An adaptive amplifier for cuff imbalance correction and interference reduction in nerve signal recording

Damage to the central nervous system because of spinal cord injury (SCI) or stroke results in loss of motor function in various parts of the body, mainly these located below the area of injury. Some of the most common consequences include loss of mobility and continence, degrading the quality of life of anyone affected. Currently, conventional medicine does not offer any method for curing the damage and fully restoring function, and therapy is focused on rehabilitation. Neuroprostheses may assist in partial restoration of function and mobility using functional electrical stimulation (FES), which involves the process of passing pulses of electrical current into the muscle tissue of interest or into nerve branches, which cause the activation of the muscle. Fully implantable FES systems may use naturally occurring nerve signals (electroneurogram or ENG) as feedback or control inputs. However, the usefulness of ENG signals recorded from nerve cuffs depends on the amount of muscle signal (electromyogram or EMG) interference present. The EMG amplitude can be as large as three orders of magnitude greater than the V ENG and their spectra overlap. The cuff electrode, used for measuring ENG, is appropriate for chronic implantation and reduces interference when made tripolar. Tripolar cuff amplifier configurations include the "quasi tripole" (QT) and the "true tripole" (TT). However, as a result of parameters including cuff asymmetries and impedance irregularity due to tissue growth, cuff performance suffers from imbalance, which causes widely reported EMG contamination at the output of these amplifier configurations. Therefore, in order to avoid the use of high order filters for removing the EMG, a system has been developed to balance the cuff and remove EMG interference. This thesis describes the design, implementation and evaluation, both in-vitro and in-vivo, of an adaptive amplifier configuration, named the "adaptive tripole" (AT). The AT performs both tasks of imbalance correction and interference reduction simultaneously and the extracted ENG can be used in neuroprosthetic devices for the improvement of FES systems. The system has been built both in discrete-component and in integrated forms, the first one used to evaluate the principle and to form the specifications for the latter. Prior to the full description of the system, some limited research is presented on the causes of imbalance, to clarify the causes of the problem and to better define the specifications of the AT. The relationship between imbalance and cuff proximity to interfering fields is also examined. The results from experiments on the discrete AT indicated that it provides up to 60 times greater output signal-to-interference ratio (SIR) than the TT for high imbalance values (>20%). Also, even when disadvantaged by the presence of two imbalances due to proximity effects it performs better than both the TT and the QT in 60% of the cases. In-vitro experiments on the IC version demonstrated the output SIR of the AT to be up to 200 to 300 times greater than the QT and TT equivalent SIRs respectively, for 40% imbalance. The average AT output SIR achieved by the IC version was 2:1 and the highest was approximately 4:1. The system will be used in the Conditional Neuromodulation (CNM) implant developed in the departments of Electronic and Electrical Engineering and Medical Physics and Bioengineering in UCL, and will contain the AT as well as a stimulator and an RF power and signal transmission transcutaneous link.