Printable microscale interfaces for long-term peripheral nerve mapping and precision control

The nascent field of bioelectronic medicine seeks to decode and modulate peripheral nervous system signals to obtain therapeutic control of targeted end organs and effectors. Current approaches rely heavily on electrode-based devices, but size scalability, material and microfabrication challenges, limited surgical accessibility, and the biomechanically dynamic implantation environment are significant impediments to developing and deploying advanced peripheral interfacing technologies. Here, we present a microscale implantable device – the nanoclip – for chronic interfacing with fine peripheral nerves in small animal models that begins to meet these constraints. We demonstrate the capability to make stable, high-resolution recordings of behaviorally-linked nerve activity over multi-week timescales. In addition, we show that multi-channel, current-steering-based stimulation can achieve a high degree of functionally-relevant modulatory specificity within the small scale of the device. These results highlight the potential of new microscale design and fabrication techniques for the realization of viable implantable devices for long-term peripheral interfacing.https://www.biorxiv.org/node/801468.fullFirst author draf

Human Sensation of Transcranial Electric Stimulation.

Noninvasive transcranial electric stimulation is increasingly being used as an advantageous therapy alternative that may activate deep tissues while avoiding drug side-effects. However, not only is there limited evidence for activation of deep tissues by transcranial electric stimulation, its evoked human sensation is understudied and often dismissed as a placebo or secondary effect. By systematically characterizing the human sensation evoked by transcranial alternating-current stimulation, we observed not only stimulus frequency and electrode position dependencies specific for auditory and visual sensation but also a broader presence of somatic sensation ranging from touch and vibration to pain and pressure. We found generally monotonic input-output functions at suprathreshold levels, and often multiple types of sensation occurring simultaneously in response to the same electric stimulation. We further used a recording circuit embedded in a cochlear implant to directly and objectively measure the amount of transcranial electric stimulation reaching the auditory nerve, a deep intercranial target located in the densest bone of the skull. We found an optimal configuration using an ear canal electrode and low-frequency (<300 Hz) sinusoids that delivered maximally ~1% of the transcranial current to the auditory nerve, which was sufficient to produce sound sensation even in deafened ears. Our results suggest that frequency resonance due to neuronal intrinsic electric properties need to be explored for targeted deep brain stimulation and novel brain-computer interfaces

Doctor of Philosophy

dissertationToday, we are implanting electrodes into many different parts of the peripheral and central nervous systems for the purpose of restoring function to people with nerve injury or disease. As technology and manufacturing continue to become more advanced, ne

Spinal cord stimulation in chronic pain: evidence and theory for mechanisms of action.

Well-established in the field of bioelectronic medicine, Spinal Cord Stimulation (SCS) offers an implantable, non-pharmacologic treatment for patients with intractable chronic pain conditions. Chronic pain is a widely heterogenous syndrome with regard to both pathophysiology and the resultant phenotype. Despite advances in our understanding of SCS-mediated antinociception, there still exists limited evidence clarifying the pathways recruited when patterned electric pulses are applied to the epidural space. The rapid clinical implementation of novel SCS methods including burst, high frequency and dorsal root ganglion SCS has provided the clinician with multiple options to treat refractory chronic pain. While compelling evidence for safety and efficacy exists in support of these novel paradigms, our understanding of their mechanisms of action (MOA) dramatically lags behind clinical data. In this review, we reconstruct the available basic science and clinical literature that offers support for mechanisms of both paresthesia spinal cord stimulation (P-SCS) and paresthesia-free spinal cord stimulation (PF-SCS). While P-SCS has been heavily examined since its inception, PF-SCS paradigms have recently been clinically approved with the support of limited preclinical research. Thus, wide knowledge gaps exist between their clinical efficacy and MOA. To close this gap, many rich investigative avenues for both P-SCS and PF-SCS are underway, which will further open the door for paradigm optimization, adjunctive therapies and new indications for SCS. As our understanding of these mechanisms evolves, clinicians will be empowered with the possibility of improving patient care using SCS to selectively target specific pathophysiological processes in chronic pain

Restoring Upper Extremity Mobility through Functional Neuromuscular Stimulation using Macro Sieve Electrodes

The last decade has seen the advent of brain computer interfaces able to extract precise motor intentions from cortical activity of human subjects. It is possible to convert captured motor intentions into movement through coordinated, artificially induced, neuromuscular stimulation using peripheral nerve interfaces. Our lab has developed and tested a new type of peripheral nerve electrode called the Macro-Sieve electrode which exhibits excellent chronic stability and recruitment selectivity. Work presented in this thesis uses computational modeling to study the interaction between Macro-Sieve electrodes and regenerated peripheral nerves. It provides a detailed understanding of how regenerated fibers, both on an individual level and on a population level respond differently to functional electrical stimulation compared to non-disrupted axons. Despite significant efforts devoted to developing novel regenerative peripheral interfaces, the degree of spatial clustering between functionally related fibers in regenerated nerves is poorly understood. In this thesis, bioelectrical modeling is also used to predict the degree of topographical organization in regenerated nerve trunks. In addition, theoretical limits of the recruitment selectivity of the device is explored and a set of optimal stimulation paradigms used to selectively activate fibers in different regions of the nerve are determined. Finally, the bioelectrical model of the interface/nerve is integrated with a biomechanical model of the macaque upper limb to study the feasibility of using macro-sieve electrodes to achieve upper limb mobilization

Functional Electrical Stimulation of Peripheral Nerve Tissue Via Regenerative Sieve Microelectrodes

Functional electrical stimulation (FES) of peripheral nervous tissue offers a promising method for restoring motor function in patients suffering from complex neurological injuries. However, existing microelectrodes designed to stimulate peripheral nerve are unable to provide the type of stable, selective interface required to achieve near-physiologic control of peripheral motor axons and distal musculature. Regenerative sieve electrodes offer a unique alternative to such devices, achieving a highly stable, selective electrical interface with independent groups of regenerated nerve fibers integrated into the electrode. Yet, the capability of sieve electrodes to functionally recruit regenerated motor axons for the purpose of muscle activation remains largely unexplored. The present dissertation aims to examine the potential role of regenerative electrodes in FES applications by testing the unifying hypothesis that sieve electrodes of various design and geometry are capable of selectively stimulating regenerated motor axons for the purpose of controlling muscle activation. This hypothesis was systematically tested through a series of experiments examining the ability of both micro-sieve electrodes and macro-sieve electrodes to achieve a stable interface with peripheral nerve tissue, electrically activate small groups of regenerated motor axons, and selectively recruit motor units present in multiple distal muscles. Custom sieve electrodes were fabricated via sacrificial photolithography. In vivo testing in rat sciatic nerve validated the ability of chronically-implanted regenerative sieve electrodes to support motor axon regeneration and integrate into peripheral nerve tissue. Sieve electrode geometry was shown to strongly modulate axonal regeneration, muscle reinnervation, and device functionality, as high-transparency macro-sieve electrodes facilitated superior neural integration and functional recovery compared to low-transparency micro-sieve electrodes. Inclusion of neurotrophic factors into sieve electrode assemblies increased axonal regeneration through implanted electrodes and improved the quality of the sieve/nerve interface in low-transparency devices. In vivo testing in rat sciatic nerve further validated the ability of chronically-implanted regenerative sieve electrodes to facilitate FES of regenerated motor axons and selective recruitment of distal musculature. Selective stimulation of regenerated motor axons using implanted micro- and macro-sieve electrodes enabled effective, external control of muscle activation within anterior and posterior compartments of the lower leg (e.g. ankle plantarflexion / dorsiflexion). Selective activation of distal musculature was achieved through modulation of stimulus amplitude, channel activation, and field steering. In summary, the present body of work provides initial evidence of the utility of regenerative electrodes as a means of selectively interfacing peripheral nerve tissue for the purpose of restoring muscle activation and motor control. These findings further highlight the clinical potential of implantable microelectrodes capable of intimately integrating into host neural tissue

Evaluating the Use of Engineered Nervous Tissue Constructs in the Repair of Peripheral Nerve Lesions and Amputations

Severe trauma to the limbs can often result in the lesioning, or even amputation, of the underlying peripheral nerves. In these cases, endogenous neural repair mechanisms are compromised and a path to the end target may be lost, resulting in the need for surgical intervention. Current repair strategies are incapable of maintaining this regenerative pathway, or providing a bridge to a surrogate end target, often resulting in incomplete repair. This thesis describes the development and evaluation of a novel method of addressing peripheral nerve lesions and amputations that utilizes living tissue-engineered neural grafts. These grafts are created by the controlled mechanical separation of axons spanning integrated neuron populations in vitro, resulting in axon tracts spanning several centimeters in length. Techniques were developed to encapsulate and transplant these tracts, with the goal of providing structural and nutrient support, while minimizing macrophage infiltration. The efficacy of these constructs in the treatment of lesions and amputations was then assessed using a rat sciatic nerve transection model. In the first study, the ability of neural constructs to (a) encourage host regeneration from the proximal stump, while also (b) attenuating distal pathway degeneration, was evaluated. At the 4-week time point, the axonal constructs were observed to promote more robust host axonal and tissue regeneration across the graft when compared to unstretched grafts. A measurement of nerve conduction velocities also revealed a statistically significant improvement in the stretch-grown group, correlating with the observed increased fiber regeneration. At the distal pathway, neural constructs were observed to prevent the atrophy of the support cells, and maintain the alignment of the Schwann cell columns for up to 4 months. These results suggest that the use of neural grafts may expand the time window within which successful nerve regeneration can occur. The axon grafts were then shown to support and maintain regenerating host axon fibers for up to 4 weeks in the absence of a distal end target. Finally, axon grafts pre-attached to an implantable electrode substrate were shown to encourage host ingrowth to the vicinity of the substrate, showing promise for the development of a chronic brain-machine interface

Design and development of an implantable biohybrid device for muscle stimulation following lower motor neuron injury

In the absence of innervation caused by complete lower motor neuron injuries, skeletal muscle undergoes an inexorable course of degeneration and atrophy. The most apparent and debilitating clinical outcome of denervation is the immediate loss of voluntary use of muscle. However, these injuries are associated with secondary complications of bones, skin and cardiovascular system that, if untreated, may be fatal. Electrical stimulation has been implemented as a clinical rehabilitation technique in patients with denervated degenerated muscles offering remarkable improvements in muscle function. Nevertheless, this approach has limitations and side effects triggered by the delivery of high intensity electrical pulses. Combining innovative approaches in the fields of cell therapy and implanted electronics offers the opportunity to develop a biohybrid device to stimulate muscles in patients with lower motor neuron injuries. Incorporation of stem cell-derived motor neurons into implantable electrodes, could allow muscles to be stimulated in a physiological manner and circumvent problems associated with direct stimulation of muscle. The hypothesis underpinning this project is that artificially-grown motor neurons can serve as an intermediate between stimulator and muscle, converting the electrical stimulus into a biological action potential and re-innervating muscle via neuromuscular interaction. Here, a suitable stem cell candidate with therapeutic potential was identified and a differentiation protocol developed to generate motor neuron-like cells. Thick-film technology and laser micromachining were implemented to manufacture electrode arrays with features and dimensions suitable for implantation. Manufactured electrodes were electrochemically characterised, and motor neuron-like cells incorporated to create biohybrid devices. In vitro results indicate manufactured electrodes support motor neuron-like cell growth and neurite extension. Moreover, electrochemical characterisation suggests electrodes are suitable for stimulation. Preliminary in vivo testing explored implantation in a rat muscle denervation model. Overall, this thesis demonstrates initial development of a novel approach for fabricating biohybrid devices that may improve stimulation of denervated muscles