Biomedical Engineering Reference
In-Depth Information
straightforward metal right-angle bend (so-called USP/Ph.Eur. design) described in
both regional pharmacopeias.
It is important to note that this inlet is not intended to be an accurate representa-
tion of the upper airway but rather a simplified arrangement that reproducibly col-
lects incoming aerosol for the purpose of product quality assessment. Chapter 12
contains information about more anatomically appropriate inlets, including the new
generation of “idealized” geometries that have been developed at the University of
Alberta, Canada.
In its role as a model of the entrance to the respiratory tract, the induction port
collects almost all the fast moving “ballistic” component of MDI-produced aerosols
that is formed by flash evaporation of the propellant and therefore likely to deposit
in the oropharynx [ 53 ]. It also serves to remove larger and often aggregated particles
generated by most DPIs. The USP/Ph.Eur. design is intended to provide a common
benchmark to compare different formulations by standardizing its critical dimen-
sions (internal diameter and unobstructed path length). Unfortunately, unlike a CI
stage, its collection efficiency is not easily determined, since there is significant
turbulence at flow rates encountered typically for inhaler testing [ 54 ], and particles
in the ballistic component from MDIs have velocities greater than that of the sur-
rounding airflow when entering the induction port. Nevertheless, recently, Zhou
et al. reported calibration data with monodisperse particles for this inlet (Fig. 2.17 ),
from which they estimated d 50 sizes to be 14.4- and 20.2-
m aerodynamic diameters
for flow rates of 60 and 30 L/min, respectively, by fitting their experimental data to
flow rate-dependent empirical relationships linking deposition fraction and d ae [ 55 ].
Their term “deposition fraction” can be considered equivalent to collection effi-
ciency. They found that d 50 at the lowest flow rate investigated (15 L/min) could not
be obtained, as the collection efficiency was found to be <50% regardless of particle
size to an upper limit close to 30
μ
m aerodynamic diameter (Fig. 2.17 ). It should be
noted that Zhou et al. grease coated the interior surfaces to mitigate particle bounce
and re-entrainment, a practice that is possible but rarely undertaken with OIP
testing.
In summary, although several attempts have been made to develop an under-
standing of how this particular induction port performs, given the complexity of the
fluid dynamics and particle motion through its flow path, particularly when captur-
ing the ballistic fraction emitted by MDIs, it should not be regarded as an additional
impactor stage in routine work.
A preseparator is often required to be located immediately after the induction
port when sampling aerosols produced from DPIs, since these formulations in many
instances contain the API attached to the surface of much larger (lactose) carrier
particles. The shear forces generated by inhalation from the DPI detach some, but
not all, of the API particles from the carrier material, but the detached particles are
sufficiently fine to penetrate beyond the oropharyngeal region into the lungs [ 56 ].
Both pertinent USP and Ph.Eur. compendial monographs refer to the use of a prese-
parator for DPI-based particle size distribution measurements with the ACI, recom-
mending that its interior surfaces be coated either with a tacky agent in the same
μ
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