Stretchable electronic platform for soft and smart contact lens applications

A smart contact lens, envisioned to correct or improve vision, entails the integration of several electronic components such as: Si chips, a power source and an electro-optic module. All of them being interconnected by non-conventional electrical layouts in a fully stretchable platform. Such a platform must be designed with strict geometrical requirements and material limitations, to attain compulsory characteristics such as: biocompatibility, oxygen/light transparency, and being imperceptible by the human eye. To favor fabrication throughput, our approach encompasses the development of the thermoplastic platform on a planar manner, in order to thermoform it afterwards into a curvilinear spherical shape by means of metallic molds. Thermoforming induces mechanical stress resulting in distributed strain regions (mainly localized at the edges), which directly affects the integrity of the components. For this reason, here we present a finite element model FEM (using COMSOL) of the thermoforming step corroborated by experimental data, in order to analyze the strain development on the lens surface making emphasis on the wrinkle formation at the edge. The thermoplastic was modelled in the static domain, in 2D-axial symmetry and 3D spaces with defined contact to the molds and free boundary conditions elsewhere. The thermoforming process was performed at several temperatures (i.e. from 80 °C to 140 °C) for two 100 μm-thick thermoplastic carriers (i.e. polyethylene terephthalate – PET and polyurethane PUT) using molds of 8 mm of radius. The measured strain and shape after the thermoforming were in good agreement with the FEM models, showing compressive hoop strains in the order of -10±2% at the border of the lens (radius of 6.5 mm), and close to zero radial strain. Non-axial symmetrical crumpling and wrinkles at the border were found out for temperatures below 100°C and radii bigger than 5 mm, and were reproduced and analyze with 3D FEM models. Finally, the output trends of the modelling were employed as guidelines to design and optimize "horse shoe" meander interconnections to increase the robustness and reliability of the whole system. Such modeling and designing approach could be applied for diverse types of thermoforming steps of soft materials (i.e. thermoplastic polymers) in order to enhance the mechanical integrity and proper component location.

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