Biomedical Engineering Reference
In-Depth Information
charged PDDA layer has about
+
40 mV zeta-potential reading, while a polyanion PSS layer is about
-
37 mV [51].
10.2
MULTILAYERED BIOFILMS THROUGH L
B
L SELF-ASSEMBLY
10.2.1 I
NTRODUCTION
As biomaterials, ultrathin fi lms have unique advantages such as precise control over the fi lm's overall
“molecular architecture,” thickness, surface charge, biocompatibility, and biodegradability. Formu-
lation of appropriate polyion/polyion and polyion/inorganic particles may achieve superior physical
and chemical properties that natural materials do not have. The principle of ultrathin fi lms on a fl at
substrate through LbL self-assembly has been illustrated in Figure 10.1a. A standard approach for
fi lm preparation involves the following: (1) taking aqueous solutions of polycation and polyanion
at concentration of 0.01 m/L (practically it is 1-3 mg/mL) and adjusting pH in such a way that both
polyions are ionized; (2) preparing a substrate of interest carrying a surface charge; (3) carrying out
alternate immersion of the substrate in polyion solutions for 30 min with 1 min intermediate water
washing. To wash a sample, use a solution of pH that keeps polyions ionized; and (4) drying the
sample in a nitrogen stream when it is necessary for ellipsometry, x-ray, UV, QCM, or other analyti-
cal methods. It should be mentioned that drying may disturb the assembly process and that it is not
necessary for the procedure.
For the applications of LbL self-assembled thin fi lms as biomaterials, we have special interest
in their biocompatibility and biodegradability. Understanding these properties would be benefi cial
to us not only for designing better fi lms for coating medical implants and in tissue engineering, but
also useful for constructing polyelectrolyte shells in microencapsulation.
The key to achieve a satisfi ed biocompatible interface is largely dependent on which material to
be used for self-assembly. There are a few natural polymers, including collagen, gelatin, heparin, and
CH that are widely used in LbL self-assembly with outstanding biocompatibility [27,34,35,57,58].
The outermost layer largely decides the biocompatibility of a fi lm. For example, in silicone rubber
modifi cation, a couple of nonbiocompatible but strongly charged polyelectrolyte layers were fi rst
used to form a precursor fi lm, then biocompatible materials such as gelatin or polylysine were further
established [27,35,59]. Cell adhesion on those composite nanofi lms was successful and no obvious
cytotoxicity was observed. Judging the biocompatibility of a multilayered fi lm depends on specifi c
applications, which involve different physiological environments, duration of tissue-material contact,
etc. Usually,
in vitro
environment is much simpler and the time frame of application is relatively
short. On the contrary, the
in vivo
system is more complicated and may involve systemic immune
responses. Some common
in vivo
tests, including hemocompatibility, carcinogenicity, and immune
response can be employed to evaluate an LbL composite fi lm. For detailed information on the bio-
compatibility of biomaterials, please refer to an excellent review by Anderson [60].
10.2.2 M
ULTILAYERED
P
OLYELECTROLYTE
F
ILMS
FOR
C
ELL
A
DHESION
Cell adhesion, migration, and other behaviors on a biointerface are crucial for both biological
studies and biomaterials development. Such a biointerface is usually a thin fi lm deposited on cer-
tain substrates. Generally, in LbL assembly of biocompatible coating for cell adhesion, there are two
approaches which have to complement one another. In the fi rst method, synthetic polyelectrolytes
such as PSS, PAA, and PAH were assembled with such architecture that they gained biocompatible
properties [61]. In the second approach, originally biocompatible materials such as polysaccharides,
polypeptides, and proteins were used to provide biocompatible patterns and PDDA outermost was
used to prevent cell adhesion [27,62]. Table 10.1 summarizes some LbL fi lms that were involved in
cell adhesion studies; all the fi lms listed here are well-attached by different cells [27,35,59,63-69].
Sel f-assembly cond it ions such as pH va lue, choice of polyelect roly tes, a nd crossl i n k i ng a re i mpor t a nt
