Novel Soft Materials with Nonlinear Mechanical Response

Abstract

Abstract
As the population ages, the number of elderly people walking outside is also expected to
increase. Due to natural process of aging, a direct consequence is weakening of muscle
strength, which in turn leads to a decrease in walking speed and increases the risk of falling.
[1,2]
The purpose of this study is to develop a soft material with a nonlinear mechanical response
by Hybrid structural design using multiple materials, which can be applied as a shoe bottom
material that supports keeping walk speed while reducing the risk of falling, and robot hands
that resemble human fingers. Soft materials have been used in a wide scope of fields such as
soft robotics, medicine, and welfare, because they are safer and closer to biological
characteristics than conventional metallic materials. In this research, we focus on giving an
extra-viscosity to silicone rubber by embedding a flow channel of viscous fluid.
Experiment
In this experiment, we fabricated 20 mm cubic silicone samples, having a flow channel in the
central 15 mm cubic area, and evaluated the dynamic viscoelasticity of the samples.
A water-soluble flow channel (Parallel crosses structure) model was 3D printed with an FDM
3D printer (QIDITECH X-pro) using polyvinyl alcohol (PVA) filament. Figure 1 shows the flow
channel structure. In the XY-plane the beam structures are printed in parallel at equal intervals,
and their orientation is rotated 90° with each layer.
The channel model was fixed in the center of a mold, then silicone resin (Ecoflex00-30,
Smooth-on) was poured and cured. After the curing, the silicone part is removed from the mold
and dipped in water to dissolve the PVA flow channel model. The sample was fabricated in
two pieces to enable smooth dissolution and diffusion by allowing the edges of the PVA part
to expose the water. The dissolved hollow part in the silicone was filled with viscous fluid.
(hydroxyl propyl cellulose (HPC 150~400, Wako) solution in water). When filling the viscous
fluid, we used a manual centrifuge machine to apply high gravity for smooth entering of the
viscous fluid and degassing.
The fluid was colored with black pigment so that the fluid can be visually recognized through
the translucent silicone part. Figure.1 and Fig.2 show optical images of fabricated samples.
Table 1 Shows the details of fabricated samples.
As an evaluation, we measured the viscosity-shear rate dependence of viscous fluids with a
rheometer. (Anton Paar, MCR302). In addition, we fabricated a DMA testing machine capable
of large deformation at low frequencies and measured the dynamic viscoelasticity of fabricated
samples at 1 Hz and 5 mm deformation. Table.2 shows the Measurement results of the DMA
test.
Result and discussion
In this result, we verified that the tangent loss of the two samples embedded with a viscous
fluid is larger than that of simple silicone rubber. The tangent loss of the fabricated sample (B)
is the largest, suggesting that only the viscosity difference between the silicone and the fluid
affects the tangent loss.
We considered that the flow in the channel would cause viscous resistance and increase the
tangent δ. However, we could not verify any significant tangent loss in the fabricated sample
(A). We guess that the viscosity of the fluid is too high to occur fluid flow in the channel or/and
the current flow channel structure easily dissipate the force so that viscous resistance is too
low. In the future, we study how the channel structure and fluid filling volume rate affects the
viscous resistance of the filled viscous fluid.
In this conference, we will report the fabrication method and evaluation results of the fabricated
samples.