Scientists Create “Shapeshifting” Inorganic Hydrogels

Andrew Sasser, Physical Sciences, Winter 2021

This is an example of a hydrophilic hydrogel polymer. Hydrogels are normally very elastic materials composed of organic or inorganic materials mixed with water. However, some hydrogels can become more rigid when exposed to heat or light (Source: Wikimedia Commons)

In the development of new technologies and materials, one of the main sources of inspiration is the natural world. Many scientists have sought to develop biomimetic materials –  materials that mimic some of the properties of plants or animals – through the careful construction of artificial nanostructures (Kumar et. al, 2020). On example is windmill blades with serrated edges, which are inspired by the tubercles, or bumpy edges, on whale fins. Similarly, the material Velcro is inspired by the sticking ability of burrs (Goddard, 2016).

One particular area of research has focused on the development of so-called mechanically “adaptive” materials that can change their rigidity in response to a stimulus. Adaptive materials are significantly more versatile than standard materials and are thus of great interest to scientists. Materials scientists have sought to mimic the ability of sea cucumbers to reversibly convert their skin from a rigid layer to a jelly-like form (RIKEN, 2020). Sea cucumbers are able to maintain a hard skin by aggregating a large network of collagen fibers on top of an underlying hydrogel matrix. This dual structure forms a tight, hydrophobic barrier to increase surface rigidity. Scientists had previously developed polymer-based systems from organic materials like vinyl alcohols and cellulose nanocrystals that could change rigidity, but had yet to do so from inorganic materials. Using inorganic compounds would expand the scope of substrates that could be used for the development of adaptive materials (de Espinosa et. al, 2017).

To this end, recent work by Sano et. al demonstrates the development of a mechanically active hydrogel from inorganic titanium nanosheets and water that is able to reversibly change its elasticity at different temperatures. To build a hydrogel that could change its elasticity, the group hypothesized that they would need to develop a material that contained charged nanosheets that could repel or attract one another. The group believed that in the “stiff” mode, electrostatic attraction would cause the nanosheets to come together; if electrostatic repulsion forces dominated, the sheets would dissociate, allowing the material to assume a “gel” state (Sano et. al, 2020).

The group chose to use titanate nanosheets – composed of titanium anions – due to their high density of negative charges of up to 1.5 Columbs/m² (Sano et. al, 2020). The group centrifuged these nanosheets with water, producing a gel material composed of 14% titanate nanosheets and 86% water (RIKEN, 2020). X-Ray scattering measurements confirmed that at room temperature, the nanosheets assumed a diffuse state, with each sheet separated by up to 11.8 nm. However, when heated up to 55˚C, the group found that the gaps between sheets had decreased to 3 nm, and that the material had become far more rigid as a result (Sano et. al, 2020). Additionally, they found that repeated heating and cooling of the material did not significantly degrade its ability to interconvert between forms.

Finally, the group decided to modify the material to make it responsive to light as well. They opted to use Gold nanoparticles because of their ability to concentrate light and raise temperatures at a localized site. When the material was irradiated with a 445-nm laser light for 40 seconds, the temperature of the material was raised from 25 to 82˚C, causing it to assume its rigid mode (Sano et. al, 2020). Ultimately, the group concluded that this work presents an alternative to the organic polymers previously used for the developed of mechanically active materials. Co-author Yashuiro Ishida noted that this may even allow the development of a form of “inorganic life” that can respond to a number of different stimuli (RIKEN, 2020).

References

1. Espinosa, L. M., Meesorn, W., Moatsou, D., & Weder, C. (2017). Bioinspired Polymer Systems with Stimuli-Responsive Mechanical Properties.Chemical Reviews, 117(20), 12851-12892. doi:10.1021/acs.chemrev.7b00168
2. Goddard, G. (2016). Biomimetic design: 10 examples of nature inspiring technology. Retrieved January 11, 2021, from https://www.sciencefocus.com/future-technology/biomimetic-design-10-examples-of-nature-inspiring-technology/
3. (2020). A shapeshifting material based on inorganic matter. Retrieved January 11, 2021, from https://www.riken.jp/en/news_pubs/research_news/pr/2020/20201127_1/index.html
4. Sano, K., Igarashi, N., Ebina, Y.et al. A mechanically adaptive hydrogel with a reconfigurable network consisting entirely of inorganic nanosheets and water. Nat Commun 11, 6026 (2020). https://doi.org/10.1038/s41467-020-19905-4
5. Suresh Kumar, N., Padma Suvarna, R., Chandra Babu Naidu, K. et al.A review on biological and biomimetic materials and their applications.  Phys. A 126, 445 (2020). https://doi.org/10.1007/s00339-020-03633-z