| During the manufacture of dry powder
inhalers (DPIs) it is necessary to ensure that
the filling process puts the right weight or
volume of powder into each pocket. The
DPI product consists of a metal foil into
which small pockets are pressed. Fine
powder with a granular size of between 50
and 100 microns is placed into each pocket
in a filling process and then a sealing foil lid
is placed over the top to contain it. Once a
user receives the product, contained in a
plastic carrier, the powder carrying the drug
is inhaled by exposing the pocket to the
atmosphere, usually by peeling the lid
back inside the plastic carrier.
It is important that the right quantity of powder is inhaled so that
the user gets the right dose each time the product is used. In order
to make sure that the user gets the right dose, each batch of
powder is checked for consistency before being placed into the
pockets. Following this, the weight of the powder in each pocket
is checked to make sure that each pocket contains the same
amount. It is possible to measure the volume of powder, but this
is hard to do accurately as the volume can vary depending on
the compression of the powder into the pocket due to the
fact that the powder is fairly compressible. As such,
volume is regarded as a poor indicator of dose. As
a quality control check, performing a weighing
operation at regular stages in the production
process ensures that the dose in each pocket
is within specification.
CHECKWEIGHING ALTERNATIVES
There are a number of basic methods of
weighing the powder in each pocket. One is
to pour the pocket content out into a suitable
vessel and then weigh the content. This has
the advantage of weighing exactly what you
want to weigh. This approach assumes that all
the powder falls out of the pocket – and from
manual experimentation this was found not to be the case. Some powder formulations are slightly sticky so that some
powder remains on the foil, both on the lid and in the pocket. When
the user inhales using the plastic carrier, the air flow generated by
the user helps the powder leave the pocket. |
