Implantable Microelectronic Devices

Transcription

1 ECE 8803/4803 Implantable Microelectronic Devices Fall Maysam Ghovanloo (mgh@gatech.edu) School of Electrical and Computer Engineering Georgia Institute of Technology 2015 Maysam Ghovanloo 1 Outline Source: Review of Last Lecture Neural Recording Microelectrodes Glass Micropipettes Metal Microelectrodes Micromachined Electrodes Robinson D.A., The Electrical Properties of Metal Microelectrodes, IEEE Proceedings, 66 (6): June Maysam Ghovanloo 2 1

2 Neural Signal Recording In vivo: Inside the animal or human body (inserting or implanting) In vitro: Inside the animal or human body (cell culture or tissue slice) Intracellular recording: Measuring the potential across the cell membrane. Electrode tip inside, ground outside. If a small electrode is placed in the vicinity of an active neuron, the potential changes in the medium resulting from AP ionic movements can be measured with respect to a nearby or distant ground electrode. This is extracellular neural recording. Extracellular recording is the most widely used method to study the nervous system at the cellular level Maysam Ghovanloo 3 Extracellular Neural Signal Recording Individual Neurons Recording Sites 0.5-5MΩ at 1kHz Probe Shank Typical Neural Signal Amplitude: µV p-p BW: kHz DC level: ± 50mV, drifting 2015 Maysam Ghovanloo 4 2

3 Micropipette Electrodes One type of recording electrode is the glass micropipette. The device is formed by heating and pulling a 1~ 2 mm diameter glass capillary into two pieces. The pulled pipette is filled with a solution such as KCl to form a conductive link to the tissue. A large-area metal wire is inserted in the solution from the top of the pipette. Equivalent circuit is dominated by the series resistance of the sharp tip and the capacitance of the pipette wall. Suitable for DC and low-frequency measurements only Maysam Ghovanloo 5 Micropipette Electrodes (cont.) Diameter of the tip can be 0.5 um or less. So micropipettes are suitable for intracellular recording. The electrical characteristics of a glass microelectrode can be designated as a parallel resistance and capacitance in series with a source of dc voltage. Measurement of these parameters depends upon the following factors: concentration, ionic species, ph, and the hydrostatic pressures of the electrolyte solution inside the glass microelectrode and surrounding its tip. The physicochemical mechanism underlying microelectrode resistance and tip potential is interpreted as interaction of the surface charges of the glass and the conductivity of the solution inside the microelectrode. Equivalent circuit of a micropipette: R EL = microelectrode resistance, C EL = microelectrode capacitance, TP = tip potential. Schanne et al., Proc. IEEE, Maysam Ghovanloo 6 3

4 Metal Microelectrodes or Microwires The preferred tool for measuring extracellular neural action potentials is a metal microelectrode also known as microwire. They are formed by electrolytically sharpening a small-diameter metal wire to a fine tip (< 1µm). The body of the metal electrode is insulated with Polyimide or Parylene-C (polymer) except at the tip, which is exposed ~3 um from insulation. Metals such as tungsten, platinum, iridium, or stainless steel are more common. Metal microelectrodes form a capacitive interface to the electrolyte (aqueous tissue) and track the variations of the potential field caused by the extracellular ionic currents. Therefore, they have fairly similar electrical characteristics and equivalent circuit models Maysam Ghovanloo 7 Metal Microelectrodes (cont.) Electrode is cut to a length of ~15 mm for penetration in the brain. The only exposed region is ~3 um at the tip. Shaft diameter is only ~100 µm or less for minimal damage to the surrounding neurons and the tip angle is Some metal microelectrodes called concentric electrodes have a grounded metal shielding for electromagnetic interference protection. Concentric metal microelectrode 2015 Maysam Ghovanloo 8 4

5 Multichannel Recording Recording from a single site can reveal characteristics of only one or a handful of neurons around that recording site. Neurophysiologists would like to know how networks of 10s to 100s of cells work together and process information. Multichannel microelectrode arrays are needed to study the temporal and spatial relationships between groups of neurons forming a neural network. For multichannel recordings, several individual metal microelectrodes are bundled together and insulated from one another using epoxy glue. These arrays are usually handmade in a painstaking process. All electrodes should be soldered to a small connector. Max reported = 80 wires 2015 Maysam Ghovanloo 9 Multichannel Recording Arrays of bundled metal microelectrodes have proven to be very useful for studying neural circuits. However, they do have some disadvantages. Their added volume introduces more tissue damage than does a single conductor electrode. There is one hole in the tissue for every site. No recording site along the electrode shank for recording from the brain multi-layered structures. They are generally hand-built. The exact geometrical configuration is hard to reproduce Similar experiments may give different results. Electrodes are completely passive and only operate as simple conductors. Solution: Photoengraved microelectrodes, built using IC technology, enable us to increase the number of recording or stimulating sites without increasing the volume of the array. These photolithographically-defined electrodes can be implemented on silicon, polymers, or ceramics using MEMS technology Maysam Ghovanloo 10 5

6 Micromachined Electrodes Wise et al., Proc. IEEE, Maysam Ghovanloo 11 Extracellular Neural Signal Recording Individual Neurons Recording Sites 0.5-5MΩ at 1kHz Probe Shank Typical Neural Signal Amplitude: µV p-p BW: kHz DC level: ± 50mV, drifting 2015 Maysam Ghovanloo 12 6

7 Metal Microelectrodes or Microwires They are formed by electrolytically sharpening a small-diameter metal wire to a fine tip (< 1µm). The body of the metal electrode is insulated with Polyimide or Parylene-C (polymer) except at the tip, which is exposed ~3 um from insulation. Made of metals such as tungsten, platinum, iridium, or stainless steel. Metal microelectrodes form a capacitive interface to the electrolyte (aqueous tissue) and track the variations of the potential field caused by the extracellular ionic currents. Therefore, they have fairly similar electrical characteristics and equivalent circuit models, which is described in Robinson s paper. Electrode is cut to a length of ~15 mm for penetration in the brain. The only exposed region is ~3 um at the tip. Shaft diameter is only ~100 µm or less for minimal damage to the surrounding neurons and the tip angle is Some metal microelectrodes called concentric electrodes have a grounded metal shielding for electromagnetic interference protection Maysam Ghovanloo 13 Properties of Metal Microelectrodes Robinson D.A., IEEE Proc., What are the main challenges in dealing with Metal Microelectrodes? 2. What does it mean to isolate and record from a single unit? Why is it difficult to do this? 3. Why are the results so different from one lab or investigator to another? Why should we be concerned about this issue? 4. Why investigating the electrode characteristics in the neural tissue is more complicated and difficult than doing the same thing in isotonic saline? 5. What is the significance of building a model or equivalent circuit for the metal microelectrode? 6. Considering that Robinson has written this paper in late 60 s, what has been changed today? 7. Extrapolating the metal microelectrode properties on to MEMS electrodes, what differences or similarities are you expecting? 2015 Maysam Ghovanloo 14 7

8 Equivalent Circuit of a Metal Microelectrode Robinson D.A., IEEE Proc., What is the basis for each of these elements? 9. What physical phenomenon are they representing? 10. Which one is the most important? Why? 11. What is a rough value for each element? 12. What are the parameters affecting the value of these elements? 2015 Maysam Ghovanloo 15 Capacitance Across Insulating Material 13. What other elements could be added to the equivalent circuit? 14. In order to improve the quality of neural recording, which parameters should be changed? In which direction? How? 15. How can C s be reduced? Any suggestions other than those in the paper? 16. Why a shielded (coax) cable should be avoided between the electrode and amplifier input? 17. What can be used/done instead? Negligible 2015 Maysam Ghovanloo 16 8

9 Double Layer Capacitance and Resistance 18. How is the electric double layer being formed at electrodeelectrolyte interface? 19. Why R e and C e change with frequency? How do they change with freq.? 20. Why is it more accurate to model the electrode impedance using transmission line theory? 21. How is the behavior of the R e C e combination at low frequency (DC) and spike frequency range (100Hz ~ 10kHz)? Huge! i 0 = 4.5 x 10-6 A/cm 2 R e = 1.33 x 10 6 M.µm 2015 Maysam Ghovanloo 17 Metal Microelectrode Electrode Tip 22. Why these tips are very noisy before platinizing? 23. How do you calculate the input referred noise of a metal microelectrode? 24. How does platinizing help? 25. What are some of the other advantages of platiniznig an implantable devices? 26. Why R e cannot be measured with an ohmmeter or by applying a DC voltage? Platinizing Setup 2015 Maysam Ghovanloo 18 9

10 Spreading Resistance R s 27. Why R s could be negligible in saline? 28. Is R s negligible in the neural tissue as well? Why? 29. Why does the electrode impedance change (up and down) as it is being inserted into the neural tissue? 30. What is the effect of microelectrode metal tip on the potential field around an excited neuron? 31. How is a 1µm tip different from 10µm in recording from a nearby neuron? 32. Why is it quite possible for a microelectrode inserted into the neural tissue not to record at all? 33. What would you do if this happens? Nicolelis et al., PNAS The resistance of a thin spherical shell of saline at radius r of thickness dr of specific resistivity is: Integrating from r e to infinity yields: 2015 Maysam Ghovanloo 19 10