Impedance spectroscopy for the assessment of hermetic barrier layers in smart contact lenses

A Smart Contact Lens (SCL) is a medical device that, as a traditional contact lens, is meant to be worn on the eye, with the difference that their capabilities are enhanced by the presence of several electronic components like sensors, antennas and microprocessors embedded into it. Their characteristics make SCL suited for many applications, from medical diagnosis to virtual and augmented reality. Azalea Vi- sion works on a solution for vision assistance with an active light management system inside the contact lens meant for patients with high light sensitivity and ocular disorders, automatically adjusting to the surrounding environment. Nevertheless, these devices are continuously exposed to fluids and ions, such as the tears when they are worn on the eye or the saline solution in which they are stored when not used. However, the materials of which they are made are not hermetic to these species. So, these can permeate through the different layers, reaching the electronic platform and leading to malfunctions or even system failure. To avoid this, the so-called barrier layers are used to protect the electronic circuit. In this work, a system for the barrier layers evaluation has been designed and developed. The system considered is an interdigitated electrode (IDE) that has been specifically designed for a 100 microns multi- layered structure deposited on top of it, made of a first Polyimide 5 microns-thick layer, an Allumina (Al2O3) 50 nm-thick layer and a thermoplastic polyurethane (TPU) 100 microns-thick layer, meant to protect the electronic part. Interdigitated Electrodes are well-known sensors used in many applications characterised by two sets of metallic fingers interlocked one in the other. Thanks to their high scalability, high sensitivity, and the possibility of single-side measurements, they have been extensively used in testing barrier layers in implantable devices. Four different designs have been considered in this work, in which different geometrical parameters are used. In particular, the widths of the finger, ratios between the width of the finger and the gap between two, or the number of fingers have been varied to analyse the influence of these on the system’s sensitivity. A Matlab script implementing an analytical method for the IDE’s capacitance calculation of multi-layered structures has been developed to aid the design process. The fabrication of the electrode has been performed in the cleanroom facility of Ghent University, con- cerning several microfabrication techniques. The long-term reliability of the barriers has been performed through an accelerated ageing test in Phosphate Buffer Saline (PBS) solution at 60°. Electrochemical Impedance Spectroscopy (EIS) measurements in a two-electrode configuration were used to test the layers’ integrity. The data were collected using the BioLogic VSP potentiostat and analysed with the EC-Lab software. The impedance spectra have been measured periodically for 32 days on samples with a single Polymide layer and for six days on samples with the three-layer barrier. On the former, initial measurements in the air were done, and the spectra were fitted with different equivalent circuits to assess which one resembled the system better. The capacitance values obtained in this way were compared with the predicted values, showing acceptable results. Loop analyses were conducted in both cases in the period immediately after the submersion of the electrode in the solution to understand the behaviour in the first phases.