Rational engineering of composite micro structures for advanced dry adhesion system

Abstract

In this thesis, we describe the bio-inspired design and fabrication methods to mimic scalable hierarchical structure in Nature and exploited 3 representative structures

lotus life, geckos feet and extra cellular matrix (ECM). Multiscale, hierarchically patterned surfaces such as lotus leaves, butterfly wings, adhesion pads of gecko lizards are abundantly found in nature, where microstructures are usally used to strengthen the mechanical stability while nanostructures offer the main functionality, i.e., wettability, structural color, or dry adhesion. To emulate such hierarchical structures in nature, multiscale, multilevel patterning has been extensively utilized for the last few decades towards various applications ranging from wetting control, structural colors, to tissue scaffolds. In this thesis, we suggested simple yet robust fabrication method to scalable multiscale patterning to bring about improved functions that can even surpass those found in nature, with particular focus on the analogy between natural and synthetic architectures in terms of the role of different length scales. First, we present here an enhanced dry adhesive skin patch with composite micropillars

the stem region of the pillars is formed by a relatively rigid material like hard polydimethylsiloxane (h-PDMS) (Youngs modulus: ~8.2 MPa) or PDMS with a higher amount of curing agent, e.g., 15% (Youngs modulus: ~2.8 MPa). The top layer is additionally integrated by transferring a soft PDMS layer with a lower amount of curing agent, e.g., 5% (Youngs modulus: ~0.8 MPa). In this way, monolithically integrated composite PDMS micropillars can be prepared with better adhesion strength and durability. Next, we present a simple method of fabricating robust dry adhesives by coating a soft polydimethyl siloxane (PDMS) thin layer on rigid backbone micropillars of polyurethane acrylate (PUA). These core-shell type micropillars demonstrated enhanced durability both in normal and shear adhesion over more than 100 cycles of attachment and detachment. Relatively strong normal (~11.4 N/cm2) and shear (~15.3 N/cm2) adhesion forces were observed, which were similar to or even larger than those of homogeneous PDMS micropillars. A simple theoretical model based on beam deflection theory was used to explain the experimental results. Finally, Inspired from exceptional climbing ability of gecko lizards, rtificial fibrillar adhesives have been extensively studied over the last decade both experimentally and theoretically. Therefore, a new leap towards practical uses beyond the academic horizon is timely and highly anticipated. To this end, we present a fibrillar adhesive in the form of bridged micropillars and its application to a transportation system with the detachment mechanism inspired by the climbing behaviour of gecko lizards. The adhesive shows strong normal attachment (~30 N/cm2) as well as easy and fast detachment within 0.5 sec without involving complex dynamic mechanisms or specific stimulus-responsive materials. The fabrication of the bridged micropillars consists of replica moulding of polydimethylsiloxane (PDMS) micropillars, transfer of the PDMS precursor to the heads of micropillars, and inverse placement on an inert Teflon-coated surface. Owing to spontaneous interconnections of low viscosity PDMS precursor, bridged micropillars with a uniform capping nanomembrane (~800 nm thickness) are formed over a large area. Interestingly, macroscopic adhesion in normal direction can be immediately switched between on and off states by changing the two detachment modes of pulling and peeling, respectively. To prove the potential of the fibrillar adhesive for practical use, an automated transportation system is demonstrated for lifting and releasing a mass of stacked glass slides over 1000 cycles of attachment and detachment.