Introduction:: Natural objects have developed unique characteristics in the process of evolution. By mimicking these properties in nature, researchers developed synthetic materials with various structures or functions. Liesegang rings, the self-organized mineral patterns in sedimentary rocks and trees, are formed as ions precipitate into crystals at the supersaturated condition(1). The structure of the periodic rings in the liesegang pattern is determined by the concentration of the ions and the composition of the ion containing electrolytes(2). Many researchers attempted to mimic this structure in a synthetic system to utilize it as a patterning technique(3, 4). However, there was only a few studies focusing on the physical properties of this unique structure such as mechanical and rheological properties. As ionic precipitates are formed according to the ring pattern, the pattern has the hard mechanical properties of the precipitated minerals which affect the physical properties of the whole material. In this study, we developed an inorganic/organic hybrid material with liesegang ring pattern through biomineralization. We controlled the pattern formation with various ionic strength of the mineral ions. Then, the effect of the liesegang structure in the mechanical properties of the material was studied. Surprisingly, we found out that the liesegang ring pattern imparts viscoelasticity in the material which is a common property of biological tissues in living organisms. In conclusion, we easily fabricated the bioinspired material that has biological properties by mimicking the beauty of nature.
Materials and Methods:: In this study, we developed an inorganic/organic material with a unique liesegang pattern. The pattern formation was performed in the organic hydrogel composed of two types of polymer networks: Covalently crosslinked polyacrylamide network and ionically crosslinked alginate network. Then, the hydrogel was immersed in the supersaturated ionic solutions containing calcium and phosphate ions. As these counterions interact with each other when diffuse into the hydrogel, the ion precipitation occurs. When calcium and phosphate ions precipitate, biomineralization proceeds to create hydroxyapatite crystals. The areas where hydroxyapatite crystal forms become opaque. Because the ionic concentration is at supersaturation, liesegang phenomenon occurs creating the opaque concentric rings of hydroxyapatite in the polymer hydrogel. With this method, we could easily obtain the hydrogel with a bioinspired mineral pattern.
Results, Conclusions, and Discussions:: During the liesegang ring formation of the biominerals, we confirmed that the separation between the rings is determined by the ionic strength. When the ionic strength was lower than 3.3 M, only mineral shell was formed at the surface of the hydrogel. At this level, the increase in ionic concentration only affected the thickness of the shell. When the ionic strength increases to 3.3 M, the unique ring pattern appeared. However, the rings were not completely separated. The complete and distinct liesegang rings were only fabricated at the supersaturated condition where ionic strength reached to 33 M. The rheological analysis was then performed. Frequency sweep was conducted to compare the storage modulus (G’) and loss modulus (G”) of the material depending on the angular frequency. Interestingly, a point where G’ and G” became the same appeared only in the biomineralized hydrogels with liesegang pattern. Moreover, a crossover point of G’ and G” was shown in the specimen in which the liesegang pattern was completely created. This behavior is completely different from the elastic behavior of bare hydrogel (HD), which has no modulus change with frequency. These results suggest that introducing the liesegang-patterned biomineralization in the elastic material can change the mechanical properties to viscoelastic. Viscoelasticity is a mechanical property that living tissues have in common, and many researchers have tried to simulate it in synthetic materials(5-7). Also, the periodic biomineral pattern is composed of hydroxyapatite which is highly cytophilic. Therefore, we believe that our strategy with bioinspired mineral pattern suggests a breakthrough to easily provide a biocompatible environment as well as the biophysical properties.
Acknowledgements (Optional): :
References (Optional): : 1. K. H. Stern, The Liesegang Phenomenon. Chemical Reviews 54, 79-99 (1954). 2. H. Nabika, M. Itatani, I. Lagzi, Pattern formation in precipitation reactions: The Liesegang phenomenon. Langmuir 36, 481-497 (2019). 3. Z. Xing et al., Liesegang Phenomenon of Liquid Metals on Au Film. Advanced Materials 35, 2209392 (2023). 4. H. Qiao, S. Sun, P. Wu, Non‐equilibrium‐Growing Aesthetic Ionic Skin for Fingertip‐Like Strain‐Undisturbed Tactile Sensation and Texture Recognition. Advanced Materials, 2300593 (2023). 5. O. Chaudhuri, J. Cooper-White, P. A. Janmey, D. J. Mooney, V. B. Shenoy, Effects of extracellular matrix viscoelasticity on cellular behaviour. Nature 584, 535-546 (2020). 6. O. Chaudhuri, Viscoelastic hydrogels for 3D cell culture. Biomaterials science 5, 1480-1490 (2017). 7. Y. Ma et al., Viscoelastic cell microenvironment: hydrogel‐based strategy for recapitulating dynamic ECM mechanics. Advanced Functional Materials 31, 2100848 (2021).
