Biomedical Engineering Reference
In-Depth Information
3.2.5
Layer-by-layer assembly
The LbL technique is a powerful tool to assemble multilayer and multimaterial thin films.
The alternate deposition of oppositely charged particles to form multiple-layer thin films was initially
reported by Iller in 1966
[120]
but picked up popularity after the work of Decher and coworkers
[121,122]
in the mid-1990s. The technique involves immersing a negatively (or positively) charged
substrate in an oppositely charged polyelectrolyte which is adsorbed onto the substrate. After equilib-
rium is reached, the substrate is removed, rinsed, dried, and immersed in a negatively charged
polyelectrolyte solution. This process is repeated until the desired thickness is achieved. The absorp-
tion of the polyelectrolyte is irreversible and charge overcompensation leads to charge reversal at
the surface
[123]
. Different materials can be inserted between layers as long as they have the
opposite charge. LbL assembly can also be performed on a colloidal substrate
[11,124]
. It enables the
coating of various different shapes and sizes by uniformly layered materials with controllable
thickness.
Initial results from multilayered film assembly
[125]
showed linear growth of mass and film
thickness. Examples of linearly growing systems include poly (styrene sulfonate) and poly (allyamine
hydrochloride)
[126,127]
. In these films, each polyelectrolyte interpenetrates only its neighboring
layers. However, films that experience exponential growth have also been reported
[128
130]
. This
exponential growth pattern was attributed to the vertical diffusion of polyelectrolyte into the film.
Diffusion is controlled by the molecular weight (MW) of the polyelectrolyte, with higher MW diffus-
ing much more slowly. Other factors that affect diffusion include polymer charge density and nature
of chemical groups present on the polymer.
LbL assembly initially focused on construction films based on electrostatic interaction; subse-
quent works have focused on developing LbL composites based on hydrogen bonding
[10,11]
,
charge
transfer interactions
[131,132]
, coordination bonding, and covalent bonding
[133,134]
.
Through hydrogen bonding, a number of additional materials can be incorporated into multilayered
composites in a water solution or organic phase. A number of polymers can act as donors and accep-
tors for hydrogen bonding. Hydrogen-bonding multilayer film assembly is based on the alternate
deposition of polymers containing a hydrogen bond acceptor and a hydrogen bond donor, respec-
tively. This enables the incorporation of biodegradable and biocompatible natural and synthetic
polymers to be incorporated as they cannot be assembled via the electrostatic LbL approach. The
double stranded DNA is a combination of hydrogen bonds between the bases and the
stacking
of the aromatic rings contained in the bases. While DNA multilayer systems have been produced
using electrostatic attractions
[135,136]
, it does not utilize the interactions between the base pairs
which can be used to manipulate the structure of the multilayer film
[137,138]
. Compared to films
assembled using electrostatic attractions, the pH range where hydrogen bonded multilayer form
stable films is limited. Hence, the ability of these materials to disassemble within a narrow range of
pH offers new possibilities for drug delivery applications. Disassembly can be achieved through fine
tuning the pH by varying the hydrogen-bonding pairs or the conditions under which the layers are
assembled. However, the films can also be made stable by cross-linking using chemical, thermal, and
photochemical techniques.
Covalently bonded multilayered films have also been assembled using the LbL techniques. The
presence of covalent bonds imparts stability to the films. The strength of the composites depends
on the strong adhesion between the two polymers. The films can be assembled in organic solvents.
ππ
