OREGON STATE UNIVERS ITY – PHYSICS DEPARTMENT Thesis Characterizing a Coupled Photodiode -LED System for Neuro-Sensing Applications Gregg Stevens Advisor: Dr. Ethan Minot 5/6/2016 I investigate the feasibility of a micron -sized biosensor designed to measure neural activity. The circuitry of the biosensor is composed of graphene, some photodiodes, and a light-emitting diode. Many of these biosensors would be injected into a subject’s brain, and when powered by near-infrared light, they would glow in response to a neuron’s action potential. A model for the circuitry of these biosensors is developed and tested to predict the behavior of the biosensor. The ratio of optical output power to optical input power is greatest when the resistance of the graphene is minimized. However, the signal- to-noise ratio is better when the graphene has a higher resistance. The trade-off between these two goals is optimized by a specific resistance value that provides the greatest sensitivity of the instrument. In a test run, this model achieved a 13% energy conversion rate. This efficiency could be increased by using higher-quality components. These micro-biosensors could solve several of the current challenges facing neuro-sensing technology, including measuring the behavior of a single neuron. Gregory Stevens Thesis 1 Table of Contents Table of Figures ....................................................................................................................................... 3 1. Introduction ........................................................................................................................................ 4 1.1 Importance .................................................................................................................................... 5 1.2 Exposure Limitations ...................................................................................................................... 5 1.3 Prior Research ............................................................................................................................... 5 1.3.1 Acoustically-powered devices ................................................................................................. 5 1.3.2 Light absorption ...................................................................................................................... 5 1.3.3 Optoelectronic micro-sensor is feasible ................................................................................... 6 1.3.4 Graphene responds to neuron’s action potential ..................................................................... 6 1.4 Scope of project ............................................................................................................................. 6 2. Theory ................................................................................................................................................. 7 2.1 LED-Graphene Sub-circuit .............................................................................................................. 7 2.2 Photodiode Sub-circuit .................................................................................................................. 8 2.3 The Combined Circuit ................................................................................................................... 10 3. Experimental Methods and Results .................................................................................................... 10 3.1 Photodiode Series Resistance Measurements .............................................................................. 10 3.2 Series Resistance Results ............................................................................................................. 10 3.3 Photodiode Shunt Resistance Measurements .............................................................................. 11 3.4 Shunt Resistance Results .............................................................................................................. 11 3.5 Photodiode I-V curve measurements ........................................................................................... 12 3.6 Photodiode I-V Curve ................................................................................................................... 13 3.7 Linear Approximation of LED ........................................................................................................ 13 3.8 Linear Approximation Results ...................................................................................................... 13 3.9 Complete Circuit Current ............................................................................................................. 14 3.10 Complete Circuit Results ............................................................................................................ 14 4. Computational methods .................................................................................................................... 15 4.1 Photodiode Model ....................................................................................................................... 15 4.2 I-V curve of Photodiode ............................................................................................................... 15 4.3 LED Model ................................................................................................................................... 16 4.4 I-V curve of LED............................................................................................................................ 16 4.5 Solving the circuit ........................................................................................................................ 17 Gregory Stevens Thesis 2 4.6 System at various resistances ....................................................................................................... 18 5. Discussion/Analysis............................................................................................................................ 18 5.1 Interpreting Current as Intensity .................................................................................................. 18 5.2 Using the Normalized Change in Resistance – ......................................................................... 19 5.3 Maximizing the Change in Light .................................................................................................... 19 5.4 Maximizing Normalized Change in Light ....................................................................................... 20 5.5 Total Light vs. Normalized Light ................................................................................................... 21 5.6 Efficiency ..................................................................................................................................... 21 5.7 Increasing Efficiency .................................................................................................................... 22 6. Conclusion ......................................................................................................................................... 22 6.1 Future projects ............................................................................................................................ 23 6.1.1 Maximize graphene’s percentage change in resistance ......................................................... 23 6.1.2 Increase range by subcutaneous/subdural emitters and receivers ......................................... 23 7. Acknowledgements ........................................................................................................................... 23 8. Appendix ........................................................................................................................................... 23 8.1 System voltage at various resistances .......................................................................................... 23 8.2 Python code ................................................................................................................................. 24 9. References ........................................................................................................................................ 25 Gregory Stevens Thesis 3 Table of Figures Figure 1: Model of a micro-biosensor ...................................................................................................... 4 Figure 2: Penetration depth of near-infrared light through a human head ............................................... 6 Figure 3: The biosensor circuit can be split up into two components ....................................................... 7 Figure 4: LED Sub-circuit .......................................................................................................................... 7 Figure 5: Model of a photodiode ............................................................................................................. 8 Figure 6: Typical I-V curve with low shunt resistance ............................................................................... 9 Figure 7: Typical I-V Curve with various series resistance values .............................................................. 9 Figure 8: Circuit to measure series resistance ........................................................................................ 10 Figure 9: Circuits to measure shunt resistance ......................................................................................