Biomedical Engineering Reference
In-Depth Information
HA nucleation; hence, negatively charged PLGA was used in their study. The precipi-
tation occurs in the following manner: The drop in pH and Ca
2+
activity triggered the
simultaneous addition of two titrants containing PO
4
3-
and Ca
2+
ions to keep the reac-
tion solution at a constant high supersaturation. By this means, the ionic activities of the
two ions were maintained at a fixed level. The same coating method has been applied
to liposomes as well [118]. Most recently, a biomimetic method has been utilized to coat
minerals onto microspheres [119]. The biomimetic method uses a simulated body fluid
to mimic physiological HA formation process. When incubating the seeding materials
(microspheres for instance) in the simulated body fluid, a layer of HA will be formed onto
the surface of the seeding at 37°C in a duration of a few days.
DrugDeliverySystemswithCoating
Coating as both drug carrier and rate controller is widely used in biomedical devices,
such as stents, scaffold [120], microneedles [121], and microparticles (drug containing
coating on a magnetic microparticle for targeting [122]). However, in most devices with
drug containing coating, burst release is usually observed (majority of the drug is released
during the first few hours). The reason for the burst release is largely due to the quick
release of the drug entrapped near the surface of the coating. When a drug containing
film is coated on the surface of biomedical devices (i.e., drug eluting stent), it also suffers
from burst release; hence, it may be difficult to sustain the release. A practical approach
to enhance the long-term effectiveness of a drug-containing film/matrix is to capsulate
it with another layer, which is essentially a reservoir-membrane system as well [123].
Venkatraman and Boey [124] reviewed several reservoir-membrane systems in the appli-
in the appli-
cation of cardiology stents. In one of the applications where drug is supposed to be released
over a longer period, a drug-free coating is applied on the surface of the drug-containing
layer (Figure 5.11). Since drug molecules have to pass through an extra layer, its release rate
is supposed to be reduced; otherwise, a faster release profile can be found in a design that
has no drug-free layer.
Sriamornsak and Kennedy [125]
studied the alginate and pectin coating in controlling
the release of a few water insoluble drugs. The drug pellets were prepared by blending
drug, cellulose, and calcium acetate. Pellets were coated by solution coating of pellets in an
alginate/pectin solution and cross-linking of alginate/pectin on the surface of pellets with
soluble calcium acetate. It was found that the alginate/pectin coating lowered the release
rate by four- to six-folds, regardless of the type of the drug, whereas the release rate was
slowest for drugs having low water solubility.
reservoir-membrane systems in the appli-
membrane systems in the appli-
Drug free layer
Drug-containing layer
Stent
Stent
Adhesive layer
FIGURE 5.11
Multilayer-coated stent.
