ABSTRACT

55.1 Introduction Functional electrical stimulation (FES) is a rehabilitation technique for the restoration of lost neuro logical function, resulting from conditions such as stroke, spinal cord injury, cerebral palsy, head inju ries, and multiple sclerosis. FES utilizes low-level electrical current applied in programmed patterns to dierent nerves or reex centers in the central nervous system to produce functional movements. e stimulation may be triggered by a single switch (open-loop) or from sensor(s) or neuronal activity (closed-loop). While FES has been used successfully to pace the heart 1 and to restore hearing 2 in the past, it has not been widely adopted as a means of reanimating paralyzed limbs that result from stroke and spi nal cord injury (SCI). It is estimated by the U.S. National Institutes of Health (NIH) that there are more than 600,000 people who experience a stroke each year in the United States, with an associated comprehensive cost of $43 billion per year. 3 Of the more than 4 million stroke survivors alive today, many experience permanent impairments of their ability to move, think, understand and use language, or speak—losses that compromise their independence and quality of life. Furthermore, stroke risk J. Schulman Alfred Mann Foundation for Scientific Research P. Mobley Alfred Mann Foundation for Scientific Research J. Wolfe Alfred Mann Foundation for Scientific Research Ross Davis Florida Institute of Technology I. Arcos Alfred Mann Foundation for Scientific Research experiencing a stroke is increasing. ere are also estimated to be 250,000 Americans living with spinal cord injuries with 10,000–12,000 new spinal cord injuries reported every year in the United States. e cost of managing the care of SCI patients approaches $4 billion each year. 4 e potential of FES to restore function in these areas has been largely unfullled mostly due to the limitations of the FES devices currently available. FES could also be used in limb loss applications to reduce phantom pain and to restore the functional movement of prosthetic limbs. In 2000/2001, about 130,000 lower-limb amputations were performed each year in the United States. 5,6 An optimal FES system should have the following fundamental characteristics. It should (1) provide both stimulating and sensing capabilities, (2) be fully implantable, (3) be minimally invasive, (4) have real-time communication capability, (5) allow a practically unlimited number of stimulation and sensing channels, and (6) function without external equipment or interconnected leads between components. is chapter describes a network of wireless implantable microstimulators/microsensors, also known as battery-powered BION •* (BIOnic Neuron) devices for functional electrical stimulation and sensing (FES-BPB system). is new platform was designed to overcome the limitations of the current FES tech nology by providing 1. Microdevices that can be programmed to be either stimulators or sensors for use in closed-loop applications 2. Minimally invasive implantation procedures to reduce labor-intensive surgery and associated patient risks and to provide rapid recovery 3. Wireless bidirectional communications and telemetry to all stimulators and sensors, which eliminates the use of both transcutaneous leads (which are susceptible to infection), and surface applied coils and stimulators 4. Real-time communication between the stimulators, sensors, and control unit, to maintain continuous closed-loop control 5. Flexibility and functional expandability since there are no leads and each implant has a full complement of programmable stimulators and sensors 6. A large number of channels, which allows the same system to be used for a variety of applications without interference in the same patient 7. Self-powered operation using rechargeable batteries to power the implantable devices. External equipment (e.g., power antennas) are only needed during battery recharging 8. Wireless sensors capable of measuring biopotentials, angle, position, pressure, temperature, and permanent magnet elds 55.2 Evolution of the Implantable BION Devices In 1988, J. Loeb proposed and W.J. Heetderks showed mathematically that the concept of a wireless net work of injectable microstimulators powered by an external antenna/coil was possible. 7 It was thought that these microstimulators would eliminate many of the problems associated with the use of percutane ous electrodes, since they do not incorporate leads. J.H. Schulman, G.E. Loeb, and P.R. Troyk, under sup port contracts from the U.S. NIH (contract N01-NS-9-2327), the Alfred Mann Foundation (AMF, Santa Clarita, CA), and the Canadian Network for Neural Regeneration and Functional Recovery, developed an injectable, glass-enclosed microstimulator (Figure 55.1) that is powered and controlled by an external alternating magnetic eld generated by a coil connected to a control unit. is rst 255-channel stimulat ing system, later called the radio-frequency (RF) BION device, allowed instantaneous control of stimula tion pulse amplitude, frequency, pulse width, pulse position timing, and pulse charge-recovery current. 8 AMF continued to develop and improve the wireless RF BION device. As a result of these eorts, a second-generation RF BION device, which incorporates a ceramic case, an output capacitor, and Zener * BION is a registered trademark of the Advanced Bionics Corporation, a Boston Scientic company. diodes to protect the device against electrostatic discharges, was developed (Figure 55.1). 9 ese RF BION devices are currently being used or have been used in several clinical studies being conducted by AMF and its aliated organizations, the Alfred Mann Institute (University of Southern California, Los Angeles, CA) and Advanced Bionics Corporation (Santa Clarita, CA). As of September 2004, 33 patients have been implanted with RF BION devices for the treatment of urinary incontinence, obstructive sleep apnea, pain associated with shoulder subluxation, knee osteoarthritis, forearm contracture, and foot drop applications. 10–13 While the wireless RF BION device eliminates the need for the leads associated with the percutane ous electrodes, it requires the patient to wear an external coil during use, to transmit power and data to the implanted device. To both improve the patient acceptance of this technology and increase the reliability of the system, it was necessary to eliminate the need to constantly use an external coil to power and control the device. us, the idea for a battery-powered BION device (hereinaer BPB) was conceived at the AMF. AMF developed, with guidance from Jet Propulsion Lab (JPL), a small cylindrical lithium ion rechargeable battery. A new company (Quallion, Inc.) was formed and nanced by Alfred Mann to manufacture and improve these unique highly reliable batteries. Today, these batteries can be safely recharged if discharged to 0 V and are expected to operate for over 10 years. ese batteries were designed specically for the BPB (Figure 55.2). AMF licensed the BPB technology to Advanced Bionics Corp., which designed and implemented the rst BPB (stimulator only) for urinary incontinence (referred to as the UI–BPB). is was the rst BION RF Powered Length (mm) Diameter (mm) BION Family Glass RF BION 15 2 16.7 2.4 28 3.1 25 3.15 Ceramic RF BION UI-BPB BPB for FE S Battery Powered FIGURE 55.1 Evolution of the BION devices. It shows four BIONS to the same scale. FIGURE 55.2 Quallion battery for the BPB. to have two-way telemetry (Figure 55.1). e telemetry receiver in this UI-BPB turns on for a very short time interval every 1.5 s, to conserve battery power. us, rapid synchronization for limb control is not feasible with the UI-BPB. As of September 2004, the UI-BPB has been implanted in 35 patients for the treatment of urinary incontinence and migraine headaches. 14 Owing to its lack of sensing capabilities and slow communication response time, the UI-BPB is not well suited for FES applications. AMF is currently developing the next generation of the BPB. is BPB (Figure 55.1) allows the cre ation of a wireless FES network, including both stimulation and sensing in each BPB for fully implant able closed-loop applications; data processing for sensed signals; high-speed bidirectional telemetry; wireless oscilloscope monitoring for tting purposes, via back telemetry of voltage-sensed signals; a rechargeable battery (enabling prolonged operation without external power); and capability of commu nications with over 850 BPBs simultaneously (eectively 100 communications/s). 15 e ceramic BION devices (Figure 55.1) use an extremely strong zirconia with 3% yttrium ceramic case. e FDA pointed out that long-term immersion in water signicantly weakens this ceramic (Joe Schulman, personal communication). Over a 3-year period, AMF came up with a process to improve the longevity of this ceramic. Today, accelerated life testing has shown that this ceramic will retain 80% of its strength aer 80 years of soaking in saline solution. 16–18 55.3 Battery-Powered BION System for Functional Electrical Stimulation and Sensing e FES-BPB system is a wireless, multichannel network of separately implantable battery-powered BION devices that can be used for both stimulation and sensing. e system is composed of a master control unit (MCU), a clinician’s programmer, a recharging subsystem (charger and coil), and BPBs. Additional equipment, dissolvable suture material, and surgical insertion tools (used only during the implantation procedure) are also part of the system. e FES-BPB system can be set up for use in two congurations: tting mode and stand–alone mode. A block diagram of the FES-BPB system is shown in Figure 55.3. e MCU is the communication and control hub for the FES-BPB system. e MCU transmits com mands to and receives data from all the BPBs and the recharging subsystem. ere are two versions contemplated for the MCU packaging: (1) an external MCU, which will be outside the body and have a few controls accessible to the patient, and (2) an implantable MCU, which will be implanted in a conve nient location in the body. e implantable MCU version will have a small patient control unit (PCU). e clinician’s programmer consists of soware loaded on a computer that allows the clinician to congure and test the FES-BPB system for each patient. e recharging subsystem (charger and coil) is used when the rechargeable battery of the BPB needs to be recharged. e charging process requires the placement of the coil close to the area on the patient’s body where the BPB is implanted. Recharging is mandatory when the battery of the BPB is low. Depending on the frequency of use and stimulation levels delivered by the BPB, the battery could potentially run down in 1–8 days. Under normal stimulation conditions (nerve stimulation of 1–2 mA pulse amplitude, 15–100 μs pulse width, and 20 pulses per second), charging for about 5–20 min per day is required to charge the battery. e BPB maximum stimulation capability (20 mA pulse amplitude at 14 V compliance with 200 μs pulse width and 125 pulses per second) can rapidly discharge the battery in a very short time and would result in the need to recharge the battery for longer periods of time and much more frequently. e external charger coil transmits only power to the implanted devices. Charging and battery status are transmitted by each BPB to the MCU. Each BPB can accept a charging eld 20 times the nominal eld to recharge, without overheating. Upon completion of the charging process, the MCU then issues a stop-charging command to the charger. e BPB has a sensor to detect the eld from a permanent magnet. e function of the magnet is to hold the stimulation o. is is a safety feature in case the BPB is stimulating in an undesired manner and the patient does not have access to his control unit. When the magnet is positioned on the patient’s body over the area where the BPB is implanted, a magnetic sensor inside the BPB detects the external magnetic eld and holds the stimulation o. When the magnet is removed, the stimulation turns back on. is is the default mode of the magnetic detector. Other modes can be programmed during tting. e BPB implantation is performed in a minimally invasive procedure and is accomplished using a combination of specially designed insertion tools and commercially available items. e functional description of the FES-BPB system components is presented here: 1. Master control unit e MCU is the communication and control hub for the FES-BPB system. e MCU transmits commands to and receives data from each one of up to a total of 850 BPBs in the system within one-hundredth of a second. When the patient uses the FES-BPB system (stand-alone mode), the MCU coordinates the activity of the BPBs by receiving data from implanted devices programmed as sensors, transmitting stimulation commands, and monitoring the overall system status. It also serves as the basic user interface for the patient, providing system ON/OFF control and alarms, as well as program selection and limited parameter control. During tting, the MCU acts as a conduit between the clinician’s programmer and the rest of the system, enabling the transparent setup of each BPB and the coordination needed among BPBs to implement the desired functional movement. e MCU also manages the recharging subsystem. e charger communicates with the system in the same manner that the BPBs do, and it can be turned ON/OFF or checked for correct operation via the MCU. PCU Patient control unit E q u i p m e n t a n d t o o l s f o r i m p l a n t a t i o n Skin CP Clinician’s programmer Stim. pattern For fitting mode only Patient selection MCU Master Control unit For FES-BPB device’s battery charging only Charger Magnet safety shut-off S e n s e d d a t a t o M C U s y s t e m c o n t r o l d a t a S t i m u l a t i o n p a r a m e t e r s t o F E S B P B s s y s t e m c o n t r o l d a t a Devices selected as stimulators F E S B P B z F E S B P B B F E S B P B A F E S B P B 3 F E S B P B 2 F E S B P B 1 Devices selected as sensors For battery-charging only Used at all times Fitting mode only For implantation only Battery-charging only Wireless Wired Dashed arrows Solid arrows Coil Stim. On/Off FIGURE 55.3 Functional electrical stimulation and sensor system (FES-BPB). e MCU contains the following safety mechanisms: • An emergency STOP button on the external MCU, which when depressed, immediately issues a “stop stimulation” command to all BPBs. • During recharging, if any BPB overheats or overcharges and cannot protect itself, it would communicate this information to the MCU, which would issue a “stop charging” command to the charger and alert the patient. If an external MCU is being used, it would produce a sound to alert the patient. In the case where an implantable MCU is being used, the MCU would send a command to the BPBs to produce a specic stimulation pattern to alert the patient and would also communicate with the external PCU, if it is within the communication range. e PCU would then generate an audible alert. e MCU also stores patient usage data for the clinician. is data can be used to verify compliance and to analyze the stimulation and sensing parameters of each session. e approximate location of each BPB in the body is also maintained in this database. 2. Soware and rmware e soware for the FES-BPB system is divided into two components. One component is the clinician’s programmer application, which runs on a laptop computer. e other component is the rmware running on the MCU. e PC with the clinician’s programmer interfaces with the MCU via a serial communication link. During the tting of the system to a particular patient, the two components work in concert to facilitate measurement and storage of the stimulation and sensor calibration parameters. Once these parameters have been gathered in a tting session, the essential information can be stored in the MCU, so that the MCU can operate in a stand-alone mode to facilitate the desired functional movement. Ultimately, the MCU will modulate the stimulation output in response to the information it receives from the BPBs programmed as sensors. e clinician’s programmer uses a graphical interface that contains screens to perform the following essential functions: • Gather basic personal information for the patient, including information about the location in the body of his/her BPBs • Establish the stimulation range for each implanted BPB and allow selection of the stimulation parameters • Specify the details of the activity sequences that will be involved in the FES algorithm • Gather the trigger information that will be used to generate transitions between activity sequences in response to the sensor inputs • Compose the nite-state machine functions that will drive a routine and download the complete program to the MCU for either immediate execution or later use 3. Battery-powered BION device (BPB) e BPB is a battery-powered microdevice that is capable of both delivering electrical stimulation and acting as a general-purpose sensor for recording biopotential signals, pressure, distance, or angle between two BPBs and temperature. e following sections describe the functional building blocks of the BPB, battery, and packaging. e specications of the BPB are provided in Table 55.1, Section 6. Figure 55.4 shows the internal components of the BPB and Figure 55.5 shows a cross section of an assembled BPB. e BPB has the following subsystems: 3.1 Stimulation e BPB is a single-channel, constant-current, charge-balanced stimulator. e stimulation output is capacitance-coupled, which also prevents direct connection between the battery or battery-generated DC voltages and the tissue. Stimulation pulse amplitude, width, and frequency can be independently adjusted. In addition, triggering events can cause the stimulation to be delivered continuously or in a pulse burst, which can be ramped up and/or down with a variety of start/stop times. TABLE 55.1 Battery-Powered BION Device Specications 1. Physical Implant weight 0.6 g Implant length with eyelet ++ and diameter 25 mm max length/3.15 mm max diameter Electrodes area 5.2 sq. mm (0.008 sq in.) stimulation electrode 12.8 sq. mm (0.019 sq in.) return electrode Case materials Yttria-stabilized zirconia; titanium 6Al4V alloy Electrode material Iridium 2. Stimulation parameters Pulse amplitude 5 μA to 20 mA in 3.3% exponential steps (255 levels) Pulse width 7.6–1953 μs in 7.6 or 15.2 μs steps Pulse frequency 1–4096 pps Stimulation control response time 10.6 ms maximum Capacitor recharge current 10–500 μA Compliance voltage Up to 14 V automatically adjusted Stimulation output capacitor 4 μF Delay to start from a trigger 0–42.4 h in 15.6 ms, 125 ms, 2 s, 1 min, 10 min steps Burst On/O time Min Max Step Range 1 0.031 s 0.9996 s 0.0156 s Range 2 0.25 s 8.00 s 0.125 s Range 3 4 s 128 s 2 s Range 4 2 min. 64 min. 1 min. 3. Sensors Temperature 16°–50°C with 0.3% accuracy Magnetic eld to trigger shut o 10.0 Gauss threshold Goniometry (number of frequency channels) 8 Range 1–20 cm Repeatable accuracy error Less than 1% for 1–10 cm Pressure (range) Readout = AC-coupled. 300–900 mm-Hg absolute Accuracy ±10 mm-Hg Biopotential sensing (amplication) 10, 30, 100, 300, 1000 Low frequency roll-o 1, 10, 30, 100, 300 Hz High frequency roll-o 300, 1 K, 3 K, 10 K Hz Notch lter 50 or 60 Hz Input referred noise 5 μVrms 4. Communication Number of implants per patient Up to 850 at 10 ms ID, MCU/BPB 27/30 bits Bandwidth 5 MHz Sense-to-stimulate delay 10.6 ms maximum Frequency band 100–500 MHz MCU to BPB data rate 15 bits/6 μs (15-bits data + 16-bits FEC)/6 μs BPB to MCU data rate 8 bits/5 μs (8-bits data + 8-bits FEC)/5 μs Data streaming (oscilloscope mode) 39.8 K samples/s × 3 channels (8-bit resolution) 5. Charging Frequency of charging eld 127 kHz Excessive magnetic eld permissible 20 times nominal continued 3.2 Communication e bidirectional propagated wave RF communication between the MCU and the BPBs is established through a dipole antenna (Figure 55.5). is link operates at a frequency in the band 100–500 MHz, using Quad phase modulation with a 5 MHz bandwidth. e BPB communication module includes a crystal-controlled transmitter, receiver, and digital processing unit that synchronizes with and processes the MCU transmissions. e digital processing unit in the BPB also corrects small numbers of errors in the received data, decodes the MCU commands, and generates the responses to the MCU, including the reporting of higher numbers of communication errors that cannot be mathematically corrected. In this latter situation, the MCU would resend the message. e communication protocol between the MCU and BPBs is shown in Figure 55.6. e timing of the frame is completely controlled by the MCU and every BPB will synchronize to its MCU’s clock. e header and trailer elds are used for frame synchronization and for carrying frame control data intended for all BPBs and/or for other MCUs. When an MCU detects another MCU, the one with the higher ID number shis the time slots of all the BPBs it is controlling, Battery Charge/ Goniometry coil ASIC chips Stimulation capacito r pressure sensor (Optional) Stimulation electrode and communication antenna Biopotential and temperature sensors Magnetic sensor Return el ectrode and communication antenna FIGURE 55.5 Battery-powered BION device cross section. TABLE 55.1 (continued) Battery-Powered BION Device Specications 6. Battery: Lithium ion rechargeable, hermetically sealed Battery length and diameter 13 mm length, 2.5 mm diameter Battery weight 0.21 g Battery capacity 3.0 mAh, 10 mWh Cell voltage range 3.0–4.0 V (3.6 V nom.) Battery life Nominally 10 years (usage dependent) FEC = Forward error correction. Brazed case Battery case Battery Electronics Coil on ferrite tube Stim. cap 300,000 Transistors 4 ASIC’s 1 Magnetic sensor* 1 Quartz crystal *Only non-custom part 11 €in film capacitors 6 Inductors 6 Discrete capacitors Ceramic case FIGURE 55.4 Battery-powered BION internal components. to avoid communication interference. Once the MCU assigns the time slots for the downlink and uplink data packets to each one of the BPBs in the net, each BPB turns on its receiving or transmitting circuitry for only a few microseconds at the assigned times in each frame to save battery. e downlink data packets contain stimulation and/or sensing control data and forward error correction (FEC) bits to correct up to 4- or 5-bit errors. Bit errors beyond that number are reported to the MCU, which will then resend the message. If some messages are vital, the message would be sent twice or the value would be sent back to the MCU for the MCU to verify and authorize the command. Uplink data packets are transmitted by each of the BPBs and are used to carry information to the MCU (e.g., sensed data). e FEC in the uplink data packet only corrects 1- or 2-bit errors. 3.3 Power (battery and charging) e main power source for the BPB is a 10 mWh rechargeable lithium ion battery that allows the implanted device to operate as a stand-alone stimulator/sensor. Its special nonammable lithium ion chemistry provides long life and permits the voltage to go to zero and be recovered safely without damage to the battery. e recharge process is achieved via a low frequency (127 kHz) magnetic link with an external coil worn or placed nearby when charging. Assuming continuous stimulation pulses at 20 pps with 100 μs pulse width and 2 mA pulse amplitude into a 2 kΩ load, the battery of a BPB selected as a stimulator will provide 100 h of continuous operation. For a BPB selected as a sensor, the battery will also provide 100 h of continuous operation. e lithium ion battery is specied to have a cycle life of 2000 cycles for a standard charge/ discharge cycle, which is a fairly deep discharge of the battery before recharge occurs. e nominal stimulation/sensing requirements in many applications are such that the battery would not be discharged to the standard low level (if recharged daily). us, the 2000 cycles represent a lifetime of over 10 years if the battery is recharged daily. 3.4 Safety e BPB includes the following safety features: • A miniature magnetic sensor that detects the magnetic eld from an external magnet and holds o the stimulation if, for some reason, it needs to be turned o. • A temperature sensor that communicates with the charger, via the MCU, to terminate charging, if appropriate and disconnects the battery when the temperature rises above a predetermined threshold. • Battery safety circuitry that protects the battery from overvoltage, overdischarge, and overcharging. • BPBs can protect themselves from magnetic elds in excess of 20 times the eld necessary for maximum charging and, for a short time, for elds in excess of 50 times the eld for maximum charging. is short time is more than sucient for the BPB to send a message to the MCU to turn o the charger and to alert the patient of the risk. 3.5 Biopotential sensing, data display, and data analysis e biopotential function is implemented to record neural or muscular electrical signals (EMG signals). Biopotential sensing is accomplished using a low-noise amplier and band– pass lter circuit, followed by a digital postprocessing circuit. e amplier is adjustable from a gain of 10 to a gain of 1000. e low-frequency setting of the band–pass lter is adjustable from below 1 to 300 Hz. e high-frequency setting is adjustable from 300 Hz to 10 kHz. Input referred noise is less than 5 μVrms (20 μV peak). 3.5.1 Data display: Oscilloscope mode 15-WD RD Header 14-WD RD Trailer Uplink Silence 0 1 Downlink Silence 1 2 3 4 5 6 7 8 A A 1 2 3 4 5 6 7 8 FIGURE 55.6 MCU: BPB communication protocol. During tting, the analog signal from the amplier/lter section can be digitized and transmitted from the BPB to the MCU to the clinician’s programmer screen at a rate of 40,000 samples per second. is “oscilloscope mode” (Figure 55.7) can be used when evaluating the placement of the BPB and during tting, but it is not suitable for longterm use due to its high power demands. 3.5.2 Data analysis e analog output of the biopotential sensor also passes to a programmable window detection circuitry that can be set by the clinician to (1) count pulses that fall within (or above or below) the set thresholds (Figure 55.8, le), or (2) rectify and integrate the sensed signal (Figure 55.8, right). (1) Counting pulses: e neuronal pulses that occur are accumulated every 10 ms and relayed to the MCU. (2) Rectify and integrate: Every 10 ms, if required, the circuit can rectify the amplitude of the biopotential sensor’s analog output and sum up the average rectied signal. An output between 0 and 255 will be generated, indicating the average energy occurring every 10 ms. 3.6 Pressure sensing Some BPBs will be fabricated with a pressure transducer mounted at one end. e initial version of this sensor is about 3 mm in diameter and is sensitive to pressures along the axial dimension of the BPB. Future versions will be sensitive to lateral pressure, and may be mounted remotely from the BPB. e present full-scale absolute pressure range is 400– 900 mm-Hg. is signal can be read out either AC- or DC-coupled. When DC-coupled, it reads the absolute pressure. Since ambient pressure varies with altitude changes, this oset can be accounted for by placing a reference sensor of the same type in the MCU, then subtracting o this baseline. 3.7 Angle/position sensing (goniometry) e same internal coil that is used to receive the magnetic eld to charge the BPB battery, may also be programmed as a transmitter in any selected BPB or as a receiver in another selected BPB. e goniometry function is implemented using one BPB as a transmitter, and the other BPB as a receiver. e BPB programmed as a receiver, detects and measures the Pulse 1 Th reshold Pulse 2 Pulse 3 FIGURE 55.8 Data analysis with the BPB biopotential sensing module. Le: Counting pulses above threshold line. Right: Rectify and integrate neural signal. FIGURE 55.7 Biopotential sensing module, oscilloscope display mode. signal strength of the received signal (Figure 55.9). e distance between two BPBs is derived from the intensity of the received magnetic eld, which falls o approximately with the cube of the distance between the devices. ere are eight dierent programmable transmitter–receiver frequencies available for goniometry use. e eight frequencies are clustered around 127 kHz. is permits eight parallel goniometry systems consisting of one transmitter and any number of receivers. ere is no limit on the number of BPB receivers that can process the signal strength to give distance measurements (from each of the transmitters). Each BPB receiver is able to send back a measurement 100 times per second. A goniometry pair (transmitter–receiver) can be used to measure distances between 1 and 20 cm. 3.8 Temperature sensing An internal temperature sensor is incorporated as an additional safety mechanism to guard against overheating of the BPB and to provide temperature data to patients, such as certain quadriplegics who do not sense temperature. e sensor is accurate to within one-third degree Celsius and is operable over the range from 16°C to 50°C. In the event a signicant temperature rise is detected, the BPB can be shut down and/or communication with the MCU can be made to initiate appropriate external action (such as shutting down the charging eld, if present). Readings are taken once per second and can be read by the MCU. 4. Recharging subsystem (charger and external coil) e charger produces a 127 kHz signal that generates a magnetic eld in the charging coil. e MCU communicates with the charger to indicate when to turn on a charging eld. e MCU interrogates each BPB to determine which BPB is going to be charged and when the BPB is fully charged. e MCU determines which BPBs are not being charged and indicates to the patient where the coil has to be moved to charge those BPBs. If the charger is coupled to several coils but can only power one coil at a time, the MCU can then cause the charger to switch coils so the uncharged BPBs can be charged. e MCU can also determine the state of the charge in each BPB and can initially select the most discharged devices to be charged rst. FIGURE 55.9 Use of BPBs for distance/angle measurements. e recharging subsystem includes a temperature sensor that stops the recharging process if the external coil temperature adjacent to the patient skin, rises over 41°C. 5. Magnet e patient can stop stimulation by placing an external magnet near the location of the implanted device(s). A neodymium magnet is being used because it is small and lightweight, and because it produces a very strong magnetic signal. e default mode of this magnet is to hold the stimulation o when the magnet is positioned on the patient’s body, over the area where the BPB is implanted. When the magnet is removed, the stimulation turns back on. Other magnet control modes are available. 6. FES-BPB system specications 7. Minimally invasive procedure to implant BPBs To implant a BPB, a minimally invasive procedure is followed. e implantation procedure can be done in a clean procedure room, where the patient’s implant sites can be surgically cleansed and draped with sterile towels and covered with adherent sterile plastic drapes. e implant physician scrubs his/her forearms and hands, is gowned and gloved, and wears a cap and mask. e implantation (insertion) tools are shown in Figure 55.10. Under local anesthesia, a 5-mm skin incision is made. A sterile probe electrode (0.71 mm OD; insulated except at the tips, Figure 55.10) connected to an external stimulator is directed into the tissues to excite and nd the target nerve/motor-point. With adjustments to the probe electrode, the optimal target muscle contraction is located. A customized introducer (dilator plus sheath) is then slid over the probe electrode. Stimulation with the probe electrode is repeated to ensure a similar optimal response and correct location. e probe electrode and dilator are then withdrawn, leaving the sheath in position. e BPB has a dissolvable suture attached to the return electrode. e BPB’s stimulation electrode end is inserted into the sheath and gently pushed by the ejection tool to the sheath tip so that only the BPB stimulation electrode end protrudes. From the ejection tool tip, saline is infused into the sheath to allow the anodal end of the BPB to have electrical connection to the tissues through small holes in the distal sheath. e BPB is activated to test and conrm its optimal position relative to the target nerve/motor-point. By withdrawing the sheath over the ejection tool, the BPB is deposited into the tissues (Figure 55.11). e sheath and the ejection tool are then removed. e BPB is retested to conrm that the optimal response is achieved. If this position is not satisfactory in regard to the responses to stimulation or recording, then the BPB can be retrieved by pulling on the suture attached to the BPB (Figure 55.12) and then reinserted. e emerging sutures are cut at the subcutaneous tissue level, and the wound is then closed. e implanted BPB is tested 1 week aer implantation to conrm that the responses are still adequate. If an inadequate response is observed, the wound could be reopened and the BPB retrieved by pulling on the sutures. A new BPB could then be reinserted to obtain proper response. A: Probe electrode B: Dilator C: Sheath, with holes D: Ejection tool (2 marks) E: 3 mL syringe, with normal saline FIGURE 55.10 BPB implantation tools. 55.4 Applications e dierent functions of the BPB (as a stimulator, biopotential signal sensor, goniometry sensor, pres sure or temperature sensor) and the availability of multiple BPBs in one patient (up to 850 BPBs) gives the clinician many opportunities to restore neurological function, especially in poststroke syndrome, spinal cord injury, cerebral palsy, multiple sclerosis, traumatic brain injury, and for limb sensing in amputees to control tted prostheses. Take, for example, the case where a paralyzed upper extremity is implanted with multiple BPBs placed near motor-points or nerves of muscles in the arm, forearm, and hand. It will be possible to trig ger sequential functional muscle actions to extend the arm and forearm, and open the hand to grasp an object. e limits of each functional action can be controlled from implanted BPBs working as goniom etry sensors, measuring the angles of the elbow (see Figure 55.9) and wrist and implanted BPBs working as pressure sensors, measuring the pressure at the nger tip (Figure 55.13). e reverse of this extension can be similarly achieved using this stimulating and sensing system to bring the grasped object, for example, to the mouth. Similar closed-loop controls of stimulation could be used in FIGURE 55.11 BPB implantation technique. FIGURE 55.12 Retrieval of BPB. FIGURE 55.13 BPBs measuring pressure in ngers. the lower extremities for standing and ambulation. For partially paralyzed extremities, sensing of the mus cle activities using BPBs would act as triggers to other BPBs to stimulate the motor-points of these muscles, thus augmenting the total action. Goniometry sensors would add the closed-loop controls to reduce or stop the actions. is approach could be used to augment swallowing, bladder control, and respiration. Where pressure points need to be monitored, for example, at the heel (as a trigger for improving walk ing on stroke patients) or buttock (to avoid pressure sores) or hand (to detect the grasping of an object), BPB devices placed in these sites can measure the pressure and either trigger motor-point functional stimulation to activate muscles or stop a functional stimulation sequence. 8. FES-BPB system in amputee patients: Controlling articial limbs For amputee patients (Figure 55.14), BPBs working as biopotential sensors, are inserted in the “stump” to pick up motor nerve signals, which can be used to control the movement of the articial limb exible components. 9. Cortical interface device: A cortical stimulator and sensor using the BPB system technology Individuals with spinal cord injury or disease that limits control over voluntary motion or sensing may be able to regain some of the ability of voluntary motion by monitoring the motor cortex and feeding back sensed response signals to the sensory cortex. Voluntary motion is expressed as neural activity in the motor cortex. e sensory cortex depends on muscle spindles and other sensors to help control the limb movement. By feeding back signals to the sensory cortex, the psychological use of the limb would be given back to the patient. e motor cortex signals can also be used to control wheel chairs and other helpful devices. e miniaturized components developed for the BPB are used to create a cortical interface device (CID) with multiple stimulation and sensing electrodes within a single implantable package (Figure 55.15). e CID has the capability of monitoring up to several hundred electrodes that can be implanted or positioned in the motor cortex, sensory cortex, or a combination of both. e CID system consists of a base unit implanted in the skull, underneath the scalp, and one or several electrode arrays placed on the sensory or motor cortices. e CID is equivalent to a group of 64 BPBs in its communication ability. It also includes an additional switching matrix that allows any amplier to sense voltages either unipolar or bipolar from any two electrodes. e CID base unit dimensions and internal components are shown in Figure 55.16. e CID is constructed with the same technology developed for the BPB. It contains the same electronics as those used in the BPB as far as the communication, charging, power management, biopotential sensing, and stimulation modules are concerned. A CID base unit contains FIGURE 55.14 Use of BPBs in amputee patients. 64 biopotential sensing modules attached to one or more electrode arrays. e battery used in the CID provides 50 mAh at 3.6 V. e electrode arrays could be congured for sensing or stimulating purposes. e sensing electrode array includes signal processing capabilities by using the same electronics as those in the biopotential sensing module in the BPB. e stimulating electrode array contains the same stimulation electronics as those in the BPB stimulation module. e CID contains a powerful microprocessor to analyze the signals from the motor cortex and to reduce the data to 64 eight-bit messages that the MCU can use to control up to 64 muscles. 1. Heart Disease and Stroke Statistics—2004 Update, American Heart Association. 2. https://www.bionicear.com/support/clinical_papers/supp_research_demo2.html, https://www. bionicear.com/ support/clinical_papers/supp_research_demo1.html, https://www.bionicear.com/ printables/Bilateral.pdf, https://www.nidcd.nih.gov/health/hearing/coch_moreon.asp, https://www. cochlear.com/896.asp. 3. Stroke Testimony before the House Committee on Energy and Commerce Subcommittee on Health. NINDS opening statement to the House Committee on Energy and Commerce Subcommittee on Health, June 6, 2002. https://www.ninds.nih.gov/about_ninds/2002_stroke_testimony.htm#background Cortical interface Motor cortex electrode Skin Base unit Electrode array FIGURE 55.15 Cortical interface device. 30 mm Charging coil Radio antenna element and indifferent electrode Ceramic seal plate Battery Electronics Feed-through plate Radio antenna element and indifferent electrode Feed-through plate 7.42 mm 20.1 mm 4.92 mm FIGURE 55.16 Cortical interface device. Le: Dimensions. Right: Cross section. 4. Facts and Figures at a Glance. May 2001. National Spinal Cord Injury Statistical Center. Spinal Cord Injury: Hope through Research https://www.ninds.nih.gov/health_and_medical/pubs/sci.htm 5. Complications of Diabetes in the United States. National Diabetes Statistics. https://www.diabetes. niddk.nih.gov/dm/pubs/statistics/ 6. 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