Soft Mechanical Actuators: Applications in Artificial Implants, Biomimicking Devices, and Stem Cell

1
Soft Mechanical Actuators Applications in
Artificial Implants, Biomimicking Devices, and
Stem Cell Culture. Han Liang Lim, Tuan Tran,
Chao Zhang, Shyni Varghese Department of
Bioengineering, University of California, San
Diego Department of Chemical Engineering,
University of Wisconsin, Madison
Figure 7. The deswelling kinetics of the hydrogel
after 1 minute of swelling. The gel showed
significant swelling over the 1 minute in which
it was exposed to the electric field. The gel
swelled anisotropically more on the side
exposed to the positively charged terminus, which
facilitated bending towards the negative
terminus. When the stimulus was removed, the gel
swelling is reversed, and the gel deswells
slowly. The deswelling is attributed to the
potential difference across the gel reverts back
to zero, and the water diffuses out.
Hydrogels are three-dimensional (3D) polymer
matrices that can imbibe large quantities of
aqueous solution. Their high water content and
viscoelastic properties are amenable to that of
soft tissues. With the advent of biomedical
research, there has been renewed interest in
hydrogels as static vessels for cell culture,
providing a stationary 3D support for cellular
functions. However, it is advantageous to use a
dynamic hydrogels system providing biophysical
cues, electrical and mechanical, to regulate stem
cell behavior within a 3D niche. With this in
mind, we have asked a question if we can create a
biocompatible device that can simultaneously
provide these cues. Such bio-mimicking actuators
can act as artificial muscles (Fig 1A), drug
delivery devices (Fig 1B), and stem cell culture
systems (Fig 1C).
The Electric Field
Figure 8. The reproducibility of the hydrogel
swelling and deswelling after 2 hours of
continuous pulsing. The gel also showed high
amounts of reversibility, as it always swells to
produce the same amount of strain, and deswells
to its original size (i.e. volume and weight)
Fig 1A. A gel hand as an artificial muscle
Fig 1B. A gel used in drug delivery
Cell-laden polymer network
Fig 5. Top left. Circuit diagram of experimental
setup. Top right. Overhead schematic of
experimental setup. Bottom Left. Electrolytic
reactions taking place in our setup. Bottom
right. Actual experimental setup.
Fig 1C. Gel used as a stem cell culture device.
Cells embedded in a polymer network.

• Hydrogel Experimental Procedures
• First Experiment (fig. 7)
• Swelled for 1 minute in E-field
• Turned off E-field, allowing gel to deswell
• Weighed the gel at every 1 minute interval

E-field
h?

• Second Experiment (fig. 8)
• Swelled for 1 minute in E field. weighed
• Turned off E-field, Allow gel to deswell for 9
  minute. Weighed
• Repeat 1. and 2. for 2 hours.

Figure 9. Left Mesenchymal stem cells in
growth medium at day 2. Right. Mesenchymal stem
cells cultured in medium containing 0.1M
concentration of AMPS, demonstrating continued
cell viability in the presence of AMPS. As seen
from the figure, AMPS promoted cell
proliferation. Cells were observed using 10x
magnification
Effect of AMPS on cell viability Bone marrow
derived mesenchymal stem cells were incubated
with medium containing 0.1 molar AMPS. The
cellular response to the medium was evaluated
qualitatively observing cell proliferation and
cell adhesion periodically.

• Conclusion
• developed a bio-mimicking soft actuator that can
  simultaneously provide mechanical and electrical
  stimuli
• The system is designed so as to culture cells in
  a 3D environment where the electrolytic waste is
  discharged in separate electrolyte reservoirs
• In 60 seconds of being exposed to the electric
  field, the gel bends to about 40 degrees, and
  swells to generate an internal tensile strain of
  15.
• This electrical stimuli dependent
  bending-stretching of the gel was repeatable over
  a large period of time.
• Preliminary cell studies indicate the monomers
  are cell compatible and support cellular
  proliferation

Analysis Data was analyzed with OriginPro v8.0.
Curves were fit with an exponential decay
function.
Results The overall resistance of the setup is
between 750 to 1000 ohms. There is a potential
difference of 1.0 V across each gel, allowing us
to determine its resistance in the circuit. The
resistance across each gel is between 12.5 ohms
and 16.7 ohms.
Fig 3. Anisotropic swelling in and electric
field induces bending/stretching in polyionic gels
Materials and Methods Synthesis of hydrogels
Fig 4. Top chemical structure of the monomers
used for hydrogel synthesis Bottom Left
Hydrogel as synthesized Bottom Right Schematic
representation of hydrogel network
Future Work We aim to encapsulate cells into our
gel constructs, and observe the combined effects
of electrical pulsing and dynamic tensile strain
on cellular behavior in a 3D environment.
Acknowledgments I would like to thank Calit2 and
UCSD for giving me this opportunity to conduct
and present my research.
Fig 6. Left. The gel resting before the power
supply is turned on. Middle. The gel bends
slightly after the power supply is turned on for
one minute. Right. The gel bends significantly
after the power supply is turned on for two
minutes. With increased exposure to the electric
field, there is also increased liberation of
alkaline discharge, which turned our indicator
(phenolphthalein) pink. The gel is stained with
alizarin red and thus appears pink when
photographed.
References Osada Y., Okuzaki, H. and Hori, H.,
1992. A polymer gel with electrically driven
motility. Nature 355, pp. 242244. De Mattei M,
Caruso A, Pezzetti F, Pellati A, Stabellini G,
Sollazzo V, et al. Effects of pulsed
electromagnetic fields on human articular
chondrocyte proliferation. Connect Tissue Res
200142269.