Biomedical Engineering Reference
In-Depth Information
The first layer is passive for surface protection, the bridging layer for the coupling of active
agents, and the transition layer for good adhesion. This layer is sometimes deleted from
the coating specification. The second layer is the matrix for drug loading and controlled
release. In a drug-free stent, only active agents are incorporated in this layer. The final
layer is the top coating, which is often used to minimize the burst release and sustain the
long-term release for a significant duration. It is a drug-free layer of matrix material.
Passive Coating
Table 6.7 summarizes the compositions of major elements in alloys for coronary stents. DES
with FDA approval are manufactured from 316L stainless steel. Stainless steel and small
amounts of nickel, chromium molybdenum, and other contaminants tend to produce a
foreign body reaction when implanted in human coronary arteries (Colombo and Airoldi,
2003). Passive coatings (Figure 6.13) have been introduced to provide a biologically inert
barrier between the stent surface, circulating blood, and endothelial wall. These coatings
are a thin, continuous layer of gold, heparin, carbon, silicon carbide, titanium-nitride-oxide,
or PC (Menown et al., 2005). The Janus CarboStent (Bartorelli et al., 2003) consists of an inte-
gral Carbofilm coating combined with the capability to load and release the antirestenotic
drug from deep sculptures on the external surface of the stent. Phosphorylcholine-coated
BiodiVsio Stent (Menown et al., 2005) has the capacity to load and release the rapamycin
analogue ABT-578. A similar coating is found on the CYPHER stent, which has a prime
layer of Parylene C.
The Parylene C layer on the Cypher stent is formed by chemical vapor deposition fol-
lowing the Gorham process (Figure 6.14) (Chang et al., 2007), which has three main stages:
vaporization, pyrolysis, and deposition. Briefly, dichloro-di(
p
-xylylene) is first vaporized at
150°C at 1 Torr, then pyrolyzed at 690°C and 0.5 Torr to form chloro-
p-
xylylene, the mono-
mer of Parylene C. At 25°C and 0.1 Torr, this monomer condenses onto the device surface
to form the final Parylene C film. The thickness of the film is determined by the amount
of dimer fed into the furnace: 1 g of dimer adds 0.5
μ
m to the thickness of the Parylene C
film (Wright et al., 2007).
Parylene C (Figure 6.15) is a member of the Parylene family (Fortin and Lu, 2004). The
basic member of the series, Parylene N (poly-
para
-xylylene), is a completely crystalline
material. In 1947,
para
-xylylene was first identified as the gaseous precursor for the forma-
tion of parylene film, followed by the use of di-
para
-xylylene, the dimer, for more efficient
deposition of parylene film, the so-called Gorham process. Parylene C is the most widely
used dimer and provides a useful combination of properties, plus a very low permeability
to moisture, chemicals, and other corrosive gases. It is used in a conformal layer, pinhole-
free coating. The material is applied at 5
μ
m/h in a thickness of 0.100-76
μ
m in a single
operation. Because of its unique properties, Parylene C conforms to virtually any shape,
including sharp edges, crevices, points, or flat and exposed internal surfaces.
TABLE 6.7
Compositions of Major Elements in Alloys for Coronary Stents
Alloy
Iron(%)
Cobalt(%)
Nickel(%)
Chromium(%)
Tungsten(%)
Titanium(%)
316 L Stainless
steel
60-65
12-14
17-18
L605 CoCr
<3
50
10
20
15
Nitinol
55
45
