Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 70

themselves, although this may well depend on the exact conditions of the medium
(Chapter 3 ).

2.6.2.7 Drug Delivery

There is a drive to develop new methods for delivering drugs in the human body.
One of the key issues in developing a drug is that many are poorly soluble in
aqueous media. Whilst solubility may be enhanced by modifi cation of the drug
molecule, such modifi cations have the risk of altering the action of the drug itself.
There is, therefore, a need to develop carriers which will deliver the active drug
without modifi cation and allow active doses to be delivered. One method for doing
this is to encapsulate the drug in a polymer particle. The drug can then exit the
particle by simple diffusion or by the gradual degradation of the particle itself. A
whole range of polymers have been exploited as carriers for drugs such as polyes-
ters (poly(glycolic acid), poly (lactic acid) and poly-

- caprolactone), polyamides
and polyphosphazenes. These polymers have been prepared as co-polymers with
polyethylene oxide, polyethylene glycol and polypropylene oxide in order to
prepare drug carrier particles (Uhrich et al. , 1999; Pillai and Panchagnula, 2001).
It is well known that the response of the body to particles can vary from organ
to organ. The liver tends to fi lter out particles on the 1- 5

ε

m scale and the spleen
tends to remove particles greater than 200 nm. However, particles smaller than
100 nm tend to penetrate blood vessel walls. Generally then, the aim is to produce
particles of around 100 nm.
Generally the nanoparticles are prepared by either a living polymerisation,
macromonomer initiation or polymer coupling, this results in a copolymer with two
sections with different solubilities in water (Section 2.5.13). A dilute solution of the
polymer is then prepared and carefully added to stirred water. As the polymer
precipitates the segments which are insoluble in the water aggregate to form a
hydrophobic core and the water soluble segments remain on the outside giving
steric stabilisation. Particles of polycaprolactone- block - polyethylene oxide pre-
pared by this method have diameters ranging from 20- 50 nm (Hua and Dong, 2007 ).
An alternative conformation for the nanoparticle is a polymersome where the
centre of the particle also contains a hydrophilic area. It has been shown that
degradable systems such as these can greatly affect the delivery of drugs. A cocktail
of taxol (hydrophobic) and doxorubicin (hydrophilic) were placed in a polymer-
some and used to treat tumors on mice. The results showed that the polymersome
was much more effective than simple administration of the cocktail without the
carrier. It should be noted that the exact structure of the copolymer affects both
the shape and form of the nanoparticle, its degradability and its activity.

µ

2.6.2.8 Cosmetics/Sunscreens

The use of titanium dioxide (TiO 2 ) or zinc oxide (ZnO) nanoparticles in sunscreens
and anti-ageing creams is already prevalent. These nanomaterials offer the added
benefi t of being transparent to visible light because the crystal sizes are too small
to cause signifi cant scattering of visible light. Generally, these particles are of a core
shell nature and have an outer layer of silica or alumina which will hinder the