Biomedical Engineering Reference
In-Depth Information
13.1 INTRODUCTION
13.1.1 B
ackgrouNd
Nanotechnology can be considered to be the application of science that “steps across the limit” of
miniaturization, where “new rules” become valid (Schmidt et al. 2003). More specifically, when the
dimensions of a solid material become incredibly small, its physical and chemical properties can
become very different from the larger, bulk form of the same material. This is one of the hallmarks
of nanotechnology, it can be described as a research area in which this limit is reached and strate-
gies are developed to exploit the regime of size-controlled properties (Paul et al. 2006). In the last
couple of years, the term “nanotechnology” has been inflated and has almost become synonymous
for things that are innovative and exceedingly promising. Alternatively, it is also the subject of
considerable debate regarding the open question on the toxicological and environmental impact of
nanoparticles (NPs) and nanotubes (Donaldson et al. 2004; Oberdörster et al. 2005).
The term “nanomaterial” is used to describe materials with one or more components that have at
least one dimension in the range of 1-100 nm, including NPs, nanotubes and nanofibers, nanostruc-
tured surfaces, and composite materials. These include NPs as a subset of nanomaterials, currently
defined by consensus as single particles with a diameter <100 nm. Agglomerates of NPs can be
larger than 100 nm in diameter, but will be included in the discussion since they may break down
in solvents or from weak mechanical forces. Nanofibers are a subclass of NPs (including nanotubes)
that have two dimensions <100 nm, but the third (axial) dimension can be much larger (Paul et al.
2006).
The unique size-dependent properties of nanomaterials mean that they behave like new chemical
substances in some ways. For instance, NPs can scatter and absorb short-wavelength UV (ultra-
violet) radiation but leave longer, wavelength-visible light almost unaffected, a property that is
exploited in transparent sunscreens. Fluorescent NPs absorb UV radiation while they emit visible
light, and the color of the emitted light is different for NPs of different diameters. This result is
exploited when NPs are designed as color-coded, fluorescent labels that can be used as diagnostic
markers or attached to target molecules. The changes in optical and transport properties become
very pronounced for NPs smaller than about 30 nm. Particles smaller than 30 nm are often called
“quantum dots” because the size controls the separation (or quantization) of energy levels inside the
particle (Paul et al. 2006).
Some nanomaterials have been produced in large volumes for a very long time. NP-sized carbon
blacks have been in production for more than a century and are used for the manufacture of pig-
ments and rubber products. Oxides such as titanium, alumina, and zirconium, and fumed silica have
been produced as nanomaterials for over half a century and used as thixotropic agents in cosmetics
and pigments. In recent times, they have been used as the basis for fine polishing powders in the
microelectronics industry. To a great extent, this high-volume production is based on vapor-phase
flame or plasma reactions carried out under highly controlled conditions (Paul et al. 2006).
For new nanomaterials, surfaces and interfaces are very important. The proportion of atoms
found at the surface increases relative to the volume as soon as particles become smaller. This
means that NPs can be more reactive, such as creating more efficient filler materials or more effec-
tive catalysts that allow for the reduction in the weight of composite materials. The higher surface
energy can also make NPs stick together and interact strongly. If the nanomaterial building blocks
are synthesized in such a way that some parts of the surface are sticky but other parts are nonsticky
and passive, random Brownian motion in a fluid can cause the blocks to stick together in defined
ways to make larger structures (Paul et al. 2006).
However, as the applications of nanomaterials increase, the risk of exposure to the general pub-
lic will grow. It will be necessary to monitor products that incorporate NPs and nanofibers from
manufacture to disposal to estimate the probability of environmental emissions, particularly from
waste-management processes and disposal. A few products may involve the direct delivery of NPs
