Biomedical Engineering Reference
In-Depth Information
3.4
NANOPARTICLES
Usage of nanoparticles is advantageous as active antibacterial groups since their surface area is
exceedingly outsized relative to their size and scope. Thus, nanoparticles may provide high activity
even in cases when only a small amount of the particles is added. Particles should be homogeneously
distributed in a restorative composite in order to generate an outer surface concentration of about
one particle per square micrometer, thereby achieving effective antibacterial activity against bacteria
which have an average size of 1-3 µm.
Synthesis of QPEI nanoparticles provides particles comprising an aliphatic polymer having anti-
bacterial quaternary chemically bound ammonium groups, wherein on the surface of the particle there
are hypothetically at least one antibacterial quaternary ammonium group per square nanometer. These
particles have an inner structure and an outer surface. Both the inner and outer structures comprise
quaternary ammonium group consisting of a nitrogen atom chemically bound to the polymer and to
three other groups, one of which being a long alkyl group. Theoretically, the inner and outer regions
of a particle are made of the same compound; therefore, the inner region cannot be dissociated from
the outer region, as the two areas of a particle are continuous and integrated with each other.
Branched polymers are preferred for crosslinking as small amount of crosslinking is required to
form insoluble nanoparticles, which are essential in materials anticipated to withstand exposure to an
aqueous environment such as restorative composites. Additionally, crosslinking prevents the unfold-
ing of the polymer and separation of the various polymeric chains that form the particle. Furthermore,
incorporation of nanoparticles is considered to be beneficial in durability over utilization of restor-
ative composites containing leachable antibacterial agents. As opposed to restorative composites con-
taining water-soluble antimicrobial antibacterial nanoparticles, when incorporated are not released,
consequently resulting in additional stability in a wet environment.
3.4.1
Polyethyleneimine Nanoparticles
According to previous reports, antibacterial activity of cationic biocides can be achieved due to hydro-
phobic nature and positive charge.
N
-alkylation of PEI crosslinked nanoparticles with alkyl halides of
various lengths results in formation of hydrophobic nanoparticles, and by subsequent
N
-methylation
of PEI's primary, secondary, and tertiary amino groups to become quaternary ammonium groups.
3.4.1.1 Synthesis
QPEI particles can be prepared in several steps using 1,5-dihalidopentane as crosslinking agent,
which causes intramolecular crosslinking
[44]
. Subsequently, the crosslinked PEI particles are
N
-alkylated with linear alkyl halides of different chain lengths varying from butyl to hexadecyl fol-
lowed by neutralization with sodium bicarbonate, and further
N
-methylation is carried out with
methyl iodide to obtain QPEI nanoparticles. In order to increase the rate of quaternization of PEI,
neutralization should be repeated with an additional quantity of sodium bicarbonate and the quaterni-
zation step should be continued for another 24 h
[45]
(
Scheme 3.1
).
Antibacterial activity of PEI is dependent on the quality and quantity of quaternary amines.
Therefore, in order to increase the degree of amino group substitution, it is beneficial to increase the
alkylation with alkyl halides of various lengths followed by methylation. Alkylating agents impart
hydrophobic nature to the hydrophilic PEI. This
N
-alkylation makes the polymer more hydrophobic
