Bioelectronics for Millimeter-Sized Model Organisms

Abstract

Advances in microfabrication technologies and biomaterials have enabled a growing class of electronic devices that can stimulate and record bioelectronic signals. Many of these devices have been developed for humans or vertebrate animals, where miniaturization allows for implantation within the body. There are, however, another class of bioelectronic interfaces that exploit microfabrication and nanoelectronics to record signals from tiny, millimeter-sized organisms. In these cases, rather than implanting a device inside an animal, animals themselves are loaded in large numbers into bioelectronic devices for neural circuit and behavioral interrogation. These scalable interfaces provide platforms to develop new therapeutics as well as better understand basic principles of bioelectronic communication, neuroscience, and behavior. Here we review recent progress in these bioelectronic technologies and describe how they can complement on-chip optical, mechanical, and chemical interrogation methods to achieve high-throughput, multimodal studies of millimeter-sized small animals.

Keywords: Bioelectronics; Electronic Materials; Systems Neuroscience; Techniques in Neuroscience.

Graphical abstract

Figure 1

Tradeoffs of Small-Model Organisms for Neurobiology Common model organisms in neurobiology and estimated tradeoffs between experimental throughput and the number of neurons in the nervous system. On the top axes, we also denote the approximate percentage of single neurons that can be simultaneously measured during experiments. Although the structure of the nervous system in millimeter-sized animals is significantly different than the mammalian brain, many molecular and cellular properties are conserved, and these animals show orders of magnitude improvements in experimental throughput and the number of neurons that can be simultaneously recorded.

Figure 2

Examples of Bioelectronic Interfaces Top row depicts conventional technologies for recording from the nervous system of Hydra and C. elegans. Bottom row shows these same recording modalities but adapted for on-chip recordings in fluidic microdevices that significantly increase experiment throughput. (A) Suction electrodes for recording neuromuscular activity from the C. elegans pharynx. (B) Electrodes pressed into the Hydra body can record local field potentials and contraction bursts. (C) Nano-SPEAR electrodes tightly pressed into the C. elegans body wall record similar spiking activity as patch-clamp electrophysiology. Panel A (top) adapted from Lockery et al. (2012) with permission from the Royal Society of Chemistry and (bottom) adapted from Raizen and Avery (1994). Panel B (top) adapted from Passano and McCullough (1964) with permission from The Company of Biologists and (bottom) adapted from Badhiwala et al. (2018). Panel C (top) adapted from Gao and Zhen (2011) and (bottom) adapted from Gonzales et al. (2017).

Figure 3

Bioelectronics for Studying Neural Circuits, Behavior, Disease Models, and the Effects of Drug Candidates (A) Adult C. elegans partially immobilized in a microfluidic chamber for muscle cell recordings with nano-SPEARs. These recordings led to the discovery of a behavioral state transition between sleep and wakefulness. (B) Bioelectronic recordings in (A) are complementary to imaging methods for behavioral monitoring and calcium imaging. These techniques were used to further dissect the neural mechanisms underlying microfluidic-induced sleep. (Top) Adult C. elegans partially immobilized in a microfluidic chamber during whole-brain GCaMP6s imaging (inset scale bar is 5 μm). (Bottom) The partial immobilization enables simultaneously monitoring animal behavior and neural activity during spontaneous sleep-wake state transitions. (C) Hydra partially immobilized in a hybrid bioelectric-fluidic device for simultaneous behavior, electrical recordings, and whole-animal calcium imaging of neural activity. Contraction bursts correspond to dramatic increases in calcium activity in specific groups of neuronal cells. These bursts in fluorescence correspond to simultaneously recorded bursts of electrical activity in the muscle cells. Electrical measurements can be captured with high temporal resolution using nano-SPEARs. (D) Nano-SPEAR muscle cell recordings in a C. elegans model for Parkinson disease. Protein aggregation deteriorates spiking activity as adults progress from day 1 to day 2; however, this phenotype can partially be rescued by a clioquinol drug treatment. (E) C. elegans partially immobilized in a microfluidic device for pharyngeal recordings during drug stimulation. Ivermectin inhibits muscle and neural activity by activating glutamate-gated chloride channels, therefore inhibiting pharyngeal pumping and dramatically changing the waveform shape and frequency. (F) Similar to (A) and (B), on-chip studies are not limited to bioelectronic recordings. Here, microfluidic immobilization of Hydra enables chemical stimulation without mechanically perturbing the animal while simultaneously monitoring behavior. Reduced glutathione (GSH) induces the feeding response, which leads to inhibition of contractile movements. Figures adapted from: A (Gonzales et al., 2017), B (Gonzales et al., 2019), C (Badhiwala et al., 2018), D (Gonzales et al., 2017), E (Hu et al., 2013, Ménez et al., 2019), F (Badhiwala et al., 2018).

Figure 4

Multimodal Bioelectronic Platforms for Small Organisms Future bioelectronic platforms will enable new discoveries by combining multiple cutting-edge technologies onto a single high-throughput platform. Electrophysiology and calcium-imaging data adapted from Gonzales et al. (2017) and Gonzales et al. (2019), respectively.