Nicolas 

Water-responsive (WR) materials undergo evaporation-induced mechanical deformations in response to changes in relative humidity (RH), thereby characterizing them as energy-conversion materials. Thus, WR materials can be harnessed for applications in soft robotics and clean energy generation. Currently, WR materials lack predictability due to complex evaporation-induced mechanical deformations; however, dipeptide crystal WR properties depend solely on peptide sequence, topology, and crystal-structure interactions, making them sufficiently simple to serve as a testbed for developing predictive design rules. In this research, our predictive design rule hypothesized that dipeptide crystals with higher peptide-water (PW) or water-water (WW) interactions than peptide-peptide (PP) interactions at 90% RH will exhibit stronger evaporation-induced mechanical deformations. To test this hypothesis, L-Phenylalanyl-L-alanine (FA), L-Valine-L-alanine (VA), and L-Leucine-L-alanine (LA) were co-crystallized with water. Optical light microscopy was used for crystal imaging; interactions were calculated using Mercury® Crystal Visualisation Software; PXRD (Powder X-ray diffraction) analysis was performed to investigate WR; and a composite of a dipeptide crystal and glue was deposited on Mylar film to assess dipeptide crystal energy density. Crystal analysis revealed that FA has greater PW interactions than PP, opposite to the trend observed in VA and LA. Based on PXRD analysis, our results support the hypothesis that only FA exhibits water responsiveness. However, after the Mylar Film tests, FA’s low energy density of 0.32 J/m³ raises questions about the completeness of our hypothesis. Nevertheless, this work is an essential step toward enabling low-cost renewable energy harvesting.