Biomedical Engineering Reference
In-Depth Information
The ability to change physical properties in a proper manner
happens through external stimuli, which may include pressure,
temperature, pH, radiation, magnetic, or electrical ield. Associated
variable physical properties may be viscosity, stiffness, or shape.
Intelligence of a system manifests in designed chemical composition,
appropriate manufacture process, and intended defects initiation or
microstructure modiication in such way that it may easily adapt
to a different levels of stimuli in a controlled manner. One signal
transforms to another in a controlled transformation mechanism,
and the level of conversion is always higher than for conventional
materials.
In this speciic and interesting group, the following materials are
identiied:
• Chromic materials
• Thickness-changing luids such as magneto and electro-
rheological luids
• Self-repairing materials
• Self-organized materials
• Thermoelectric materials
• Light-emitting materials
• Shape- and size-changing materials
Materials that possess shape memory abilities are classiied as
active structure and can be used to generate motion or force. They
can also accumulate energy, and most important, they exhibit shape
recovery effect.
The materials that possess actuation properties are useful in
many interesting applications, depending on many factors. The
most important factor that engineers have to pay attention to during
designing is the actuation energy density — available work per unit
volume and actuation frequency. The best situation for an active
material perhaps is to have both of them, as high as possible [22].
The actuation energy density is shown in Fig. 8.2 by dotted lines,
which is a result of the relation of actuation stress to actuation strain,
assuming that the material is operating at a constant stress level. For
a better understanding of signaled relations, we make a comparison:
Imagine a 1 cm
3
of shape memory alloy that can exert enough force
to move an object weight 4650 kg.
Speciic value of actuation energy density expressed by work
output per unit mass can be easy calculated for the material by
