled number of times (Wolf et al., 2008). Coating of
tacrolimus onto ceramic stents is achieved by dipping in a 3-mg/mL solution of the drug
