Implantable Neuromuscular Stimulators: Technology & Operation

1. FACULTY OF ENGINEERING DEPARTMENT OF BIOMEDICAL ENGINEERING BME 312 BIOMEDICAL INSTRUMENTATION IILECTURER: ALİ IŞIN LECTURE NOTE 5 ImplantableStimulators forNeuromuscularControl BME 312-BMI II-L5- ALİ IŞIN 2014

2. Functional Electrical Stimulation • Implantable stimulators for neuromuscular control are the technologically most advanced versions of functional electrical stimulators. • Their function is to generate contraction of muscles, which cannot becontrolledvolitionally because of the damage ordysfunction in the neural paths of the central nervous system (CNS). BME 312-BMI II-L5- ALİ IŞIN 2014

3. Their operation is based on the electrical nature of conducting information within nervefibers, from the neuron cell body (soma),along the axon, where a travelling action potential is the carrier of excitation. • While the action potential is naturally generated chemically in the head of the axon, it mayalso be generated artificially by depolarizing the neuron membrane with an electrical pulse. BME 312-BMI II-L5- ALİ IŞIN 2014

4. A train of electrical impulses with certain amplitude, width, and repetition rate, applied to a muscle innervatingnerve (a motor neuron) will cause the muscle to contract, very much like in natural excitation. • Similarly,a train of electrical pulses applied to the muscular tissue close to the motor point will cause musclecontraction by stimulating the muscle through the neural structures at the motor point. BME 312-BMI II-L5- ALİ IŞIN 2014

5. The System for Delivering Stimulation Pulses to Excitable Tissue • Thesystem used to stimulate a nerve consists of three components; • A pulsegeneratortogeneratea train of pulses capable of depolarizing the nerve 2. A leadwire, the function of which is to deliver thepulses to the stimulation site 3. An electrode, which delivers the stimulation pulses to the excitabletissue in a safe and efficient manner. BME 312-BMI II-L5- ALİ IŞIN 2014

6. In terms of location of these threecomponents of an electrical stimulator,stimulation technologycan be described in the following terms: • Surface or transcutaneous stimulation, where all three components are outside the body and theelectrodes are placed on the skin above or near the motor point of the muscle to be stimulated. BME 312-BMI II-L5- ALİ IŞIN 2014

7. This method has been used extensively in medical rehabilitation of nerve and muscle. • The inability of surface stimulationto reliably excite the underlying tissue in a repeatable manner and to selectively stimulatedeep muscles has limited the clinical applicability of surface stimulation. BME 312-BMI II-L5- ALİ IŞIN 2014

8. Percutaneous stimulation, employs electrodes which are positioned inside the body close to the structures to be stimulated. • Their lead wires permanently penetrate the skin to be connected to the external pulse generator • State of the art embodiments of percutaneous electrodes utilize a smalldiameterinsulated stainless steel lead that is passed through the skin. • The electrode structure isformed by removal of the insulation from the lead and subsequent modification to ensure stability within the tissue. This modification includes forming barbs or similar anchoring mechanisms. BME 312-BMI II-L5- ALİ IŞIN 2014

9. implantable stimulation; • refers to stimulation systems in which all three components, pulsegenerator, lead wires, and electrodes, are permanently surgically implanted into the body and theskin is solidly closed after the implantation procedure. • Any interaction between the implantable partand the outside world is performed using telemetry principles in a contact-less fashion. We will focus on implantable neuromuscular stimulators. BME 312-BMI II-L5- ALİ IŞIN 2014

10. Stimulation Parameters • In functional electrical stimulation, the typical stimulation waveform is a train of rectangular pulses. • This shape is used because of its effectiveness as well as relative ease of generation. • All three parametersof a stimulation train, i.e., frequency, amplitude, and pulse-width, have effect on muscle contraction. BME 312-BMI II-L5- ALİ IŞIN 2014

11. Generally, the stimulation frequency is kept as low as possible, to prevent muscle fatigue and to conserve stimulation energy. • The determining factor is the muscle fusion frequency at which a smooth muscleresponse is obtained. This frequency varies; however, it can be as low as 12 to 14 Hz and as high as 50 Hz. • In most cases, the stimulation frequency is kept constant for a certain application. This is true both forsurface as well as implanted electrodes. BME 312-BMI II-L5- ALİ IŞIN 2014

12. In implantable stimulators and electrodes, the stimulation parameters greatly depend on the implantation site. • When the electrodes are positioned on or around the target nerve, the stimulation amplitudesare on the order of a few milliamperes or less. Electrodes positioned on the muscle surface (epimysialelectrodes) or in the muscle itself (intramuscular electrodes), employ up to ten times higher amplitudes BME 312-BMI II-L5- ALİ IŞIN 2014

13. For muscle force control, implantable stimulators rely either on pulse-width modulation or amplitudemodulation. • For example, in upper extremity applications, the current amplitude is usually a fixedparamter set to 16 or 20 mA, while the muscle force is modulated with pulse-widths within 0 to 200 μs. BME 312-BMI II-L5- ALİ IŞIN 2014

14. Implantable Neuromuscular Stimulators • Implantable stimulation systems use an encapsulated pulse generator that is surgically implanted and hassubcutaneous leads that terminate at electrodes on or near the desired nerves. • In low power consumptionapplications such as the cardiac pacemaker, a primary battery power source is included in the pulsegenerator case. When the battery is close to depletion, the pulse generator has to be surgically replaced. BME 312-BMI II-L5- ALİ IŞIN 2014

15. Most implantable systems for neuromuscular application consist of an external and an implantedcomponent. Between the two, an inductive radio-frequency link is established, consisting of two tightly coupled resonant coils. • The link allows transmission of power and information, through the skin, fromthe external device to the implanted pulse generator. In more advanced systems, a back-telemetry link isalso established, allowing transmission of data outwards, from the implanted to the external component. BME 312-BMI II-L5- ALİ IŞIN 2014

16. Ideally, implantable stimulators for neuromuscular control would be stand alone, totally implanteddevices with an internal power source and integrated sensorsdetecting desired movements from themotor cortex and delivering stimulation sequences to appropriate muscles, thus bypassing the neural damage. BME 312-BMI II-L5- ALİ IŞIN 2014

17. At the present developmental stage, they still need a control source and an external controllerto provide power and stimulation information. • The control source may be either operator driven,controlled by the user, or triggered by an event such as the heel-strike phase of the gait cycle. BME 312-BMI II-L5- ALİ IŞIN 2014

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19. Figure in the previous slide depicts a neuromuscular prosthesis for the restoration of hand functions using an implantableneuromuscular stimulator. • In this application, the patient uses the shoulder motion to control openingand closing of the hand. BME 312-BMI II-L5- ALİ IŞIN 2014

20. The internal electronic structure of an implantable neuromuscular stimulator is shown in above figure. • It consists of receiving and data retrieval circuits, power supply, data processing circuits, and output stages. BME 312-BMI II-L5- ALİ IŞIN 2014

21. Receiving Circuit • The stimulator’s receiving circuit is an LC circuit tuned to the resonating frequency of the externaltransmitter, followed by a rectifier. • Its task is to provide the raw DC power from the receivedrf signal and at the same time allow extraction of stimulation information embedded in therf carrier. BME 312-BMI II-L5- ALİ IŞIN 2014

22. Power Supply • The amount of power delivered into an implanted electronic package depends on the coupling betweenthe transmitting and the receiving coil. The coupling is dependent on the distance as well as the alignmentbetween the coils. • The power supply circuits must compensate for the variations in distance for differentusers as well as for the alignment variations due to skin movements and consequent changes in relativecoil-to-coil position during daily usage. • The power dissipated on power supply circuits must not raisethe overall implant case temperature. BME 312-BMI II-L5- ALİ IŞIN 2014

23. Data Retrieval • Data retrieval technique depends on the data-encoding scheme and is closely related to power supplycircuits and implant power consumption. • Most commonly, amplitude modulation is used to encode the in-going data stream. BME 312-BMI II-L5- ALİ IŞIN 2014

24. Data Processing • Once the information signal has been satisfactorily retrieved and reconstructed into logic voltage levels,it is ready for logic processing. • For synchronous data processing a clock signal is required. • It can begenerated locally within the implant device, reconstructed from the incoming data stream, or can be derived from the rf carrier. BME 312-BMI II-L5- ALİ IŞIN 2014

25. A crystal has to be used with a local oscillator to assure stable clock frequency. • Local oscillator allows for asynchronous data transmission. Synchronous transmission is best achievedusing Manchester data encoding. Decoding of Manchester encoded data recovers the original clock signal, which was used during data encoding. BME 312-BMI II-L5- ALİ IŞIN 2014

26. Another method is using the downscaledrf carrier signal as the clock source. In this case, the information signal has to be synchronized with therf carrier. BME 312-BMI II-L5- ALİ IŞIN 2014

27. Complex command structure used in multichannel stimulators requires intensive data decoding and processing andconsequentlyextensive electronic circuitry. • Custom-made, application specific circuits (ASIC) are commonly used tominimize the space requirements and optimize the circuit performance. BME 312-BMI II-L5- ALİ IŞIN 2014

28. Output Stage • The output stage forms stimulation pulses and defines their electrical characteristics. • Even though a mere rectangular pulse can depolarize a nervous membrane, such pulses are not used in clinical practice dueto their noxious effect on the tissue and stimulating electrodes. BME 312-BMI II-L5- ALİ IŞIN 2014

29. These effects can be significantly reducedby charge balanced stimulating pulses where the cathodic stimulation pulse is followed by an anodicpulse containing the same electrical charge, which reverses the electrochemical effects of the cathodic pulse. BME 312-BMI II-L5- ALİ IŞIN 2014

30. Charge balanced waveforms can be assured by capacitive coupling between the pulse generatorand stimulation electrodes. • Charge balanced stimulation pulses include symmetrical and asymmetricalwaveforms with anodic phase immediately following the cathodic pulse or being delayed by a short, 20 to60 μs interval. BME 312-BMI II-L5- ALİ IŞIN 2014

31. The output stages of most implantable neuromuscular stimulators have constant current characteristics,meaning that the output current is independent on theelectrode or tissue impedance. • Practically,the constant current characteristics ensure that the same current flows through the excitable tissuesregardless of the changes that may occur on the electrode-tissueinterface, such as the growth of fibrous tissue around the electrodes. BME 312-BMI II-L5- ALİ IŞIN 2014

32. The stimulus may be applied through either monopolar or bipolar electrodes. • The monopolar electrodeis one in which a single active electrode is placed near the excitable nerve and the return electrode isplaced remotely, generally at the implantable unit itself. BME 312-BMI II-L5- ALİ IŞIN 2014

33. Bipolar electrodes are placed at the stimulationsite, thus limiting the current paths to the area between the electrodes. • Generally, in monopolar stimulationthe active electrode is much smaller than the return electrode, while bipolar electrodes are the same size. BME 312-BMI II-L5- ALİ IŞIN 2014

34. Packaging of Implantable Electronics • Electronic circuits must be protected from the harsh environment of the human body. The packaging ofimplantable electronics uses various materials, including polymers, metals, and ceramics. • The encapsulation method depends somewhat on the electronic circuittechnology. BME 312-BMI II-L5- ALİ IŞIN 2014

35. Older devices may still use discretecomponents in a classical form, such as leaded transistors and resistors. • The newer designs, depending on the sophistication of the implanted device, may employ application-specific integrated circuits (ASICs)and thick film hybrid circuitry for their implementation. Such circuits place considerable requirementsfor hermeticity and protection on the implanted circuit packaging. BME 312-BMI II-L5- ALİ IŞIN 2014

36. Epoxy encapsulation was the original choice of designers of implantable neuromuscular stimulators. • Polymers do not provide an impermeable barrier andtherefore cannot be used for encapsulation of high density, high impedance electronic circuits. Themoisture ingress ultimately will reach the electronic components, and surface ions can allow electricshorting and degradation of leakage-sensitive circuitry and subsequent failure. BME 312-BMI II-L5- ALİ IŞIN 2014

37. Hermetic packaging provides the implant electronic circuitry with a long-term protection from theingress of body fluids. Materials that provide hermetic barriers are metals, ceramics, and glasses. • Metallicpackaging generally uses a titanium capsule machined from a solid piece of metal or deep-drawn froma piece of sheet metal. BME 312-BMI II-L5- ALİ IŞIN 2014

38. Electrical signals, such as power and stimulation, enter and exit the packagethrough hermetic feedthroughs, which are hermetically welded onto the package walls. The feedthroughassembly utilizes a ceramic or glass insulator to allow one or more wires to exit the package withoutcontact with the package itself. BME 312-BMI II-L5- ALİ IŞIN 2014

39. Metallic packaging requires that the receiving coil be placedoutside the package to avoid significant loss ofrf signal or power, thus requiring additional space withinthe body to accommodate the volume of the entire implant. • Generally, the hermetic package and thereceiving antenna are jointly imbedded in an epoxyencapsulant, which provides electric isolation for themetallic antenna and stabilizes the entire implant assembly. Below figure shows such an implantable stimulator BME 312-BMI II-L5- ALİ IŞIN 2014

40. Photograph of a multichannel implantable stimulator telemeter. Hybrid circuit in titanium packageis shown exposed. Receiving coil (left) is imbedded in epoxy resin together with titanium case. Double feedthroughsare seen penetrating titanium capsule wall on the right. BME 312-BMI II-L5- ALİ IŞIN 2014

41. More recently, alumina-based ceramic packages havebeen developed that allow hermetic sealing of the electronic circuitry together with enclosure of thereceiving coil. • This is possible due to therf transparency of ceramics. • The advantage of this approach is that the volume of theimplant can be reduced, thus minimizing the biologic response, which is a function of volume. BME 312-BMI II-L5- ALİ IŞIN 2014

42. Leads and Electrodes • Leads connect the pulse generator to the electrodes. • They must be sufficiently flexible to move across thejoints while at the same time sufficiently sturdy to last for the decades of the intended life of the device. BME 312-BMI II-L5- ALİ IŞIN 2014

43. They must also be stretchable to allow change of distance between the pulse generator and the electrodes,associated with body movements. • Ability to flex and to stretch is achieved by coiling the lead conductorinto a helix and inserting the helix into a small-diameter silicone tubing. BME 312-BMI II-L5- ALİ IŞIN 2014

44. Several individually insulated multi-strand conductors can be coiled together,thus forming a multiple conductor lead wire. • Most lead configurations include a connector at some pointbetween the implant and the terminal electrode, allowing for replacement of the implanted receiver orleads in the event of failure. • Materials used for lead wires are stainless steels, MP35N (Co, Cr, Ni alloy), and noble metals and their alloys. BME 312-BMI II-L5- ALİ IŞIN 2014

45. Electrodes deliver electrical charge to the stimulated tissues. • Those placed on the muscle surface arecalled epimysial, while those inserted into the muscles are called intramuscular. • Nerve stimulating electrodes are called epineural when placed against the nerve, or cuff electrodes when they encircle the nerve. BME 312-BMI II-L5- ALİ IŞIN 2014

46. Electrodes are made of corrosion resistant materials, such as noble metals (platinum or iridium) and their alloys. • For example, a platinum–iridium alloy consisting of 10% iridium and 90% platinum iscommonly used as an electrode material. • Epimysial electrodes use Ø4 mm Pt90Ir10discs placed on Dacron reinforced silicone backing. BME 312-BMI II-L5- ALİ IŞIN 2014

47. Figure shows Implantable electrodes with attached lead wires. • Intramuscular electrode (top) has stainless steel tipand anchoring barbs. • Epimysial electrode has PtIr disk in the center and is backed by silicone-impregnated Dacron mesh. BME 312-BMI II-L5- ALİ IŞIN 2014

48. Safety Issues of Implantable Stimulators • The targeted lifetime of implantable stimulators for neuromuscular control is the lifetime of their users,which is measured in tens of years. • Resistance to premature failure must be assured by manufacturing processes and testing procedures. BME 312-BMI II-L5- ALİ IŞIN 2014

49. Appropriate materials must be selected that will withstand the working environment. • Protection against mechanical and electrical hazards that may be encountered during thedevice lifetime must be incorporated in the design. BME 312-BMI II-L5- ALİ IŞIN 2014

50. Manufacturing and testing • Production of implantable electronic circuits and their encapsulationin many instances falls under the standards governing production and encapsulation of integrated circuits • To minimize the possibility of failure, the implantable electronic devices aremanufactured in controlled clean-room environments, using high quality components and strictly defined manufacturing procedures. • Finished devices are submitted to rigorous testing before being released for implantation. BME 312-BMI II-L5- ALİ IŞIN 2014