Page 360 - Control Theory in Biomedical Engineering

326 Control theory in biomedical engineering

5 Fabrication methodology

Since our design will be subjected to tensile and compressive loads and
undergo deformation, we require our material to be compliant and flexible.
We used two different methods of fabrication to achieve our tubular struc-
tures. The first method involves cutting a two-dimensional sheet of soft
material and rolling it into a tube. The second method involves direct 3D
printing of the tubular structures. Each of the methods are detailed in the
sections that follow.

5.1 Rolling two-dimensional sheets

As it is possible to create tubes by rolling materials on two-dimensional
sheets, we use a standard re-entrant planar design on sheets of silicone
and laser cut the pattern. We use Dragon Skin 30 Silicone rubber to create
the sheets. We then spread out the mixture over a flat surface for a sheet
thickness of 1mm. After curing and cutting the sheets, they are rolled into
a cylinder. When stretched, we did not observe much of an expansion or
auxetic activity because of the fully compliant nature of silicone. Auxetic
materials display their properties best when expansion is only constrained
to the hinge of rotation while the other parts remain relatively rigid. This
is not possible with silicone rubber as all parts undergo equal stretch, thereby
giving a Poisson’s ratio of close to zero.
Similar tests were conducted using the following varieties of silicone rub-
ber with different Young’s modulus and thickness. Upon observation, all
these tests result in little to no auxetic activity. Hence, we abandoned the
idea of using 2D planar sheets to fabricate auxetic materials.

5.2 3D printed auxetics

We took a different approach to the fabrication of auxetic materials than in
the previous section. In this method, we first created a computer aided
design (CAD) model of a sheet metal and superimposed the required sketch
of the auxetic design. We then extruded the design and joined the bends of
the fold to get our required structure. This part was then directly printed
using different flexible materials. We used five different auxetic designs
and three different materials for each design to conduct our tests. The mate-
rials and designs are described below.