Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Functionally useful reanimation of paralyzed limbs generally requires reliable, finely graded control of muscle recruitment and force with minimal fatigue. We used force and electromyographic (EMG) recordings in combination with myofibrillar adenosine triphosphatase activity and glycogen depletion analysis to investigate the recruitment properties of intramuscular (IM) and nerve cuff (NC) stimulating electrodes implanted acutely or chronically in cat hindlimbs. Overall, 32 muscles were submaximally stimulated with current intensities producing approximately 20% of maximal twitch force using 330 ms trains of pulses at 20 and 40 pps. Both the glycogen-depletion and fatigue-test results were found to be difficult to interpret because NC stimulation resulted in surprisingly unstable re- cruitment during such trains. Fluctuations of force and M-waves within trains of identical stimuli were significantly greater for NC than for IM stimulation. NC stimulation produced much steeper recruitment curves and a reduced tetatnus/twitch ratio compared to IM stimulation. IM stimulation produced more reliable and less fatigable recruitment of a mix of motor unit types that tended to be localized in neuromuscular compartments containing, or adjacent to, the IM electrode. We hypothesize that trains of submaximal stimulation applied through NC electrodes resulted in fluctuating recruitment because this electrode configuration magnifies the effects of refractoriness and small changes in axonal excitability during pulse trains. <s> BIB001 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Clinical studies have shown that spinal or cortical neurostimulation could significantly improve pain relief. The currently available stimulators, however, are used only to generate specific electrical signals without the knowledge of physiologically responses caused from the stimulation. We thus propose a new system that can adaptively generate the optimized stimulating signals base on the correlated neuron activities. This new method could significantly improve the efficiency of neurostimulation for pain relief. We have developed an integrated wireless recording and stimulating system to transmit the neuronal signals and to activate the stimulator over the wireless link. A wearable prototype has been developed consisting of amplifiers, wireless modules and a microcontroller remotely controlled by a Labview program in a computer to generate desired stimulating pulses. The components were assembled on a board with a size of 2.5 cm x 5 cm to be carried by a rat. To validate our system, lumbar spinal cord dorsal horn neuron activities of anesthetized rats have been recorded in responses to various types of peripheral graded mechanical stimuli. The stimulation at the periaqueductal gray and anterior cingulate cortex with different combinations of electrical parameters showed a comparable inhibition of spinal cord dorsal horns activities in response to the mechanical stimuli. The Labview program was also used to monitor the neuronal activities and automatically activate the stimulator with designated pulses. Our wireless system has provided an opportunity for further study of pain processing with closed-loop stimulation paradigm in a potential new pain relief method. <s> BIB002 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Vagus nerve stimulation (VNS) is an approved therapy for the treatment of adult patients and adolescents aged 12 years and older who have partial onset seizures refractory to antiepileptic medications. More than 50,000 patients worldwide have been implanted with the VNS system. Work continues to understand the mechanism of action of VNS with the goal of improving the treatment, particularly to identify patients who will be helped by VNS, to develop a closed-loop seizure detection system, and to improve the selection of stimulation parameters. VNS has also been approved for treatment–resistant depression, and it may have utility in the treatment of a variety of other medical disorders. <s> BIB003 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Neural interfaces are connections that enable two-way exchange of information with the nervous system. These connections can occur at multiple levels, including with peripheral nerves, with the spinal cord, or with the brain; in many instances, fundamental biophysical and biological challenges are shared across these levels. We review these challenges, including selectivity, stability, resolution versus invasiveness, implant-induced injury, and the host-interface response. Subsequently, we review the engineered solutions to these challenges, including electrode designs and geometry, stimulation waveforms, materials, and surface modifications. Finally, we consider emerging opportunities to improve neural interfaces, including cellular-level silicon to neuron connections, optical stimulation, and approaches to control inflammation. Overcoming the biophysical and biological challenges will enable effective high-density neural interfaces for stimulation and recording. <s> BIB004 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Peripheral nerve injury causes sensory dysfunctions that are thought to be attributable to changes in neuronal activity occurring in somatosensory cortices both contralateral and ipsilateral to the injury. Recent studies suggest that distorted functional response observed in deprived primary somatosensory cortex (S1) may be the result of an increase in inhibitory interneuron activity and is mediated by the transcallosal pathway. The goal of this study was to develop a strategy to manipulate and control the transcallosal activity to facilitate appropriate plasticity by guiding the cortical reorganization in a rat model of sensory deprivation. Since transcallosal fibers originate mainly from excitatory pyramidal neurons somata situated in laminae III and V, the excitatory neurons in rat S1 were engineered to express halorhodopsin, a light-sensitive chloride pump that triggers neuronal hyperpolarization. Results from electrophysiology, optical imaging, and functional MRI measurements are concordant with that within the deprived S1, activity in response to intact forepaw electrical stimulation was significantly increased by concurrent illumination of halorhodopsin over the healthy S1. Optogenetic manipulations effectively decreased the adverse inhibition of deprived cortex and revealed the major contribution of the transcallosal projections, showing interhemispheric neuroplasticity and thus, setting a foundation to develop improved rehabilitation strategies to restore cortical functions. <s> BIB005 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Optogenetic control of the peripheral nervous system (PNS) would enable novel studies of motor control, somatosensory transduction, and pain processing. Such control requires the development of methods to deliver opsins and light to targeted sub-populations of neurons within peripheral nerves. We report here methods to deliver opsins and light to targeted peripheral neurons and robust optogenetic modulation of motor neuron activity in freely moving, non-transgenic mammals. We show that intramuscular injection of adeno-associated virus serotype 6 enables expression of channelrhodopsin (ChR2) in motor neurons innervating the injected muscle. Illumination of nerves containing mixed populations of axons from these targeted neurons and from neurons innervating other muscles produces ChR2-mediated optogenetic activation restricted to the injected muscle. We demonstrate that an implanted optical nerve cuff is well-tolerated, delivers light to the sciatic nerve, and optically stimulates muscle in freely moving rats. These methods can be broadly applied to study PNS disorders and lay the groundwork for future therapeutic application of optogenetics. <s> BIB006 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> INTRODUCTION ::: There is no therapeutic approach that provides precise and rapidly reversible inhibition of motor nerve and muscle activity for treatment of spastic hypertonia. ::: ::: ::: METHODS ::: We used optogenetics to demonstrate precise and rapidly reversible light-mediated inhibition of motor nerve and muscle activity in vivo in transgenic Thy1::eNpHR2.0 mice. ::: ::: ::: RESULTS ::: We found optical inhibition of motor nerve and muscle activity to be effective at all muscle force amplitudes and determined that muscle activity can be modulated by changing light pulse duration and light power density. ::: ::: ::: CONCLUSIONS ::: This demonstration of optical inhibition of motor nerves is an important advancement toward novel optogenetics-based therapies for spastic hypertonia. <s> BIB007 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Viral delivery of opsins to peripheral nociceptors allows optogenetic stimulation and inhibition of pain in non-transgenic freely moving mice. <s> BIB008 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> The process of pain perception begins in the periphery by activation of nociceptors. From here nociceptive signals are conveyed via the dorsal horn of the spinal cord to multiple brain regions, where pain is perceived. Despite great progress in pain research in recent years, many questions remain regarding nociceptive circuitry and behavior, in both acute nociception and chronic pain states. Techniques that allow for selective activation of neuronal subpopulations in vivo can provide a better understanding of these complex pathways. Here we review the studies to date that have employed novel optogenetic tools to improve our understanding of the pain pathway at the peripheral, spinal and supraspinal levels. <s> BIB009 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Monitoring neuronal activity in the rodent in vivo brain is commonly done using micro-electrode arrays but these devices are not normally compatible with optical technologies. Here the authors design a transparent and flexible electrode array based on graphene that allows them to combine electrophysiological recordings with optogenetic and imaging experiments. <s> BIB010 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Objective. Closed-loop operation of neuro-electronic systems is desirable for both scientific and clinical (neuroprosthesis) applications. Integrating optical stimulation with recording capability further enhances the selectivity of neural stimulation. We have developed a system enabling the local delivery of optical stimuli and the simultaneous electrical measuring of the neural activities in a closed-loop approach. Approach. The signal analysis is performed online through the implementation of a template matching algorithm. The system performance is demonstrated with the recorded data and in awake rats. Main results. Specifically, the neural activities are simultaneously recorded, detected, classified online (through spike sorting) from 32 channels, and used to trigger a light emitting diode light source using generated TTL signals. Significance. A total processing time of 8 ms is achieved, suitable for optogenetic studies of brain mechanisms online. <s> BIB011 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Optogenetic techniques enable precise excitation and inhibition of firing in specified neuronal populations and artifact-free recording of firing activity. Several studies have suggested that optical stimulation provides the precision and dynamic range requisite for closed-loop neuronal control, but no approach yet permits feedback control of neuronal firing. Here we present the ‘optoclamp’, a feedback control technology that provides continuous, real-time adjustments of bidirectional optical stimulation in order to lock spiking activity at specified targets over timescales ranging from seconds to days. We demonstrate how this system can be used to decouple neuronal firing levels from ongoing changes in network excitability due to multi-hour periods of glutamatergic or GABAergic neurotransmission blockade in vitro as well as impinging vibrissal sensory drive in vivo. This technology enables continuous, precise optical control of firing in neuronal populations in order to disentangle causally related variables of circuit activation in a physiologically and ethologically relevant manner. ::: ::: DOI: http://dx.doi.org/10.7554/eLife.07192.001 <s> BIB012 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Optogenetics offers promise for dissecting the complex neural circuits of the spinal cord and peripheral nervous system and has therapeutic potential for addressing unmet clinical needs. Much progress has been made to enable optogenetic control in normal and disease states, both in proof-of-concept and mechanistic studies in rodent models. In this Review, we discuss challenges in using optogenetics to study the mammalian spinal cord and peripheral nervous system, synthesize common features that unite the work done thus far, and describe a route forward for the successful application of optogenetics to translational research beyond the brain. <s> BIB013 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> Spatially targeted, genetically-specific strategies for sustained inhibition of nociceptors may help transform pain science and clinical management. Previous optogenetic strategies to inhibit pain have required constant illumination, and chemogenetic approaches in the periphery have not been shown to inhibit pain. Here, we show that the step-function inhibitory channelrhodopsin, SwiChR, can be used to persistently inhibit pain for long periods of time through infrequent transdermally delivered light pulses, reducing required light exposure by >98% and resolving a long-standing limitation in optogenetic inhibition. We demonstrate that the viral expression of the hM4D receptor in small-diameter primary afferent nociceptor enables chemogenetic inhibition of mechanical and thermal nociception thresholds. Finally, we develop optoPAIN, an optogenetic platform to non-invasively assess changes in pain sensitivity, and use this technique to examine pharmacological and chemogenetic inhibition of pain. <s> BIB014 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Stimulation and Closed-Loop Implantable Neural Devices <s> We present a wireless electro-optic headstage that uses a 0.13- ${\mu}$ m CMOS custom integrated circuit (IC) implementing a digital neural decoder (ND-IC) for enabling real-time closed-loop (CL) optogenetics. The ND-IC processes the neural activity data using three digital cores: 1) the detector core detects and extracts the action potential (AP) of individual neurons by using an adaptive threshold; 2) the data compression core compresses the detected AP by using an efficient Symmlet-2 discrete wavelet transform (DWT) processor for decreasing the amount of data to be transmitted by the low-power wireless link; and 3) the classification core sorts the compressed AP into separated clusters on the fly according to their wave shapes. The ND-IC encompasses several innovations: 1) the compression core decreases the complexity from O(n $^{2}$ ) to O(n $\cdot$ log(n)) compared to the previous solutions, while using two times less memory, thanks to the use of a new coefficient sorting tree; and 2) the AP classification core reuses both the compressed DWT coefficients to perform implicit dimensionality reduction, which allows for performing intensive signal processing on-chip, while increasing power and hardware efficiency. This core also reuses the signal standard deviation already computed by the AP detector core as threshold for performing automatic AP sorting. The headstage also introduces innovations by enabling a new wireless CL scheme between the neural data acquisition module and the optical stimulator. Our CL scheme uses the AP sorting and timing information produced by the ND-IC for detecting complex firing patterns within the brain. The headstage is also smaller (1.13 cm $^{3}$ ), lighter (3.0 g with a 40 mAh battery) and less invasive than the previous solutions, while providing a measured autonomy of 2h40, with the ND-IC. The whole system and the ND-IC are first validated in vivo in the LD thalamus of a Long-Evans rat, and then in freely-moving CL experiments involving a mouse virally expressing ChR2-mCherry in inhibitory neurons of the prelimbic cortex, and the results show that our system works well within an in vivo experimental setting with a freely moving mouse. <s> BIB015
For stimulation, electrical or focal brain stimulation is the conventional technique used in BMI. For instance, epilepsy is now treated by electrical stimulation of vagus nerve. However, electrical stimulation of undesired neurons introduces shortness of breath, cough, throat pain, thereby restricting the extent of this approach BIB003 . This method has also been applied for motor control in patients having stroke and spinal cord injury to excite the paralyzed muscles BIB004 . Such stimulation of the paralyzed muscles, nevertheless, wear out the muscle strength due to the disorganized enlisting of unwanted motor elements BIB001 , and unable to confine muscle contractions in spasticity. Recently, optogenetics  , BIB013 , where light is used to stimulate the genetically modified neurons, has enlighten with another unique option for neuromodulation. This genetic modification of neurons is engineered by using light-responsive proteins named as opsins to realize light-based stimulations or inhibition. Light-sensitive proteins such as Channelrhodopsin 2 (ChR2) initiates action potential, whereas Halorhodopsin (Halo) triggers neuron inhibition, and Archaerhodopsins (Arch) prompts action potential inhibition. Upon light stimulation (blue), the ChR2 depolarizes the targeted neuron by opening the cation channel. Then again, upon yellow light illumination, another protein NpHR (Halo) results in an inhibitory effect due to injection of chloride ions into the neuron. Optogenetics enables cell specificity BIB006 as this method can inhibit BIB008 , BIB007 and/or stimulate cells BIB009 and capable of treating brain diseases such as nerve injury BIB005 and neuropathic pain BIB014 to name a few. In addition, light-based neural modulations can be carried out effortlessly as it is free of electromagnetic interference. Furthermore, as compared to electrical stimulation, light-based stimulation can be confined to only genetically modified neurons, as illustrated in Fig. 6 . As a result, optogenetics ensures immaculate manoeuvre of neural modulations, which has also been another key challenge. However, optogenetics is still in the development phase, mostly tested on animals, and requires genetic modifications. Simultaneous capability of neural recording and stimulation is actively pursued to ensure versatile and long-term recording and stimulation implantable neural systems BIB002 ,  . Such closed-loop neuromodulation system, in general, may include a power management system, recording electrodes, signal processing core, electrophysiology unit as well as stimulation system that can be either optogenetics or electrical as shown in Fig. 7 . Optogenetics modulation offers less interference with Fig. 7 . Schematic of a head stage closed loop neural modulation system. simultaneous electrical recordings and optical stimulation than does electrical stimulation. However, a typical complication associated with electrical recordings and optogenetics is undesirable electrodes' response to the light BIB010 . This is due to the fact that in case the light strikes a metal electrode, it introduces an artefact due to the photovoltaic or Becquerel effect. The amplitude of these artefacts can be significant and long-lasting, causing potential data loss and/or distorts recorded neuronal signal. Therefore, electrode recordings during optogenetic stimulation are complicated for neural modulation. Recently, several artefact-free closed-loop battery-powered optogenetics/optoelectrophysiology systems have been introduced  BIB015  BIB012 BIB011  . Some of the strategies that can reduce light artefacts are using graphene electrodes  , covering the electrode with opaque polyimide BIB015 , or reducing photopotential  . Additionally, the battery-powered head-mounted optogenetics closed-loop devices permits majority of the system to stay outside the body. As a result, these systems provide the options for modification to perceive diverse multi-modal platforms with less restrictions on size and scale, powering methods, and electronic designs. Nevertheless, due to the relative bulkiness and size, this method limits its application for chronic implantation and is more susceptible to physical injury due to the external mass. In the next section, fully implantable wireless neural devices will be discussed.