Porous and Lattice Structures for Biodevices with Advanced Properties - Advanced Design and Manufacturing Technologies for Biomedical Devices - page 123

very special attention is being paid to the development of auxetic structures designed
and controlled on a molecular scale (Griffi n et al. 2005 ).
Auxetic geometries are being progressively employed in the development of
novel products, especially in the fi elds of intelligent expandable actuators, shape
morphing structures and minimally invasive implantable devices. Regarding smart
actuators based on an auxetic structure, it is important to cite some recent progress
linked to auxetic shape-memory alloys (SMA) for developing deployable satellite
antennas (Scarpa et al. 2010 ) and some research on the characterisation of polyure-
thane foams with shape-memory behaviour and auxetic properties, promoted thanks
to several post-processing stages (Bianchi et al. 2010 ).
In the area of medical devices, recent research has also assessed the behaviour of
a few auxetic geometries for implementing expandable stents (Tan et al. 2011 ), and
their application to other implantable biodevices is clearly a matter of research.
The use of auxetic devices in the biomedical fi eld is started to be assessed as use-
ful for the following possibilities:
• Adapting the stiffness of prostheses to that of tissues and organs with which the
biodevice will interact, for promoting more adequate contact phenomena and
minimising stress shielding and related aspects as bone resorption
• Improving implantability of biodevices and promoting minimally invasive surgi-
cal procedures, as explained towards the end of present section
• Obtaining sheet structures for cell growth that do not let wrinkles appear when
stressed, helping implanted tissue to progressively grow and adapt
Figure 7.5 provides a brief extract from a CAD library recently developed by our
team, aimed at providing auxetic cell units for complementing future design inno-
vation tasks linked to their application to implantable devices and scaffolds for tis-
sue engineering (Álvarez Elipe and Díaz Lantada 2012 ; Álvarez Elipe 2012 ) . Some
examples of 2D and 3D auxetic structures or metamaterials are provided.
Based on the computer-aided design fi les, structural calculations using the FEM
method can allow us to estimate the Poisson modules and compression ratios of the
auxetic structures, even from the design stage. Aspects such as stiffness and density
can also be assessed rapidly.
Additional information regarding the FEM-based simulation (elements used,
loads and boundary conditions applied) of these and several more structures,
together with a comparative study considering stiffness, density, maximum com-
pression ratio and Poisson ratio, can be found at our more specifi c report on auxetic
geometries (Álvarez Elipe and Díaz Lantada 2012 ; Álvarez Elipe 2012 ), even
though it is not so aimed at medical devices as present section.
Once obtained, CAD fi les can be also manufactured for carrying out in vitro and
in vivo trials, following the approach mentioned in Sect. 7.4 .
As previously introduced, these auxetic structures have also remarkable potential
for the development of implants and medical devices for promoting minimally inva-
sive surgical procedures. A minimally invasive medical procedure is defi ned as one
that is carried out by entering the body through the skin or through a body cavity or
anatomical opening, but with the smallest damage possible to these structures. The