What's the “Buzz”? Welcome to a new(s) feature, highlighting interesting articles. We are calling it “Bioelectricity Buzz” because it aims to be “newsy” as well as to reflect the multifaceted nature and dynamism of the bioelectricity field (research and applications). We hope this feature will stimulate conversations, innovative projects, and new collaborations. Here is a selection of recent articles, and we look forward to sharing more exciting discoveries and news in future issues of Bioelectricity.
Injectrodes: Injectable Composite Electrodes
James K. Trevathan, Ian W. Baumgart, Evan N. Nicolai, Brian A. Gosink, Anders J. Asp, Megan L. Settell, Shyam R. Polaconda, Kevin D. Malerick, Sarah K. Brodnick, Weifeng Zeng, Bruce E. Knudsen, Andrea L. McConico, Zachary Sanger, Jannifer H. Lee, Johnathon M. Aho, Aaron J. Suminski, Erika K. Ross, Jose L. Lujan, Douglas J. Weber, Justin C. Williams, Manfred Franke, Kip A. Ludwig, Andrew J. Shoffstall. An injectable neural stimulation electrode made from an in-body curing polymer/metal composite. Adv Healthc Mater 2019; 8:1900892.
In this article, the authors developed what they term an “Injectrode,” a soft, injectable polymer gel electrode with applications for neuromodulation and other therapies that require implantation of electrodes into soft tissues.
Conceptually, the “Injectrode” can be injected as a prepolymer silicone carrier combined with a conductive silver element, which then conforms to the tissue anatomy, curing to produce a mechanically compliant somewhat porous electrode in vivo. In this study, the authors characterize the electrochemical, mechanical, and microscopic properties of the Injectrode material. Furthermore, they describe preclinical proof-of-concept tests using a cuff electrode constructed of silver wire or wire mesh embedded in the Injectrode material to stimulate rat peripheral nerve. The potential for minimally invasive electrode insertion and the ability of the flowing polymer to conform to individual anatomy make the Injectrode an attractive concept. Mechanical mismatch between soft neural tissues and traditional electrodes is increasingly recognized as problematic and this technology addresses the issue directly. However, as the authors point out, there is plenty of scope for further development, in particular, the selection of silver as the conductive component. The known toxicity of particulate silver and silver chloride, especially for long-term use, invites exploration of safer conductive materials. In addition, the authors tested injection of the Injectrode material in an open cut in porcine cadaveric materials and have not yet tested how the injected Injectrode performs as an electrode in a preclinical animal model. That will certainly be crucial, and I will be watching for that with interest.
Potential for Bionic Neural Implants: Toward Solid-State Real-Time Neurons
Kamal Abu-Hassan, Joseph D. Taylor, Paul G. Morris, Elisa Donati, Zuner A. Bortolotto, Giacomo Indiveri, Julian F.R. Paton, Alain Nogaret. Optimal solid state neurons. Nat Commun 2019. DOI:10.1038/s41467-019-13177-3
Whereas the “Injectrode” article already described seeks an improved electrode neuron material for stimulation of existing neurons, here the emphasis is on creating microcircuitry to integrate biological neural stimuli and to replicate natural neural responses in silico.
The exciting proposal in this article is that neuronal function can be replicated in silico to compensate for circuitry damaged by chronic disease states or injury. Long-standing problems with this technically demanding approach are the complexity of modeling key biological functions, such as nonlinear voltage thresholds and recovery times, the diversity of neuronal subtypes that have specific functionality, and complex dynamic feedback networks. Translating those biological contexts to an in silico model is additionally complicated by hardware constraints, especially the need for the model to adapt rapidly to a wide variety of incoming current patterns and intensities. The advance in this article is that it describes an analog model to address those issues, modeling generic ion channels in a way that is consistent with the classical Hodgkin–Huxley model and that would permit low power implants to be built. Importantly, the authors demonstrated its functionality by building a three ion-channel solid-state neuron model that predicted the spike timings of the Hodgkin–Huxley model with 96% accuracy. They took this further by constructing a six-channel model of CA1 hippocampal and respiratory neurons, which incorporated details such as the axonal or dendritic location of specific ion channels. The models were in 94–97% agreement with membrane potential oscillations to a wide variety of stimulation protocols, suggesting that they could simulate these neurons faithfully. The ability to use such biomimetic in silico devices has wide potential utility for situations that require dynamic biofeedback to regulate nervous system function. For example, the feedback regulating respiratory sinus arrhythmia, which is linked to sleep apnea and heart failure, and is sometimes related to aging or disease. The very low power consumption of the analog model is estimated to be 109 times less than the equivalent digital version, making it an attractive proposition for indwelling implants that rely on real-time physiological feedback and low power requirements. It will be fascinating to see how this technology moves forward.
It Is All in Your Mind: Endogenous Electric Fields in the Brain Guide Neural Stem Cells
Stephanie N. Iwasa, Abdolazim Rashidi, Elana Sefton, Nancy X. Liu, Milos R. Popovic, Cindi M. Morshead. Charge-balanced electrical stimulation can modulate neural precursor cell migration in the presence of endogenous electric fields in mouse brains. eNeuro 2019. DOI: 10.1523/ENEURO.0382-19.2019
Iwasa et al. demonstrate that the migration path of adult neuronal precursor cells in the mammalian brain is influenced by an endogenous electric field and can be altered by an applied field.
It has long been reported that stem cells injected therapeutically in animal models of stroke tend to accumulate in the damaged region, but the mechanism for their targeted migration has not been determined precisely. That the damaged region would inevitably have a naturally occurring injury potential has been postulated by proponents of bioelectricity as one cue that may underpin homing to the intended target. In this study, the authors show that a natural electric field exists in an area of the brain called the rostral migratory stream. This is significant because neural precursor cells migrate through this region toward the olfactory bulb, where they give rise to interneurons. Coupling previous reports from other laboratories identifying electric fields in the rostral migratory stream with this laboratory's previous evidence that undifferentiated (but not differentiated) neural precursor cells migrate rapidly toward the cathode of an electric field in vitro, the authors now extend these findings. Importantly, they demonstrate that the endogenous electric field contributes to neural precursor cell migration in the rostral migratory stream and that perturbing it disrupts neuronal precursor cell migration patterns in a predictable way. Experimentally, fluorescently labeled neural stem cells injected into the brain cortex were observed to migrate laterally, which correlated with the cathode of the electric field measured by a pair of electrodes inserted into the relevant brain region. Platinum stimulation electrodes inserted into the brain were then used to attempt to reroute the default migration path of neural precursor cells. After 3 days of stimulation, the endogenous migration path was not influenced significantly; however, a longer (6 days) stimulation regime could disrupt the default migration path. The use of platinum stimulation electrodes, which have inherent limitations for long-term stimulation protocols (akin to the silver used in the Injectrode technology already featured), is perhaps not ideal, but the authors addressed that by using a charge-balanced stimulation regime. The ability to disrupt the injected neural precursor migration path was subtle, but this might not be surprising as multiple endogenous physical and chemical gradients coexist with the electrical gradient. This article confirms that targeted electrical control of cell migration is an exciting and feasible technology. However, it will require tweaks to account for the complex in vivo brain environment and to define more exactly the optimum stimulation waveform, electrode placement, stimulation duration, and electrode materials. Careful modeling and simulation would be genuinely instructive for the design of clinical therapies using (stem) cell-based injection therapies coupled with electrical stimulation.
SCHEEPDOG: An In Vitro Tool for Herding Cells Electrically
Tom J. Zajdel, Gawoon Shim, Linus Wang, Alejandro Rossello-Martinez, Daniel J. Cohen. On demand spatiotemporal programming of collective cell migration via bioelectric stimulation. bioRxiv 2019. DOI: 10.1101/2019.12.20.884510
Speaking of modeling, here is a really nice article with the potential to aid understanding of the cellular mechanisms that underpin cell electrotaxis.
During the many years I have worked on neuronal axon guidance, I thought it would be fun to find the right spatiotemporal electric field conditions that would coax an axon to spell out my name as it grew in vitro (at least my first name). This article rekindled that idea because the authors describe an interactive programmable system that permits precise spatial and temporal control of an electric field to control cell migration en masse in vitro. The ability to control large number of cells collectively inspired the name for the device: SCHEEPDOG, which stands for spatiotemporal cellular herding with electrochemical potentials to dynamically orient galvanotaxis. It is a mouthful and I am sure it generated some mirth in the laboratory. Historically, most in vitro studies of cells' electric field responses used a steady direct current electric field because it mimics the endogenous fields present in tissues. But that has limitations when asking questions about the mechanisms that underpin cellular electrotaxis (galvanotaxis). In this study, the authors present a self-contained device with four orthogonally arranged programmable electrodes that also incorporates microfluidic flow to provide metabolic support for the cells. In addition, the design accounts for the need to separate the silver/silver chloride electrodes from the culture medium using agar salt bridges (due to toxicity issues as mentioned in the context of the Injectrode) and the need to define the shape of the cell culture area and chamber boundaries that determine the electric field geometry precisely. The authors used monolayer cell cultures of two cell types to validate the system: a cell line of MDCK cells and primary cultures of neonatal mouse keratinocytes. Cells were herded collectively toward the cathode as predicted, but then the cells were challenged to make turning maneuvers by alternating the active electrode pair by 90°. Both cell types displayed an L-shaped trajectory, adjusting track to migrate to the new cathode, but the keratinocytes were more responsive to the shift in cue than the MDCK cell line. The authors were able to program the device to stimulate cells to migrate in a roughly circular pattern by shifting the pairs of active electrodes regularly for an 8-h period. Excitingly, this device has the potential to aid dissection of the temporal scale of different aspects of the electrotaxis mechanism. Indeed, the authors identified two spatially distinct phases of cellular responses: a rapid initial electric field sensing and a slower directional migration response. A long-standing question in this larger research field is how do cells detect the electrical gradient in the extracellular space and translate it into a cell-type appropriate directional response? This tool coupled with innovative cell imaging and molecular techniques may help to answer this question. It is unlikely, however, that a one-size-fits-all cellular mechanism will be identified due to the heterogeneity of cell responses to electrical cues, even as demonstrated in this article. But comparisons of different cell types will be instructive in unpicking the key intracellular signaling mechanisms. Although used here for monolayer cultures, which are relevant to processes such as wound healing and migration of cell sheets during embryogenesis, it will be very interesting to use the system for other cell types, including neurons.
Keeping an Eye on the Kids: Bacteria Monitor Offspring Quality Using Membrane Potential
Teja Sirec, Jonatan M. Benarroch, Pauline Buffard, Jordi Garcia-Ojalvo, Munehiro Asally. Electrical polarization enables integrative quality control during bacterial differentiation into spores. iScience 2019;16:378–389.
Here is a change of scale. Even bacteria need to keep an eye on what their kids get up to and they use electrochemical signaling to do it.
Some bacteria form spores as a survival strategy in harsh environments, with subsequent germination of successfully formed spores once conditions are again suitable for growth. During sporulation, the mother cell forms an endospore by asymmetric division followed by engulfment of the forespore by the mother cell. This results in the spatially close forespore and mother cell membranes having opposing polarities. In this study, the authors explored sporulating Bacillus subtilis because the mechanism by which cells control the quality of their offspring while also undergoing differentiation is not well understood, especially in the emerging context of the electrical nature of communication within biofilms. Using time lapse microscopy and a membrane permeable cationic dye (ThT), the authors showed that cation accumulation was coupled with successful sporulation, with the outer surface of the endospore membrane becoming negative during late sporulation. From ThT studies using mutants lacking structural proteins of the outer spore coat, the authors concluded that the outer protective layers of forespores attract cations as they develop. Cation accumulation (electrical polarization) detected by ThT labeling appeared to prevent premature spore germination in single cell imaging experiments, so this idea was expanded and developed into a mathematical simulation. The model predictions were tested in experiments using the potassium ionophore valinomycin, which revealed that the electrochemical gradient of K+ during the late sporulation stage prevents premature germination. So quality control of spore formation is related intimately and spatiotemporally to the cation gradient. Importantly, the cation accumulation provides information about the bacterial environment. These results have implications for understanding the fundamentals of biofilm communication and bacterial cell growth, but they also have practical implications for control of bacterial growth in food or medical settings.
Flash Graphene: Not an Interstellar Superhero, But an Electrode Material Made from Rubbish
Duy X. Luong, Ksenia V. Bets, Wala Ali Algozeeb, Michael G. Stanford, Carter Kittrell, Weiyin Chen, Rodrigo V. Salvatierra, Muqing Ren, Emily A. McHugh, Paul A. Advincula, Zhe Wang, Mahesh Bhatt, Hua Guo, Vladimir Mancevski, Rouzbeh Shahsavari, Boris I. Yakobson, James M. Tour. Gram-scale bottom-up flash graphene synthesis. Nature 2020;577:647–651.
My final choice is something completely different. It is not strictly bio-electrical but is an innovative way to repurpose spent coffee grounds, hair clippings, and old rubber tires in an electrical context.
In this study, the authors loosely pack the carbon source material of interest (even pieces of plastic bottles) inside a quartz or ceramic tube situated between two electrodes. High-voltage discharge from a capacitor bank raises the temperature of the carbon source material rapidly (∼100 ms) to temperatures >3000 K. This flash Joule heating process can produce high yields of “flash graphene,” with the yield depending on the starting material. A high carbon source material gives 80–90% yield, but even humble coffee grounds can yield 35% flash graphene. This might not sound remarkable, but because coffee contains ∼40% carbon, this represents an impressive 85% conversion into graphene. By optimizing pressure, heating, and cooling conditions, high-quality flash graphene was produced. The authors demonstrate that it can be made in large quantities and that it has practical uses. For example, when prepared as composites, it served to strengthen concrete. Importantly, the authors tested flash graphene's utility in electrochemical storage devices by using it as electrodes in a Li-ion capacitor and in a Li-ion battery. This low energy technology, utilizing essentially free and widely available starting materials, is not surprisingly already being commercialized and scaled up for industrial purposes. I wonder whether making high-quality electrodes from rubbish has the potential to lower bioelectricity laboratory running costs.
Ann M. Rajnicek, BS, PhD, FRSB
School of Medicine, Medical Sciences and Nutrition Institute of Medical Sciences
University of Aberdeen
Aberdeen AB25 2ZD
United Kingdom
Media Editor
Bioelectricity
Email: a.m.rajnicek@abdn.ac.uk