In Vivo Characterization of Individual Electrode-Retina Interface Impedance: Physical Fitting Process of HARP4k Subretinal Implant
L.S. Fan, Y.C. Chen, S.C. Liu. S. Shen, J. Sheu
Iridium Medical Technology Corporation, Hsinchu, Taiwan, R.O.C.
An implanted subretinal prosthesis device is intended to convert from image projected onto the retina to retinal stimulation that will create the appropriate percept. The physical part of this process includes those represented by the transfer functions of the light-to-photocurrent transduction and the electrical-signal-to-current injection. Although these characteristics can be well-characterized for individual pixel of the retina chip in factory, the actual characteristics will depend on the local biological environment of the implant. Therefore, it is desirable to measure in vivo the actual electrical stimulation delivered to the retina. As the number of electrodes increases, an automatic process to quickly measure and wirelessly transmit the measured impedances out during the fitting process is also needed. We implemented an automatic impedance measurement function in the HARP4k subretinal implant and measured/transmitted all of the 4,000 individual working electrode-retina interface impedance in less than 4 minutes using mini pigs as the animal model two months after the implants. The measurements were wirelessly transmitted out using a load-shifting keying (LSK) protocol. We found that the in vivo electrical interface impedance maps in high spatial resolution (working electrodes to counter electrodes 25.4m center-to-center, to neighboring working electrode 44m center-to-center) correlated to retina features observed in OCT optical images. The average impedance value in vivo is compared to that measured in saline solutions in vitro.
Financial disclosure: LF, YC: IMTC patent (P), financial interest (I), employment (E); SL, SS, JS, DC: IMTC employment (E)