Semisolid dosage form:

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.

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.

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

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,

Examples of transdermal drug delivery

Adnavtages and disadvantages of TDDS:

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

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.

 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 .

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

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.

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

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%

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.

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.

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.

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).

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

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.

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.

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.

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.

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 43°C to 45 °C .

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. 

Equipments used in semisolid mixing: Planetary mixer

Sigma blade mixer

Roll mill: