Biomedical Engineering Reference
In-Depth Information
FIGURE 6.24
Formation of droplets in an ink-jet printing process.
droplets on demand (drop-on-demand jetting). In a continuous mode ink-jet printer, pres-
surized fluid is forced through an orifice, typically 50-80
μ
m in diameter, to form a liquid
jet. In a drop-on-demand ink-jet printer, the fluid is maintained at an ambient pressure,
and a transducer is used to create a drop only when needed (Figure 6.23).
Matrix Free Ink-Jet Printing
Using ink-jet printing, fenofibrate was dissolved in isobutanol at a concentration of
20 mg/ mL. The ink-jet microdispenser was programmed to jet 21,900 droplets and aimed
to deliver a target dosage of 150
μ
g per stent tube. Analysis of the stent demonstrated a
coating of 140.0
±
2
μ
g (Figure 6.24).
Jet Spray Process
Jet spraying has been used to coat microporous stent surfaces with sirolimus (Wessely et
al., 2005). Sirolimus (0.75%) in ethanol was sprayed onto sandblasted 316L stainless steel
stents. The stent surface is dried by removing the ethanol with pressured air. The coating
process could be repeatedly run for the purpose of increasing drug dosage.
ControlledRelease
Restenosis has been a major limitation of angioplasty since its introduction in 1978, but
it can be controlled by administering effective agents to the stented artery (Faxon, 2002).
The absence of a clear overview of the complete pathophysiology of restenosis has ham-
pered progress in precise treatment options. In the early 1980s, it was largely thought
that platelets and thrombi were the primary factors responsible for restenosis. Subsequent
studies demonstrated the important role of smooth muscle cell proliferation. However,
the importance of inflammation and extracellular matrix turnover is now recognized as
a key component of the condition. However, the most important observation has been the
critical role of vascular remodeling in restenosis (Lafont and Topol, 1997). By eliminating
this phenomenon, the pharmacological approach has been largely simplified by allowing
the primary treatment focus to reduce smooth muscle cell proliferation (Farb et al., 1999).
In this way, the immediate postoperative period (less than 2 weeks) is associated with the
appearance of fibrin, platelets, and acute inflammatory cells, followed by the emergence of
a neointima containing smooth muscle cells. In longer-term stents (>30 days after implant),
success was characterized by neointimal growth within the stent and was not influenced
by artery or stent size.
The choice of drugs in the treatment of restenosis and the manner in which they are
administered is of considerable interest. Systemic drugs with bare metal stents are cheap
and applicable to blood vessels of a broad range of sizes. However, patients have to toler-
ate the systemic toxicity. Local delivery of drugs through release from stents brings with it
