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Deformable platform with thin-film based circuits and ultra-thin Si chips for smart contact lens applications

Periodic Reporting for period 1 - STRETCHLENS (Deformable platform with thin-film based circuits and ultra-thin Si chips for smart contact lens applications)

Reporting period: 2016-09-01 to 2018-08-31

A smart contact lens is a medical device in direct contact with the eye, having integrated electronic functionalities in order to improve the well-being of the user. In that respect, these devices are envisaged to address diverse complex aspects, such as providing augmented reality, performing biomedical sensing and correcting or improving vision. For the first two application areas, possible approaches have already been demonstrated. However, the use of smart contact lenses to correct vision has only been recently proposed through the help of integrated liquid crystal (LC) cells. The integration of these LC cells in a contact lens is particularly appealing for ophthalmological disorders like iris disorders and presbyopia; the latter alone affects more than 1 billion people. The smart contact lens platform envisages the hybrid integration of electro-optic capabilities (e.g. LC cells), RF transmission (e.g. antenna, ultra-thin Si chip - UTC), specific biomarker sensing (e.g. to identify some types of cancer cells) and thin-film based stretchable electrical interconnections. The platform involves many different challenges regarding the required stretchable nature, due to the spherical shape of the eye and manipulations during insertion/extraction, and coupling of many different microsystems technologies.
The STRETCHLENS project has focused on those challenges in order to realize an autonomous miniaturized system capable of interacting with the environment and providing better vision for the users. Additionally, it has developed new knowledge through technological advancement and models of thermoforming in order to optimize the assemblies through design. Thanks to the combination of these different models and fabrication techniques to develop such highly integrated stretchable systems, the project has open up diverse research opportunities in the fields of biomaterial science, stretchable micromechanics, and autonomous biomedical and conformal electronics smart systems.
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).
Highly hybrid and complex integration techniques gave rise to a very versatile, stretchable and potentially autonomous platform for smart contact lens vision correction applications. The innovative combination of thin-film processes, thermosetting and thermoplastic polymers was at the foundations of the developed platform. Specific FEM models were developed to optimize the thermoforming process which converts a 2D design (regular electronics fabrication steps) to a 3D design, required for conformal medical devices such as contact lenses. The developed platform is both compatible with soft and rigid contact lenses, thus increasing the potential adoption of the technology for commercial medical devices. Finally, the combination of numerical/analytical models with microsystems fabrication was proven by a functional prototype with all the required building blocks for vision correction devices, i.e. wireless power, silicon chip as controller, liquid crystal cell to modulate the incoming light and stretchability for the application.
The concept was already approached by different industrial partners at IMEC, whom showed high interest to develop commercial products. Additionally, the work performed during STRETCHLENS was the basis for two new projects on smart contact lenses funded by the Belgian/Flemish government (VLAIO) and Ghent University Research fund (IOF), where different vision correction applications are explored in order to bring them to the market.