Pharmaceutical Capsule

Pharmaceutical Capsule

Capsules are solid dosage form in which the drug substance is enclosed within a hard or soft soluble shell generally formed from gelatin

Capsule shell composition

Capsule shell composition is made of Gelatin.

Gelatin is a heterogenous product derived by irreversible hydrolytic extraction of treated animal collagen.

Common sources of collagens are ⇨ animal bones, hide portion, frozen pork skin.

Types of gelatin 

1. Type A gelatin 

2. Type B gelatin

Type A gelatin is derived from an acid-treated precursor and exhibits an isoelectric point in the region of pH 9.

Type B gelatin is derived from an alkali treated precursor and has its isoelectric point in the region of pH 4.7.

Bloom or gel strength of gelatin

Bloom or gel strength of gelatin is a measures of the cohesive strength of the cross-linking that occurs between gelatin molecules and is proportioned to the molecular weight of the gelatin.

Bloom is determined by measuring the weight in grams required to move a plastic plunger of 0.5 inches in diameter, 4 mm deep into a (20÷3)% gelatin gel that has been held at 10°C for 17 hours.

In general, the higher the Bloom strength of the gelatin used, the more physically stable is the resulting capsule shell.

The cost of gelatin is directly proportional to its Bloom or gel strenght.

Viscosity of gelatin

Viscosity of gelatin is determined on a 6% concentration of gelatin in water at 60°C. Viscosity of gelatin is a measure of the molecular chain length and determines the manufacturing characteristics of the gelatin film. The viscosity of gelatin can range from 25 to 45 milipoise.

Non gelatin capsule shell

Non gelatin capsule shells are made form starch, hydroxypropylmethyl cellulose and Pallatab.

Thickness of capsule wall

Thickness of the capsule wall is controlled by the viscosity of the gelatin solution and the speed and time of deeping.

Optimum moisture content of capsule shell

The optimum moisture content of capsue shell range between 13% and 16%

Glidants/Lubricants used in capsules are:

⇨ Glycol esters

⇨ Silicons

⇨ Silicon dioxide

⇨ Metallic stearates

⇨ Stearic dioxides

⇨ Talc

Bioavailability of a drug for absorption usually decreases in the following order:

⇨ Solution > suspension > Soft gelatin capsule > Powdered filled capsule > Compressed tablet > Coated tablet

Capsule filling Calculation

Capsule fill weight = Tapped density of powder × Capsule volume (ml)

Additional diluent required= (Volume unoccupied × Tapped density of powder)

Problem: A formulation has a theoretical fill weight of 350 mg and tapped density of 0.75 gml-1. What size capsule is required?

Solution:

We know, capsule fill weight = Tapped density of powder × capsule volume

So, Capsule volume = Capsule fill weight ÷ Tapped density = 0.50÷0.75 = 0.47 ml

So, Capsule size = 1 (Since 1 size equal to 0.48 ml)

Relation between capsule size and Volume occupied

Capsule size ⇨ Volume occupied

* 000 ⇨ 1.36 ml

* 00 ⇨ 0.95 ml

* 0 ⇨ 0.67 ml

* 1⇨ 0.48 ml

* 2 ⇨ 0.37 ml

* 3 ⇨0.27 ml

* 4 ⇨ 0.20 ml

* 5 ⇨ 0.13 ml

Largest capsule size ⇨ 000 size

Smallest capsule size ⇨ 5 size

Microencapsulation

Microencapsulation is a process by which solids, liquids or even gases may be enclosed in microscopic particles by formation of thin coatings of wall material around the substance.

A capsule with a size range of about 200 micro is termed as microscapsule.

Process of microencapsulation:

1. Air suspension

2. Pan coating

3. Spray drying and spray congealing

4. Solvent evaporation