Temperature-Induced Switchable Adhesion using Nickel-Titanium-Polydimethylsiloxane Hybrid Surfaces.

A switchable dry adhesive based on a nickel-titanium (NiTi) shape-memory alloy with an adhesive silicone rubber surface has been developed. Although several studies investigate micropatterned, bioinspired adhesive surfaces, very few focus on reversible adhesion. The system here is based on the indentation-induced two-way shape-memory effect in NiTi alloys. NiTi is trained by mechanical deformation through indentation and grinding to elicit a temperature-induced switchable topography with protrusions at high temperature and a flat surface at low temperature. The trained surfaces are coated with either a smooth or a patterned adhesive polydimethylsiloxane (PDMS) layer, resulting in a temperature-induced switchable surface, used for dry adhesion. Adhesion tests show that the temperature-induced topographical change of the NiTi influences the adhesive performance of the hybrid system. For samples with a smooth PDMS layer the transition from flat to structured state reduces adhesion by 56%, and for samples with a micropatterned PDMS layer adhesion is switchable by nearly 100%. Both hybrid systems reveal strong reversibility related to the NiTi martensitic phase transformation, allowing repeated switching between an adhesive and a nonadhesive state. These effects have been discussed in terms of reversible changes in contact area and varying tilt angles of the pillars with respect to the substrate surface.

Bioinspired polymeric surface patterns for medical applications.

PURPOSE: A powerful principle in nature is the presence of surface patterns to improve specific characteristics or to enable completely new functions. Here, we present two case studies where bioinspired surface patterns based on the adhesive system of geckos may be applied for biomedical applications: residue-free adhesion to skin and gecko-inspired suture threads for knot-free wound closure. METHODS: Gecko-inspired skin adhesives were fabricated by soft lithography of polydimethylsiloxane with successive inking and dipping steps. Their adhesion was measured using a home built adhesion tester designed for patterned surfaces. Preliminary lap shear tests on the back of a human hand were also performed. Commercial suture threads from different materials were patterned in the group of A. del Campo at the Max-Planck-Institute for Polymer Research (Mainz, Germany) using oxygen plasma. The treated threads were pulled through artificial skin in both directions measuring the peak force and the pull through force. RESULTS AND CONCLUSIONS: Unpatterned reference samples of the skin adhesive did not stick to human skin, while the patterned samples all showed notable adhesion up to 1.2 Newton for a sample size of approximately 3 cm². First results with the patterned suture threads indicated that the surface patterning of the thread has only a minor effect on the pull-through forces. To achieve knot-free sewing the surface geometry of the suture threads needs to be optimized and more realistic testing procedures, e.g. testing on human skin, are necessary.

Adhesion of biocompatible and biodegradable micropatterned surfaces.

We studied the effects of pillar dimensions and stiffness of biocompatible and biodegradable micropatterned surfaces on adhesion on different compliant substrates. The micropatterned adhesives were based on biocompatible polydimethylsiloxane (PDMS) and biodegradable poly(lactic-co-glycolic) acid (PLGA) polymer systems. Micropatterned and non-patterned compliant PDMS did not show significant differences in adhesion on compliant mice ear skin or on gelatin-glycerin model substrates. However, adhesion measurements for micropatterned stiff PLGA on compliant gelatin-glycerin model substrates showed significant enhancement in pull-off strengths compared to non-patterned controls.