In recent times, numerous investigations have been conducted to explore the area of additive manufacturing (AM) to fabricate hierarchical surfaces with high surface area-to-volume (SA/V) ratios. Nevertheless, designing and fabricating structures with tunable SA/V ratios possessing desired functionalities remains challenging.

In this project, we have systematically established intricate correlations between the hierarchical surface structure geometry, SA/V ratio, functionality, and manufacturing feasibility using the two-photon polymerization (TPP) process. Drawing inspiration from many natural structures, our approach introduces a 3-level hierarchical design along with a mathematically derived model for the SA/V ratio. By incorporating geometric and manufacturing constraints, we have formulated a framework for creating three-dimensional hierarchical surfaces with remarkable geometric accuracy (>96%). Subsequently, we have developed a flowchart, which guides the design of the proposed surface structures, facilitating the realization of predefined functional targets, SA/V ratios, and geometric accuracy. A wide range of surfaces with varying SA/V ratios and hierarchy levels has been conceptualized and subsequently fabricated through the TPP process.

For a proof-of-concept study, the wetting characteristics and antireflection properties of the fabricated surfaces have been characterized. Evidently, the 3-level design exhibits a considerable degree of adaptability in tuning wetting and antireflection properties, achieved through strategic adjustments of design parameters and hierarchy levels. Remarkably, the proposed surface architecture showcases an inherent capacity to transform an initially hydrophilic surface (CA: 63°) into one displaying near-superhydrophobic behavior (CA: 138°). Furthermore, the 3-level design enables geometrical light-trapping effects, substantially elevating the antireflection efficacy to a noteworthy extent (>80% reduction in reflection).

These findings emphasize the immense potential of the proposed bio-inspired hierarchical surface structures across a diverse array of applications, including but not limited to microfluidics, optics, energy systems, and interfacial interactions.

Gecko feet-inspired hierarchical structures exhibiting stimuli-responsiveness, programmable and reversible adhesion, and adaptability to irregular surfaces have attracted substantial interest for diverse applications such as microgrippers and soft robots. Despite notable advancements in design and manufacturing, rapid shape transformation, non-contact control, and reversible switching between attachment and detachment modes remains challenging.

In this project, we present an innovative and rapid manufacturing methodology capable of fabricating locally-controlled particle distribution and gecko-inspired hierarchical surface features across multiple lengths. The localized material control enables precise shape morphing through external magnetic forces, while the inherent adaptability of the flexible hierarchical surface structures fosters swift attachment and detachment dynamics. This synergistic interplay of material and structural components enables reversible adhesive characteristics while eliminating the need for intricate surface treatments and external power sources.

To replicate gecko-like adhesive characteristics, we developed a photocurable magnetic composite material and examined the adhesion and magnetic-responsive attributes of the printed films. Two compelling test cases are illustrated to validate the effectiveness of the printed films: soft robots and untethered actuators. Firstly, we employed the printed films to create magnetic soft robots, investigating the impact of surface structure on adhesive properties and showcasing the resulting crawling locomotion of the robots, which exhibited a 15-fold increase in crawling speed compared to robots without surface structures. The second test case involves the development of a gripper designed for lifting and releasing objects. Here, we successfully demonstrate the ability of printed films to grasp objects of varying dimensions across diverse environments, encompassing both air and water.

Empirical results substantiate the efficacy of the fabricated gecko-inspired films and highlight the promising potential of proposed films, underlining their advantages in terms of programmable adhesion, locally tailored flexibility, rapid non-contact actuation, and reversible adhesion.