Abstract:
Improved implantable microstimulators are covered with a biocompatible polymeric coating in order to provide increased strength to the capsule and to capture fragments of the microstimulator should it become mechanically disrupted. Such coating also makes the microstimulator safer and easier to handle. The coating may include one or more diffusible chemical agents that are released in a controlled manner into the surrounding tissue. The chemical agents, such as trophic factors, antibiotics, hormones, neurotransmitters and other pharmaceutical substances, are selected to produce desired physiological effects, to aid, support or to supplement the effects of the electrical stimulation. Further, microstimulators in accordance with the invention provide systems that prevent and/or treat various disorders associated with prolonged inactivity, confinement or immobilization of one or more muscles. Such disorders include pressure ulcers, venous emboli, autonomic dysreflexia, sensorimotor spasticity and muscle atrophy. The microstimulator systems include external control for controlling the operation of the microstimulators. The control include memory for programming preferred stimulation patterns for later activation by the patient or caregiver.

Description:
This application is a Divisional Application of U.S. application Ser. No. 9/077,662, filed May 29, 1998, now U.S. Pat. No. 6,051,017 which application is incorporated herein by reference, and which application was filed in the U.S. under 35 U.S.C. §371 based on international application number PCT/US97/02576, filed Feb. 19, 1997; which international PCT application claimed priority to U.S. Provisional Applications Nos. 60/011,870; 60/012,019; 60/011,868; and 60/011,869; all filed Feb. 20, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     Muscles serve a number of functions, most of which are dependent upon their regular contraction, which is in turn dependent upon their strength and health. For example, in addition to the well known functions of supporting the skeleton and permitting movement, muscles serve to pad the force of bone protuberances against the skin, and they promote blood flow, particularly through deep blood vessels. In response to repeated contractions against a load, muscle fibers grow in cross-sectional area and develop more force, and in response to repeated contraction over a long period of time, the oxidative capacity and blood supply of the fibers is enhanced. 
     In normal individuals, muscles are activated to contract by electrical signals that are communicated from the brain and spinal cord by way of muscle nerves. Many medical diseases, physical disabilities and cosmetic disfigurements arise from abnormal or absent electrical signals to the muscles. Such abnormal or absent electrical signals may be pathological or may simply be due to prolonged immobility or confinement that restricts or prevents the voluntary movement of one or more muscles. Without normal, routine electrical stimulation, muscles atrophy, that is lose their normal size and strength. Also contributing to muscle atrophy may be a wide range of other pathophysiological mechanisms, including absence of sustaining hormones and other endogenous trophic substances. 
     Many situations exist in which voluntary muscle contraction cannot be used effectively to operate, condition or strengthen muscles. The most extreme loss of voluntary muscle function occurs when the brain or spinal cord is injured by trauma, the growth of tumors or cerebrovascular accidents. In patients suffering from these conditions, muscles become wholly or partially paralyzed because the electrical commands that are normally generated in the nervous system are no longer available to stimulate muscle contractions. Less extreme degrees of muscle weakness and atrophy can come about when some of the nerve fibers supplying a muscle are damaged by disease or injury or when the muscle is immobilized or voluntarily rested, for example by casting or bedrest, in order to recover from an injury or surgical intervention involving a nearby body part, or other prolonged confinement or immobilization. 
     With respect to prolonged physical confinement or immobilization, the affect of muscle non-use and atrophy frequently leads to two disorders that are particularly difficult to avoid and expensive to treat, pressure ulcers of the skin and subcutaneous tissues and retardation of the normal circulation of blood through deep vessels. Continual, unrelieved pressure on localized regions of skin can result in the development of pressure ulcers of the skin and subcutaneous tissues, also known as bed sores or decubitus ulcers. Pressure ulcers are thought to occur when tissues underlying a site of pressure are deprived of oxygen and nutrients because blood flow is impeded, and when the area is subjected to frictional and shearing forces associated with continuous rubbing and movement. Pressure ulcers vary in size and degree of damage from small regions of redness to deep craters of tissue erosion passing through skin, connective tissues, muscle and even bone that can threaten the life of a patient by providing portals of entry for pathogenic organisms. They are often exacerbated in chronically paralyzed or bedridden patients because of atrophy of the unused muscles that normally provide a degree of padding between the skin and underlying bony protuberances. The treatment of pressure ulcers often requires prolonged, intensive medical care and occasionally extensive surgery, usually entailing further restrictions in the posture of the patient, which may further complicate medical and nursing care and cause other complications. 
     As mentioned above, prolonged immobilization or physical confinement of a body part often also results in retardation of circulation of blood through deep vessels, particularly the veins in an around muscles. For example, the failure to contract muscles in the limbs at regular intervals, as occurs normally when walking or standing, is known to cause stasis of blood in some veins. Venous stasis is a predisposing factor in the formation of clots in the veins. Such deep venous thrombosis further compromises blood flow to the immobilized body part and can be the source of dangerous emboli to the heart and lungs. Thrombosed veins may also become chronically infected, posing a danger of septicemia. Examples of particular populations of patients that are especially at risk for development of pressure ulcers and venous emboli include comatose and obtunded patients, patients who are confined by paralysis to bed or wheelchairs, bedridden patients who have medical or surgical conditions that limit their activity, and elderly patients with limited mobility. To reduce complications in these patients, it is necessary to reestablish movement of the vulnerable body parts; however, these patients are either incapable of voluntary movement or severely restricted in their ability to voluntarily move. Therefore, therapists often spend considerable time manipulating the passive limbs of these patients, but this is expensive and relatively ineffectual because it is the active contraction of muscle that tends to pump blood through the veins and to maintain the bulk of the muscle. 
     It has long been known that muscle contractions can be elicited involuntarily by stimulating muscles and their associated motor nerves by means of electrical currents generated from electronic devices called stimulators. This has given rise to various therapies that seek to prevent or reverse muscle atrophy and its associated disorders by the application of electrical stimulation to the muscles and their nerves via these stimulators. For example, the field of research known as functional neuromuscular stimulation (FNS) or functional electrical stimulation (FES) has begun, which seeks to design and implement devices capable of applying electrical currents, in order to restore functional movement to paralyzed limbs. Similarly, therapies employing stimulators to regularly apply specific patterns of electrical stimulation to muscles in order to prevent or reverse atrophy are known. 
     Many of the earliest stimulators were bulky and relied upon the delivery of large current pulses through electrodes affixed to the skin, a procedure that requires careful positioning and fixation of the electrodes to the skin and frequently produces disagreeable cutaneous sensations and irritation of the skin. Additionally, such transcutaneous stimulation produces relatively poor control over specific muscles, particularly those that lie deep in the body. Thus, this procedure can be time-consuming, uncomfortable, and is generally useful only for muscles located immediately beneath the skin. 
     It is also possible to stimulate muscles more directly by passing electrodes through the skin into the muscles or by surgically implanting self-contained stimulators and their associated leads and electrodes in the body. These devices have many configurations, but most are large and have numerous leads that must be implanted and routed through the body to the desired muscles using complex surgical methods. Further, they are expensive to produce and the invasive procedures required for their implantation are impractical for most patients because they increase rather than decrease the required care and the danger of infection and other sources of morbidity in patients who are already seriously ill. Thus, such devices have been used primarily in patients with severe paralysis in order to demonstrate the feasibility of producing purposeful movements such as those required for locomotion, hand-grasp or respiration. 
     More recently a new technology has been described whereby electrical signals can be generated within specific tissues by means of a miniature implanted capsule, referred to as a “microstimulator”, that receives power and control signals by inductive coupling of magnetic fields generated by an extracorporeal antenna rather than requiring any electrical leads. See, U.S. Pat. Nos. 5,193,539; 5,193,540; 5,324,316; and 5,405,367, each of which is incorporated in its entirety by reference herein. These microstimulators are particularly advantageous because they can be manufactured inexpensively and can be implanted non-surgically by injection. Additionally, each implanted microstimulator can be commanded, at will, to produce a well-localized electrical current pulse of a prescribed magnitude, duration and/or repetition rate sufficient to cause a smoothly graded contraction of the muscle in which the microstimulator is implanted. Further, operation of more than one microstimulator can be coordinated to provide simultaneous or successive stimulation of large numbers of muscles, even over long periods of time. 
     While originally designed to reanimate muscles so that they could carry out purposeful movements, such as locomotion, the low cost, simplicity, safety and ease of implantation of these microstimulators suggests that they may additionally be used to conduct a broader range of therapies in which increased muscle strength, increased muscle fatigue resistance and/or increased muscle physical bulk are desirable; such as therapies directed to those muscle disorders described above. For example, electrical stimulation of an immobilized muscle in a casted limb may be used to elicit isometric muscle contractions that would prevent the atrophy of the muscle for the duration of the casting period and facilitate the subsequent rehabilitative process after the cast is removed. Similarly, repeated activation of microstimulators injected into the shoulder muscles of patients suffering from stroke would enable the paretic muscles to retain or develop bulk and tone, thus helping to offset the tendency for such patients to develop subluxation at the shoulder joint. Use of microstimulators to condition perineal muscles as set forth in applicant&#39;s copending patent application Ser. No. 60/007,521, filed Nov. 24, 1995, entitled “Method for Conditioning Pelvic Musculature Using an RF-Controlled Implanted Microstimulator”, incorporated herein by reference, increases the bulk and strength of the musculature in order to maximize its ability to prevent urinary or fecal incontinence. 
     In addition to the therapeutic use of microstimulators to promote contraction of specific, isolated muscles in order to prevent or remedy the disorders caused or contributed to by inactive muscles, the administration of hormones, trophic factors and similar physiologically active compounds may also be useful. It is known that the extent to which a muscle will grow in response to any stimulation regime is affected by the hormonal and chemical environment around the muscle. Muscle fibers have receptors for many physiologically active compounds that circulate normally in the blood stream or are released from nerve endings. These trophic factors have significant effects on the nature, rate, and amount of growth and adaptation that can be expected of the muscle in response to stimulation, whether such is produced voluntarily or by electrical stimulation. Perhaps the best known of these hormones are the androgenic steroids often used by athletes to increase muscle bulk and strength; but other hormones such as estrogens and growth hormones are also known to affect muscle properties. For example, the dramatic reductions in circulating estrogens and androgens that occur in women following menopause appear to account for decreases in the mass of muscles and bones, which can be slowed or even reversed by administering the deficient hormones systemically. 
     Thus, the beneficial strengthening effects of electrical stimulation can be maximized by providing the affected muscles with a supportive hormonal environment for growth. These compounds can be provided systemically by administering them orally or by injection. However, many such compounds are rapidly metabolized by the liver, so that high doses must be administered to achieve a desirable therapeutic effect. This can expose all tissues of the body, including the liver, to high and perhaps poorly controlled levels of the compound, resulting in undesirable side-effects that may outweigh the desired actions of the agent. In one aspect, the present invention recognizes that this problem could be circumvented by using a more selective method of drug delivery directed specifically to the electrically exercised muscles. Even if the introduced compound were ultimately to be cleared by absorption into the bloodstream, high concentrations would be produced only in the tissue around the target. A steep dilutional gradient would ensure that other regions of the body were exposed to much lower levels of the administered compound. By providing a more conducive chemical environment in the early stages of electrical therapy, it is expected that muscle atrophy could be reversed more rapidly and effectively. After muscle function has been reestablished, longer-term performance of the muscle could be more easily maintained at the desired level by electrical stimulation alone or in combination with low-dose systemic replacement therapy. 
     The microstimulators described and claimed herein are elongated devices with metallic electrodes at each end that deliver electrical current to the immediately surrounding biological tissues. The microelectronic circuitry and inductive coils that control the electrical current applied to the electrodes are protected from the body fluids by a hermetically sealed capsule. This capsule is typically made of a rigid dielectric material, such as glass or ceramic, that transmits magnetic fields but is impermeable to water vapor. 
     Encapsulation in glass is an effective and inexpensive way to ensure a hermetic seal between the electronic components and the biological tissues. Methods for forming similar hermetic seals within the confined dimensions of the overall device are well-known in the fabrication of industrial magnetic reed relays and diodes and have been described specifically for implantable microstimulators. See, e.g., U.S. Pat. Nos. 4,991,582; 5,312,439; and 5,405,367, each of which is incorporated in its entirety by reference herein. Such a hermetic barrier is important both to ensure good biocompatibility with the body and to protect the sensitive electronics from the body fluids that might destroy their function. 
     Unfortunately, however, glass and similarly brittle materials such as ceramic may crack or shatter as a result of externally applied forces or even residual stress in the crystalline structure of the material itself. If such an event occurs within the body or during a surgical procedure, it is desirable to retain or capture the sharp fragments of the capsule and any internal components so that they do not irritate or migrate into the surrounding tissues. In a testing or surgical environment in which devices are handled repeatedly, the hard, slippery surface of the glass capsule makes the device difficult to handle, and could increase the likelihood that the device will be dropped or pinched with a force sufficient to break the glass. Therefore, in one aspect, the present invention provides a well-chosen biocompatible coating for the glass which would decrease the lubricity of the device and ensure that glass pieces resulting from device fracture would be contained/captured in a protective sleeve. 
     The reaction of a living body to an intact foreign body such as an implanted microstimulator depends at least in part on the shape and texture of the surface of the foreign body, as described, e.g., by Woodward and Saithouse (1986). The surfaces left by the manufacturing processes used for the implanted microstimulator are constrained by the nature of the materials and processes required to achieve the desired electronic and mechanical characteristics of the device. Therefore, modification of the microstimulators&#39; chemical nature and/or superficial physical contours to avoid, prevent and/or discourage an immunological response by the body, would be advantageous. Additionally, in selecting an appropriate coating material the opportunity arises for the introduction of various chemical compounds, such as trophic factors and/or hormones, as discussed above, into or onto the coating. Such compounds could then diffuse from the surface of the coating into the surrounding tissues for various therapeutic and diagnostic purposes, as previously mentioned. 
     SUMMARY OF THE INVENTION 
     The present invention provides for the prevention and treatment of various disorders caused or exacerbated by abnormal or absent electrical signals to the muscles and apparatus useful therefore. In one aspect, the invention provides an improved microstimulator having a biocompatible polymeric coating on portions of its exterior, thereby reinforcing the mechanical strength of the microstimulator such that it may optionally be implanted deeply into the muscle, while also providing a means for capturing the fragments of the microstimulator should mechanical disruption occur. In preferred embodiments, the coatings provided herein are selected to improve the nature of the foreign body reaction to the implanted microstimulator by modifying its chemical surface, texture and/or shape. 
     The implantable microstimulators disclosed and claimed herein are preferably of a size and shape that allows them to be implanted by expulsion through a hypodermic needle or similar injectable cannula. The microstimulator includes a hermetically sealed housing, at least two exposed electrodes, and electronic means within the housing for generating an electrical current and applying the electrical current to the exposed electrodes. The coating, as described in detail herein, is formed on at least a portion of the exterior of the microstimulator in contact with the hermetic seal. 
     In another aspect of the present invention, the improved microstimulator, in addition to providing electrical stimulation to the muscle within which it is implanted, is modified to provide a locally high level of one or more desired chemical agents or drugs. In a preferred embodiment, the polymeric coating covering a portion of the microstimulator&#39;s surface contains a chemical agent that is released gradually from the coating. Thus, when the microstimulator is implanted within or adjacent to a muscle it produces an electrical current that activates the motor nerves and/or muscle fibers of the muscle while simultaneously dispensing the chemical agent(s) in the vicinity of the active muscle fibers. 
     Further, in preferred embodiments, the improved microstimulator is designed to provide electrical stimulation over a period of many years and to provide elution of the chemical agent(s) over a period of many days, weeks or longer without any percutaneous connections to the external world. Release of the chemical agent from the coating of the microstimulator may be by diffusion or, alternatively, may be at a predetermined rate controlled by electrical signals produced by the implantable device. 
     In yet another aspect of the present invention, systems providing involuntary movement to muscles for the purpose of preventing, treating and/or slowing the progress of various complications associated with muscle inactivity, especially inactivity due to prolonged physical confinement or immobilization, are provided. These systems employ one or more microstimulators non-surgically implanted in or near one or more inactive muscles. Once implanted, the prescribing physician uses an external controller to command each of the implanted microstimulators to produce various output stimulation pulses in order to determine a pattern of stimulation that produces the desired muscle contraction pattern. The external controller retains the programmed stimulation routine and, thereafter, administers the therapy on a regularly scheduled basis and/or whenever commanded to do so by the patient or any caregiver. 
     The systems provided herein are particularly useful for maintaining or improving the functional capacity of paralyzed, weak, immobilized or under-exercised muscle without requiring voluntary exercise and for preventing various complications of prolonged physical confinement, including but not limited to pressure ulcers, deep venous thrombosis, autonomic dysreflexia and sensorimortor spasticity. For example, the implantable microstimulators are employed to stimulate specific muscles in order to reduce the incidence and accelerate the healing of pressure ulcers on the sacrum heels and other bony protuberances of bedridden or immobilized patients. Alternatively or additionally, the systems are employed to reduce the possibility of venous stasis and embolus formation by eliciting regular muscle contractions in the legs of the bedridden or otherwise immobilized patient. Advantageously, these systems may be employed to produce the desired pattern of regular contractions in one or more muscles for periods of days or weeks without the need for ongoing, continuous patient or caregiver supervision. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
     FIG. 1 diagrammatically illustrates one embodiment of a microstimulator coated with a polymeric coating in accordance with the present invention; 
     FIG. 2 shows another embodiment of a microstimulator coated with a polymeric coating wherein the coating extends over a portion of the electrodes; 
     FIG. 3 illustrates another embodiment of the implantable device of the present invention that provides both electrical stimulation and release of a chemical agent; 
     FIG. 4 shows another embodiment of an implantable device in accordance with the present invention; 
     FIG. 5 diagrammatically illustrates an implanted microstimulator in muscle tissue and its control using an external controller; 
     FIG. 6 shows a variation of the invention wherein a battery is included within the implantable device to allow it to operate independently of the external controller; 
     FIG. 7 illustrates a preferred manner used to implant a microstimulator in accordance with the present invention; 
     FIG. 8 illustrates the general circumstances that give rise to pressure ulcers, and illustrates one preferred manner in which an implanted microstimulator, in accordance with the present invention, may be used to reduce pressure ulcer formation; and 
     FIG. 9 illustrates the general circumstances that give rise to venous stasis, and further shows a preferred manner in which an implanted microstimulator may be used and controlled, in accordance with the present invention, to prevent and treat such a condition. 
    
    
     Corresponding reference characters indicate corresponding components throughout the views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
     An implantable device  9  made in accordance with the present invention is illustrated in FIG.  1 . The device  9  includes a narrow, elongated capsule  2  containing electronic circuitry  4  connected to electrodes  6  and  8 , which pass through the walls of the capsule at either end together forming a microstimulator of the type disclosed and fully described in U.S. Pat. Nos. 5,193,539; 5,193,540; 5,324,316 and 5,405,367, each of which is incorporated herein, in its entirety, by reference. A coating  10  is applied over the longitudinal extent of the surface of the capsule  2 . In the particular embodiment of FIG. 1, the ends of the coating  11  do not extend over the surface of electrodes  6 ,  8 , so that the coating does not change the overall profile of the microstimulator. The device  9  is shaped to permit its insertion through a tubular insertion cannula, such as a syringe, that can be passed transcutaneously into a target muscle with or without fluoroscopic guidance, as described further below. 
     The capsule  2  may be made of glass or a similar dielectric material, such as ceramic, that can provide a hermetic barrier to the permeation of body fluids and water vapor into circuitry  4 . The basic design of the current-generating circuitry  4  is the same or similar to that described in the above-referenced patents, in which electrodes  6  and  8  may be continuously charged (through inductive coupling) by a programmable magnitude of direct current and may be occasionally discharged so as to produce a large, brief stimulation pulse with a programmable magnitude and duration, which stimulation pulse is used for the activation of nearby motor nerve and/or muscle fibers. 
     The coating  10  of the improved microstimulator is selected to both be biocompatible and to be elastic enough to provide some reinforcement to the capsule  2 . Additionally, it is advantageous and preferred that the material chosen to form the coating  10  serve to reduce the risk of injury from and to provide means for the capture of capsule fragments in the event the capsule is broken. Finally, it is desirable that the coating  10  chosen reduces the lubricity of the device, as glass and ceramic materials, of which the capsule  2  is most often constructed, are slippery. It will be appreciated by those of skill in the art that several different coatings are available having these characteristics. By way of example only and in no way to be limiting, the polymeric coating  10  may be formed of a silicone elastomer or a thermoplastic material, such as polyethylene, polyester, polyurethane or a fluorinated carbon chain from the TEFLON family. 
     The preferred method of application of coating  10  depends on its chemical composition and physical properties. For example, in one embodiment, the coating  10  is formed from a thin-walled extrusion of silicone elastomer whose inside diameter is slightly smaller than the outside diameter of capsule  2 . The extruded tubing is cut to the desired length and its diameter temporarily expanded by absorption of an appropriate solvent such as heptane, toluene or xylene. The expanded silicone tubing is then slipped over the microstimulator, subsequently shrinking tightly onto the surface of the microstimulator as the solvent is evaporated from the silicone elastomer, thereby forming the desired coating  10 . 
     In another embodiment, the coating  10  is made from a thermoplastic material such as a polyethylene, polyester, polyurethane or a fluorinated carbon chain from the Teflon family. A thin-walled extrusion of such thermoplastic material whose inside diameter is smaller than the outside diameter of capsule  2  is mechanically expanded so as to temporarily increase its inside diameter. The expanded extrusion is then cut to the desired length, slipped over the microstimulator, and caused to shrink onto the surface of the microstimulator by briefly heating it to the temperature at which it contracts toward its unexpanded dimensions, thereby forming the desired coating  10 . 
     In another embodiment, coating  10  is made from a liquid solution containing melted, dissolved or unpolymerized material which is applied to the surface of the microstimulator by dip-coating, injection molding, or other suitable methods known to those of skill in the coating art. After covering the desired portions of the microstimulator, the coating  10  is allowed or caused to harden by appropriate means. 
     FIG. 2 shows an alternative embodiment of a microstimulator  16  in accordance with the present invention. The microstimulator  16  of FIG. 2 is similar to the microstimulator  9  of FIG. 1 except that in FIG. 2 the ends  12  of coating  10  extend over electrodes  6 ,  8 , thereby preventing concavities  14  from coming into direct contact with tissues surrounding the implanted microstimulator. Advantageously, concavities  14  may be filled with a solid material, such as silicone or other material, to eliminate the presence of pockets of fluid that may act as a nidus of chronic infection. 
     FIG. 3 shows another embodiment of an improved microstimulator  18  in accordance with the present invention. The microstimulator  18  of FIG. 3 is similar to the microstimulator  9  of FIG. 1 except that the coating  10  in FIG. 3 contains a chemical agent  20  which diffuses from the surface of the coating  10  into the surrounding tissues. The chemical agent  20  may be any of a large number of pharmacologic and diagnostic agents whose presence in the tissue surrounding the implantable microstimulator is desired as part of the treatment received by the patient. Examples of suitable chemical agents  20  include anti-inflammatory or antibiotic compounds intended to reduce the foreign body reaction, hormones, neuromodulators and neurotransmitters intended to potentiate the effects of the electrical currents, or dyes intended to mark the original location of the implanted microstimulator. This list of agents provides only examples and is not intended to limit the scope of the invention set forth in the claims. 
     The method of introduction of the chemical agent  20  into or onto the coating  10  depends upon the chemical nature of the agent and the selection of an appropriate coating material. In general, the types of agents and compatible coatings that may be used therewith are known to those of skill in the arts of chemical binding and diffusion and the design of sustained release pharmaceuticals. 
     In a preferred embodiment, the chemical agent  20  comprises a long-acting compound of testosterone, such as testosterone propionate, cypionate or enanthate. This agent  20  is mixed with or adsorbed onto a silicone elastomer that is injection-molded or dip-coated and subsequently polymerized to provide a thin coating  10 , which coating  10  is spread over a substantial portion of the surface area of the capsule  2 . It should be appreciated that silicone is a highly biocompatible compound that has been used previously to administer steroids to experimental animals without exposing the animals to the trauma of repeated injections. However, it should also be appreciated that coating  10  could be formed from a variety of other materials, or by using a variety of other processes, as described above. 
     It is thus seen that in this preferred embodiment, agent  20  comprises a trophic compound used to enhance muscle development, specifically a testosterone derivative. It should be appreciated that such compounds have been used for many years in humans to treat endocrine disorders or to retard the development of estrogen-sensitive mammary tumors, and that a single intramuscular bolus of the compound will exert its actions for 2 to 4 weeks. The chemical agent  20  associated with the external coating  10  of the present invention, however, could be selected from a variety of trophic chemicals with actions on muscle or connective tissues, and could be bound to the coating in any manner that advantageously affects its rate of release. The rate of release may be designed to be anywhere from a few hours to a few days or weeks. Furthermore, agent  20  might actually consist of a multiplicity of active compounds, various of which affect or influence muscle fibers, nerve fibers, connective tissue, or inflammatory cells so as to modify many aspects of the response of the tissues to the presence and activation of the device. 
     Certain composite materials, such as the drug-filled polymeric matrix that may be used for coating the device, have the property that electrical voltage influences a change in the rate at which the fillers diffuse from the matrix. Where it is desirable to use such compositions, the microstimulator illustrated in FIG. 2 is particularly useful, as the electrical output signals generated by circuit  4  are applied, at least in part, to the coating  10  by its contact with the electrodes  6 ,  8  of the device. Such electrical output signals are systematically varied so as to produce the desired rate of elution of the chemical agent  20  into the tissues surrounding the implanted device. Thus, it is seen that the electrical currents produced by electrodes  6  and  8  in the process of stimulating the muscle could also advantageously have the effect of increasing the elution rate of agent  20  simultaneously with the electrically-induced muscle contraction. 
     As illustrated in FIG. 4, rate control of the elution of the chemical agent  20  from the coating  10  may alternatively be managed using additional electrodes  26  which are affixed to the capsule  2  and connected to the circuitry  4  of the device. Such additional electrodes provide for separate control of the electrical currents and voltages applied to stimulate the muscle electrically and to control the rate of elution of chemical agent  20  from the polymeric coating  10 . Advantageously, such multiple electrodes facilitate the use of electrophoretic current through coating  10  to effect the release of agent  20 , independent of the currents required to charge and discharge those electrodes associated with muscle or nerve stimulation. As illustrated in FIG. 4, electrode  26  is entirely covered by the polymeric coating  10 , whereas electrodes  6  and  8  are exposed to the body fluids. Electrical current applied between electrodes  26  and  8  would pass through coating  10  to effect electrophoretic release of chemical agent  20 . Electrical current applied between electrodes  6  and  8 , on the other hand, would pass unobstructed through the body fluids and tissues to effect electrical stimulation of nearby nerve or muscle fibers. 
     Referring now to FIG. 5, an improved microstimulator  28  is shown implanted into muscle  30 . In this embodiment, as well as in those of FIGS.  1 - 3 , the improved microstimulator receives power from an external control device  40 . The external control device  40  generates an alternating magnetic field, illustrated symbolically by the lines  36 , through an external coil  38 , which coil may advantageously be located underneath the patient in a seat or mattress pad or in a garment or item of bedclothes. The magnetic field  36  is coupled with an implanted coil  33 , which forms part of the microstimulator device  28 , and induces a voltage and current within the coil  33 . The induced voltage/current in the coil  33  is used to power the electronic circuitry  4 , and fluctuations (e.g., modulation) of the varying magnetic field  36  are used to control operation of the electronic circuitry  4 . That is, the device  28  delivers current to its electrodes  6 ,  8  according to instructions encoded in fluctuations of the magnetic field  36 . In this preferred embodiment, electrical current emitted from electrodes  6  and  8  stimulates motor nerve fibers  32 . Muscle fibers themselves are relatively difficult to activate via such electrical currents, but the motor nerve fibers are more readily stimulated, particularly if the microstimulator is located near them in the muscle. Each time a motor nerve fiber is excited, it conveys an electrical impulse through its highly branched structure to synaptic endings on a large number of muscle fibers, which results in the activation of essentially all of those muscle fibers. Electronic circuit  4 , then, controls the amplitude and duration of the electrical current pulse emitted by the microstimulator  28 , thereby determining the number of such motor nerve fibers that are excited by each pulse. 
     As an example of a preferred use of the improved microstimulator, the prescribing physician uses a programming station  44  to command external controller  40  to produce various stimulation pulses, during the initial treatment session after implantation of the improved microstimulator  28 . This is done in order to determine an exercise program that will provide the desired therapeutic muscle contraction program for the individual patient. The exercise program is down-loaded into a memory element  42  of the external controller  40 , where it can be reinitiated at will by, for example, manually activating control  46 . This manual control may be performed, e.g., by the patient or an attending caregiver. In the preferred embodiment, programming station  44  is a personal computer, external controller  40  contains a microprocessor, and memory element  42  is a nonvolatile memory bank such as an electrically programmable read-only memory (EEPROM). However, it will be appreciated by those of skill in the art that many different systems, architectures and components can achieve a similar function. 
     In accordance with a variation of the invention, shown in FIG. 6, a battery  46  is included within the implanted device (microstimulator), and is employed as a continuous source of power for the electronic circuit  4 . Such battery also provides storage and production means for a program of output currents and stimulation pulses that may then be produced autonomously by the implanted device without requiring the continuous presence of extracorporeal electronic components, i.e., without the need for an external control device  40 . In such instance, means would be provided for transmitting the desired program to each microstimulator and for commanding each microstimulator to begin or to cease operating autonomously. Advantageously, such an embodiment as shown in FIG. 6 may provide for continuous biasing current or voltage applied to coating  10  (when at least one of the electrodes is positioned to contact the coating  10 , as shown in FIG. 2 above, or when a separate electrode is embedded in the coating  10  as illustrated in FIG. 4, above) so that the rate of elution of agent  20  would always be well-controlled. 
     In a preferred implantation method, the microstimulator is injected into the muscle of interest through an insertion device whose preferred embodiment is shown in FIG.  7 . The external cannula  110  of the insertion tool is comprised of a rigid, dielectric material with sufficient lubricity to permit the easy passage of the microstimulator without scratching its external surface. The central trocar  120  of the insertion tool is an electrically conductive rod whose sharpened point extends beyond the insertion cannula, where it can be used to deliver current pulses to the biological tissue near its point. The initial insertion of the tool is directed either by a knowledge of musculoskeletal landmarks or radiographic imaging methods to approach the region of muscle  30  in which motor nerve fibers  32  enter. Optimally, the insertion device is advanced into the muscle in parallel with the long axis of muscle fiber fascicles. Electrical stimuli may be delivered through the metallic trocar by connecting a conventional electrical stimulator (not shown) to connector  122  on the trocar. By observing the contractions of the muscle  30 , these test stimuli can be used to ensure that the tip of the insertion device is situated sufficiently close to motor nerve fibers  32  to permit activation of a substantial portion of the muscle  30  without undesirable activation of other muscles or nerves. Failure to elicit the desired muscle contractions would suggest a poor site of placement for the microstimulator and a need to reposition the insertion tool closer to the site of motor nerve entry. 
     When the desired position is reached, the trocar  120  is removed from cannula  110 , taking care to keep the cannula  110  in position within muscle  30 , and a microstimulator is pushed through cannula  110  and into muscle  30  using a blunt-ended push-rod  130 . 
     As stated above, the microstimulators provided herein are particularly useful in the prevention and treatment of various disorders associated with prolonged immobilization or confinement; such as muscle atrophy, pressure ulcers and venous emboli. Referring to FIG. 8, there is illustrated, in diagrammatic form, the general circumstances that give rise to pressure ulcers and a preferred embodiment whereby one or more microstimulators maybe employed to reduce the incidence of and/or contribute to the healing of such pressure ulcers. As depicted in FIG. 8, force  52  applied between bone  50  and firm support surface  56  is transmitted through intervening soft tissues of the skin  34  and muscle  30 , resulting in compression of skin region  54 . Skin region  54  is thus in danger of developing a pressure ulcer. Active contraction of muscle  30  is induced by electrical stimulation applied by microstimulator  48  and its associated electrodes  6  and  8 . Such active contraction makes muscle  30  stiffer, causing force  52  to be dissipated over a larger region of the skin  34 . Further, active contraction of muscle  30  tends to shift the position of the body with respect to surface  56 , causing force  52  to be directed to a fresh region of skin  34 . Regular active contraction of muscle  30  induces various trophic mechanisms in the muscle that maintain or even enhance the bulk and tone of muscle  30  in its passive state, thereby reducing the concentration of force  52  on skin region  54 . 
     To further aid in the prevention and/or treatment of the pressure ulcer, the microstimulator as described above and illustrated in FIGS.  3 - 6  employing a coating  10  having a chemical agent  20  associated therewith, may be used. In this alternative, the chemical agent  20  may be a trophic factor employed to improve the bulk and tone of the muscle  30  or may be an antibiotic or similar therapeutic drug useful for preventing infection of the pressure ulcer, or the chemical agent may be a combination of the two different agents. Increasing the bulk and tone of the muscle  30 , can provide additional padding between the bone  50  and support surface  56 , thereby lessening the force  52  against the skin region  54 . 
     In the embodiment illustrated in FIG. 8, a microstimulator  48  has been injected into muscle  30  at (or very near) the skin region site  54  where a potential pressure ulcer may develop. However, it may be satisfactory (or even preferred in some instances) to inject one or more microstimulators into adjacent muscles or near various nerves that control muscle  30  and/or other muscles that can affect the magnitude and direction of force  52  upon various regions of skin  34 . 
     It should be appreciated that contraction of many different muscles and groups of muscles tends to lift the prominence of bone  50  so as to distribute the load of the body more evenly across the skin  34 , thereby reducing the amount of force  52  applied at a particular skin region  54 . Optimally, a particular temporal pattern of stimulation applied by one or more microstimulators generates a sustained contraction of the respective muscles that is maintained for several seconds to permit blood flow into vulnerable tissues. Such is accomplished by the extracorporeal components illustrated in FIG.  5  and described above. Thus, upon an external command, or at predetermined intervals, power and command signals sent from controller  40  cause the various microstimulators to emit a series of electrical current pulses (i.e., a pulse train) at the desired frequency and amplitude sufficient to cause the muscles to lift the body for the duration of the pulse train. 
     Further movement of the body part typically occurs after cessation of such pulse-train stimulation because of various nervous reflexes or voluntary movements that are triggered by the concomitant activation of various sensory nerve fibers resulting either from direct electrical stimulation of the sensory fibers or the mechanical consequences of the directly stimulated muscle activity. Such triggered movements are generally just as important, and may even be more important, than the directly stimulated muscle activity caused by the microstimulator generated pulse train for shifting body posture. 
     FIG. 9 illustrates the general circumstances that give rise to venous stasis and a particular embodiment for the use of microstimulators to reduce such stasis. Blood flow  64  in veins  66  running through and between muscles  30 ,  62  depends in part on compressive forces  52  and general metabolic stimulation resulting from the occasional active contraction of muscles  30  and  62 . In the absence of such contractions, flow is reduced, resulting in stasis and an increased likelihood of the formation of clots or thrombi in the veins. One common site for this problem is in the calf muscles of the lower leg, which extend the ankle. In accordance with one aspect of the present invention, therefore, one or more microstimulators are injected into the extensor muscles of the ankle, and one or more microstimulators are injected into the flexor muscles of the ankle. The programmed sequence of stimulation stored in memory bank  42  is used by controller  40  to create the necessary transmission of power and command signals from coil  38  to cause the microstimulators injected into the ankle muscles to generate a prescribed stimulation sequence. Ideally, this prescribed sequence elicits muscle contractions sufficient to shift the position of the foot alternately into extension and flexion for several seconds. The interval between the various muscle contractions and the strength and duration of the contraction in each muscle is set by an attending physician or physiotherapist using a programming station  44  that downloads the desired program into memory bank  42 . The rhythmic intermittent muscle contractions produced each time the program is activated causes compressive forces to act on deep veins  66 , augmenting venous flow  64  out of the muscle by a pumping action that reduces venous stasis. 
     It should also be noted that a particular pattern of stimulation applied through a particular microstimulator, or combination of microstimulators, may also be effective at reducing the incidence of both pressure sores and venous stasis simultaneously, as well as generating other useful trophic effects on the muscles themselves, metabolic stimulation of the cardiorespiratory system, and improvements in the functioning of nervous pathways responsible for various reflexive and autonomic functions commonly affected adversely by prolonged immobilization. Other specific dysfunctions that have been reported to be reduced by regular electrical stimulation of nerves and muscles include autonomic dysreflexia and sensorimotor spasticity, particularly in patients suffering from spinal cord injury. 
     It should also be noted that the particular complications of pressure sores and venous stasis illustrated respectively in FIGS. 8 and 9 are intended only to provide specific examples of the beneficial effects of regular, active muscle exercise that can be induced by microstimulators, and are not intended to limit the scope of the invention set forth in the claims regarding the utility of stimulation applied in this manner. The present invention pertains generally to all beneficial effects that a caregiver might achieve by the appropriate implantation and programming of one or more microstimulators in any patient immobilized for a period of more than a few days. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention as set forth in the claims.