Second Generation Retinal Implant Chip for the Blind

Neural prosthetic devices offer a means of restoring function that have been lost due to neural damage. The first part of this work investigates the design of a 15-channel low-power fully implantable stimulator chip. The chip is powered wirelessly and receives wireless commands. The chip features a CMOS only ASK detector, a single-differential converter based on a novel feedback loop, a low-power adaptive bandwidth DLL and 15 programmable current source that can be controlled via four commands. Though, it is feasible to build an implantable stimulator chip the amount of power required to stimulate more than 16 channels is prohibitively large.Clearly there is a need for a fundamentally different approach. The ultimate challenge is to design a self-sufficient neural interface. The ideal device will lend itself to seamless integration with the existing neural architecture. This necessitates that communication with the neural tissue should be performed via chemical rather than electrical messages. However, catastrophic destruction of neural tissue due to the release of large quantities of a neuroactive species precludes the storage of quantities large enough to suffice for the lifetime of the device. The ideal device then should actively sequester the chemical species from the body and release it upon receiving appropriate triggers in a power efficient manner.This work proposes the use of ionic gradients, specifically K+ ions, as an alternative chemical stimulation method. The required ions can readily be sequestered from the background extracellular fluid. The parameters of using such a stimulation technique are first established by performing in-vitro experiments on rabbit retinae. The results show that modest increases (~10mM) of K+ ions are sufficient to elicit a neural response.The first building block of making such a stimulation technique possible is the development of a potassium selective membrane. To achieve low-power the membranes must be ultrathin to allow operation in the diffusive transport regime. One method of achieving this is to use lyotropic self-assembly, unfortunately conventional lipid bilayers cannot be used since they are not robust enough. Furthermore the membrane cannot be made potassium selective by simply incorporating ion carriers since they would eventually leach away from the membrane.A single solution that solves all the above issues was then investigated in this work. A novel facile synwork of self-assembling receptor functionalized polymers was achieved. By combining the properties of hydrophobic and hydrophlic interactions of two polymers a triblock co-polymer was synthesized. The middle hydrophobic block is composed of biocompatible polysiloxanes and was further derivatized to posses ion recognition capabilities via pendant crown ether chains. The hydrophilic blocks were composed of biocompatible polyoxazoline. The membrane properties were studied by self-assembling them into vesicular structures. The ion responsive properties of these polymers were then examined. These polymers also show emergent behavior, such as spontaneous fusion and shape transformation to ionic stimuli, due to the synergy between form and function.The results from the work show that it is feasible to build a renewable chemically based neural proswork based on supramolecular architectures. However, there remains a lot of fundamental work that needs to be pursued in the future to bring the idea to complete fruition.