Inspired by actuation mechanisms in plant structures and motivated by recent advances in electro-chemically driven micro-pumps, this paper is concerned with a novel concept for active materials based on distributed hydraulic actuation. Due to the similarity of the actuation principles seen in plants undergoing nastic motion, we refer to this class of active materials as nastic materials. We present a mechanical modeling approach for nastic materials representing the effects of pressure generation and fluid transport by incompressible eigenstrains. This model is embedded into a two-level macro/micro topology optimization procedure. On a macroscopic level, the integration of nastic material into a structural system is optimized. The placement and distribution of nastic material on a flexible substrate are optimized to generate target displacement and force distributions. On a microscopic level, the stress and strain generation is tailored to desired macroscopic material properties by optimizing the layout of vascular fluid channels embedded in an elastic matrix. For the layout optimization of vascular fluid channels, a novel topology optimization procedure is presented that models the effects of pressure along the fluid channels via an analogy with thermal conduction and convection. For this purpose an auxiliary heat transfer problem is solved. The macro-scale optimization procedure is studied for plate structures patterned by nastic materials in order to generate target bending and twist deformations. The results show the significant differences of the optimal distributions of active material depending on the strain model used for representing the actuation concept. The micro-scale vascular design methodology is verified with plane-stress examples. The results show that the layout of fluid channels can be optimized such that target strains are generated.

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