SYNTHETIC GECKO INSPIRED DRY ADHESIVE THROUGH TWO- PHOTON POLYMERIZATION FOR SPACE APPLICATIONS

This work aims to develop an advanced and cost-effective fabrication process to produce a simplified gecko-inspired microstructure with two-photon polymerization and polymer molding, aimed to improve the adhesive properties of microstructures. Such adhesive microstructures can be implemented for multi-purpose adhesive grasping devices, which have recently gained significant interest in the space exploration sector. Previous gecko-inspired microstructures were reviewed, and the new gecko-inspired microstructures have been developed with the adaptation of additive manufacturing methods for facile fabrication. The examined microstructures in this thesis were the tilted mushroom-shaped and wedge-shaped designs, which could both maximize adhesion by shearing the micropillars toward the tilted direction when preload force is applied. The improved microstructure fabrication process could produce micropillars in the height of 270 μm with soft polymer without defects. However, the miniaturized micropillars in the height of 40 μm, frabricated with the same process, had broken tips and missing structures. The effects of the scale, height, and shape of the micropillars in controllable dry adhesion were investigated through the experiments. The adhesion of the microstructures with artificial gecko setae in the height of 270 μm was 2 times higher than the microstructures with 40 μm of height. Meanwhile, the microstructures that consisted of long and short artificial gecko setae had inferior adhesive performance to the microstructures having uniform long setae on all tested surfaces. Meanwhile, the result showed no direct correlation between the surface roughness of the attached surface and the adhesive performance of the microstructures. The wedge-shaped design was determined to have higher adhesion than the tilted mushroom-shaped design due to lower structural resistance on bending and higher effective contact area

Functional surface microstructures inspired by nature : From adhesion and wetting principles to sustainable new devices

In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

Functional surface microstructures inspired by nature – From adhesion and wetting principles to sustainable new devices

In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

Biomimetic adhesion for transfer printing via microstructured surfaces

Demand for robust engineering techniques on the micro and nano scales has been steadily growing in the age of modern technology, not only because of the driving force to fit electronics into smaller form factors, but also for a variety of other applications, from devices with microfluidic functions to components whose interfacial behaviors are key features. In our research we attempted to develop a tool that facilitates assembly of a wide variety of devices on both conventional and novel surfaces in the hopes of both improving modern capabilities of technological fabrication, as well as opening up possibilities for new classes of devices that can be easily assembled on surfaces and in form factors that were not previously possible. In summary, primary benefit of this technology is the potential ability to fabricate a variety of electronic devices on any surface – thus expanding the versatility and ability to integrate different classes of technology in way that is not possible using modern, competing fabrication methods for micro and nano-scale chemical/electronic/mechanical devices. In the first two chapters, I will discuss background information relating to the basis and motivation for this technology, beginning with a summary of adhesion – how different types of adhesion occur and what their applicability is, with a focus on dispersive, or van der Waals adhesion – followed by a discussion of the field of biomimetics and how the study of naturally occurring dry adhesion techniques employed by animals such as geckos and insects has inspired a field of research into the use of dispersive intermolecular forces as an engineering solution for limitations of nanofabrication and assembly. In the following chapters I will describe our own group’s design, fabrication, and iii testing of a variety of microstructured surfaces intended to control adhesive strength by increasing it and decreasing it, as needed. Finally, I will present the results of our experiments and draw conclusions about the effectiveness and future potential of transfer printing via kinetically controlled microstructured stamps

High Quality Bioreplication of Intricate nanostructures from a Fragile Gecko Skin Surface with Bactericidal Properties

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Shape memory polymers as direct contact dry adhesives for transfer printing and general use

For most diminutive life on Earth, control over external adhesive forces is crucial for survival. As humans, we pay little notice because at our scale inertial forces typically overwhelm adhesive forces by a wide margin. Nonetheless, the study and development of dry adhesives, which rely on ubiquitous intermolecular attractions to repeatedly form and break attachment to their adherends, have garnered substantial interest in recent decades. High performance artificial dry adhesives may unlock the door for many exciting new technologies from nanoscale manufacturing to wall climbing robots, but thus far the challenges have proven substantial and few successful commercial applications have come to fruition. This dissertation represents an initial investigation into the benefits and potential limitations of developing shape memory polymer (SMP)-based dry adhesives. Prior to the presentation of experimental results, a review of the current state of dry adhesive knowledge including both theory, observations of the natural world, and lessons learned by other researchers in their attempts to develop a wide variety of synthetic dry adhesives is provided. It is concluded that dry adhesives fundamentally function through careful control of elastic energy, an idea that is very well suited to explore using SMPs which offer a large change in compliance across their thermal transition temperature. Thermoset epoxy SMPs are identified as an ideal choice for the investigation due to their mechanical strength, chemical resistance, manufacturability and convenient glass transitions among other features. The dry adhesive performance of a selected SMP is first evaluated for the purpose of microscale transfer printing, wherein micro-objects are assembled through precise control of adhesive surface forces. Significant benefits over existing solutions in terms of maximum adhesive strength during loading (~7 MPa), minimum strength for release (~0 MPa), and process versatility are confirmed, culminating in demonstrations of several challenging assemblies. The increase in adhesive strength is explained by invoking arguments from linear fracture mechanics and considering the dramatic compliance change experienced by the SMP between bond and load events. Advanced methods of heating and meaningful steps towards commercial-scale parallel printing processes are demonstrated. The suitability of SMP for larger-scale applications is considered next. Strength rivaling or exceeding known alternatives is demonstrated, showing adhesion exceeding 2 MPa for 6 mm diameter adhesives while retaining excellent releasability through the use of microstructuring. A method of internally heating the SMP by adding conductive carbon nanoparticles is explored, including quantitative analyses of conductivity and the SMP composite's storage and loss moduli. The resulting flexible and conductive bi-layer SMP adhesive supports load while attached to surfaces of varied curvature. Variations on the SMP formula have their adhesive and mechanical properties tested, and are used to produce a self-contained SMP prototype wall-hanging adhesive

Adhesion modulation In bio-inspired micropatterned adhesives by electrical fields

With steps towards Industry 4.0, it becomes imperative to the development of next-generation industrial assembly lines, to be able to modulate adhesion dynamically for handling complex and diverse substrates. The inspiration for the design and functionality of such adhesive pads comes from gecko’s remarkable ability to traverse rough and smooth topographies with great ease and agility. The emphasis in this thesis was to equip artificial micropatterned adhesives with such functionalities of tunability and devise an on-demand release mechanism. The project evaluates the potential of electric fields in this direction. The first part of this work focusses on integrating electric fields with polymeric micropatterns and studying the synergistic effect of Van der Waals and electrostatic forces. An in-house electroadhesion set up was built to measure the pull-off forces with and without electric fields. As a function of the applied voltage, adhesion forces can be tuned. The second part of the work demonstrates a novel route that exploits the in-plane actuation of the dielectric elastomeric actuators integrated with microstructure to induce peeling in them. Voltage-dependent actuation has been harnessed to generate the requisite peel force to detach the micropatterns. Overall, the findings of this thesis combine disciplines of electroadhesion, electroactuation, and reversible dry adhesives to gain dynamic control over adhesion.Im Einklang mit dem Fortschreiten in Richtung Industrie 4.0, wird es auch für die Entwicklung von industriellen Montagelinien der nächsten Generation unerlässlich sein, die Handhabung komplexer und unterschiedlicher Objekte zu flexibilisieren. Bioinspirierte Haftpads nach dem Vorbild des Gecko könnten zukünftig hierzu wesentlich beitragen. Der Schwerpunkt dieser Arbeit bestand darin, künstliche mikrostrukturierte Haftpads mit einem elektrisch schaltbaren Adhäsions- und Ablösemechanismus zu funktionalisieren, um die Grundlage für einen schnell schaltbaren, intelligenten Greifer zu schaffen. Der erste Teil dieser Arbeit konzentriert sich auf die Kombination elektrischer Felder mit elastomeren Mikrostrukturen und die Untersuchung der synergistischen Wirkung von Van der Waals- und elektrostatischen Kräften. Zur Messung der Adhäsion wurde ein individueller Aufbau realisiert und mit diesem die Feldstärkeabhängigkeit der Haftkräfte nachgewiesen. Der zweite Teil der Arbeit demonstriert einen neuartigen Ablösemechanismus unter Ausnutzung der lateralen Bewegung dielektrischer elastomerer Aktuatoren, um so ein Abschälen der Haftpads vom Substrat zu induzieren. Durch Variation der elektrischen Spannung wurde untersucht, wie sich diese auf die Ablösegeschwindigkeit der Haftpads auswirkt. Insgesamt kombinieren die Ergebnisse dieser Arbeit die Disziplinen Elektroadhäsion, Elektroaktuation und reversible trockene Klebstoffe, um so eine dynamische Kontrolle über die Adhäsion zu erhalten