Page 219 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: ZBU Final Pages
Encyclopedia of Physical Science and Technology EN011J-141 July 31, 2001 15:14

796 Pharmaceuticals, Controlled Release of

FIGURE 5 Biodegradation of poly(lactic acid-co-glycolic acid) to its simple acid precursors. The rate of biodegradation
is a function of the copolymer composition. The pure homopolymers are both relatively crystalline and biodegrade
slowly, but the more amorphous copolymers biodegrade more rapidly.

devices that actually use osmotic effects to control the re- the salt chamber. The volume of the salt chamber increases
lease of drugs have been developed. These devices, called because of this water flow, which distends the latex di-
osmotic pumps, use the osmotic pressure developed by aphragm separating the salt and drug chambers, thereby
diffusion of water across a semipermeable membrane into pumping drug out of the device.
a salt solution to push a solution of the active agent from The pumping rate of the Rose–Nelson pump is given
the device. Osmotic pumps of various designs are widely by the equation
applied in the pharmaceutical area, particularly in oral
dM t dV
tablet formulations. = · c, (10)
dt dt
The forerunner of modern osmotic devices was the
Rose–Nelson pump. Rose and Nelson were two Australian where dM t /dt is the drug release rate, dV/dt is the vol-
physiologists interested in the delivery of drugs to the gut ume flow of water into the salt chamber, and c is the con-
of sheep and cattle. Their pump, illustrated in Fig. 7, con- centration of drug in the drug chamber. The osmotic water
sists of three chambers: a drug chamber, a salt chamber flow across a membrane is given by the equation
containing excess solid salt, and a water chamber.The drug
dV Aθ π
and water chambers are separated by a rigid, semiperme- = , (11)
dt l
able membrane. The difference in osmotic pressure across
the membrane moves water from the water chamber into where dV/dt is a water flow across the membrane of
3
area A, thickness l, and osmotic permeability θ (cm · cm/
2
cm · hr · atm), and π is the osmotic pressure difference
between the solutions on either side of the membrane.
This equation is only strictly true for completely selective

FIGURE 7 Mechanism of action of a Rose–Nelson osmotic
FIGURE 6 Various types of biodegradable drug delivery sys- pump, the precursor of today’s osmotic controlled release delivery
tems. systems.