PDMS-based stretchable multi-electrode and chemotrode array for epidural and subdural neuronal recording, electrical stimulation and drug delivery

Abstract

An implantable device for the electrical and/or pharmaceutical stimulation of the central nervous system, especially the spinal cord, is suggested. The device comprises a conformable substrate which is primarily composed of a flexible and stretchable polymer, and a plurality of flexible electrodes and conductive leads embedded in the conformable substrate. Not only the substrate, but also the leads are stretchable. The substrate may consist of PDMS, and the leads may consist of a conductive PDMS, in particular, PDMS with an electrically conductive filler material, and may optionally be metal-coated. The device defines a multi-electrode array which may be employed for neurostimulation in the epidural or subdural space of an animal or human.

Claims

1. An implantable device for electrical and pharmaceutical stimulation of a central nervous system which is configured and arranged for implantation in an animal or human in an epidural or subdural space of a spinal cord, comprising: a conformable substrate which is primarily composed of a flexible and stretchable polymer; a plurality of flexible and stretchable electrodes and stretchable conductive leads embedded in the conformable substrate; and microfluidic channels that are embedded in the conformable substrate, wherein the electrical and pharmaceutical stimulation is applied to the spinal cord, epidurally or subdurally, wherein the stretchable conductive leads are essentially made of a polymer matrix with an electrically conductive filler material, wherein the filler material is at least one of Ag, Au, Cu, Pt, Ir, Al, Fe, Cr, Ni, C, In, Sn, and Ti; and wherein the filler material is present in fibrous, particle, or nanotube form, further configured such that pharmaceuticals to the epidural or subdural space can be delivered through the microfluidic channels.

2. The device according to claim 1, wherein at least one of the electrodes and leads are stretchable by at least 20% while maintaining their conductivity.

3. The device according to claim 1, wherein the conformable substrate essentially consists of poly(dimethylsiloxane).

4. The device according to claim 3, wherein at least one of the electrodes and leads essentially consist of conductive poly(dimethylsiloxane).

5. The device according to claim 1, further comprising at least one of the following: a reception means for contactless energy supply to the plurality of flexible and stretchable electrodes; an electrical circuit embedded in the conformable substrate or attached to the conformable substrate; or a battery configured to supply electricity to the plurality of flexible and stretchable electrodes.

6. The device according to claim 1, having a total thickness of less than 1 mm.

7. The device according to claim 1, wherein at least one of the electrodes and conductive leads is functionalized with a drug releasing coating.

8. The device according to claim 1, further comprising a plurality of holes or other microstructures to improve fluid circulation and thermoregulation and/or to promote tissue ingrowth and/or to prevent inflammation when the device is implanted.

9. The device according to claim 1, wherein the electrodes form a multi-electrode array for stimulating neural tissue.

10. The device of claim 1, wherein at least one of the electrodes and leads are stretchable by more than 50% while maintaining their conductivity.

11. The device of claim 1, wherein the microfluidic channels embedded in the conformable substrate are horizontal channels.

12. The device of claim 1, wherein a first layer of the conformable substrate is non-conducting.

13. A method of neurostimulation, comprising: implanting a device in an epidural or subdural space of a spinal cord of an animal or human, the device comprising: a conformable substrate which is primarily composed of a flexible and stretchable polymer; and a plurality of flexible and stretchable electrodes and stretchable conductive leads embedded in the conformable substrate, wherein the stretchable conductive leads are essentially made of a polymer matrix with an electrically conductive filler material, wherein the filler material is at least one of Ag, Au, Cu, Pt, Ir, Al, Fe, Cr, Ni, C, In, Sn, and Ti; and wherein the filler material is present in fibrous, particle, or nanotube form, further configured such that drugs to the epidural or subdural space can be delivered through microfluidic channels embedded within the device; the method further comprising: stimulating neurons in the spinal cord by providing electrical signals to the electrodes or the conductive leads of the device configured and arranged for implantation in the epidural or subdural space of the spinal cord.

14. A method of drug delivery, comprising: implanting a device in an epidural or subdural space of a spinal cord of an animal or human, the device comprising: a conformable substrate which is primarily composed of a flexible and stretchable polymer; and a plurality of flexible and stretchable electrodes and stretchable conductive leads embedded in the conformable substrate, wherein the stretchable conductive leads are essentially made of a polymer matrix with an electrically conductive filler material, wherein the filler material is at least one of Ag, Au, Cu, Pt, Ir, Al, Fe, Cr, Ni, C, In, Sn, and Ti; and wherein the filler material is present in fibrous, particle, or nanotube form, further configured such that drugs to the epidural or subdural space can be delivered through microfluidic channels embedded within the device; the method further comprising: delivering drugs to the epidural or subdural space through the microfluidic channels embedded within the device.

15. A method of recording neuronal signals, comprising: implanting a device in an epidural or subdural space of a spinal cord of an animal or human, the device comprising: a conformable substrate which is primarily composed of a flexible and stretchable polymer; and a plurality of flexible and stretchable electrodes and stretchable conductive leads embedded in the conformable substrate, wherein the stretchable conductive leads are essentially made of a polymer matrix with an electrically conductive filler material, wherein the filler material is at least one of Ag, Au, Cu, Pt, Ir, Al, Fe, Cr, Ni, C, In, Sn, and Ti; and wherein the filler material is present in fibrous, particle, or nanotube form, further configured such that drugs to the epidural or subdural space can be delivered through microfluidic channels embedded within the device; the method further comprising: recording neuronal signals from the spinal cord received by the electrodes or the conductive leads of the device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

(2) FIG. 1 illustrates the layout of a MEA having six electrodes, together with its intended position relative to the spinal cord;

(3) FIG. 2 illustrates the layout of a MEA having eight electrodes, together with its intended position relative to the spinal cord;

(4) FIG. 3 illustrates the layer sequence for the preparation of a PDMS layer on a glass slide carrier;

(5) FIG. 4 illustrates a first preferred process for manufacturing a SMEA;

(6) FIG. 5 illustrates a second preferred process for manufacturing a SMEA;

(7) FIG. 6 illustrates a third preferred process for manufacturing a SMEA;

(8) FIG. 7 illustrates a fourth preferred process for manufacturing a SMEA;

(9) FIG. 8 illustrates a fifth preferred process for manufacturing a SMEA;

(10) FIG. 9 illustrates a sixth preferred process for manufacturing a SMEA;

(11) FIG. 10 illustrates a seventh preferred process for manufacturing a SMEA;

(12) FIG. 11 shows part of a SMEA produced using the sixth process, carrying an array of eight platinum electrodes;

(13) FIG. 12 shows detail (A) of FIG. 11 on an enlarged scale;

(14) FIG. 13 illustrates the connection of the SMEA to a head connector;

(15) FIG. 14 shows a diagram in which stress and resistance are plotted over elongation of the SMEA of FIGS. 12 and 13;

(16) FIG. 15 shows implantation of a SMEA with six electrodes into a rat;

(17) FIG. 16 illustrates implantation in the epidural space (part A) as compared to the subdural space (part B);

(18) FIG. 17 illustrates a protocol for subdural implantation of a SMEA; and

(19) FIG. 18 illustrates improved fixation of the SMEA by multiple holes.

DESCRIPTION OF PREFERRED EMBODIMENTS

(20) General Remarks

(21) In the following, preferred embodiments of PDMS-based stretchable microelectrode arrays (SMEA) for spinal cord stimulation are described. Such SMEAs were fabricated and tested in vivo on the spinal cord of rats. However, the invention is by no means limited to SMEAs suitable for implantation in rats or other rodents, and SMEAs of the type described in the following may also be employed with other vertebrates, including humans. Use of such SMEAs is not limited to implantation at the spinal cord. SMEAs of the type described here may also be employed in other body regions where they are subjected to strains.

(22) The monolithic PDMS structure of the SMEA contained conductive PDMS (cPDMS) tracks, gold or platinum or platinum-iridium electrodes and contact pads connected to a head connector via medical fine wires. A PDMS substrate was used to reduce the mismatch between the mechanical properties of the implant and the spinal cord. The elastic and electrical properties of cPDMS promote high flexibility and stretchability to the implant, thus providing an electronic skin over the tissue. The SMEAs were implanted and tested chronically (2 months) in rats. Electrical epidural and subdural stimulation could induce spinal reflexes even at low current level (10 A).

(23) Polydimethylsiloxane (PDMS) was used as a substrate material because of its excellent biocompatibility and mechanical properties. Biological tissues like rat spinal dura mater were reported to have similar elastic modulus than PDMS (1 MPa), whereas parylene and polyimide are 1000 times stiffer. This makes it challenging to produce conductive tracks that can stretch together with the PDMS substrate. It is assumed that strain greater than 20% can occur during chronic implantation in rats, while sputtered gold tracks on PDMS can generally only survive up to 1% strain. Conductive PDMS prepared by dispersing silver particles or carbon nanotubes in the PDMS matrix can remain conductive at strains >60%. In the preferred embodiments of the present invention, described in more detail below, cPDMS tracks were integrated into a PDMS structure to produce a monolithic PDMS-based MEA that remains conductive while being stretched above 30% or even above 50%.

(24) Preparation of cPDMS

(25) To prepare the conductive PDMS, silver particles were dispersed into a PDMS mixture until a smooth paste was obtained. 5 g of PDMS (Sylgard 184, Dow Corning) was prepared by mixing base and curing agent in a 10:1 (w/w) ratio. 12-14 g of silver powder (2-3 m near-spherical particles) were progressively dispersed into the PDMS mixture by using a miller or sonication tip. Hexane was used to lower the viscosity of the preparation in order to facilitate the milling. The resulting smooth paste was degassed for 15 min in a desiccator. The cPDMS mixture could be kept over months in a freezer at 20 C.

(26) Preparation of Copper Stencils

(27) To prepare the stencils, prior to screen-printing the cPDMS electrodes, copper foils of 75 m thickness were wet etched from both sides. Copper foils of dimension 76 mm26 mm were rinsed in acetone, isopropanol, blow dried with a nitrogen gun, activated in Oxytron 15 (Oxy Metal Industries) solution, rinsed in milliQ filtered and deionized water and again blow-dried. Positive photoresist 1805 was spin coated at 500 rpm for 30 s and cured at 85 C. for 10 min on both sides. The foils were precisely positioned and fixed between 2 masks, exposed to UV from one side for 45 s, returned and exposed for another 45 s. The photoresist was developed in Microposit Developer for 3 s and rinsed in milliQ water. The copper substrate was etched from both sides in a copper etching solution until complete dissolution of the copper. The photoresist was then stripped in acetone. The copper stencils were rinsed in milliQ water and blow-dried.

(28) Array Layout

(29) Different layouts were used in this study. Array with six or eight electrodes were produced. The term electrode is to be understood in the usual manner to relate to an electrically conducting element that is exposed to the environment. FIG. 1 illustrates the layout for the six-electrode array. Six electrodes 11 are arranged on an imaginary array 12 with three rows and eight columns in the following positions (first number indicates row, second number indicates column): (1, 2), (3, 2), (2, 4), (2, 5), (1, 7), (3, 7). The rows and columns are not necessarily equidistant and not even necessarily parallel. In the present example, the distance between the first and second column is d.sub.1=3.0 mm, while the distance between subsequent columns is d.sub.2=d.sub.3=d.sub.4=d.sub.5=d.sub.6=d.sub.7=2.5 mm. The distance between rows decreases in the present example from left to right, with a row-to-row center distance of w.sub.1=1.0 mm at the position of the second column and w.sub.2=0.7 mm at the position of the seventh column. In particular, the present layout and dimension are adapted to neurostimulation at the spinal cord of adult rats, in the position near vertebrae T12, T13, L1 and L2 and spinal segments L1-L6 and S1-S4, as shown in FIG. 1. Typical length dimensions of these vertebrae are L.sub.T12=6.8 mm, L.sub.T13=7.2 mm, L.sub.L1=7.5 mm, and L.sub.L2=7.8 mm. The electrodes of the array are arranged such that they span a plurality of spinal segments, here six such segments. Of course, other layouts and dimensions are conceivable, depending on the intended application. By the way of example, if the array is intended to be used with humans, the arrangement of electrodes will be adapted to the size of the human vertebrae and spinal segments accordingly so as to span a plurality of human spinal segments.

(30) FIG. 2 illustrates a corresponding layout for an eight-electrode array. The layout corresponds to the layout of FIG. 1, with two additional electrodes at positions (2, 1) and (2, 8).

(31) Fabrication of the SMEA

(32) In the following, various processes that may be employed for the manufacture of SMEAs according to the present invention are described by the way of example.

(33) In general terms, to prepare the SMEA, cPDMS structures were applied on a PDMS substrate by using custom-made copper stencils or screen-printed with commercially available screens. A second layer of PDMS was deposited as an insulation layer.

(34) 100 m thick Kapton foils of dimensions 76 mm26 mm were deposited on microscopy glass slides with same dimensions. Adhesion was realized by adding a drop of water between the Kapton foil and the glass substrate. The Kapton foils were used as anti-adhesion layer for the PDMS. FIG. 3 schematically illustrates the resulting layer sequence after application of the first PDMS layer. A carrier 31 in the form of a microscopy slide carries an anti-adhesion layer 32 in the form of a Kapton foil, which in turn carries a first layer of a conformable substrate, here in the form of a PDMS layer 33. As an alternative to a polyimide layer such as a Kapton layer, a thin gold film, a Ti/Au film, a Teflon coating, a polymeric coating like PMMA or polyimide or a layer of alkoxysilane molecules may be deposited as an anti-adhesion layer 32 on the carrier 31 (here a glass slide).

(35) A first layer (30-50 m thick) of PDMS was spin coated at 3000 rpm for 30 s on the Kapton or gold layer and cured at 100 C. for 30 minutes on a hotplate. There are different curing protocols for PDMS: temperatures ranging from 25 C. up to 150 C. with curing times ranging from 10 minutes up to 2 days in an oven or on a hotplate can be used.

(36) These initials steps are the same for the following different processes.

(37) First Process

(38) A first exemplary process is illustrated in FIG. 4. The copper stencils, as described above, were gently pressed against the PDMS substrate 33. cPDMS was spread with a blade over the stencils. The stencils were carefully peeled off, leaving cPDMS structures 34 on the PDMS substrate, cleaned in toluene, rinsed in isopropanol, milliQ water and blow-dried. As an alternative, commercially available screens with stainless steel mesh of 30 m aperture and 40 m thick photoresist are also suitable for this process. The screen is then positioned parallel to the substrate at a distance of 30 mm, and the structures are printed using standard techniques. The cPDMS structures were cured at 100 C. for 3 hours in an oven or on a hotplate. In an optional step, the stencils were put back on the PDMS substrates and used as shadow masks to sputter Ni and Ag layers of 15 nm and 100 nm thickness, respectively, on the cPDMS structures. The optional Ag layer 35 increases the conductivity of each track and promotes an adhesive substrate for making the contact pads (FIG. 4(a)). Gold or platinum may be used instead of silver for the layer 35. Drops of conductive silver epoxy 36 were manually deposited on each contact pad and cured at 130 C. for 1 hour in an oven. Drops of SU-8 negative photoresist 37 were manually deposited on each electrode and cured at 95 C. for 1 hour on a hotplate (FIG. 4(b)). A second layer of PDMS was spin coated at 1200-1400 rpm for 30 s to make an insulation layer. SU-8 bumps were mechanically removed (FIG. 4(c)) and the holes were either manually filled with cPDMS, and cured at 100 C. for 2 hours in an oven or on a hotplate in order to make bumpy electrodes 38, or manually filled with silver epoxy, and cured at 130 C. for 1 hour in an oven in order to make flat electrodes 40. Ti/PtIr layers 39 or 41 of 15 nm/300 nm were sputtered on each electrode by using shadow masks similar to that previously described (FIG. 4(d) and FIG. 4(d), respectively). The resulting SMEA was peeled off from the carrier 31 and anti-adhesion layer 32 (FIG. 4(e)).

(39) Second Process

(40) A second process is illustrated in FIG. 5. As in the first process, cPDMS structures 34 were obtained on the PDMS substrate 33 and cured at 100 C. for 3 hours in an oven or on the hotplate, and optionally Ni/Ag layers 35 of 15 nm/100 nm were sputtered onto the cPDMS structures 34 (FIG. 5(a)). Drops of conductive silver epoxy 36, 42 were manually deposited on each contact pad and electrode and cured at 130 C. for 1 hour in an oven (FIG. 5(b)). A second layer of PDMS was spin coated at 1200-1400 rpm for 30 s to make an insulation layer (FIG. 5(c)). The thin PDMS insulation layers over the electrodes were manually removed and Ti/PtIr layers 43 of 15 nm/300 nm were sputtered on each electrode by using shadow masks similar to those previously described (FIG. 5(d)). Finally, the resulting SMEA was removed from the carrier 31 and anti-adhesion layer 32 (FIG. 5(e)).

(41) Third Process

(42) A third process is illustrated in FIG. 6. Holes of 350 m were manually punched through the PDMS layer 33 at the location of the pads and electrodes (FIG. 6(a)). The copper stencils were then carefully positioned and gently pressed against the PDMS substrates 33. cPDMS was spread with a blade over the stencils. The stencils were carefully peeled off leaving the cPDMS structures 34 on the PDMS substrate, cleaned in toluene, rinsed in isopropanol, milliQ water and blow-dried. The cPDMS structures 34 were cured at 100 C. for 3 hours in an oven or on the hotplate. The stencils were put back on the PDMS substrates and used as shadow masks to sputter Ni/Ag layers 35 of 15 nm/100 nm on the cPDMS structures 34 (FIG. 6(b)). A second layer of PDMS was spin coated at 1200-1400 rpm for 30 s to make an insulation layer (FIG. 6(c)). The array was removed from the carrier 31, 32, flipped and put back onto the carrier. Ti/PtIr layers 44, 45 of 15 nm/300 nm were sputtered on each electrode and pad by using shadow masks similar to those previously described (FIG. 6(d)).

(43) Fourth Process

(44) A fourth process is illustrated in FIG. 7. This process is used to produce 3D structures with twice more electrodes. A structure as shown in FIG. 6(c) was manufactured as described in conjunction with the third process. These steps were repeated once more in order to get structures (second cPDMS structure 46, second Ni/Ag layers 47) in two different planes with a doubled amount of electrodes (FIGS. 7(b) and (c)). The array was removed from the carrier 31, 32, flipped and put back onto the carrier. Ti/PtIr layers 48, 49 of 15 nm/300 nm were sputtered on each electrode and pad by using shadow masks similar to those previously described (FIG. 7(d)).

(45) Fifth Process

(46) A fifth process is illustrated in FIG. 8. In this process, platinum is used as a filler to produce cPDMS. As a result, cPDMS can directly be used as an electrode material. Holes of 350 m were manually punched through the PDMS layer 33 at the location of the pads and electrodes (FIG. 8a). The copper stencils were then carefully positioned and gently pressed against the PDMS substrates. cPDMS was spread with a blade over the stencils. The stencils were carefully peeled off leaving the cPDMS structures 34 on the PDMS substrate 33, cleaned in toluene, rinsed in isopropanol, milliQ water and blow-dried. The cPDMS structures 34 were cured at 100 C. for 3 hours in an oven or on the hotplate. The stencils were put back on the PDMS substrates and used as shadow masks to sputter Ni/Ag layers 35 of 15 nm/100 nm on the cPDMS structures (FIG. 8(b)). A second layer of PDMS was spin coated at 1200-1400 rpm for 30 s to make an insulation layer (FIG. 8(c)). The array was removed from the carrier 31, 32. The array does not need an extra electrode coating. Ni/Ag layers 50, 51 are sputtered on the pads to promote adhesion for silver epoxy (FIG. 8(d)).

(47) Sixth Process

(48) A sixth process is illustrated in FIG. 9. cPDMS structures 34 with Ni/Ag layers 35 were prepared as before (FIG. 9(a)). Drops of conductive silver epoxy 36, 42 were manually deposited on each contact pad and electrode and cured at 130 C. for 1 hour in an oven. Prior to curing, platinum disks 52 of 350 m diameter, obtained from a 12.5 m thick platinum foil were manually placed over each electrode (FIG. 9(b)). The adhesion and electrical contact between the cPDMS and Pt disks are provided by the silver epoxy. Drops of SU-8 photoresist 53 were manually deposited on each electrode and cured at 95 C. for 1 hour on a hotplate (FIG. 9(c)). A second layer of PDMS was spin coated at 1200-1400 rpm for 30 s to make an insulation layer. SU-8 bumps were mechanically removed to expose the Pt electrodes (FIG. 9(d)), and the SMEA was peeled off from the carrier 31, 32 (FIG. 9(e)).

(49) Seventh Process

(50) In the following, a process for preparing a SMEA containing fluidic channels (microchannels defining chemotrodes) is described by the way of example with reference to FIG. 10. PDMS is first casted on a structured carrier whose structures correspond to the geometry of the desired microchannels. The structured carrier 31 can be produced either by making structures of photoresist like SU8 on a carrier like glass or silicon using standard photolithography or by wet etching, dry etching or laser ablating the carrier (FIG. 10(a) and (b)). The structured carrier 31 is then coated with an anti-adhesive layer 32 that facilitates the later peeling of the PDMS layer (FIG. 10(c)). The anti-adhesive layer 32 can be a metallic layer e.g. Ti/Au, a Teflon coating, a polymeric coating like PMMA or polyimide or a layer of alkoxysilane molecules. PDMS is spin-coated on the structured substrate and then cured (FIG. 10(d)). The PDMS is then peeled off from the carrier. The obtained micro-structured PDMS layer is flipped and bonded on the back of a device 54, which may have been prepared according of one of the processes described before. The bonding of the two PDMS layers is made by first treating the surface to be bonded in air plasma, placing the layers against each other, pressing and waiting until the two layers are bonded (symbolized by connection 57). This results in horizontal channels 56 between the two layers. Holes 58 for liquid outlet and inlet are drilled with a laser.

(51) Eighth Process

(52) In an alternative process for preparing a SMEA with fluidic channels, a device fabricated, e.g., according to one of the processes described before is flipped and its bottom surface is micromachined with a laser. Holes and microchannels are drilled with a laser. Then a layer of PDMS is bonded as described before to close the microchannels.

(53) Regardless of how the microchannels were produced, a small stainless steel cannula may be used to make the interconnection between a small tube of the pump and the inlet of the microchannels. The diameter of the cannula is bigger than that of the tube and inlet to avoid leakage. The interconnection is sealed in PDMS.

EXAMPLE

SMEA Produced by Sixth Process

(54) FIGS. 11 and 12 show an array of eight platinum electrodes 62 in the arrangement of FIG. 2, produced using the sixth process as shown in FIG. 9. The width of each conductive track 61 is 150 m. The minimum distance between two next-neighbor tracks is 150 m. The diameter of each electrode 62 is 350 m. Holes 63 of 350 m diameter are visible. The thickness of the array is within the range of 100-200 m. It does not exceed 300 m. The width of the array is within the range of 2.8-3.0 mm.

(55) Omnetics circular connectors were used as headplugs for the rats. Medical fine wires 65 with stainless steel core and PTFA insulation were used to connect the array to the headplug at the contact pads 64, as shown in FIG. 13. To this end, the insulation was removed from the tip of each fine wire 65. Each tip was then placed on top of a contact pad 63. Ag-epoxy was used to electrically connect wires and pads. The Ag-epoxy was cured for 1 hour at 130 C. in an oven. A PDMS layer was then casted on the region of the contact pads to cover the contact pads and the Ag-epoxy.

(56) The electrical properties of the SMEA were measured with contact probes. Electrical resistivity of conductive tracks was measured with a multimeter between contact pads and respective electrodes. For Ag-coated tracks, the resistivity did not exceed 50. For non Ag-coated tracks, the resistivity was in the range 100-200.

(57) A non Ag-coated track was stretched and its resistivity was measured with a multimeter as a function of elongation. The stretching speed was 0.1 mm/s. The results are shown in FIG. 14. Stress increased linearly with elongation, as expected. Resistivity first increased with elongation, then slowly dropped again to reach a local minimum at about 40% elongation, before slowly rising again. The resistivity changed by less than a factor of 3 for an elongation range of 0-100%.

(58) Preliminary investigations of AC impedance showed that the impedance of the tested electrodes was in the range 2-50 k at a frequency of 1 kHz.

(59) Epidural and Subdural Implantation

(60) A SMEA 1 as described above was positioned on the spinal cord of a rat (FIG. 15) in the epidural or subdural space (FIG. 16). The advantages of epidural implantation are that epidural implantation is less invasive, technically simpler and less traumatic. However, the specificity of the epidural spinal cord stimulations is limited by the relative large distance between the electrodes and the neural elements. Neural activity has been recorded from the epidural space, but the obtained signals showed poor specificity due to the large distance between the electrode and the source of the neural signal. It is therefore desirable to position the electrodes closer to and more accurately from the targeted neural structures. This can be achieved by positioning the electrodes subdurally to reduce the distance between the electrode and the targeted neural elements. The presently proposed thin and flexible MEA allows the stable positioning of the electrodes close to spinal circuits and pathways while limiting the mechanical stress imposed on neural structures. In addition, the location of the MEA subdurally allows the delivery of drugs to cerebrospinal fluid through chemotrodes defined as microfluidic channels embedded within the MEA bypassing the blood-brain barrier.

(61) FIG. 17 illustrates an exemplary subdural implantation technique. To pass the electrode between the spinal cord 75 and vertebral column 73, rostral and caudal laminectomies 74 were made (i.e. incisions of dura mater spinalis were performed), exposing the spinal cord 75. Ethilon 4.0 suture 72 was used for guiding the SMEA 71 and pulling the SMEA into the subdural space between the spinal cord and the vertebral column. Multiple holes 76 were prepared in the SMEA 71 between the electrodes for fixation of the implant to the dura (FIG. 18(A)). Connective tissue 77 grows through the holes in 1-2 weeks after implantation and efficiently stabilizes the implant (FIG. 18(B)). In addition the holes maintained fluid circulation and thermo-regulation throughout the MEA.

(62) The positioning of the array subdurally allowed a markedly improved fixation of the array by the dura mater, which reduces the risk of migration over time. The close and accurate positioning of the electrodes close to targeted neuronal structures enabled delivery of more specific stimulations and improved recordings of neural activity. A stress-release loop placed intramuscular before entrance of the MEA under the vertebrae additionally saved the stable position of the implant on the cord.

(63) To improve biocompatibility the appropriate size, form and thickness of PDMS in MEA components (connector, release loop, electrode array) were chosen according to the different steps of the surgical procedure. Properties of the electrode array were further optimized on basis of feedback from in-vivo experiments, dissection and histological evaluation of the biological tissue around the implant. The materials did not adversely affect the integrity of tissue culture.

(64) Preliminary testing in rats with chronically implanted MEA over lumbosacral segments showed no sign of inflammation and preserved implant integrity two weeks after surgery. As early as one week after a complete spinal cord transection, EES applied at the various electrodes of the MEA could encourage continuous locomotion on the treadmill. For testing this, the rats were positioned over a treadmill. Drugs were first injected. Then electrical stimulation was applied to each electrode and the minimal current amplitude for which response in the hind limbs' muscles was observed (using EMG recordings) was determined as well as the specific pattern of muscle activation. The electrical stimulus that was applied had the following parameters: Monopolar stimulation between one epidural electrode and a common counter electrode located in the back. Bipolar stimulation between two epidural electrodes. Current amplitude ranging from 10 A up to 1 mA. Biphasic square pulse, cathodic first, 200 s up to 1 ms for each phase. Frequency between 20 and 100 Hz.

(65) The experiments showed that, by using single-site EES, paralyzed rats were able to walk. However the locomotion was not optimal meaning that there is a significant difference with a non-injured rat. In order to improve the locomotion of the paralyzed rats, multi-site EES was used. It had already been shown that simultaneous monopolar stimulations at two locations can improve the locomotion. By the help of the presently proposed SMEAs, it could be shown that it is possible to do even better by applying monopolar stimulations at different sites and at different moments. For example, after figuring out which electrode is responsible for the right leg flexion, electrical stimulation may be applied to that electrode only during the swing phase of the right limb. By doing so, the superiority of the new stimulation paradigm could be demonstrated, which is made possible by the presently proposed electrode arrays.