Semisolid dosage form:

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• Semisolid dosage form

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Skin Structure

• The skin consists of three distinct layers as
  follows
• 1. The epidermis,
• 2. The dermis,
• 3. The sub-cutaneous layer.
• Besides the different layers, there are some
  other features in the skin
• sweat gland
• sebaceous gland
• hair
• nail
• nerve, etc.

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• Outermost is the protective epidermis, which is
  continually worn off (shed off) from the top and
  replenished from below.
• Under this, is the dermis, which contains blood
  vessels, hair follicles, glands, and sense
  receptors responsible for the sensations of
  touch, temperature, and pain.
• Below the dermis is the subcutaneous layer, which
  contains fat cells. Hair and nails are extensions
  of the skin made of a tough protein called
  keratin.

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Layers of epidermis

• the epidermis consists of five layers as follows
  
• a) Stratum basale
• b) Stratum spinosum
• c) Stratum granulosum
• d) Stratum lucidum
• e) Stratum corneum

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Transdermal drug delivery

• Transdermal is a route of administration wherein
  active ingredients are delivered across the skin
  for systemic distribution. Examples include
  transdermal patches used for medicine delivery,

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Examples of transdermal drug delivery
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Adnavtages and disadvantages of TDDS
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Types of skin preparation

• There are 3 types of skin preparation based on
  their physical nature
• 1.solid example
• 2. liquid example
• 3. semisolid example

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Semi solid dosage form   

• Ointments   
• Are composed of fluid hydrocarbon meshed in a
  matrix of
• higher melting solid hydrocarbon. Most Ointment
  are
• based on mineral oil and petrolatum.
•  Pastes
•  Are ointments in to which a high parentage of
  insoluble
• Solid has been added.

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•  Cream    
• Are Semi solid emulsion systems with opaque
• appearances. Their consistency and rheologic
• character depend on whether the emulsion is a o/w
  or
• w/o type.
•   Gels
• Are semisolid system in which a liquid  phase is
• constrained within a three dimensional polymeric
  matrix.
• The polymer used includes tragacanth  , pectin
  ,agar,
• algenic acid and some synthetic material such as
  methyl
• cellulose  ,carboxy methyl cellulose, hydroxy
  ethyl
• cellulose and the carbopols .

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Percutaneous absorption

• Percutaneous", via its Latin roots means, 'by
  way of the skin'. An example would be
  percutaneous drug absorption from topical
  medications

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Routes of penetration

• There are 3 potential portals of entry 
• 1.  Through the folliculate region.
• 2.  Through the sweat ducts.
• 3.   Through the unbroken stranum.

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Factor in skin penetration

• 1. Physicochemical properties of the drug.
• 2. Vehicle, pH , concentration of drug
• 3.Physiological factor-
• a) Condition of the skin e g- intact or
   injured
•   b) Skin age      
•   c) Area of skin treated
• d) Thick ness of skin barrier phase
•   e) The species variation
• f) Skin moisture content

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• The principal physicochemical factor in skin
  penetration is the hydration state of the stratum
  corneum , which affects the rate of passage of
  all substances that penetrate the skin. 
  Hydration result from water diffusion from
  underlying epidermal layers after application of
  an occlusion vehicle or covering  on the surface
  .Under  occlusive condition ,the moisture content
  of stratum corneum rises from 5-15 to 50

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Raw materials

•  More raw materials are available for use on the
  skin than for oral use . Substance such as
  isopropyl  myristate  , butyl stearate  may be
  used topically without toxic effect ,yet these
  esters may not be used orally because hydrolysis
  of the esters by digestive enzyme yield poorly 
  tolerated alcohol.

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1.Oil

• Any good quality vegetable oil can be used to
  make your
• ointments and creams.
• Almond oil is very luxury for face creams.
• The most luxury oil is jojoba wax,
• For healing the skin olive oil is perfect because
  it heals by itself.
• For dry skin sunflower oil is very good.
• Rapeseed oil is fine and neutral, it is very
  liquid and doesn't have a strong smell by itself.
• Sesame oil is a nourishing oil and it protects a
  little bit against too much sun.

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2.Thickener

• Beeswax, good protection, a film stays on the
  skin. Strong thickening.
• Cocoa butter, made from cocoa beans, rather
  greasy. Soft thickening.
• Wool wax, made from the wool of sheeps. Very soft
  but healing effects.it is a soft emulsifier. Soft
  thickening.
• Cetyl alcohol, a substance similar to skin
  substances. White and not greasy. Stabilizes
  emulsions. Very strong thickening.

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3) Emulsifiers-

• When you want to prepare a cream you need to
  combine oily and watery substances. Usually water
  and oil doesn't connect, they separate as soon as
  possible.
• To help both types of substances to connect, you
  need an emulsifier.
• The emulsifier connects at one side with the
  water and at the other side with the oil. This is
  called an emulsion.
• There are different kind of emulsions
• When there is a lot of oil around some water, it
  is called water-in-oil-emulsion (W/O).
• When there is a lot of water around some oil, it
  is called oil-in-water-emulsion (O/W).

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• 1.Lanolin anhydrous-Lanolin is a natural wax,
  that is produced by the sheeps to fatten their
  wool. It is brown and sticky. Sometimes it's
  called woolwax.
• 2.anionic surfactant-sodium lauryl sulfate
• 3.Cationic surfactant-cetrimide
• 4.Non ionic surfactant- most widely used

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4) Polyols-

• Glycerine , propylene, glycol, sorbitol  70 and
  lower molecular  weight polyethylene glycol are
  used as humectants (prevent the cream from drying
  out)in creames .they also has effect on the
  texture and viscosity of the preparation.

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5.Preservative

• If you prepare creams, the combination of water
  and fat makes the cream sensible to mold and
  bacterias.
• Without conservating substances a cream keeps
  good about one week, in the refrigerator it could
  be two weeks.
• When you want to keep the cream for a longer
  time, you need substances to conservate the cream.

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6.water

• Water is an important part of creams. Usually
  water from the tap is not good enough, because
  creams made by this kind of water get mildew
  faster than other creams.

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    Preparation of semisolids-

•  1.The component of the oil or fat mixture are
  placed in to a stainless steel stem jacketed
  kettle,?? melted, and ?? mixed. 
• 2.The component of the aqueous phase are
  dissolved in the unfired water and filtered .A
  soluble drug may be added  to the aqueous phases
  .
• 3.the above two phases are mixed. mixing is done
  in planetary mixer or sigma blade mixer.

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Cooling-

• 4.Following  the addition of the phases ,the rate
  of cooling is generally slow to allow for
  adequate mixing while the emulsion is still
  liquid .
• 5.if perfume is to be added it is best done at a
  temperature of 43C to 45 C .

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Homogenization-

• 6.The creams or ointments that require further
  treatment are then transferred or pumped to the
  proper homogenous  eg low shear  gear pump,
  roller mill ,colloid mill. 

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Equipments used in semisolid mixing

• Planetary mixer

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Sigma blade mixer
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Roll mill