Aerosols and Foams
Chapter 14
ANSEL'S Pharmaceutical Dosage Forms and Drug Delivery Systems
Eleventh Edition
Objectives:
After reading this topic, the student will be able to:
• Define aerosols
• Understand the types and applications of aerosols
• Identify the main advantage of aerosols
• Define foams
• Explore the types and applications of foams
• Identify the main advantage of foams
• Differentiate between aerosols and foams
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Pharmaceutical Aerosols
Pharmaceutical aerosols are pressurized systems that, upon valve
actuation, emit a fine dispersion (either continuous or metered) of liquid
and/ or solid materials containing one or more active ingredients in a
gaseous medium.
Pharmaceutical aerosols are similar to other dosage forms because they
require the same types of considerations with respect to formulation,
product stability, and therapeutic efficacy. However, pharmaceutical
aerosols unlike other dosage forms differ in their dependence upon the
function of the container, its valve assembly, and an added component the
propellant-for the physical delivery of the medication in proper form.
The term pressurized product.
• Pressure is applied to the
aerosol system through the
use of one or more liquefied
or gaseous propellants.
• Upon activation of the valve
assembly of the aerosol, the
pressure exerted by the
propellant forces the contents
of the package out through
the opening of the valve.
• The physical form in which
the contents are emitted
depends on the formulation of
the product and the type of
valve.
Aerosol products may be designed to expel their contents as a fine mist; a
coarse, wet, or dry spray; a steady stream; or a stable or a fast-breaking
foam. The physical form selected for a given aerosol is based on intended
use.
There are two types of aerosols.
1. Space sprays
Aerosols used to provide an airborne mist.
For example, inhalation therapy, as in the treatment of asthma or
emphysema, must present particles in the form of a fine liquid mist or as
finely divided solid particles. Particles less than 6 μm will reach the
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respiratory bronchioles, and those less than 2 μm will reach the alveolar
ducts and alveoli, also, room disinfectant’s, room deodorizers fall in this
list
The particle size of the released product is generally quite small, usually
below 50 μm, and must be carefully controlled so that the dispersed
droplets or particles remain airborne for a long time.
The valve opening is small, so the particle released are of small size.
The formulation is either solution or suspension.
2. Surface sprays or surface coatings.
Aerosols intended to carry the active ingredient to a surface of the body
(dermatologic aerosols). Included in this group many cosmetic
preparations like personal deodorant sprays,
By contrast, the particle size for a dermatologic spray intended for
deposition on the skin is coarser and generally less critical to the
therapeutic efficacy of the product. Some dermatologic aerosols present
the medication in the form of a powder. a wet spray, a stream of liquid
(usually a local anaesthetic), or an ointment-like product.
Property Space Sprays Surface Sprays
Product Concentrate (%) 2 – 20 20 – 75
Propellant (%) 70 – 98 25 – 80
Pressure (psig at 20°C) 30 – 40 25 – 55
Particles (μm) <1 – 50 50 – 200
Types of Aerosols According to The Route of Administration
1. Inhalation aerosols, commonly known as metered-dose inhalers
(MDIs), are intended to produce fine particles or droplets for inhalation
through the mouth and deposition in the pulmonary tree. The design of
the delivery system is intended to release measured quantities and of
the appropriate quality of the active substance with each actuation.
2. Nasal aerosols, commonly known as nasal MDIs, produce fine particles
or droplets for delivery through the nasal vestibule and deposition in the
nasal cavity. Each actuation of the valve releases measured mass and
appropriate quality of the active substance.
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3. Oral (buccal and sublingual aerosols are intended to produce fine
particles or droplets for deposition on the surface of the tongue. The
design of the delivery system releases one dose with each actuation.
4. Topical aerosols produce fine particles or droplets for application to the
skin. Topical aerosol drug products may be designed, as needed. to
deliver a metered amount of formulation upon actuation of the designed
valve or continuous release of formulation during depressed status of
the valve.
Advantages of Aerosols
The main advantages of aerosols are:
1. The use of aerosols provides the patient a means of applying the drug
in a convenient manner, to the desired surface area without the use of
the fingertips, making the procedure less messy than with most other
types of topical preparations. The rapid evaporation of the propellent
also provide cooling and refreshing effect
2. The drug is withdrawn from the container without contamination or
expose of the remaining material. If the product is sterile, sterility can
be maintained throughout the product’s shelf life.
3. Protect the medicinal agent from environment oxygen and light
because the container is opaque
4. The formulation and the valve control may affect the physical form
and particle size of the emitted product therefore the efficacy of the
aerosol especially in MDI
5. Application is a clean process (no need for wash after application)
The Aerosol principle
An aerosol formulation consists of two components:
1. The product concentrate is the active drug combined with additional
ingredients or co-solvents required to make a stable and efficacious
product. The concentrate can be a solution, suspension, emulsion,
semisolid, or powder, in addition to solvents and surfactant and
antioxidants to prepare a stable and efficient product.
2. The propellant provides the force that expels the product concentrate
from the container and additionally is responsible for the delivery of the
formulation in the proper form.
A. Provide the driving force to expel the product from the container
B. Responsible for developing proper pressure inside the container
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Types of propellent
Type I - Propellent A Liquefied gas
a- Oral and Inhalation (fluorinated HC) Cl₃ F C, Cl₂ F₂C
b- Topical Pharmaceutical Aerosol (HC) propane, butane
When the propellant is a liquefied gas or a mixture of liquefied gases, it
can also serve as the solvent or vehicle for the product concentrate.
They are immiscible with water and have a density less than 1.
When the propellant is in the external phase, foams are not created but
sprays or wet streams result.
The propellent exist as liquids under pressure, but also as gas in the head
space. As the valve is opened some of the liquid propellent turn to gas to
keep the head space full of gas. The pressure in the container remains
essentially constant.
Type II Propellent B Compressed gas N₂, CO₂
Compressed gas propellent occupy the head space above the liquid in the
container. When the aerosol valve is opened the gas pushes the liquid out
of the container
The amount of gas in the headspace remains the same but it has more
space and as a result the pressure will drop during the life of the product
Spray performance is maintained by careful choice of the aerosol valve
and actuator.
Aerosol container and valve assembly
The effective of a pharmaceutical aerosol depends on achieving the
proper combination of formulation, container, and valve assembly.
A. Containers are made of a) glass b) metals tin plated steel,
aluminium, stainless steel and c) plastic
Selection depends formula compatibility, ability to sustain the
required pressure, and cost.
B. Valve assembly allows expulsion of the contents in the desired
form at the desired rate and for metered valves the proper amount,
they are two types
Continuous used in topical applications
Metered used for inhalation of potent medication
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Pump spray
Pump spray means a packaging system in which the product ingredients
within the container are not under pressure and in which the product is
expelled only while a pumping action is applied to a button, trigger or
other actuator manually on the pump to deliver the product in the form of
a spray.
Use of special actuator according to the site of administration
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Pharmaceutical Foams
A foam is a coarse dispersion of a gas in a liquid.
Foams are emulsified systems packaged in special dispensing
devices that contain dispersed gas bubbles, usually in a liquid continuous
phase, that when dispensed has a fluffy, semisolid consistency. The foams
are generally o/w emulsions resembling light creams. They are water
miscible and non-greasy.
The product concentrate in an emulsion consists of the active
ingredient, aqueous and/or non-aqueous vehicles, and a surfactant.
Foams are produced when the product concentrate is dispersed
throughout the propellant and the gas is in the internal phase, i.e., the
emulsion behaves like o/w emulsions.
Advantages of foams
Probably the most convincing argument for the use of foams
1) Foam formulations are generally easier to apply, are less dense, and
spread more easily than other topical dosage forms,
2) Used without the use of fingertips especially for places hard to
reach (vagina, rectal) or that cause irritation when applied (for burn
dressing)
3) Patient and consumer acceptance because it is easy to use.
4) Foams may be formulated in various ways to provide emollient or
drying functions to the skin, depending on the formulation
constituents.
Product concentrate
In addition to the formula components, the main constituents that
differentiate foams are surfactants such as:
1. Anionic surfactants: Fatty acids saponified with triethanolamine,
2. Nonionic surfactants: such as the Polyoxyethylene fatty esters,
Polyoxyethylene sorbitan esters (Tweens) , alkyl phenoxy ethanol, and
alkanolamide.
The nonionic surfactants are present fewer compatibility problems
because they charge no electronic charge
Types of Foams
1) Foam Type 1 Breaking Foam
A quick breaking foam creates a foam when emitted from the container
but the foam collapses in a relatively short time.
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This type of foam is used to apply the product concentrate to a large area
without having to manually rub or spread the product. Also, the active
drug is more rapidly available because the foam quickly collapses.
2) Foam Type 2 Stable Foam
Depending on the components, the emitted product can be a stable
foam (shaving cream type)
Stable foams are produced when surfactants are used that have limited
solubility in both the organic and aqueous phases.
Surfactants concentrate at the interface between the propellant and the
aqueous phase forming a thin film referred to as the "lamella." It is the
specific composition of this lamella that dictates the structural strength
and general characteristics of the foam.
Thick and tightly layered lamellae produce very structured foams,
which are capable of supporting their own weight.
Site of application of foams
A. Topical Foams
B. Vaginal Foams
The foams are used intravaginally in the same manner as for creams,
using a special type of applicator
The aerosol package contains an inserter that is filled with foam and the
contents placed in the vagina through activation of the plunger.
C. Rectal Foams are usually stable foams.
Packaging of foams
Foam valves have only one orifice that leads to a single expansion
chamber. The expansion chamber also serves as the delivery nozzle or
applicator. The chamber is the appropriate volume to allow the product
concentrate to expand into a ball of foam. Foam valves are used for viscous
product concentrates such as creams and ointment
ents because of the large orifice and chamber. Foam valves also are used
to dispense rectal and vaginal foams. If the size of the orifice and expansion
chamber are appropriately reduced, a product concentrate that would
produce a foam will be emitted as a solid stream. In this case, the ball of
foam begins to develop where the stream impinges on a surface.
Types of packaging
a) Aerosol Foam pressurised containers which contain propellent which
aid in expulsion of the product concentrate (usually a stable foam)
produces a thick cream
b) Foam Pump without the use of propellent, but special type of valve
and actuator (usually a breakable foam) produces a light emulsion
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