Understanding the mechanical behavior and failure mechanisms of stretchable electronics is key in developing reliable and long-lasting devices. In this work a micron-scale stretchable system consisting of an aluminum serpentine patterned interconnect adhered to a polyimide substrate is studied. In-situ experiments are performed where the stretchable sample is elongated, while the surface topography is measured using a confocal microscope. From the resulting height profiles the microscopic three-dimensional deformations are extracted using an adaptive isogeometric digital height correlation algorithm. The displacement information is compared to realistic numerical simulations, in which the interface behavior is described by cohesive zone elements. It is concluded that despite fitting the traction separation law parameters, the model fails to correctly capture the distinct out-of-plane buckling (with magnitude of a few micron) of the interconnect. The model is updated with residual stresses resulting from processing and crystal plasticity induced behavior (decreased yield strength) in the aluminum layer, but both measures are not resulting in the experimentally observed deformations. Finally, mixed-mode cohesive zones are implemented, in which the properties are different in the shear and normal direction. After fitting the corresponding parameters to the experimental data, the model shows realistic in-plane and out-of-plane deformations. Also a predictive simulation for a different geometry results in the correct experimentally measured behavior. It is concluded that the aluminum-polyimide interface mode-angle dependency explains the observed microscopic failure mode of local delamination and buckle formation.

A flexible nickel-phosphorus (NiP) amperometric electrochemical glucose sensor is proposed. A thin layer of NiP was deposited by electroless technique directly onto the Polyethylene Terephthalate (PET) substrate. NiP electroless plating has proved to be a low cost, simple and versatile fabrication technique. The electrochemical behavior of the sensor was in agreement with the literature value, with anodic peak at 0.54 V vs Ag/AgCl. Voltammetric response of glucose was characterized, showing a good sensitivity of the device. The influence of mechanical stress (bending) on the electrochemical response and on the glucose sensing performance of the PET/NiP sensor was examined. Bending the PET/NiP electrode to small radii of curvature (up to 5mm) has no effect upon the electrochemical behavior, but an increase in sensitivity with respect to flat sensors was found.

A novel adaptive isogeometric digital height correlation (DHC) technique has been developed in which the set of shape functions, needed for discretization of the ill-posed DHC problem, is autonomously optimized for each specific set of profilometric height images, without a priori knowledge of the kinematics of the experiment. To this end, an adaptive refinement scheme is implemented, which refines the shape functions in a hierarchical manner. This technique ensures local refinement, only in the areas where needed, which is beneficial for the noise robustness of the DHC problem. The main advantage of the method is that it can be applied in experiments where the deformation mechanisms are unknown in advance, thereby complicating the choice of suitable shape functions. The method is applied to a virtual experiment in order to provide a proof of concept. A second virtual experiment is executed with stretchable electronics interconnects, which entail localized buckles upon deformation with complex kinematics. In both cases, accurate results were obtained, demonstrating the beneficial aspects of the proposed method. Moreover, the technique performance on profilometric images of a real experiment with stretchable interconnects was demonstrated.

MECHANICAL RESPONSE OF ALUMINUM/POLYIMIDE STRETCHABLE UNITS: A COHESIVE ZONE MODEL APPROACH R. Lucchini, E. Cattarinuzzi, D. Gastaldi and P. Vena 1 CMIC, LaBS, Politecnico di Milano, Milano, Italy, riccardo.lucchini@polimi.it 2 CMIC, LaBS, Politecnico di Milano, Milano, Italy, emanuele.catterinuzzi@polimi.it 3 CMIC, LaBS, Politecnico di Milano, Milano, Italy, dario.gastaldi@polimi.it 4 CMIC, LaBS, Politecnico di Milano, Milano, Italy, pasquale.vena@polimi.it