Biomedical Engineering Reference
In-Depth Information
ring-opening polymerization of trimethylene carbonate and initiated by dihydroxyl poly(ethylene
glycol), have been reported and investigated for the release of anticancer drugs.
49
It is generally
used as a plasticizer and has been incorporated in Maxon sutures.
50
More recently, nanoparticles of
poly(trimethylene carbonate) (PTMC) and monomethoxy PEG-
block
-PTMC (mPEG-PTMC), with
poly(vinyl alcohol) as a stabilizer, were synthesized with particles sizes in the range between 95 and
120 nm. The particles were reported to be stable over 20 weeks, and dexamethasone release studies
conducted by using these nanoparticles showed that the release was diffusion controlled and, sus-
tained release over 14 to 60 days was possible.
51
15.2.5 P
OLY
(
α
-A
MINO
A
CIDS
)
Poly(amino acids) widely occur in nature and synthetic analogs have been investigated for biomedical
applications. Poly(α-amino acids) are synthesized from aspartic acid and glutamic acid, or can be
fermented. Pure poly(α-amino acids) are highly crystalline, which results in slow rates of degradation
and the antigenicity of these polymers can make
in vivo
use more challenging. Poly(esteramides)
are the most commonly investigated of these group of polymers. Drug delivery applications have
reported wherein covalent attachment of drugs to the polymer backbone has been achieved.
52
15.2.6 P
OLY
(A
LKYL
2-C
YANOACRYLATES
)
Poly(alkyl 2-cyanoacrylates) are synthesized from cyanoacrylates. They have excellent adhesive
properties as a result of the strong bonds that can form with polar substrates including living tissues
and skin.
53
-
55
They are widely used as surgical adhesives.
56,57
Polycyanoacrylates are synthesized
from cyanoacrylates as shown in Figure 15.7.
Polyalkylcyanoacrylates are used in several biomedical applications
58
and more recently
with increasing interest in the fi eld of nanotechnology, these polymers have also been prepared
in nanoparticulate sizes and utilized in drug delivery.
59
These polymers have been investigated
extensively for drug delivery. There have been three main generations of polyalkylcyanoacrylates
drug delivery systems, the fi rst being the conventional drug carriers where the molecules are rap-
idly eliminated from the blood stream by the macrophages, the second being stealth drug carriers
in which the drug carriers avoid the macrophages due to the PEG chains on their surface;
60
thus,
they are able to circulate for a longer time, however still do not target the tissue. The third genera-
tion drug carriers are called targeted drug delivery systems that can target a tissue or a cell due to
the chemical grafting of PEG chains.
61
There is an increasing interest in this group of polymers
for therapeutics, especially for cancer treatments, which involve highly toxic molecules in con-
tact with healthy tissues.
62
-
64
Polymer colloidal drug delivery systems are considered promising
for targeted drug delivery and compounds such as nucleic acids and peptides are being tethered
to the poly(alkylcyanoacrylates), and it is one of the most promising group of polymers that are
being investigated. This is due to the fact that cyanoacrylate monomers are able to form polymeric
materials with biodegradable characteristics that may easily be controlled depending on the nature
of the cyanoacrylic monomer used. Heparin-coated poly(isobutylcyanoacrylate) nanoparticles have
C
N
C
N
CH
2
C
H
2
CC
C
O
C
O
O
O
CH
3
CH
3
n
Methylcyanoacrylate
Poly(methylcyanoacrylate)
FIGURE 15.7
Polymerization of methylcyanoacrylate to poly(methylcyanoacrylate).
