WP1 focused on the study of stretchable non-conventional 2D electrical interconnections, which was the basis of the interconnection technology used throughout the duration of the project. This was studied first by means of finite element models (FEM’s), where new models and knowledge was created specially when using thermoforming techniques. The later were used to transform the 2D circular platform to a 3D spherical cap. An interesting optimization process occurred where meanders or horse shoe designs were found as optimal. The knowledge of IMEC on 2D stretchable electronics was the basis for the current developments. The developed models allowed the optimization of the thermoforming processing parameters, such as radius of curvature, temperature and duration. As main result, the first mockup prototypes were fabricated based on the developed models and fabrication methods at IMEC (thin-film electronics and lamination techniques).
WP2 was focused on the multilayer stretchable assemblies with 3D electrical interconnections for the ultra-thin silicon chips. This work was based on the previous experience of the researcher on isotropic conductive adhesives (ICA) and blind vias for flip-chip interconnections. The 3D interconnections were achieved by printing ICA pastes based on silver particles onto pre-determined contact pads on the stretchable circuitry. Different methods to open the pads were investigated, for instance, laser ablation and reactive ion etching (RIE). Although, laser ablation was proven as a more versatile and fast technique, the precision of RIE was much higher, and it was kept for the subsequent versions of the demonstrators. By using the FEM models and the optimized fabrication techniques from WP1 the second prototype was validated with a commercial near-field communication (NFC) chip and a custom-made radio frequency (RF) antenna. The communication with the integrated chip was validated and even enough power was transferred to power up a micro LED, also integrated on the lens.
By having developed the electronic substrate with thin silicon chips and wireless RF power, the second main task was to investigate the liquid crystal cells (LC) for vision correction and their implementation on stretchable substrates. This was the focus of WP3, where the LC technology available at IMEC was used as a starting point. First of all, the main requirements for the LC cells and contact lens were drafted, for instance maximum thicknesses, oxygen transmission, optical transparency, optical contrast and electrical driving signals (voltage, current, frequency and waveform). Second, the FEM models developed in WP1 were employed here again, this time considering a different thermoplastic substrate (from polyurethane to polyethylene terephthalate), since the equations and process were the same. The models allowed the optimization of the gap thickness and location of the spacers, which are used to keep the top and bottom electrodes at a constant distance. Finally, the techniques developed in WP2 (i.e. ICA vias for electrical interconnections) were used to integrate the LC cell to the main electronics platform. This gave rise to the first semi-passive smart contact lens prototype with a LC cell in the line of sight for vision correction applications. The embedding of such prototype on soft lenses (i.e. based on hydrogels) and rigid lenses (i.e. based on gas permeable polymers) was explored by partnerships with industry (i.e. contact lens manufacturers).
