Micropillar with Radial Gradient Modulus Enables Robust Adhesion and Friction.

The gradient modulus in beetle setae plays a critical role in allowing it to stand and walk on natural surfaces. Mimicking beetle setae to create a modulus gradient in microscale, especially in the direction of setae radius, can achieve reliable contact and thus strong adhesion. However, it remains highly challenging to achieve modulus gradient along radial directions in setae-like structures. Here, polydimethylsiloxane (PDMS) micropillar with radial gradient modulus, (termed GM), is successfully constructed by making use of the polymerization inhibitor in the photosensitive resin template. GM gains adhesion up to 84 kPa, which is 2.3 and 4.7 times of soft homogeneous micropillars (SH) and hard homogeneous micropillars (HH), respectively. The radial gradient modulus facilitates contact formation on various surfaces and shifts stress concentration from contact perimeter to the center, resulting in adhesion enhancement. Meanwhile, GM achieves strong friction of 8.1 mN, which is 1.2 and 2.6 times of SH and HH, respectively. Moreover, GM possesses high robustness, maintaining strong adhesion and friction after 400 cycles of tests. The work here not only provides a robust structure for strong adhesion and friction, but also establishes a strategy to create modulus gradient at micron-scale.