Biomedical Engineering Reference
In-Depth Information
(a)
Proteins or
nanoparticles
Polyanion
Polycation
(2)
(3)
(1)
Polyanion
Proteins or
nanoparticles
Polycation
(b)
Proteins or
nanoparticles
Polyanion
Polycation
(1)
(2)
(3)
FIGURE 10.1
(a) Schematic illustration of fi lm deposition process through LbL self-assembly on a fl at sub-
strate and (b) encapsulation of polyelectrolyte/nanoparticles multilayers on micro/nanotemplates.
To reach a surface charge reversion during linear polyion adsorption one needs a concentra-
tion higher than 10
−
5
M [23]. The dependence of polyion layer thickness on concentration is not
strong; thus, in the concentration range 0.1-5 mg/mL, polystyrenesulfonate/polyallylamine (PSS/
PAH) pair gave a similar bilayer thickness [24]. A large decrease of polyion concentration (using
0.01-0.1 mg/mL) slightly decreases the layer thickness of adsorbed polyion [25,26].
Microencapsulation is the continuation of LbL self-assembly from ultrathin fi lm coating on
fl at substrates to micro/nano 3-D objects. All kinds of materials, charged or noncharged, includ-
ing small molecule drug crystals, protein aggregates, inorganic particles, and even biological cells,
can be chosen as templates for encapsulation. The coating procedure is simple and straightforward
as illustrated in Figure 10.1b. The fi rst step is coincubation of templates with excessive amount of
polyelectrolytes, similar to the procedure on 2-D substrates (Step 1, Figure 10.1b). Then, a washing
step is required to remove free polymers before coating the second layer. This step can easily be per-
formed through appropriate centrifugation or fi ltration. By repeating the fi rst two steps, multilayers
of polyelectrolytes can be assembled on micro/nanotemplates with precise control of thickness and
molecular structures. The modifi ed surface represents new physical and chemical properties, which
was designed to fi t into different applications.
10.1.3 M
ATERIALS
FOR
L
B
L S
ELF
-A
SSEMBLY
Among all the charged compounds for LbL self-assembly, polyions are the most important
compounds for building biomaterials; especially, while assembling protein and nanoparticle
multilayers, an alternation with linear or branched polyion layer is necessary [4]. Flexible polyions
can penetrate between protein globules and function as “electrostatic glue,” which keeps together
neighboring arrays of proteins or nanoparticles. Both synthetic and natural polymers can be chosen
