1. Field of the Invention
The present invention is directed to piezoelectric stimulation of a predetermined body part, e.g., a nerve.
2. Description of Related Art
Nerve and muscle cells have membranes that are composed of lipids and proteins, and have unique properties of excitability such that an adequate disturbance of the cell's resting potential can trigger a sudden change in the membrane conductance. A neuronal process can be divided into unit lengths, which can be represented in an electrical equivalent circuit. Each unit length of the process is a circuit with its own membrane resistance, membrane capacitance and axonal resistance.
A nerve cell can be excited by increasing the electrical charge within the nerve, thus increasing the membrane potential inside the nerve with respect to the surrounding extracellular fluid. This fundamental feature of the nervous system i.e., its ability to generate and conduct electrical impulses, can take the form of action potentials (AP), which are a single electrical impulse passing down an axon. This action potential (nerve impulse or spike) is an “all or nothing” phenomenon. That is, once the threshold stimulus intensity is reached, an action potential will be generated.
Nerve stimulation may be realized by applying electrical pulses having different frequencies, amplitudes and waveforms. Stimulating electrical signals may be generated by electrodes disposed close to the target nerve or tissue of interest. Transcutaneous Electrical Nerve Stimulators (TENS) produce an electrical signal at frequencies up to approximately 200 Hz to stimulate nerves for relatively small periods of time. TENS use a small electrical device to deliver low frequency (10 Hz to 100 Hz) electrical impulses through the skin via electrode pads affixed to the skin. Electrodes are located at selected locations on the patient's skin and the electrical energy is transferred between the two electrodes. Electrical energy is generally applied in the form of low frequency electrical impulses. The impulses pass through the skin and interact with the nerves that lie beneath the skin. A typical TENS device includes a stimulator, lead wires and electrodes attached to the surface of the skin of the patient. The stimulator is an electrical pulse generator that delivers electrical pulses at a predetermined or selectable frequency. TENS devices are only effective in treating nerves very close to the surface of the skin because the low frequency electrical impulses diminish in strength very quickly due to tissue impedance and thus are not sufficient in intensity to stimulate nerves deep beneath the skin. As an alternative to surface electrodes, implantable electrodes may be surgically implanted proximate a target nerve or tissue of interest to be stimulated. The need for invasive surgery makes such implanted electrodes undesirable.
Aside from electrical stimulation, a nerve cell can also be excited by mechanical vibration which increases the membrane potential inside the nerve with respect to the surrounding extracellular fluid. This mechanical vibration or resonance can be detected by nerve endings if above a certain threshold frequency; that is, a minimum threshold level of stimuli is required before the action potential is triggered or fired. If the threshold stimulus intensity is reached an electrical signal is passed along the axon of the nerve and an action potential is fired.
PCT International Publication WO 2005/079909 discloses a method and apparatus for the detection and treatment of respiratory disorders using implanted devices to mechanically stimulate afferent nerves so as to indirectly cause an increase of the tone of upper airway muscles normally involved with maintenance of upper airway patency. The tone of the upper airway muscles typically decrease during Obstructive Sleep Apnea (OBS), contributing to a collapse and obstruction of the airway. During wakefulness reflexes work to maintain tone in upper airway muscles thereby preventing airway collapse. This reflex mechanism is substituted or enhanced during sleep to restore or maintain airway patency by the application of electrical or mechanical stimulation applied to the afferent nerves. In the case of mechanical stimulation a mechanical element, for example, a piezo-electric element, is implanted at a site in the vicinity of the upper airway, for example, within or adjacent to the base of the genioglossus muscle. A controller sends an electrical signal to the piezo-electric element thereby eliciting a vibration. Vibration of the element elicits stimulation of mechanoreceptor afferent nerve endings within the upper airway. The amplitude, frequency and duration of the mechanical stimulation are controlled such that sufficient stimulation of afferent nerves is achieved without sensory stimulation sufficient to cause arousal from sleep. The mechanical stimulation of afferent nerves would typically be achieved by a period of several seconds of vibration at frequencies in the range of 10-50 Hz, and is tuned to the frequency at which the target receptors are most sensitive. As is evident from the required low frequency range stimulus of 10-50 Hz, the electrical signal used to invoke the mechanical vibrations must inherently be produced by an implanted pulse generator. Otherwise, transcutaneous delivery of such a low frequency electrical signal generated by a non-invasive signal generator would not be sufficient in strength or intensity to trigger stimulation of the afferent nerve beneath the skin as most of the energy would be dissipated at the level of the skin. As noted above, such a pulse generator that generates only low frequency would require surgery to be implanted. It is therefore, desirable to develop a piezoelectric neurostimulator implanted in proximity to a nerve of interest and employing a non-invasive signal generator to cause electrical stimulation of the piezo element resulting in mechanical vibration/resonance of the piezo element thereby stimulating the nerve.
U.S. Patent Application Publication No. 2006/0167500 discloses a neurostimulator using an implanted piezo-electric chip as an electrode. The neurostimulator includes driving circuitry connected to an ultrasound transducer and at least one piezoelectric chip located proximate a nerve fiber. The ultrasound transducer is positioned to create a pressure wave that is incident on the piezoelectric chip. The excitation of the piezoelectric materials in the piezoelectric chip generates an electric current that can then be used to stimulate an action potential or inhibit the creation of an action potential in the nerve. Here a mechanical signal is transmitted through the skin and is converted to an electrical signal by the piezoelectric chip.
It is therefore desirable to develop a neurostimulator in which a high frequency electrical signal is generated externally and transmitted through the skin causing a mechanical disturbance in an implanted piezoelectric element disposed in proximity to a nerve of interest of sufficient intensity to cause firing of an action potential in the nerve.