Polymer gels have been studied for decades thanks to their unique and highly tunable physiochemical properties. Today, their applications range from drug delivery systems to energy storage devices, and with their recently developed color-changing abilities, further applications could be imminent.
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What are Polymer Gels?
Polymer gels are three-dimensional cross-linked polymer networks swollen in solvents. They are soft and capable of large deformations, with the network of long polymer molecules holding the solvent in place and giving the gel its solidity.
Arguably their most interesting property is the substantial swelling and collapsing they can undergo as a function of their environmental conditions, such as temperature, pH, or ionic strength. This phenomenon, termed volume phase transition, has led to great interest in the application of stimuli-responsive gels in the biomedical sector, for artificial muscles, glucose sensors, and tissue scaffolds for example.
Their inhomogeneous nature and low volume fraction of polymer chains mean that polymer gels are inherently weak, yet there has been a growing demand for mechanically robust gels for use in wearable electronics, soft robotics, and programmable mechanically active materials. To achieve this, various strategies are available to augment their mechanical strength, such as incorporating filler nanoparticles, creating a double-network gel structure, or increasing gel homogeneity through geometric constraints.
Developing the notation of mechanically robust polymer gels has led researchers Takuya Yamakado and Shohei Saito to create a new ‘force probe’ which is compatible with such gels.
A force probe allows reaction rates to be measured as a function of the restoring force in a molecule that has been stretched or compressed, enabling the quantitative mapping of local stress distribution at the molecular level. This can be shown through color changes and luminescence responses displayed by the probe. Yamakado and Saito, (2022) thus identified the considerable value of incorporating these probes in the weak and fragile polymer gels which can often split and crack without warning.
Their recent research is the continuation of previous work carried out by Saito and his colleagues in developing a V-shaped molecule, called a flapping molecular force probe (FLAP), which changed color from blue to green fluorescence when its two side structures, or wings, were flattened with sufficient force (in the pico to nanonewton range).
This FLAP worked well with polymer films, but immediately turned polymer gels a fluorescent green without any external force. Therefore, Yamakado and Saito decided to modify the FLAP’s molecular design to enable the probe to be used in conjunction with common polymer gels.
The New Force Probe
The persistent fluorescent green color in polymer gels was due to the spontaneous excited-state planarization (flattening) of the solvated FLAP molecule. The researchers hypothesized that by replacing the FLAP’s two anthraceneimide wings with pyreneimide ones, the excited state energy profile would be tuned to suppress this unwanted planarization and enable the probe function of the FLAP in gels as was seen in polymer films.
The pyreneimide wings, themselves a pyrene dione derivative, were synthesized by a condensation reaction of pyrene dione and aromatic amines. They were then attached to the same flexible cyclooctatetraene (COT) joint as used previously through a reaction in a mixed solvent of chloroform and acetic acid whilst being heated.
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The resulting structural modifications in the FLAP gave the same blue-to-green color change in polymer films and raised the excited-state energy barrier as hypothesized. This not only enabled the molecule to be used as a force probe in the wet conditions in polymer gels, but also broadened the dynamic range of the stress-induced fluorescence spectral change in polymer films.
Finally, Yamakado and Saito demonstrated the force probe function of their newly developed FLAP in a polymer gel when under a non-uniform stress distribution from supporting a selection of small metal stamps. The color variation in the compressed gel was just visible with the naked eye and with ratiometric fluorescence imaging, which was sufficient to give a quantitative evaluation of true compression stresses.
Implications of the Research
The researchers expect the enhanced functionality of this FLAP to allow closer studying of polymer chain dynamics and the development of tougher polymer gels. They also stated that using the probe to increase the hydrophilicity (the affinity for water) of certain gels would enable tension probes for cell membranes.
Similar research by Kotani et al., (2022) which investigated FLAPs in other polymer materials, came to the same conclusion. They found that the molecular level monitoring of stress concentrations enabled new insights into the rheology of polymers. Namely, they identified a strain-hardened region in which the local stress concentration in the polymer crosslinks was almost twice that of the main chain. This insight can clearly guide the rational design of tougher polymer materials, which now, thanks to Yamakado and Saito’s research, can include polymer gels.
Tough polymer gels have a wide range of applications. In the biomedical sector, for example, such gels could be used for cartilage and tendon tissue engineering, for developing artificial skin and smart prosthetics, or for creating tough drug delivery vehicles.
Elsewhere, tough polymer gels could alleviate the increasing pressure on precious raw materials for the manufacturing of lithium-ion batteries. Miguel et al., (2020) presented the benefits of aluminium-ion batteries as a sustainable alternative and the further potential benefits of using a tough polymer gel for the battery’s electrolyte.
Tension probes for cell membranes could be just as fascinating. The cell membrane tension is crucial in regulating many cell functions including cell division and cell migration, but it has proved extremely hard to measure with existing techniques invasive to the cell and often inaccurate.
Evaluating the membrane tension and other cellular mechanotransduction mechanisms through tension probes is expected to lead to a deeper understanding of the development of diseases such as skin disorder, muscular dystrophy, and cancer.
References and Further Reading
Baek, K., Kim, S. and Koh, H., 2022. Molecular Tension Probes to Quantify Cell-Generated Mechanical Forces. Molecules and Cells, 45(1), pp.26-32. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8819489/
Colom, A., Derivery, E., Soleimanpour, S., Tomba, C., Molin, M., Sakai, N., González-Gaitán, M., Matile, S. and Roux, A., 2018. A fluorescent membrane tension probe. Nature Chemistry, 10(11), pp.1118-1125. https://www.nature.com/articles/s41557-018-0127-3
Dannert, C., Stokke, B. and Dias, R., 2019. Nanoparticle-Hydrogel Composites: From Molecular Interactions to Macroscopic Behavior. Polymers, 11(2), p.275. https://www.mdpi.com/2073-4360/11/2/275
Fuchs, S., Shariati, K. and Ma, M., 2019. Specialty Tough Hydrogels and Their Biomedical Applications. Advanced Healthcare Materials, 9(2), p.1901396. https://onlinelibrary.wiley.com/doi/abs/10.1002/adhm.201901396
Gu, Z., Huang, K., Luo, Y., Zhang, L., Kuang, T., Chen, Z. and Liao, G., 2018. Double network hydrogel for tissue engineering. WIREs Nanomedicine and Nanobiotechnology, 10(6). https://wires.onlinelibrary.wiley.com/doi/abs/10.1002/wnan.1520
Kotani, R., Yokoyama, S., Nobusue, S., Yamaguchi, S., Osuka, A., Yabu, H. and Saito, S., 2022. Bridging pico-to-nanonewtons with a ratiometric force probe for monitoring nanoscale polymer physics before damage. Nature Communications, 13(1). https://www.nature.com/articles/s41467-022-27972-y
Li, X., Nakagawa, S., Tsuji, Y., Watanabe, N. and Shibayama, M., 2019. Polymer gel with a flexible and highly ordered three-dimensional network synthesized via bond percolation. Science Advances, 5(12). https://www.science.org/doi/10.1126/sciadv.aax8647
Mano, J., 2008. Stimuli-Responsive Polymeric Systems for Biomedical Applications. Advanced Engineering Materials, 10(6), pp.515-527. https://onlinelibrary.wiley.com/doi/10.1002/adem.200700355
Miguel, Á., García, N., Gregorio, V., López-Cudero, A. and Tiemblo, P., 2020. Tough Polymer Gel Electrolytes for Aluminum Secondary Batteries Based on Urea: AlCl3, Prepared by a New Solvent-Free and Scalable Procedure. Polymers, 12(6), p.1336. https://www.mdpi.com/2073-4360/12/6/1336
Yamakado, T. and Saito, S., 2022. Ratiometric Flapping Force Probe That Works in Polymer Gels. Journal of the American Chemical Society, 144(6), pp.2804-2815. https://pubs.acs.org/doi/abs/10.1021/jacs.1c12955