Page 405 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

340 Membranes, Synthetic, Applications

FIGURE 51 Schematic of artificial pancreas under development based on a membrane unit (Circe Biomedical, Inc., Lexington, MA).

certain minimum level to obtain the desired therapeutic is controlled by the size of the patch. The patch may be
effect, but below the point where toxic side effects occur. removed to discontinue drug delivery instantly. Devices
Once the desired level is reached, it should be maintained of this type have been used to administer nitroglycerine
over an extended period without further intervention from for treating angina.
the user. Controlled release technology based on the use of Still another dosage form is microcapsules, which are
synthetic membranes is an effective approach to address- tiny droplets of an active agent coated with a permselective
ing these issues. barrier polymer, all in the form of a suspension. Micro-
capsules are conveniently formulated for injection. In such
cases the barrier polymer is often chosen to be degradable
1. Passive Controlled Release into innocuous products in the body after the active agent
In one type of controlled-release device, an active agent is has been dispensed.
Osmotic pumps operate by a different principle. In
dissolved at its solubility limit in a reservoir surrounded
one design, the active agent is mixed with an osmotic
by a membrane. By providing excess agent in the reser-
agent such as salt or sucrose, and covered with a water-
voir (e.g., by physical mixing or compounding), a constant
permeable membrane. When the device is immersed in
permeation driving force is exerted on the membrane. In
water and the infusion of water generates a pressure
this way a constant release rate is achieved until the agent
inside the membrane, the active agent is forced out of
concentration drops below saturation. Release rate is de-
the device through metering holes or capillaries on the
termined by the solubility and diffusivity of the agent in
surface of the membrane. Alternatively, the active agent is
the membrane. Tailoring the chemistry and geometry of
packaged within a sac that separates it from the osmotic
the device provides different release profiles and capac-
agent, which in turn is confined by the water-permeable
ities. Devices of this type have been developed for ad-
membrane. The pressure generated by the wetted osmotic
ministering steroids for fertility control or hormone ther-
agent compresses the sac and exudes the active agent from
apy, or intraocular dispensing of pilocarpine for glaucoma
the device.
control. For nonmedical applications, similar devices are
available in multilayer laminate form for dispensing insect
2. Active Controlled Release
pheromones, insecticides, fragrances, and bactericides.
Sometimes the human skin is used as the rate- A novel approach to controlled release integrates biosens-
controlling membrane for certain pharmaceuticals that ing and control functions in a single membrane device.
are readily absorbed. A removable skin patch containing As an example, a membrane responds to changes in glu-
the drug is applied directly on the body to provide a cose level in the body by automatically changing its per-
reservoir of specific capacity; the rate of administration meability to insulin. The response mechanism is shown