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 fig1 . 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 fig1 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 said 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 . fig2 shows an alternative embodiment of a microstimulator 16 in accordance with the present invention . the microstimulator 16 of fig2 is similar to the microstimulator 9 of fig1 except that in fig2 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 . fig3 shows another embodiment of an improved microstimulator 18 in accordance with the present invention . the microstimulator 18 of fig3 is similar to the microstimulator 9 of fig1 except that the coating 10 in fig3 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 compounds 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 10 the device , have the property that electrical voltage influences a change in the rate at which the fillers diffuse form the matrix . where it is desirable to use such compositions , the microstimulator illustrated in fig2 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 fig4 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 fig4 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 fig5 an improved microstimulator 28 is shown implanted into muscle 30 . in this embodiment , as well as in those of fig1 - 3 , the improved microstimulator receives power form 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 fig6 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 fig6 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 fig2 above , or when a separate electrode is embedded in the coating 10 as illustrated in fig4 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 fig7 . 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 trochar 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 trochar 120 by connecting a conventional electrical stimulator ( not shown ) to connector 122 on the trochar . 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 trochar 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 fig8 there is illustrated , in diagrammatic form , the general circumstances that give rise to pressure ulcers and a preferred embodiment whereby one or more microstimulators may be employed to reduce the incidence of and / or contribute to the healing of such pressure ulcers . as depicted in fig8 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 fig3 - 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 fig8 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 fig5 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 . fig9 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 64 . 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 52 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 fig8 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 .
improved implantable microstimulators covered with a biocompatible polymeric coating in order to provide increased strength to the capsule thereof and to capture fragments of the microstimulator should it become mechanically disrupted and to make the microstimulator safer and easier to handle are provided here . 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 , provided herein are systems employing the improved microstimulators to 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 . these systems include external control means for controlling the operation of the microstimulators , said control means having memory means for programming preferred stimulation patterns for later activation by the patient or caregiver .