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Volumetric size numbers for capsules

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Dr Milind Biyani, Director, Aspire Advisors, gives an insight on how managing capsules by volume number will be more convenient during whole life cycle of the product management

Capsule is a container to carry the material. Hard capsules are usually cylindrical in shape consisting of the body and cap whereas empty capsules are supplied in partially pressed form. These can be opened and the material can be filled and closed completely. Some capsule types after filling are tightly closed while some types can be opened by pulling out the cap even after filling the material. Capsules can be sealed using band sealing to make them tamper proof and leak proof and are available as transparent or in different colours. These can be pre-printed up to four colours, vertically or across the circumference with product name, company name, logo, etc.(1) Capsules are soluble or bio-degradable when they are made from gelatin, HPMC (Hydroxy Propyl Methyl Cellulose), modified starch or pulullan. Capsules are available in large variety of sizes. Capsule sizes widely used in pharma, nutra and dietary supplements industry are given in Figure 1 and Table 1 (2). Capsule sizes given in Table 2 are mainly used in veterinary practice (mice to elephant). Capsule sizes in 36 ml, 90 ml and 120 ml are also available (3).

It is possible to fill in the capsules with one or more of powders, granules, different pellets, mini/micro tablets, another capsule or even non-aqueous liquids. Non-aqueous liquids could be solutions, suspensions, semi-solids, self emulsifying systems or hot melts. Acidic aqueous solutions can be filled in enteric capsules. Fill weight can be obtained after considering bulk density of the material to be filled and the volume that can be filled in a capsule.

Non-pharma uses are combination with medical devices as soluble aid to insertion, test kits, products for water treatment, soil treatment, for reagents, bath products, toys, optics etc.; for special effects in movies, television (capsules for gun, container for simulated blood/when broken in mouth – the effect of blood created, ‘birthing effect’ etc). Use of transparent capsules enable to see contents whenever desirable.

As seen from Table 1, 2 and the Figure 1, capsule sizes are described by some number. The first criteria for selection of particular capsule size is the volume that can be filled in the capsule. Capsule number does not give any clue about capacity or the volume that can be filled in the capsule. Capsule size number inversely increases with capsule volume for Table 1 but does not hold good for Table 2. Hence, this system of numbering is not convenient and is confusing.

Soft gelatin capsules are one piece and hermetically sealed. Filled volume when expressed in minims, it is closer with the capsule size numbers as shown in Table 3. A minim is equivalent to the amount of water in a drop, which is also equal to one grain. We believe that origin of size of hard capsules expressed as numbers were based on the standard rod dia (mesh size) used for making pin bards for dip moulding. Hence, the present system has been derived from engineering background and not from pharma. Therefore, it will be useful to identify hard capsule size by its volume in ml. Capsule size number can be mentioned in the bracket to correlate with the existing use and specifications. This will help users to quickly identify desired size of the capsule. Accordingly, volumetric sizes will be as per Table 4 and Table 5 when compared to existing size nomenclature by numbers.

The weight of the product to be filled in gram can be converted in ml volume based on the bulk density of the product to be filled and nearest capsule size in ml can be easily selected. For example, if the quantity to be filled is 500 mg and bulk density is 0.5, then volume will be 1 ml and capsule of 1 ml size (Number 00 as per existing system) can be selected. If the quantity to be filled is 50 mg of the bulk density 0.5, volume will be 0.1 ml and the capsule of 0.1 ml volume can be selected (Number 5 as per existing system).

The volume in ml = Weight in gm / Bulk density. Accordingly, the capsules of similar volume or slightly higher volume can be selected to accommodate the excipients. Soft gelatin capsules’ size may also be indicated by volume that can be filled in minim on higher side of the range as shown in Table 3. The volume to be filled in ml can be converted to minims by multiplying it by 16.231 to find out desired minims number of the soft gelatin capsule.

Thus in the proposed new system of calling capsule size by volume, the volume number increases with increase in size of the capsule. In the present system, capsule size increases inversely with the volume of the capsule which is confusing. The volumetric size number will enable formulator to quickly select the capsule size based on the volume of the product to be filled. For packaging personnel also, it will be easier and faster to select the pocket size for blister pack or bottle pack. It will be useful for pharma as well as veterinary industry. The practice will be equally useful for non-pharma users. Non-pharma uses include combination with medical devices, products for water treatment, soil treatment, reagents, bath products, toys, optics etc. Use of the transparent capsule enables to see contents whenever desirable. For capsule manufacturers also, managing capsules by volume number will be more convenient during whole life cycle of the product management.

1. Milind Biyani, Choosing Capsules: A Primer, Pharmaceutical Technology, 41(10), 36-41, 2017.

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