Biomedical Engineering Reference
In-Depth Information
For example, alternating PDDA with inorganic montmorillonite clay platelets made it possible to
build natural biocomposites such as artifi cial analogs of nacre [36]. Another type of clay-50 nm
diameter halloysite nanotubes—enabled assembly of tubule composites perspective for tissue
engineering and as a depot system [37,38].
10.1.4 C
HARACTERIZATION
OF
L
B
L S
ELF
-A
SSEMBLY
To monitor and to study the overall structures in LbL self-assembled biomaterials, the following
methods are available: QCM, Fourier transform infrared (FTIR), x-ray and neutron refl ectivity,
scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force
microscopy (AFM), confocal laser scanning microscopy (CLSM), contact angle measurement,
dynamic light scattering, surface (zeta) potential, ellipsometry, light guiding attenuation, and
UV-vis absorbency. Recently, fl ow cytometry was used to monitor polyelectrolyte multilayer fi lm
formation on particles [39].
QCM is a simple, cost-effective, high-resolution mass sensing technique based on the piezoelec-
tric effect [40]. The QCM method is extremely suitable for a time-dependent control of adsorption
and monitoring of the assembly
in situ.
The multilayer assemblies can be characterized by the QCM
technique in two ways: (1) after drying a sample in nitrogen stream, one can measure the resonance
frequency shift and calculate a total amount of adsorbed mass by Sauerbrey equation or (2) by con-
tinuously monitoring of resonator frequency during the adsorption process onto one side of the reso-
nator, which is in permanent contact with polyion solutions. For the situation of pure elastic mass
added to the surface, the well-known linear Sauerbrey equation (Equation 10.1) was fi rst observed
[41] and used to precisely quantify, with ng sensitivity, the quantity of elastic mass added to the sur-
face where ∆
fi
is the measured resonant frequency decrease (Hz),
fi
is the intrinsic crystal frequency,
∆
m
is the elastic mass change (g),
A
is the electrode area,
ρ
q
is the density of quartz (2.65 g/cm
3
),
and
µ
is the shear modulus (2.95
10
11
dyn/cm
2
).
×
2∆
m
·
fi
2
-
__________
∆
fi
=
A
·
√
______
μ
·
ρ
q
= -
C
fi
· ∆
m
(10.1)
d
(
A
)
≈ −
0.16∆
fi
(Hz)
(10.2)
d
(
A
)
≈ −
1.5∆
fi
(Hz)
(10.3)
With the help of dynamic QCM study, it was suggested that polyion adsorption occurs in two
stages: quick anchoring to a surface and slow relaxation. The ultrasensitivity of QCM measurement
is obvious: for a 9 MHz crystal electrode (USI-System Inc., Japan), 10 Hz frequency change equals
8.7 ng mass adsorption, which for organic fi lms corresponds to ca 0.2 nm [18]. The thin fi lm thick-
ness can be estimated by taking into account the fi lm density (Equation 10.2) [18]. The density was
assumed to be 1.2
0.1 g/cm
3
for proteins, while for
a 5 MHz quartz crystal electrode (QCM100, Stanford Research System, Inc., Sunnyvale, CA), the
relationship between fi lm thickness and frequency shift is given in Equation 10.3 [42].
The internal structure of LbL polyelectrolyte multilayers can be investigated by measuring the
x-ray and neutron refl ectivity from such a fi lm. X-ray refl ectivity measurements of LbL-adsorbed
polyelectrolyte fi lms show patterns with profound intensity oscillations (so-called Kiessig fringes,
where the steepest gradients in electron density occur, due to interference of x-ray beams refl ected
from interfaces solid support/fi lm and air/fi lm) [13,43]. From the periodicity of these oscilla-
tions, one can calculate the fi lm thickness with the help of Bragg-like equation and taking into
account refraction phenomena that are essential at small angles [13,24]. For a poly(vynilsulfate)/
poly(allylamine) (PVS/PAH) assembly, x-ray analysis of the fi lm was dried at 20, 26, 32, 39, and
41 cycles of the assembly and showed a linear increase of the fi lm thickness with the number of
0.1 g/cm
3
for polyelectrolyte fi lms and 1.2
±
±
