Overview, Classification Of Pharmaceutical Powder
Before we talk about pharmaceutical powder and it's classification, let us define powder. Powder is a homogenous mixture of more or less finely divided particulate material in dry form. On the other hand, pharmaceutical powders are solid dosage forms of medicament in which one or more drugs are dispensed in a finely divided state with or without excipients. They are available in crystalline or amorphous form.
Although the use of powders as a solid dosage form has declined, they are the starting material in the manufacture of many dosage forms. Powders are one of the oldest dosage forms and are used both internally and externally. They impart flexibility, stability, rapid therapeutic effect and ease of administration to all categories of patients.
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However, the preparation of powders is not a suitable dosage form for unpleasant-tasting, hygroscopic and deliquescent drugs.
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Powders are subdivided solids which are classified in the BP according to the size of their constituent particles ranging from 1.25 mm to 1.7 mm in diameter. A good powder formulation has a uniform particle size distribution. If the particle size distribution is not uniform, the powder can segregate as per to particle size which may result in inaccurate dosing or inconsistent performance.
A uniform particle size distribution ensures a uniform dissolution rate if the powder is to dissolve, a uniform sedimentation rate if the powder is used to remain in a suspension, and minimizes stratification when powders are stored or transported. Reduction in particle size of a powder results in a uniform distribution of particle size. The process of reducing the particle size is called comminution.
Classification Of Powders
Powders can be classified in various ways and these include:
Classification Of Powders Based On Particle Size
a. Very coarse (No. 8) powder: All particles pass through a No. 8 sieve (2.38 mm) and not more than 20% pass through a No. 60 sieve.
b. Coarse (No. 20) powder: All particles pass through a No. 20 sieve (0.84 mm) and not more than 40% pass through a No. 60 sieve.
c. Moderately coarse (No. 40) powder: All particles pass through a No. 40 sieve (0.42 mm) and not more than 40 % pass through a No. 80 sieve.
d. Fine (No. 60) powder: All particles pass through a No. 60 sieve (0.25 mm) and not more than 40% pass through a No. 100 sieve.
Classification Of Powders According To The Manner Of Their Dispensing
1. Divided powders
2. Simple and compound powders for internal use
3. Powders for reconstitution
4. a) Bulk powders for internal use
i. Oral bulk powders
ii. Antacid Oral bulk powders for reconstitution
iii. Injectable bulk powders for reconstitution
4. b) Bulk powders for external use
i. Dust
ii. Douches
iii. Dental
iv. Insufflations
v. In snuff
vii. Effervescent granules
Classification Based On Use
1. Powders for internal use
(a) Divided powders
(i) Simple powders
(ii) Compound powders
(iii) Powders enclosed in cachet
(iv) Tablet triturates
(b) Bulk powders
(i) Antacid
(ii) Laxative
2. Powders for external use
(a) Dusting powders
(i) Medicated dusting powders
(ii) Surgical dusting powders
(b) insufflations
(c) Douche powder
(d) Dentifrices
3. Special powders
(a) Eutectic mixtures
(b) Effervescent powders
1. Powders For Internal Use/Oral Powder
Oral powders are finely divided powders that contain one or more medicaments with or without auxiliary substances including, where specified, flavouring and coloring agents. However, addition of saccharin or its salts is not permitted in the preparations meant for pediatric use. They are intended to be taken internally with or without the aid of water or any other suitable liquid.
(a) Divided Powders
Divided powders (or charta) are single doses of powdered drugs individually wrapped in cellophane, metallic foil or paper.
The divided powder is a more accurate dosage form than bulk powder because the patient is not involved in measurement of the dose. These are powders used for drugs that are effective in low concentrations. Cellophane and foil-enclosed powders are better protected from the external environment until the time of administration than paper-enclosed powders.
(ii) Compound powder: It consists of a mixture of more than one active ingredient and other constituents.
(iii) Powders enclosed in cachet: Cachets consist of a dry powder enclosed in a shell, usually prepared from a mixture of rice flour and water by molding into a suitable shape and drying. They are quite useful for administering the drugs with nauseating and unpleasant taste and a large dose can be enclosed in a cachet than in a tablet or capsule.
A cachet offers little protection against light and moisture. Now-a-days cachets are replaced by capsules. There are two types of catches:
1. Wet cachets: Lower half of the cachet is filled with powdered drug, then the flange of the empty upper half of the cachet is moistened with water and pressed over the lower half. The cachet is dried for 15 minutes.
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2. Dry cachets: Drug powder is filled in the lower half and the upper half is pressed over it just like a capsule. They are used for administering the drug with unpleasant taste and a large dose. Before administration, a cachet should be immersed in water for a few seconds and then placed on the tongue and swallowed with water.
(iv) Tablet triturates: Tablet triturates are powders molded into tablets. Tablet triturates are generally prepared by mixing the active drug with lactose, dextrose, sucrose, mannitol, or some other appropriate diluents that can serve as the base. This base must be readily water soluble and should not degrade during the tablet's preparation. Lactose is the preferred base but mannitol adds a pleasant, cooling sensation and additional sweetness in the mouth.
The base ordinarily used for molded tablet triturates is lactose containing 10%- 20% sucrose, the latter being added to make a firmer tablet. Drugs that react chemically with sugars require special bases such as precipitated calcium carbonate, precipitated calcium phosphate, kaolin or bentonite.
A liquid is added to moisten the powder mixture so it will adhere while being pressed into the mold cavities. Mixtures of alcohol and water in varying proportions (typically about 50-80% alcohol) are employed; the alcohol will speed-up the drying of the liquid and the water will cause the sugars to dissolve and bind the tablet. If the tablet contains ingredients that are very soluble in water, water can be omitted altogether and alcohol alone can be used.
Tablet triturates are used for oral administration or sublingual use. They may also be used in compounding procedures by pharmacists in the preparation of other solid or liquid dosage forms. They can be inserted into capsules, and this eliminates the problems of measuring the accurate amount of potent drugs in the powder form.
(b) Bulk Powder
Bulk powders are non-potent and can be closed with acceptable accuracy and safety using measuring devices such as the teaspoon, cup or insufflators. The mixed ingredients are packed into a suitable bulk container, such as a wide-mouthed glass jar.
Because of the disadvantages of this type of preparation, the constituents are usually relatively non-toxic medications with a large dose. This practically limits the use of orally administered bulk powders to antacids, dietary supplements, laxatives, and a few analgesics.
Powders For External Use
(a) Dusting Powders
Dusting powders are externally used bulk powders. They are free flowing very fine powders containing antiseptics, antipruritics, astringents, antiperspirants, absorbents, lubricants etc.
(i) Medicated dusting powders: Medicated dusting powders are sterile ones and meant for application on superficial skin. Body dusting powders have a wide appeal because of the smooth feel and cooling effect, which they impart while they temporarily absorb moisture. The cooling effect is due to extra heat loss due to the large surface area of talc particles.
Talc is a major ingredient in medicated dusting powder formulation, which should have good slip characteristics, covering power and body adhesion. The slip and adhesion properties of medicated dusting powder essentially depend on talc. It is essential to use grid and alkali free high quality cosmetic talc in preparation of medicated dusting powder.
Talc should be free from bacteria and therefore sterilized grades should only be used. In order to improve adhesion properties, metallic stearates such as zinc stearate or magnesium stearate and kaolin are incorporated. To improve absorbency, magnesium carbonate, starch, kaolin and precipitated chalk are used in combination.
Zinc oxide and titanium dioxide, at low levels along with earth colors can be incorporated and should be sufficiently powerful to cover the base odour. Other ingredients sometimes included are boric acid to act as skin buffering agent and fused silica to give powder a lower density; salicylic acid for antibacterial action. Aluminum chloride is also incorporated as an antiperspirant.
(ii) Surgical dusting powders: Surgical dusting powders are intended to be used deep in the skin and also on major wounds as a result of burns and umbilical cords of infants. Surgical dusting powders must be free from pathogenic microorganism and hence it must be sterilized before their use.
The dusting powders are mainly used for their antiseptic, astringent, absorbent, antiperspirant, and antipruritic action. It mainly contains antimicrobial agents like chlorhexidine and hexachlorophene. They are generally prepared by mixing two or more ingredients one of which must be starch, talc or kaolin as one of the ingredients of the formulation. Talc is more commonly used because of its chemical inertness.
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However, since such ingredients are readily contaminated with pathogenic bacteria, these must be sterilized by dry heat method before use. Dusting powders are dispensed in Sifter-top containers or aerosol containers. It may also be applied with powder puff or sterilized gauze pad.
(b) Insufflations
They are medicated dusting powders meant for introduction into the body cavities such as the nose, throat, ears, etc. with the help of an apparatus known as insufflators (powder blower). lt sprays the powder into a stream of finely divided particles all over the site of application.
The insufflations are used to produce a local effect, as in the treatment of ear, nose and throat infection with antibiotics or to produce a systemic effect from a drug that is destroyed in the gastrointestinal tract. As like aerosols, uniform dose may not be obtained by insufflations.
(c) Douche Powder
These powders are intended to be used as antiseptics or cleansing agents for a body cavity; most commonly for vaginal use although they may be formulated for nasal, otic or ophthalmic use also. They usually are used after being dissolved in water. Douche powder formulation often includes aromatic oils.
It becomes necessary to pass them through a sieve 40 or 60 to eliminate agglomeration and to ensure complete mixing. They can be dispensed either in wide mouth glass bottles or in powder boxes but the former are preferred because of protection afforded against air and moisture.
(d) Dentifrices
Dentifrices are preparations meant to clean the teeth and other parts of the oral cavity (gums) using a finger or a toothbrush. They are available as tooth powder, toothpastes, gels, dental creams and even as dental foams. They are meant to enhance the personal appearance of the teeth (daily removal of pellicles) by maintaining cleaner teeth.
Reduction of bad odour (removal of putrefying food particles from spaces between teeth) and also make the gum healthy. They contain a suitable detergent or soap, some abrasive substance and a suitable flavour. The abrasive agents such as calcium sulphate, magnesium carbonate, sodium carbonate and sodium chloride are used in fine powder form. A strong abrasive substance should not be used as it may damage the tooth structure.
The main components of tooth powders are solid particles of very fine size and the end product is also a very dry powder.
Since the main components like abrasives and surface active agents are solid powders, it is required that they all are in very fine particle size. That means comminuted, if desired, passed through a sieve and mixed in a mortar in the lab scale and in blenders on an industrial scale.
The flavoring oils are added at the end either by spraying on the powder mixture or first blending with one of the components and then mixing this blend to the rest of the mixture by the method of dilution or geometric proportion.
3. Special Powders
(a) Eutectic Mixtures
Eutectic mixtures are mixtures of low melting point ingredients which on mixing together turn to liquid form due to depression in melting point of the mixture below room temperature. They are mixtures of substances that liquefy when mixed, rubbed or triturated together. The melting points of many eutectic mixtures are below room temperature.
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Examples of the substances which tend to liquefy on mixing are camphor, thymol, menthol and salol. Any two of these drugs turn to liquid when mixed. This problem during formulation of powders of such material can be solved by using inert absorbents such as starch, talc, lactose to prevent dampness of the powder and dispensing the components of the eutectic mixture separately.
(b) Effervescent Powders
Effervescent powders contain materials which react in presence of water-evolving carbon dioxide. This class of preparations can be supplied either by compounding the ingredients as granules or dispensed in the form of salts.
For evolution of the gas, two constituents are essential. A soluble carbonate such as sodium bicarbonate and an organic acid such as citric or tartaric acid. The preparation can be supplied either as a bulk powder or distributed in individual powders.
There are three alternative methods of dispensing effervescent powders based upon the nature of prescription.
(i) If the effervescent salts are prescribed to be dispensed in bulk form, no granulation is necessary. The ingredients are mixed uniformly and directions stated on the label to add the prescribed quantity to water, before use.
(ii) If the effervescent salt is prescribed in divided doses, the ingredients which cause effervescence on mixing with water are enclosed separately in papers of different color. The patient is advised to take one powder of each color and add it to water before use. Quantities of the sodium bicarbonate and the organic acid, citric acid, tartaric, are equimolecular in proportion.
(iii) In the third case, the product contains all the ingredients mixed together in granular form.
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