Hydrogels with quadruple-shape capability


The increase in the number of shape shifts for temperature‐sensitive shape‐memory materials enabling complex movements addresses a requirement from applications. Hydrogels are an interesting material category according to their ability to enable diffusion processes through the material and their tissue‐like mechanical properties. In contrast to non‐swollen systems, macroscopic movements in hydrogels are limited by the applicable temperature range (phase transitions of water). So far, only two different shape shifts could be implemented in hydrogels by means of two types of crystallizable side chains. Here, we explored whether quadruple‐shape hydrogels enabling three shape transformations could be created. A temperature independent swelling capacity in the hydrophilic polymer network is required to ensure that directed movements are not related to swelling effects. Semi‐crystalline oligomeric side chains were introduced in the hydrogel matrix by monomethacrylated oligo esters via radical copolymerization. A demonstrator device was prepared using a casting mold fabrication by 3D printing utilizing computer supported design. Polymer network forming components were copolymerized within the mold, which was subsequently dissolved in water. The demonstrator object was obtained after equilibrium swelling of the copolymer network in water. Three directed movements were successfully realized when the temperature of the hydrogel system was increased from 5 °C to 90 °C resulting in an overall recovery ratio related to the original shape above 90%. Accordingly, a quadruple‐shape effect triggered by heat as stimulus as new record for complex movements in hydrogels was realized. For this system, the applicable temperature range was limited by water as swelling media, distinctly separated thermal transitions were necessary, and the overall elasticity indispensable for consecutive deformations was reduced as result of partially chain segment orientation induced by swelling in water. Conclusively, the challenges for more complex macroscopic shape shifts (e.g. penta‐ or hexa‐shape) hydrogels are demanding for material systems providing higher elastic deformability and enabling distinct shape shifts within narrow temperature ranges.
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