Sequential Coupling of Functions in Hydrogels Enables Shape-Memory Hydrogels with pH, Enzyme- or an Inverse Temperature Sensitivity

The integration of functions in materials to gain macroscopic effects in response to environmental changes is an ongoing challenge in material science. When functions on different hierarchical levels are sequentially linked by combining a suitable polymer morphology in combination with a proper microstructuring of the macroscopic device, the processes occurring on the molecular level can be translated to the macroscopic level to induce directed movements in hydrogels.<br/>Here we describe microporous hydrogel systems based on covalently cross-linked polymer networks, which are capable to respond to the change of pH, the presence of enzymes or a decrease of temperature with a macroscopic shape shift. The micro-porosity permitted swelling into the pores and thus reduced volumetric changes on a macroscopic level, which otherwise could interfere with the shape shift. The translation of the presence of protons on the molecular level to the macro level was achieved by implementing lysine-rich peptide molecules, which change their conformation into a b-hairpin structure due to the reduced electrostatic repulsion amongst deprotonated amino groups when the pH is increased.^[1] Coupled to this conformation change is the capability of the b-hairpin motifs to subsequently assemble into aggregates acting as reversible crosslinks, which are used as controlling units to fix a temporary macroscopic shape. When such controlling units are equipped with enzyme-specific cleavable bonds, a transformed shape is achieved by the release of internal stresses because of the enzymatic reaction.^[2] In this case the increase in entropy goes along with a swelling-supported stretching of polymer chains within the microarchitectured hydrogel. <i>Vice versa</i>, the directed formation of micelles in hydrogels based on covalently crosslinked oligo(ethylene glycol)-oligo(propylene glycol)-oligo(ethylene glycol) (OEG-OPG-OEG) segments upon heating is capable to fix a deformation. Upon cooling, the micelles dissociate again, the deformation is reversed and the permanent shape is obtained. In this way an inverse shape-memory effect (iSME) can be obtained.<br/>In this presentation the challenges in the coupling of function are discussed and examples are provided for potential application of such multifunctional hydrogels ranging from sensors to scaffolds for tissue reconstruction.<br/><b>References</b><br/>Zewang You, Marc Behl, Stephan L. Grage, Jochen Bürck, Qian Zhao, Anne S. Ulrich, and Andreas Lendlein <i>Biomacromolecules</i> <b>2020</b> <i>21</i> (2), 680-687.<br/>Maria Balk, Marc Behl, Ulrich Nöchel, and Andreas Lendlein <i>ACS Applied Materials & Interfaces</i> <b>2021</b> <i>13</i> (7), 8095-8101.<br/>Lucile Tartivel, Anna M. Blocki, Steffen Braune, Friedrich Jung, Marc Behl, and Andreas Lendlein, Advanced Material and Interfaces <b>2021</b>, 2101588 DOI: 10.1002/admi.202101588.