The properties of nanomaterials that are unique relate to their structural stability
and quantum-scale reactivity, which open up many exploitable possibilities, includ-
ing targeted drug delivery and the creation of microscale biological sensors. Auto-
fluorescent quantum dots (QD) are currently being developed for use in imaging
applications. In the field of biosensors, a chemical reaction on the surface of a
nanofibre may alter the electrical properties of the material, rendering them ideal
for use as biological sensors when linked to specific antibodies. In theory, such
devices could be used to provide an early warning of the onset of many diseases or
for the presence of toxic chemicals. Nanomaterials may also be developed to
contain tissue-targeting biomolecules, enabling a reduction in the dose required to
provide efficacy. Multifunctional nanoparticles containing tumour-seeking sensors,
imaging agents and receptor-triggered release of toxins that could revolutionise the
therapy of cancer are currently under consideration (Service 2005 ). Some workers
have conceptualised what are essentially synthetic viruses that would function as
intelligent cellular delivery devices but could not replicate to produce adverse
effects.
Nanomaterials may also be constructed from block-polymers of drugs them-
selves, as drug carrying dendrimers (repeatedly branching polymers) or, alterna-
tively, incorporated into nano-shells that would allow accurate controlled release of
drug at the tissue target site. The selective, or in some cases restricted, transport of
nanomaterials further increases their potential for targeted or restricted drug deli-
very. The extremely high surface reactivity of nanomaterials on a weight basis
also creates the possibility of scavenging applications, ranging from the removal of
toxic substances to scavenging free-oxygen radicals generated in various disease
processes.
As well as facilitating implantation into animals, nanoscale material science will
further reduce the size of microprocessors and microfluidic devices (discussed
above) and thereby facilitate the development of nanoscale computer processors.
These will increase further the power as well as decreasing the size and cost of
analytical instruments, microarray platforms and computers.
A cautionary comment must also be made, as the toxicology of nanomaterials
has yet to be clearly defined (Monteiro-Riviere and Tran 2007 ). The parameters for
characterising the properties of nanomaterials must be determined to ensure that
any therapeutic platform utilising nanoscale material does not result in adverse
events.
A specific issue that has been recognised is that the pharmacokinetic properties
of nanomaterials thus far studied are significantly different from small organic
molecules (Riviere 2009 ; Lee et al. 2009 ). Using a PBPK model for QD in mice
and rats, our group has shown that tissue uptake cannot be modelled by flow-limited
approaches but rather should take into consideration actual mechanism of cellular
uptake. This is most likely to be due to vesicular cellular transport mechanisms.
Another issue is that for many nanoparticles, excretion from the body does not
readily occur, making concepts such as bioaccumulation and tissue withdrawal
periods problematic. What are the dose metrics for nanoparticles: mass, particle
number, particle size, surface charge? Self-agglomeration of nanomaterials or
202 J.E. Riviere