3D-Printed Photothermal Hydrogels with Gold Nanorods for Remote Actuation

The convergence of advanced 3D printing technologies and functional materials has paved the way for the creation of smart, responsive systems with potential applications spanning from biomedical engineering to soft robotics. Among these materials, hydrogels stand out due to their exceptional capacity for mimicking natural tissue properties, making them ideal for various applications including tissue engineering, drug delivery, and actuation systems. The integration of photothermal properties into hydrogels, facilitated by the incorporation of gold nanorods (GNRs), presents a promising avenue for developing remotely controllable, stimuli-responsive systems. This study, conducted by researchers from the University of North Carolina and the Leibniz Institute of Polymer Research Dresden, delves into the fabrication and functionality of 3D-printed hydrogels embedded with GNRs to act as photothermal actuators.

One of the primary challenges in creating effective 3D-printed photothermal actuators lies in achieving the right balance of material properties and structural integrity. Traditional hydrogels, while highly biocompatible and flexible, often lack the mechanical strength and stability required for intricate 3D printing. Additionally, embedding functional nanoparticles such as GNRs into these hydrogels without compromising their properties poses a significant challenge. The GNRs need to be uniformly distributed and well-integrated to ensure consistent photothermal response and prevent aggregation, which can adversely affect the material’s optical and mechanical characteristics. Moreover, the rheological properties of the hydrogel precursor solutions are crucial for successful 3D printing. Many thermoresponsive hydrogels like poly(N-isopropylacrylamide) (PNIPAAm) have precursor solutions that are too liquid-like, complicating the extrusion-based printing processes. Ensuring that the printed structures maintain their designed shape and functionality during and after the printing process is another critical hurdle.

New study published in Polymers and conducted by PhD candidate Melanie  Ghelardini, Professor Martin Geisler, Niclas Weigel, Jameson Hankwitz, Nicolas Hauck, Jonas Schubert, Andreas Fery, Joseph Tracy, and Professor Julian Thiele from the University of North Carolina and Leibniz Institute of Polymer Research Dresden addressed these challenges and advance the field of smart materials and 3D printing. The researchers aimed to develop a method for fabricating 3D-printed hydrogels that incorporate GNRs for photothermal actuation while maintaining structural integrity and mechanical robustness. By leveraging the unique properties of PNIPAAm and the photothermal capabilities of GNRs, the team sought to create a material system that can be remotely triggered to undergo shape changes, thus enabling applications in soft robotics, tissue engineering, and beyond. The study specifically explores a macromer-based approach for preparing PNIPAAm hydrogels, which is hypothesized to enhance the homogeneity and optical transparency of the material. Additionally, the use of a sacrificial support matrix during the 3D printing process was investigated to address the low viscosity of the hydrogel precursor solution, providing the necessary mechanical stability during printing and curing. Through this innovative approach, the researchers aimed to demonstrate the feasibility of creating complex, free-standing hydrogel structures capable of reversible shape changes upon photothermal stimulation.

The initial step involved the synthesis of GNRs using a cetyltrimethylammonium bromide (CTAB) stabilization method, yielding rods with dimensions of approximately 81 × 23 nm. To enhance their biocompatibility and stability within the hydrogel matrix, the GNRs were further functionalized with bovine serum albumin. The success of this functionalization was confirmed through transmission electron microscopy and optical extinction spectra, which showed a shift in the LSPR peak and a change in ζ-potential, indicating effective coating of GNRs with BSA. This functionalization step was crucial as it prevented the aggregation of GNRs and ensured their uniform dispersion within the hydrogel. Next, the team focused on preparing the hydrogel ink by combining the BSA-functionalized GNRs with a PNIPAAm macromer and a thioxanthone photosensitizer. The incorporation of the photosensitizer enabled photocrosslinking during the 3D printing process. A sacrificial support matrix made of acetylated gelatin microgel was used to provide the necessary mechanical stability for the low-viscosity ink during printing. This innovative approach allowed the researchers to overcome the challenge of ink spreading and deformation, ensuring the structural integrity of the printed hydrogels. During the printing process, the hydrogel ink was extruded into the support matrix, and simultaneous photocrosslinking was performed using UV light. The printed structures exhibited a crosshatched design with dimensions of 13 mm × 13 mm and a height of 3.6 mm, achieving a high level of detail and fidelity. Confocal fluorescence microscopy and bright-field microscopy images revealed clear and well-defined crosshatched patterns, indicating successful 3D printing and curing.   The photothermal properties of the printed hydrogels were tested by exposing them to NIR light, triggering the photothermal heating of the embedded GNRs. This exposure caused the hydrogels to transition from a swollen to a shrunken state as the temperature rose above the LCST of PNIPAAm. The extent of shrinkage was measured, with all samples shrinking to approximately 20-30% of their initial area. This significant and consistent shrinkage demonstrated the effectiveness of GNRs in enabling photothermal actuation. Compression testing was performed to assess the mechanical properties of the hydrogels. Cylindrical samples were prepared using the same ink and UV crosslinking conditions as the 3D-printed structures. The tests showed that the GNR-loaded hydrogels were mechanically softer than their unloaded counterparts due to the interference of UV light absorption during photopolymerization, which affected crosslinking density. The loaded samples exhibited a fracture strength of 22 kPa compared to 39 kPa for the unloaded samples, highlighting the impact of GNRs on the mechanical properties.

To compare photothermal and convective heating, the researchers conducted experiments where the printed hydrogels were subjected to both types of heating. During convective heating, the hydrogels were immersed in a water bath and heated above the LCST, causing them to shrink over a period of 5-8 minutes. For photothermal heating, NIR light was used to locally heat the GNRs, achieving similar shrinkage in a shorter time. Thermal imaging confirmed the rapid and localized heating capability of the GNRs, with consistent collapse occurring around 29 °C. The photothermal heating experiments demonstrated that even low concentrations of GNRs were sufficient for effective actuation, with higher loadings slightly increasing the extent of shrinkage. The consistency of the shrinkage across multiple cycles of heating and cooling highlighted the robustness of the photothermal actuation mechanism. Additionally, the BSA-GNRs remained well-dispersed and did not leach out of the hydrogels during repeated cycles, maintaining the material’s functionality and integrity.

An unexpected finding during the reswelling process was the observation of inward buckling in certain samples (OD 5 and OD 20) after photothermal heating. This buckling resulted in a sinusoidal ligament pattern, indicative of auxetic behavior, where the structures exhibited an effective negative Poisson ratio. This behavior was not observed in the unloaded samples or those subjected to convective heating alone. The researchers hypothesized that this buckling could be attributed to the non-uniform LED illumination during photothermal heating, causing localized stresses that led to this unique structural response.

The study represents a significant advancement in the field of smart materials and 3D printing, demonstrating the successful integration of gold nanorods within thermoresponsive hydrogels to create photothermal actuators. By incorporating GNRs into poly(N-isopropylacrylamide) (PNIPAAm) hydrogels, the researchers developed a material system that can undergo controlled shape changes upon exposure to near-infrared light. This integration enables precise, remote actuation, which is a substantial leap forward in the design of stimuli-responsive materials. The study showcases the use of direct ink writing combined with a sacrificial support matrix to fabricate complex, free-standing hydrogel structures. This approach overcomes the challenges associated with the low viscosity of hydrogel precursor solutions and ensures high-fidelity printing. The photothermal actuation of the 3D-printed hydrogels was consistent across multiple cycles of heating and cooling, demonstrating the robustness and reliability of the material system. The non-leaching behavior of GNRs further underscores the stability and durability of the hydrogels.

The ability to remotely control hydrogel actuators opens up new possibilities for minimally invasive medical devices, targeted drug delivery systems, and tissue engineering. For instance, these hydrogels could be used to develop soft robotic tools that can navigate and operate within the human body, guided by external NIR light sources. The photothermal actuators can be employed in soft robotics to create flexible, adaptive robots that can change shape and perform complex tasks in response to external stimuli. This capability is particularly useful in environments where traditional rigid robots are less effective. The study provides a blueprint for developing smart materials that can respond to environmental changes, such as temperature and light. These materials can be used in various sensors and actuators for applications in environmental monitoring, wearable technology, and beyond. The techniques developed in this study for 3D printing hydrogels with embedded functional nanoparticles can be extended to other material systems, paving the way for the creation of multifunctional materials with tailored properties for specific applications.

Through these experiments, the researchers successfully demonstrated the feasibility of creating 3D-printed hydrogels with embedded GNRs for photothermal actuation. The innovative use of a sacrificial support matrix, combined with precise control over the hydrogel ink composition and 3D printing parameters, enabled the fabrication of complex, free-standing structures capable of reversible shape changes. The findings highlight the potential of these materials for applications in soft robotics, biomedical devices, and other fields requiring remote-controlled actuation and stimuli-responsive behavior.