A rigid-flexible neural probe and its preparation method

Abstract

The present invention discloses a rigid-flexible neural probe and a preparation method thereof. At present, although flexible neural probes have smaller mechanical mismatch and tissue damage, they have problems of insufficient rigidity and inaccurate positioning during implantation. Although rigid probes can be implanted quickly and accurately, they have a large mechanical mismatch with tissues. The present invention proposes a method for preparing and converting a rigid-flexible neural probe having a rigid skeleton and a flexible substrate. By setting stress concentration points on the rigid skeleton, the rigid skeleton can be transformed into a rigid-flexible mechanical transformation in vivo under ultrasonic excitation. By utilizing the mechanical characteristics of the probe's rigidity before ultrasound and its flexibility after ultrasound, the probe can be quickly and accurately implanted in vivo and its mechanical mismatch with tissues can be reduced, thereby improving the safety and stability of implantable neural probes.

Description

Technical Field

[0001] The present invention belongs to the technical field of MEMS biosensors, and specifically relates to a rigid-flexible neural probe for neural signal recording and neural stimulation and a preparation method thereof; the neural probe is prepared on an SOI substrate by MEMS micromachining technology, and realizes rigid-flexible conversion in vivo by applying ultrasonic excitation. Background Art

[0002] In the 1970s, the University of Michigan in the United States achieved large-scale production of Michigan electrodes by introducing integrated circuit technology. Using planar photolithography technology, Michigan electrodes have achieved extremely high spatial resolution. However, the Michigan electrode has a high Young's modulus and poor biocompatibility, which makes its long-term implantation stability insufficient. In recent years, with the rapid development of flexible electronic technology, flexible neural microelectrode arrays based on polymer substrates have become a new research hotspot. Compared with silicon-based Michigan electrodes, polymer electrodes have a lower Young's modulus and can significantly reduce the risk of inflammatory response. Therefore, conducting microfabrication research on flexible neural microelectrode arrays is crucial to achieve high temporal and spatial resolution and long-term stable neural recordings.

[0003] In order to achieve the recording and stimulation of complex neural networks in multiple brain regions, it is necessary to significantly increase the number of channels and probes of flexible neural microelectrode arrays. As the number of probes increases, the rapid implantation and precise positioning of flexible neural microelectrode arrays in the brain become extremely difficult. Although the use of degradable materials such as polyethylene glycol (PEG) and silk protein to harden flexible electrodes can achieve assisted implantation, the thickness of the hardened coating will increase the implantation damage (Guan S., Wang J., Gu X., et al. Elastocapillary self-assembled neurotassels for stable neural activity recordings [J]. Science Advances, 2019, 5 (3): 2842. Xiang Z., Yen SC, Xue N., et al. Ultra-thin flexible polyimide neural probe embedded in a dissolvable maltose-coated microneedle [J]. Journal of Micromechanics & Microengineering, 2014, 24 (6): 065015.). In addition, the use of microneedles and surgical robots for assisted implantation also has the problems of high cost and poor flexibility (Hara SA, Kim BJ, Kuo J., et al. Long-term stability of intracortical recordings using perforated and arrayed Parylene sheathelectrodes[J]. Journal of Neural Engineering, 2016, 13(6): 066020. Zhao Z., Lan L., Wei X., et al. Nanoelectronic Coating Enabled Versatile Multifunctional Neural Probes[J]. Nano Letters, 2017, 17(8): 4588.). In order to overcome these shortcomings of flexible neural microelectrodes, it is urgent to develop a new rigid-flexible neural microelectrode system that can realize the rigid-flexible transformation of the probe under external excitation. By utilizing the mechanical characteristics of the probe's preoperative rigidity and postoperative flexibility, the purpose of rapid and accurate implantation and reduction of mechanical mismatch can be achieved at the same time. Summary of the invention

[0004] The purpose of the present invention is to address the defects in the prior art, use MEMS micromachining technology to realize the preparation of a rigid-flexible neural probe with a rigid skeleton and a flexible substrate, and use ultrasonic excitation to achieve rigid-flexible conversion after the probe is implanted.

[0005] A rigid-flexible neural probe in the present invention comprises a rigid skeleton and a flexible substrate. The flexible substrate comprises an upper insulating layer, a conductive layer and a lower insulating layer stacked in sequence. The conductive layer is provided with a plurality of electrode points and wires for leading out the electrode points. The rigid skeleton is arranged in the lower insulating layer; a plurality of stress concentration points are arranged on the rigid skeleton; when the rigid skeleton vibrates under the action of ultrasonic excitation, some or all of the stress concentration points are broken, so that the rigid skeleton is divided into multiple sections that are not connected to each other.

[0006] Preferably, the side of the rigid skeleton away from the conductive layer is flush with the back surface of the lower insulating layer and is exposed outside the lower insulating layer.

[0007] Preferably, the rigid skeleton is a protruding structure formed by deep silicon etching, and has a width of 1-80 microns and a thickness of 1-30 microns.

[0008] Preferably, the stress concentration point on the rigid skeleton is a notch that is larger on the outside and smaller on the inside;

[0009] Preferably, the rigid skeleton comprises an end connection segment and a plurality of branch segments arranged side by side and at intervals. The ends of each branch segment are connected to the end connection segment; the width of the branch segment is 1-80 microns and the thickness is 1-30 microns. Multiple stress concentration points aligned with each other are arranged on both side edges of each branch segment. The depth of the stress concentration point is less than or equal to 1 / 4 of the width of the branch segment.

[0010] Preferably, the flexible substrate has a width of 2-100 microns and a thickness of 1-50 microns.

[0011] Preferably, when in use, continuous ultrasonic excitation is applied to the cerebrospinal fluid during implantation in the brain, or instantaneous ultrasonic excitation is applied to the cerebrospinal fluid after implantation in a designated position; the portion of the rigid skeleton that enters the cerebrospinal fluid or brain tissue vibrates under the action of ultrasonic excitation and breaks at the locations of various stress concentration points, so that the portion of the probe implanted in the brain changes from a rigid structure to a flexible structure.

[0012] Preferably, the ultrasonic sound pressure range for causing the rigid skeleton entering the liquid to break at the stress concentration point is 0.1 MPa to 10 MPa.

[0013] Preferably, the electrode points include recording electrode points and stimulation electrode points for neuron regulation.

[0014] The method for preparing the rigid-flexible neural probe comprises the following steps:

[0015] (1) Clean and dry the SOI silicon wafer.

[0016] (2) Using photoresist as a mask, deep silicon etching technology is used to form a 5-10 micron thick silicon skeleton on the front side of the SOI silicon wafer.

[0017] (3) A 5-15 micron thick polymer film is deposited on the silicon skeleton as a lower insulating layer.

[0018] (4) A layer of metal is deposited on the surface of the polymer film using a magnetron sputtering system as a conductive layer. Subsequently, the conductive layer is patterned into electrode points, wires, and pads using photolithography and ion beam etching processes.

[0019] (5) A 2-5 μm thick polymer film is deposited on the conductive layer as an upper insulating layer.

[0020] (6) Use photolithography and reactive ion etching to remove the polymer film above the electrode points, pads and contour lines to form a conductive window and electrode contour.

[0021] (7) Use reactive ion etching to remove the silicon oxide on the front contour line of the probe.

[0022] (8) Use double-sided alignment lithography and reactive ion etching to pattern the silicon oxide on the back side of the SOI silicon wafer.

[0023] (9) Use photolithography and deep silicon etching to pattern the silicon on the back side of the SOI silicon wafer.

[0024] (10) Using the silicon oxide on the back of the SOI silicon wafer as a mask, deep silicon etching is used to thin the silicon on the back of the SOI silicon wafer to the buried oxide layer to obtain a rigid-flexible neural probe.

[0025] The present invention has the following beneficial effects:

[0026] 1. The present invention combines a rigid skeleton with a flexible substrate to enhance the rigidity of the flexible probe without increasing the cross-sectional area of ​​the probe. After the rigid skeleton is used to realize rapid and accurate implantation of the neural probe in the body, a method of applying ultrasonic excitation to the cerebrospinal fluid is provided to break the part of the rigid skeleton in the brain, so that the part of the probe in the brain becomes a flexible structure, thereby reducing the possibility of the brain being damaged by the probe.

[0027] 2. The present invention sets stress concentration points on the rigid skeleton to cause it to break at multiple designated locations under ultrasonic excitation, thereby restoring the flexibility of the probe. The rigid-flexible transformation after the probe is implanted can reduce the inflammatory response caused by mechanical mismatch between the probe and the tissue interface, thereby improving the stability of the implantable neural probe. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the overall structure of the rigid-flexible neural probe of the present invention;

[0029] Figure 2 Schematic diagram of two routes of rigid-flexible conversion of the rigid-flexible combined neural probe under ultrasonic excitation in the present invention;

[0030] Figure 3 The flowchart is a method for preparing a rigid-flexible neural probe in the present invention. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings.

[0032] like Figure 1 As shown, a rigid-flexible neural probe includes two parts, a rigid skeleton 4 and a flexible substrate. The flexible substrate includes an upper insulating layer 1, a conductive layer and a lower insulating layer 2 stacked in sequence from top to bottom. Among them, the conductive layer is provided with recording electrode points 5, stimulation electrode points 6 for neuron regulation, and wires 3 for connecting the electrode points and the rear-end pads. The rigid skeleton 4 is wrapped inside the lower insulating layer 2 of the flexible substrate, and a plurality of stress concentration points 7 arranged at equal intervals are provided on its edge. The stress concentration point 7 is specifically a triangular notch that is larger outside and smaller inside.

[0033] like Figure 1 As shown, the rigid skeleton 4 has a hollow grid structure, and its side away from the conductive layer is flush with the back surface of the lower insulating layer 2 and exposed outside the lower insulating layer 2. The rigid skeleton 4 is a protruding structure formed by deep silicon etching, and the height of the protruding structure formed after etching is the thickness of the rigid skeleton 4.

[0034] The rigid skeleton 4 includes an end connection segment and a plurality of branch segments arranged side by side and at intervals. The ends of each branch segment are connected to the end connection segment; the width of the branch segment is 1-80 microns and the thickness is 1-30 microns. Multiple stress concentration points 7 aligned with each other are arranged on both side edges of each branch segment. The depth of the stress concentration point 7 is less than or equal to 1 / 4 of the width of the branch segment. Each stress concentration point 7 forms a fragile area arranged in sequence along the length direction on the branch segment; when the external load on the rigid-flexible neural probe exceeds the threshold, the fragile areas on the branch segment will be the first to break. The fragile area can be actively broken by applying ultrasonic excitation.

[0035] The upper insulating layer 1 and the lower insulating layer 2 in the flexible substrate are both polymers formed by spin coating curing or vapor deposition. The width of the flexible substrate is 2-100 microns, and the thickness is 1-50 microns.

[0036] like Figure 2As shown, the working process of the rigid-flexible combined neural probe for rigid-flexible conversion is divided into two routes, which are as follows:

[0037] Route 1 (dynamic rigid-flexible transformation): an implantation port is opened at the location where the probe needs to be implanted in the head; the tip of the probe is inserted into the implantation port; the rigid skeleton 4 provides sufficient rigidity for the probe, which is convenient for doctors or staff to insert the probe inward, and enables the probe to pass through the cerebral cortex and enter the brain tissue; during the implantation of the probe, an ultrasonic probe is used to continuously apply ultrasonic excitation to the cerebrospinal fluid on the surface of the cerebral cortex; when the ultrasonic probe generates sufficient sound pressure (0.1MPa to 10MPa) at the "probe-cerebrospinal fluid" interface, the location of the stress concentration point 7 in the part of the probe implanted in the brain breaks under the vibration caused by ultrasound. As the implantation depth increases, the stress concentration points 7 that contact the cerebrospinal fluid break one by one. After the implantation is completed, the probe can realize the dynamic rigid-flexible transformation process. After the fracture, the rigid skeleton 4 loses its rigid support for the probe, so that the part of the probe that enters the brain becomes a flexible structure, thereby avoiding damage to the brain tissue by the probe while realizing electrical signal acquisition and electrical stimulation. In addition, ultrasound waves with a sound pressure lower than 10MPa will not cause damage to human brain tissue.

[0038] Route 2 (static rigid-flexible conversion): an implantation port is opened at the location where the probe needs to be implanted in the head; the tip of the probe is inserted into the implantation port; the rigid skeleton 4 provides sufficient rigidity for the probe, which is convenient for doctors or staff to insert the probe inward, and enables the probe to pass through the cerebral cortex and enter the brain tissue; after the probe is completely implanted in the predetermined brain area, an ultrasonic probe is used to apply instantaneous ultrasonic excitation to the cerebrospinal fluid on the surface of the cerebral cortex; the part of the probe that contacts the brain tissue vibrates with the ultrasonic excitation; when the ultrasonic probe generates sufficient sound pressure (0.1MPa to 10MPa) at the "probe-cerebrospinal fluid" interface, the stress concentration point 7 in the part of the probe implanted in the brain is broken under the vibration caused by ultrasound. After the ultrasound is completed, the probe can realize the static rigid-flexible conversion process. After the fracture, the rigid skeleton 4 loses its rigid support for the probe, so that the part of the probe that enters the brain becomes a flexible structure, thereby avoiding damage to the brain tissue by the probe while realizing electrical signal acquisition and electrical stimulation.

[0039] Example 1

[0040] like Figure 3 As shown, the method for preparing the rigid-flexible neural probe comprises the following steps:

[0041] (1) Using SOI silicon wafer (top silicon layer 5-10 microns, buried oxide layer 0.5-2 microns, bottom silicon layer 200-500 microns) as the substrate, the silicon wafer was ultrasonically cleaned in acetone, ethanol and deionized water for 5 minutes each. Then, it was dried with nitrogen and baked in an oven at 180°C for 3 hours.

[0042] (2) Spin-coat a 5-micron thick positive photoresist on the front of the SOI silicon wafer and pre-bake it, and use a photolithography machine to pattern the photoresist into a mask structure. Subsequently, use deep silicon etching technology to form a 5-10 micron thick silicon skeleton on the front of the SOI silicon wafer. Next, place the SOI silicon wafer in acetone to remove the glue and clean it in ethanol and deionized water. Finally, dry it on a hot plate and set it aside.

[0043] (3) Spin-coat a 5-15 μm thick PI film on the silicon skeleton and heat-cure it to serve as the lower insulating layer.

[0044] (4) A layer of chromium / gold (30 / 300nm) is deposited on the surface of the lower insulating layer using a magnetron sputtering system as a conductive layer. Subsequently, a 5-micron thick positive photoresist is spin-coated on the front of the SOI silicon wafer and pre-baked, and the photoresist is patterned into a mask structure using a photolithography machine. Next, the conductive layer is patterned using an ion beam etching process to form electrode points, wires, and pads. Then, the SOI silicon wafer is placed in acetone to remove the glue and cleaned in ethanol and deionized water in turn. Finally, it is dried on a hot plate and set aside.

[0045] (5) Spin-coat a 2-5 μm thick PI film on the conductive layer and heat and cure it to form an upper insulating layer.

[0046] (6) Spin-coat a 5-micron thick positive photoresist on the front of the SOI silicon wafer and pre-bake it, and use a photolithography machine to pattern the photoresist into a mask structure. Subsequently, use a reactive ion etching system to remove the PI above the electrode points, pads, and contour lines to form a conductive window and electrode contour. Next, place the SOI silicon wafer in acetone to remove the glue and clean it in ethanol and deionized water. Finally, dry it on a hot plate and set it aside.

[0047] (7) Reactive ion etching is used again to remove the silicon oxide on the front contour line of the probe.

[0048] (8) Spin-coat a 5 μm thick positive photoresist on the back of the SOI silicon wafer and pre-bake it. Use double-sided alignment photolithography to pattern the photoresist into a mask structure. Next, use reactive ion etching to pattern the silicon oxide on the back of the SOI silicon wafer. Subsequently, place the SOI silicon wafer in acetone to remove the glue and clean it in ethanol and deionized water. Finally, dry it on a hot plate and set it aside.

[0049] (9) Spin-coat a 5-micron thick positive photoresist on the back of the SOI silicon wafer and pre-bake it. Use double-sided alignment photolithography to pattern the photoresist into a mask structure. Next, use a deep silicon etching system to pattern the silicon on the back of the SOI silicon wafer. Subsequently, place the SOI silicon wafer in acetone to remove the glue and clean it in ethanol and deionized water. Finally, dry it on a hot plate and set it aside.

[0050] (10) Using the back side silicon oxide as a mask, a deep silicon etching system is used to thin the back side silicon to the buried oxide layer. Subsequently, the SOI silicon wafer is cleaned in acetone, ethanol and deionized water. Finally, it is dried on a hot plate and set aside.

[0051] Example 2

[0052] like Figure 3 As shown, the specific preparation method of the rigid-flexible neural probe includes the following steps:

[0053] (1) Using SOI silicon wafer (top silicon layer 5-10 microns, buried oxide layer 0.5-2 microns, bottom silicon layer 200-500 microns) as the substrate, the silicon wafer was ultrasonically cleaned in acetone, ethanol and deionized water for 5 minutes each. Then, it was dried with nitrogen and baked in an oven at 180°C for 3 hours.

[0054] (2) Spin-coat a 5-micron thick positive photoresist on the front of the SOI silicon wafer and pre-bake it, and use a photolithography machine to pattern the photoresist into a mask structure. Subsequently, use deep silicon etching technology to form a 5-10 micron thick silicon skeleton on the front of the SOI silicon wafer. Next, place the SOI silicon wafer in acetone to remove the glue and clean it in ethanol and deionized water. Finally, dry it on a hot plate and set it aside.

[0055] (3) Using a chemical vapor deposition system, a 5-15 μm thick parylene C (polyparaxylene) film is deposited on the silicon skeleton as a lower insulating layer.

[0056] (4) A layer of Ti (titanium) / Au (gold) is deposited on the surface of the lower insulating layer using a magnetron sputtering system as a conductive layer. Subsequently, a 5-micron thick positive photoresist is spin-coated on the front of the SOI silicon wafer and pre-baked, and the photoresist is patterned into a mask structure using a photolithography machine. Next, the conductive layer is patterned using a wet etching process to form electrode points, wires, and pads. Then, the SOI silicon wafer is placed in acetone to remove the glue and cleaned in ethanol and deionized water. Finally, it is dried on a hot plate and set aside.

[0057] (5) A 2-5 μm thick parylene C film is deposited on the conductive layer using a chemical vapor deposition system as an upper insulating layer.

[0058] (6) Spin-coat a 5-micron thick positive photoresist on the front of the SOI silicon wafer and pre-bake it, and use a photolithography machine to pattern the photoresist into a mask structure. Subsequently, use reactive ion etching to remove the parylene C film above the electrode points, pads, and contour lines to form a conductive window and electrode contour. Then, place the SOI silicon wafer in acetone to remove the glue and clean it in ethanol and deionized water. Finally, dry it on a hot plate and set it aside.

[0059] (7) Reactive ion etching is used again to remove the silicon oxide on the front contour line of the probe.

[0060] (8) Spin-coat a 5 μm thick positive photoresist on the back of the SOI silicon wafer and pre-bake it. Use double-sided alignment photolithography to pattern the photoresist into a mask structure. Next, use reactive ion etching to pattern the silicon oxide on the back of the SOI. Then, place the SOI silicon wafer in acetone to remove the glue and clean it in ethanol and deionized water. Finally, dry it on a hot plate and set it aside.

[0061] (9) Spin-coat a 5-micron thick positive photoresist on the back of the SOI silicon wafer and pre-bake it. Use double-sided alignment photolithography to pattern the photoresist into a mask structure. Next, use a deep silicon etching system to pattern the silicon on the back of the SOI silicon wafer. Then, place the SOI silicon wafer in acetone to remove the glue and clean it in ethanol and deionized water. Finally, dry it on a hot plate and set it aside.

[0062] (10) Using the back silicon oxide as a mask, use deep silicon etching to thin the back silicon to the buried oxide layer. Then, wash the SOI silicon wafer in acetone, ethanol and deionized water. Finally, dry it on a hot plate and set aside.

Claims

1. A rigid-flexible neural probe, comprising a rigid skeleton (4) and a flexible substrate; the flexible substrate comprises an upper insulating layer (1), a conductive layer and a lower insulating layer (2) stacked in sequence; wherein: The conductive layer is provided with a plurality of electrode points and wires (3) for leading out the electrode points; the characteristics are: the rigid frame (4) is arranged in the lower insulating layer (2); a plurality of stress concentration points (7) are arranged on the rigid frame (4); when the rigid frame (4) vibrates under the action of ultrasonic excitation, some or all of the positions where the stress concentration points (7) are located are broken, so that the rigid frame (4) is divided into a plurality of sections that are not connected to each other; The rigid skeleton (4) is a protruding structure formed by deep silicon etching; the width of the rigid skeleton (4) is 1-80 microns and the thickness is 1-30 microns; the stress concentration point (7) on the rigid skeleton (4) is a notch that is larger on the outside and smaller on the inside; the rigid skeleton (4) includes an end connection section and a plurality of branch sections arranged side by side and at intervals; the ends of each branch section are connected to the end connection section; the width of the branch section is 1-80 microns and the thickness is 1-30 microns; the edges of both sides of each branch section are provided with a plurality of stress concentration points (7) aligned with each other; the depth of the stress concentration point (7) is less than or equal to 1 / 4 of the width of the branch section; the ultrasonic sound pressure range for the rigid skeleton (4) entering the liquid to break at the stress concentration point (7) is 0.1 MPa to 10 MPa.

2. The rigid-flexible neural probe according to claim 1, characterized in that: The side of the rigid skeleton (4) away from the conductive layer is flush with the back surface of the lower insulating layer (2) and is exposed outside the lower insulating layer (2).

3. The rigid-flexible neural probe according to claim 1, characterized in that: The flexible substrate has a width of 2-100 microns and a thickness of 1-50 microns.

4. The rigid-flexible neural probe according to claim 1, characterized in that: When in use, continuous ultrasonic excitation is applied to the cerebrospinal fluid during implantation in the brain, or instantaneous ultrasonic excitation is applied to the cerebrospinal fluid after implantation in a designated position; the portion of the rigid skeleton (4) that enters the cerebrospinal fluid or brain tissue vibrates under the action of ultrasonic excitation and breaks at the locations of the stress concentration points (7), so that the portion of the probe implanted in the brain changes from a rigid structure to a flexible structure.

5. The rigid-flexible neural probe according to claim 1, characterized in that: The electrode points include recording electrode points (5) and stimulation electrode points (6) for neuron regulation.

6. A method for preparing a rigid-flexible neural probe according to any one of claims 1 to 5, characterized in that: The steps include: (1) Clean and dry the SOI silicon wafer; (2) Using photoresist as a mask, deep silicon etching technology is used to form a 5-10 micron thick silicon skeleton on the front side of the SOI silicon wafer; (3) Depositing a 5-15 μm thick polymer film on the silicon skeleton as a lower insulating layer; (4) Using a magnetron sputtering system to deposit a layer of metal on the surface of the polymer film as a conductive layer; Subsequently, the conductive layer is patterned into electrode points, wires, and pads using photolithography and ion beam etching processes; (5) depositing a 2-5 μm thick polymer film on the conductive layer as an upper insulating layer; (6) Using photolithography and reactive ion etching to remove the polymer film above the electrode points, pads and contour lines to form a conductive window and electrode contour; (7) Reactive ion etching is used to remove silicon oxide on the front contour of the probe; (8) Patterning the silicon oxide on the back side of the SOI wafer using double-sided alignment lithography and reactive ion etching; (9) Patterning the silicon on the back of the SOI wafer using photolithography and deep silicon etching; (10) Using the silicon oxide on the back of the SOI silicon wafer as a mask, deep silicon etching is used to thin the silicon on the back of the SOI silicon wafer to the buried oxide layer to obtain a rigid-flexible neural probe.

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