Tablets are the most widely used dosage form in the world.Tablets are manufactured in different shapes and dimensions.Double layer tablet was a unique concept in the history of tablet manufacturing where two API (Active Pharmaceutical Ingredient) are compressed together in two different layers to form a single tablet.
When to Use
Double layer tablet system may be used for the following
- Combining Different Release Patterns.
- Incompatible APIs
- Avoid Multiple Tablets intake
Combining Different Release Patterns
- Double layer tablets are used for the situations where we want to obtain two release patterns in a single tablet i.e one is controlled release and other is Immediate release.
- In this case, first, we compress one layer that may be controlled release and then the second layer is compressed to form two layers of tablets.
- Double layer tablets are produced for those APIs which are not compatible with each other and we can not manufacture granules of these two APIs in a single granulation stage.
- We manufacture granules of these APIs in 2 steps.
- In one step we prepare granules of one API and in the second step we manufacture granules of the second API.
- These granules are then compressed two form a double layer.
Avoid Multiple Tablets intake
- By Combining two APIs in a single tablet it reduces the number of tablets to take by the patient at a time.
- One tablet having two APIs in different layers can be used instead of two tablets.
Mechanism of Double-layer
- To compress a double layer tablet, a double feed system or double hopper compression machine is used.
- Usually, D type machines are used to compress double layer tablets.
- Granules or powder of one layer are fed in one hooper and granules or powder of the second layer is fed into the second hopper.
- Powder or granules of the first layer to be compressed are filled in dies and pre-pressure is applied over it to form a thin fragile tablet.
- The tablet formed is not strong enough because less amount of pressure is applied.
- The weight of this layer is adjusted according to BMR (Batch Manufacturing Record)
- When the turret rotates the dies pass below the second feeder containing powder or granules of the second layer and it is filled over the pre-formed tablet.
- Suitable pressure is applied to form a tablet by combining both layers.
- At this stage main compression rollers are used to apply the pressure.
- The hardness of the tablet is adjusted according to BMR and weight of both layers is verified according to BMR.
How To Check Layers Weight
- First layer weight is checked by ejecting the tablet after pre-compression.
- As the second layer is compressed over the first layer so its weight can not be checked separately after compression.
- To check the weight of the second layer a Separating Knife or cutter is used.
- Carefully apply pressure with a sharp edge of separation knife at the spot where two layers combine.
- This will separate both layers and the weight of each layer can be checked.
Experience and skilled person separate the layers more accurately.
Defects of Double Layer
The following defect may occur during compression of double-layer tablets
- Color Bleeding
- Colour bleeding is a defect also known as colour contamination,in which color granules or powder of one layer are leaked from the feeder and are mixed with a second layer.
- It may affect the content uniformity of individual layers if color mixing occurs in excess.
- In color bleeding physical appearance of tablets looks bad.
Following solution may be used
- Feeder Adjustment
- Vacuum Suction
- Double or Triple Granulation
- Feeder of compression machine should be in such a way that there should be no chances of leakage of colour powder or granules of one layer to the second.
- Proper vacuum suction systems should be installed to remove any leakage of powder or granules.
Double or Triple Granulation
- Try to keep one layer in more granular form to achieve maximum granules because leakage of product from the feeder is reduced when granules are in more ratio.
- For this, wet granulation method for a single layer is repeated for two or three times i.e after drying we sieve the granules through a suitable sieve and fines or less granular material retained on the specific sieve is kneaded (Granulated) again, dried and sieved.
- This produces more granules and less fine so color bleeding is prevented by this method.
- Delamination is also known as layer separation.
- In this defect,two layers are not bound well and are separated after compression.
- Granules size may be large.
- Pre-pressure of the first layer may be high.
- Reduce the size of granules.
- Reduce the pre pressure to make a proper substrate for second layer binding.
Demerits of Double Layer Tablet
Following are the demerits of double layer compression.
- Low Output
For double layer tablets cost of operation is increased because in normal practice we can granulate two APIs in a single granulation stage.
But for Double-layer due to incompatible APIs, we have to manufacture two layers of API separately.
Speed of the compression machine for double layers is slower than normal compression so it results in reduced tablet output.
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