Biomedical Engineering Reference
In-Depth Information
configuration that lends itself to semi- or full automation more readily than the
vertical stack configuration associated with ACI designs, and the process of abbre-
viating the full-resolution configuration is not as daunting as might be considered
at first sight.
Two distinctly different approaches are feasible; however, there is relatively lim-
ited data on either reported thus far in the open literature. The most straightforward
method involves the use of deep cups to make certain stages inoperable; particles
fail to impact on the collection surface and simply pass to the next stage. In addition,
an insert can be fitted to the stage 1 nozzle to reduce jet diameter to give a desirable
stage cut point. However, since the internal flow pathway through the NGI is not
reduced, the so-called deep-cup approach has the potential drawback that losses to
the internal surfaces may increase. Future studies validating this option will there-
fore need to address this concern.
In the second, more radical approach to the abbreviation of the full-resolution
NGI (Fig.
10.64a
) first adopted by Svensson and Berg [
59
], the NGI itself was
abbreviated by moving “active” stages followed by the back-up filter so that the
flow passed through these components before being returned to the NGI body.
Daniels and Hamilton also adopted this arrangement for their reduced NGI (abbre-
viated to rNGI)-based studies [
46
], and their data have already been discussed in the
previous section in connection with understanding how internal dead space affects
start-up flow characteristics in DPI testing with the FSI. Their particular rNGI set-
up involved moving the filter collection stage containing a bespoke filter to follow
directly before full-resolution NGI stage 3, where separation of fine from coarse
subfractions takes place (Fig.
10.45
). This change could be made without altering
the type of collection cup used or making penetrations through the body of the NGI
itself to remove flow from unused stages. Svensson and Berg termed this apparatus
the “internal filter” configuration (Fig.
10.64b
).
The rNGI approach adopted by Daniels and Hamilton [
46
] overcomes the need
for specialized collection cups, as would be the case if external connections had
needed to be made between components to achieve an abbreviated design, as origi-
nally proposed by Svensson and Berg in one of their configurations (Fig.
10.64c
).
Daniels and Hamilton [
46
] recovered the deposited API by rinsing stages 1 and 2
together (representing the
LPM
) and separately from the recovery of API from the
bespoke filter located before stage 3 (representing the
SPM
, i.e.,
FPM
<4.46μm
). The
comparative data for their particular DPI obtained with this rNGI configuration
compared favorably with measurements by full NGI. However, the comparison of
data from either of the two NGI configurations and an FSI was less good (Fig.
10.46
).
After their follow-up study using the eLung™ to replicate patient inhalation profiles
(Fig.
10.48
), this relatively poor agreement was attributed to the possibility that the
original FSI configuration in its simpler set-up (constant
Q
of 60 L/min with a 4-s
“inhalation” time) may have affected the ramp-up profile of the DPI.
The ability to move the cut size between large and small particle fractions by
insertion of the filter stage in different stage positions in the rNGI, so that it is close
to the MMAD of the OIP of interest, is a significant advantage for product QC appli-
cations. Svensson and Berg evaluated three different stage positions of the filter in
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