Micromeritics is the study of finer particles which are smaller in size.
The merits and demerits of fine particles as dosage forms are given below:
Advantages of fine powder as dosage form:
1. Flexibility or activity:
In powder form, all quantity of a drug becomes active but in tablet or capsule form all quantity of drug is not active and small quantity is excreted out as inactive drug for example, in powder form if we take 100 mg drug then all of 100 mg will be active while in 500 mg tablet 350 or 400 mg drug will be activated and remaining will be excreted out.
2. Stability:
Some drugs are more stable in dry powder form and their activity is reduced in solution form.
Moreover, drugs in powder form are completely hydrolyzed where as in tablets form it may be possibility that no total drug is hydrolyzed.
3. Rapid therapeutic effect:
The effect of a tablet or capsule is given below:
If drug is already available in powder form then it is rapidly absorbed and show rapid therapeutic effect than tablets.
4. Ease of administration:
In powder form smaller surface area of drug particles can be easily administered but in tablets form there is a difficulty to swallow it.
Disadvantages of fine powder as dosage form:
1. Prevention due to acidity:
Some drugs become inactive due to acidity in stomach pH.
So we take them in enteric coating to prevent from acidity instead of taking in powder form.
2. Efflorescent, Deliquescent and hygroscopic:
Some drugs become efflorescent, deliquescent or hygroscopic in nature and we can not use them in powder form, so we take them in tablets or in capsule forms for preventing them.
3. Bitter in taste:
Some drugs are bitter in taste and we take them in tablet or in capsule form instead of powder form.
Their bitter taste is masked by coating.
Comminution:
It is a process by which we can reduce the size of a substance.
There are various methods of size reduction.
1. Manual Size reduction
2. Mechanical size reduction
Manual size reduction:
It can be done by one of the following methods or their combination.
These are used mostly on small scale.
1. Cutting:
In this method, sharp blades or knives are used to cut a material into small particles.
This method is used for those materials which are elastic in nature.
2. Compression:
By applying pressure larger molecules can be broken down into smaller pieces.
3. Impaction:
In this method, large particles are broken by heavy mass.
If we hit large particle with heavy mass or hit heavy mass with stationary large particle, the large particle is broken into pieces.
4. Attrition:
In this method, large particles are crushed down into small pieces between two opposite moving surfaces.
5. Trituration:
Grinding of solid into powder is done by continuous stirring or rubbing the particle in mortar with pestle.
Trituration is also used for mixing of two or more substances.
Here different types of mortar and pestles can be used.
6. Levigation:
It is also called as wet grinding.
In this size of substance is reduced to finer state by triturating the substance with few drops of liquid in which it is insoluble.
Substances are used: oil, water, glycerine and alcohol.
This process is often used to incorporate the solid substances into dermatological and ophthalmic preparations.
After mixing, levigation continue for this purpose, mortar and pestle, spatula and glass slabs are used.
In this you will get fine paste.
7. Pulverization by intervention:
In this method, solid substance is reduced to a fine powder by means of triturating the solid with a suitable solvent which is easily removed at the end for example camphor is difficult to reduce in size because as it reduces in size. It cohere with each other but when few drops of alcohol, ether, chloroform, it can be powdered easily and these substances evaporate.
Mechanical method:
On large scale, size reduction is carried out mostly by using different kinds of mills which have capacity of producing powder of wide range of particle size. As different mills are used to reduce the particle size hence it is also known as milling.
Various mills are ball mill, hammer mill, colloidal mill etc.
Grading of powders:
1. Very coarse:
All particles pass through sieve number 8 and not more than 20% through sieve number 60.
2. Coarse:
All particles pass through sieve number 20 and not more than 40% through sieve number 60.
3. Moderately coarse:
All particles pass through sieve number 40 and not more than 40% pass through sieve number 80.
4. Fine:
All particles pass through sieve number 60 and not more than 40% through sieve number 80.
5. Very fine:
All particles pass through sieve number 80 and there is no limit for greater finest.
Methods used to determine the size:
The following methods are used to determine the size of particles and to separate them:
1. Seiving method:
This is the most common applicable method. By this method, we can get 50-5000micrometer.
In this method, different sieves are arranged together, when material is put over the sieves and sieves are shaken. We get finest powder at the last while the remaining material is present in above sieves which entrap them according to size of their holes. So in this way size of powder is determined.
2. By microscopy:
We can measure the size of particles of powder by microscope.
The particles whose size ranges from 1-25 micron can be measured under microscope.
When a prepared slide of material is examined under microscope then the microscope has a micrometer by which we can determine the size of particles.
The advantage of this method is that we can measure the size of particle directly.
3. Sedimentation method:
This method is based on sedimentation time i.e. How much time is taken to settle down the particles of powder from dispersion medium.
The apparatus used in this method is called as Andreasen apparatus.
The larger particles settle down rapidly while smaller particles take sometime to settle down.
So, larger sedimented particles are taken out by the tube from the bottom and remaining are the fine particles.
4. Elutriation method:
This method is used for the separation of different sized particles of the powder.
Importance of particle size in pharmacy:
1. Size reduction increases the surface area of drugs, which helps in solution formation and bioavailability increases.
2. Mixing is easier and uniform.
3. Rate of drying is enhanced, if particle size is reduced.
4. Stability of emulsion is increased by decreasing the size of globules.
5. Rate of absorption will be greater when the particle size is small.
6. In case of suspension, very small particles result in formation of cake and very large particles settles quickly but we should have to form loose cake because it redisperse upon shaking.
7. Physical appearance of ointments, paste and creams can be improved.
8. Stability of some drugs increase in powder form and decrease in solution form.
9. Powdered drugs are easily administered than solid dosage forms.
