Adhesion and contact in soft and hard materials: From organic electronics to medical devices

Abstract

Contact and adhesion between different components in many structures are crucial for the performance and life-time of these structures. This study presents the results of a combined experimental and computational study of adhesion and contact between soft and hard materials in small structures that are relevant to biomedical applications and organic electronics. These structures include: drug-eluting stents that are used for the treatment of cardiovascular disease, bio-inspired functionally graded nanocomposite dental multilayers, and large-area organic light emitting diodes. In each of these areas, this work combined nano-scale experimental measurements of adhesion with analytical and computational models that enabled the prediction of adhesion at the micro- and macro-scales. Also, a range of materials/mechanical characterization techniques were used to obtain useful inputs into mechanics models of contact and adhesion.

The study of drug-eluting stents explored new ways of characterizing and modeling the adhesion of polymeric drug-containing coatings to metallic substrates. Atomic force microscope (AFM) was used to measure the pull-off force between bi-materials pairs. The adhesion energies were then obtained from the measured pull-off forces and adhesion theories. Subsequently, the adhesion energies were incorporated into an interfacial fracture mechanics zone model that was used to determine the mode mixity dependence of the interfacial fracture toughness. The mode mixity dependent fracture toughness conditions were then integrated into finite element method (FEM) models that were used to compute the critical push-out forces of the suspended polymeric films. The predicted push-out forces were found to be in good agreement with the results obtained from the experiments. The implications of the results were then discussed for the design of drug-stent interfaces.

Subsequently, contact-induced deformation was explored in bio-inspired dental multilayers that were inspired by the functionally graded structure of the detin-enamel-junction (DEJ). FEM was used to explore the effects of layer thickness and layer architecture on the contact-induced stresses that are induced in bio-inspired dental multilayers with functionally graded architectures that mimic the DEJ in natural teeth. Occlusal contact was idealized as a Hertzian contact problem in which bio-inspired layered structures were used to simulate DEJ-inspired structures. A layered nanocomposite structure was then fabricated by the sequential rolling of micro-scale nanocomposite layers with local moduli that increase from the side near the soft dentin-like polymer composite foundation to the side near the top ceramic layer. The loading rate dependence of the critical failure loads was shown to be well predicted by a slow crack growth model that integrates the actual layer mechanical properties that are obtained from nanoindentation experiments. The model bio-inspired FGM structures explored in this study could be used to engineer significant improvements (~ 20-40 %) in the critical loads of dental multilayers.

Also, adhesion and contact models were developed for the optimization of lamination processes that are being explored for the next generation of organic electronics. The study presents the results of finite element simulations of the lamination process for the fabrication of organic light emitting devices (OLEDs). The simulations utilize mechanical properties of the individual layers of the OLED structures that are obtained using nanoindentation techniques. The simulations show that applied pressure can cause contact evolution and sink-in around dust particles that are interposed between the organic materials layers, or the organic/inorganic layers. The implications of such improved contact are then discussed for the development of organic light emitting devices with improved performance characteristics.