Biomedical Engineering Reference
In-Depth Information
engineering [
103
-
105
]. We evaluated the poly(DP-
co
-LA)s having carboxylic acid
and amino groups as scaffold for tissue engineering with controllable degradation
rates, various physicochemical properties, and chemical modification abilities
[
108
-
111
].
2.2 Block Copolymers
The ROP of cyclic esters, including LA, GA, CL, and cDPs, can be initiated with
alcoholic hydroxyl groups so that biodegradable polyesters having terminal alcoholic
residues can be obtained. Using this principle, block copolymers containing aliphatic
polyester segments can easily be obtained by polymerization of cyclic esters with
polymers having terminal hydroxyl groups as macro-initiators. Amphiphilic block
copolymers can also be synthesized by a coupling reaction of hydrophobic polymers
and hydrophilic polymers with reactive termini. Some amphiphilic block copolymers
were also obtained using polyesters having terminal initiating groups. For example,
alkylbromide can be used as initiating group for atom transfer radical polymerization
(ATRP) to synthesize vinyl-type block copolymers, and primary amino groups can be
used for ROP of amino acid
N
-carboxy anhydride (NCA) to synthesized
polypeptide-
block
-polyesters. Aliphatic polyesters are usually hydrophobic and com-
bine with hydrophilic polymers to give amphiphilic block copolymers. Such amphi-
philic block copolymers have been used in biomedical fields as polymeric micelles,
temperature-responsive materials and so on (see Sects.
4
and
8
). The preparation and
application of PLA-based amphiphilic block copolymers were reviewed recently [
16
].
The most popular hydrophilic polymers used in conjugation with aliphatic
polyesters to prepare amphiphilic block copolymers are polyethers, especially
PEG, which can also be called polyoxyethylene or poly(ethylene oxide) (PEO).
PEG is nonionic, nonimmunogenetic, nontoxic and biocompatible, soluble in water
and common organic solvents, and FDA-approved for clinical use. PEG can provide
a prolonged blood circulation and diminished reticuloendothelial system (RES)
uptake [
112
]. Although PEG is not biodegradable, low molecular weight (below
ca. 30,000 Da) PEG can be excreted mainly from the kidneys. Because of such
favorable properties, PEG has been used in the biomedical field. There are many
reports on the preparation of amphiphilic block copolymers of aliphatic polyesters
and polyethers that include PEG [
113
-
134
]. Most of the research on amphiphilic
block copolymers of aliphatic polyesters and polyethers has been carried out using
AB-type diblock copolymers and ABA (hydrophobic-hydrophilic-hydrophobic)-
type or BAB (hydrophilic-hydrophobic-hydrophilic)-type triblock copolymers.
In addition, multiblock copolymers of polyesters and polyethers have also been
synthesized, mainly by polycondensation of the hydrophilic and hydrophobic
segments, and investigated as biomedical materials such as antiadhesive membranes
and temperature-responsive injectable polymers [
135
-
137
].
