Biomedical Engineering Reference
In-Depth Information
With respect to nanoparticulate metals, the antimicrobial properties of silver
[27]
and copper
[28]
have received the most attention. Both of these have been coated onto or incorporated into various
base materials
[29]
, including PMMA
[30]
and hydrogels
[31]
. An inverse relationship between the
size of nanoparticles and antimicrobial activity has been clearly demonstrated, where particles in the
size range of 1
10 nm have been shown to possess the greatest biocidal activity against bacteria
[3,32]
. Indeed, it has been shown that smaller silver nanoparticles are more toxic than larger particles,
more so when oxidized
[33]
. At the nanoscale, Ag
1
ions are known to be released (leached) from the
surface
[34]
.Sotiriouetal.
[35]
proposed that the antimicrobial activity of small (
10 nm) nanosil-
,
ver particles is dominated by Ag
1
ions, while for larger particles (
15 nm) the contributions of Ag
1
ions and particles to the antibacterial activity are comparable, the Ag
1
ion release being proportional
to the exposed nanosilver surface area.
Particular nanoparticles, as a result of their small size, may be able to offer other advantages to
the biomedical field through improved biocompatibility
[36]
. Also, it appears that bacteria are far
less likely to acquire resistance to metal nanoparticles than they are to other conventional and
narrow-spectrum antibiotics
[37]
. This is thought to occur because metals may act on a broad range
of microbial targets, and many mutations would have to occur in order for the microorganisms to
resist their antimicrobial activity. Shape may also affect the activity of nanoparticles. It has been
demonstrated that the shape of silver nanoparticles can influence antimicrobial activity, as has been
shown in the case of Escherichia coli
[37]
. Truncated triangular silver nanoplates with a {111}
lattice plane as the basal plane showed the greatest biocidal activity compared with spherical and
rod-shaped nanoparticles. The differences appear to be explained by the proportion of active facets
present in nanoparticles of different shapes.
Exploitation of the toxic properties of nanoparticulate metals and metal oxides, such as titanium
dioxide (TiO
2
;
Figure 10.1B
) and zinc oxide (ZnO;
Figure 10.1C
), in particular those that produce
reactive oxygen species (ROS) under UV light, are finding increased use in antimicrobial formulations,
with silver metal nanoparticles (5
.
40 nm) having been reported to inactivate most microorganisms,
including HIV-1
[38]
. The high reactivity of nano-titanium dioxide and nano-silicon dioxide (SiO
2
)is
exploited extensively for their bactericidal properties in filters and coatings on substrates such as poly-
mers, ceramics, glasses, and alumina
[39]
. Significant activity using metal and metal oxide nanoparti-
cles and their compound clusters against fungal and bacterial pathogens such as methicillin-resistant
Staphylococcus aureus (MRSA) and E. coli has recently been demonstrated. These have also shown
the capability to inactivate viruses, including SARS (severe acute respiratory syndrome), H1N1 swine
flu, and H5N1 bird flu. For example, new broad-spectrum materials (5
60 nm) can reduce virus levels
by 80
100% through direct or indirect contact. Nanoparticle preparations, including those based upon
nickel (Ni, NiO), zirconium (ZrO
2
), copper (Cu, CuO, and Cu
2
O), titanium (TiO
2
), zinc (ZnO), alumi-
num (Al
2
O
3
), silicon (IV) nitride (Si
3
N
4
), silver (Ag), and tungsten carbide (WC) have been compared
in regards to their antimicrobial potential. Significant activity with Ag, ZnO, TiO
2
(in the presence of
UV light), SiO
2
,Cu,Cu
2
O, and CuO against bacterial pathogens, including MRSA and Pseudomonas
aeruginosa, has been demonstrated
[40]
. MBCs were found to be in the range of 0.1
5mg/mL.In
comparison, traditional antibiotics are effective at concentrations 1000-fold lower. NiO, Ni, Al
2
O
3
,
TiO
2
(in the absence of UV light), Si
3
N
4
, WC (tungsten carbide), and ZrO
2
were found to lack antimi-
crobial activity at the concentrations tested. The oral pathogens P. gingivalis, F. nucleatum, Prev.
intermedia, and A. actinomycetemcomitans were also found to be susceptible to Ag and CuO nano-
particles under anaerobic conditions with MBC values in the range 0.025
2.5 mg/mL
[41]
.
