Porous and Lattice Structures for Biodevices with Advanced Properties - Advanced Design and Manufacturing Technologies for Biomedical Devices - page 122

The apparent stiffness of such lattice structures can be also obtained and the dif-
ferent structures represented in form of Ashby s diagrams showing, for instance,

adimensional apparent Young modulus (effective Young modulus/Young modulus
of the bulk material) versus adimensional apparent density (effective density/den-
sity of the bulk material).
Such graphics or diagrams (grouping materials by families and subfamilies) pro-
vide a very useful tool for material selection processes, in parallel to the whole
product development, helping to consider different factor to be optimised, such as
stiffness, resistance, weight, cost and manufacturability, among others (Ashby and
Jones 1996 , 1998 ; Ashby 2005 ).
It is also relevant to cite the development of material selection software based on
the Ashby

'

s methodology for helping engineers and designers to handle these infor-
mation and graphics in an easier way. The mentioned software is commercialised
under the name “CES Selector” by Granta ( www.grantadesign.com ), and several
teaching resources and professional packages oriented to different industries,
including the medical device industry, are offered.
The combination of CES Selector with more specifi c databases of material prop-
erties, for example, “CampusPlastics” ( www.campusplastics.com ) for polymer
technology, can be a source of valuable information for novel product development
projects, even though quality control and verifi cation through personal trials that the
materials provided by the suppliers fulfi l specifi cations is always a very advisable
procedure.
In many cases, in projects linked to medical devices, novel or special functional-
ities are provided by the use of recently discovered biomaterials, what normally
requires special attention to characterisation tasks for obtaining the valuable infor-
mation needed for the design process.

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7.3 Design of Lattice Auxetic Structures and Devices:

Application to Expandable Devices

When a material is stretched, there is normally an accompanying reduction in width,
normally linked to mass conservation. A measure of this dimensional change can be
defi ned by Poisson axial the transverse and

axial strains when the material is stretched or compressed in the axial direction. In
a more general case,

' s ratio, ν = −d ε trans /d ε axial , being ε trans and ε

ij is the Poisson ratio that corresponds to a contraction in
direction “j” when an extension is applied in direction “i”.
For most materials this value is positive and refl ects a need to conserve volume.
Auxetic materials (or metamaterials) are those with a negative Poisson ratio (NPR)
and display the unexpected property of lateral expansion when stretched, as well as
an equal and opposing densifi cation when compressed (Lakes 1987 ; Evans 1991 ;
He et al. 2005 ; Liu and Hu 2010 ). Natural (some minerals, skins, etc.) and man-
made (foams, Gore-Tex®, polymeric foams) auxetics have been described, and

ν