Scroll down below this main CNCT page for additional details transcribed by one of our CAHRA members.
The underlying goal for all of the work conducted by the CNCT is to make multichannel recording and stimulation devices available to researchers which will enable them to more efficiently communicate with the brain. Micromachined electrodes offer the potential to extend small ensemble studies to tissue volume studies consisting of dozens if not hundreds of cells.
Internal Research ProjectsThere are three internal research projects under the Center which are designed to enhance the capability of the base technology:
Collaborative Research Projects
In addition to internal projects, there are multiple collaborative research projects with investigators outside the Center. Collaborators contribute significantly toward improving and expanding the use of the technology in areas including optimization of device designs, evaluation of chronic connectors, interconnects and cranial chambers, improvement of implantation methods, development of protocols for chronic electrode site maintenance, testing of advanced devices, and exploration of new application areas.Here are links to webpages of some of our collaborators (more to come.....):
ServiceService to investigators outside of the CNCT is provided in the forms of distribution of probes, and training in their use.
TRANSCRIPTION OF FURTHER CNCT SITE DETAILED TEXT:
------------------------------------------------------------------------ With support from the NIH/NCRR, the Center for Neural Communication Technology (CNCT) is able to offer a variety of probes to neuroscientists. The fabrication process for these probes gives great freedom in their design for recording and stimulation in the central nervous system. Their small size and fine features permit multichannel interaction with the tissue on a scale that approaches the size of the cells of interest. While the probes are not the perfect tool for every application, they do offer several advantages over conventional electrodes. These include batch fabrication which leads to very reproducible geometrical and electrical characteristics, the ability to include multiple sites on a single substrate at accurate dimensional relationships to one another, the capacity to integrate a multichannel interconnect for chronic applications, the potential for inclusion of on-chip electronics for signal conditioning and/or stimulus generation, and the ability to produce virtually any two-dimensional shape. The extensive design freedom offered by the technology has resulted in a variety of different probes which should satisfy the needs of investigators in many different disciplines. If you would like to receive probes from the CNCT, please fill out an application form . The distribution of passive probes to investigators both internal and external to the University of Michigan began in late 1988. One of our goals has been to obtain information on how the probes perform in a variety of applications, and to use this information to further optimize their design. The probes are currently available free-of-charge through support from the NIH National Center for Research Resources. The only method of "payment" which is currently required from users is feedback on probe performance to aid in the continuing effort to better understand and improve the technology. Acute Probes Since acute probes are generally available in greater numbers, are simple to package, and are easy to handle, they are a good entry point to using the technology. A schematic of an acute probe and its basic features is shown in Figure 1. The different acute designs offered in the catalog (available for download in pdf format, or email email@example.com for a hardcopy) vary in the number of shanks, the length and width of the shanks, and in the spacing and surface area of the conductive sites. Acute probes are mounted and electrically connected to PC boards using ultrasonic bonding as shown in Figure 2. Exposed connections are stabilized and insulated with epoxy. The pins on the PC board mate directly to standard integrated circuit DIP sockets permitting easy handling and connection. Such a socket is mounted on the front end of our custom-designed preamplifier (see below) and connected to high-input impedance amplifiers. The preamplifier is designed to be mounted on a microdrive directly above the animal preparation. In this way, the entire electrode package can be lowered to the preparation with minimal handling and precise electrode insertion can be achieved. This acute packaging scheme has proven to work well for most preparations and is provided as a standard item. We will, however, provide custom acute packaging for users who provide their own connectors. Once packaged, the probe site impedances are tested at 1kHz in saline. The investigator is provided with the impedance characteristics for each probe, and a site map which relates the sites on the probe to the pins on the PC board. Maps are also available for download. Probes are typically provided in groups of 6-10. Chronic Probes Many investigators are interested in performing chronic experiments. We currently offer chronic probes only to CNCT collaborators and to investigators who have gained experience with acute probes and who are willing to work with us to understand and improve chronic recordability. Recording sites on chronic probes tend to increase in impedance and degrade in recording quality over time. Internal Research Project 2 is aimed at understanding this degradation, and developing ways to prevent or remedy it. We hope to bring chronic probes into the general distribution effort in the near future. For a floating electrode configuration, chronic assemblies utilize a probe with an integrated flexible silicon ribbon cable as the interconnect (Figure 3). In this configuration, the probe shank is inserted into tissue and the flexible cable forms the interconnect to the percutaneous connector. Probes have also been packaged for investigators in non-floating chronic configurations. In this case, a non-cabled probe is attached directly to the percutaneous connector. Custom Design Some investigators wish to obtain devices which are designed specifically for their application. In fact, many of the devices in our catalog are based on designs that were submitted by investigators external to the CNCT. Custom design is a service which is offered by the Center to investigators who, through experience with existing designs, have determined that a special design is required for their study. New design runs occur approximately once a year with up to 20 designs per run. Site Impedance: Testing and Reduction The probes in the CNCT catalog have sites of two surface areas: 177 and 1250 sq micrometer. Typically, the smaller sites are used for recording and the larger for stimulation. All sites are made of sputtered iridium. Typical impedance ranges are 2 to 3 megohms for recording sites and several hundred kohms for stimulation sites. When you receive packaged probes from the CNCT, you will also receive a data sheet with 1kHz site impedances. The measurements are made in phosphate buffered saline using an HP 4194A Gain/Phase analyzer. If users wish to bond their own probes or modify the sites in any way, an AC impedance tester is recommended. In choosing or building a system, it is important that current passed through the probe site is very low. There are several suitable systems commercially available including one by Frederick Haer (#40-60-2) which uses 10nA measurement current). Iridium sites can be modified electrochemically to increase their current passage capabilities, or to decrease their impedance. This is done through formation of an oxide on the surface by cycling a voltage across the iridium/electrolyte interface. The process is known as activation . The resulting iridium oxide has a high charge capacity, is resistant to dissolution and corrosion during stimulation, and has a lower impedance than pure iridium. Activation is required for those users who will be passing appreciable current through the sites to prevent deterioration of the metal. The charge injection limit of activated iridium is several hundred times that of unactivated iridium. Activation also has merits when applied to recording electrodes; for those users who wish to reduce and impedance, small sites can be reduced by several orders of magnitude. This may be especially important when minimizing crosstalk is critical such as for CSD analysis and when a long probe shank is required. Amplifiers Use of an appropriate headstage amplifier is critical to maximize signal quality from the small, high impedance sites on the probe. Important characteristics for such a headstage include high input impedance and close proximity to the probe to minimize signal loss and crosstalk, and low bias current to prevent damage to the sites. At the University of Michigan Kresge Hearing Research Institute, probes are used with custom built high impedance buffer amplifiers (Figure 4). The design uses the Texas Instruments TLC2274CD quad op amp in a DC coupled, non-inverting, unity gain configuration. The design incorporates SOIC packages and surface-mounted passive devices on a double-sided printed circuit board to minimize the size of the final package. The probe can be connected to the board with a standard DIP socket. A 10kohm resistor in each input circuit protects the op amp from damage by static discharge. A 100ohm resistor in each output circuit prevents oscillation when driving long cables. To prevent oscillations, increase slew rates, and lower output noise, 4.7 mfd tantalum capacitors, in parallel with 0.1 mfd ceramic capacitors, are connected as close as possible between each of the TLC2274 power supply pins, and the power supply common. The headstage can be powered from an AC/DC power supply, or from batteries. The power supply voltage range can be +/- 2.2 VDC to +/- 8 VDC at 1.5mA per channel. The power supply common can be connected to earth, or isolated. If isolated, an isolation stage must be provided in a secondary amplifier. An aluminum case protects the circuit and provides electrical shielding. The case is mounted on a non-conductive rod to isolate it from the micromanipulator. The headstage case and connecting cable shield should be connected to earth for best shielding from 60 Hz pickup. If the headstage power supply is to be isolated, the case should be electrically isolated from the experimental animal. While the CNCT currently does not offer these headstages, questions about their design and construction, or about choosing an appropriate commercial headstage, can be directed to the technical support email group (firstname.lastname@example.org). ----------------------------------------------------------------------