Objective: Select two components from a
stethoscope and go through the material selection process. This analysis will focus on an acoustic
stethoscope, similar to one used in the medical field. A stethoscope is comprised of several
components such as earpieces, tubing, end piece, diaphragm, and the bell; as
well as different adaptors for earpieces and pediatric diaphragm and
bells. This analysis will focus on the
earpiece and tubing. See figure 1 for
layout of a typical stethoscope.1
Figure
1: Typical Stethoscope
1) Translation
A.
Function – Stethoscopes are a critical
device in the medical field that allow the user to listen to various internal
sounds inside the human body ranging from the heart and lungs to blood flow and
intestines. Listening is very critical
when using a stethoscope so that the doctor and identify any issues happening
with the patient. The earpieces must fit
inside the users ears and allow for the best acoustic function as well as be
comfortable because doctors may have to use a stethoscope countless times
throughout the day.2 A proper
fit in the ear is critical so that there is a complete seal from the earpieces
inside the ear so that the acoustical sounds transmitted from the stethoscope
are heard with precision and accuracy by the user. The tubing is the second component selected;
the tubing carries pressure waves from either the diaphragm or the bell to the
user’s ears. The tube is hollow and
filled with air. Additionally the tube
is flexible which allows for greater function (quickly and easily moving the chest
piece to various places on the body without the user having to reposition).
B.
Objective – It is essential that the
earpieces give an airtight seal in the users ears in order to accurately
transmit the sound. Earpieces that don’t
seal completely may not transmit all of the sound to the user, and could mean
that the doctor doesn’t hear the full range of acoustics that is gathered by
the stethoscope. The earpiece must be
flexible so that it will fit different size ears. The earpieces should have a low cost because
dirt and bacteria can build up over time causing the earpieces to be unpleasant
to the user. Cost effective earpieces
are important because over many uses the earpiece and break down and will no
longer provide an airtight seal in the ear.
The tubing must remain completely sealed without any cracks to transmit
the sound (pressure waves). Bending and
twisting will be common functions the tube will experience on a daily basis and
must still remain functional.
C.
Constraints – The earpiece must fill
the users ear hole up to create an airtight seal and the main constraint for
this would be the elasticity. The
earpiece undergoes compression when the user fits it into their ear. This compression must happen in the elastic
zone of the material and not have any permanent (plastic) deformation. The tubing is hollow so it can transmit sound
waves and has a similar constraint. The
tube will experience bending and twisting which causes tensile and compressive
forces at the point of bending or twisting.
The material for the tube must function in the elastic range and not
plastically deform. The modulus of
elasticity is the slope on the stress-strain curve before permanent
deformation; it describes the material’s response when a force is applied
either in tension or compression.3
The equation for the modulus of elasticity is: E= stress/strain
Figure 2: Stress-Strain Curve
At point 1 on the graph in figure 2 the material starts to deform plastically. The material selected for the earpiece and tube must be able to support a large strain but not necessarily a large stress; keeping in mind the yield strength should be kept in mind.
D.
Free Variables – Parameters that are
not constrained include the type of material to be used and the length of the tube.
2) Screening
– From the equation above it is necessary to minimize the Modulus of elasticity. Seen on the graph below, figure 3, the horizontal line is set at around 1 GPa and materials below this line will be
considered due to their low Modulus of elasticity. Additionally, for the design of both
components of the stethoscope the strength can’t be too low because a
stethoscope is used daily and is subjected to dropping, bumping into objects,
and being handled by the user. If the
strength is too low, the user can damage it by accidentally dropping the stethoscope
or by the user handling it too roughly.
Therefore, materials below 1.1 MPa will be neglected. From the chart in figure 3, the list of materials is narrowed down to elastomers and some foams. Elastomers are more desirable of the two due to their lower Modulus of elasticity and higher strength. Elastomers are able to stretch, bend, twist, and give an airtight seal in a person’s ear without failing at moderate loads.
Figure 3: Young’s Modulus vs. Strength
3) Ranking
– With the material selection narrowed down to elastomers, the next step in the
material selection generally looks at the cost of the material. Below in figure 4, the chart displays the
relative cost of the range of elastomers.
Data from some elastomers is tabulated in table 1 to show a comparison
among different materials.
Figure 4: Young’s Modulus vs. Relative Cost
Table 1: Comparison of Elastomers
The final selection is neoprene for both tubing and ear pieces. Neoprene offers excellent flexibility and good strength. Even though the cost is higher for neoprene, the Young’s modulus and strength outweighs the cost.
4) Supporting
Information – As seen in most doctors’ offices, the stethoscope appears to have
a rubber-like material. Additionally,
websites offer the sale of stethoscopes and the material is a rubber for both
components.
5) References
1.
Mosby, I. (2009). Mosby's dictionary of
medicine, nursing & health professions (9th ed.). St. Louis, Mo.:
Mosby/Elsevier.
2. How to
Use a Stethoscope. (n.d.). : 3M™ Littmann® Stethoscopes: 3M US.
Retrieved , from
http://www.littmann.com/wps/portal/3M/en_US/3M-Littmann/stethoscope/littmann-learning-institute/about-stethoscopes/stethoscope-use/
3. Ashby,
M. F. (2011). Materials selection in mechanical design (4th
ed.). Amsterdam: Butterworth-Heinemann.
