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B. C. Basinger, A. P. Rowley, A. Beremesh, M. S. Humayun, J. D. Weiland; Mechanics During Retinal Tacking of an Epiretinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2547.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the mechanical interface between retinal prosthesis microelectrode arrays of varying stiffness and the eyewall during retinal tacking.
The force required to insert a retinal tack 2mm into the eyewall, both manually and using computer-controlled displacement, was recorded. Computer-controlled insertion speed was varied from 0.5 to 2 mm/sec. Tacks were re-used up to 10 times. Mechanical characteristics of polyimide, silicone and parylene were measured using a Bose Electroforce 3100 dynamic mechanical analysis instrument. Samples strips, 2mm x 8mm, of each polymer with thickness varying from 20-200 µm were subjected to tensile force. Force and displacement were measured, from which stress, strain and elastic modulus were calculated. Materials were assumed to be isotropic. Microelectrode array and eyewall models were developed using commercial software (SolidWorks). Retina, choroid and sclera were modeled with elastic moduli of 0.1, 8.6 and 2.3 MPa, respectively, based on a previously established porcine ocular model, while electrode array material properties were varied to represent the three polymers examined. The effect of a 1.5 N tacking force on the electrode array/eyewall interface was evaluated using Finite Element Analysis techniques.
Required tacking force increased with insertion speed, tack usage and manual insertion. Polymer elastic moduli ranged from 0.4 GPa - 7.1 GPa, which is comparable to published data for similar materials. The modulus of silicone and parylene increased when submerged in 37° saline. Finite Element Analysis indicates that a tacking force of 1.5 N produces stress up to 3.75 kPa in the retina (46 kPa in the sclera). Strain energy density reaches 0.11 kJ/m³ in the retina layer (3.3 kJ/m³ in the sclera). Stress is concentrated along the edges of the array for all polymers. Materials with a lower elastic modulus generate lower stress and strain values.
Stresses generated during the tacking process are concentrated along the edges of an electrode array but can be reduced somewhat by using electrode array materials with a low elastic modulus. This model may be extended to analyze post-tacking forces.
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