Biomedical Engineering Reference
In-Depth Information
for LbL self-assembly. The most commonly used synthetic polymers include polycations such
as linear or branched poly(ethyleneimine) (PEI), poly(dimethyldiallyl ammonium) (PDDA), and
poly(allylamine) hydrochloride (PAH), and polyanions such as PSS and poly(acrylic acid) (PAA)
(Figure 10.2). Most of these polymers are strongly charged and can be used to form LbL fi lms in
a wide range of pH environments. These synthetic polymers have been used as substrates for cell
adhesion or for building microcapsules for protein encapsulation [27-30]. Incorporation of PEI into
LbL self-assembly would be interesting since it has been widely used in gene delivery.
Natural polymers offer great advantages because of their biocompatibility and biodegradability;
most of them are water soluble, which are essential for LbL self-assembly. The three major nat-
ural polymers are (1) proteins and enzymes (albumin, protamine sulfate, glucose oxidase, etc.);
(2) polypeptides (polylysine, poly(α,l-glutamic acid), poly(aspartic acid)); and (3) polysaccharides
(hyaluronan (HA), dextran, alginate, heparin, chondroitin, carboxymethyl cellulose, and chitosan
[CH]). Modifi cation of natural polymers gives more choices in the self-assembly of biomaterials.
For example, HA can be amine-modifi ed to have positive charges [31]. Among all natural polymers,
collagen (more than 16 different types) is the most widely available polypeptide, with a unique
triple-helical structure, existing in many different tissues. A single molecule of type I collagen
has a molecular mass of about 285 kDa, a width of 1.5 nm, and a length of
300 nm [32]. These
molecules can self-assemble into collagen fi brils with diameters ranging from 20 to 200 nm and
length of 1-20 µm under various temperatures and assembly time [33]. Collagen-based biomateri-
als have been widely used in studies on biomaterials and even in clinical applications. Negatively
∼
N
N
y
x
n
NH
2
COOH
Poly(ethyleneimine) (PEI)
Poly(acrylic acid) (PAA)
n
n
N
Cl
−
H
3
C
CH
3
SO
3
Na
Poly(dimethyldiallyl ammonium chloride)
(PDDA)
Sodium poly(styrenesulfonate) (PSS)
n
NH
+
Cl
−
Poly(allylamine) hydrochloride (PAH)
FIGURE 10.2
Structural formula of commonly used synthetic polymers.
