Influence of transdermal current flow in tDCS-induced cutaneous adverse events

Significant contributors to the broad application of transcranial direct current stimulation (tDCS) are portability, ease-of-use, and tolerability; with adverse events limited to transient and mild cutaneous sensations (e.g. perception of burning, itching, and tingling) and erythema. However, the fundamental questions remain about the mechanism of transdermal current flow during transcranial electrical stimulation, including tDCS. Example of previously unexplained questions in tDCS include: 1) the relationship between tDCS-induced skin reddening (erythema) profile and local current density profile predicted by the model; 2) the source of burning sensation during tDCS and whether it is related to an actual skin heating; 3) the role of skin multi-layers and ultrastructures (blood vessels, sweat glands, and hair follicles) in current flow. The finite element modeling (FEM) of current flow using simplified tissue geometries predict higher current density at the electrode edge, but the experimental evidences for the cutaneous effects of tDCS (skin heating or skin reddening) are unclear. Prior skin models of cutaneous current flow lacked anatomical details that will a priori be expected to govern current flow patterns. In this dissertation we address the aforementioned questions by: first quantifying tDCS-induced skin erythema profile alongside FEM predicting local current density profile; then assess the extent of skin heating during tDCS, including the role of joule heating, and relate temperature increase (if any) to burning sensation; and finally develop a realistic skin model to address the role of complex skin tissue layers and ultrastructures in current flow. In the first study, we conclude that the tDCS-induced skin reddening profile is diffuse, higher in active stimulation than sham stimulation, and does not occur at the electrode edges suggesting two alternate hypothesis: 1) skin reddening profile is not related to local current density; and 2) skin current density is relatively uniform, so prior FEM models are incorrect. Next, we conduct phantom measurement suggesting no significant temperature increase due to joule heat as expected at the skin during tDCS. The in vitro human skin temperature measurement suggests that independent of tDCS polarity, temperature increases by about 1oC; an increase during tDCS that is less than the cooling produced following a room-temperature sponge application during the set-up. We conclude that any incremental temperature increase by tDCS may reflect vascular flare response due to current flow, cannot exceed the core body temperature, and is more than the offset by sponge-material coolness, thus, the sensation of skin “burning” during tDCS is not related to an actual increase in temperature. In the final study, we develop a detailed multi-layer skin model including sweat glands, hair follicles, and vasculature, and assess the role of multi-layers and ultrastructures in current flow. The FEM analysis predict that sweat glands eliminates localized current density around the electrode edges, and blood vessels uniformly distribution current across the modeled vasculature under the electrode. We expect that a current flow and bioheat model of such a detailed skin would increase the uniformity of current density and temperature predicted at the skin - consistent with the experimental measurement of skin reddening and skin heating

An Efficient Noninvasive Neuromodulation Modality for Overactive Bladder Using Time Interfering Current Method

Objective: The present study aimed to evaluate a new tibial nerve stimulation (TNS) modality, which uses interferential currents, in terms of the stimulation electric field penetration efficiency into the body and physiological effectiveness. Methods: In silico experiments were performed to analyze the penetration efficiency of proposed interferential current therapy (ICT). Based on this, we performed in vivo experiments to measure excitation threshold of ICT for the tibial nerve, which is related to stimulation field near the nerve. Regarding analysis of the physiological effectiveness, in vivo ICT-TNS was performed, and changes in bladder contraction frequency and voiding volume were measured. The penetration efficiency and physiological effectiveness of ICT were evaluated by comparison with those of conventional TNS using transcutaneous electrical nerve stimulation (TENS). Results: Simulation results showed that ICT has high penetration efficiency, thereby generating stronger field than TENS. These results are consistent with the in vivo results that nerve excitation threshold of ICT is lower than that of TENS. Moreover, ICT-TNS decreased contraction frequency and increased voiding volume, and its performance was profound compared with that of TENS-TNS. Conclusion: The proposed ICT is more efficient in inducing the stimulation field near the tibial nerve placed deep inside the body compared with conventional TENS and shows a good clinical effectiveness for TNS. Significance: The high efficiency of ICT increases the safety of noninvasive neurostimulation; therefore, it has clinical potential to become a promising modality for TNS to treat OAB and other peripheral neurostimulations.11Nsciescopu

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

Determination of a Clinically Relevant Tissue Phantom for Transcutaneous Ultrasound Stimulation of Piezoelectric Discs for Current Density Applications

Ventral hernia repairs, one of the most common surgeries, have a recurrence rate ranging from 24-43% even with the use of a prosthetic mesh. Over 1 million Americans will undergo a hernia repair surgery each year according to the FDA [1]. The number of surgeries and recurrences leads to a burden of an estimated $700 million annually for U.S. hospitals [2, 3]. Many of these complications related to the surgery are a result of poor healing and mesh failure. Many patients report chronic abdominal pain, mesh erosion, and mesh migration [4, 5]. The research solution proposed and ongoing is an electrically active hernia repair mesh. The electrical stimulation generator will consist of a piezoelectric disc connected to a circuit and fully encapsulated in medical grade silicone. The piezoelectric discs will be mechanically stimulated by therapeutic ultrasound through the layers of skin, fat, and muscle. Therapeutic ultrasound has been the chosen mode for mechanical loading as it is readily available and a medically safe application. Previous research has shown therapeutic ultrasound successful in the mechanical stimulation of piezoelectric discs [6]. The use of tissue phantoms is common in imaging ultrasound studies but has not been used in the research of stimulating piezoelectric discs with ultrasound. By comparing the power and voltage output of the same PZT disc with different tissue phantoms the effect of the tissue phantom type on the ultrasound stimulation can be examined. Therefore, the different types of tissue phantoms attenuation and speed of sound difference can be compared to that of porcine tissue to achieve a more clinically relevant tissue phantom for a specific application use. A thickness of 40 mm was chosen based on literature on abdominal wall thickness of hernia repair patients. It was determined that a phantom using Humimic® medical gelatin #0 with the combination of a fiber supplement (Metamucil®) was a reusable phantom that allowed for similar voltage output at a resistance of one kiloohm (the expected electrical resistance of muscle). The determined phantom can be utilized with the single piezoelectric disc for future studies to advance research for an electrically active hernia repair mesh

Brain and Human Body Modeling

This open access book describes modern applications of computational human modeling with specific emphasis in the areas of neurology and neuroelectromagnetics, depression and cancer treatments, radio-frequency studies and wireless communications. Special consideration is also given to the use of human modeling to the computational assessment of relevant regulatory and safety requirements. Readers working on applications that may expose human subjects to electromagnetic radiation will benefit from this book’s coverage of the latest developments in computational modelling and human phantom development to assess a given technology’s safety and efficacy in a timely manner. Describes construction and application of computational human models including anatomically detailed and subject specific models; Explains new practices in computational human modeling for neuroelectromagnetics, electromagnetic safety, and exposure evaluations; Includes a survey of modern applications for which computational human models are critical; Describes cellular-level interactions between the human body and electromagnetic fields

Evoked Somatosensory Feedback for Closed-Loop Control of Prosthetic Hand

Somatosensory feedback, such as tactile and proprioceptive feedback, is essential to our daily sensorimotor tasks. A lack of sensory information limits meaningful human-machine interactions. Different somatosensory feedback strategies have been developed in recent years. Non-invasive sensory substitutional approaches often evoke sensations that are unintuitive, requiring extensive sensory training. Alternatively, invasive neural stimulation can elicit intuitive percepts that are interpretable readily by prosthetic hand users; however, the invasive nature of the procedure limits wide clinical applications. To overcome these issues, we developed a multimodal sensory feedback approach that delivers tactile and proprioceptive information non-invasively. We used a skin-surface nerve stimulation array to target afferent fibers in the peripheral nerves, which can elicit intuitive tactile feedback at the fingertips. We used a vibrotactile array to deliver proprioceptive percepts encoding kinematic information of prosthetic joints. First, we evaluated whether the peripheral nerve stimulation technique could be used for the recognition of object properties. Evoked tactile sensations were modulated using forces recorded by a sensorized prosthesis not actively controlled by the users. We demonstrated that the elicited tactile sensation at the fingertips can enable recognition of object shape and surface topology. Second, we evaluated how evoked tactile feedback can be integrated into the functional utility of a prosthetic hand. We quantified the benefits of tactile feedback under different myoelectric control strategies, when participants performed an object manipulation task. We showed an improved task success rate and reduced muscle activation effort when tactile feedback was provided. Finally, we investigated whether multimodal (tactile and proprioceptive) feedback can enable the recognition of more complex object properties during active control of a prosthetics hand. We found that integrated tactile and proprioceptive feedback allowed for simultaneous recognition of multiple object properties (size and stiffness) in individuals with and without an arm amputation. Overall, this work demonstrates that artificially evoked somatosensory feedback can be utilized effectively to improve the closed-loop control of prostheses. These outcomes highlight the critical role of somatosensory feedback during human-machine interactions, which can enhance functional utility of prosthetic devices and promote user experience and confidence.Doctor of Philosoph

Designing sensory feedback approaches for restoring touch and position feedback in upper limb amputees

Upper limb amputation disrupts most daily activities and reduces the quality of life of affected individuals. Building a suitable prosthetic limb, which can restore at least some of the lost capabilities, is a goal which has been pursued for centuries. In the last few decades, our rapidly expanding understanding of the human nervous system has unlocked impressive advances in artificial limbs. Today, commercial prosthetic hands can be controlled intuitively through voluntary muscle contractions. Nevertheless, despite leaps in the quality of modern prostheses, sensory feedback remains one of the major omissions, forcing users to rely on vision to accomplish basic tasks, such as holding a plastic cup without crushing it. Several sensory feedback strategies have recently been developed to restore tactile and proprioceptive feedback to amputees, demonstrating benefits in important areas, such as higher functional performance and increases in the sense of prosthesis ownership. Sensory feedback strategies can be distinguished based on whether the sensation they restore matches the quality (homologous feedback) or the location (somatotopic feedback) of the original sensation. Despite promising results, somatotopic tactile feedback strategies often result in unnatural sensations (e.g. electricity). Furthermore, restoration of more than a single sensory modality is rarely reported, despite being necessary to create artificial limbs capable of delivering realistic sensorimotor experiences during use. In this work, I proposed three novel and complementary strategies to improve sensory feedback restoration in upper limb prostheses. I begin by describing a non-invasive transcutaneous electrical nerve stimulation (TENS) approach aimed at restoring somatotopic tactile sensations, which is potentially applicable to all trans-radial amputees. This stimulation strategy was shown to lead to high performance during functional tasks, and compared favorably to more invasive approaches, despite a few key differences. Considering that there is no such thing as a one-size-fits-all solution for amputees, I concluded that TENS represents a viable alternative to invasive systems, especially in cases where an implant is not possible or desirable. In the second part, I proposed a sensory substitution approach to multimodal feedback, which delivered somatotopic tactile and remapped proprioceptive feedback simultaneously. This stimulation strategy relied entirely on implantable electrodes, simplifying the overall system by delivering two streams of sensory information with the same device. Using this feedback system, two amputees were able to perform interesting functional tasks, such as understanding the size and compliance of various objects, with high accuracy. Finally, I proposed a novel stimulation technique for sensory feedback designed to desynchronize induced neural activity during electrical stimulation, leading to more biomimetic patterns of activity. I discussed how this strategy could be combined with the results obtained in a recent study which I contributed to, in which we demonstrated that a model based encoding strategy resulted in more natural sensations of touch. This thesis provides evidence that advances in electrical stimulation protocols can lead to more capable prosthetic limbs. These new methods enable the delivery of multimodal, biomimetic sensory feedback and will help bridge the gap between scientific discovery and clinical translation

Brain and Human Body Modeling

This open access book describes modern applications of computational human modeling with specific emphasis in the areas of neurology and neuroelectromagnetics, depression and cancer treatments, radio-frequency studies and wireless communications. Special consideration is also given to the use of human modeling to the computational assessment of relevant regulatory and safety requirements. Readers working on applications that may expose human subjects to electromagnetic radiation will benefit from this book’s coverage of the latest developments in computational modelling and human phantom development to assess a given technology’s safety and efficacy in a timely manner. Describes construction and application of computational human models including anatomically detailed and subject specific models; Explains new practices in computational human modeling for neuroelectromagnetics, electromagnetic safety, and exposure evaluations; Includes a survey of modern applications for which computational human models are critical; Describes cellular-level interactions between the human body and electromagnetic fields

Functional magnetic resonance imaging : cerebral function alterations in subthreshold and suprathreshold spinal cord stimulation

Peer reviewedPublisher PD

A simulation environment for studying transcutaneous electrotactile stimulation

Transcutaneous electrical nerve stimulation (TENS) allows the artificial excitation of nerve fibres by applying electric-current pulses through electrodes on the skin's surface. This work involves the development of a simulation environment that can be used for studying transcutaneous electrotactile stimulation and its dependence on electrode layout and excitation patterns. Using an eight-electrode array implementation, it is shown how nerves located at different depths and with different orientations respond to specific injected currents, allowing the replication of already reported experimental findings and the creation of new hypotheses about the tactile sensations associated with certain stimulation patterns. The simulation consists of a finite element model of a human finger used to calculate the distribution of the electric potential in the finger tissues neglecting capacitive effects, and a cable model to calculate the excitation/inhibition of action potentials in each nerve