4D Printing of Programmable Structures with Triboelectric Sensing Capabilities

Shape morphing is a natural phenomenon in living organisms critical for their survival by adapting to an ever-changing environment. Ever since the research community is attempting to create such bio-inspired manmade materials in response to external stimuli like moisture and light. Shape changes are governed by intricate mechanisms involving the formation of gradients and anisotropic swelling within the organism. However, such mechanodynamic behaviour has never been applied to develop a sensing platform.

The proposed work aims to employ 4D printing technology to fabricate a novel spatially graded multi-component hydrogel system. IIT Madras team will prepare two gels by varying the composition of alginate (Alg) and methylcellulose (MC) with different swelling properties, which will be printed over each other via multi-material extrusion-based 3D printing. As prepared films will be placed on a droplet that will undergo shape changes in response to ionic crosslinking (Ca2+). When the droplet comes in contact with the film unilaterally, differential strain within the structure will facilitate diffusion, due to the presence of sparsely and densely cross-linked regions as designed in the films. For the development of a chemo-mechanical sensing platform, these hydrogel inks will be modified with ionophores to create binding sites in the printed films for the detection of physiologically relevant ions in biofluids. Upon contact, ions diffuse across the film, it will cause bending, as a result, contact electrification will take place at the solid-liquid interface owing to the differential charge transfer between the film and water droplets. As the ionic droplet interacts across the triboelectric film sensor surface, the surface charge induced will be balanced with the charges formed in the ionic droplet due to electrical double-layer formation. Subsequently, the competitiveness between the surface charge and the double-layer charge will generate a characteristic output voltage peak which will serve as a sensing signal for the ions. The charge generated will be collected through the microelectrode array patterned at the edges of the film by the Deakin’s team. In addition to electrical output, bending angle is another signal which can be correlated with ion concentration.

IITM team will investigate the mechanics of in-plane and out-of-plane bending and other complex deformation of these multilayer systems subjected to different system inputs using a combination of experimental and theoretical techniques. Using high-speed imaging, we will quantify the dynamics of shape change in the films and use these data raw data to develop analytical models based on Timoshenko’s bilayer beam theory to predict the changes in curvature that depend on the stiffness of each gel, their swelling behaviour as well as geometrical factors such as gel thickness. These results will be used to develop multiscale computational models for the 4D printed soft actuators with complex geometries to predict their deformations and mechanical behaviour under various physical stimuli that mimic real-life conditions thereby furthering the applicability of these systems for sensing and structural health monitoring.

The multidisciplinary nature of the proposed work leads to the initiation of this collaborative effort between IIT Madras towards the successful development of chemo-mechanical sensing platform using 4D films for essential biological indicators in biofluids, which may offer exciting possibilities for real-time health monitoring.;;

Objectives;

1. To design, fabricate, and engineer a multi-component hydrogel system with differential swelling gradients and characterize their structural, rheological, and mechanical properties.;

2. To investigate 4D effects, including real-time moisture-responsive, unidirectional/bidirectional, reversible shape-morphism, and solvent-responsive behaviours. ;

3. Deposition and patterning of microelectrode array onto the 3D printed dynamic films to enable charge generation and collection via solid-liquid contact electrification.;

4. To develop a chemo-mechanical sensing platform based on solid-liquid contact electrification (triboelectric effect) at the 4D film-ionic droplet interphase by modifying the polymeric film with ionophores for specific binding of target ions present in the biofluid.;

5. Utilize advanced image processing techniques and data analysis methods to quantify the dynamics of shape evolution during in and out of plane bending of the 4-D films and correlate them with their mechanical and morphological parameters like gel stiffness, porosity, substrate thickness, using analytical and computational mechanics (FEA) based models.;