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THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE DEPARTMENT OF MECHANCIAL ENGINEERING Biomolecular Materials Emulate Short-Term Synaptic Plasticity for Signal Processing AHMED ALMATAR SPRING 2021 A thesis submitted in partial fulfillment of the requirements for a baccalaureate degree in Mechanical Engineering with honors in Mechanical Engineering Reviewed and approved* by the following: Joseph Najem Assistant Professor of Mechanical Engineering Thesis Supervisor Jean-Michel Mongeau Assistant Professor of Mechanical Engineering Honors Adviser * Electronic approvals are on file. i ABSTRACT The amount of digital data we are producing is increasing at rapid rates and might soon exceed our current capacity to process it, mostly due to energy limitations. Therefore, to process this vast amount of data, optimizing computing per unit energy is key. Currently, neuromorphic computing, a model for computing that borrows key computational aspects of the human brain, is a leading solution to optimizing computing. However, despite major progress, solid-state neuromorphic computing hardware bears little resemblance to biological neurons and synapses, and thus, is still lagging in performance and energy efficiency compared to the brain. This study investigates a different approach to neuromorphic systems by using biomolecular memory elements as opposed to silicon-based elements. Replacing silicon with biomembranes to form memristors enables the emulation of short-term synaptic plasticity—a signal filtering property of biological synapses. The performance of the biomembrane has been tested experimentally using a DC signal as the input and a solid-state neuron circuit as the load. The biomembrane was determined to be exhibiting short-term synaptic plasticity by controlling the firing rate of the neuron. The results suggest that our biomolecular memristor is capable of performing basic signal processing tasks, namely, high-pass filtering. However, the behavior of networks of biomembranes remains unknown and is of great interest. ii TABLE OF CONTENTS LIST OF FIGURES ..................................................................................................... iii LIST OF TABLES ....................................................................................................... iv ACKNOWLEDGEMENTS ......................................................................................... v Chapter 1 Introduction ................................................................................................. 1 Chapter 2 Background ................................................................................................. 3 2.1 Smart Materials .................................................................................................. 3 2.2 Mimicking the Nervous System ......................................................................... 4 2.3 Constructing a Biomembrane Synapse Mimic ................................................... 5 2.4 Neuron Circuity .................................................................................................. 7 2.5 Research goals .................................................................................................... 9 Chapter 3 Methods ....................................................................................................... 10 3.1.1 Droplet Interface Bilayer/Biomembrane Formation ....................................... 10 3.1. 1 Droplet Interface Bilayer/Biomembrane Model ............................................. 12 3.2.1 Integrate and Fire Neuron Model .................................................................... 12 3.2.2 Integrate and Fire Neuron Simulation ............................................................. 16 3.2.2 Integrate and Fire Neuron Assembly ............................................................... 18 Chapter 4 Results ......................................................................................................... 19 4.1 Simulation Results .............................................................................................. 19 4.2 Experimental Results .......................................................................................... 22 Chapter 5 Discussion ................................................................................................... 27 5.1 Short-term Synaptic Plasticity and Firing Frequency ........................................ 27 5.2 Alamethicin Peptides .......................................................................................... 28 5.3 Computing Abilities ........................................................................................... 28 Chapter 6 Conclusion and Future Work ...................................................................... 29 BIBLIOGRAPHY ........................................................................................................ 30 iii LIST OF FIGURES Figure 1: A schematic describing the assembled synapse-neuron model. The biomolecular synapse receives a signal and processes it using short-term synaptic plasticity. The processed signal controls the firing frequency of the neuron model, emulating neural sensitization. 2 Figure 2: The presynaptic voltage spikes represent stimuli entering the synapse. The sensitized and adapted voltage spikes are shown above it [7]. ......................................................... 4 Figure 3: (A) biological neuron composition (B) artificial neuron composition [13].............. 5 Figure 4: An artificial synapse constructed using biomembranes. Two droplets of water with a lipid layer in an oil solution. The lipid layer contains proteins and protein-like chemicals [5].6 Figure 5: The sensory neuron receives the signal from stimuli in the environment, if the stimuli is large enough, the action potential (AP) is going to start traveling down the neuron. (image credit: Scott Clarke, Monash University). ........................................................................ 7 Figure 6: Each of circuit branches with a resistor is an analog to the three currents types of currents discovered by Hodgkin and Huxley. The variable resistors are the equivalent to ion channels opening and closing (which is how the neuron moves action potentials). The constant resistor represents “leakage”, which is the permissibility of the neuron walls. Each branch has a voltage source equivalent to potentials related to their operation (when they are open and closed for channels). Lastly the capacitor represents the neuron’s membrane ability to hold charge [18]. .......................................................................................................... 8 Figure 7: Snapshot of the assembled and formed DIB with thickness in the order of 100-101 nm. 11 Figure 8: The three main components to make an integrate and fire neuron. The capacitor is an analog to the neuro’s charge accumulating capacity, the resistor is an analog to the neuron’s ion channels, and the comparator is an analog to the threshold voltage needed to fire the neuron signal. ................................................................................................................... 13 Figure 9: An analog integrate-and-fire neuron with a specified threshold voltage VThr. The circuit is divided into 4 parts: input, integrating amplifier, comparator, and output [14]. .......... 14 Figure 10: Final desgin of the neuron circuitry. The design parameters are identical to Table 1. The input part of the circuit here is a constant current (dc voltage + resistance), but it will be the DIB current for the model. ......................................................................................... 16 Figure 11: The neuron voltage vs time for constant current input. This is the result of the simulation run for the circuit in Figure 10. ...................................................................... 17 Figure 12: The neuron circuit assembly. .................................................................................. 18 Figure 13: Simulated DIB current vs time for an input voltage of 145 mV. ........................... 20 Figure 14: Simulated Neuron voltage vs time for an input voltage of 145 mV. ...................... 20 iv Figure 15: Simulated DIB current vs time for an input voltage of 150 mV. ........................... 21 Figure 16: Simulated Neuron voltage vs time for an input voltage of 150 mV. ...................... 21 Figure 17: Experimental DIB current vs time with no alamethicin peptides present. ............. 22 Figure 18: Experimental Neuron voltage vs time for an input voltage of 160 mV. ................. 23 Figure 19: Experimental DIB current vs time for an input voltage of 145 mV. ...................... 24 Figure 20: Experimental firing frequency of the neuron vs time for an input voltage of 145 mV. 24 Figure 21:Experimental DIB current vs time for an input voltage of 150 mV. ....................... 25 Figure 22: Experimental firing frequency of the neuron vs time for an input voltage of 150 mV. 25 Figure 23: Experimental DIB current vs time for an input voltage of 160 mV. ...................... 26 Figure 24: Experimental firing frequency of the neuron vs time for an input voltage of 160 mV. 26 Figure 25: A hypothesized model for the parallel biomembrane synapses network. The droplets are connected in parallel. .................................................................................................. 29 v LIST OF TABLES Table 1: The desgin parameters for this project’s