Biomedical Engineering Reference
In-Depth Information
2.5 DENDRIMERS
One of the most versatile and most studied nanostructures as carriers of various
molecules is dendrimers.
189
Dendrimers have complex structures consisting of
an inner core surrounded with branches decorated with functional groups at the
ends. Dendrimers become progressively larger with the addition of each genera-
tion (shell) of branches and can be built from the molecular level to nanoscale
size (1 nm to over 10 nm). The generation (shell) number and the chemical
composition of the core, branches, and surface functional groups determine the
size, shape, and reactivity of dendrimers. The ability to precisely control their
size, shape, and surface functionality during synthesis makes dendrimers one of
the most versatile and customizable nanotechnologies. Dendrimers have myr-
iad applications, including solubility enhancement
190,191
and gene therapy
192,193
drug delivery
194,195
,
196
nanocomposites
197
-
199
), photodynamic therapy,
200,201
and
bioimaging of and the treatment of cancer.
202,203
Dendrimers, also known as arborols, or cascade, cauliflower, or starburst
polymers are attracting have unique structures and properties.
204-206
These syn-
thetic molecules are constructed from AB
n
monomers (
n
usually 2 or 3) result-
ing in hyper-branched structures rather than the sAB monomers, which produce
linear polymers. They are synthesized in an iterative fashion leading to stepwise
synthetic growth, wherein the number of monomer units incorporated in each
successive iteration roughly doubles (AB2) or triples (AB3) that in the previous
cycle.
204
Using vide infra, one of the synthetic approaches to dendrimers each
repetition cycle leads to the addition of additional layer of branches,
generation
,
to the dendrimer framework. In this process, the generation number is equal to
the number of repetition cycles performed that is determined by counting the
number of branch points from the core to the periphery.
204
But, an activated
AB
n
monomer may be polymerized in a single step resulting in a polymer with
a higher polydispersity (PD) and a lower degree of branching called
hyper-
branched polymers
.
204,207
The ability to synthesize dendrimer in a controlled manner to yield very
high molecular weight polymers with narrower molecular weight distributions
is desired.
204
Smaller dendrimers (e.g.,
M
w
10-50 kDa) are often produced from
short syntheses leading to single compounds (PD) and not mixtures of poly-
mer molecular weights. Dendrimers with molecular weights over 103 kDa and
molecular dimensions in the 1-100 nm range have been synthesized in a few
steps which are not as homogeneous as the smaller dendrimers.
204
Since the
initial report about dendrimers by Vogtle in 1978, a multitude of dendrimers of
different structural classes have been reported ranging from pure hydrocarbons
to peptides to coordination compounds.
The two most commonly used dendrimers, the poly(amidoamine) (PAMAM)
dendrimers and poly(propylene imine) dendrimers, are commercially avail-
able.
204
There are two main synthetic strategies for synthesizing dendrimers:
the divergent and convergent approaches.
