Biomedical Engineering Reference
In-Depth Information
into account, HBPs are one of the most promising materials to be designed as
functional vehicles for drug and gene delivery. By means of various synthetic
methodologies, HBPs can be blessed with numerous specific functions, such as
responsiveness (pH, temperature, magnetic field, redox sensitivity, etc.),
7-12
recognition (saccharide, enzyme, folic acid, antibody, etc.),
13-15
and imaging
(quantum dots, conjugated polymers, bioluminescent proteins, etc.).
16-19
These
excellent performances of HBPs can, to a considerable degree, improve in vivo
delivery behavior and increase the related auxiliary functions of drug and gene
vehicles. Moreover, these functions for drug delivery and gene transfection can
be controlled and optimized through modulating the branched structures of
HBPs.
2,6
In a nutshell, the design and preparation of functional HBPs are of
great significance for the exploitation of novel drug and gene delivery systems.
This field has been extending into its own niche and more valuable works are
underway. The combination of recent progress, the synthetic approaches, the
functionalization of HBPs, and their applications in drug and gene delivery are
summarized in this chapter.
d
n
4
y
3
n
g
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1
5.2 Preparation of Functional HBPs
A variety of synthetic strategies for HBPs have been proposed and achieved
during the past decades.
3-5
As previously outlined, several different
methodologies have been developed. Firstly, the main methods of synthesizing
the highly branched architecture will be briefly discussed. Subsequently, the
functionalization strategies of HBPs will be presented.
5.2.1
Preparation of HBPs
HBPs have versatile chemical structures (linear units, dendritic units, and
terminal units) and exhibit unique properties. The majority of HBPs are
prepared through two pivotal methods, namely the step growth approach and
the chain growth approach. Step growth was initially developed according to
Flory's theory. With the discovery of self-condensing vinyl polymerization by
Frech´t, more and more attention was focused on the chain growth method.
Occasionally, copolymerization of different monomers requires a hybrid of
two synthetic methods. Furthermore, supramolecular interaction has also been
introduced to construct and control the highly branched architecture as well.
5.2.1.1
Step Growth
There have emerged two kinds of monomers for the step growth method,
namely AB
x
and A
2
+ B
y
. A broad portfolio of suitable A and B structures can
result in different HBPs. On the one hand, polycondensation of AB
x
monomers is the classical way to prepare HBPs with and without core
moieties. However, there only exist a limited number of AB
x
monomers, which
are not easy to obtain. On the other hand, HBPs can be prepared by the A
2
+
