Turgor Pressure Reinforces Mineral-Based Composites

Nature fabricates hard functional materials from soft organic scaffolds that are mineralized. To overcome the brittleness of inorganic components, biominerals are arranged in hierarchical structures and sometimes develop local compressive or tensile stresses. For example, the precipitation of hydroxyapatite minerals leads to contractile stresses in collagen fibrils, and eventually reinforces mechanical properties of bones ^[1]. The fibril contraction comes from the dehydration of the polymer matrix, even though it happens in a hydrated environment. Similarly in plant cells, osmotic pressure is used to stiffen leaves by exploiting a strong turgor pressure ^[2]. Osmotic pressure is well-known to change the mechanical properties of living biological materials. Nevertheless, this strategy has thus far not been leveraged in mineral-based composites. This shortcoming might in part be related to the inferior mechanical behavior of synthetic composites compared to natural counterparts due to the limited structural control over different length scales.<br/><br/>Here, we demonstrate a mineral-based composite whose strength and stiffness can be repeatably adjusted through osmotic pressure driven swelling and de-swelling of confined hydrogels. Specifically, we employ a synthetic porous matrix with a similar degree of structural control from the nanometer up to the centimeter length scale made from an emulsion-based, 3D printable ink ^[3]. Those pores in the matrix are back-filled with a hydrogel, which can generate internal compressive or tensile stresses triggered by osmotic pressure gradients. The matrix can selectively bind certain cations such as Ca²⁺ ions, which can serve as mineralization sites upon addition of CO₃^2-. The hydrogel contractions induced through the mineral precipitations and osmotic pressure gradients lead to a synergetic control over the mechanics of the composite. This work provides an exciting concept for enhancing mechanical properties of mineral-based composites through turgor pressure similar to biological systems, which can be used as soft actuators or intelligent devices for underwater applications.<br/><br/>References:<br/>[1] Ping, H., Wagermaier, W., Horbelt, N., Scoppola, E., Li, C., Werner, P., Fu, Z. & Fratzl, P. (2022). Mineralization generates megapascal contractile stresses in collagen fibrils. <i>Science, 376</i>(6589), 188-192.<br/>[2] Fung, Y. C. (2013). <i>Biomechanics: mechanical properties of living tissues</i>. Springer Science & Business Media.<br/>[3] Zhao, R., Wittig, N., De Angelis, G., Yuan, T., Hirsch, M., Birkedal, H., & Amstad, E. (2023). Additive manufacturing of porous biominerals. <i>Advanced Functional Materials,</i> accepted.