PHR 221: Pharmaceutical Technology II

Course Teacher: Mohammad Nasir Uddin

Lecturer, Department of Pharmacy, NSU

Tablet Coating

SUGAR COATING

Sugar coating is a multistage process and can be divided into the

following steps:

1. Sealing

2. Subcoating

3. Smoothing

4. Coloring

5. Polishing

6. Printing

Sealing

• To prevent moisture penetration into the tablet core, a seal coat

is applied. This is specially needed in pan-ladling processes, in

which localized over wetting of a portion of the tablet occurs.

• Without a seal coat, the over wetted tablets would absorb

excess moisture, leading to tablet softening or disintegration

and affecting the physical and chemical stability of the

finished product.

• In spray process, over wetting does not occur and the

application of subcoats and further coats can be adjusted which

can eliminate the seal coating step.

Sealing

• Shellac is an effective sealant, but tablet disintegration and

dissolution times tend to lengthen on aging because of the

polymerization of the shellac.

• Zein has also been used as an another effective sealant which

does not lengthen tablet disintegration and dissolution time.

• Besides, some synthetic polymers such as cellulose acetate

phthalate (CAP) or polyvinyl acetate phthalate (PVAP) are

also being used as sealant.

Subcoating

• The subcoating is applied to round the edges and build up the

tablet size. Sugar coating can increase the tablet weight by 50

to 100%

• The subcoaing step consists of alternatively applying a sticky

binder solution to the tablets followed by a dusting of

subcoating powders and then drying.

• Subsequent subcoats are applied in the same manner until the

tablet edges have been covered and the desired thickness is

achieved.

Subcoating

• For spray processes, a subcoating suspension containing both

the binder (e.g. sucrose) and the insoluble powder (e.g.

calcium carbonate) is sprayed intermittently on the tablet bed.

• Antiadherents such as talc may be used to prevent tablets

sticking together.

• Polysaccharide gums, such as gum acacia, may also be added

as a binder in order to reduce brittleness.

Smoothing

• The purpose of the smoothing step is to cover and fill in the

imperfections in the tablet surface caused by the subcoating

step and to impart the desired color to the tablet.

• To facilitate the application of the coloring layer which

requires a smooth surface, subcoated tablets are usually

smoothed out by applying a sucrose coating that is often

colored with titanium dioxide to achieve the desired level of

whiteness.

Coloring

• Nearly all sugar coated tablets are colored.

• Color coatings usually consist of sucrose syrups containing the

requisite coloring materials.

• Traditionally, water soluble dyes have been used for color

coating.

• Nowadays, to speed up the coating process and minimize

color migration problems, dyes have been replaced with

pigments.

Polishing

• Once the color coatings layers have been applied and dried, the

tablet surface tends to be smooth but somewhat dull in

appearance.

• To achieve the glossy finish, a final stage involving the

application of waxes is employed.

• Suitable waxes include beeswax, carnauba wax or candelila

wax, applied as finely ground powders or as

suspensions/solutions in an appropriate organic solvents.

Printing

• For sugar coated tablets identification (manufacturer logo,

product name or other code) can be achieved by means of a

printing process, which is typically an offset gravure process,

using special edible inks.

• Alternative printing processes, such as inkjet and pad printing

processes, have also gained acceptance.

Sugar coating defect

Common sugar coat defects include:

Tablets that are rough in appearance

Tablets that are smooth but dull in appearance

Tablets that have debris stuck to the surface

Tablets exhibiting poor color uniformity

Tablets that splits on storage as a result of inadequate drying

COATING OF MULTIPARTICULATES

• Coated multiparticulates, often referred to as pellets or beads,

commonly form the basis for a wide range of modified-released

dosage forms.

• Typically used for extended and delayed released products

• In these systems, the dosage of the drug substances is divided

on a plurality of subunit, typically consisting of thousands of

spherical particles with diameter of 0.5-2.00 mm

Benefits of coated multiparticulates

• Capitalizing on small size (typically 0.5-2.0 mm)

• Minimizing irritant effects

• Reducing the consequences of imperfect coatings

• Reducing the impact of poor coating uniformity

Please go through the book (Pharmaceutics by Aulton page 511-512)

for details.

Types of multiparticulates

• Drug crystals

• Irregular granules

• Spheronized granules

• Drug-loaded non-pareils

• Mini tablets

Figure: page 512 (Pharmaceutics by Aulton)

Drug-loaded non-pareils

• These are preformed spherical particles about 1 mm in

diameter consisting primarily of sucrose and starch.

Application of the drug uses any of following two methods:

• A powder dosing technique involving alternate dosing of

powder (containing the drug substances) and binder liquid

onto the surface of the non pareils until the required dose of

the drug has been achieved

Drug-loaded non-pareils

• Spray application of drug, either suspended or dissolved in a

suitable solvent (usually water) containing also a polymer

binder (e.g. HPMC or PVP) onto the surface of the non

pareils.

Coated drug loaded non-pareil

Mechanisms of drug release from multiparticulates

Diffusion

• Diffusion is primarily a process whereby drug will partition

into the film coat membrane and permeate through it.

• The rate at which the drug is released by this mechanism is

influenced by the drug concentration gradient across the

membrane, the thickness of the membrane, the solubility of

the drug in the membrane and the permeability coefficient

governing passage of the drug through the membrane.

Mechanisms of drug release from multiparticulates

Osmosis

• Once water has passed through the film coating, dissolution

of soluble components (excipients and drug) within the core

can allow an osmotic pressure to be generated inside the

coated particle that will influence the rate at which the drug

will be pushed out through pores or a preformed aperture in

the membrane.

Mechanisms of drug release from multiparticulates

Dialysis

• Dialytic effects describe conditions where water-filled

channels are formed in a micro porous membrane (often

created by the imperfections common to many applied film

coatings) through which drug in solution can pass. The key

factors influencing drug release by this mechanism include

the length and tortuosity of these channels, as well as the

solubility of the drug in water.

Mechanisms of drug release from multiparticulates

Erosion

• Some coatings are designed to erode gradually with time,

thereby releasing the drug contained within the pellet in a

controlled manner.

• Examples of these types of coatings are usually those that

consist of natural materials such as shellac or waxes and fats

that become soft enough to facilitate erosion as the coated

multiparticulates are subjected to intense agitation as they

pass through the GIT.