Multifunctional 3D-Printed Pollen Grain-Inspired Hydrogel Microrobots for On-Demand Anchoring and Cargo Delivery.

While a majority of wireless microrobots have shown multi-responsiveness to implement complex biomedical functions, their functional executions are strongly dependent on the range of stimulus inputs, which curtails their functional diversity. Furthermore, their responsive functions are coupled to each other, which results in the overlap of the task operations. Here, a 3D-printed multifunctional microrobot inspired by pollen grains with three hydrogel components is demonstrated: iron platinum (FePt) nanoparticle-embedded pentaerythritol triacrylate (PETA), poly N-isopropylacrylamide (pNIPAM), and poly N-isopropylacrylamide acrylic acid (pNIPAM-AAc) structures. Each of these structures exhibits their respective targeted functions: responding to magnetic fields for torque-driven surface rolling and steering, exhibiting temperature responsiveness for on-demand surface attachment (anchoring), and pH-responsive cargo release. The versatile multifunctional pollen grain-inspired robots conceptualized here pave the way for various future medical microrobots to improve their projected performance and functional diversity.

Enhanced Flexible Mold Lifetime for Roll-to-Roll Scaled-Up Manufacturing of Adhesive Complex Microstructures.

Various functional complex 3D patterned surfaces with micro- or nanostructures have been developed and their superior performances over non-patterned smooth surfaces proven. However, it is challenging to mass-produce such complex micro-/nanopatterned surfaces, which limits their commercialization drastically. Although roll-to-roll (R2R) manufacturing using flexible molds has been implemented for mass-production of such functional surfaces, the poor mold repeatability issue has not been resolved yet. Here, a strategy to significantly improve the repeatability of the micropatterned flexible silicone molds over 1000 cycles against highly adhesive polyurethane acrylates (PUAs) in UV light curing based R2R systems by using a two-step curing process is reported. The mold repeatability is drastically increased from 10s of cycles to over 1000 cycles through the proposed strategy in spite of the complicated 3D undercut geometry and high tackiness of the microstructure. This two-step process would enable scaled-up production of micro-/nanostructured adhesives, such as gecko-inspired microfiber adhesives as demonstrated in this study, as well as various other functional micro-/nanostructured surfaces by enhancing the flexible mold lifetime.

Wireless soft millirobots for climbing three-dimensional surfaces in confined spaces.

Wireless soft-bodied robots at the millimeter scale allow traversing very confined unstructured terrains with minimal invasion and safely interacting with the surrounding environment. However, existing untethered soft millirobots still lack the ability of climbing, reversible controlled surface adhesion, and long-term retention on unstructured three-dimensional (3D) surfaces, limiting their use in biomedical and environmental applications. Here, we report a fundamental peeling-and-loading mechanism to allow untethered soft-bodied robots to climb 3D surfaces by using both the soft-body deformation and whole-body motion of the robot under external magnetic fields. This generic mechanism is implemented with different adhesive robot footpad designs, allowing vertical and inverted surface climbing on diverse 3D surfaces with complex geometries and different surface properties. With the unique robot footpad designs that integrate microstructured adhesives and tough bioadhesives, the soft climbing robot could achieve controllable adhesion and friction to climb 3D soft and wet surfaces including porcine tissues, which paves the way for future environmental inspection and minimally invasive medicine applications.

Amplification factor in shear-activated adhesives: effect of elasticity.

In contrast to common adhesives used in joining, directional adhesives are designed for a dynamic attachment/detachment operation and, hence, their performance is assessed not only in an activated mode but also in a disactivated mode. Consequently, in addition to a peak adhesive strength, a ratio of maximum over minimum attachment force, which can be called a shear-driven amplification factor, is utilized for their characterization. Considering that the peak adhesive response and the amplification factor both depend on the elasticity of the microstructured surface, here we report a combined numerical and experimental study of the effect of the Young's modulus on the attachment properties of flap-shaped contact elements drawn from polyurethane. Our results demonstrate that while the activated and disactivated pull-off forces decrease, the amplification factor increases as the Young's modulus increases. Given that both the activated pull-off force and the amplification factor should be maximized for the best performance, the Young's modulus has to be carefully chosen based on the characteristics of the objects to be manipulated.

Drawing-Based Manufacturing of Shear-Activated Reversible Adhesives.

Although biomimetic technologies for dry reversible adhesion seem to be maturing, the costs, complexity, and time expenditures associated with the current template-based molding techniques call for research on other fabrication methods. In this paper, we report a novel cost-effective, simple, and flexible drawing-based technique for manufacturing the soft elastomeric thin-film-based microstructures needed for successful implementation of the principles of biological shear-activated adhesion. Several different types of adhesive microstructures are fabricated, and the best of them demonstrate shear-driven amplification of pull-off force by a factor of 40, which significantly outperforms known molded analogues. A simple gripper based on a manual center-clamping vise is shown to enable pick-and-place manipulation of various flat and curved objects of everyday use.

Contact splitting in dry adhesion and friction: reducing the influence of roughness.

Splitting a large contact area into finer, sub-contact areas is thought to result in higher adaptability to rough surfaces, stronger adhesion, and a more uniform stress distribution with higher tolerance to defects. However, while it is widely believed that contact splitting helps to mitigate the negative effects of roughness on adhesion- and friction-based attachment, no decisive experimental validation of this hypothesis has been performed so far for thin-film-based adhesives. To this end, we report on the behavior of original and split, wall-shaped adhesive microstructures on different surfaces ranging across four orders of magnitude in roughness. Our results clearly demonstrate that the adhesion- and friction-driven attachment of the wall-shaped microstructure degrades, regardless of the surface waviness, when the surface roughness increases. Second, splitting the wall-shaped microstructure indeed helps to mitigate the negative effect of the increasing surface unevenness by allowing the split microstructure to adapt more easily to the surface waviness and by reducing the effective average peeling angle. These findings can be used to guide the development of biomimetic shear-actuated adhesives suitable for operation not only on smooth but also on rough surfaces.

Biomimetic wall-shaped adhesive microstructure for shear-induced attachment: the effects of pulling angle and preliminary displacement.

To date, a handful of different gecko-like adhesives inspired by spatula-shaped attachment hairs have been suggested based on wedge and flap geometry of contact elements. However, while these surface designs have been shown to have directionality in adhesion, high friction, long lifetime and the ability to work in vacuum, an experimental verification of the very basic concept of the pulling angle effect has not yet been reported. To close this gap, here we use wall-shaped adhesive microstructures of three different flap heights to systematically study the effect of pulling angle on the normal and tangential components of the pull-off force tested at different preliminary tangential displacements. In accord with the prediction of the Kendall model for the normal component of peeling force, there is an optimal normal force that is required to detach the wall-shaped adhesive microstructure. The optimum is obtained at about half the distance needed to initiate sliding and at pulling angles that range within 60-90°, which suggests that the wall-shaped microstructure can tolerate relatively large inaccuracies in the loading direction. The increase of the attachment force with increasing flap height is found to correlate with the flap thickness, which decreased with increasing flap height.

Rough-Surface-Enabled Capacitive Pressure Sensors with 3D Touch Capability.

Fabrication strategies that pursue "simplicity" for the production process and "functionality" for a device, in general, are mutually exclusive. Therefore, strategies that are less expensive, less equipment-intensive, and consequently, more accessible to researchers for the realization of omnipresent electronics are required. Here, this study presents a conceptually different approach that utilizes the inartificial design of the surface roughness of paper to realize a capacitive pressure sensor with high performance compared with sensors produced using costly microfabrication processes. This study utilizes a writing activity with a pencil and paper, which enables the construction of a fundamental capacitor that can be used as a flexible capacitive pressure sensor with high pressure sensitivity and short response time and that it can be inexpensively fabricated over large areas. Furthermore, the paper-based pressure sensors are integrated into a fully functional 3D touch-pad device, which is a step toward the realization of omnipresent electronics.