Biomedical Engineering Reference
In-Depth Information
OH
HO
O
n
FIGURE 15.6
Polyethylene glycol.
repeating unit, however in addition, it has hydroxyl groups at each end of the molecule. Typically,
high molecular weight polyethylene oxide polymers are used as biomaterials whereas PEG is
frequently used with low degree of polymerization with “
n
” ranging from 12 to 400, although
high molecular weight polymers are commercially available. PEO and PEG are highly biocom-
patible, which makes them attractive as biomaterials, and in addition, PEG with its hydroxyl end
groups (Figure 15.6) can be synthetically linked with other polymeric units providing versatility
in the properties of the modifi ed polymers. PEG is soluble in water, methanol, dichlorometh-
ane, diethyl ether, and hexane. It can be covalently linked to hydrophobic molecules that yield
surfactants.
PEG is a water-soluble and fl exible polymer, the backbone similar to that of silicones. It has
little specifi c interaction with biological molecules and is very useful in applying osmotic pressure.
PEG
42,43
is nontoxic and used in a variety of medicinal and cosmetic products. Hydrogels of PEG
are also good candidates for tissue engineering scaffolds because they are highly biocompatible and
exhibit versatile physical characteristics based on their weight percent, molecular chain length, and
crosslinking density.
44
It has been used in drug delivery and proteins attached covalently to PEG,
enhances the effi cacy of the drug due to improved dosing intervals and slow clearance from the
blood. PEG is also a constituent in skin creams, laxatives, and toothpaste, etc.
PEGylation is a term that is increasingly being used to describe the ability to covalently
link the PEG unit with other larger molecules, especially useful in the fi eld of pharmaceutics.
PEGylated proteins are commonly used, for example, PEGylated interferon as an injectable drug for
Hepatitis C infection. PEG-fi brinogen hydrogels made up of protein fragments covalently bound to
PEG-DA and crosslinked using UV photoinitiation have been reported, which allow the fi brinogen
constituent to be covalently linked to functionalized PEG. A Michael-type addition reaction is used
to form the ester bond between the free thiols in the fi brinogen cysteines and acrylate end groups on
the PEG-DA (PEGylation).
45
PEG-i brinogen hydrogels have also been synthesized using denatured
fi brinogen fragments that are PEGylated with PEG-diacrylates, mixed with photoinitiator, and
exposed to UV light to form a hydrogel material in the presence of a cell suspension. The advantage
of this type of material lies in its mechanical properties while maintaining biological functional-
ity. The elastic modulus of such PEG-fi brinogen hydrogel has been shown to be dependent on the
molecular weight of the PEG constituent and proportional to the percent polymeric composition.
These type of scaffolds can be very useful for cultivating cells in a 3D environment for tissue regen-
eration therapies.
46
Recent research has shown that it can aid in both peripheral and central nerve
repair
47
and improve in spinal healing in animal models. Several groups have investigated PEO as
an injectable polymeric device for chondral defects. The high degree of hydration in PEO gels acts
as barrier to macromolecules involved in the immune response and it has an established biocompat-
ibility. The potential for PEO in the repair of articular cartilage is high because of its ability to retain
water and its crosslinking under facile conditions. PEG is currently under investigation for newer
clinical applications such as colorectal cancer and paraplegia.
15.2.4 T
RIMETHYLENE
C
ARBONATE
A ring-opening polymerization of cyclic carbonate or lactones yields trimethylene carbonate.
A variety of organocatalysts have been surveyed for the ring opening polymerization and excellent
control on molecular weight and polydispersities can be obtained.
48
ABA-type triblock copolymers,
poly(trimethylene carbonate)-poly(ethylene glycol)-poly(trimethylene carbonate), obtained via the
