Biomedical Engineering Reference
In-Depth Information
10-50 K
M
f
A
s
martensite
100%
austenite
100%
A
f
M
s
5-30 K
temperature
Fig. 10.11
Volume transformed of nitinol as function of temperature
apparent elasticity of NiTi is possible through crystallographic exchange between
two phases of different volume for the same number of atoms. For this reason,
the phenomenon is qualified by the term
pseudoelasticity
. Orthodontic wires are
an obvious application of pseudoelasticitiy.
A few words about Shape Memory Effect (SME), another biomedically exploit-
able characteristic of a group qualified as Shape Memory Alloys (SMA). NiTi
is a member of this group and till present the most widely used in biomedical
devices. The martensite induction is activated by change of temperature as shown in
Fig.
10.11
. The transformation is characterized by four temperatures:
M
s
martensite
A
s
start of reverse transformation and
A
f
finish of reverse transformation of martensite. The temperatures at which it all
happens for SM as well as for pseudoelastic behavior depend on composition and
thermomechanical processing.
When a spring shaped wire is stretched at a temperature below
M
f
start and
martensite finish temperature,
M
f
, it will return
to its original shape when heated at a temperature above
A
f
but will not return
to the stretched state when cooled below
M
f
:the
one-way SME
. With appropriate
training, ignoring here the crystallographic background, two shapes can be memo-
rized: one shape existing below
A
f
. The process can be
repeated indefinitely provided strain levels and temperatures are not exceeding cer-
tain limits (twice the pseudoelastic strain domain, some tens of degrees above
M
f
and the second above
A
f
).
NiTi staples for immobilizing bone fractures are a good example of a one-way SME.
The staples remember the former closed shape and tighten at body temperature, thus
compressing the matching parts of the fracture. Another orthodontic application is
showninFig.
10.12
, a spring which closes at body temperature.
