DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

2.2. Reentrant Structures and Ceramic-Metal Composites
The primary funding for this dissertation work is in support of a collaborative
effort with Dr. Lisa Friis' group in the Mechanical Engineering Department at the
University of Kansas. The broad objective of the project is to create a biomedical scaffold
with properties that mimic natural bone. That is, the scaffold should have porosity on the
order of 50%, should be a non-linear elastic material and should display slight
piezoelectric behavior. The following describes the biomedical significance in greater
detail.
Non-linear elastic behavior is displayed by negative Poisson's ratio (NPR)
structures while piezoelectricity can provide a self-sustaining electrical signal. Combined,
these are hypothesized to mimic biologic bone to promote cell development through a
combination of mechanical stress, electrical, and ionic fluid flow environments. Poisson's
ratio ν, as illustrated in Figure 2.2, is the ratio of transverse strain to the axial strain, when
an object is stretched or compressed. Most materials have a positive Poisson's ratio, i.e.
expanding in transverse direction when compressed in axial direction. A NPR indicates
the reverse trend, i.e., a negative transverse strain along with a negative axial strain. Few
materials in nature exhibit NPR behavior and natural cork is a good example of a zero
Poisson’s ratio material. Usually NPR behavior is induced in a material as a result of
structural modification. The term "reentrant" refers to one type of cellular NPR structure,
where struts defining the unit cell of the structure point inward. The reentrant structure
has many unique mechanical properties, including nonlinear load-deformation response
and good vibration damping.
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