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R Hornig, R Eckmiller; Retina Implant Stimulator with On-chip Memory for Complex Stimulus Profiles . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4489.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To develop and test an implantable, multi-channel retina stimulator chip for stimulus profile-dependent, selective stimulation of ganglion cells or fibers. Methods: Stimulus pulse shape optimization was demonstrated to increase the number of distinguishable epiretinal stimulation sites (Hornig et al. ARVO 2001). Based on this concept, a pulse memory stimulator (PMS) chip for eight channels in CMOS 0.7 µm mixed signal technology was designed, implemented, and tested. Circuit simulation was done by transient analysis. Most parts (digital and analog) of the microchip were developed in full custom design technology. However, current source- and memory-modules had to be designed in analog technology. For digital design, standard cells and specially adjusted full custom cells were used. Following a chip manufacturing process of several weeks, the circuit function was analysed in a computer controlled test environment. Results: The PMS chip was successfully tested to generate variable stimulus current profiles with various time courses, amplitudes, and time delays between corresponding cluster stimulation pulses. For this purpose, the eight memory modules for storage of four pulse profiles each were addressed simultaneously by an appropriate, word-structured serial data stream. The embedded memory with its stored choices of pulse profiles, allowed PMS operation via a wireless communication system with low bandwidth. The data stream words specified the simultaneously selected channels and corresponding pulse profiles e.g. for cluster stimulation. Currents could be generated with a precision of seven bit. For higher accuracy, the range of the current sources could be globally tuned between 50 and 200 µA. PMS baseline power consumption was found to be well below 10 µW due to the channel-parallel architecture, which allowed typical clock frequencies as low as 100 kHz. Conclusion: Retina stimulator chips with channel-parallel architecture and memory for complex pulse profiles offer a highly effective combination of low power consumption and cluster stimulation in order to significantly increase the number of separately adressable stimulation sites with a given small number of implanted microcontacts.
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