Layer Jamming Enhanced by Micro-Structured Biomimetic Adhesive Sheets for Soft Robotics Applications

Layer jamming, in which the shape and the stiffness of a micro-structured sheets is modulated by regulating the friction force between layers, is a promising technology in the field of stiffness-tunable soft robotics. Biomimetic dry adhesive technologies utilizing the distinctive microstructures found on a gecko's foot, have been applied in applications ranging from climbing robots, pick and place assembly, skin-adhesives, and microfluidics. The biomimetic dry adhesive technology can be utilized to augment the friction force that induces layer jamming in reversible manner without the need of vacuum.<br/>In this project, we create dry adhesive sheets employing Kraton G1645 - a styrene-ethylene-butylene-styrene (SEBS) thermoplastic elastomer. G1645 is used as the primary structural material and then modified by blending different weight percentages of polypropylene (PP) which is miscible and lower cost than SEBS. The blend is then used for micro-molding using a modified recipe of our previously reported compression molding processes. <br/>The Setex gecko adhesive materials (DA910B) has proven to be a promising candidate for jamming applications, demonstrating favorable results as a layer jamming actuator in prior studies. While the material composition remains proprietary, it seems to be a relatively viscoelastic cured polyurethane. Our objective is to transition from the proprietary Setex adhesive to a more cost-effective and customizable alternative by employing a blend of SEBS (a recyclable, easy process and low-cost thermoplastic elastomer) and PP. Despite SEBS exhibiting commendable dry adhesive properties and strength on smooth surfaces which makes it an excellent reversible adhesive, challenges arise due to its low melt flow index, hindering the production of ultra-thin sheets through micro-molding techniques. Furthermore, its strength may not be sufficient to endure numerous cycles of attachment and detachment, as it is prone to mechanical tearing in low thickness and deformation of micro features under shear stress during detachment processes. Polypropylene (PP) as a cost-effective thermoplastic offers chemical compatibility for blending, and may increase the mechanical stability of individual fibers, allowing taller fibers to space out jammed layers which should in theory increase the effectiveness of the sheets at stiffness switching. Previous reports have also used PP microfibers for self-cleaning and directional adhesion properties which inspires the idea of using it in a blend with SEBS to improve mechanical properties and preserve adhesion ability of SEBS simultaneously.<br/>The manufacturing process of gecko adhesive layers with a mushroom-shaped microstructure (the same structure as the Setex commercial type) involves the hot embossing technique using negative PDMS molds replicated from Setex masters. The SEBS/PP blends are produced using a single-screw extruder and then, using a hydraulic hot press, compressed into thin sheets with approximately 60µm thickness. The SEBS/PP blends are compared to pure SEBS using microscale tests based on JKR models to measure the adhesion properties of the samples. Additionally, mechanical tests are conducted to provide a complete comparison with commercial samples and to identify the optimum composition of SEBS/PP for maximum repeatability and longevity of the adhesive layers. The experimental results demonstrate good compatibility between SEBS and PP and the adhesive layers can be thermally bonded to thin PP films to complete a jamming actuator. These research results hold promising implications for the large-scale production of low-cost materials suitable for various applications, especially as stiffness tunable actuator in soft robotics.