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

Materials and Methods

Synthesis of hydrogels

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).

Fig 3. Anisotropic swelling in and electric field induces bending/stretching in polyionic gels

Fig 4. Top: chemical structure of the monomers used for hydrogel synthesis; Bottom Left: Hydrogel as synthesized; Bottom Right: Schematic representation of hydrogel network

Fig 2. picture of a swollen hydrogelResults

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.

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.

Analysis: Data was analyzed with OriginPro v8.0. Curves were fit with an exponential decay function.

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.

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)

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 reservoirsIn 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

Acknowledgments

I would like to thank Calit2 and UCSD for giving me this opportunity to conduct and present my research.

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.

The Electric Field:

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.

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

As exposure time increases, bending angle increases

References

Osada Y., Okuzaki, H. and Hori, H., 1992. A polymer gel with electrically driven motility. Nature 355, pp. 242–244. 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 2001;42:269.

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Hydrogel Experimental Procedures:

First Experiment (fig. 7)1. Swelled for 1 minute in E-field2. Turned off E-field, allowing gel to deswell3. Weighed the gel at every 1 minute interval

Second Experiment (fig. 8)1. Swelled for 1 minute in E field. weighed2. Turned off E-field, Allow gel to deswell for 9

minute. Weighed3. Repeat 1. and 2. for 2 hours.

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 1A. A gel hand as an artificial muscle

Fig 1B. A gel used in drug delivery

Fig 1C. Gel used as a stem cell culture device. Cells embedded in a polymer network.

h

Cell-laden polymer network

initiator

Polymer with functional groups

cellE-field