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• Osmosis can be defined as the net movement of water

across a selectively permeable membrane driven by a

difference in osmotic pressure across the membrane.

• It is driven by a difference in solute concentrations across

the membrane that allows passage of water, but rejects

most solute molecules or ions.

• Osmotic pressure is the pressure which, if applied to the

more concentrated solution, would prevent transport of

water across the semipermeable membrane.
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These systems are fabricated by encapsulating an
osmotic drug core containing an active drug (or a
combination of an osmotically inactive drug with an
osmotically active salt eg. NaCl) within a semi
permeable membrane made from biocompatible
polymer, e.g. cellulose acetate.

A gradient of osmotic pressure is then created,

under which the drug solutes are continuously
pumped out over a prolonged period of time through
the delivery orifice.

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This type of drug system dispenses drug solutes
continuously at a zero order rate.

A gradient of osmotic pressure is created, under which the drug

solutes are continuously pumped out over a prolonged period of

time through the delivery orifice.
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Osmotic pressure activated drug delivery system

• In this type of controlled drug delivery system the

release of the drug takes place due to osmotic


• Drug reservoir which can be either a solid or a

suspension is contained in a semipermeable housing

• The release is activated through a specially formed

orifice and rate of release is modulated by controlling

the osmotic gradient

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• Thus release rate is dependent on water

permeability of membrane, solubility of osmogen,

effective surface area of semipermeable housing as

well as osmotic gradient

• Representative example of this type of implantable

controlled release drug delivery system is alzet

osmotic pump

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• This pump comprises a rigid, rate controlling outer semipermeable

membrane surrounding a solid layer of salt coated on the inside by

an elastic diaphragm and on the outside by the membrane. In use,

water is osmotically drawn by the salt chamber, forcing drug from

the drug chamber.


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• The membrane allows free diffusion of water but
not of drug.

• Because of osmotic pressure difference, volume
flow rate dv/dt of water into tablet is

dv/dt = kA/h (Δπ – ΔP)
K, A, H are – membrane permeability, area,

thickness resp.
Δπ – osmotic pressure difference.
ΔP – hydrostatic pressure difference.

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• But ΔP <<< Δπ
• So eqn. becomes dv/dt = kA/h. Δπ
• Volume flow rate of water into tablet is determined by

membrane permeability, area, thickness, osmotic
pressure difference.

• Drug will be pumped out through the orifice at
controlled rate, dM/dt which is given by

• dM/dt = dv/dt. Cs
• Release rate will be constant until conc. Of drug inside

tablet falls below saturation.

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Criteria for selection of a drug

• Short biological Half-life (2- 6 hrs)

• High potency

• Required for prolonged treatment

• (e.g: Nifedipine, Glipizide, Verapamil and Chlorpromazine


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Basic components

• Drug

• Osmotic agent

• Semipermeable membrane

• Coating material

• Hyrophilic and hydrophobic polymers


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• The delivery rate of zero-order is achievable with osmotic systems.

• Delivery may be delayed or pulsed, if desired.

• The release rate of osmotic systems is highly predictable and can be

programmed by modulating the release control parameters.

• For oral osmotic systems, drug release is independent of gastric pH

and hydrodynamic conditions.

• The release from osmotic systems is minimally affected by the

presence of food in gastrointestinal tract.

• A high degree of in vivo- in vitro correlation (IVIVC) is obtained

in osmotic systems.

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Osmotic system

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Marketed formulations

Brand name API Manufacturer

Novartis / Pfizer /
Efidac 24® Chlorpheniramine


Acutrim ® Phenylpropanolamine Novartis

McNeil Consumer
Sudafed 24® Pseudoephedrine


Minipress XL® Prazocine Pfizer / Alza

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