March 13, 2008
Interschool Lab, Room 750 CEPSR
Speaker: Luke Theogarajan, Massachusetts Institute of Technology
Retinal Prostheses are being developed around the world in hopes of restoring useful vision for patients suffering from certain types of diseases like Age Related Macular Degeneration and retinitis pigmentosa. This talk will examine two approaches to developing such a retinal prosthesis. The first is an electrically based retinal prosthesis and the second is a novel bio-ionic neural interface.The central component of an electrical retinal prosthesis is a wirelessly powered and driven stimulator chip. In this talk we will discuss 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 sources that can be controlled via four commands.
Though electronics offer a superior computational platform, the electrical interface to neural tissue is not always optimal. The key limitation of the electrical interface is fundamental: electronics and the natural neural environment are incompatible in both, form and function. One of the main issues with the electrical interface is the need for large amounts of current. This further necessitates the use of large electrodes, due to safety issues, that leads to stimulation of large populations of neurons rather than a few.
Clearly there is a need for a fundamentally different approach to neural interfaces. 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. Communication with the neural tissue should then be performed via chemical rather than electrical messages. However, catastrophic destruction of neural tissue due to 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.
The use of ionic gradients, specifically K+ ions as an alternative chemical stimulation method will be examined in this talk. The key advantage being that the required ions can readily be sequestered from the background extracellular fluid. Results from in-vitro stimulation of rabbit retina show that modest increases in K+ ion concentration are sufficient to elicit a neural response.
The talk will then outline the different components that will be needed to build a neural interface using the ionic stimulation method. One of the key components is the development of a self-assembling potassium selective membrane. To achieve low-power the membranes must be ultrathin to allow for efficient operation in the diffusive transport limited 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 will be then discussed. The talk will then conclude by discussing some of the exciting opportunities and challenges that lie in this intersection of biology, chemistry and electrical engineering.