(Nanowerk Spotlight) Organogels, polymer networks infused with organic liquids, have long tantalized scientists with their potential for diverse applications due to the wide range of properties achievable by combining different polymer networks, solvents, and 3D shapes. However, progress has been hampered by the inherent limitations of current fabrication methods, which restrict solvent choice and composition, thereby constraining the spectrum of properties, applications, and innovation possible with these unique materials.
Historically, the fabrication of organogels has been largely confined to films, coatings, and bulk nanostructured gels. While recent advances have allowed some structure through photomask irradiation to create liquid channels, gel photoresists, and even reversible holograms, the incorporation of 3D printing has remained elusive. The primary obstacle has been the incompatibility of many organic solvents with existing 3D printing technologies, especially when high solvent content is required, as this results in slow cross-linking and reduced mechanical strength.
Now, a team of researchers from the Karlsruhe Institute of Technology in Germany has developed an innovative method to overcome these challenges and unlock the full potential of 3D-printed organogels. In a paper published in Advanced Functional Materials (“Solvent-Independent 3D Printing of Organogels”), the team presents a universal, tunable approach for solvent-independent 3D printing of organogel structures using digital light processing (DLP).
Solvent-independent 3D printing of organogels with preserved control over properties. 3D-printing provides free choice of shape and rapid production, but is limited in the choice of solvent. Printing with a sacrificial solvent and with subsequent swelling in the solvent of interest allows solventindependent fabrication of 3D organogels with a wide range of functionalities, such as extreme thermal stability and response, and enhanced surface properties. (Image: Reprinted from DOI:10.1002/adfm.202403694, CC BY)
The key innovation lies in decoupling the printing process from the choice of solvent. The researchers first 3D print a base organogel structure using a non-volatile sacrificial mineral oil. This structure can then be infused with the desired organic solvent through a post-printing solvent exchange and swelling step. This elegant approach allows high solvent content to be achieved in a solvent-independent manner without compromising print resolution.
Using this method, the team demonstrated the ability to create complex 3D organogel structures with feature resolutions down to 40 μm, the single mirror size of their DLP system, which is comparable to typical DLP 3D printing of hydrogels. By reducing the crosslinker content or increasing the solvent-to-monomer ratio in the initial ink, they could further tune the swelling ratio and therefore the final solvent content, achieving up to 90% liquid fractions.
Remarkably, the researchers discovered that by simply varying the infused solvent, they could dramatically alter the properties of the organogel while keeping the 3D geometry and polymer network unchanged. For example, swelling with hydrophobic solvents like medium-chain alcohols, toluene, and oils transformed a highly adhesive polymer network into an extremely slippery surface. Temperature-dependent rheology measurements revealed that the choice of solvent could modulate the thermal stability and mechanical properties.
Organogels swollen with n-hexadecane (melting point 18 °C) exhibited a rapid thermo-responsive switch from soft viscoelastic to stiff behavior at the solvent’s melting point, while those swollen with a mineral oil (melting point ≈ −18 °C) remained flexible down to −15 °C. The lowest thermal stability limit was observed for organogels swollen with butyl disulfide (melting point −94 °C), which remained stable down to ≈−28 °C.
These findings highlight the immense and previously untapped influence of solvent selection on organogel functionality. By granting access to the full diversity of organic solvents, this 3D printing method substantially expands the design space and potential applications for these versatile materials.
Furthermore, the solvent-swelling process was found to significantly improve the surface smoothness of the printed organogels, reducing surface roughness from ≈5.4 µm to less than 1 µm. This simultaneously enhanced the optical transparency of the gels, with the maximum absorbance dropping from 0.41 to 0.03 a.u. in the 400–700 nm range, pointing to swelling as a potential post-processing technique to ameliorate printing artifacts in DLP-printed viscoelastic materials.
The findings of this work suggest that solvent-independent 3D printing could have far-reaching implications beyond the already substantial achievement of fabricating organogels with high solvent content. By enabling precise control over mechanical, surface, and thermal properties through solvent selection, this approach opens new avenues for application-specific optimization of organogels.
The tunable thermo-mechanical characteristics are especially relevant for fields such as soft robotics, where the ability to 3D print actuators and sensors with programmable responses could enable more sophisticated and adaptive systems.
Moreover, the ability to prefabricate standard organogel geometries for on-demand property modification through solvent infusion presents intriguing possibilities for streamlining manufacturing and minimizing waste. This is particularly advantageous for leveraging solvents that are incompatible with direct 3D printing or pose health and safety concerns during handling.
The groundbreaking work by Kuzina et al. represents a major stride forward in the development of functional organogels and additive manufacturing. By unlocking solvent-independent 3D printing and demonstrating the profound impact of solvent selection on organogel properties, they have laid the foundation for a new era of innovation in this exciting domain. As these novel materials and fabrication techniques continue to evolve, we can anticipate a proliferation of 3D organogel structures with finely tuned properties, empowering a broad array of previously inaccessible applications across diverse fields such as soft robotics, sensors, actuators, and beyond.
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