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Long-Sheng Fan, Frank Yang, Ching-Yu Liu, Chih-Chiao Teng, Mao-Yen Chang, Fu-Min Wang, Ta-Ching Chen, Chang-Hao Yang, Chung-May Yang; Thermal Mechanical Evaluation of a Contact-Lens-Shaped Flexible Retinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1824.
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We previously reported a 16,384-pixel flexible retinal prosthesis chip using a 180 nm mixed-signal CIS technology with pixel array sensing image and generating electrical stimulations 30 μm in pitch to enable the possibility of high visual acuity and large field of view. The image-sensing retinal prosthesis is made into a contact lens shape conforming to the surface of a 25 mm eyeball for a better stimulation resolution and a lower stimulation threshold. The current study evaluates the thermal and mechanical reliability of such contact-lens-shaped retinal prosthesis through in vivo and in vitro experiments.
To evaluate the temperature rise from the operation of an implanted high-density retinal prosthesis in the subretinal space, we integrate resistance temperature detectors into a retinal MEA device. We surgically implanted this device in the subretinal space of a rabbit model and measure the temperature rise in vivo. To evaluate the mechanical reliability of the contact-lens-shaped retinal chip, an accelerated test of repeated mechanical force is applied to an eye-and-orbit physical model with the retinal chip embedded in the corresponding sub retinal space. An automatic tester is built and a force of ~4 Kgf acting upon the physical model is repeated for >5 million times, and the strain and electrical data are acquired automatically to assess any sign of mechanical failure of the retinal chip.
Experiments show the in vivo temperature rise versus power dissipation of the device has a slope of 0.84°C/(mW*mm^-2) in the subretinal space of the animal model. The maximum measured temperature rise in the animal model under maximum operating power is 0.84°C, which is consistent with the calculated values from the finite element method. The test of the mechanical reliability of the contact-lens-shaped retinal chip has been repeated > 5 million times under the peak stress and no sign of failure have been observed.
This study shows that to ensure the temperature rise within 1.0°C, the maximum power dissipation of the retinal implant should be kept within 1.2 mW/mm^2, which is compatible with the power requirement of our high-density retinal chip. The initial accelerated test of the mechanical reliability of the contact-lens-shaped retinal chip equivalent to operating a retinal prosthesis for ~10 years under the extreme mechanical stress condition has shown no sign of mechanical failure.
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