require external stimuli such as high electric fields, or high temperatures and pressures. [1,2] These materials can be driven by perturbations to the ambient humidity or with natural fluctuations. [7,8] A variety of materials, such as graphene oxide, [9][10][11][12] polymers, [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] liquid crystals, [14,29,30] paper, [31,32] and biologically based materials [6][7][8][33][34][35][36] can serve as hygroscopic actuators. These actuators exhibit curling and folding, [10,14,19] walking, [17,21,24,37] gripping, [18] and power generation functions. [7,8,38] Despite the potential of humidityresponsive materials, relatively low work densities and slow responses limit the scope of applications. Studies of hygroscopic properties of Bacillus spores have shown that these dormant microorganisms exhibit actuation capabilities, with energy densities up to about 20 MJ m −3 , which raise the possibility of using spores as building blocks of macroscopic actuators. [8] However, assembly of spores into larger materials while retaining their actuation performance presents challenges. In particular, because spores do not adhere strongly to each other, approaches that strengthen spore-spore interactions are needed. Therefore, initial demonstrations of spore-based actuators used mixtures of spores with water soluble adhesives. These mixtures were then deposited onto plastic films to create various actuators and energy harvesting devices. [7] However, the resulting materials exhibited low work density and specific work in comparison to individual spores. Importantly, while water soluble adhesives simplify the fabrication process, they are prone to degradation when placed in direct contact with water, making it difficult to use in waterdriven applications. Therefore, different approaches are needed to enhance the work density and robustness of spore-based materials.Here, we report spore-based actuators that use waterresistant UV-curable adhesives and achieve work densities up to 0.44 MJ m −3 , which is an order of magnitude higher than the synthetic humidity-responsive polymers. [22][23][24][25] The water resistance of the spore-based actuators allowed direct waterdriven actuation, which reduced the response times by nearly 100-fold. We also leveraged the UV curability of these adhesives to facilitate rapid fabrication of complex dynamic and/or adaptive structures using photolithography.Bacillus subtilis spores exhibit hygroscopic actuation capabilities (Figure 1c) and have shown high energy densities Active materials and surfaces that are less intrusive and more compatible with humans and their environment are critical for robotic applications. Humidity-and water-responsive materials are emerging as versatile alternatives to commonly used actuators in robotic systems, due to ease of actuation with humidity gradients and pervasiveness of water in the environment. However, these materials exhibit relatively low work densities and slow responses, and they are degraded when placed in ...