ECE 8803/4803 Implantable Microelectronic Devices Fall - 2015 Maysam Ghovanloo (mgh@gatech.edu) School of Electrical and Computer Engineering Georgia Institute of Technology 2015 Maysam Ghovanloo 1 Outline Brain-Computer Interfaces Brain Neurons Action Potentials Neural Signal Recording Neural Recording Microelectrodes 2015 Maysam Ghovanloo 2 1
Review of Last Lectures Implantable microelectronic devices have become possible due advancements in miniaturization, wireless interfacing, and low power circuit design as evident in small pacemakers with batteries lasting more than 10 Years. Neural prostheses generally refers to a class of implanted devices that restore lost sensory or motor functions in a patient via stimulation of the nervous system or muscles. Multichannel neural recording is a powerful tool to employ, verify, and optimize the design of implantable stimulators. It can also help neurophysiologists understand the mechanisms behind neural activities. Recording can map theareaintowhichthestimulatordevicewillbe implanted and indicate the optimal positioning of the electrodes and adjustment stimulation parameter. Example: cochlear implant. Hybrid brain-machine interfaces (HBMI) can employ cortical recordings to control the 3-D trajectory of artificial prosthetics such as a robotic arm. The loop can be closed by adding artificial sensors and stimulators. This class will focus on different aspects of implantable devices especially sensors and circuits for neural recording and stimulation. 2015 Maysam Ghovanloo 3 A Distributed Network of Wireless 3-D Implants for the Central Nervous System "The blind see, the lame walk... the deaf hear." Ghovanloo, JSSC 2004 2015 Maysam Ghovanloo 4 2
Brain Machine Interfacing (BMI) Real-time direct interfaces between the brain and electronic and mechanical devices could one day be used to restore sensory and motor functions lost through injury or disease. Hybrid brain machine interfaces also have the potential to enhance our perceptual, motor and cognitive capabilities by revolutionizing the way we use computers and interact with remote environments. Actions from thoughts by Miguel Nicolelis Nature, vol. 409, 1/18/2001 2015 Maysam Ghovanloo 5 Brain Pacemaker for Epilepsy Treatment A brain pacemaker monitors neural activity using a VLSI chip designed to detect seizure activity. When seizure is detected, the VLSI chip sends a signal to an implanted stimulus generator that drives either a nerve cuff electrode or a mini-pump for drug delivery, either of which can stop the seizure activity. Nicolelis, Nature 2001 2015 Maysam Ghovanloo 6 3
Controlling A Robotic Prosthetic Arm Using Brain-Derived Signals Multiple, chronically implanted, intracranial microelectrode arrays would be used to sample the activity of large populations of single cortical neurons. The combined activity of these neural ensembles would then be transformed by a mathematical algorithm into continuous threedimensional arm-trajectory signals that would be used to control the movements of a robotic prosthetic arm. A closed control loop would be established by providing the subject with both visual and tactile feedback signals generated by movement of the robotic arm. Nicolelis, Nature 2001 2015 Maysam Ghovanloo 7 Restoring Memory In a healthy hippocampus, signals travel from DG to CA3 to CA1. If one of those regions is damaged, signals could be rerouted through a chip that mimics the processing duties of thousands of neurons, thereby completing the circuit, which is critical to memory. MIT Technology Review May 2003 2015 Maysam Ghovanloo 8 4
The Universe Inside Your Head! Human brain is the most complex living structure in the universe (so far). Cerebral cortex is divided into 4 section (lobes): Frontal, occipital, temporal, and parietal Internal brain structures: Forebrain, midbrain, hindbrain Courtesy of Prof. Gary Duncan, Dept. Psychiatry, UNC-CH Brain Facts, SFN 2005 2015 Maysam Ghovanloo 9 Is a cell specialized in transmitting information. Is the basic working unit of the brain. Consists of cell body, axon, nerve terminals, and dendrites Brain has 10 9 to 10 12 neurons depending on the species. The Neuron Courtesy of Prof. Gary Duncan 2015 Maysam Ghovanloo 10 5
Neurons Communicate with each other through an electrochemical process. Dendrites bring information to the cell body and Axons take information away from the cell body. Most neurons have many dendrites but only one axon. Courtesy of Prof. Gary Duncan Brain Facts, SFN 2005 2015 Maysam Ghovanloo 11 Neurons (cont.) Neurons are categorized by direction in which information flows in them: Sensory (or afferent) neurons send information from sensory receptors (in skin, eyes, nose, tongue, ears) toward central nervous system (CNS). Motor (or efferent) neurons send information away from CNS to muscles or glands. Interneurons, mostly located in CNS, send information between sensory and motor neurons. A neuron fires by transmitting electrical signals along its axon. When signals reach the end of the axon, they trigger the release of neurotransmitters. Neurotransmitters bind to receptor molecules that are present on the surfaces of adjacent neurons. The point of virtual contact is known as the synapse. 2015 Maysam Ghovanloo 12 6
Brain Cross Section White Matter: Nerve Fibers Grey Matter: Nerve Cell Bodies Courtesy of Prof. Gary Duncan 2015 Maysam Ghovanloo 13 Cerebral Cortex The cerebral cortex is the extensive outer layer of gray matter of the cerebral hemispheres. Cortex is responsible for many higherorder functions, including language, information processing and memory, and is also involved in sensation and voluntary muscle movement. Association cortex: those parts that are not involved in sensory or motor functions but in advanced processing. 2015 Maysam Ghovanloo 14 7
Courtesy of Prof. Gary Duncan 2015 Maysam Ghovanloo 15 Dominance of Functions in Cerebral Hemispheres 2015 Maysam Ghovanloo 16 8
Neurons as Excitable Cells Cells that are able to produce and respond to electrical signals Neurons Muscles Heart Secretory cells (such as pituitary, insulin producing cells of pancreas, cells in adrenal medulla) 2015 Maysam Ghovanloo 17 Neuronal Operation Most important ions in CNS are Sodium (Na+), Potassium (K+), Calcium (Ca++), and Chloride (Cl-). There are also some negatively-charged Protein molecules (A-). Nerve cells are surrounded by a semi-permeable membrane that is polarized by active (charge) pumps. When a neuron is not sending any signal it is at rest. The inside of the neuron is has more negative charges relative to the outside (resting membrane potential < 0V). Resting membrane potential of a neuron is between -50mV to -90mV depending on the living system and its environment. Potassium ions can easily cross the cell membrane but Sodium and Chloride ions are slower. At rest, there are more K+ ions inside the neuron and more Na+ ions on the outside. Negatively-charged Protein molecules cannot cross the membrane. 2015 Maysam Ghovanloo 18 9
Action Potentials Cell membrane is like a capacitor that can be discharged through ion channels (holes) and create an action potential (AP). An AP is also called a spike. An AP occurs when a neuron propagates information down an axon away from the cell body. Ion channels have voltage controlled Na + and K + gates similar to MOSFETs. During an AP, Na + channels open and Na + flows into the cell (low concentration) from outside (high concentration). Therefore, membrane potential go towards 0V and even becomes positive. The AP amplitude does not reduce as it propagates. AP is always transient (otherwise it does not carry any information) and membrane potential returns back to its resting potential. Initial depolarization : Voltage necessary to open voltage gated Na+ channel has to come from some source: 1- Generator potential in primary sensory neurons (heat, stretch, light, ) or 2- Activation by neurotransmitter synaptic transmission from pre-synaptic neuron. 2015 Maysam Ghovanloo 19 Action Potentials (cont.) When a neuron receives sufficient stimuli from other cells, a depolarizing current flows, causing the resting membrane potential to go towards 0mV (from its initial value of ~ -70mV). When it passes a certain threshold (~ -55mV), the neuron fires an AP of a fixed amplitude. If the threshold level is not reached, there will be no AP. Detailed AP generation and propagations mechanism: Web Assignment! 2015 Maysam Ghovanloo 20 10
Action Potentials (cont.) Movement of ions are local and do not change the general membrane potential. An AP lasts for 1~2 ms and all APs from the same cell are identical. Neural activity recorded from a rat ganglion cell when a single whisker was moved and then held in position. Neuron was firing at around 100 APs per second. 2015 Maysam Ghovanloo 21 11