Implantable Neural Stimulators

Physical Design and Materials for the Stimulator

The physical form of the neurostimulator is designed based on constraints, requirements, and ideals from both engineering and clinical realms. The device design must balance the need for a biocompatible, hermetically sealed, and mechanically robust package that is capable of housing all the stimulator components and meeting the clinical demands for minimal invasiveness, conformation to anatomy, facilitation of surgical installment, and device cosmesis.

Most implantable neurostimulators conform to a fundamental organization consisting of three primary components: a centralized implantable pulse generator (IPG), one or more leads, and one or more electrodes (for stimulating and possibly recording). Some of the many commercially available stimulators following this organizational paradigm include all cardiac pacemakers and defibrillators, deep brain stimulators, spinal cord stimulators, vagus nerve stimulators, and cochlear implants. The stimulator houses some or all of the stimulation circuitry (some devices may have external components), and the leads carry the stimulus current to the electrodes, which provide the electrochemical interface to the nervous system. The leads and electrodes are discussed in Chapter 20 .

Stimulator size can range from quite small (Nucleus 24 by Cochlear Ltd, Lane Cove, NSW, Australia, is approximately 6.9 mm thick × 22 mm wide × 50.5 mm long, with much of the length resulting from an external inductive coil ( )) to relatively large (implantable defibrillators with a volume greater than 200 cm ³ have been commercially deployed). The NDI Medical Micropulse stimulator, a fully implantable programmable IPG, has dimensions of 48 mm × 29 mm × 7.5 mm (volume of 8 cm ³ ) and weighs 17 g. The shape of the device will depend on where it is to be implanted. Most neurostimulators are implanted in a subclavicular pocket or in the abdomen and have a familiar flat, rounded shape. The flat shape is designed to minimize the device profile under the skin, and the rounding facilitates surgical insertion and minimizes tissue erosion at the implantation site. For devices that are implanted in locations other than the thoracic or abdominal regions, device shape and size can vary significantly to fit the anatomy relevant to the particular application. For example, cochlear implants must be small enough for implantation in the mastoid process of the temporal bone of the skull (and often have the telemetry/powering coil external to the device can, which is located in a shallow subcutaneous pocket behind the ear). Another device exhibiting a conformational shape is the NeuroPace RNS stimulator (NeuroPace Inc., Mountain View, CA), which is designed for intracranial implantation and therefore has a thin curved profile for conformation to the cortex.

Notable exceptions to the centralized stimulator–lead–electrode design have been developed in response to particular application demands. The Alfred Mann Institute’s Bion (Advanced Bionics/Boston Scientific, Valencia, CA), shown in Fig. 21.1D , is a single-channel stimulator that is fully encapsulated in a glass or ceramic package and is small enough to be inserted via injection into a target muscle for motor functional electrical stimulation (FES) applications or near to a target nerve for applications such as stimulation for migraine headache ( ) or urinary control ( ). At Case Western Reserve University (CWRU, Cleveland, OH) we have developed a distributed motor FES system (see Fig. 21.1 ) that consists of multiple stimulating and recording modules networked together to form an intrabody network. This system, called the Networked Neuroprosthesis (NNP), is designed to provide a scalable and flexible neuroprosthesis platform to meet the variable stimulation needs of those suffering from paralysis or paresis ( ). The NNP design enables recording from and delivery of stimulation to remote sites and implementation of multiple neural implants by the one platform. Retinal implants, which are working toward the restoration of visual function, also depart from the standard form (see Fig. 21.1F ), and are compact systems designed to be implanted on to the retinal surface within the eye ( ).

A standard centralized stimulator consists of two major components: the hermetic package, and the interconnect header for connection of the leads to the stimulator. The primary function of the hermetic package is to keep bodily fluids from reaching the stimulator circuitry and prevent the body from being exposed to the potentially harmful chemicals present inside of the stimulator (especially those in the battery, if present, which is sealed in a hermetic can of its own). Most IPG designs use titanium as the material of choice for the hermetic package, because of its biological inertness, very attractive strength properties, and light weight.

There are several implantable-grade titanium qualities and alloys. Most devices use commercially pure titanium (e.g., grades 1, 2), although the increasing use of transcutaneous powering and recharge systems (see the section below on the power system) is making some of the more power-efficient alloys more attractive (e.g., grade 23). These alloys tend to be more mechanically brittle than pure titanium, which translates into larger bend radius constraints for can molding, but results in less magnetic eddy current loss during inductive power transfer. The increased inductive coupling afforded by these materials can translate into increased power efficiency, decreased device heating, or deeper implantation depths. Other packaging approaches to maintaining power transfer efficiency include using more magnetically transparent packaging material such as ceramic, and locating the powering coil external to the hermetic metal package using hermetic feedthroughs. Ceramic packaging has had an increasing large impact in the packaging of neuromodulation devices, because enclosure of the receiving coil inside the package allows efficient transmission through the package and therefore a smaller package. This approach was pioneered by Erwin Hochmair (1997) and adopted for the Med-El Cochlear implant, and leads to a package size of 33.5 mm × 23.4 × 3 mm. For the external powering coil option, the coil and package tend to be potted together in a biocompatible epoxy (e.g., Medtronic’s Mattrix stimulator (Medtronic Inc., Minneapolis, MN) and the CWRU and NeuroControl Corp. Freehand System (Cleveland, OH) ( ) and IST ( ) motor FES system) or plastic polymer (some cochlear implants). For systems implementing monopolar stimulation, the titanium package (“can”) is often used as the return (common) anode. When this is the case, the stimulator can is often partially coated with (or partially potted in) an insulating material to restrict return path current flow to a particular region of the case which will not be in contact with electrically activatable tissue. This is done to prevent unintended activation of muscle (e.g., the pectoralis muscles if a subclavicular pocket is used) or sensory fibers ( ). The conductors carrying the stimulation current are transferred through the hermetic metal can using glass or ceramic feedthroughs. These feedthroughs insulate the conductors from each other and from the conductive stimulator package and maintain the overall stimulator hermeticity. On the external side of the feedthrough the conductors either connect directly to permanently attached leads or pass into a header which contains connectors for a detachable lead(s) (the header is often cast biocompatible epoxy). The detachable lead(s) is fixed into the connector by either set screws or spring locks. The design of the header and connector takes into account the fact that some assembly is required by the surgeon/implanter: connectors tend to be large for easy handling, and orientation is designed to be unambiguous. Some devices include radiopaque markers in the header to facilitate postimplant identification using radiography. It is also of note that implantable stimulators, leads, and electrodes can only be sterilized using chemical processes (e.g., ethylene oxide), due to the damaging moisture and high temperatures associated with autoclaving.