Non-Genetic Modulation of Electrical Behavior in Cardiomyocytes and Neurons
By: Nivedina A. Sarma
The mitochondria get all the credit for being the powerhouse of the cell, but an often-overlooked, equally important player in cellular activity is the cell membrane. The cell membrane is composed of a fluid phospholipid bilayer that is studded with ion pumps through which calcium, potassium, and sodium ions flow between the intracellular and extracellular environments. This dynamic ion flow generates a membrane potential, which is responsible for regulating the action potentials that drive activities such as communication and beating in electrically active cells.
Functions such as neuronal cell communication and cardiomyocyte beating are essential for living systems to conduct daily functions. Therefore, when electrical activity goes awry in such cells, the result is debilitating diseases such as epilepsy, Parkinson’s disease, and cardiac arrhythmia. In the face of this challenge, photoelectric devices–tools that transduce energy from light into an electric current–promise to address aberrant electrical activity by modulating membrane voltage in cells such as cardiomyocytes and neurons.
The Electrical Basis for Cellular Activity
To understand the basis of this claim, we must venture to 18th century Italy, where physiologist Luigi Galvani installed a lightning rod,splayed a frog corpse across a table in his back garden, and waited for it to rain. Galvani’s hypothesis was that naturally-occurring electricity was somehow related to limb movement. One evening, in the middle of a storm, lighting struck the rod and the frog’s leg began to twitch, providing evidence for Galvani’s claim. This experiment, which gave us the word “galvanized,” was monumental because it established an electrical basis for biological activity [1]. Today, materials scientists build upon Galvani’s discovery to design nanoscale biomimetic devices that operate in close contact with neurons and cardiomyocytes and transduce energy from light to influence behavior at cell, tissue, and organ scales.
Since Galvani’s initial experiment, decades of research in physiology, cell biology, and material-cell interfaces have uncovered that the frog’s leg twitched because contact with lightning initiated action potentials–millisecond-long spikes in cell membrane voltages–between neurons and muscle cells. At rest, a nerve cell’s phospholipid bilayer separates the negatively charged intracellular environment from the positively charged extracellular space. In this role, the cell membrane works like the dielectric of a capacitor and the intracellular potential hovers 70 mV below the extracellular charge. Changes in current, pressure, or temperature initiate ion flow amongst the selective channels within the cell membrane, and the ion flow lowers the membrane voltage to -55mV. The drop in membrane voltage triggers an action potential as Na+ ion channels open and nerve impulses travel along the axon [2]. At the tissue level, individual action potentials work in concert to create local field potentials that coordinate activity across the brain and the peripheral nervous system. By studying new ways of modulating membrane voltage amongst single cells, researchers stand to gain insight that could promote advances in regenerative medicine.
Silicon Nanomaterials and the Photoelectric Effect
In the quest to design devices capable of modulating membrane voltage, researchers have worked with a host of materials. Silicon nanomaterials are a particularly promising toolkit because silicon’s biocompatibility and semiconductor properties allow devices to be fabricated so they can modulate and record electrical activity in neurons and cardiomyocytes with high spatial and temporal specificity [3].
Silicon’s electrical conductivity can be tuned by introducing electronic impurities into the silicon lattice – a process known as doping. For example, doping with boron introduces holes to the silicon lattice (creating a “p-type” material) because boron has one less valence electron than silicon. By the same logic, doping with phosphine introduces excess electrons to the lattice (resulting in an “n-type” material) because phosphorus has one more valence electron than silicon. When p-type and n-type materials are interfaced and stimulated with light, holes flow from the p-type lattice to the n-type lattice and electrons flow from the n-type lattice to the p-type lattice. This generates an electric current, which can be applied at the nanoscale to open voltage-gated ion channels within the cell membrane. Once the ion channels are opened, dynamic ion flow proceeds to regulate a cell’s electrical activities.
One such nanoscale device is the silicon nanowire, which works as a coaxial p-i-n junction and can be internalized into mammalian cells through phagocytosis to create active interfaces with cytoskeletal components. The 50nm diameter of the nanowire is advantageous for interfacing with single cells, and it enables the nanowire to target specific locations on cell membranes in order to control calcium ion flow. These effects have been demonstrated among cardiac and neuron cultures and are promising for applications in regenerative medicine such as heart tissue repair and deep brain stimulation.
Silicon Nanomaterials to Influence Cardiomyocyte Behavior
Devices to modulate the heart’s electrical activity are extremely pressing because heart disease is the leading cause of death, and cardiomyocytes are incapable of regeneration as a result of their high level of specialization. Consequently, after heart failure, dense and electrically inactive scar tissue forms on the heart’s surface. The scar tissue impedes regular heart contractions, introduces arrhythmia, and perpetuates risk of heart failure. Current therapies for heart disease rely largely on transplants, but this solution is unsustainable because there will always be fewer donor hearts than there are people who need them. In light of this challenge, researchers have developed nanoscale photovoltaic devices that are capable of interacting with organelles and single cells to restore function to the heart.
Silicon nanowires have been applied to treat cardiac conduction disorders and induce maturation in cardiomyocytes differentiated from stem cells. In a study published in the Proceedings of the National Academy of Sciences, the Tian group at the University of Chicago showed that silicon nanowires implemented into a tissue scaffold efficiently modulate beating frequency in a cardiomyocyte culture. The photoelectric effect from the nanowires trained cardiomyocytes to beat at an applied frequency after they were periodically exposed to optical pulse trains in vitro. The project also saw optical stimulation applied to ex vivo rat hearts, which resulted in sustained heart beats at various frequencies [4].
A second project, led by the Mei group at Clemson University, used silicon nanowires to induce maturation in cardiomyocytes derived from human induced-pluripotent stem cells (hiPSC-CMs). The current biochemical processes for differentiating stem cells into heart cells are only able to arrive at immature heart cells, which lack efficient cell communication pathways and the cohesive beating required for clinical applications. The study observed that hiPSC-CMs developed stronger contractile complexes and that their beating profiles became aligned following photoelectric stimulation with p-i-n silicon nanowires [5].
Silicon Nanomaterials for Neuronal Activity and Motor Control
Silicon nanowires also have promising applications for neuromodulation. In February 2018, the Tian group demonstrated that silicon nanowires elicit action potentials in primary rat dorsal root ganglion (DRG) neurons through a photoelectrochemical process. When the nanowires were introduced to DRG neuron culture, they formed tight junctions with the neuronal membranes. These junctions enabled depolarization at specific locations along the cell membrane, and the depolarization triggered action potentials in the neurons. Researchers found that as the duration of laser stimulation increased, the necessary laser power to create an action potential decreased [6]. From this information, they were able to determine the minimum energy to trigger an action potential.
Other forms of silicon, such as mesoporous silicon, are also able to modulate brain activity through photoelectric effects. Mesoporous silicon has a deformable structure and is minimally invasive to biological systems. A 2018 study conducted by Jiang et al. investigated mesoporous silicon’s material structure and found that the photovoltaic and Faradic effects enable such materials to modulate brain activity. The study concluded with interfacing a mesoporous silicon mesh device with a mouse brain. Photoelectric stimulation on the right side of the forelimb primary motor context resulted in controlling the mouse’s left forelimb while stimulation of the left forelimb primary motor context controlled motion in the right forelimb [7].
Breakthroughs with nanoscale photoelectric devices introduce a non-genetic approach to modulating cell activity and promise impacts in regenerative medicine for nervous and cardiac systems at both the tissue and organ levels. In designing devices that pace cardiomyocytes to target frequencies and induce maturation in hiPSC-CMs, researchers can restore function to the heart after trauma, which is a state that the heart cannot achieve alone. Furthermore, using silicon nanowires to trigger action potentials in individual neurons and stimulate local field potentials that modulate brain tissue activity illuminates new methods to perform deep brain stimulation and restore motor function. This opens the door for using light to correct aberrant electrical behavior in electrically active cells, and in so doing promote tissue and organ repair in ways never seen before.
References
The mitochondria get all the credit for being the powerhouse of the cell, but an often-overlooked, equally important player in cellular activity is the cell membrane. The cell membrane is composed of a fluid phospholipid bilayer that is studded with ion pumps through which calcium, potassium, and sodium ions flow between the intracellular and extracellular environments. This dynamic ion flow generates a membrane potential, which is responsible for regulating the action potentials that drive activities such as communication and beating in electrically active cells.
Functions such as neuronal cell communication and cardiomyocyte beating are essential for living systems to conduct daily functions. Therefore, when electrical activity goes awry in such cells, the result is debilitating diseases such as epilepsy, Parkinson’s disease, and cardiac arrhythmia. In the face of this challenge, photoelectric devices–tools that transduce energy from light into an electric current–promise to address aberrant electrical activity by modulating membrane voltage in cells such as cardiomyocytes and neurons.
The Electrical Basis for Cellular Activity
To understand the basis of this claim, we must venture to 18th century Italy, where physiologist Luigi Galvani installed a lightning rod,splayed a frog corpse across a table in his back garden, and waited for it to rain. Galvani’s hypothesis was that naturally-occurring electricity was somehow related to limb movement. One evening, in the middle of a storm, lighting struck the rod and the frog’s leg began to twitch, providing evidence for Galvani’s claim. This experiment, which gave us the word “galvanized,” was monumental because it established an electrical basis for biological activity [1]. Today, materials scientists build upon Galvani’s discovery to design nanoscale biomimetic devices that operate in close contact with neurons and cardiomyocytes and transduce energy from light to influence behavior at cell, tissue, and organ scales.
Since Galvani’s initial experiment, decades of research in physiology, cell biology, and material-cell interfaces have uncovered that the frog’s leg twitched because contact with lightning initiated action potentials–millisecond-long spikes in cell membrane voltages–between neurons and muscle cells. At rest, a nerve cell’s phospholipid bilayer separates the negatively charged intracellular environment from the positively charged extracellular space. In this role, the cell membrane works like the dielectric of a capacitor and the intracellular potential hovers 70 mV below the extracellular charge. Changes in current, pressure, or temperature initiate ion flow amongst the selective channels within the cell membrane, and the ion flow lowers the membrane voltage to -55mV. The drop in membrane voltage triggers an action potential as Na+ ion channels open and nerve impulses travel along the axon [2]. At the tissue level, individual action potentials work in concert to create local field potentials that coordinate activity across the brain and the peripheral nervous system. By studying new ways of modulating membrane voltage amongst single cells, researchers stand to gain insight that could promote advances in regenerative medicine.
Silicon Nanomaterials and the Photoelectric Effect
In the quest to design devices capable of modulating membrane voltage, researchers have worked with a host of materials. Silicon nanomaterials are a particularly promising toolkit because silicon’s biocompatibility and semiconductor properties allow devices to be fabricated so they can modulate and record electrical activity in neurons and cardiomyocytes with high spatial and temporal specificity [3].
Silicon’s electrical conductivity can be tuned by introducing electronic impurities into the silicon lattice – a process known as doping. For example, doping with boron introduces holes to the silicon lattice (creating a “p-type” material) because boron has one less valence electron than silicon. By the same logic, doping with phosphine introduces excess electrons to the lattice (resulting in an “n-type” material) because phosphorus has one more valence electron than silicon. When p-type and n-type materials are interfaced and stimulated with light, holes flow from the p-type lattice to the n-type lattice and electrons flow from the n-type lattice to the p-type lattice. This generates an electric current, which can be applied at the nanoscale to open voltage-gated ion channels within the cell membrane. Once the ion channels are opened, dynamic ion flow proceeds to regulate a cell’s electrical activities.
One such nanoscale device is the silicon nanowire, which works as a coaxial p-i-n junction and can be internalized into mammalian cells through phagocytosis to create active interfaces with cytoskeletal components. The 50nm diameter of the nanowire is advantageous for interfacing with single cells, and it enables the nanowire to target specific locations on cell membranes in order to control calcium ion flow. These effects have been demonstrated among cardiac and neuron cultures and are promising for applications in regenerative medicine such as heart tissue repair and deep brain stimulation.
Silicon Nanomaterials to Influence Cardiomyocyte Behavior
Devices to modulate the heart’s electrical activity are extremely pressing because heart disease is the leading cause of death, and cardiomyocytes are incapable of regeneration as a result of their high level of specialization. Consequently, after heart failure, dense and electrically inactive scar tissue forms on the heart’s surface. The scar tissue impedes regular heart contractions, introduces arrhythmia, and perpetuates risk of heart failure. Current therapies for heart disease rely largely on transplants, but this solution is unsustainable because there will always be fewer donor hearts than there are people who need them. In light of this challenge, researchers have developed nanoscale photovoltaic devices that are capable of interacting with organelles and single cells to restore function to the heart.
Silicon nanowires have been applied to treat cardiac conduction disorders and induce maturation in cardiomyocytes differentiated from stem cells. In a study published in the Proceedings of the National Academy of Sciences, the Tian group at the University of Chicago showed that silicon nanowires implemented into a tissue scaffold efficiently modulate beating frequency in a cardiomyocyte culture. The photoelectric effect from the nanowires trained cardiomyocytes to beat at an applied frequency after they were periodically exposed to optical pulse trains in vitro. The project also saw optical stimulation applied to ex vivo rat hearts, which resulted in sustained heart beats at various frequencies [4].
A second project, led by the Mei group at Clemson University, used silicon nanowires to induce maturation in cardiomyocytes derived from human induced-pluripotent stem cells (hiPSC-CMs). The current biochemical processes for differentiating stem cells into heart cells are only able to arrive at immature heart cells, which lack efficient cell communication pathways and the cohesive beating required for clinical applications. The study observed that hiPSC-CMs developed stronger contractile complexes and that their beating profiles became aligned following photoelectric stimulation with p-i-n silicon nanowires [5].
Silicon Nanomaterials for Neuronal Activity and Motor Control
Silicon nanowires also have promising applications for neuromodulation. In February 2018, the Tian group demonstrated that silicon nanowires elicit action potentials in primary rat dorsal root ganglion (DRG) neurons through a photoelectrochemical process. When the nanowires were introduced to DRG neuron culture, they formed tight junctions with the neuronal membranes. These junctions enabled depolarization at specific locations along the cell membrane, and the depolarization triggered action potentials in the neurons. Researchers found that as the duration of laser stimulation increased, the necessary laser power to create an action potential decreased [6]. From this information, they were able to determine the minimum energy to trigger an action potential.
Other forms of silicon, such as mesoporous silicon, are also able to modulate brain activity through photoelectric effects. Mesoporous silicon has a deformable structure and is minimally invasive to biological systems. A 2018 study conducted by Jiang et al. investigated mesoporous silicon’s material structure and found that the photovoltaic and Faradic effects enable such materials to modulate brain activity. The study concluded with interfacing a mesoporous silicon mesh device with a mouse brain. Photoelectric stimulation on the right side of the forelimb primary motor context resulted in controlling the mouse’s left forelimb while stimulation of the left forelimb primary motor context controlled motion in the right forelimb [7].
Breakthroughs with nanoscale photoelectric devices introduce a non-genetic approach to modulating cell activity and promise impacts in regenerative medicine for nervous and cardiac systems at both the tissue and organ levels. In designing devices that pace cardiomyocytes to target frequencies and induce maturation in hiPSC-CMs, researchers can restore function to the heart after trauma, which is a state that the heart cannot achieve alone. Furthermore, using silicon nanowires to trigger action potentials in individual neurons and stimulate local field potentials that modulate brain tissue activity illuminates new methods to perform deep brain stimulation and restore motor function. This opens the door for using light to correct aberrant electrical behavior in electrically active cells, and in so doing promote tissue and organ repair in ways never seen before.
References
- Lai, A. The Experiment That Shocked the World. Northwestern Helix (2017) at https://helix.northwestern.edu/article/experiment-shocked-world
- What is an action potential? Molecular Devices (2019) at https://www.moleculardevices.com/applications/patch-clamp-electrophysiology/what-action-potential
- Jiang, Y. and Tian, B. Inorganic semiconductor biointerfaces. Nat Rev Mater 3, 473-490 (2018)
- Parameswaran, R., Koehler, K., Rotenberg, M.Y., Burke, M.J., Kim, J., Jeong, K-Y., Hissa, B., Paul, M.D., Moreno, K., Sarma, N., Hayes, T., Sudzilovsky, E., Park, HG., and Tian B. Optical stimulation of cardiac cells with a polymer-supported silicon nanowire matrix. Proceedings of the National Academy of Sciences 116 (2) 413-421 (2018)
- Tan, Y., Richards, D., Xu, R., Stewart-Clark, S., Mani, S.K., Borg, T.K., Menick D.R., Tian, B., and Mei, Y. Silicon Nanowire-induced Maturation of Cardiomyocytes Derived from Human Induced Pluripotent Stem Cells. NanoLetters 15, 5, 2765-2772 (2015)
- Parameswaran, R., Carvalho-de-Souza, J.L., Jian, Y., Burke, M.J., Zimmerman, J.F., Koehler, K., Phillips, A. W., Yi, J., Adams, E.J., Bezanilla, F., and Tian, B. Photoelectrochemical modulation of neuronal activity with free-standing coaxial silicon nanowires. Nature Nanotechnology 13, 260-266 (2018)
- Jiang, Y., Li, X., Liu, B., Yi, J., Fang, Y., Shi, F., Gao, X., Suzilovsky, E., Parameswaran, R., Koehler, K., Nair, V., Yue, J., Guo, K., Fang, Y., Tsai, H.M. Greyermuth, G., Wong, R.C.S., Kao, C.M., Chen, C.T., Nicholls, A.W., Wu, X., Shepherd, G.M.G., and Tian, B. Rational design of silicon structures for optically controlled multiscale biointerfaces. Nature Biomedical Engineering, 2, 508-521 (2018)
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