3D-Printing of Conductive Supramolecular Hydrogels and Eutectogels

Supramolecular gels formed from low molecular weight gelators (LMWGs) have emerged as potential soft materials for conductive applications due to their self-assembled networks formed through non-covalent interactions, chemical tunability and inherent stimuli responsive behaviour. Emerging applications in soft and bioelectronics, neuromorphic, and tissue-interfacing systems requires conductive pathways to be positioned with defined architectures, which can be achieved through three-dimensional (3D) printing. The limited literature on inducing conductivity into 3D-printed supramolecular gel systems is addressed in this project.
Herein, we report the 3D printing of conductive supramolecular gels using a wet-spinning technique that employs a solvent-switch mechanism to trigger gelation. Two LMWGs, DBS-CONHNH2 and DBS-COOH were used to print the gel patterns. The selective in situ reduction of Au(III) to form gold nanoparticles (AuNPs) by the hydrazide group in DBS-CONHNH2, which is absent in DBS-COOH, enabled the fabrication of multi-component hydrogel patterns with differential conductivity as a result of AuNP loading.
Ionic conductivity was subsequently introduced into both DBS-CONHNH2 and DBS-COOH systems using a deep eutectic solvent (DES) to print eutectogel patterns. The initial liquid phase comprises a mixture of DES and water, which further dried within the printed gel via evaporative loss of water to yield a stable DES-rich composition, resulting in stable high conductivities of ca. 5.0 mS/cm. DBS-CONHNH2 retained its ability to selectively reduce Au(III) to form AuNPs within the eutectic composition, and could even be loaded with AuNPs during the printing process itself. This demonstrates the consistency in retaining the molecular-scale functionality in a differing printing medium.
The ability to form reversible acylhydrazone bonds via the hydrazide group in DBS-CONHNH2 was next explored to cross-link self-assembled gel fibres using dialdehydes within printed patterns. The formation of acylhydrazone linkages was confirmed within the printed patterns and shown to alter both the mechanical and conductivity properties of the printed DBS-CONHNH2 eutectogels, indicating how the performance of these 3D-printed gels can be dynamically modified in situ.
Finally, attempts were made to introduce conductivity via fabrication of a conductive gel network by incorporating thiophene units into DBS-CONHNH2 fibres using 2,5-thiophene dicarboxaldehyde and subsequent doping using iodine vapour. Although thiophene units were successfully incorporated into the printed gel, iodine doping of these single thiophene systems did not lead to a selective enhancement in conductivity.