Implantable Transducers for Neurokinesiological Research and Neural Prostheses

The objective of this thesis was to develop a family of advanced electrical and mechanical interfaces to record activity of nerves and muscles during natural movements. These interfaces have applications in basic research and may eventually be refined for used in restoring voluntary control of movement in paralyzed persons.I) A muscle length gauge was designed that is based on piezoelectric crystals attached at the ends of a fluid filled extensible tubing. The in-vivo performance of these gauges was equal to previous length gauge designs. In addition, the ultrasound based design provided for the first time a direct muscle length calibration method.2) An innovative nerve cuff closing technique was devised that does not reqmre suture closures. The new design uses interdigitated tubes to lock the opening and fix the lumen of a nerve cuff. The cuffs were tested in long-term mammalian implants and their performance matched or surpassed previous closure designs. The nerve cuff was further redesigned to include a more compliant cuff wall and wire electrodes.3) Floating microelectrodes previously used for central nervous system recordings were adapted for chronic use in the peripheral nervous system. These electrodes proved disappointing in terms of signal quality and longevity. The reasons for failure are thought to be of both electrical and mechanical origin.4) An innovative silicon micromachined peripheral single unit electrode was designed and tested. In the in-vivo tests, a limited number of recording sites successfully established short-term neural interfaces. However, the quality of the electrode performance, in terms of signal amplitude and ability to discriminate single unit potentials, was insufficient.5) Using a finite difference model, a numerical simulation of static and dynamic electrical interactions between peripheral axons and microelectrode interfaces was derived. The model consisted of resistive and capacitive elements arranged in a 3-dimensional conductive universe (two spatial dimensions and time). Models of intrafascicular fine wire or silicon based electrodes were used to record simulated propagating action potentials. It was confirmed that electrode movement affected the recorded signal amplitude and that a dielectric layer on a silicon electrode accentuated the recorded potential field. A conducting back plane facing away from axon sources did not have a significant effect on the electrode recording properties.In conclusion, several novel implantable transducers were developed for use in neurokinesiological research. A numerical simulation of the axonal potentials recorded by intrafascicular electrodes helped interpret various shortcomings found in the in-vivo electrode performance. Although not attempted in the present thesis some of the developed technologies may have potential of transferring to clinical neural prostheses applications.