ELECTRODE-TISSUE INTERFACE
The replacement of the charge carrier from conduction electron in the metal electrode to ion in the tissue must occur at the electrode-tissue interface, a consideration of the electro-chemistry and its relationship to electrode material and shape is important.
The interface can be done by considering the equivalent electric circuit. The capacitance reflects the double layer of charge that arises at the metal-electrolyte interface; the single layer in the metal arises because of its connection to the battery, whereas that in the electrolyte is due to the attraction of ions in the electric field. These layers are separated by the molecular dimensions of the water molecule so the effective capacitance (being inversely proportional to charge separation) is quite high.
At sufficiently low levels the current will be primarily capacitive with little or no flow through the pathway represented by the Faradic impedance Z. But for high currents that exceed the capabilities of the capacitance channel, irreversible chemical reactions will take place, and these are reflected by a much reduced impedance Z. The consequences of these chemical reactions are undesirable since they are detrimental to the tissue or electrode or both.
In general, the goal is to operate stimulating electrodes to utilize exclusively their capacitance and thereby to keep within their operating (linear) range. Under these conditions the electrode current is, essentially capacitive and reversible; in this way one can avoid detrimental chemical reactions. For example, if a stainless steel electrode is excessively anodic one gets
Fe Fe2+ + 2e¯
and the dissolution of the electrode results. For an excessively cathodic condition the result is
2H2O + 2e¯ H2 + 2OH¯
and increasingly negative electrode may result in
O2 + H2O + 2e¯ OH¯ + O2H¯
In either case a consequent rise in pH results which can produce tissue damage.
Fig.2.2 The electrode-tissue interface includes a series ohmic resistance Rs, a series capacitance Cs, and a Faradaic impedance Z. The capacitance represents a double layer that arises at the metal-electrolyte interface, whereas the Faradaic impedance includes several physical and chemical processes that may occur.
Operating an electrode in its linear (capacitive) range is desirable since it is entirely reversible and results in neither tissue damage or electrode dissolution. To do so require that the electrode be positioned as close as possible to the target nerve (so that the maximum required current will be minimized) and, if possible, to have a large area for a low current density. Finally the electrode material should be chosen to have maximum capacitance (other things being the same). A roughened surface appears to result in a decrease in microscopic current density without affecting the overall size of the electrode.
The motor nerve is most susceptible to stimulation at the point it branches to enter the muscle, called the motor point. Therefore, the closer the electrode is to the motor point, the less current it takes to stimulate the muscle through its nerve. Coincidentally, the motor point has the greatest density of sodium channels and therefore the lowest impedance and is the easiest point to stimulate. In practice, by moving an impedance probe over the muscle, the point where the motor point enters the muscle can be easily found. This point produces less pain and the strongest muscle contraction when applying electrical stimulation. Further the lower the density of current under an electrode, the less pain will be felt and therefore, electrode size is another important consideration [12].
A spinal cord stimulator is a device used to exert pulsed electrical signals to the spinal cord to control chronic pain. Electricity can have various effects on living tissues or cells. The possibility of exciting the action potential in a neural cell appears to be very attractive. The stimulus then propagates along ramified pathways of the nervous system. In this way, almost any organ of the human body can be influenced by electrical pulses.
Fig.2.3 Spinal Cord stimulator
A stimulator is a potentially excellent option as a last resort for patients with chronic pain. It may provide exceptional pain relief and a decreased need for pain medications for those suffering from failed back surgery syndrome, failed neck surgery syndrome, diabetic neuropathy, peripheral neuropathy, chronic pelvic pain, chronic abdominal pain, post laminectomy syndrome, chronic testicular pain, coccydynia, RSD and CRPS.
Functional electrical stimulation (FES) is described as electrical stimulation of muscle deprived of nervous control with a view of providing muscular contraction and producing a functionally useful movement [13].
Apart from provoking contraction of heart and skeletal muscles, electrical currents can be applied to alleviate pain, help inhibit or void the urinary bladder, reduce epileptic seizures, improve blood circulation in a certain part of the body, excite spinal cord neurons, control breathing etc.
Stimulation of the spinal cord has been shown to have great potential for improving function after motor deficits caused by injury or pathological conditions. Using a wide range of animal models, many studies have shown that stimulation applied to the neural networks intrinsic to the spinal cord can result in a dramatic improvement of motor ability, even allowing an animal to step and stand after a complete spinal cord transection. Clinical use of this technology, however, has been slow to develop due to the invasive nature of the implantation procedures, the lack of versatility in conventional stimulation technology, and the difficulty of ascertaining specific sites of stimulation that would provide optimal amelioration of the motor deficits. Moreover, the development of tools available to control precise stimulation chronically via biocompatible electrodes has been limited [10].
The location of the electrodes on the spinal cord, the specific stimulation parameters, and the orientation of the cathode and anode determines the body part to be stimulated. The evoked potentials are critical tools to study selective activation of interneuron circuits via responses of varying latencies.
Distinct patterns of stepping and standing produced by stimulation of different combinations of electrodes on the array located at specific spinal cord levels and by specific stimulation parameters, i.e., stimulation frequency and intensity, and cathode/anode orientation.
Therefore the high density electrode array allows high spatial resolution and the ability to selectively activate different neural pathways within the lumbosacral region of the spinal cord to facilitate standing and stepping in adult spinal rats and provides the capability to evoke motor potentials and thus a means for assessing connectivity between sensory circuits and specific motor pools and muscles.
The specificity and high-density features of the electrode array enable to capitalize on two key features of the spinal cord circuitries that are believed to be essential for rehabilitating posture and locomotion after spinal cord injury (SCI). Firstly, the spinal circuitry can be neuromodulated and the stimulation can be carefully delimited to affect only relevant areas of the spinal cord, thus optimizing the motor outcome. Secondly, as locomotor circuitries are highly plastic and adapt when provided with sensory cues during motor training , the density and versatility of the multi-electrode array allows for rapid adjustments of stimulation protocols and adaptations to physiological changes that may occur in the spinal cord over time after injury.
The replacement of the charge carrier from conduction electron in the metal electrode to ion in the tissue must occur at the electrode-tissue interface, a consideration of the electro-chemistry and its relationship to electrode material and shape is important.
The interface can be done by considering the equivalent electric circuit. The capacitance reflects the double layer of charge that arises at the metal-electrolyte interface; the single layer in the metal arises because of its connection to the battery, whereas that in the electrolyte is due to the attraction of ions in the electric field. These layers are separated by the molecular dimensions of the water molecule so the effective capacitance (being inversely proportional to charge separation) is quite high.
At sufficiently low levels the current will be primarily capacitive with little or no flow through the pathway represented by the Faradic impedance Z. But for high currents that exceed the capabilities of the capacitance channel, irreversible chemical reactions will take place, and these are reflected by a much reduced impedance Z. The consequences of these chemical reactions are undesirable since they are detrimental to the tissue or electrode or both.
In general, the goal is to operate stimulating electrodes to utilize exclusively their capacitance and thereby to keep within their operating (linear) range. Under these conditions the electrode current is, essentially capacitive and reversible; in this way one can avoid detrimental chemical reactions. For example, if a stainless steel electrode is excessively anodic one gets
Fe Fe2+ + 2e¯
and the dissolution of the electrode results. For an excessively cathodic condition the result is
2H2O + 2e¯ H2 + 2OH¯
and increasingly negative electrode may result in
O2 + H2O + 2e¯ OH¯ + O2H¯
In either case a consequent rise in pH results which can produce tissue damage.
Fig.2.2 The electrode-tissue interface includes a series ohmic resistance Rs, a series capacitance Cs, and a Faradaic impedance Z. The capacitance represents a double layer that arises at the metal-electrolyte interface, whereas the Faradaic impedance includes several physical and chemical processes that may occur.
Operating an electrode in its linear (capacitive) range is desirable since it is entirely reversible and results in neither tissue damage or electrode dissolution. To do so require that the electrode be positioned as close as possible to the target nerve (so that the maximum required current will be minimized) and, if possible, to have a large area for a low current density. Finally the electrode material should be chosen to have maximum capacitance (other things being the same). A roughened surface appears to result in a decrease in microscopic current density without affecting the overall size of the electrode.
The motor nerve is most susceptible to stimulation at the point it branches to enter the muscle, called the motor point. Therefore, the closer the electrode is to the motor point, the less current it takes to stimulate the muscle through its nerve. Coincidentally, the motor point has the greatest density of sodium channels and therefore the lowest impedance and is the easiest point to stimulate. In practice, by moving an impedance probe over the muscle, the point where the motor point enters the muscle can be easily found. This point produces less pain and the strongest muscle contraction when applying electrical stimulation. Further the lower the density of current under an electrode, the less pain will be felt and therefore, electrode size is another important consideration [12].
A spinal cord stimulator is a device used to exert pulsed electrical signals to the spinal cord to control chronic pain. Electricity can have various effects on living tissues or cells. The possibility of exciting the action potential in a neural cell appears to be very attractive. The stimulus then propagates along ramified pathways of the nervous system. In this way, almost any organ of the human body can be influenced by electrical pulses.
Fig.2.3 Spinal Cord stimulator
A stimulator is a potentially excellent option as a last resort for patients with chronic pain. It may provide exceptional pain relief and a decreased need for pain medications for those suffering from failed back surgery syndrome, failed neck surgery syndrome, diabetic neuropathy, peripheral neuropathy, chronic pelvic pain, chronic abdominal pain, post laminectomy syndrome, chronic testicular pain, coccydynia, RSD and CRPS.
Functional electrical stimulation (FES) is described as electrical stimulation of muscle deprived of nervous control with a view of providing muscular contraction and producing a functionally useful movement [13].
Apart from provoking contraction of heart and skeletal muscles, electrical currents can be applied to alleviate pain, help inhibit or void the urinary bladder, reduce epileptic seizures, improve blood circulation in a certain part of the body, excite spinal cord neurons, control breathing etc.
Stimulation of the spinal cord has been shown to have great potential for improving function after motor deficits caused by injury or pathological conditions. Using a wide range of animal models, many studies have shown that stimulation applied to the neural networks intrinsic to the spinal cord can result in a dramatic improvement of motor ability, even allowing an animal to step and stand after a complete spinal cord transection. Clinical use of this technology, however, has been slow to develop due to the invasive nature of the implantation procedures, the lack of versatility in conventional stimulation technology, and the difficulty of ascertaining specific sites of stimulation that would provide optimal amelioration of the motor deficits. Moreover, the development of tools available to control precise stimulation chronically via biocompatible electrodes has been limited [10].
The location of the electrodes on the spinal cord, the specific stimulation parameters, and the orientation of the cathode and anode determines the body part to be stimulated. The evoked potentials are critical tools to study selective activation of interneuron circuits via responses of varying latencies.
Distinct patterns of stepping and standing produced by stimulation of different combinations of electrodes on the array located at specific spinal cord levels and by specific stimulation parameters, i.e., stimulation frequency and intensity, and cathode/anode orientation.
Therefore the high density electrode array allows high spatial resolution and the ability to selectively activate different neural pathways within the lumbosacral region of the spinal cord to facilitate standing and stepping in adult spinal rats and provides the capability to evoke motor potentials and thus a means for assessing connectivity between sensory circuits and specific motor pools and muscles.
The specificity and high-density features of the electrode array enable to capitalize on two key features of the spinal cord circuitries that are believed to be essential for rehabilitating posture and locomotion after spinal cord injury (SCI). Firstly, the spinal circuitry can be neuromodulated and the stimulation can be carefully delimited to affect only relevant areas of the spinal cord, thus optimizing the motor outcome. Secondly, as locomotor circuitries are highly plastic and adapt when provided with sensory cues during motor training , the density and versatility of the multi-electrode array allows for rapid adjustments of stimulation protocols and adaptations to physiological changes that may occur in the spinal cord over time after injury.