Biomedical Engineering Reference
In-Depth Information
in extremely thin membranes of, for example, silicon nitride. These micro-
sieves can be used to filter bacteria from milk or yeast cells from beer, and for
other food applications, but can also be employed in water cleaning (van Rijn
2004). Infected water is a very big problem in less developed countries with
a high death toll, especially among young children (WHO 2002). Sieving
out bacteria from water would greatly reduce this risk. Unfortunately, this
can only be done effectively with high-technology equipment, and therefore
expensive membranes from microtechnology, because it requires the combi-
nation of high porosity to achieve sufficient throughput and a narrow pore
size distribution to prevent the passage of some bacteria through larger holes
and into the clean water stream. High-volume production of these mem-
branes would make such an application also available to poor economies but
it would probably require extensive subsidization from developed countries
to reach an economically viable scale.
By reducing the size of the holes in the sieve, we automatically enter the
realm of nanofiltration and the possibility to start separating smaller biologi-
cal organisms, such as viruses, or even larger molecules from water. However,
this principle can also be achieved with certain materials that by nature are
only permeable for very small molecules. Semipermeable membranes are
often employed in biology to maintain a certain pressure on structures: tur-
gor. This principle, although in a small scale, is also used in military applica-
tions to extract potable water from a dirty water source. A bag partly made
of a semipermeable membrane containing a sugar solution through osmosis
extracts water from the dirty water source. After some time, the bag contains
clean, though sweetened, water. With nanotechnology, this principle can be
improved by replacing sugar with magnetic nanoparticles. Because of their
small size, they create a high concentration solution on the inside of the bag
but because of their magnetic properties they can easily be extracted from
the water inside the bag before consumption.
Microsieves, but especially nanosieves, have a high surface area that comes
in contact with the water to be sieved. This surface can be exploited to attach
enzymes or other substances that can convert certain unwanted molecules to
more desirable ones. These catalytic properties are also used in combination
with nanoparticles, as we will see below.
5.5.2 Remediation
Many of the polluting substances are higher-level organic molecules that are
by-products of industrial activity, or that come into the aqueous environment
after their useful lifetime. Not only do they threaten the life of the organ-
isms living in this environment, they are also a problem in water purification
installations. These installations in part rely on microorganisms to do some
part of the cleaning process. Nanoparticles can help break down these sub-
stances. It has been known already for a long time that some metallic com-
pounds strongly speed up certain chemical reactions without being used
