A flexible optical genetic probe based on SU-8 waveguide and a preparation method thereof

Abstract

The application discloses a preparation method of a flexible optical genetic probe based on an SU-8 waveguide, which comprises the following steps: preparing an SU-8 waveguide on a hard substrate with a deposited sacrificial layer through a photoetching process, immersing the waveguide chip into a sacrificial layer etching solution to partially release the SU-8 waveguide, fixing one end of the SU-8 waveguide in a groove of a packaging platform, so that the released SU-8 waveguide extends outside the groove, placing an optical fiber at the other end of the packaging platform, so that the core of the optical fiber is coupled with the SU-8 waveguide, dropping ultraviolet curing glue to immerse the optical fiber and the SU-8 waveguide in the groove, and curing the ultraviolet curing glue by using an ultraviolet lamp, and finally filling the whole groove with opaque black glue to obtain the flexible optical genetic probe based on the SU-8 waveguide. The probe has a small inflammatory reaction to a living body in a long-term implantation process, and through the design of the waveguide size, the emergent light of the probe can satisfy the irradiation of certain brain regions, so that the intervention on the behavior of the living body is realized. The process flow of the application is simple, and the preparation efficiency of the probe is high.

Description

Technical Field

[0001] This invention relates to the field of micro-nano photonic device fabrication, specifically to a flexible optogenetic probe based on an SU-8 waveguide and its fabrication method. Background Technology

[0002] Optogenetic probes are micro / nano photonic devices that can transmit light to the vicinity of target neurons within biological tissues, serving as brain-computer interfaces. Implantable flexible optogenetic probes overcome the problems of poor biocompatibility and tissue damage associated with traditional rigid probes, effectively reducing immune responses and tissue inflammation, and improving the stability and durability of neural interfaces. This provides crucial technical support for fields such as human rehabilitation, treatment of neurological diseases, brain science research, and human-computer interaction. Waveguide-integrated optogenetic probes, benefiting from rapidly developing micro / nano fabrication technologies, possess excellent optical control capabilities. The probe uses a waveguide as the light-guiding pathway. Due to the inherent flexibility of polymer materials, transparent polymers with good biocompatibility in the visible light band, such as Parylene C, polyimide, polymethyl methacrylate, and SU-8, can be selected to fabricate the waveguide. Current flexible polymer waveguide integrated optogenetic probes mostly use Parylene C (Young's modulus of 1.5-4 GPa) (Microsystems & Nanoengineering, 2020, 6, 85) as the core layer and polydimethylsiloxane (Young's modulus of 1.32-2.97 MPa) (Microsystems & Nanoengineering, 2020, 6, 85) as the cladding. The Young's modulus of the cladding and the relatively small size suitable for implantation result in low bending stiffness of the probe. When implanted into brain tissue, the flexible probe will bend, making it impossible to implant directly and accurately. A rigid implantation guide device is required.

[0003] The Young's modulus of SU-8 is approximately 2 GPa (Microchem, SU-8 2000 series), which is less than that of hard probes (Young's modulus is 130-170 GPa) (Journal of Microelectromechanical Systems, 2010, 19, 229-238) but greater than that of polydimethylsiloxane. As a flexible waveguide, SU-8 can remain horizontal within a certain aspect ratio range without bending. At suitable implantation sizes, the bending stiffness of the probe meets the requirements for direct and accurate implantation into brain tissue. SU-8 is a negative photoresist with strong adhesion to the substrate, exhibiting good mechanical, thermal, and chemical stability and biocompatibility. SU-8 waveguides can be directly fabricated using a one-step photolithography process. Combined with a transfer method based on etching a silicon dioxide sacrificial layer, flexible SU-8 waveguides can be easily fabricated. From the perspective of fabrication process, the flexible waveguide integrated SU-8 probe reduces one step in the patterning process, two steps in the dry etching process, and one step in the resist removal process compared to the hard SU-8 probe. Furthermore, it eliminates the need for deep reactive ion etching to remove the back silicon, significantly reducing the complexity and cost of the probe process. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a method for fabricating a flexible optogenetic probe based on an SU-8 waveguide. SU-8 is used as the waveguide material, and the waveguide is partially peeled off from a rigid substrate via hydrofluoric acid release, releasing the flexible waveguide probe. The probe is then encapsulated by end-face coupling with an optical fiber and a waveguide chip to obtain the flexible optogenetic probe. Compared to the previously reported Parylene C flexible waveguide integrated probe, the fabrication technology based on the SU-8 waveguide is less complex, has a shorter fabrication time, higher probe fabrication efficiency, and lower waveguide optical transmission loss. Because the probe employs a multimode waveguide design, it can support the transmission of multiple wavelengths of light, resulting in a wide spectral range and facilitating multi-wavelength optical stimulation, thereby enabling simultaneous activation and inhibition of neurons.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] On the one hand, a method for fabricating a flexible optogenetic probe based on an SU-8 waveguide is provided, the method comprising the following steps:

[0007] Step 1: On a hard substrate with a sacrificial layer deposited, an SU-8 waveguide is fabricated using photolithography to obtain a waveguide chip.

[0008] (1.1) When the hard substrate itself has a sacrificial layer, execute (1.2) directly; otherwise, a sacrificial layer needs to be deposited on the hard substrate first.

[0009] (1.2) Spin-coating a certain thickness of SU-8 photoresist onto the sacrificial layer.

[0010] (1.3) SU-8 waveguides are fabricated on the SU-8 photoresist layer using photolithography to obtain waveguide chips.

[0011] Step 2: Immerse the waveguide chip in the sacrificial layer etching solution to partially release the SU-8 waveguide.

[0012] (2.1) Immerse the waveguide chip in the sacrificial layer etching solution to partially release the SU-8 waveguide;

[0013] (2.2) Disassemble the SU-8 waveguide portion at the end of the waveguide chip that is not immersed in the sacrificial layer etching solution to obtain a clean and flat SU-8 waveguide end face.

[0014] Step 3: Fix the released waveguide chip of the SU-8 waveguide to one end of the groove in the rigid slot packaging platform, so that the released SU-8 waveguide extends out of the groove; place an optical fiber at the other end of the rigid slot packaging platform so that the fiber core is coupled to the SU-8 waveguide; then drop UV curing adhesive to immerse the optical fiber and SU-8 waveguide in the platform groove, and cure with a UV lamp to ensure the stability of the coupling state;

[0015] Step 4: Fill the entire groove with opaque black glue to enhance the integration of the packaged device and block the light reflected between the optical fiber and the waveguide chip, thus obtaining a flexible optical genetic probe based on the SU-8 waveguide.

[0016] Furthermore, the sacrificial layer material is silicon dioxide; the sacrificial layer etching solution is selected as hydrofluoric acid solution.

[0017] Furthermore, the sacrificial layer material is germanium dioxide, and the sacrificial layer etching solution is water or hydrogen peroxide solution.

[0018] Furthermore, to facilitate the stripping process, the thickness of the sacrificial layer is 500-5000 nm. The thickness of the SU-8 photoresist used to fabricate the SU-8 waveguide is not less than 0.5 μm. To fabricate a relatively large SU-8 waveguide cross-section and reduce the difficulty of fiber coupling, the width of the SU-8 waveguide is not less than 50 μm.

[0019] Furthermore, in order to enhance the control of the reaction between the sacrificial layer etchant and the sacrificial layer, achieve partial release of the flexible probe, and avoid the probe from completely detaching from the hard substrate, the volume fraction of the hydrofluoric acid solution is approximately 10-30%, the immersion depth of the waveguide chip is 5% to 30% of the length of the SU-8 waveguide, and the immersion time is 30-120 seconds.

[0020] The concentration of the hydrogen peroxide solution is 5%-30%, and the immersion depth of the waveguide chip is 5%-30% of the length of the SU-8 waveguide, with an immersion time of 3-10 minutes; for water, the immersion time is 20-30 minutes.

[0021] Furthermore, since the packaging platform is not implanted into a biological body in optogenetic experiments and does not require flexibility, to ensure that the waveguide chip and optical fiber remain relatively fixed in position within the packaging platform structure after end-face coupling is achieved, thus preventing changes in the coupling state, and to prevent light leakage to the platform where the waveguide chip is placed, the grooved packaging platform needs to be a rigid, opaque groove structure made of an opaque, biocompatible material. This rigid groove structure can be fabricated from polymer materials using laser engraving, 3D printing, or chemical etching; alternatively, it can be fabricated from bioceramic materials using CNC machine tools or laser cutting machines followed by sintering; or it can be obtained by laser cutting or chemical etching of silicon materials.

[0022] Furthermore, in order to ensure that the adhesive fixing the waveguide chip does not melt due to the temperature rise during probe operation and to maintain the bonding state when the working environment temperature is high, an adhesive with thermosetting properties made of any one of epoxy resin, polyurethane, phenolic resin, acrylic resin, or polyimide is used to fix the waveguide chip released from the SU-8 waveguide part to one end of the groove in the packaging platform with groove.

[0023] Furthermore, in order to ensure that the optical properties of the adhesive in the fixed coupling state do not affect the light transmission and that rapid curing can be achieved under artificial control to avoid changes in the coupling state over time, the UV-curable adhesive is selected from any one of aromatic amine adhesives (e.g., p-formylaniline), silicone-based adhesives (e.g., organosilicon composite UV curing agents), acrylic and its derivative adhesives, epoxy adhesives (e.g., epoxy resins), and polyurethane adhesives.

[0024] Furthermore, there is a certain amount of reflected light between the chip and the optical fiber. This light will leak into the transparent UV-curable adhesive. In order to prevent this light from illuminating non-target tissues after the probe is implanted in the body, and in order to further enhance the integration of the chip, platform and optical fiber, the opaque black adhesive is selected from any one of the following: acrylic adhesive doped with carbon black pigment, black epoxy resin adhesive, silicone adhesive with mixed dyes, and polyurethane adhesive.

[0025] On the other hand, a flexible optical genetic probe based on SU-8 waveguide is provided, including a rigid groove structure packaging platform, a partially released SU-8 waveguide chip, and an optical fiber;

[0026] The unreleased end of the SU-8 waveguide on the SU-8 waveguide chip is fixed to one end of the groove in the packaging platform by thermosetting adhesive, and the released end of the SU-8 waveguide extends out of the packaging platform as the implantation end;

[0027] The optical fiber is located at the other end of the slot in the packaging platform and is coupled to the SU-8 waveguide; the coupling state of the SU-8 waveguide and the optical fiber is fixed by UV-curing adhesive.

[0028] The groove of the packaging platform is filled with opaque black glue that blocks light reflected between the optical fiber and the waveguide chip.

[0029] The encapsulation platform is made of opaque, biocompatible material.

[0030] The beneficial effects of this invention are as follows:

[0031] (1) In the probe preparation method of this invention, a sacrificial layer is used to partially release the waveguide, resulting in a flexible optogenetic probe. The material for implanting the waveguide probe is SU-8, which has a lower Young's modulus and bending stiffness than rigid silicon-based probes, resulting in better biocompatibility, less inflammatory response, and less damage to biological tissues during long-term implantation as the rigid encapsulation platform is not implanted in the biological body. Its Young's modulus is much greater than that of the ultra-flexible material polydimethylsiloxane, resulting in greater bending stiffness when implanting small-sized device structures like probes. This makes it easier to directly penetrate brain tissue during implantation, achieving relatively precise implantation without the need for rigid implantation aids.

[0032] (2) The probe has simple process steps, low process difficulty, short process cycle and high device fabrication efficiency, which can serve as a reference for the fabrication of other flexible photonic devices. Attached Figure Description

[0033] Figure 1 This is a process flow diagram of the fabrication method of the flexible optogenetic probe based on the SU-8 waveguide of the present invention.

[0034] Figure 2 This is a schematic diagram of the finished product of the flexible optogenetic probe based on the SU-8 waveguide of the present invention. Detailed Implementation

[0035] The present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. The purpose and effects of the present invention will become clearer. It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0036] This invention proposes a flexible optogenetic probe based on an SU-8 waveguide and its fabrication method. The method involves etching a sacrificial layer with a sacrificial etchant to release a portion of the SU-8 waveguide, fixing the waveguide chip in a groove of a slot-shaped rigid packaging platform, end-face coupling the waveguide on the chip with optical fiber, and then encapsulating it to achieve the fabrication of a complete flexible optogenetic probe. The overall process flow diagram of this fabrication method is shown below. Figure 1 Specifically, the steps include the following:

[0037] Step 1: Deposit a sacrificial layer on the hard substrate; if the hard substrate itself has a sacrificial layer, proceed directly to Step 2.

[0038] The hard substrate here can be a silicon wafer. On the silicon wafer, a silicon dioxide thin film can be prepared as a sacrificial layer using techniques such as magnetron sputtering, thermal oxidation, plasma-enhanced chemical vapor deposition, and atomic layer deposition. The thickness of the sacrificial layer is 500-5000 nm.

[0039] Step 2: Spin-coat a certain thickness of SU-8 photoresist onto the sacrificial layer.

[0040] In this step, based on the commonly used SU-8 photoresist's coating thickness, the thickness of the SU-8 photoresist is selected as 0.5-50μm.

[0041] Step 3: Fabricate the SU-8 waveguide on the photoresist layer using photolithography.

[0042] In this step, the SU-8 photoresist is exposed to light through a photolithography machine, causing cross-linking. After baking and development, only the cross-linked photoresist remains, forming the final waveguide pattern. The pattern width of the transparent portion of the selected mask is 50-500 μm, resulting in a relatively large final waveguide cross-sectional size, which facilitates easier coupling.

[0043] Step 4: Immerse one end of the waveguide chip obtained in Step 3 into the sacrificial layer etching solution to partially release the SU-8 waveguide.

[0044] In this step, only the waveguide chip is placed in the container, and sacrificial layer etching solution is dripped into the container. The immersion depth of the waveguide chip is 5% to 30% of the length of the SU-8 waveguide, and the immersion time is 30-120 seconds.

[0045] Step 5: Disassemble the SU-8 waveguide portion at the end of the waveguide chip that is not immersed in the sacrificial layer etching solution to obtain a clean and flat SU-8 waveguide end face.

[0046] In this step, before dissociation, the waveguide end face is located in the middle of the chip. Therefore, taking the side of the SU-8 waveguide that is not immersed in hydrofluoric acid solution as a reference, a part of the hard chip is cut off parallel to this side. The resulting SU-8 waveguide end face is clean and flat, and easy to couple.

[0047] Secondly, if there are multiple waveguides on the waveguide chip, the waveguide chip can be further divided, and the final single chip will have only a single waveguide.

[0048] Step Six: To ensure that the adhesive fixing the waveguide chip does not melt due to the temperature rise during probe operation and to maintain the bonding state at high operating temperatures, a thermosetting adhesive made of any one of epoxy resin, polyurethane, phenolic resin, acrylic resin, or polyimide is used to fix the waveguide chip released from the SU-8 waveguide portion to one end of the groove in the packaging platform with grooves, so that the released SU-8 waveguide extends out of the groove.

[0049] Step 7: Place an optical fiber at the other end of the rigid groove encapsulation platform so that the fiber core is coupled to the SU-8 waveguide; then drop UV curing adhesive into the platform groove to immerse the optical fiber and SU-8 waveguide, and cure with a UV lamp to ensure the stability of the coupling state.

[0050] In this step, the result of coupling in step seven is fixed by dripping glue that can cure quickly under ultraviolet light and then irradiating it with ultraviolet light. The coupling state may change during the curing process, and the relative positions of the optical fiber and waveguide end face should be adjusted in time.

[0051] Secondly, UV-curable adhesives that are transparent in the visible light band can include aromatic amine adhesives such as p-formylaniline, silicone-based adhesives such as organosilicon composite UV curing agents, acrylic and its derivative adhesives, epoxy adhesives such as epoxy resins, polyurethane adhesives, etc.

[0052] Step 8: Fill the entire groove with opaque black glue to enhance the integration of the packaged device and block the light reflected between the optical fiber and the waveguide chip, thus obtaining a flexible optical genetic probe based on the SU-8 waveguide.

[0053] In this step, the opaque black adhesive can be made of acrylic adhesives containing carbon black pigments, black epoxy resin adhesives, silicone adhesives mixed with dyes, and polyurethane adhesives.

[0054] Example 1

[0055] 2500 nm silicon dioxide was deposited on a silicon wafer using a thermal oxidation method. A 50 μm thick SU-8 layer was spin-coated onto the silicon dioxide layer, and a 400 μm wide SU-8 waveguide was obtained through photolithography. The waveguide chip was immersed in a 10% (v / v) hydrofluoric acid solution to a depth of 5 mm for 120 s to remove the silicon dioxide from the immersed portion of the chip. The hard substrate portion etched by the hydrofluoric acid solution was cut away, releasing the flexible SU-8 waveguide. The portion of the chip not immersed in hydrofluoric acid was then cut away, exposing the clean rear end face of the SU-8 waveguide. The waveguide chip was placed in a platform groove based on bioceramic material, obtained through laser cutting, with the flexible portion of the probe suspended outside the platform. The waveguide chip was fixed to the platform using polyurethane thermosetting adhesive. An optical fiber was placed at the rear end of the platform and end-face coupled to the rear waveguide of the waveguide chip. UV-curable p-formyl aniline adhesive is dropped into a tank, and acrylic adhesive doped with carbon black pigment is applied on top of the UV-curable adhesive to obtain a flexible optogenetic probe based on the SU-8 waveguide. Figure 2 As shown.

[0056] Example 2

[0057] 1500 nm germanium dioxide was deposited on a silicon wafer using thermal oxidation. A 30 μm thick SU-8 layer was spin-coated onto the germanium dioxide layer, and a 250 μm wide SU-8 waveguide was obtained through photolithography. The waveguide chip was immersed in a 25% hydrogen peroxide solution to a depth of 3 mm for 300 s to remove the germanium dioxide from the immersed portion of the chip. The hard substrate portion etched by the hydrogen peroxide solution was cut away, releasing the flexible SU-8 waveguide. The portion of the chip not immersed in hydrogen peroxide was then cut away, exposing the clean rear end face of the SU-8 waveguide. The waveguide chip was placed in a polymer hard platform groove obtained by laser engraving, with the flexible portion of the probe suspended outside the platform. The waveguide chip was fixed to the platform using epoxy thermosetting adhesive. An optical fiber was placed at the rear end of the platform and end-face coupled to the rear waveguide of the waveguide chip. By dripping UV-curable silicone composite photocuring agent into the tank and applying silicone adhesive mixed with black dye on top of the UV-curable adhesive, a flexible photogenetic probe based on the SU-8 waveguide can be obtained.

[0058] It will be understood by those skilled in the art that the above descriptions are merely preferred examples of the invention and are not intended to limit the invention. Although the invention has been described in detail with reference to the foregoing examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. All modifications and equivalent substitutions made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for fabricating a flexible optogenetic probe based on an SU-8 waveguide, characterized in that, The method includes the following steps: Step 1: On a hard substrate with a sacrificial layer deposited, an SU-8 waveguide is fabricated using photolithography to obtain a waveguide chip; Step 2: Immerse the waveguide chip in the sacrificial layer etching solution to partially release the SU-8 waveguide; Step 2 includes the following sub-steps: (2.1) Immerse the waveguide chip in the sacrificial layer etching solution to partially release the SU-8 waveguide; the immersion depth of the waveguide chip is 5%~30% of the length of the SU-8 waveguide; (2.2) Disassemble the SU-8 waveguide portion at the end of the waveguide chip that is not immersed in the sacrificial layer etching solution to obtain a clean and flat SU-8 waveguide end face; Step 3: Fix the released waveguide chip of the SU-8 waveguide to one end of the groove in the rigid slot packaging platform, so that the released SU-8 waveguide extends out of the groove; place an optical fiber at the other end of the rigid slot packaging platform so that the fiber core is coupled to the SU-8 waveguide; then drop UV curing adhesive to immerse the optical fiber and SU-8 waveguide in the platform groove, and cure with a UV lamp to ensure the stability of the coupling state; Step 4: Fill the entire groove with opaque black glue to enhance the integration of the packaged device and block the light reflected between the optical fiber and the waveguide chip, thus obtaining a flexible optical genetic probe based on the SU-8 waveguide. The thickness of the sacrificial layer is 500-5000 nm; the thickness of the SU-8 photoresist used to fabricate the SU-8 waveguide is not less than 0.5 μm, and the width of the SU-8 waveguide is not less than 50 μm.

2. The method for fabricating a flexible optogenetic probe based on an SU-8 waveguide according to claim 1, characterized in that, The sacrificial layer material is silicon dioxide; the sacrificial layer etching solution is hydrofluoric acid solution.

3. The method for fabricating a flexible optogenetic probe based on an SU-8 waveguide according to claim 1, characterized in that, The sacrificial layer material is germanium dioxide, and the sacrificial layer etching solution is water or hydrogen peroxide solution.

4. The method for fabricating a flexible optogenetic probe based on an SU-8 waveguide according to claim 2, characterized in that, The hydrofluoric acid solution has a volume fraction of 10-30%, and the waveguide chip is immersed for 30-120 seconds.

5. The method for fabricating a flexible optogenetic probe based on an SU-8 waveguide according to claim 3, characterized in that, The concentration of the hydrogen peroxide solution is 5%-30%, and the immersion time of the waveguide chip is 3-10 min; the immersion time for water is 20-30 min.

6. The method for fabricating a flexible optogenetic probe based on an SU-8 waveguide according to claim 1, characterized in that, The grooved encapsulation platform is a rigid groove structure made of opaque, biocompatible material.

7. The method for fabricating a flexible optogenetic probe based on an SU-8 waveguide according to claim 1, characterized in that, A thermosetting adhesive made of any one of epoxy resin, polyurethane, phenolic resin, acrylic resin, or polyimide is used to fix the waveguide chip released from the SU-8 waveguide section to one end of the groove in the rigid slot packaging platform. The UV-curable adhesive is selected from any one of aromatic amine adhesives, silicone adhesives, acrylic and its derivative adhesives, epoxy adhesives, and polyurethane adhesives. The opaque black adhesive is selected from any one of the following: acrylic adhesives containing carbon black pigment, black epoxy resin adhesives, silicone adhesives with mixed dyes, and polyurethane adhesives.

8. A flexible optogenetic probe based on an SU-8 waveguide, characterized in that, The flexible optical genetic probe is prepared by the method for preparing a flexible optical genetic probe based on an SU-8 waveguide as described in any one of claims 1 to 7; the flexible optical genetic probe includes a rigid groove-type packaging platform, an SU-8 waveguide chip partially released from the SU-8 waveguide, and an optical fiber; The unreleased end of the SU-8 waveguide on the SU-8 waveguide chip is fixed to one end of the groove in the packaging platform by thermosetting adhesive, and the released end of the SU-8 waveguide extends out of the packaging platform as the implantation end; The optical fiber is located at the other end of the slot in the packaging platform and is coupled to the SU-8 waveguide; the coupling state of the SU-8 waveguide and the optical fiber is fixed by UV-curing adhesive. The groove of the packaging platform is filled with opaque black glue that blocks light reflected between the optical fiber and the waveguide chip. The encapsulation platform is made of opaque, biocompatible material.

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