Medical Design Briefs - September 2021 - 14

Wearable Injector Systems
occurred, including the color change in
the viewing window of the device or any
light emissions.
Fig. 2 - This monitor shows the image taken by an optical micrometer, determining the insertion
depth of a rigid needle during a test.
preference or physician suggestions. In
the case of diabetes care, the devices can
even interface with other external
devices
like glucose meters, actively
monitoring glucose levels to adjust the
insulin delivery profile. These closed
loop systems represent a tremendous
leap forward in utilizing machine in -
telligence to help patients. Providing
greater treatment visibility can also
improve patient outcomes in many
cases, allowing physicians to catch po -
tential issues before they occur.
Product Testing
Along with this myriad of benefits also
comes a set of challenges for drug-device
manufacturers and designers related to
testing, which are currently addressed
primarily through internal specifications
and procedures. International standards
organizations have only recently begun
to address the need for well-defined
criteria for testing wearable devices.
The International Organization for
Standardization (ISO) released a draft
subsection of ISO 11608 to address the
unique requirements of these devices,
referred to in the standard as On Body
Delivery Systems (OBDS). This definition
is more than likely kept purposefully
broad in anticipation that these
devices will continue to develop in
terms of total output and design variations.
In order to meet these growing
requirements, manufacturers need
universal testing systems flexible
enough to perform the wide range of
tests outlined by standards and internal
specifications.
14
Intro
Cov
The most critical test criteria are related
to the dosing of the device. Like
more traditional devices, dose accuracy
and dose volume are measured, but for
OBDS the ejection profile must also be
characterized. The ejection profile helps
visualize the drug delivery over time,
and it often has specific limitations
based on the drug type and viscosity.
The graph in Figure 1 shows a sample of
scale data for a typical test with functional
bounds on the dose delivery time and
volume. A precision scale is required to
capture the dispensed volume after the
device is activated. To capture the profile,
the data capture rate of the scale is
a critical parameter needing enough
data points to properly plot the curve.
This an important distinction from pen
injector tests, where the final volume is
captured along with the start and stop
time for the delivery.
When it comes to the activation and
completion of the ejection, a range of
tests are needed. The activation force is
measured using a universal testing system
to simulate the pressing of the button.
Different button designs may be
evaluated to find the optimal activation
force based on the 5th to 95th percentile
of hand strength for the target patient.
Being able to detect all device feedback
is important, requiring test systems capable
of capturing audio and visual cues of
activation as well. Auditory sensors can
be added to perform click detection,
syncing the events up with the scale and
force data to characterize the event.
Integrated cameras should be utilized as
well to confirm that visual cues have
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Evaluating the adhesive properties of
the patch can be broken down into tack,
peel, and shear tests. Existing ASTM and
ISO standards can and should be used as
building blocks for developing these test
methods. The main difference should
be the inclusion of additional test environments
and preconditioning to better
simulate the real-world usage of these
products. Many adhesives can strengthen
over time, a property that can cause
pain for the patient at removal. Creating
time studies for adhesive tests is a guaranteed
way to avoid these issues. Beyond
time studies, adhesives should be evaluated
after experiencing use-case conditioning.
This can include axial torsional
cyclic loading to replicate the stretching
and twisting of the skin or the application
of water or lotions to the substrate
prior to removing the adhesive. The
change in adhesive properties with re -
spect to the preconditioning can assist
with identifying adhesives able to withstand
continued patient use.
Finally, evaluating the needle or cannula
insertion distance is especially important
for a wearable device. Patient movement
could cause small variations in insertion
depth throughout the delivery, meaning
the optimal insertion range must be
validated on the system. Optical micrometry
systems can be integrated on universal
test frames to accurately determine
needle extension past the device housing.
Conclusion
It is imperative to create a robust
design evaluation process for developing
the next generation of injection devices.
Identifying universal testing systems that
can efficiently evaluate all the product
requirements and simplify data analysis
is a key part of this process. Based on
current trends, the global demand for
wearable injectors will continue to rise,
meaning that both R&D and quality labs
will need more and more testing capabilities.
In response to these needs, equipment
manufacturers will need to further
specialize their test systems, working collaboratively
with global device manufacturers
to stay ahead of the curve.
This article was written by Landon
Goldfarb, Biomedical Market Manager at
Instron, Norwood, MA. He can be reached at
Landon_Goldfarb@instron.com. For more
information, visit http://info.hotims.com/
79417-340.
Medical Design Briefs, September 2021
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Medical Design Briefs - September 2021

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