Abstract:
A bio-probe having a base and a tip, and comprising a core of substantially rigid, high-strength material, said core tapering inwardly from the base to the tip and a set of conductors extending longitudinally about said core. In addition, dielectric material, substantially electrically isolates each conductor from its surroundings. Also, a set of apertures is defined by the dielectric material to the set of conductors, thereby defining a set of electrodes.

Description:
STATEMENT OF GOVERNMENTAL SUPPORT  
       [0001] This invention was made with government support under grant No. 1R43MH59502-01 awarded by the Small Business Research Program of the Department of Health and Human Services of the Public Health Service. The government has certain rights in the invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The assembly of a brain probe assembly employed in brain research is quite challenging from both a structural and an electrical standpoint.  
           [0003]    Structurally, probes must not fray or in any way come apart when pushed through the dura, a tough membrane covering the brain, and other brain tissue. Probe should have enough strength and rigidity to broach the dura without the need for assistance by, for example, a guide tube or an initial incision.  
           [0004]    Moreover, probes must not break, running the risk of leaving a fragment in the brain. Also, they must not cause undue damage to tissue at the sensing site. Inevitably, the tissue separating the sensing site from the brain exterior will suffer some damage as a probe is pushed to its destination.  
           [0005]    Electrically, one should note that field signals to be detected in the brain, are typically of the order of 100 to 500 μvolts. The low amplitude of these signals makes it necessary to amplify them as physically close as possible to their source. In fact, the signals involved are so minute that variations in circuit geometry could well affect significantly the detection processing of the signals. It is also highly desirable to minimize cross-talk between any two signals. Given the tight geometries allowable for brain probe design, these requirements are difficult to meet simultaneously.  
         SUMMARY OF THE INVENTION  
         [0006]    In a first separate aspect the present invention is a bio-probe having a base and a tip, and comprising a core of substantially rigid, high-strength material. The core tapers inwardly from the base to the tip and a set of conductors extend longitudinally about the core. In addition, dielectric material, substantially electrically isolates each conductor from its surroundings. Also, a set of apertures are defined by the dielectric material to the set of conductors, thereby defining a set of electrodes.  
           [0007]    In a second separate aspect, the present invention is a method of producing a bio-probe. This method includes the step of providing a tapering core of substantially rigid material. The core is then coated with dielectric material and this dielectric material is coated with a first layer of conductive material. The conductive material is then divided into longitudinal traces, extending from the base into proximity to said tip. The conductive material is then coated with a second layer of dielectric material. Finally, portions of the second layer of dielectric material are removed to form apertures to the conductive material, thereby forming electrodes.  
           [0008]    In a third separate aspect, the present invention is a bio-probe assembly for measuring bio-electrical signals, comprising, a probe portion having a distal end and a proximal end and a set of electrodes at said distal end for detecting the bio-electrical signals. Each electrode is connected to a longitudinal conductor, extending to the proximal end and a set of substantially identical amplifier circuit cards connected to the longitudinal conductors. Accordingly, each bio-electrical signal is amplified in substantially the same manner.  
           [0009]    The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the preferred embodiment(s), taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is an exploded perspective view of bio-probe assembly according to the present invention.  
         [0011]    [0011]FIG. 2 is a front view of the circuit card assembly of the bio-probe assembly of claim 1.  
         [0012]    [0012]FIG. 3 is an expanded perspective view of the tip of the bio-probe assembly of FIG. 1.  
         [0013]    [0013]FIG. 4 is a greatly expanded cross-sectional view of the tip of the bio-probe assembly of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    A preferred embodiment of a brain probe assembly  10 , according to the present invention is composed of a probe core  12  and a handle core  14 . The probe core  12  is made of tungsten, chosen for its material stiffness and tensile strength. Probe core  12  must be absolutely straight. To achieve this end, a straightening machine that pulls on core  12 , thereby creating tensile stress and annealing core  12  may be used. A tip or distal end  20  of probe core  12  has a diameter of 200 microns (8.0 mils) and a base or proximal end  24  of core  12  has a diameter of 600 microns (24 mils). In addition, core  12  is 89 mm (3.5″) long. The tip  20  is preferably formed by way of centerless grinding. Probe core  12  should be electro polished so that the deposition of materials onto it (see below) can be accomplished efficiently and so that the finished assembly  10  can pass through brain tissue as smoothly as possible.  
         [0015]    For ease of assembly and so that operating personnel may more easily handle assembly  10 , the handle core  14  is expanded in cross-section relative to probe core  12 . Although the handle core  14  is preferably a unitary piece of medical grade 304 stainless steel, it may be conceptually divided into a cylinder  15 , having a diameter of 4.826 mm (0.19″), and a frustum  17 . The frustum  17  tapers inwardly at 15° angle from the sides of cylinder  15 . A 600 μm (24 mil) aperture (not shown) at the narrow end of frustum  17  permits introduction of the base of probe core  12 , after which probe core  12  is joined to handle core  14 , by way of an epoxy, to form joint core  26 . The epoxy used must be conductive, so that the probe core  12  is grounded to the base core  14 , heat resistant, so that it withstands the sterilization process that the probe  10  must undergo in use. It must also be able to withstand the different degrees of expansion that stainless steel and tungsten undergo during the sterilization process. An epoxy that is available from Epoxy Technology, Inc. of Billerica, Mass. under the designation E3084 appears to meet these requirements. In an alternative preferred embodiment, the probe core  12  is laser-welded to the base core  14 .  
         [0016]    After joint core  26  is produced, it is dip coated with a dielectric epoxy, which has been premixed with a surfactant to promote an even coating, to form an insulating coat  30 . The desirable characteristics for an epoxy to be used are biocompatibility, heat tolerance to withstand the sterilization process, low viscosity to produce a thin film, a heat accelerated cure and a high bulk resistivity and a low dielectric coefficient to avoid electrical losses and withstand electrostatic charges. One epoxy that appears to meet these requirements is available as #377 from Epoxy Technology, Inc. of Billerica, Mass. A suitable surfactant is available as FC-430 from 3M of St. Paul, Minn. In an additional preferred embodiment quartz crystal, glass or a similar dielectric material is vacuum deposited to form coat  30 . In this preferred embodiment, in order to gain adherence, however, a 200 Å coat of chrome (not shown) is first applied, also through vacuum deposition on core  26  to promote the adhesion of coat  30 . The thickness of coat  30  is chosen to minimize the capacitance between core  26  and the conductive traces  50  (see below) deposited over it.  
         [0017]    On top of coat  30 , a 0.5 μm thick plate of conductive material (not shown as such but later rendered into a set of traces  50 ) is, preferably, vacuum deposited. This plate  50  also may be adhered by way of a 200 Å layer of vacuum deposited chrome (not shown). Plating  50  must be highly conductive and, if vacuum coating is used, must be an element of the periodic table. Accordingly, gold, platinum and iridium are among the materials that may be used. Other deposition techniques, such as chemical deposition, may permit the application of other highly conductive materials, such as a conductive polymer. The material used to create plating  50  must also be susceptible to removal by laser ablating or an etching process.  
         [0018]    Next, plate  50  is sectioned into 24 longitudinal traces  50  (other numbers of traces  50  are possible) extending from approximately the tip  20  to the proximal end of base core  14 . Accordingly, near the tip  20  the traces  50  have a pitch of about 27 μm, near the base  24  have a pitch of about 80 μm at the proximal end of handle  14  have a pitch of about 630 μm. Of particular utility for performing task of sectioning the conductive plate into traces  50  is a frequency multiplied ND:YAG laser, which can cut kerfs to separate the traces on the order of 5-10 μm width.  
         [0019]    In one preferred embodiment there are just four traces  50 . Using this embodiment a compound probing device may be built that incorporates an array of probe assemblies  10  to sense and or stimulate a number of neural sites separated not just in depth, but also transversely to probe assembly  10  longitudinal dimension.  
         [0020]    Next, the conductive traces  50  are coated with an outer layer  60  of high coefficient dielectric material. An additional dip coat of epoxy #377 is one way of accomplishing this. Another method is a vacuum deposition of glass or quartz crystal placed, again over an intermediate 200 Å layer of chrome. Dielectric layer  60  preferably has a thickness of from 10 to 40 μm to avoid damage by static electric discharge. A laser is used to ablate this outer layer to create several apertures extending through layer  60 , having a diameter of about 10 μm at each prospective microelectrode site. A platinum-iridium electrode is built up, preferably by electroplating, at each of these sites.  
         [0021]    Base  14  is attached to a plate  70  that includes outwardly extending conductive traces (not shown) that connect traces  50  to a set of connector pins  72 . In turn a set of connectors  72  on plate  70  attach to a matching set of connectors  74  on a circuit card assembly  80 . Assembly  80  includes a set of 24 circuit cards  82 , one for each trace, each bearing an identical amplification circuit for processing each signal from each trace  50  in an identical manner.  
         [0022]    The advantages of the present invention should now be apparent. Probe assembly  10  is strong, smooth and sleek, for moving through brain tissue to the site of interest. The cross capacitance between traces  50  is minimized due to the shape of the traces  50 , which are curved solid rectangles, on the order of 0.5 μm thick but varying between 10 μm and 50 μm wide. Finally, identical circuits  82  ensure equal treatment for each trace signal.  
         [0023]    The terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation. There is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.