MEMS neuroprosthetic system outlines

From the literature, it is possible to generalize three different situations for MEMS-based neuroprosthetic systems and to identify similarities and differences between them that affect the circuit design. These are systems located in the central nervous system (CNS), systems located in the peripheral nervous system (PNS), and those located in the eye (retinal prostheses). With this in mind, it is possible to outline generic systems, and use these as a basis for discussion of the individual circuit elements involved. Many of these circuit elements will overlap from one system to another, but there are some unique problems to be solved for the different applications; particularly when one considers the retinal prosthesis.

13.1.3.1 CNS applications In this generalization, prostheses applied to the brain are considered. The spinal cord represents something of a transitional area, exhibiting some of the characteristics one may find relating to prosthetics applied to the brain and some of those relating to the PNS. Specific examples of such prostheses include visual prosthetics applied to the visual cortex,5,17,18 prosthetics applied to the auditory brain stem,19,20 and those applied to the motor cortex.21,22 It is useful to note that the latter represent, to some extent, an attempt to link CNS and PNS prosthetics in the case of spinal cord injury. It has been proposed that lost sensation and control in paralyzed limbs can be restored by recording from the motor cortex and interpreting and transmitting these commands to a PNS element of the prosthesis. This would, in turn, electrically stimulate the PNS/muscles to achieve the desired effect. Proprioceptive signals recorded from the PNS would be used to control this stimulation and those returned to the CNS element of the prosthesis would provide conscious feedback of the process by stimulating the sensory cortex of the patient.23 This is, perhaps, one of the most challenging systems so far proposed for MEMS-based neuroprosthetics. The principle characteristics of the environment encountered by prosthetics applied to the brain, from an electronics point of view, are the relative mechanical stability, the potential of mounting connectors on the skull, the relative lack of EMG and other interference, and complexity.

We can, therefore, identify a generic CNS neuroprosthetic system consisting of a fairly substantial two- or three-dimensional electrode array, penetrating up to a few millimeters into the brain. Although the complexity of brain means that the signal processing and identification overhead involved can be relatively large, the location of the device in the relatively stable and shielded environs of the skull means that a relatively large area of silicon real estate can be located relatively near to the electrode structure itself. Additionally, the skull provides a good anchor point for percutaneous connectors, which can be employed (e.g., work of Dobelle et al.18), although some may still prefer the radiofrequency (RF) induction coil or other transcutaneous approaches. The length of wiring is of less concern when all elements of a system can be mounted in close proximity, but the number of connections required, particularly to percutaneous connectors, is still a matter of interest. This is because of the pulsation of the brain with blood flow and the potential tethering effect of the connector or other elements of the system. The sort of generic system that we consider can be illustrated by the works of Dobelle18 and Kennedy et al.22 (see Figure 13.1a).

13.1.3.2 PNS applications The PNS differs from the CNS in that the complexity of the system decreases as one considers more distal locations; there is even some evidence for functional organization within the nerve trunk itself. Nerve cuff solutions have been applied to the PNS with considerable success in the past, but more complex systems for standing and walking have suffered from problems of cable failure.25 Where such systems have employed more proximally located cuff electrodes (at the ventral roots, for example), the success of the system has been limited by the selectivity of the electrodes employed.26

It is noted that while the PNS has evolved to withstand many decades of repeated mechanical stress from joint flexion and muscle contraction, the same is not true of metal wires. Locating prosthetic systems beyond the relatively protected confines of the spinal column can be problematic; even here, space is at a premium, implying a requirement for a miniaturized system.

The PNS prosthesis will typically employ two-dimensional regeneration electrodes or probe-type devices,162728 although at least one group has proposed that a three-dimensional array would be required to interface in a highly selective manner to individual nodes of Ranvier.29 One of the applications typically proposed for such a system is standing and walking in paraplegics. Here, it is proposed, the highly selective nature of the MEMS-based microelectrode structures can be drawn upon. The system could be located in or near the spinal column with electrodes situated in the ventral roots for stimulation and devices situated in the dorsal roots providing sensory feedback for closed-loop control.630 Another proposed application is to provide control and sensation for prosthetic limbs.10 2728

The generic PNS prosthesis therefore consists of a number of microelec-trode structures at several discrete locations within the PNS. These will typically perform minimal signal processing, due to the difficulty of supplying

Figure 13.1 (a) CNS (brain) prosthesis, characterized by co-location of recording and stimulating sites with control and signal processing circuitry and a relatively stable situation for per- or transcutaneous signal transmission. (b) PNS prosthesis, characterized by distributed microelectrode devices with the requirement for more distributed intelligence and localized power regulation; per- or transcutaneous power and data transmission is still required. (c) The retina prosthesis. For the purposes of this work, the spinal cord lies somewhere between the CNS and PNS outline.

Figure 13.1 (a) CNS (brain) prosthesis, characterized by co-location of recording and stimulating sites with control and signal processing circuitry and a relatively stable situation for per- or transcutaneous signal transmission. (b) PNS prosthesis, characterized by distributed microelectrode devices with the requirement for more distributed intelligence and localized power regulation; per- or transcutaneous power and data transmission is still required. (c) The retina prosthesis. For the purposes of this work, the spinal cord lies somewhere between the CNS and PNS outline.

such locations with cabling for power and signal transmission. Ideally, such devices would not project from the nerve trunks into which they had been implanted, due to mechanical considerations, and wireless communication and power would be employed. Control would be centralized, typically in a unit implanted beneath the skin with a coil for RF coupling; this latter item is not dealt with in this chapter (see Figure 13.1b). Injectable stimulators31,32 provide a well-developed example of this approach.

0 0

Post a comment