Bioprocess Engineering
45
monoclonal antibodies may be used to de-
liver drugs to cancer cells only through
specifc ligand–receptor binding. Gene
therapy relies on a better understanding
oF developing vectors to deliver genes and
proteins to specifc targets, to deliver vac-
cines, For example. Animal models are
improving and the Future may hold more
gene therapy For humans, although this is
another area oF controversial research and
application.
Understanding cellular transport also
aids in environmental remediation. Once
microorganisms have been injected into
the environment, they must move From
the
point
oF
injection
to
the
con-
tamination site. Cell receptors known
as
pili
or
fmbriae
are responsible For
some cell adhesion to the soil, min-
erals, and other contaminants in soil.
Polysaccharide
production
is
also
re-
sponsible
For
cell
adhesion.
IF
pilin
and polysaccharide synthesis and bind-
ing mechanisms are understood, the eF-
fciency oF using biodegradation to re-
mediate soil and water can be deter-
mined through process engineering anal-
ysis and compared to other existing tech-
nologies such as chemical oxidation and
incineration.
A growing area oF concern is the under-
standing oF pathogen transport through
water systems. Once a bacterial or viral
pathogen enters a water supply, it is im-
portant to understand how its transport
can be limited. EFfcient, cost-eFFective al-
ternatives For pathogen death and removal
are currently being investigated, as well
as the Fundamental principles influenc-
ing pathogen transport such as the eFFect
oF minerals, organics, and temperature.
Since the environment in which we live
is complex and changing, how a pathogen
behaves in a particular environment is still
not well understood.
6
Bioseparations
Although a large quantity oF work has been
done on Fermentation and other upstream
processing, the main cost oF making a
biotechnological product is in the down-
stream processing or separation steps.
Improved methods are always needed,
since most oF the separation equipment
employed are simple scale-ups oF ana-
lytical and micro-preparative techniques
pioneered in liFe sciences laboratories.
AFter a product is made by a cell or a
plant, it needs to be separated From the rest
oF the by-products. The standard industrial
separation techniques For separating cells
From spent media entail either centriFu-
gation or cross-flow fltration. IF the cell
product is an intracellular product, the
cells must be lysed through homogeniza-
tion, osmotic shock, or bead milling. Then
the cell debris is removed by centriFuga-
tion or fltration. Bacterial products tend
not to secrete protein products and re-
quire this step, whereas mammalian cells
typically do secrete. The expensive part
is Further purifcation oF the product. ±or
soluble protein products, all oF the cel-
lular protein is usually precipitated as a
frst step by changing the temperature
or ionic strength or by adding a solvent
such as ethanol. The purity and eFfciency
oF the process is governed by thermody-
namics – in particular, by the chemical
potential oF the precipitant and the so-
lution. A number oF technologies exist
For Further product separations, depend-
ing on the purity oF the product required.
These include aFfnity, ion exchange, re-
versed phase, size exclusion and other
Forms oF chromatography; electrophore-
sis; liquid–liquid extraction; membranes;
and distillation. There are process de-
sign heuristics For purifcation/separation,
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