Abstract
Although this edition of Bioelectricity Buzz starts off in the mud, with selections about the surprising utility of electrical bacteria found there, it finishes on a high with an example of how electrical levitation of SARS-CoV-2 could be useful to determine how long it survives in the air. The selections are relevant to this Bioelectricity special issue on the Methodology of Research and Applications of Electric Fields because they highlight how improved understanding of bioelectrical concepts is increasingly central to innovative diagnostic and clinical therapies.
Next time you have a stimulating mud treatment at a spa give this some thought; there is much more going on in mud than meets the eye. In fact, Science devoted a special issue to mud in August 2020.
Pennisi E. The mud is electric. Science 2020;369: 902–905. DOI: 10.1126/science.369.6506.902.
Beuth L, Pfeiffer CP, Schröder U. Copper bottomed: electrochemically active bacteria exploit conductive sulphide networks for enhanced electrogeneity. Energy Environ Sci 2020; DOI: 10.1039/d0ee01281e.
Liu X, Gao H, Ward JE, Liu X, Yin B, Fu T, Chen J, Lovley DR, Yao J. Power generation from ambient humidity using protein nanowires. Nature 2020;578: 550–554. DOI: 10.1038/s41586-020-2010-9.
This set of articles demonstrates the growing appreciation for harvesting electricity from our own surroundings, highlighting the utility of electrochemically active bacteria of the genus Geobacter found in muddy sediments.
Geobacter sulfurreducens have unique electrical properties; they can join end to end, forming cables that carry current over several centimeters through muddy environments, as biofilms on copper electrodes they can generate copper sulfides that form a filamentous network that produces electric current, and nanoscale protein wires harvested from them can generate useful amounts of sustained electricity under ambient conditions.
The cable properties of the networks in mud allow the bacterial population in the deeper relatively oxygen-depleted areas to tap into the metabolic processes operating in distant oxygen-rich regions. Electrochemically, the hydrogen sulfide arising from natural decay of organic matter in the deep mud layers is oxidized, with electrons shuttled along the conducting surface of the bacterial cable toward the surface layer where electrons are transferred to electron acceptors, such as iron, to generate iron oxide and water. A by-product of this bacterial chemistry is a pH gradient, with surface layers more alkaline and deeper layers more acidic, which further influences the bioelectrical properties of microbes in different layers.
Other bacterial types that generate electricity in mud have been identified in both fresh water and saltwater environments, suggesting that this phenomenon exists beneath our feet in many ecosystems and is a ripe area for future exploration. Beuth et al. (2020) demonstrate that biofilms dominated by Geobacter growing on metallic copper can develop networks of highly conductive copper sulfides that boost current generation by the biofilm substantially.
Geobacter can also extend “nanowires” from its surface that shuttle electrons and allow distant bacterial populations to interact in mutually beneficial ways, thus resolving their relative redox requirements. How electrons move through the nanowires is still under investigation, but their practical utility has been demonstrated by Gao et al. (2020). A thin (∼7 μm) film of protein nanowires harvested from Geobacter sulfurreducens was deposited onto a gold electrode and a smaller gold electrode was positioned in contact with the upper surface of the nanowire film.
When connected to a load resistor, the device delivered continuous electrical current for at least 20 h before self-recharging and was able to provide current up to 2 months (1,500 h). Current production was linked to ambient humidity due to a moisture gradient present within the nanowire layer: less moisture near the larger basal electrode and more moisture at the upper surface in contact with the relatively small electrode. The moisture gradient creates an ionization gradient and charge separation within the film to produce an electric field.
An open circuit voltage of ∼0.5 V was maintained using a device as small as 1 mm2. Connecting 17 devices in series generated 10 V, so there is obvious potential for their use in powering “green” electronic toys, wearable electroceutical devices such as those described hereunder, or perhaps one day being able to charge a mobile phone using ambient air. The ability to genetically alter the nanowires suggests that they could be tuned for optimization and to better understand the underlying mechanism.
Diabetes is a significant health problem globally and as its incidence increases, the need to find novel effective therapies increases accordingly. Management often involves invasive therapies, such as injections and monitoring devices, but Carter et al. propose that noninvasive electrical and magnetic fields could be used to control type 2 diabetes.
Carter CS, Huang CS, Searby CC, Cassaidy B, Mille MJ, Grzesik WJ, Piorczynsk TB, Pak TK, Walsh SA, Acevedo M, Zhang Q, Mapuskar KA, Milne GL, Hinton AO Jr, Guo D-F, Weiss R, Bradberry K, Taylor EB, Rauckhorst AJ, Dick DW, Akurath V, Falls-Hubert KC, Wagner BA, Carter WA, Wang K, Norris AW, Rahmoun K, Buettner GR, Hansen JM, Spitz DR, Abel ED, Sheffield VC. Exposure to static magnetic and electric fields treats type 2 diabetes. Cell Metabol 2020;32: 561–576. https://doi.org/10.1016/j.cmet.2020.09.012
Hallmarks of diabetes include insulin resistance and hyperglycemia. In type 2 diabetes, a redox imbalance contributes, with relatively high levels of pro-oxidants, such as superoxide and hydrogen peroxide, and relatively low levels of antioxidants (mainly glutathione). This imbalance disrupts insulin function, but it has proven difficult to target redox homeostasis for diabetes clinically because the drugs tested have very short half-lives and undesirable side effects.
Carter et al. (2020) explored the influence of electromagnetic fields on glucose regulation in vivo using three mouse models of type 2 diabetes. Free moving mice were exposed to a combination of static magnetic and electrostatic fields (3 mT magnetic field, vertically oriented electric field of 7 kV/m) for 30 days. Intriguingly, testing them separately revealed that the magnetic field alone worsened glycemia and glucose tolerance, whereas the electric field alone did not have a significant influence compared with sham controls. This indicates that the beneficial effect is induced only with combined stimulation.
When the reason for enhanced insulin sensitivity was explored, Carter et al. (2020) found that the combined field treatment elevated liver glycogen in mice and in human hepatocytes in tissue culture, implicating liver function mechanistically. The team identified a redox switch that mediates the insulin sensitizing effects due to the combined fields, but the proteins that mediate the response have yet to be identified.
Although the mice exposed continuously to the combined fields for 30 days showed no histopathology, changes in cardiac function, or changes in blood pressure, there is a question of how well the therapy would translate to humans. Although the positive glycogen response of cultured human cells is promising, the stimulation parameters used on mice would have to be scaled up considerably and the magnitude and intensity of the stimulation parameters would need optimization. This may present safety challenges for the more intense stimulation regimes, but hopefully they can be met.
Perhaps the smart watch on your wrist just got smarter in a move toward personalized medicine.
Lin S, Yu W, Wang B, Zhao Y, En K, Zhu J, Cheng X, Zhou C, Lin H, Wang Z, Hojaiji H, Yeung C, Milla C, Davis RW, Emaminejad S. Noninvasive wearable electroactive pharmaceutical monitoring for personalized therapeutics. PNAS 2020;117(32): 19017–19025. https://doi.org/10.1073/pnas.2009979117
Individuals can metabolize medications with different kinetics depending on physical and physiological circumstances. The ability to deliver an appropriate dose of medication at the appropriate time, or conversely, to prevent overdosing, would benefit from a system to monitor drug kinetics and drug concentration noninvasively and accurately. Lin et al. (2020) describe a voltammetric sensor with a boron-doped diamond interface positioned on the skin surface that can be worn conveniently as a smart watch.
Acetaminophen was used as the electroactive test drug because it is used widely as a treatment for pain and fever, it can cause liver failure if overused, and it has a reported correlation between saliva and blood concentrations. For the drug sensing interface, undesired signals were controlled by tuning the working skin surface electrode to the electron transfer kinetics of the redox reaction of acetaminophen and incorporating a polymer membrane to minimize noise. The device combined capabilities for sampling sweat or saliva, signal acquisition, and data display with a temporal resolution of minutes.
Initial measurements from sweat or saliva samples doped with acetaminophen were followed by successful measurements of the drug in human subjects wearing the sensor as a modified smart watch. The ability to monitor pharmacokinetics of drugs in real time and to record compliance with prescribed treatments have the potential to deliver truly personalized medicine.
Electrical stimulation by cochlear implants can bypass nonfunctioning hair cells of the ear to transmit auditory signals to the brain, but lack of signal specificity can be problematic. Keppeler et al. combined cutting edge optogenetic and light emitting diode (LED) technologies to design an implant device that improves signal resolution.
Keppeler D, Schwaerzle M, Harczos T, Jablonski L, Dieter A, Wolf B, Ayub S, Vogl C, Wrobel C, Hoch G, Abdellatif K, Jeschke M, Rankovic V, Paul O, Ruther P, Moser T. Multichannel optogenetic stimulation of the auditory pathway using microfabricated LED cochlear implants in rodents. Sci Transl Med 2020;12:eabb8086. DOI: 10.1126/scitranslmed.abb8086
The inability to hear can be alleviated by implant devices that convert sound waves to electrical impulses that stimulate the neurons in the ear and transmit auditory signals to the brain. This technique typically reproduces sound using a microphone detector with the resulting signal split into banded frequency channels that stimulate an array of 12–24 electrodes to deliver electrical impulses to ganglia neurons in the cochlea. Because the fluid in the ear is electrically conductive, the resulting electrical stimulation signal tends to spread laterally. The result is a loss of spatial specificity that can manifest in poor sound quality, especially where there is a lot of background noise, such as crowded rooms.
Keppeler et al. (2020) have leveraged the technological advances of optogenetics and miniaturized LED technologies to develop a cochlear implant that when tested in rodents stimulated neurons more selectively and with higher spatial resolution. Spiral ganglion neurons in the cochlea of rats were modified genetically to express channelrhodopsin. The neurons were then stimulated using a linear array of 10 power-efficient blue LED chips integrated onto a thin polyimide layer, with fiber optical connections to individual LEDs.
Rats were trained to respond to different sounds with distinct behaviors and then the hair cells in their ears were disabled and the device was implanted. Multichannel optogenetic stimulation of spiral ganglion neurons was assessed electrophysiologically, with recordings from the auditory midbrain indicating improved selectivity with the implants. In behavioral tests, the animals showed similar responses to the sounds as demonstrated during preimplant training.
This report is exciting because it could improve the performance of auditory implants but there are several challenges ahead. In a human ear, the implants would require many more LED chips, and the stability and continued performance of the chip need further testing. A significant hurdle, however, might be the issue of the requirement to genetically modify the nerve cells to respond to the light stimulus. However, this limitation is also the strength of the technique, permitting signal delivery to specific neurons.
Surgically implanted electrodes can be used to treat advanced Parkinson's disease, but whether the type of anesthesia (awake or asleep) used during surgery influences the outcome has not been studied. Here, Senemmar et al. (2020) demonstrate that asleep surgical methods improve the treatment window.
Senemmar F, Hartmann CJ, Slotty PJ, Vesper J, Schnitzler A, Groiss SJ. Asleep surgery may improve the therapeutic window for deep brain stimulation of the subthalamic nucleus. Neuromodulation 2020. https://doi.org/10.1111/ner.13237
Although levodopa is an effective treatment for motor symptoms in Parkinson's disease, long-term treatment leads to adverse effects. Surgical implantation of devices for deep brain stimulation (DBS) is now a recognized therapy for advanced Parkinson's disease in patients with motor complications that are not resolved by drug treatment. DBS can be delivered using ring electrodes, or with the more recently developed segmented electrodes that deliver directional DBS (dDBS) to target neuronal populations more precisely.
Whether the type of anesthesia (awake or asleep) used during surgery influences the outcome has not been studied, especially for dDBS. Here, Senemmar et al. (2020) studied a total of 104 patients (80 asleep and 24 awake) undergoing subthalamic nucleus DBS. The outcome measure was the therapeutic window, defined as the difference between therapeutic threshold current (minimum stimulation amplitude leading to clinical motor improvement) and the side effect threshold (minimum stimulation amplitude leading to persistent side effects).
They demonstrated that asleep surgical methods improved the treatment window compared with awake surgery, but that there was no influence on the motor score outcome. The treatment window was improved further by using segmented electrodes for dDBS as opposed to using ring electrodes to deliver omnidirectional DBS. These data suggest that asleep surgery may be preferred over awake surgery, but this needs to be followed up in a longer term study.
Here is a chance to get really creative with your wearable bioelectronics! Ershad et al. (2020) describe a highly conformable adhesive electrode material that can be drawn freehand onto skin. Excitingly, it was shown to accelerate wound healing in mouse skin.
Ershad F, Thukral A, Yue J, Comeaux P, Lu Y, Shim H, Sim K, Kim N, Rao Z, Guevara R, Contreras L, Pan F, Zhang Y, Guan Y, Yang P, Wang X, Wang P, Wu X, Yu C. Ultra-conformal drawn-on-skin electronics for multifunctional motion artifact-free sensing and point-of-care treatment. Nat Commun 2020;11: Article number 3823. https://doi.org/10.1038/s41467-020-17619-1
With the advent of electroceutical technologies, wearable devices are used increasingly for clinical diagnosis, monitoring, and treatment. However, a distinct challenge for real-world applications is maintaining device performance in a moving subject, including avoidance of artifactual signals. Surface applied electrodes are prone to being dislodged from moving or sweating skin, especially if they have insufficient elastic or adhesion properties, so there is a need to develop more robust, highly adhesive, and elastic conductive materials.
Ershad et al. (2020) prepared inks from composites of silver flakes and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (Ag-PEDOT:PSS), poly(3-hexylthiophene-2,5-diyl) nanofibrils (P3HT-NF), and ion gel as the conductive, semiconducting, and dielectric inks, respectively. By using stencils and a modified pen, it is possible to draw any desired shape or circuit repeatedly and rapidly with the inks. Tests showed that the electronic devices and their performance were unaffected by sweat or skin movement and that they could be linked wirelessly to monitor a moving subject's heart rate.
As an example of a potential therapeutic use, the ink was used to apply electrical stimulation to wounded mouse skin. After hair removal with a depilatory cream, an epidermal wound was made surgically. The conductive ink was drawn on two sides of one half of the wound to mimic capacitor plates, leaving the other half without ink as a control. On days 1 and 3, pulsed DC stimulation (30 μA DC pulses, 100 μs in duration, every 15 ms) was applied for 1 h per day. By day 5, the wound width was reduced significantly in the half of the wound with the electronic ink compared with the control half.
This experiment suggests practical advantages in addition to the ability to accelerate healing and the properties of the ink itself. Its portability indicates that it could be used in areas with minimal resources, for example, in battlefield (or surgical) situations to draw around wounds of any shape. This technology is very exciting and is likely to find many uses.
The inability to maintain penile erection has many underlying causes and the condition can have devastating personal and interpersonal consequences. Existing pharmacological treatments have focused on neurotransmitters, second messengers, reactive oxygen species, growth factors, and hormones but ion channels are now emerging as promising targets.
Diniz AFA, Ferreira RC, de Souza ILL, da Silva BA. Ionic channels as potential therapeutic targets for erectile dysfunction: A review. Front Pharmacol 2020;11:1120. DOI: 10.3389/fphar.2020.01120. eCollection 2020.
Erectile dysfunction usually affects men >40 years of age and is estimated to affect ∼150 million men worldwide. As a persistent condition, it is desirable to find novel effective treatments that are safe. The most common first-line therapy drugs are phosphodiesterase inhibitors, such as sildenafil, tadalafil, vardenafil, and iodenafil, but these can have adverse side effects. Diniz et al. (2020) reviewed the etiological, physiological, and psychological basis of erectile dysfunction and compiled evidence implicating a variety of ion channel types in its underlying physiology. Specifically, they explored potential contributions of large-conductance Ca2+-activated K+ channels, small-conductance Ca2+-activated K+ channels, KCNQ-encoded voltage-dependent K+ channels, transient receptor potential channels, and calcium-activated chloride channels. They concluded that modulation of these channels is an attractive area for development. However, these channels also control key functions in a wide variety of tissues. Therefore, if they are to be considered safe, it might be desirable to find a way to target delivery to avoid complications related to cardiac arrhythmia, hypotension, or hypertension.
How long does the virus responsible for COVID-19 remain viable in the air? The answer has been difficult to determine but using electricity to suspend the virus under a variety of controllable conditions may resolve this important question.
Otero Fernandez M., Thomas R., Garton N., Hudson A., Haddrell A., Reid J. Assessing the airborne survival of bacteria in populations of aerosol droplets with a novel technology. J Roy Soc Interface 2020;16:20180779. https://doi.org/10.1098/rsif.2018.0779.
Like it or not, we are increasingly expected to wear face coverings in public places to curb transmission of COVID-19. The effectiveness of this strategy depends on how well masks constrain aerosolized droplets carrying the virus, but also on how long the virus survives in air, which remains controversial.
Existing methods used to generate aerosolized droplets in laboratories have inherent limitations that prevent extrapolation to real-life conditions. The rotating drum method used most frequently is limited by inevitable gravitational deposition of droplets on the walls of the drum, and creating suspensions of droplets similar in diameter to those generated by a cough or sneeze is difficult. An alternative technique suspends droplets on spider silk “microthreads,” but they can be lost due to turbulence, they are not truly in suspension, and residual chemicals from silk processing may influence viability of the biological cargo.
Use of nebulizers to generate bioaerosol droplets experimentally also has limitations, including reduced microorganism viability caused by physical damage and nonuniform droplet diameters, which both affect cell viability. A further difficulty is controlling the conditions in which droplets are suspended to prevent unintended influence by laboratory conditions, including temperature, humidity, and light (visible and UV wavelengths), which impact aerosolized microbe survival. An improved method is, therefore, needed to generate bioaerosols of uniform biologically relevant diameter that permits suspension of droplets for prolonged periods under experimenter-controlled conditions.
Otero Fernandez et al. (2019) describe a novel system they named CELEBS (controlled electromagnetic levitation and extraction of bioaerosol onto a substrate). The Bioelectricity relevance is that it couples a piezoelectric droplet dispenser with an electrodynamic trap to create bioaerosol droplets that are subsequently suspended between two horizontal ring electrodes for times ranging from seconds to days, but theoretically, they could be held in suspension indefinitely.
The CELEBS system produces individual droplets (∼28 μm radius), and then oscillating forces from the electrodynamic field created by applying an alternating current (AC) potential to the ring electrodes confine the particles in the center of the electrodynamic trap. Accidental exposure of operators to the aerosol is minimized by using very small volumes (<10 μL) and by sealing the unit in a sealed box in which temperature, humidity, and light conditions can be controlled. At the end of the experiment, the bioaerosol droplets can be deposited onto a surface of choice for subsequent viability tests or other analysis.
Otero Fernandez et al. (2019) used the CELEBS system to explore the viability of bacteria, but it is currently being used by Dr. Allen Haddrell at the University of Bristol in proof of concept tests on levitated mouse virus particles, with the intention to study the longevity of Sars-CoV-2 in aerosols soon. (https://www.theguardian.com/world/2020/sep/25/uk-scientists-begin-study-of-how-long-covid-can-survive-in-the-air). This could be extremely valuable both to aid basic understanding of how the virus spreads and to inform public policy about the utility of face coverings and other personal protective equipment in the context of COVID-19.
A caveat might be whether the electrical conditions used to levitate the droplets could themselves influence virus viability, especially during extended levitation. Otero Fernandez et al. (2019) found no influence on viability of the AC field used to levitate E coli bacteria for 5 s, but whether virus viability is affected needs scrutiny, especially after prolonged levitation.
This is the last installment of Bioelectricity Buzz for 2020, a year we will long remember. I look forward to sharing many more exciting developments and innovations with you in 2021.