Towards an Implantable Vestibular Prosthesis: The Surgical Challenges

Published versio

A Partial-Current-Steering Biphasic Stimulation Driver for Vestibular Prostheses

Published versio

Restoring Sensation of Gravitoinertial Acceleration through Prosthetic Stimulation of the Utricle and Saccule

Individuals with bilateral vestibular hypofunction suffer reduced quality of life due to loss of postural and ocular reflexes essential to maintaining balance and visual acuity during head movements. Vestibular stimulation has demonstrated success in restoring sensation of angular head rotations using electrical stimulation of the semi-circular canals (SCCs). Efforts toward utricle and saccule stimulation to restore sensation of gravitoinertial acceleration have been limited due to the complexity of the otolith end organs and otolith-ocular reflexes (OORs). Four key pieces of technology were developed to extend prosthetic stimulation to the utricle and saccule: a low-noise scleral coil system to record binocular 3D eye movements; a motion platform control system for automated presentation of rotational and translational stimuli; custom electrode arrays with fifty contacts targeting the SCCs, utricle and saccule; and a general-purpose neuroelectronic stimulator for vestibular and other neuromodulation applications. Using these new technologies, OORs were first characterized in six chinchillas to establish OOR norms during translations and static tilts. Results led to creation of a model that infers the axis of head tilt from measured binocular eye movements and thereby provides a context and means to assess the selectivity of prosthetic utricle and saccule stimulation. The model confirms the expectation that excitation of the left utricle and saccule primarily encodes tilts that bring the left ear down. Three of the chinchillas were implanted with electrode arrays in the left ear. Step changes in pulse rate were delivered to utricle and saccule electrodes near the maculae while measuring 3D binocular eye movements with the animal stationary in darkness. These stimuli elicited sustained ocular counter-roll responses that increased in magnitude as pulse rate or amplitude increased. Bipolar stimulation via neighboring electrodes elicited slow-rising or delayed onset of ocular counter-rolls (consistent with normal translational OOR low-pass filter behavior). Two chinchillas showed different direction of electrically-evoked ocular counter-roll between utricle versus saccule stimulation. Only near-neighbor bipolar electrode combinations elicited eye responses compensatory for tilts other than the ‘usual’ left ear down, suggesting the need for distributing multiple bipolar electrode pairs across the maculae to achieve selective stimulation and restore 3D sensation of gravitoinertial acceleration

Neuromorphic hardware for somatosensory neuroprostheses

In individuals with sensory-motor impairments, missing limb functions can be restored using neuroprosthetic devices that directly interface with the nervous system. However, restoring the natural tactile experience through electrical neural stimulation requires complex encoding strategies. Indeed, they are presently limited in effectively conveying or restoring tactile sensations by bandwidth constraints. Neuromorphic technology, which mimics the natural behavior of neurons and synapses, holds promise for replicating the encoding of natural touch, potentially informing neurostimulation design. In this perspective, we propose that incorporating neuromorphic technologies into neuroprostheses could be an effective approach for developing more natural human-machine interfaces, potentially leading to advancements in device performance, acceptability, and embeddability. We also highlight ongoing challenges and the required actions to facilitate the future integration of these advanced technologies

Advances in Microelectronics for Implantable Medical Devices

Implantable medical devices provide therapy to treat numerous health conditions as well as monitoring and diagnosis. Over the years, the development of these devices has seen remarkable progress thanks to tremendous advances in microelectronics, electrode technology, packaging and signal processing techniques. Many of today’s implantable devices use wireless technology to supply power and provide communication. There are many challenges when creating an implantable device. Issues such as reliable and fast bidirectional data communication, efficient power delivery to the implantable circuits, low noise and low power for the recording part of the system, and delivery of safe stimulation to avoid tissue and electrode damage are some of the challenges faced by the microelectronics circuit designer. This paper provides a review of advances in microelectronics over the last decade or so for implantable medical devices and systems. The focus is on neural recording and stimulation circuits suitable for fabrication in modern silicon process technologies and biotelemetry methods for power and data transfer, with particular emphasis on methods employing radio frequency inductive coupling. The paper concludes by highlighting some of the issues that will drive future research in the field

Auditory Neural Prostheses – A Window to the Future

Hearing loss is one of the commonest congenital anomalies to affect children world-over. The incidence of congenital hearing loss is more pronounced in developing countries like the Indian sub-continent, especially with the problems of consanguinity. Hearing loss is a double tragedy, as it leads to not only deafness but also language deprivation. However, hearing loss is the only truly remediable handicap, due to remarkable advances in biomedical engineering and surgical techniques. Auditory neural prostheses help to augment or restore hearing by integration of an external circuitry with the peripheral hearing apparatus and the central circuitry of the brain. A cochlear implant (CI) is a surgically implantable device that helps restore hearing in patients with severe-profound hearing loss, unresponsive to amplification by conventional hearing aids. CIs are electronic devices designed to detect mechanical sound energy and convert it into electrical signals that can be delivered to the coch­lear nerve, bypassing the damaged hair cells of the coch­lea. The only true prerequisite is an intact auditory nerve. The emphasis is on implantation as early as possible to maximize speech understanding and perception. Bilateral CI has significant benefits which include improved speech perception in noisy environments and improved sound localization. Presently, the indications for CI have widened and these expanded indications for implantation are related to age, additional handicaps, residual hearing, and special etiologies of deafness. Combined electric and acoustic stimulation (EAS / hybrid device) is designed for individuals with binaural low-frequency hearing and severe-to-profound high-frequency hearing loss. Auditory brainstem implantation (ABI) is a safe and effective means of hearing rehabilitation in patients with retrocochlear disorders, such as neurofibromatosis type 2 (NF2) or congenital cochlear nerve aplasia, wherein the cochlear nerve is damaged or absent on both sides and hence, a cochlear implant (CI) would be inef­fective. In such patients, the brainstem implant bypasses the damaged / absent cochlear nerves and directly stimulates the cochlear nucleus in the brainstem.  The auditory midbrain implant (AMI) has been designed for stimulation of the auditory midbrain, particularly the central nucleus of inferior colliculus (ICC). It is used especially in patients with large neurofibromatosis type 2 (NF2) wherein tumor induced damage to the brain­stem/cochlear nucleus often co-exists. The efficacy and safety of auditory neural prostheses is well proven. Advancements in technology will enhance the benefit provided by these prostheses.  Hearing loss is one of the commonest congenital anomalies to affect children world-over. The incidence of congenital hearing loss is more pronounced in developing countries like the Indian sub-continent, especially with the problems of consanguinity. Hearing loss is a double tragedy, as it leads to not only deafness but also language deprivation. However, hearing loss is the only truly remediable handicap, due to remarkable advances in biomedical engineering and surgical techniques. Auditory neural prostheses help to augment or restore hearing by integration of an external circuitry with the peripheral hearing apparatus and the central circuitry of the brain. A cochlear implant (CI) is a surgically implantable device that helps restore hearing in patients with severe-profound hearing loss, unresponsive to amplification by conventional hearing aids. CIs are electronic devices designed to detect mechanical sound energy and convert it into electrical signals that can be delivered to the coch­lear nerve, bypassing the damaged hair cells of the coch­lea. The only true prerequisite is an intact auditory nerve. The emphasis is on implantation as early as possible to maximize speech understanding and perception. Bilateral CI has significant benefits which include improved speech perception in noisy environments and improved sound localization. Presently, the indications for CI have widened and these expanded indications for implantation are related to age, additional handicaps, residual hearing, and special etiologies of deafness. Combined electric and acoustic stimulation (EAS / hybrid device) is designed for individuals with binaural low-frequency hearing and severe-to-profound high-frequency hearing loss. Auditory brainstem implantation (ABI) is a safe and effective means of hearing rehabilitation in patients with retrocochlear disorders, such as neurofibromatosis type 2 (NF2) or congenital cochlear nerve aplasia, wherein the cochlear nerve is damaged or absent on both sides and hence, a cochlear implant (CI) would be inef­fective. In such patients, the brainstem implant bypasses the damaged / absent cochlear nerves and directly stimulates the cochlear nucleus in the brainstem.  The auditory midbrain implant (AMI) has been designed for stimulation of the auditory midbrain, particularly the central nucleus of inferior colliculus (ICC). It is used especially in patients with large neurofibromatosis type 2 (NF2) wherein tumor induced damage to the brain­stem/cochlear nucleus often co-exists. The efficacy and safety of auditory neural prostheses is well proven. Advancements in technology will enhance the benefit provided by these prostheses. Â

Advances in Scalable Implantable Systems for Neurostimulation Using Networked ASICs

Neurostimulation is a known method for restoring lost functions to neurologically impaired patients. This paper describes recent advances in scalable implantable stimulation systems using networked application specific integrated circuits (ASICs). It discusses how they can meet the ever-growing demand for high-density neural interfacing and long-term reliability. A detailed design example of an implantable (inductively linked) scalable stimulation system for restoring lower limb functions in paraplegics after spinal cord injury is presented. It comprises a central hub implanted at the costal margin and multiple Active Books which provide the interface for stimulating nerve roots in the cauda equina. A 16-channel stimulation system using four Active Books is demonstrated. Each Active Book has an embedded ASIC, which is responsible for initiating stimulus current to the electrodes. It also ensures device safety by monitoring temperature, humidity, and peak electrode voltage during stimulation. The implant hub was implemented using a microcontroller-based circuit. The ASIC in the Active Book was fabricated using XFAB’s 0.6-µm high-voltage CMOS process. The stimulation system does not require an accurate reference clock in the implant. Measured results are provided

Towards Integrating Vestibular Implant Stimulation of the Semicircular Canals and the Otolith End Organs to Drive Posture, Gait, and Eye-Stabilizing Reflexes

Individuals who suffer from bilateral vestibular hypofunction see an increased risk of falling and a decreased quality of life due to symptoms like imbalance, difficulty keeping their eyes on target during head movement, and difficulty walking. The current standard of care to address these symptoms involves rehabilitation exercises to train the central nervous system to rely on visual and proprioceptive signals to compensate for a loss in vestibular signal. However, no sensory substitution can compensate for the extremely fast ocular and spinal reflexes driven by the vestibular nerves. Decades of research led to the development of multichannel vestibular prostheses designed to electrically stimulate the vestibular nerve endings to provide head movement information from a three-axis motion processing unit. The Johns Hopkins Multichannel Vestibular Implant Early Feasibility Study implanted nine study participants with a unilateral vestibular implant targeting the three semicircular canals. The Johns Hopkins Multichannel Vestibular Implant for clinical use does not incorporate circuitry or hardware to stimulate the remaining two sensory organs of the vestibular system, the otolith end organs, responsible for encoding gravitoinertial accelerations that contribute to ocular and spinal reflexes. Stimulation to these end organs were not the priority because they typically elicit much smaller ocular reflexes and encode slower, low-frequency information that can more easily be compensated by the visual and proprioceptive systems. Additionally, their sensory epithelia encode a wider range of information, so mapping a three-axis accelerometer’s signal to stimulation parameters is not straightforward. The research described in this dissertation first assesses the need for otolith-targeted stimulation by measuring otolith-related vestibulo-spinal reflexes via clinical tests of posture and gait in the Multichannel Vestibular Implant Early Feasibility Study with individuals receiving only semicircular canal-targeted stimulation. In the following chapters, the research expands on previous work done in normally functioning chinchillas to explore otolith-targeted stimulation in a rodent model of bilateral vestibular hypofunction and to understand and optimize the stimulation parameters that will be physiologically relevant and useful to restore otolith-specific information to the vestibular nerve endings. The work described in this dissertation is a step toward a more complete vestibular prosthesis to help restore the otolith-driven reflexes to individuals with profound vestibular loss

Potential Mechanisms of Sensory Augmentation Systems on Human Balance Control

Numerous studies have demonstrated the real-time use of visual, vibrotactile, auditory, and multimodal sensory augmentation technologies for reducing postural sway during static tasks and improving balance during dynamic tasks. The mechanism by which sensory augmentation information is processed and used by the CNS is not well understood. The dominant hypothesis, which has not been supported by rigorous experimental evidence, posits that observed reductions in postural sway are due to sensory reweighting: feedback of body motion provides the CNS with a correlate to the inputs from its intact sensory channels (e.g., vision, proprioception), so individuals receiving sensory augmentation learn to increasingly depend on these intact systems. Other possible mechanisms for observed postural sway reductions include: cognition (processing of sensory augmentation information is solely cognitive with no selective adjustment of sensory weights by the CNS), “sixth” sense (CNS interprets sensory augmentation information as a new and distinct sensory channel), context-specific adaptation (new sensorimotor program is developed through repeated interaction with the device and accessible only when the device is used), and combined volitional and non-volitional responses. This critical review summarizes the reported sensory augmentation findings spanning postural control models, clinical rehabilitation, laboratory-based real-time usage, and neuroimaging to critically evaluate each of the aforementioned mechanistic theories. Cognition and sensory re-weighting are identified as two mechanisms supported by the existing literature