Biological materials achieve directional reinforcement with oriented assemblies of anisotropic building blocks. One such example is the nanocomposite structure of keratinized epithelium on the toe pad of tree frogs, in which hexagonal arrays of (soft) epithelial cells are crossed by densely packed and oriented (hard) keratin nanofibrils. Here, a method is established to fabricate arrays of tree-frog-inspired composite micropatterns composed of polydimethylsiloxane (PDMS) micropillars embedded with polystyrene (PS) nanopillars. Adhesive and frictional studies of these synthetic materials reveal a benefit of the hierarchical and anisotropic design for both adhesion and friction, in particular, at high matrix–fiber interfacial strengths. The presence of PS nanopillars alters the stress distribution at the contact interface of micropillars and therefore enhances the adhesion and friction of the composite micropattern. The results suggest a design principle for bioinspired structural adhesives, especially for wet environments.

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The contribution of water to gecko and gecko-inspired adhesion remains a debate. Here, by investigating the adhesion performances of gecko-inspired nanopillar arrays in humid environments, the function of water in dry adhesion is discussed. Adhesion increases with the increase of relative humidity for both hydrophilic and hydrophobic nanopillar arrays. The superficial layer of both kinds of nanopillars are softened by water, forming a kind of “soft shell–stiff core” structure. The core–shell structure reduces the stress at contact perimeter, enlarges the cohesive zone, and increases the tolerance to misalignment on contacting surfaces, contributing to the enhancement of normal adhesion. The result suggests a mechanism for the function of water in gecko and gecko-inspired dry adhesions.

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Materials and devices with tunable dry adhesion have many applications, including transfer printing, climbing robots, and gripping in pick-and-place processes. In this paper, a novel soft device to achieve dynamically tunable dry adhesion via modulation of subsurface pneumatic pressure is introduced. Specifically, a cylindrical elastomer pillar with a mushroom-shaped cap and annular chamber that can be pressurized to tune the adhesion is investigated. Finite element-based mechanics models and experiments are used to design, understand, and demonstrate the adhesion of the device. Specifically, the device is designed using mechanics modeling such that the pressure applied inside the annular chamber significantly alters the stress distribution at the adhered interface and thus changes the effective adhesion strength. Devices made of polydimethylsiloxane (PDMS) with different elastic moduli were tested against glass, silicon, and aluminum substrates. Adhesion strengths (σ0) ranging from ∼37 kPa (between PDMS and glass) to ∼67 kPa (between PDMS and polished aluminum) are achieved for the nonpressurized state. For all cases, regardless of the material and roughness of the substrates, the adhesion strength dropped to 40% of the strength of the nonpressurized state (equivalent to a 2.5× adhesion switching ratio) by increasing the chamber pressure from 0.3σ0 to 0.6σ0. Furthermore, the strength drops to 20% of the unpressurized strength (equivalent to a 5× adhesion switching ratio) when the chamber pressure is increased to σ0.

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Recently, a novel concept to realize dynamically tunable dry adhesion via subsurface stiffness modulation (SSM) in a composite core–shell structure has been introduced and demonstrated for gripping and release of objects. Here, a variant form of the composite core–shell design is proposed to significantly improve the performance of dynamically tunable dry adhesion in terms of activation time and activation voltage. Specifically, composite pillars with an embedded microfluidic channel filled with a low melting point alloy (LMPA) are fabricated, and the adhesion of the pillars is characterized as a function of LMPA state: either melted or solid. The effects of the thickness and in‐plane pattern of the LMPA channel, as well as the depth at which it is embedded on tunable adhesion are investigated. Experiments show that the effective adhesion strength can be reduced up to 50%, equivalent to a 2× change in dry adhesion when the LMPA is melted. Finite element analysis of the stress distribution change under SSM shows that the experimentally observed tunable adhesion is primarily due to stiffness change close to the interface. In addition, two technology demonstrations of composite pillars picking and releasing objects with fast activation (≈1 s) and low activation voltages (≈1 V) are included.

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