WO2019200046A1 - A wire for a high density and biostable microelectrode array for neural electrode stimulation and recording - Google Patents

Abstract

An ultrafine, medical-grade wire is suitable for use in an array of wires for neural monitoring and stimulation throughout a designated area of the central nervous system including the spine and brain, or as a peripheral nerve interface for interfacing with nerves local to an amputated limb, for example. As described in further detail below, the present wire has a size small enough to avoid immune response, thus evading the chemical and physical immune foreign body response and enabling biostability. The wire is formed of a metal which is a chemically inert in vivo, and also possesses mechanical strength sufficient to enable precise tissue-penetrating placement of each electrode within the implantation site in the brain. Mechanical strength is achieved with a combination of material selection, processing and wire geometry.

Description

A WIRE FOR A HIGH DENSITY AND BIOSTABLE MICROELECTRODE ARRAY

FOR NEURAL ELECTRODE STIMULATION AND RECORDING

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial

No. 62/656,193, entitled A WIRE FOR A HIGH DENSITY AND BIOSTABLE

MICROELECTRODE ARRAY FOR NEURAL ELECTRODE STIMULATION AND RECORDING and filed on April 11, 2018, the entire disclosure of which is hereby expressly incorporated by reference herein.

BACKGROUND

1. Technical Field.

[0002] The present invention relates to wire used in biomedical applications and, in particular, relates to wires used as electrodes suitable for neural implantation, either within the central nervous system as the brain, or in the peripheral nerves, as a high-microelectrode-density and tissue-penetrating array for stimulation and recording with a high degree of spatial and temporal stability in the device recipient.

2. Description of the Related Art.

[0003] Electrodes, when inserted in the human brain, facilitate the flow of electronic signals to and from the brain. Specifically, these signals can permit neural recording and stimulation, which have great potential for clinical applications, such as treating neurological conditions like Parkinson’s disease or epilepsy, or for controlling prosthetic limbs, or to be used as an interface with integrated electronic systems, computers and the like.

[0004] However, long-term neural stimulation and/or recording may be impaired by eventual failure of implantable electrodes, such as failure due to a tissue response caused by the implantation and constant presence of the electrode, known in medicine as the foreign body response. In these cases, astrocytes and microglia (among other physical immune-based defenses in the brain) attempt to engulf the electrode, increasing the electrical impedance between the electrode and neurons, and possibly pushing neurons away from the recording site. This tissue response and resultant electrode fouling may occur over a period of weeks, to months or less than a year with present brain-computer or neural-computer interface technology. Short term fouling may thus render present neural interfaces unsuitable for high fidelity and long term application. Such challenges have inhibited long term and human-profitable benefit from what may be revolutionary technology for limb amputees, paraplegics, and neural disease-sufferers.

[0005] Further, present technological efforts may suffer from a lack of microelectrode density to adequately stimulate and record local neurons. For example, microelectrode arrays fabricated from silicon using lithographic, or microelectromechanical systems (MEMS) techniques common to the integrated chip industry are possible at individual electrode gaps, or pitches, of less than 50 microns. Silicon, however, does not possess sufficient mechanical integrity to achieve sufficient electrical conductivity and neural tissue penetration unless carbon or metallic elements are added and thicknesses are increased to substantially greater than 50 microns. Therefore, high density electrode pitches of less than 50 to 100 microns, with at least 3 to 5 mm of neural tissue penetration while maintaining a high degree of spatial accuracy are not possible using such silicon-based devices. Silicon electrodes are also susceptible to fracture during bending loads that may occur during device implantation, potentially leading to undesirable free-floating foreign material and providing insufficient mechanical durability for most long term human implants.

[0006] What is needed is an improvement over the foregoing.

SUMMARY

[0007] The present disclosure provides an ultrafme, medical-grade wire suitable for use in an array of wires for neural monitoring and stimulation throughout a designated area of the central nervous system including the spine and brain, or as a peripheral nerve interface for interfacing with nerves local to an amputated limb, for example. As described in further detail below, the present wire has a size small enough to avoid immune response, thus evading the chemical and physical immune foreign body response and enabling biostability. The wire is formed of a metal which is a chemically inert in vivo, and also possesses mechanical strength sufficient to enable precise tissue-penetrating placement of each electrode within the implantation site in the brain. Mechanical strength is achieved with a combination of material selection, processing and wire geometry.

[0008] Therefore, when a large plurality of such wires are mounted to a substrate in a neural array, the present wire design provides immune system evasion for biostability and high microelectrode density through very small electrode pitch. Through its chemical and mechanical properties, the wire material facilitates neural tissue penetration and provides chemical inertness and mechanical durability in vivo.

[0009] In one form thereof, the present invention provides a metal wire having a diameter between 5 pm and 50 pm, the metal wire formed of a medical-grade material having a yield strength which reaches 1000 MPa and a stiffness which reaches 140 GPa, whereby the metal wire is suitable for use as a microelectrode in a high-density-microelectrode and tissue- penetrating neural recording and stimulation array.

[0010] In one form thereof, the present invention provides a metal wire having a diameter between 5 pm and 50 pm, the metal wire formed of a medical-grade material having a yield strength which reaches 1000 MPa and a stiffness which reaches 140 GPa, whereby the metal wire is suitable for use as a microelectrode in a high-density-microelectrode and tissue- penetrating neural recording and stimulation array.

[0011] In one aspect, the medical-grade material is selected from the group consisting of

99% rhodium and trace impurities, a rhodium alloy having at least 50% wt.% rhodium, other alloying elements including at least one of platinum, tungsten, iridium, nickel, gold,

molybdenum and rhenium, and trace impurities, and a platinum-iridium alloy having between 10 wt.% and 30% wt.% iridium, balance platinum and trace impurities.

[0012] In another aspect, the wire defines a deviation from perfect straightness, the deviation defining an amplitude between 1/10 of the diameter of the wire and 1/1 of the diameter of the wire. The deviation may define a pitch between 50 times the diameter of the wire and 200 times the diameter of the wire.

[0013] In another aspect, the medical-grade material is electrically conducting and excludes polymeric materials, silicon and carbon.

[0014] In yet another aspect, the wire comprises a shell and core received within the shell, the core completely filling a central cavity within the shell. [0015] In still another aspect, the wire comprises a monolithic wire formed of a single piece of homogenous material.

[0016] In another aspect, the wire has an electrically insulating sheath disposed about the exterior surface of the medical-grade material, the sheath covering the exterior surface along an implant length and not covering an electrically conductive tip of the wire.

[0017] In yet another aspect, a tip of the wire forms a blunt tip. The tip of the wire forms a sharpened tip. The sharpened tip may be a rounded tip presenting a convex surface across at least 90% of an axial end surface of the wire. The sharpened tip may be a pointed tip presenting a convex surface across less than 20% of an axial end surface of the wire. The pointed tip may define an angle, in cross-section, between 5 degrees and 30 degrees.

[0018] In still another aspect, the metal wire is a monolithic wire formed of a single piece of the medical-grade material.

[0019] In another aspect, the medical-grade material is a first medical-grade material, the metal wire is a composite wire comprising a core and a concentric shell formed around the core, one of the core and the shell is formed from the first medical-grade material, and the other of the core and the shell formed from a second medical-grade material different from the first medical- grade material.

[0020] In another aspect, the yield strength is at least 1300 MPa, or at least 1500 MPa.

[0021] In another aspect, the stiffness is at least 180 GPa, or at least 200 GPa, or at least

250 GPa or at least 300 GPa, or at least 350 GPa.

[0022] In yet another aspect, the medical-grade material excludes copper, silver, lead, cadmium, 300-series stainless steel and 400-series stainless steel.

[0023] In still another aspect, the wire includes a thin coating disposed around the wire periphery and having a thickness less than 1120^ of the diameter. The thin coating may be made of gold.

[0024] In another aspect, the wire has a round cross-section. Alternatively, the wire has a non-round, polygonal cross-section. In another alternative, the wire is tubing with a hollow void running axially through the wire.

[0025] In another form thereof, the present disclosure provides a high density and implantable microelectrode array for neural tissue penetration, tissue stimulation and recording, the array including a plurality of metal wires including any of the foregoing features and aspects, or any combination of such features and aspects, and a substrate. The plurality of metal wires are arrayed and respectively connected to the substrate.

[0026] In one aspect, the plurality of metal wires each extend perpendicularly away from the substrate.

[0027] In another aspect, each of the plurality of metal wires avoids contact with any other metal wire across its entire axial extent.

[0028] In yet another aspect, the plurality of metal wires are connected to the substrate with a wire density between 10,000 and 1 million wires per square centimeter.

[0029] In still another aspect, at least some of the plurality of metal wires define a wire length different from others of the plurality of metal wires.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

[0031] Fig. 1A is a perspective, cross-section view of a monolithic wire having diameter

D_2S, in accordance with the present disclosure;

[0032] Fig. 1B is a perspective, cross-section view of a composite wire having overall diameter D₂s, in accordance with the present disclosure;

[0033] Fig. 2A is a schematic view illustrating an exemplary process of forming monolithic wire using a lubricated drawing die;

[0034] Fig. 2B is a schematic view illustrating an exemplary process of forming composite wire using a lubricated drawing die;

[0035] Fig. 2C is an elevation view of a wire in accordance with the present disclosure, before a final cold working process;

[0036] Fig. 2D is an elevation view of the wire of Fig. 2C, after the final cold working process;

[0037] Fig. 3A is a top plan view of a high microelectrode density neural array comprising a plurality of wire-based electrodes made in accordance with the present disclosure;

[0038] Fig. 3B is a side elevation view of the neural array shown in Fig. 3A; [0039] Fig. 4A is an elevation view of a monolithic wire made in accordance with the present disclosure and featuring a pointed tip;

[0040] Fig. 4B is an elevation view of a composite wire made in accordance with the present disclosure and featuring a pointed tip;

[0041] Fig. 4C is an elevation view of a monolithic wire made in accordance with the present disclosure and featuring a blunt tip and a coating to provide electrical isolation of the tissue-penetrating length with exposure of the active stimulation and recording tip region;

[0042] Fig. 4D is an elevation view of a monolithic wire in accordance with the present disclosure and featuring a rounded tip;

[0043] Fig. 5 is a schematic view of a wire made in accordance with the present disclosure, illustrating non-straight wire characteristics.

[0044] Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrates embodiments of the invention, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION

1. Introduction

[0045] The present disclosure provides an ultrafme wire, such as monolithic wire 103 shown in Fig. 1A, which is suitable for use in a multi-wire array which can be implanted at a desired area of the brain for signal monitoring, electrical stimulation, or a combination thereof.

In an exemplary embodiment, wire 103 is formed from rhodium, platinum or alloys thereof, and has a uniform size and cross-sectional geometry along its axial length. The wire has an outer diameter D₂s as shown in Fig. 1 A which is less than 50 pm, such as less than 30 pm or 20 pm, in order to avoid an immune response when implanted at a neural site. For example and as further described below, wire 103 may have an outer diameter D₂s as small as 5 pm, 9 pm, 12 pm or 15 pm, and as large as 20 pm, 25 pm, 30 pm or 50 pm, or any diameter within any range defined by any of the foregoing values.

[0046] In another embodiment, composite wire 101 (Fig. 1B) may include a separate core

14 and shell 12 with the same overall diameter D₂s as described above with respect to monolithic wire 103. For purposes of the present disclosure, all discussions of“wire 103” also apply to wire 101, unless specifically stated otherwise. For example the materials, wire diameters, other wire shapes and geometries, and wire processing parameters discussed with respect to wire 103 may also be applied to wire 101, either in shell 12, core 14 or both.

[0047] As described in further detail below, outer diameter D₂s of wires 103 may be tailored to balance competing interests in the context of an electrode designed for insertion and subsequent in vivo use at a neural site. On one hand, faster insertion speed, finer tip geometry, smaller size, and lower material stiffness has been found to decrease damage caused by the insertion process, and reduces the intensity of the reactive tissue response. On the other hand, small electrode wires may buckle during insertion as they encounter bodily tissue, particularly upon initial surface penetration and for more rigid tissues (e.g., connective tissue and tissue fascias) and/or deeper implant sites. As described in greater detail herein, wire made in accordance with the present disclosure may be sized, shaped and constructed to provide strength sufficient to resist or avoid such buckling, while also minimizing the intensity of the reactive tissue response, and offering a consistently low impedance over a long service life.

[0048] In particular, wires 101 and 103 combine a sufficiently small size for immune response avoidance with sufficient mechanical strength to meet the demands of neural implantation in densely packed wire array. Mechanical strength may be derived from a combination of material constituency, wire processing and wire geometry (e.g., straightness), to produce a wire that can be successfully integrated into a dense-packed microelectrode array 200 (Figs. 3A and 3B) and implanted into a brain or other implantation site with a high degree of precision and accuracy.

2. Terminology

[0049] As used herein, "wire" or "wire product" encompasses continuous wire and wire products which may be continuously produced and wound onto a spool for later dispensation and use, such as wire having a round cross section and wire having a non-round cross section, including flat wire or ribbon. "Wire" or "wire product" also encompasses other wire-based products such as strands, cables, coil, and tubing, which may be produced at a particular length depending on a particular application. Although round cross-sectional wire forms are shown in the Figures of the present application and described further below, non-round wire forms may also be produced in accordance with the present disclosure. Exemplary non-round forms include polygonal cross-sectional shapes such as rectangular cross-sectional shapes.

[0050] “Fine wire” refers to a wire having an outer diameter of less than 1 mm.

“Ultrafme wire” refers to a wire having an outer diameter of 50 pm or less.

[0051] “Monolithic” refers to a wire or other structure which is formed as a single piece of material.

[0052] "DFT®" is a registered trademark of Fort Wayne Metals Research Products Corp. of Fort Wayne, IN, and refers to a bimetal or poly-metal composite wire product including two or more concentric layers of metals or alloys, typically at least one outer layer or shell disposed over a core filament, and formed by drawing a tube or multiple tube layers over a solid metallic wire core element.

[0053] “Impurities,”“incidental impurities” and“trace impurities” are material constituents present in a material at less than 500 parts per million or 0.05 wt. %. Alloys“free” of or“excluding” a certain constituent are alloys having such a constituent in amounts equal to or less the 500 parts per million impurities threshold.

[0054] "OD" refers to the outside diameter of a metallic wire or outer shell. "ID" refers to the inside diameter of a metallic outer shell.

[0055] “Microelectrode array” refers to a plurality of electrically conductive elements capable of neural tissue stimulation and recording.

3. Exemplary Wires

[0056] Referring to Fig. 1A, monolithic wire 103 having diameter D₂s is sized and shaped to have sufficient strength and stiffness to facilitate penetration to a desired depth at a neural implantation site alongside an array of wires 103, without buckling occurring in any of the individual wires 103 of the array. At the same time, diameter D₂s is small enough to evade the human body’s natural defenses which might otherwise promote degradation of wire 103, or degradation of the electrical performance of wire 103 in situ.

[0057] Wires 103 may encounter interference during insertion due to initial penetration pressure requirements and potential tissue impacts during penetration into the human brain that operate to urge the wire to bend or warp, which can then lead to buckling behavior and loss of implant site location accuracy, or tissue damage. Diameter D₂s of wire 103 is sized small enough to minimize interference during travel through brain tissue, and to facilitate long-term implant functionality. For example, diameter D2S may be maintained at less than or equal to the diameter of a human hair, such that glial cells (such as strocytes and microglia, which are parts of the human brain’s physical defences) are less prone to attack the electrode. In one exemplary embodiment, wire 103 balances all of the above competing interests by having a diameter D2S as small as 5 pm, 9 pm, 12 pm or 15 pm, and as large as 20 pm, 25 pm, 30 pm or 50 pm, or any diameter within any range defined by any of the foregoing values. For example, wire 103 may have a diameter between 5 pm-9 pm, 5 pm- 12 pm, 5 pm- 15 pm, 5 pm-20 pm, 5 pm-25 pm, 5 pm-30 pm, 5 pm-50 pm, 9 pm- 12 pm, 9 pm- 15 pm, 9 pm-20 pm, 9 pm-25 pm, 9 pm-30 pm, 9 pm-50 pm, 12 pm-15 pm, 12 pm-20 pm, 12 pm-25 pm, 12 pm-30 pm, 12 pm-50 pm, 15 pm-20 pm, 15 pm-25 pm, 15 pm-30 pm, 15 pm-50 pm, 20 pm-25 pm, 20 pm-30 pm, 20 pm-50 pm, 25 pm-30 pm, 25 pm-50 pm, or 30 pm-50 pm.

[0058] At the same time, diameter D₂s cooperates with the metal or metal alloy material of wires 103 to provide the columnar strength needed for a particular implantation site (e.g., implant depth, tissue density, etc.). In particular, the material for wire 103 may be chosen to include a yield strength which reaches 1000 MPa, such as at least 1300 MPa, at least 1500 MPa, or more. At the same time, the material for wire 103 may have a stiffness which reaches 140 GPa, such as 180 GPa, 200 GPa, 250 GPa, 300 GPa, 350 GPa, or more. Strength is measured by uniaxial tensile testing in accordance with the standard defined in ASTM E8 / E8-16a of 2016. Stiffness is the Young’s elastic modulus of the material, as measured by uniaxial tensile testing in accordance with the standard defined in ASTM E8 / E8-l6a of 2016. Thus, for purposes of the present disclosure,“stiffness” refers to a tensile stiffness rather than a bending stiffness.

[0059] These strength and stiffness values are sufficient, in combination with an ultrafme wire size as described herein, to withstand implantation into a brain without buckling or other geometric instability (e.g., uncontrolled or unintentional bending). Exemplary materials capable of meeting such yield strength and stiffness thresholds include:

Substantially pure rhodium, such as 99% rhodium or greater with inevitable trace impurities.

- Rhodium alloys including greater than 50% wt.% rhodium, with alloying elements

including platinum, tungsten, iridium, nickel, gold, molybdenum and rhenium together with inevitable trace impurities. In an exemplary embodiment, rhodium may be processed and/or alloyed to achieve a yield strength between 1200 MPa and 2200 MPa, depending on thermal, mechanical and other processing parameters as discussed herein. Alloys of platinum and iridium, such as alloys containing at least 10 wt.%, 20 wt.%, 25 wt.% or 30 wt.% iridium, balance platinum and inevitable trace impurities. In an exemplary embodiment, such platinum/iridium alloys may be processed and/or alloyed to achieve a yield strength between 1000 MPa and 2400 MPa, depending on thermal and other processing parameters as discussed herein.

Titanium based alloys, such as alloys including at least 50 wt.% titanium.

Nickel titanium (NiTi or Nitinol) alloys, including binary NiTi, NiTiZr, NiTiNb, NiTiCr, and NiTiCo. All of these alloys, when used for a neural electrode in accordance with the present disclosure, may be coated with platinum or a platinum-iridium alloy to avoid reactivity in vivo.

Cobalt chromium alloys, such as alloys including at least 35 wt.% cobalt and 35 wt.% chromium, in accordance with the standard defined in ASTM F562-13 of 2013; Co-Cr- Mb alloys which can include at least 26 wt.% Cr and at least 50 wt.% Co, in accordance with the standard defined in ASTM F1537-11 of 2011; and L605 alloys which can include at least 15 wt.% Cr and at least 50 wt.% Co, in accordance with the standard defined in ASTM F90-14 of 2014. All of these alloys, when used for a neural electrode in accordance with the present disclosure, may be coated with platinum or a platinum- iridium alloy to avoid reactivity in vivo.

Iron-based absorbable alloys.

- Magnesium-based absorbable alloys.

[0060] The metal or metal alloy material of wires 103 is formed exclusively of medical- grade metals suitable for use at an in vivo neural implantation site. For example, wires 103 exclude non-metallic materials including polymeric materials, silicon and carbon fiber. Wires 103 also exclude non-medical grade materials such as copper, silver and all metals toxic to the human body such as lead and cadmium. Wires 103 further exclude stainless steels, such as 300- and 400-series stainless steels. 300-series stainless steels include 304L, 304V, 304N, 316, 316L, 316LVM, and 317. 400-series stainless steels include 420, 430, 440, 450, 455, 455 custom, 470, 470 custom, 17-4 PH and 17-7 PH. [0061] Wires 103 may, in some instances, be plated or coated with a secondary material as required or desired for a particular application. In one exemplary embodiment, wires 103 or 101 may include a thin coating 104, a portion of which is shown schematically in Fig. 1A.

Coating 104 is disposed around the wire periphery, but may not cover the tip, whether the tip is blunt/flat as shown, or pointed or rounded as shown in Figs. 4A, 4B and 4D and further described below. The thickness of coating 104 may be about l/20th of the wire diameter D₂s, or may be thinner such as l/lOOth of the wire diameter Fhs or l/lOOOth of the wire diameter Fbs, or may be thicker such as about l/lOth of the wire diameter D2S. In one example, a thin gold plate (e.g., having a thickness of about 1 pm uniformly applied to the exterior of wire 103) may be used to provide contrast for machine vision. For neural applications, a material or plating other than gold may be employed to avoid chemical interaction with the blood. For example, a polyimide insulation may be provided over wires 103 to provide for visual contrast while remaining chemically inert in vivo. Such a plating or coating process is distinct from a composite wire structure, as shown in Fig. IB and described in further detail below, in that plating or coating is applied only after wire drawing is completed.

[0062] The present wires 103 are also shaped and spatially configured to contribute to the desired strength and buckling resistance discussed above. For example, the straightness of wire 103 may be controlled within acceptable limits to contribute to buckling strength, with greater straightness associated with greater buckling resistance and vice-versa. Straightness can be more tightly controlled (i.e., held to a tight tolerance) for wires 103 in which wire size and material provide for less“natural” buckling resistance (e.g., smaller diameters or less strong/stiff materials), or straightness may less controlled (i.e., held to a looser tolerance) for wires 103 in which wire size and material provide for more“natural” buckling resistance (e.g., larger diameters or relatively stronger/stiffer materials).

[0063] Turning to Fig. 4, wire 103 is shown in a non-straight configuration for the purpose of illustration. Non-straightness can be in the form of an arc in one plane, i.e., the wire appears curved as viewed in a first plane parallel to the longitudinal wire axis (as shown in Fig.

4, for example) but appears straight from as viewed in a second plane also parallel to the axis but perpendicular to the first plane. Non-straightness can be in the form of helical twist, or of projected wave character, where the wire appears curved in all planes parallel to the longitudinal wire axis and appears similar to a sinusoid when projected on one such parallel plane. Non- straightness of the wave character can be characterized by the amplitude A of any curvature or deviation from perfectly straight, together with the pitch (or period) P of such curvature or deviation. Similarly, non-straightness of a curved wire can be considered to have a total amplitude A derived from its total curvature. Therefore, it is understood that amplitude can represent either the total deviation of wire 103 from its central axis (as shown by amplitude A in Fig. 4), or for regular repeating patterns of curvature (e.g., helical twist), the deviation from the overall central axis defined by the wire 103 (i.e., 1/2A in the context of Fig. 4).

[0064] Amplitude A may be expressed a function of wire diameter. For an exemplary embodiment of wire 103, amplitude A is less than 1/2 of the wire diameter DJS. For example, amplitude A may be less than 1/10, 1/8, 1/5, 1/3 or 1/1 of wire diameter D₂s, with smaller amplitudes A associated with a relatively greater contribution of straightness to buckling strength of wire 103 and vice-versa.

[0065] The combination of amplitude A and pitch P is also associated with buckling strength, with combinations of smaller amplitude A and shorter pitch P associated with increased buckling strength and vice-versa. Similar to amplitude A, pitch may expressed as a multiple of wire diameter Fhs. In an exemplary embodiment, wire 103 may have a pitch P at less than 200, 100, 80 or 50 multiples of diameter D₂s.

[0066] Although wires 103, 101 have a round cross-section as shown in Figs. 1A and 1B respectively, non-round wire forms may also be produced in accordance with the present disclosure. Other exemplary forms include polygonal cross-sectional shapes such as rectangular cross-sectional shapes and hollow forms such as tubing, with a hollow void running axially through wire 103. Tubing which may be used directly in an end product or as a shell in composite wire 101 (further described herein). For purposes of the present disclosure, the “diameter” of a wire form having a non-round cross-sectional shape is the smallest circle circumscribing the non-round cross-sectional shape.

[0067] Yet another contributor to buckling resistance for wire 103 is processing parameters, including cold-work and thermal processing as described in further detail below. In particular, an appropriate balance between strength and ductility can be developed and employed by such processing parameters.

[0068] Turning now to Figs. 4A and 4B, wires 103, 101 may optionally terminate in pointed tips 41, 40 respectively. Pointed tips 41, 40 are formed by machining or otherwise forming a steadily reducing diameter at the axial end of wire 103 or 101 to define a generally conical point. Pointed tips 41, 40 may be appropriate for certain applications, such as a neural electrode, by aiding insertion into the implant site. Specifically, by adjusting angle a located at pointed tips 41, 40 the sharpness of wires 103, 101 can be increased or decreased to adjust the amount and nature of the forces experienced by the distal end of wires 103, 101 during penetration into tissue. In one exemplary embodiment, angle a may be as small as 5°, 10°, or 15° and as large as 20°, 25°, or 30°, or may be any angle within any range defined by any two of the foregoing values. For example, angle a may be between 5°-10°, 5°-l5°, 5°-20°, 5°-25°, 5°-30°, 10°-15°, l0°-20°, 10°-25°, l0°-30°, l5°-20°, l5°-25°, l5°-30°, 20°-25°, 20°-30°, or 25°-30°.

[0069] As shown in Figs 1A, 1B and 4C and noted above, wires 103 (or 101) may also have a blunt tip 43, which is to say the end of the wire forms a circular flat. Fig. 4C further illustrates a sheath or coating 106 which may be applied to the exterior surface of wire 103, which may be formed from an electrically insulating (i.e., electrically non-conductive) material such as a polymer. In an exemplary embodiment, sheath 106 covers the exterior surface of wire 103 along its implant length, i.e., along the length which may potentially be exposed to tissue and potentially also more proximal portions of wire 103. However, sheath 106 is“stripped” or otherwise absent from the tip portion of wire 103, in order to facilitate electrical interface with the final implant site. The length of bare, unsheathed wire material may be varied, and may be expressed as a function of the diameter D₂s of wire 103. In an exemplary embodiment, the bare portion at the tip of wire 103 may have an axial extent between 1/2 the diameter D₂s and 50 times diameter D₂s, such as about 1/1 the diameter D₂s. Moreover, it will be appreciated that the nominal extent of the bare portion of wire 103 will vary depending on the particular neural application contemplated for wire 103, with some applications requiring a long uncoated section of wire 103 and others requiring less.

[0070] Turning to Fig. 4D, wire 103 may also have a rounded (e.g., hemispherical) tip

44. In some embodiments of array 200 (shown in Figs. 3A and 3B and described in further detail below), hemispherical or otherwise rounded tips 44 on the electrodes formed from wires 103 may facilitate implantation while also providing electric field distribution. In particular, rounded tip 44 may be“sharper” than blunt tip 43 (Fig. 4C), in that rounded tip 44 poses less of an impediment to initial insertion than blunt tip 43. At the same time, however, rounded tip 44 avoids electric field localization which may occur with a“sharp” or fully pointed tip, such as tips 40, 41 shown in Figs. 4A and 4B. Rounded tip 44 presents a convex surface across most (e.g., 90% or more) or all of the axial end surface of wire 103, while pointed tips 40, 41 are convex on a small amount (e.g., 20% or less) of the axial end surface.

[0071] As noted herein, all features described herein with respect to monolithic wire 103 may also be applied to composite wire 101. In addition, the features of Figs. 4A-4D may be combined into any single wire construct in any combination or permutation. For example, a monolithic or composite wire made in accordance with the present disclosure may have a blunt, rounded or sharpened tip, may include or exclude thin coating 104, and may include or exclude sheath 106. Any of these combinations or permutations may have any of the features described herein.

[0072] In one exemplary embodiment, a substantially pure rhodium ingot is hot drawn into a fine wire construct, then cold drawn as described herein to produce a high strength rhodium wire 103 having a finished diameter D₂s of 45 pm. Wire 103 may exhibit an amplitude A (Fig. 4) which is less than 45 pm (i.e., 1/1 of wire diameter D₂s) over a pitch P of 10 cm length, which may be the overall length of wire 103. Such a wire exhibits strength of at least 2 GPa and stiffness, measured as Young’s elastic modulus, of at least 300 GPa. This material may be suitable for use in the context of neural array 200 (Figs 3A and 3B). where very high precision is warranted.

[0073] In another exemplary embodiment, a Pt20Ir (i.e., 20 wt.% iridium with balance platinum) wire 103 is formed from an ingot to a fine wire and then to an ultrafme wire by cold drawing and repetitive anneals, as described in detail herein. This wire 103, in its finished form, has a diameter D₂s of 45 pm. Like the above-described wire 103 made of rhodium, this wire 103 may exhibit an amplitude A (Fig. 4) which is less than 45 pm (i.e., 1/1 of wire diameter D₂s) over a pitch P of 10 cm length, which may be the overall length of wire 103. Such a wire exhibits strength of at least 1.2 GPa, and stiffness, measured as Young’s elastic modulus, of at least 150 GPa. This material may be suitable for use in the context of some configurations of neural array 200, while reducing cost compared to the pure rhodium wire 103.

[0074] In yet another exemplary embodiment, a platinum shell and Nitinol core are cold drawn and repetitively annealed as described herein, producing composite wire 101 with a 45 pm diameter D₂s. The NiTi core 14 of this wire 101 occupies 95% of the cross-sectional area of the wire 101, with the platinum shell 12 occupying the remaining 5%. In this configuration, wire 101 exhibits superelastic properties due to the dominance of the superelastic NiTi material of core 14. Similar to the above-described wire 103 made of rhodium, wire 101 may exhibit an amplitude A (Fig. 4) which is less than 45 pm (i.e., 1/1 of wire diameter D₂s) over a pitch P of 10 cm length, which may be the overall length of wire 101. This wire 101 exhibits strength of at least 1.0 GPa, and stiffness, measured as Young’s elastic modulus, of at least 50 GPa. This material, with superelasticity enabling less than 0.5% permanent strain after a 6% axial tensile deformation, may be suitable for use in the context of some configurations of neural array 200 where recoverable wire deformation is of interest.

4. Drawing and Cold Work

[0075] A metal or metal alloy in accordance with the present disclosure is first formed in bulk, such by casting an ingot, continuous casting, or extrusion of the desired material. This bulk material is then formed into a suitable intermediate, or pre-form, material (e.g., a rod, plate or hollow tube) by hot-working the bulk material into the desired pre-form size and shape. For purposes of the present disclosure, hot working is accomplished by heating the material to an elevated temperature above room temperature and performing desired shaping and forming operations while the material is maintained at the elevated temperature. A coarse wire structure is then made by, for example, a schedule of drawing and annealing the intermediate material to create a structure ready for final processing into wires 101 or 103. Thereafter, the coarse wire structure may be subjected to one or more additional draws, as well as a final cold work conditioning step (Figs. 2A-2B) to form wires 101 or 103. One or more thermal processing steps such as shape setting, annealing and/or aging may then be performed in order to impart desired mechanical properties to the finished wire product, including strength and stiffness as discussed above. Further details of exemplary wire production and processing methods are further described below.

[0076] In one exemplary embodiment shown in Fig. 2A, monolithic wire 103 made of medical-grade metal material (described above) may be produced from a pre-form material into a wire of a desired diameter prior to final processing. That is, the pre-form material is drawn through one or more dies 105 (Fig. 2A) to reduce the outer diameter of the intermediate material slightly while also elongating the material, after which the material is annealed to relieve the internal stresses (i.e., retained cold work as discussed below) imparted to the material by the drawing process. This annealed material is then drawn through one or more new dies 105 with a smaller finish diameter to further reduce the diameter of the material, and to further elongate the material. Further annealing and drawing of the material is iteratively repeated until the material is formed into a drawn wire construct ready for final processing into wire 103.

[0077] To form composite wire 101 (Fig. 2B), such as DFT® brand composite wire, core

107 is inserted within shell 109 to form an intermediate construct, and an end of this intermediate construct is then tapered to facilitate placement of the end into a first drawing die 105 (Fig. 2B). The end protruding through the drawing die 105 is then gripped and pulled through the die 105 to reduce the diameter of the construct and bring the inner surface of shell 109 into firm physical contact with the outer surface of core 107. More particularly, the initial drawing process reduces the inner diameter of shell 109, such that shell 109 closes upon the outer diameter of core 107 and the inner diameter of shell 109 equals the outer diameter of core 107. After this initial drawing, the inner core 107 completely fills the central cavity of the outer shell 109 when viewed in section, as shown in Figs. 1B and 2B. Similar to monolithic wire 103 described above, this drawing process is then iteratively repeated to further reduce the diameter of the material, which also further elongates the material. Iterative annealing and drawing of the material is performed until the material is formed into a drawn wire construct ready for final processing into a drawn composite wire 101. Further detail regarding the construction and geometry of a composite wire in accordance with the present disclosure can be found in U.S. Patent Nos. 7,420,124, 7,501,579 and 7,745,732, filed September 13, 2004, August 15, 2005 and January 29, 2009 respectively and all entitled DRAWN STRAND FILLED TUBING WIRE, the entire disclosures of which are hereby expressly incorporated herein by reference.

[0078] Drawn wire constructs are structurally distinguished from constructs formed by other methods (e.g., casting, machining, coating, etc.) by their characteristic smoothness and high reflectivity. In the case of a bimetallic composite wire construct having a core and a shell, the circularity of the cross-section and the concentricity of the shell and core are substantially finer in a drawn construct as compared to, e.g., a coated construct. In addition, the

microstructure of a drawn construct may be structurally distinct from other constructs, for example by exhibiting an elongated grain structure (shown in Fig. 2D and further discussed below) or a fine-grain structure after thermal processing. [0079] Exemplary composite wires 101 may be formed using rhodium, rhodium alloys or platinum alloys in accordance with the present disclosure for either shell 109 or core 107. Other materials may be used in conjunction with the present materials (as described above) as required or desired for a particular application. For neural electrode applications of composite wire 101, high conductivity materials may be used for core 107 such as pure platinum, tantalum, rhodium and silver. These high-conductivity materials boost electrical conductivity and provide enhanced stimulation and recording signals.

[0080] The step of drawing subjects wire 101 or 103 to cold work. For purposes of the present disclosure, cold-working methods effect material deformation at or near room

temperature, e.g. 20-30 °C. ln the case of composite wire 101, drawing imparts cold work to the material of both shell 109 and core 107, with concomitant reduction in the cross-sectional area of both materials. The total cold work imparted to wire 101 or 103 during a drawing step can be characterized by the following formula (I):

rD

cw = 1- Z-l x 100% (I)

v J

wherein“cw” is cold work defined by reduction of the original material area, "D2" is the outer cross-sectional diameter of the wire (i.e., D2S for monolithic wire 103, and both D2C and D2S for composite wire 101) after the draw or draws, and "Di" is the outer cross-sectional diameter of the wire (i.e., Dis for monolithic wire 103, and both Die and Dis for composite wire 101) prior to the same draw or draws.

[0081] Referring to Figs. 2A and 2B, the cold work step may be performed by the illustrated drawing process. As shown, wire 101 or 103 is drawn through a lubricated die 105 having an output diameter D2S, which is less than diameter Dis of wire 101 or 103 prior to the drawing step. The outer diameter of wire 101 or 103 is accordingly reduced from pre-drawing diameter Dis to drawn diameter D₂s, thereby imparting cold work cw.

[0082] Alternatively, net cold work may be accumulated in wire 101 or 103 by other processes such as cold-swaging, rolling the wire (e.g., into a flat ribbon or into other shapes), extrusion, bending, flow forming, pilgering or cold-forging. Cold work may also be imparted by any combination of techniques including the techniques described here, for example, cold swaging followed by drawing through a lubricated die finished by cold rolling into a ribbon or sheet form or other shaped wire forms. In one exemplary embodiment, the cold work step by which the diameter of wire 101 or 103 is reduced from Dis to D₂s is performed in a single draw and, in another embodiment, the cold work step by which the diameter of wire 101 or 103 is reduced from Dis to D₂s is performed in multiple draws which are performed sequentially without any annealing step therebetween.

[0083] For processes where the drawing process is repeated without an intervening anneal on composite wire 101, each subsequent drawing step further reduces the cross section of wire 101 proportionately, such that the ratio of the sectional area of shell 109 and core 107 to the overall sectional area of wire 101 is nominally preserved as the overall sectional area of wire 101 is reduced. Referring to Fig. 2B, the ratio of pre-drawing core outer diameter Die to pre drawings shell outer diameter Dis is the same as the corresponding ratio post-drawing. Stated another way, Dic/Dis = D2C/D2S. Further details regarding wire drawing are discussed in U.S. Patent Application Serial No. 12/395,090, filed February 27, 2009, entitled "Alternating Core Composite Wire", assigned to the assignee of the present invention, the entire disclosure of which is incorporated by reference herein.

5. Annealing

[0084] Thermal stress relieving, otherwise known in the art as annealing, is achieved by heating the material to a nominal temperature not exceeding the melting point of the material or materials used in the construct. Annealing is used to improve the ductility of the construct between drawing steps, thereby allowing further plastic deformation by subsequent drawing steps. When calculating cold work cw using formula (I) above, it is assumed that no anneal has been performed subsequent to the process of imparting cold work to the material.

[0085] Ideating wire 101 or 103 to a temperature sufficient to cause recrystallization of grains eliminates accumulated cold work. The cold work imparted by each iterative cold work process is relieved by fully annealing the material between draws, thereby enabling the next iterative cold working process for materials which might otherwise become brittle by repeated draws or other cold work processing. In full annealing, the cold-worked material is heated to a temperature sufficient to substantially fully relieve the internal stresses stored in the material, thereby relieving the stored cold work and“resetting” cold work to zero.

[0086] On the other hand, wires 101 or 103 subject to drawing or other mechanical processing without a subsequent annealing process retain an amount of cold work. The amount of retained work depends upon the overall reduction in diameter from Dis to D₂s, and may be quantified on the basis of individual grain deformation within the material as a result of the cold work imparted. Referring to Fig. 2C, wire 103 is shown in a post-annealing state, with grains 111 shown substantially equiaxed, i.e., grains 111 define generally spheroid shapes in which a measurement of the overall length Gl of grain 111 is the same regardless of the direction of measurement. After drawing wire 101 or 103 (as described above), equiaxed grains 111 are converted into elongated grains 113 (Fig. 2D), such that grains 113 become longitudinal structures defining an elongated grain length G2 (i.e., the longest dimension across grain 113) and a relatively shorter grain width G3 (i.e., the shortest dimension across grain 113). The elongation of grains 113 results from the cold working process, with the longitudinal axis of grains 113 generally aligned with the direction of drawing, as illustrated in Fig. 2D.

[0087] The retained cold work of wire 101 or 103 after drawing can be expressed as the ratio of the elongated grain length G2 to the width G3, such that a larger ratio implies a grain which has been“stretched” farther and therefore implies a greater amount of retained cold work. By contrast, annealing wire 101 or 103 after an intermediate drawing process recrystallizes the material, converting elongated grains 113 back to equiaxed grains 111 and“resetting” the retained cold work ratio to 1 : 1 (i.e., no retained cold work) .

[0088] For the above-described rhodium, rhodium alloy and platinum alloy materials, full annealing or stress-relief annealing sufficient to tune strength and straightness properties may be accomplished at a temperature between 400 and 1000 °C for at least 2-120 seconds for wires 103 having outer diameters D₂s between 5-50 pm as described herein, with higher temperatures associated with full annealing and lower temperatures associated with stress-relief annealing that does not fully recrystallize elongated grains 113 back to equiaxed grains 111. Annealing time, also called the“dwell time” during which the wire is exposed to the annealing temperature, is dependent on the size of the wire 103 and the desired effect of the annealing process, as well- understood by a person of skill in the art of material processing.

[0089] For purposes of the present discussion, annealing time may be assumed to be positively linearly correlated with the cross-sectional area of the wire being annealed. Thus, for a given annealing temperature, a similar annealing result is assumed for a first wire having a first cross-sectional area and annealed for a first amount of time, as for a second wire having twice the cross-sectional area of the first wire and annealed for a second amount of time that is twice the first time. However, for smaller fine wires and ultrafme wires, such as those having 200 mih or less, it may be assumed that the wire material becomes quickly heated through to the desired temperature, and the time for this heating is not significantly diameter-dependent. Thus, for wires 101 and 103 having diameters Dzs less than 200 pm, the annealing time is not correlated to diameters D₂s and is instead solely determined on the desired effect, i.e., full annealing or various gradations of stress-relief annealing as described above.

[0090] Alternatively, a full anneal can be accomplished with a higher temperature, such as between 600 and 1200 °C, for a shorter time, such as at least 0.10 to 2 seconds, again depending on diameter of the material and assuming a linear relationship between annealing time and wire cross-sectional area. Of course, a relatively higher temperature annealing process can utilize a relatively shorter time to achieve a full anneal, while a relatively lower temperature will typically utilize a relatively longer time to achieve a full anneal.

[0091] Moreover, annealing parameters can be expected to vary for varying wire diameters, with smaller diameters shortening the time of anneal for a given temperature and a given wire material. Whether a full anneal has been accomplished for any given wire sample can be verified in a number of ways as well known in the art, such as microstructural examinations using scanning electron microscopy (SEM), mechanical testing for ductility, strength, elasticity, etc., and other methods.

[0092] Further discussion of cold working and annealing methods can be found in U.S.

Patent No. 8,840,735, filed September 18, 2009 and entitled FATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, the entire disclosure of which is hereby incorporated by reference.

6. Shape-Set Thermal Processing

[0093] After the initial iterative draw/anneal processing is completed, the resulting coarse wire material may then be finally processed into a final form, such as a straight individual electrode wire construct, such as wires 103 or 101.

[0094] In particular, the coarse drawn material may be subject to a final cold work and subsequent“shape setting” process to form a straight microelectrode.“Shape setting” as used herein denotes a process in which a work piece (such as a wire) is constrained to a desired shape and thermally processed to retain the desired shape (e.g. constrained as a straight length). For example, the work piece may be bent, held straight or otherwise formed into a desired shape, and held in that shape during subsequent thermal processing. In another example, the work piece may be constrained to its“natural” undeformed, pre-existing shape, which may include a straight shape. This“constraint” may not impart any stress to the material prior to thermal processing, but rather, may simply prevent the material from deforming away from the undeformed shape during subsequent thermal processing. With the work piece constrained to whatever shape is desired, the temperature of the work piece is increased in a thermal processing step until the work piece retains the desired shape, at which point the shape setting process is completed.

[0095] Shape setting may be performed on an annealed material with no stored cold work within the scope of the present disclosure. However, shape setting materials with stored cold work produces larger recoverable strain capabilities for a given material geometry and constituency of the metals and metal alloys described herein for use with neural electrode wires. In particular, using material with retained cold work in the present shape setting process raises the material’s yield strength and mitigates or eliminates plastic deformation at a given level of strain, and produces certain crystal orientations favorable to robust strain recovery as compared to material without retained cold work. In an exemplary embodiment, final cold work processing is performed to impart a cold work as little as 50% or 75% and as much as 99% or 99.9%, or any cold work defined by any two of the foregoing values. For example, retained cold work in wire 103 prior to shape setting may be between 50%-75%, 50%-99%, 50%-99.9%, 75%-99%, 75%- 99.9%, or 99%-99.9%. In one exemplary embodiment, for example, a final cold work of about 90% is imparted to the work piece prior to shape setting. Utilizing these amounts of retained cold work in conjunction with shape setting can impart a high level of straightness to wire 103, as further described herein with respect to Fig. 5.

[0096] In the shape setting process, the work piece is subjected to an ambient temperature (e.g., in an oven or other heater) between 300°C and 800°C for a time period between 2 seconds and 120 seconds. For rhodium and rhodium alloys, the temperature for this primary shape set be as little as 300°C, 350°C, or 400°C, and as much as 425°C, 450°C, or

500°C, or may be any temperature in any range defined by any two of the foregoing values. The temperature at this stage may be held for as little as 2 seconds, 10 seconds, or 2 minutes or 30 minutes or may be any period of time in any range defined by any two of the foregoing values. [0097] As with annealing, time and temperature are inversely correlated in a shape setting process in accordance with the present disclosure. That is to say, shape setting at the upper range of acceptable temperatures will require a generally shorter time for a given work piece geometry (e.g., size and configuration), while the lower range of acceptable temperatures will required a relatively longer time. Also, shape setting time is positively correlated with cross-sectional area of the work piece for any given shape set temperature, with an assumed linear relationship as discussed above with respect to annealing

[0098] Thermal processing of material as described herein, including annealing and shape setting, can be accomplished in any suitable fashion, including batch annealing of individual work pieces in an oven, fluidized bed furnace or forced convection furnace, and continuous annealing of spool-to-spool wire materials passing through a heated chamber, for example. Heating times and temperatures described herein are tailored for oven-based heating methods. Moreover, although heating methods all expose a work piece to an elevated ambient temperature for a specified time as described herein, it is also contemplated that heating of the work piece may be accomplished by any other suitable method. To the extent that such alternative heating modalities are employed, such alternative methods should be adjusted as necessary to produce an internal work piece temperature approximately equal to those reached by the ambient heating-based method described herein.

7. Wire Array

[0099] As noted above, a plurality of wires 103 may be used to create a wire array for simultaneous monitoring and/or stimulation across and area of neural sites. An exemplary microelectrode array 200, shown in Figs. 3A and 3B, includes a rigid or semi-rigid substrate 202 with each wire 103 connected to a unique position on the substrate 202 and forming an electrical connection on a printed circuit board or similar construct embedded into or attached to the substrate. Wires 103 may be arrayed across the entire surface of substrate 202, as illustrated schematically in Fig. 3A, in any desired pattern as required or desired for a particular implantation site and monitoring/stimulation modality.

[00100] As best seen in Fig. 3B, wires 103 may extend perpendicularly away from the substrate 202 such that, with a large number of wires 103 positioned in a pattern or array extending across the substrate 202, the microelectrode array 200 assembly takes on the appearance of a small wire brush. In addition to the variability of the pattern of wires 103 for the microelectrode array 200, as noted above, the lengths of the various wires 103 may be varied across the array 200 to achieve varying depth of electrode implantation across the implant area defined by substrate 202.

[00101] In an exemplary embodiment, microelectrode array 200 may include a density of wires 103 with a lineal (i.e., one-dimension) pitch density of more than 100, 200, 400 or 1000 wires per centimeter, yielding 2-D electrode densities of up to 10,000, 40,000, 160,000 or 1 million wires per square centimeter. This short pitch (i.e., high wire density) can achieve high stimulation and recording electrode density within a given implant area.

[00102] In the context microelectrode array 200, this high density of electrodes is enabled by the thinness, and precise straightness of the individual wires 103 (or 101), as discussed above with respect to Fig. 5, with increased straightness enabling higher density and vice-versa.

Electrode density is also a function of the overall size of each wire 103, except that all wires 103 must possess at least the minimum strength and stiffness for precise neural implantation as described above. In particular, array 200 may be designed such that each of the wires 103 connected to substrate 202 avoids contact with any other neighboring wire 103 along its entire axial extent, thereby avoiding any electrical connection between neighboring electrodes such that a single, clean neural signal can be transmitted by each wire 103.

[00103] While this invention has been described as having exemplary designs, the present invention may be further modified with the spirit and scope of this disclosure. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.

Claims

What is claimed is:

1. A metal wire having a diameter between 5 mih and 50 mih, the metal wire formed of a medical -grade material having a yield strength which reaches 1000 MPa and a stiffness which reaches 140 GPa, whereby the metal wire is suitable for use as a microelectrode in a high- density-microelectrode and tissue-penetrating neural recording and stimulation array.

2. The metal wire of claim 1, wherein the medical-grade material is selected from the group consisting of:

99% rhodium and trace impurities,

a rhodium alloy having at least 50% wt.% rhodium, other alloying elements including at least one of platinum, tungsten, iridium, nickel, gold, molybdenum and rhenium, and trace impurities, and

a platinum-iridium alloy having between 10 wt.% and 30% wt.% iridium, balance platinum and trace impurities.

3. The metal wire of claims 1 or 2, wherein the wire defines a deviation from perfect straightness, the deviation defining an amplitude between 1/10 of the diameter of the wire and 1/1 of the diameter of the wire.

4. The metal wire of claim 3, wherein the deviation defines a pitch between 50 times the diameter of the wire and 200 times the diameter of the wire.

5. The metal wire of any preceding claim, wherein the medical-grade material is electrically conducting and excludes polymeric materials, silicon and carbon.

6. The metal wire of any preceding claim, wherein the wire comprises a shell and core received within the shell, the core completely filling a central cavity within the shell.

7. The metal wire of any preceding claim, wherein the wire comprises a monolithic wire formed of a single piece of homogenous material.

8. The metal wire of any preceding claim, wherein the wire comprises an electrically insulating sheath disposed about the exterior surface of the medical-grade material, the sheath covering the exterior surface along an implant length and not covering an electrically conductive tip of the wire.

9. The metal wire of any preceding claim, wherein a tip of the wire forms a blunt tip.

10. The metal wire of claim 9, wherein a tip of the wire forms a sharpened tip.

11. The metal wire of claim 10, wherein the sharpened tip comprises a rounded tip presenting a convex surface across at least 90% of an axial end surface of the wire.

12. The metal wire of claims 10 or 11, wherein the sharpened tip comprises a pointed tip presenting a convex surface across less than 20% of an axial end surface of the wire.

13. The metal wire of claim 12, wherein the pointed tip defines an angle, in cross-section, between 5 degrees and 30 degrees.

14. The metal wire of any preceding claim, wherein the metal wire is a monolithic wire formed of a single piece of the medical-grade material.

15. The metal wire of any preceding claim, wherein the medical-grade material is a first medical-grade material, and wherein:

the metal wire is a composite wire comprising a core and a concentric shell formed around the core,

one of the core and the shell is formed from the first medical-grade material, and the other of the core and the shell formed from a second medical-grade material different from the first medical -grade material.

16. The metal wire of any preceding claim, wherein the yield strength is at least 1300 MPa.

17. The metal wire of any preceding claim, wherein the yield strength is at least 1500 MPa.

18. The metal wire of any preceding claim, wherein the stiffness is at least 180 GPa.

19. The metal wire of any preceding claim, wherein the stiffness is at least 200 GPa.

20. The metal wire of any preceding claim, wherein the stiffness is at least 250 GPa.

21. The metal wire of any preceding claim, wherein the stiffness is at least 300 GPa.

22. The metal wire of any preceding claim, wherein the stiffness is at least 350 GPa.

23. The metal wire of any preceding claim, wherein the medical-grade material excludes copper, silver, lead, cadmium, 300-series stainless steel and 400-series stainless steel.

24. The metal wire of any preceding claim, wherein the wire includes a thin coating disposed around the wire periphery and having a thickness less than 1720^th of the diameter.

25. The metal wire of claim 24, wherein the thin coating is made of gold.

26. The metal wire of any preceding claim, wherein the wire has a round cross-section.

27. The metal wire of any preceding claim, wherein the wire has a non-round, polygonal cross-section.

28. The metal wire of any preceding claim, wherein the wire comprises tubing with a hollow void running axially through the wire.

29. A high density and implantable microelectrode array for neural tissue penetration, tissue stimulation and recording, the array comprising: a plurality of metal wires in accordance with any of claims 1-28; and

a substrate, the plurality of metal wires arrayed and respectively connected to the substrate.

30. The array of claim 29, wherein the plurality of metal wires each extend perpendicularly away from the substrate.

31. The array of claims 29 or 30, wherein each of the plurality of metal wires avoids contact with any other metal wire across its entire axial extent.

32. The array of any of claims 29-31, wherein the plurality of metal wires are connected to the substrate with a wire density between 10,000 and 1 million wires per square centimeter.

33. The array of any of claims 29-31, wherein at least some of the plurality of metal wires define a wire length different from others of the plurality of metal wires.