BME 312 BIOMEDICAL INSTRUMENTATION II LECTURER: ALİ IŞIN
LECTURE NOTE 5
Implantable Stimulators
FACULTY OF ENGINEERING
DEPARTMENT OF BIOMEDICAL ENGINEERING
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 be controlled volitionally because of the damage or dysfunction in the
neural paths of the central nervous system (CNS).
• Their operation is based on the electrical nature of conducting information within nerve fibers, 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 may also be generated artificially by depolarizing the neuron membrane with an electrical pulse.
• A train of electrical impulses with certain amplitude, width, and repetition rate, applied to a muscle
innervating nerve (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 muscle contraction by stimulating the muscle
through the neural structures at the motor point.
The System for Delivering Stimulation Pulses to Excitable Tissue
• The system used to stimulate a nerve consists of three components;
1.A pulse generator to generate a train of pulses capable of depolarizing the nerve
2. A lead wire, the function of which is to deliver the pulses to the stimulation site
3. An electrode, which delivers the stimulation pulses to the excitable tissue in a safe and
• In terms of location of these three components of an electrical stimulator, stimulation
technology can be described in the following terms:
• Surface or transcutaneous stimulation, where
all three components are outside the body and
the electrodes are placed on the skin above or
near the motor point of the muscle to be
• This method has been used extensively in medical rehabilitation of nerve and muscle.
• The inability of surface stimulation to reliably excite the underlying tissue in a repeatable manner and to selectively stimulate deep
muscles has limited the clinical applicability of
surface stimulation.
• 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 small diameter insulated stainless steel lead that is passed
through the skin.
• The electrode structure is formed by removal of the insulation from the lead and subsequent modification to ensure stability
• implantable stimulation;
• refers to stimulation systems in which all three components, pulse generator, lead wires, and
electrodes, are permanently surgically implanted into the body and the skin is solidly closed after the
implantation procedure.
• Any interaction between the implantable part and the outside world is performed using telemetry principles in a contact-less fashion. We will focus on implantable
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 parameters of a stimulation train,
i.e., frequency, amplitude, and pulse-width,
have effect on muscle contraction.
• 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 muscle response 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 for surface as well as
implanted electrodes.
• 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 amplitudes are on the order of a few milliamperes or less. Electrodes positioned on the muscle surface (epimysial electrodes) or in the muscle itself
(intramuscular electrodes), employ up to ten times higher amplitudes
• For muscle force control, implantable stimulators rely either on pulse-width modulation or amplitude modulation.
• For example, in upper extremity applications, the current amplitude is usually a fixed
paramter set to 16 or 20 mA, while the muscle
force is modulated with pulse-widths within 0
to 200 μs.
Implantable Neuromuscular Stimulators
• Implantable stimulation systems use an encapsulated pulse generator that is surgically implanted and has subcutaneous leads that terminate at electrodes on or near the desired nerves.
• In low power consumption applications such as the cardiac pacemaker, a primary battery power source is included in the pulse generator case. When the battery is close to depletion, the pulse generator has to be surgically replaced.
• Most implantable systems for neuromuscular application
consist of an external and an implanted component. 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, from the external device to the implanted pulse generator. In more advanced systems, a back-telemetry link is also established, allowing transmission of data
outwards, from the implanted to the external component.
• Ideally, implantable stimulators for
neuromuscular control would be stand alone, totally implanted devices with an internal
power source and integrated sensors
detecting desired movements from the motor cortex and delivering stimulation sequences to appropriate muscles, thus bypassing the
neural damage.
• At the present developmental stage, they still
need a control source and an external controller to 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.
• Figure in the previous slide depicts a
neuromuscular prosthesis for the restoration of hand functions using an implantable
neuromuscular stimulator.
• In this application, the patient uses the
shoulder motion to control opening and
closing of the hand.
• 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.
Receiving Circuit
• The stimulator’s receiving circuit is an LC
circuit tuned to the resonating frequency of the external transmitter, followed by a
rectifier.
• Its task is to provide the raw DC power from
the received rf signal and at the same time
allow extraction of stimulation information
embedded in the rf carrier.
Power Supply
• The amount of power delivered into an implanted electronic package depends on the coupling between the transmitting and the receiving coil. The coupling is dependent on the
distance as well as the alignment between the coils.
• The power supply circuits must compensate for the variations in distance for different users as well as for the alignment
variations due to skin movements and consequent changes in relative coil-to-coil position during daily usage.
• The power dissipated on power supply circuits must not raise the overall implant case temperature
.
Data Retrieval
• Data retrieval technique depends on the data- encoding scheme and is closely related to
power supply circuits and implant power consumption.
• Most commonly, amplitude modulation is
used to encode the in-going data stream.
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 be generated locally within the implant device, reconstructed from the incoming data stream, or can be derived from the rf carrier.
• 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 achieved using Manchester data
encoding. Decoding of Manchester encoded
data recovers the original clock signal, which
was used during data encoding.
• Another method is using the downscaled rf
carrier signal as the clock source. In this case,
the information signal has to be synchronized
with the rf carrier.
• Complex command structure used in
multichannel stimulators requires intensive data decoding and processing and
consequently extensive electronic circuitry.
• Custom-made, application specific circuits
(ASIC) are commonly used to minimize the
space requirements and optimize the circuit
performance.
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 due to their
noxious effect on the tissue and stimulating
electrodes.
• These effects can be significantly reduced by
charge balanced stimulating pulses where the
cathodic stimulation pulse is followed by an
anodic pulse containing the same electrical
charge, which reverses the electrochemical
effects of the cathodic pulse.
• Charge balanced waveforms can be assured by capacitive coupling between the pulse
generator and stimulation electrodes.
• Charge balanced stimulation pulses include symmetrical and asymmetrical waveforms
with anodic phase immediately following the
cathodic pulse or being delayed by a short, 20
to 60 μs interval.
• The output stages of most implantable
neuromuscular stimulators have constant current characteristics, meaning that the output current is independent on the electrode or tissue impedance.
• Practically, the constant current characteristics ensure that the same current flows through the
excitable tissues regardless of the changes that may occur on the electrode-tissue interface, such as the growth of fibrous tissue around the electrodes.
• The stimulus may be applied through either monopolar or bipolar electrodes.
• The monopolar electrode is one in which a single active electrode is placed near the excitable nerve and the return electrode is
placed remotely, generally at the implantable
unit itself.
• Bipolar electrodes are placed at the
stimulation site, thus limiting the current paths to the area between the electrodes.
• Generally, in monopolar stimulation the active electrode is much smaller than the return
electrode, while bipolar electrodes are the
same size.
Packaging of Implantable Electronics
• Electronic circuits must be protected from the harsh environment of the human body. The packaging of implantable electronics uses
various materials, including polymers, metals, and ceramics.
• The encapsulation method depends
somewhat on the electronic circuit
technology.
• Older devices may still use discrete components 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 requirements for
hermeticity and protection on the implanted circuit
• Epoxy encapsulation was the original choice of
designers of implantable neuromuscular stimulators.
• Polymers do not provide an impermeable barrier and therefore cannot be used for encapsulation of high density, high impedance electronic circuits. The
moisture ingress ultimately will reach the electronic components, and surface ions can allow electric
shorting and degradation of leakage-sensitive circuitry and subsequent failure.
• Hermetic packaging provides the implant electronic circuitry with a long-term
protection from the ingress of body fluids.
Materials that provide hermetic barriers are metals, ceramics, and glasses.
• Metallic packaging generally uses a titanium
capsule machined from a solid piece of metal
or deep-drawn from a piece of sheet metal.
• Electrical signals, such as power and stimulation, enter and exit the package through hermetic feedthroughs, which are hermetically welded onto the package walls.
The feedthrough assembly utilizes a ceramic
or glass insulator to allow one or more wires
to exit the package without contact with the
package itself.
• Metallic packaging requires that the receiving coil be
placed outside the package to avoid significant loss of rf signal or power, thus requiring additional space within the body to accommodate the volume of the entire implant.
• Generally, the hermetic package and the receiving
antenna are jointly imbedded in an epoxy encapsulant, which provides electric isolation for the metallic
antenna and stabilizes the entire implant assembly.
• Photograph of a multichannel implantable stimulator telemeter. Hybrid circuit in titanium package is shown exposed. Receiving coil (left) is imbedded in epoxy resin together with titanium case. Double feedthroughs are seen penetrating titanium capsule wall on the right.
• More recently, alumina-based ceramic packages have been developed that allow hermetic sealing of the
electronic circuitry together with enclosure of the receiving coil.
• This is possible due to the rf 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.
Leads and Electrodes
• Leads connect the pulse generator to the electrodes.
• They must be sufficiently flexible to move across the joints while at the same time
sufficiently sturdy to last for the decades of
the intended life of the device.
• 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 conductor into a helix and
inserting the helix into a small-diameter
silicone tubing.
• 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 point between the implant and the terminal
electrode, allowing for replacement of the implanted receiver or leads in the event of failure.
• Materials used for lead wires are stainless steels,
MP35N (Co, Cr, Ni alloy), and noble metals and their
• Electrodes deliver electrical charge to the stimulated tissues.
• Those placed on the muscle surface are called
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.
• 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 is commonly used as an electrode material.
• Epimysial electrodes use Ø4 mm Pt90Ir10 discs placed on Dacron reinforced silicone backing.
• Figure shows Implantable electrodes with attached lead wires.
• Intramuscular electrode (top) has stainless steel tip and anchoring barbs.
• Epimysial electrode has PtIr disk in the center and is backed by silicone-
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.
• Appropriate materials must be selected that will withstand the working environment.
• Protection against mechanical and electrical
hazards that may be encountered during the
device lifetime must be incorporated in the
design.
Manufacturing and testing
• Production of implantable electronic circuits and their
encapsulation in many instances falls under the standards governing production and encapsulation of integrated circuits
• To minimize the possibility of failure, the implantable
electronic devices are manufactured 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.
Bio-compatibility
• Since the implantable stimulators operate surgically implanted in living tissue, an
important part of their design has to be dedicated to biocompatibility,
• i.e., their ability to dwell in living tissue without
disrupting the tissue in its functions, creating
adverse tissue response,or changing its own
properties due to the tissue environment.
• Elements of biocompatibility include tissue reaction to materials, shape, and size, as well as
electrochemical reactions on stimulation electrodes.
• There are known biomaterials used in the making of implantable stimulators. They include stainless steels, titanium and tantalum, noble metals such as
platinum and iridium, as well as implantable grades of selected epoxy and silicone-based materials.
Susceptibility to electromagnetic interference (EMI) and electrostatic discharge (ESD)
• Electromagnetic fields and electrostatic discharges can disrupt the operation of electronic devices,
which may be lethal in situations with life support
systems, but they may also impose risk and danger to users of neuromuscular stimulators.
• The electronic circuitry in implantable stimulators is generally protected by the metal case. However, the circuitry can be damaged through the feedthroughs either by handling or during the implantation
Implantable Stimulators in Clinical Use
• Peripheral Nerve Stimulators
Manipulation—Control of complex functions for movement, such as hand control, requires the use of many channels of stimulation Locomotion—The first implantable stimulators were designed and
implanted for the correction of the foot drop condition in hemiplegic patients.
Respiration—Respiratory control systems involve a two-channel implantable stimulator with electrodes applied bilaterally to the phrenic nerve. Activation of the phrenic nerve results in
contraction of each hemidiaphragm in response to electrical
Urinary control—Urinary control systems have been developed for persons with spinal cord injury.
Scoliosis treatment—Progressive lateral curvature of the
adolescent vertebral column with simultaneous rotation is known as idiopathic scoliosis. Electrical stimulation applied to the convex side of the curvature has been used to stop or
reduce its progression
•Stimulators of Central Nervous System
Some stimulation systems have electrodes implanted on the surface of the central nervous system or in its deep areas.
They do not produce functional movements; however, they
“modulate” a pathological motor brain behavior and by that stop unwanted motor activity or abnormality. Therefore, they can be regarded as stimulators for neuromuscular control.
Cerebellar stimulation—Among the earliest stimulators from this category are cerebellar stimulators for control of
reduction of effects of cerebral palsy in children.
Vagal stimulation—Intermittent stimulation of the vagus nerve with 30 sec on and five min off has been shown to reduce frequency of epileptic seizures.
Deep brain stimulation—an implantable stimulation device (Activa byMedtronic) can dramatically reduce uncontrollable tremor in patients with Parkinson’s disease or essential
tremor . With this device, an electrode array is placed stereotactically into the ventral intermediate nucleus of thalamic region of the brain
.
Recent Advances and Future of Implantable Electrical Stimulators
• Distributed Stimulators
• One of the major concerns with multichannel implantable neuromuscular stimulators is the
multitude of leads that exit the pulse generator and their management during surgical implantation.
• A solution to that may be distributed stimulation
systems with a single outside controller and multiple single-channel implantable devices implanted
throughout the structures to be stimulated.
• Micro-injectable stimulator modules have been developed that can be injected into the tissue, into a muscle, or close to a nerve through a lumen of a hypodermic needle.
• A single external coil can address and activate a number of these devices located within its field, on a pulse-to-pulse basis.
• Sensing of Implantable Transducer–Generated and Physiological Signals
• External command sources such as the shoulder-controller impose additional constraints on the implantable stimulator users.
• Permanently implanted control sources make neuro-
prosthetic devices much more attractive and easier to use
• An implantable joint angle transducer (IJAT) has been
developed that consists of a magnet and an array of magnetic sensors implanted in the distal and the proximal end of a
joint, respectively.
• The sensor is connected to the implantable stimulator
package, which provides the power and also transmits the sensor data to the external controller, using a back-telemetry link.
• Myoelectric signals (MES) from muscles not affected by paralysis are another attractive
control source for implantable neuromuscular stimulators. Amplified and bin-integrated EMG signal from uninvolved muscles, such as the
sternocleido-mastoid muscle, has been shown
to contain enough information to control an
upper extremity neuroprosthesis
Summary
• Implantable stimulators for neuromuscular control are an important tool in rehabilitation of paralyzed individuals with preserved neuro-muscular apparatus, as well as in the
treatment of some neurological disorders that result in involuntary motor activity.
• Their impact on rehabilitation is expected to increase with
further progress in microelectronics technology, development of smaller and better sensors, and with improvements of
advanced materials. Advancements in neurophysiological
science are also expected to bring forward wider utilization of possibilities offered by implantable neuromuscular stimulators.
