A research group focused on the development of artificial retina prototyped a system that can wirelessly supply power from an external source to an LSI in an artificial retina implanted in the eyeball.
The research group led by Mitsumasa Koyanagi, professor at graduate school of engineering, Tohoku University, presented the accomplishment at 2007 International Conference on Solid State Devices and Materials (SSDM), an international conference on semiconductors that was held from Sept. 18 to 21, 2007.
The latest system employs an electromagnetic induction type wireless power supply unit. The unit is equipped with a compact battery. It supplies power from the primary coil embedded in a lens of eyeglasses to the secondary coil in a crystalline lens serving as the lens for the eye by means of electromagnetic induction. Two major parts of the latest development were the secondary coil and the Schottky diode provided to rectify AC current supplied to the secondary coil to DC current.
The secondary coil was formed by a damascene process using copper (Cu) plating. The coil has a diameter of 1 cm to match the size of the crystalline lens and the wiring width is designed to be 1 mm. The Cu wiring provided with an insulating layer of SiO2 is wound twice.
According to Koyanagi Laboratory, the coil has a small number of turns, i.e., two, because "the main object of the development of this prototype was to investigate how many were necessary to generate an induced electromagnetic force of 4.5 V, which is estimated to be required in the secondary coil." The group intends to eventually provide roughly 20 turns.
High frequency results in heat generation in the eyeballs
According to the laboratory, the latest prototype showed that a voltage of 4.5 V is generated when the numbers of the primary and secondary coils are set to 50 and 20 turns, respectively. The voltage thus obtained was in the AC frequency range of 3 MHz to 300 MHz and particularly concentrated on approximately 100 MHz. At the same time, the group pointed out that there are many issues to be considered in selecting the frequency.
"The electromagnetic wave heats the eyeball when the frequency exceeds 100 MHz and increases closer to the GHz order. On the other hand, a lower frequency ranging from 3 MHz to 4 MHz has problems concerning the regulations specified by the Radio Law. A frequency of 13.56 MHz used in radio frequency identification (RFID) tags may be a practical choice," said a researcher at the laboratory.
Another important point in designing the power supply system is how to reduce the power consumption by the circuit in the eyeball including the LSI.
"Cells in the eyeball are damaged when the temperature increases by 3°C. Thus, we will try to reduce the power consumption to 50 mW or lower in order to ensure the safety," the researcher said.
Koyanagi's group conducted a joint experiment with researchers at graduate school of medicine of Tohoku University in 2005. The experiment proved that when an electric stimulus is applied to the distal end of the optic nerve of a rabbit, a signal having the similar waveform to that generated when the actual retina receives light flows through the nerve.