Medical Design Briefs - June 2021 - 10

Acrylic Copolymers
subjective when screening through
industry-wide choices. Foremost, the test
methods to quantify compatibility
against a chemical agent show disconnect
and widespread practices across
suppliers. Therefore, the medical device
industry should combine input from
resin producers, medical device manufacturers,
and the application environment
to define a regulated approach for
measuring ESCR and corroborating
material comparisons. As a result, the
different resin solutions available for a
given application would be assessed
through the same metrics and ranked
for performance/resistance against
these chemicals. Key aspects to consider
for universal harmonization and evaluating
ESCR are discussed below.
Preconditioning. Foremost, polymer
mechanical response to applied mechanical
stress or strain depends on its chemical
structure and morphology during
polymer processing. Test specimens
should be annealed to minimize the
effect of molded-in stress (i.e., part
design/geometry and external factors
that are application dependent). An -
nealing and preconditioning are practices
to establish comparable testing conditions
and to eliminate specimen history
(storage, processing). It is also critical
to bring the material into equilibrium by
establishing controlled temperature and
humidity conditions before testing. De -
pending on the polymer, crystalline or
amorphous, controlled conditioning
ensures reproducibility and repeatability
of analysis. Polymer-specific standards
should be followed for conditioning be -
fore testing. For example, ASTM D-4066
stipulates the need for testing to be carried
out on dry as-molded specimens for
hygroscopic materials such as nylon.
ASTM-D 618-13 specifies preconditioning
the test bars for >40h, at 23 ± 2 °C, 50
± 5% relative humidity before strain
exposure.7
Measuring Mechanical Properties. Me -
chanical strength of medical plastics can
be expressed using tensile, compressive,
flexural, impact, fatigue, weathering, and
other similar metrics, depending on the
targeted application. For ESCR evaluation
of medical plastics, the mechanical
test type studied is varied, e.g., tensile or
impact or flexural, etc.3-6 However, the
property retention upon chemical agent
exposure to establish compatibility is an
industry-wide practice. The stress-strain
curves as part of tensile testing provide
the ductile or brittle response of the
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Strain,ε
Fig. 1 - Typical tensile stress-strain curve of
plastic material showing ductile and brittle failure
behaviors. ASTM-D543-14, Section 12 and
ASTM D638-14 recommend ESCR testing
using tensile testing.
material along with modulus and elongation
at break properties (see Figure 2).
For tensile testing, ASTM-D618 lists tensile
bar dimensions and tolerances for
standard tensile specimens with regard to
size and geometry. Moreover, the test
parameters such as extensometer capacity
and crosshead speed for testing require
rigorous attention specific to a material
tested.8 ASTM D543-14 provides guidelines
for chemical exposure under
12.4 R
Ductile
Brittle
6.2 R
4.1 R
Radius of Jig, R (in inches) % Strain = (T/(2R+T)*100
12.4
6.2
4.1
0.5
1
1.5
Fig. 2. - ASTM tensile type 1 exposed to three
different strains at 0.5 percent, 1.0 percent,
and 1.5 percent. The strain is determined primarily
by the thickness of the tensile bar type,
T, and the radius of curvature of the strain jig,
R, as shown in the expression.
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ToC
strained environments and evaluating
tensile property retention.9
Percent Strain Loading and Chemical
Agents for Exposure Parameters. ESCR
testing measures the tensile property
retention of material upon simultaneous
exposure to a controlled strain and a
chemical agent. The test assembly is
designed to mimic the mechanical loads
in use and simulate the internal stresses
the material undergoes in the healthcare
environment. Figure 3 shows control
strain jigs at 0.5 percent, 1 percent,
or 1.5 percent strain, with tensile bar
held in place and exposed to chemical
agent. Notice the increase in curvature
as the strain is increased from 0.5 to 1.5
percent. Depending on the application,
chemical exposure to numerous chemical
agents is tested for the duration of
exposure (typically ranging from 5 to 24
hours), to evaluate material response in
a controlled environment.
Evidently, a material is likely to show
different response against variable
strain, chemical agents tested, and to the
extent of exposure. More importantly
these are critical variables in context to
the application, whether it is short-term
use as in the case of disposables, or longterm
medical devices as in equipment
housings. The current data reports don't
shed much insight on the selection of
strain percent or the chemicals in context
of the application and do not
address the medical device manufacturer's
unmet needs.
Chemical Exposure Method and
Duration of Exposure. ASTM D543-14
lists the wet patch method for ESCR testing
against various chemical agents. The
wet patch method allows for exposing
the strained tensile bar to the chemical
agent periodically - for example, every
30 minutes for a maximum duration of
the test exposure. Resin manufacturers
are discussing three approaches for
exposure: periodic wipe method, wet
patch exposure by periodic saturation,
and continuous immersion by creating
an isolated environment. Method comparisons
are enlightening, revealing the
potential variability in mechanical re -
sponses and property retention.
A periodic wiping test is reported to
be more aggressive than continuous im -
mersion for polycarbonate (PC) and PC
blends and linked to the added stressors
from the concentration build of highboiling
chemicals in the formulations.4
However, this does not guarantee the
same mechanical behavior for all materiMedical
Design Briefs, June 2021
Stress,σ
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Medical Design Briefs - June 2021

Table of Contents for the Digital Edition of Medical Design Briefs - June 2021

Medical Design Briefs - June 2021 - Intro
Medical Design Briefs - June 2021 - Cov4
Medical Design Briefs - June 2021 - Cov1a
Medical Design Briefs - June 2021 - Cov1b
Medical Design Briefs - June 2021 - Cov1
Medical Design Briefs - June 2021 - Cov2
Medical Design Briefs - June 2021 - 1
Medical Design Briefs - June 2021 - 2
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Medical Design Briefs - June 2021 - Cov3
Medical Design Briefs - June 2021 - CovIV
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