OSMOTICALLY CONTROLLED RELEASE SYSTEMS
• 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.
OSMOTICALLY CONTROLLED RELEASE
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.
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.
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
• 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
• 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.
• 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,
Δπ – osmotic pressure difference.
ΔP – hydrostatic pressure difference.
• 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
• 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.
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
• Osmotic agent
• Semipermeable membrane
• Coating material
• Hyrophilic and hydrophobic polymers
• 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.
Brand name API Manufacturer
Novartis / Pfizer /
Efidac 24® Chlorpheniramine
Acutrim ® Phenylpropanolamine Novartis
Sudafed 24® Pseudoephedrine
Minipress XL® Prazocine Pfizer / Alza