Flexible non-invasive implantable electrode featured by conductive through-holes and enhanced interfacial stability

Abstract

The current invention relates to a flexible electrode comprising a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface; at least one through hole in the porous flexible substrate material, where the at least one through hole extends from the first surface to the second surface, the at least one through hole having an internal surface; a conductive material attached to the first and second surfaces of the porous flexible substrate material, wherein: the at least one through hole is formed such that a first entry hole, having a first entry hole diameter, in the first surface has a larger diameter than a corresponding second entry hole in the second surface, or vice versa, such that the internal surface of the through hole is tapered; the conductive material is also attached to the internal surface of the at least one through hole to provide a continuous conductive film attached to the first and second surfaces, where the first and second surfaces are conductively connected by the conductive material in the at least one through hole; and the conductive material penetrates into the porous flexible substrate to form a mechanical interlock between the conductive material and the porous flexible substrate.

Description

[0001] FLEXIBLE NON-INVASIVE IMPLANTABLE ELECTRODE FEATURED BY CONDUCTIVE THROUGH-HOLES AND ENHANCED INTERFACIAL STABILITY

[0002] Field of Invention

[0003] The current invention relates to flexible electrodes and methods of forming the flexible electrodes.

[0004] Background

[0005] The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0006] The development of implantable and flexible electronic devices necessitates the use of materials and fabrication techniques that adhere to stringent criteria for biocompatibility, electrical performance, and mechanical resilience. Traditional electrodes on polyimide (PI) substrates often employ copper, which lacks sufficient biocompatibility and encounters deposition challenges. Also, for achieving a high-density electrode array for implantable application, conductive through-holes are essential but challenging due to insufficient metal deposition at the edges of through-holes by traditional methodologies.

[0007] Besides, creating reliable electrode interfaces on substrates such as PI poses significant challenges. These challenges mainly stem from issues related to electrode stability and effective conductivity, particularly when the device is subjected to mechanical stresses like bending.

[0008] It is apparent that there is a need to address the limitations of conventional electrode manufacturing processes.

[0009] Summary of Invention

[0010] The current invention relates to an innovative electrode fabrication method using platinum (Pt) on PI substrates, featuring a novel tapered through-hole design to enhance the continuity and uniformity of Pt deposition, thus addressing the limitations of conventional electrode manufacturing processes. It has been surprisingly found that with a porous surface of PI film, an interlocking mechanism can be realized when depositing Pt, thus improving the robustness and electrical performance of electrodes on flexible substrates. Aspects and embodiments of the invention will now be described by reference to the following numbered clauses.

[0011] 1 . A flexible electrode comprising : a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface; at least one through hole in the porous flexible substrate material, where the at least one through hole extends from the first surface to the second surface, the at least one through hole having an internal surface; a conductive material attached to the first and second surfaces of the porous flexible substrate material, wherein: the at least one through hole is formed such that a first entry hole, having a first entry hole diameter, in the first surface has a larger diameter than a corresponding second entry hole in the second surface, or vice versa, such that the internal surface of the through hole has a tapered configuration; the conductive material is also attached to the internal surface of the at least one through hole to provide a continuous conductive film attached to the first and second surfaces, where the first and second surfaces are conductively connected by the conductive material in the at least one through hole; and the conductive material penetrates into the porous flexible substrate to form a mechanical interlock between the conductive material and the porous flexible substrate.

[0012] 2. The flexible electrode according to Clause 1 , wherein the porous flexible substrate material has a plurality of pores having a diameter of from 5 nm to 5 pm, such as from 10 nm to 3 pm, such as from 20 nm to 2 pm.

[0013] 3. The flexible electrode according to Clause 1 or Clause 2, wherein the continuous conductive film has a thickness of from 0.1 to 3 pm, such as from 1 to 2 pm, such as about 1.5 pm.

[0014] 4. The flexible electrode according to any one of the preceding clauses, wherein the continuous conductive film can maintain conductive in-plane and out of plane conductivity after being subjected to scraping by a suitable implement (e.g. metallic tweezers).

[0015] 5. The flexible electrode according to any one of the preceding clauses, wherein the flexible electrode exhibits a water contact angle after 30 minutes that is less than or equal to the water contact angle of the water contact angle of the porous flexible substrate material immediately following contact with water.

[0016] 6. The flexible electrode according to Clause 5, wherein the water contact angle after 30 minutes is from 30 to 90s, such as from 65 to 85s, such as from 70 to 80e, such as about 72.1s.

[0017] 7. The flexible electrode according to any one of the preceding clauses, wherein the first entry hole and second entry hole of the at least one through hole have a first edge and a second edge respectively, where the conductive material is distributed substantially homogeneously over: the first surface and the second surface of the porous flexible substrate; and the first edge, the second edge, and the internal surface of the at least one through hole, thereby providing a continuous conductive film.

[0018] 8. The flexible electrode according to any one of the preceding clauses, wherein the conductive material is a metal, optionally wherein the metal is selected from one or more of the group consisting of tungsten, molybdenum, magnesium, titanium, and, more particularly, gold and platinum.

[0019] 9. The flexible electrode according to Clause 8, wherein the conductive material is gold or, more particularly, platinum.

[0020] 10. The flexible electrode according to any one of the preceding clauses, wherein porous flexible substrate material is selected from one or more of the group consisting of a silicone rubber (e.g. a polydimethylsiloxane (PDMS)), a styrene-ethylene-butylene-styrene (SEBS) copolymer, a styrene-butadiene-styrene (SBS) copolymer, a styrene-isoprene-styrene (SIS) copolymer, a polyurethane (PU), a polyamide (e.g. nylon), a polyvinyl alcohol (PVA), a polyethylene terephthalate (PET), and a polyimide (PI).

[0021] 11 . The flexible electrode according to Clause 8, wherein the porous flexible substrate material is a polyimide (PI).

[0022] 12. The flexible electrode according to any one of the preceding clauses, wherein the first entry hole has a diameter of from greater than 50 to 300 pm and the second entry hole has a diameter of from 100 to less than 300 pm, or vice versa, provided that the first entry hole diameter is larger than the second entry hole diameter or vice versa. 13. The flexible electrode according to any one of the preceding clauses, wherein the internal surface of the at least one through hole defines a truncated conical shape.

[0023] 14. The flexible electrode according to any one of the preceding clauses, wherein the flexible electrode has one or more of the following properties:

[0024] (I) a sheet resistance of from 0.1 to 10 Q / square, such as from 0.5 to 5 10 Q / square, such as about 1 Q / square;

[0025] (ii) a resistivity value of from 0.01 to 0.5 mQ / cm, such as from 0.05 to 0.3 mO / cm, such as from 0.1 to 0.2 mQ / cm, such as about 0.15 mQ / cm; and

[0026] (iii) a conductivity of from 3 to 10 x105S / m, such as from 4 to 8 x105S / m, such as from 5 to 7 x105S / m, such as about 6.7 x105S / m.

[0027] 15. A method of forming a flexible electrode as described in any one of Clauses 1 to 14, the method comprising:

[0028] (a) providing a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface, the porous flexible substrate further comprising at least one through hole in the porous flexible substrate material, where the at least one through hole extends from the first surface to the second surface, the at least one through hole having an internal surface; and

[0029] (b) depositing a conductive material onto the first and second surfaces of the porous flexible substrate and onto the internal surface of the at least one through hole to provide a continuous conductive film.

[0030] 16. The method according to Clause 15, wherein the porous flexible substrate material of step (a) in Clause 15 is provided by punching at least one through hole into a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface.

[0031] 17. The method according to Clause 17, wherein the porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface is obtained using one or more of mechanical punching, E-beam etching, and more particularly, laser drilling, laser etching, chemical etching, and plasma etching, optionally wherein chemical etching is used (e.g. removal of CaCOs from a flexible substrate material where CaCOs is homogeneously distributed throughout the flexible substrate material). 18. The method according to any one of Clauses 15 to 17, wherein step (b) is conducted using one or more of chemical vapour deposition, and, more particularly, thermal evaporation, and physical vapour deposition (e.g. using an electron beam and sputtering).

[0032] Drawings

[0033] FIG. 1 depicts the fabrication process of Pt electrode on PI. (A) The schematic and optical images of PI substrate with taper hole that was automatically formed by punching process. (B)-(C) Deposition of Pt on (B) one and (C) another side of PI by sputtering process. The Pt will be formed on both sides of PI.

[0034] FIG. 2 depicts the electrical validation of Pt electrode. (A) Small pieces of Pt electrode with only one through-hole in the middle (left: 200 pm and right: 300 pm in diameter). (B) The schematic of electrical conductivity validation process. (3) The top-to-bottom electrical resistance of Pt electrode was 3.2 Q and 3.3 Q with 200 pm and 300 pm through-hole, respectively.

[0035] FIG. 3 depicts the visual inspection of Pt deposition around through-holes. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) images of (A) top view and (B) cross-sectional view of Pt-deposited through-hole.

[0036] FIG. 4 depicts (A) the schematic of PI film with tapered through-hole and porous surfaces, which was proved by SEM images of the through-hole and surface; (B) the schematic of Pt coating on both side of PI film, the through-hole was fully covered and maintained a porous structure after deposition.

[0037] FIG. 5 depicts the wettability of PI with (A) porous surface and (B) porous Pt@PI. The Pt electrode shows a progressive wetting feature.

[0038] FIG. 6 depicts (A) the schematic of the mechanical interlocking structure as the Pt was deposited on PI; (B) the Atomic Force Microscope (AFM) image of the Pt electrode and PI substrate, which illustrated the thickness of Pt is ~1.5 pm; (C) the fabrication process of the current invention enables a stable Pt-Pl interface that maintained in-plane and out-of-plane conductive after scarping using tweezers.

[0039] Description In a first aspect of the invention, there is provided a flexible electrode comprising: a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface; at least one through hole in the porous flexible substrate material, where the at least one through hole extends from the first surface to the second surface, the at least one through hole having an internal surface; a conductive material attached to the first and second surfaces of the porous flexible substrate material, wherein: the at least one through hole is formed such that a first entry hole, having a first entry hole diameter, in the first surface has a larger diameter than a corresponding second entry hole in the second surface, or vice versa, such that the internal surface of the through hole has a tapered configuration; the conductive material is also attached to the internal surface of the at least one through hole to provide a continuous conductive film attached to the first and second surfaces, where the first and second surfaces are conductively connected by the conductive material in the at least one through hole; and the conductive material penetrates into the porous flexible substrate to form a mechanical interlock between the conductive material and the porous flexible substrate.

[0040] The flexible electrodes disclosed herein may be bent by the application of mechanical force (e.g. using tweezers), but the electrode will maintain its conductivity and structural integrity.

[0041] It will be appreciated that flexible electrodes of the type disclosed herein may make use of any suitable substrate material. For example, the porous flexible substrate material may be selected from one or more of the group consisting of a silicone rubber (e.g. a polydimethylsiloxane (PDMS)), a styrene-ethylene-butylene-styrene (SEBS) copolymer, a styrene-butadiene-styrene (SBS) copolymer, a styrene-isoprene-styrene (SIS) copolymer, a polyurethane (PU), a polyamide (e.g. nylon), a polyvinyl alcohol (PVA), a polyethylene terephthalate (PET), and a polyimide (PI). As will be appreciated, these listed substrate materials are polymeric and are generally regarded as being an insulating material. Therefore, as set out in the aspect of the invention above, a conductive material is attached to the first and second surfaces of the porous flexible substrate and this conductive material also extends through and is attached to the surface of the through hole used herein. It will be appreciated that the conductive material is essentially bonded to the porous flexible substrate material so that it can provide conductivity even when the electrode is flexed and / or stretched (e.g. during use). As the flexible substrate is porous in nature, the conductive material may penetrate into the pores of the flexible material so as to provide a mechanical interlock between the conductive material and the porous flexible substrate. Without wishing to be bound by theory, it is believed that this penetration may include penetration of the flexible substrate at an atomic level and / or at the micro level, where it conforms to the porous structure of the substrate.

[0042] It will be appreciated that the conductive material may be attached by any suitable means to the porous flexible substrate material. For example, the porous flexible substrate material may be subjected to one or more deposition processes so as to deposit and attach the conductive material to its surfaces.

[0043] Given the nature of the porous flexible substrate material, it will be appreciated that the conductive material attached to a surface of said material (e.g. coated onto the porous flexible substrate surface) may also be porous in nature. That is, the conductive material may have a porous structure due to the porous flexible substrate material’s morphology.

[0044] An important part of the current invention is that there is a through hole that extends from the first surface to the second surface and which has a tapered configuration for its internal surface. In the context of the current invention, this means that the internal surface of the through hole is tapered or tapered-like, with a non-straight edge. Nevertheless, the important point is that there is a difference in the sizes of the first and second entry holes that forces the internal surface of the through hole to be more exposed that may normally be expected, thereby allowing the deposition and attachment of the conductive material, so as to ensure a greater electrical connection between the first and second surfaces of the electrode that is conventionally the case. It will be appreciated, that any configuration that can achieve this functionality may be used herein.

[0045] The flexible electrode disclosed herein makes use of a porous flexible substrate material. As such, the porous flexible substrate material has a plurality of pores and these pores may have any suitable diameter. Suitable diameters for the pores that may be mentioned herein include, but are not limited to pores having a diameter of from 5 nm to 5 pm, such as from 10 nm to 3 pm, such as from 20 nm to 2 pm.

[0046] The continuous conductive film may have any suitable thickness that allows it to retain its flexibility and utility for any intended application. For example, the continuous conductive film may have a thickness of from 0.1 to 3 pm, such as from 1 to 2 pm, such as about 1.5 pm.

[0047] One of the advantages of the electrode developed in this study is its ability to maintain both in-plane and out-of-plane conductivity, even after being scratched by rigid objects — such as metallic tweezers, stones, or gravel — that would typically compromise the electrical performance of conventional electrodes. In experimental tests, the applied force can be precisely controlled. To evaluate robustness, we use the minimum force that is sufficient to break the conductivity of a standard electrode. Under such conditions, the present electrode continues to perform reliably, showing strong mechanical and electrical resilience.

[0048] Surprisingly, the flexible electrodes disclosed in the current invention may exhibit a water contact angle after (contact with water for) 30 minutes that is less than or equal to the water contact angle of the porous flexible substrate material immediately following contact with water. That is, the flexible electrodes, bearing the conductive material can replicate (after a period of time) a similar water contact angle to that of the porous flexible substrate without the conductive material on its surface. This shows that the flexible electrodes of the current invention are particularly suitable for use in wearable devices and / or on the skin surface of a subject (e.g. during exercise), thereby potentially allowing more accurate monitoring of the subject’s performance and vital signs.

[0049] The water contact angle after 30 minutes may be from 30 to 90°, such as from 60 to 87°, 65 to 85s, such as from 70 to 80s, such as about 72.13or about 72.4s.

[0050] In certain embodiments of the invention that may be mentioned herein, the first entry hole and second entry hole of the at least one through hole may have a first edge and a second edge respectively, where the conductive material is distributed substantially homogeneously over: the first surface and the second surface of the porous flexible substrate; and the first edge, the second edge, and the internal surface of the at least one through hole, thereby providing a continuous conductive film.

[0051] It is noted that the conductive material may be densely integrated and conformally attached to the substrate surface. Thus, there is no lip of conductive material visible at the trough holes.

[0052] The conductive material used in the flexible electrode disclosed herein may be any suitable conductive material. For example, it may be a conductive polymer. However, in more preferred embodiments of the invention, the conductive material may be a metal. Any suitable metal may be used herein. For example, the metal may be selected from tungsten, molybdenum, magnesium, titanium, and, more particularly, gold, platinum, or alloys thereof. In particular embodiments of the invention that may be mentioned herein, the conductive material may be gold or, more particularly, platinum. As noted hereinbefore, the porous flexible substrate material may be any suitable such substrate material. For example, the porous flexible substrate material may be selected from one or more of the group consisting of a silicone rubber (e.g. a polydimethylsiloxane (PDMS)), a styrene-ethylene-butylene-styrene (SEES) copolymer, a styrene-butadiene-styrene (SBS) copolymer, a styrene-isoprene-styrene (SIS) copolymer, a polyurethane (PU), a polyamide (e.g. nylon), a polyvinyl alcohol (PVA), a polyethylene terephthalate (PET), and a polyimide (PI). In particular embodiments of the invention that may be mentioned herein, the porous flexible substrate material may be a polyimide (PI).

[0053] As noted hereinbefore, the first entry hole is larger than the second entry hole or vice versa this enables there to be some form of taper configuration on the internal surface of the hole, thereby allowing a greater deposition / attachment of the conductive material, which in turn provides one or more of the advantages mentioned herein. In certain embodiments of the invention that may be mentioned herein, the first entry hole may have a diameter of from greater than 50 to 300 pm and the second entry hole may have a diameter of from 100 to less than 300 pm, or vice versa, provided that the first entry hole diameter is larger than the second entry hole diameter or vice versa.

[0054] In certain embodiments of the invention that may be mentioned herein, the internal surface of the at least one through hole may define a truncated conical shape.

[0055] The flexible electrodes of the current invention may have one or more of the following properties:

[0056] (i) a sheet resistance of from 0.1 to 10 Q / square, such as from 0.5 to 5 10 Q / square, such as about 1 Q / square;

[0057] (ii) a resistivity value of from 0.01 to 0.5 mQ / cm, such as from 0.05 to 0.3 mQ / cm, such as from 0.1 to 0.2 mQ / cm, such as about 0.15 mQ / cm; and

[0058] (iii) a conductivity of from 3 to 10 x105S / m, such as from 4 to 8 x105S / m, such as from 5 to 7 x105S / m, such as about 6.7 x105S / m.

[0059] In a second aspect of the invention, there is provided a method of forming a flexible electrode as described in hereinbefore, the method comprising:

[0060] (a) providing a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface, the porous flexible substrate further comprising at least one through hole in the porous flexible substrate material, where the at least one through hole extends from the first surface to the second surface, the at least one through hole having an internal surface; and

[0061] (b) depositing a conductive material onto the first and second surfaces of the porous flexible substrate and onto the internal surface of the at least one through hole to provide a continuous conductive film.

[0062] The porous flexible substrate material of step (a) may be provided by punching at least one through hole into a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface. The porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface may be obtained using one or more of mechanical punching, E-beam etching, and more particularly, laser drilling, laser etching, chemical etching, and plasma etching, optionally wherein chemical etching is used (e.g. removal of CaCOs from a flexible substrate material where CaCOs is homogeneously distributed throughout the flexible substrate material).

[0063] In certain embodiments of the invention that may be mentioned herein, step (b) may be conducted using one or more of chemical vapour deposition, and, more particularly, thermal evaporation, and physical vapour deposition (e.g. using an electron beam and sputtering).

[0064] Some advantages of the current invention over existing methods, devices, or materials may include on or more of the following.

[0065] (1 ) Biocompatibility and Chemical Stability: In certain embodiments, where platinum is used, its inherent biocompatibility and corrosion resistance may make it ideal for implantable devices, mitigating inflammatory responses and degradation over time. Pt electrode is an optimal selection for long-term implantable tissue electronics.

[0066] (2) Enhanced Deposition Efficiency: The tapered design of the through-holes addresses conventional limitations by distributing the conductive material (e.g. Pt) more evenly and extensively, particularly along the edges where continuity is critical.

[0067] (3) Improved Electrical and Mechanical Properties: The combined benefits of the tapered through-holes and the porous substrate design facilitate superior mechanical adhesion and electrical continuity, crucial for the reliable operation of high-density electronic arrays.

[0068] The fabrication technology in the current invention is particularly advantageous for high- resolution implantable medical devices such as neural interfaces, cardiac monitors, and other biomedical sensors that require precise and robust electronic components. It is highlighted by a conformal interface by the flexibility of electrode. Additionally, it holds potential for advanced flexible electronics where space optimization and device integrity under mechanical stress are critical.

[0069] Further aspects and embodiments of the invention will now be described by reference to the following non-limiting examples.

[0070] Examples

[0071] Materials

[0072] Calcium carbonate (Analytical grade, Alfa Aesar, USA)

[0073] Platinum (Pt) and Gold (Au) (Jiangyin Maideli Advanced Materials Co., Ltd, China)

[0074] Characterisation Methods

[0075] Scanning Electron Microscopy (SEM)

[0076] Scanning electron microscopy (SEM) was conducted using the JEOL JSM-7800F PRIME field-emission scanning electron microscope, operated at an accelerating voltage of 15 kV, to examine the surface morphology of the samples.

[0077] Energy Dispersive X-Ray Spectroscopy (EDX)

[0078] Energy-dispersive X-ray spectroscopy (EDX) was performed using an Oxford Instruments Aztec EDX system, integrated with the JEOL JSM-7800F PRIME field-emission scanning electron microscope (FESEM). The EDX detector is used to determine the elemental composition of selected regions.

[0079] Wettability Measurements

[0080] Water contact angles were measured using the Contact Angle Dataphysics OCA15, with 5 pL deionized water droplets at ambient conditions, to evaluate the surface wettability of the substrates.

[0081] Atomic Force Microscopy (AFM)

[0082] Atomic force microscopy (AFM) analysis was carried out with an Atomic Force Microscope NX10 in tapping mode to obtain topographic information of the electrode at the micrometrescale.

[0083] Electrical performance validation

[0084] The sheet resistance and conductivity of samples were measured by Resistivity / Sheet resistance Measurement system (AIT CMT-SR2000N / Filmetrics R50). The vertical resistance across the through hole was measured by a multimeter (UNI-T UT890C). Example 1. Fabrication and Characterisations of Flexible Implantable Electrodes

[0085] The fabrication process of Pt on PI substrate, particularly for creating through-holes with conductive plating, involves several detailed steps.

[0086] Step 1: Preparation of the PI Substrate and Punching Through-Holes (FIG. 1A)

[0087] (a) Substrate Selection: A clean, dry PI substrate was used, whereby the substrate surface was ensured to be free from any contaminants, oils, or debris.

[0088] (b) Punching Through-Holes: A precision punching tool was utilized to create through-holes in the PI substrate. These taper-like holes were typically created by the nature of the punching tool or technique, allowing the hole to have a slightly conical shape. The size of the through- holes can be adjustable (200 pm and 300 pm in diameter were applied in this Example).

[0089] Step 2: Sputtering Pt on One Side of the PI Substrate (FIG. 1B)

[0090] (a) Sputtering Setup: The flexible substrate (including Silicon Rubber (Polydimethylsiloxane (PDMS), Eco-Flex), Styrene-Ethylene-Butylene-Styrene (SEES), Styrene-Butadiene-Styrene (SBS), Styrene-lsoprene-Styrene (SIS), polyurethane (PU), Polyamide (Nylon), Polyvinyl alcohol (PVA), Polyethylene terephthalate (PET) and PI (in this Example)) was loaded into a sputtering chamber. Before starting the deposition, the chamber was ensured to be vacuum- sealed and that the target material (Au and Pt (in this Example)) was correctly positioned. The vacuum should be lower than 2 x 106Torr.

[0091] (b) Deposition Process: Then, the sputtering of target material (Pt in this Example) begun. This process involves the ejection of Pt atoms from the target material towards the substrate by the momentum transfer from ionized Argon molecules under a vacuum. The taper-like through-holes ensure a continuous conductive Pt film on the edge of through-hole. The parameters of deposition were set as depending on the required thickness of target conductive materials: power of 100-300 W, pressure of 2-10 mTorr, and deposition duration of 100-800 seconds. In this case, we applied power of 250 W, pressure of 8 mTorr, and deposition duration of 800 seconds.

[0092] Step 3: Sputtering on the Opposite Side of the Flexible Substrate (FIG. 1C)

[0093] (a) Flip the Substrate: After the first side was coated, the substrate was flipped to expose the other side to the sputtering target.

[0094] (b) Repeat Sputtering Process: The sputtering process was performed again on this other side by using the same operation parameters in Step 2. This will also achieve uniformity in target material (Pt in this Example) film coated on both sides. (c) Out-of-Plane Conductivity: The second deposition ensures that Pt forms a continuous film from top to bottom, effectively creating out-of-plane conductivity. This is critical for high- density flexible electrodes and circuit design and fabrication.

[0095] Step 4: Final Inspection and Testing (FIG. 2)

[0096] (a) Electrical Testing: Electrical tests were conducted to verify the conductivity through the substrate and the through-holes. In general, small pieces of Pt electrode with only one through-hole in the middle (left: 200 pm and right: 300 pm in diameter) were prepared for resistance testing (FIG. 2A and 2B). In this case, the top-to-bottom electrical resistance of Pt electrode was 3.2 Q and 3.3 Q with 200 pm and 300 pm through-hole, respectively (FIG. 2C). For more detailed electrical performance of Pt electrode, the four-probe station was be applied to measure the sheet resistance, resistivity, electrical conductivity of Pt electrode (Table 1).

[0097] (b) Visual Inspection: After deposition, the substrate was visually inspected for uniformity in coating. Optical microscope was used to confirm that Pt has been deposited throughout the tapered holes (FIG. 1A). For more detailed information of samples, scanning electron microscopy was applied to characterize the surface morphology and continuity of Pt electrode in-plane (FIG. 3A and 3B). Energy-dispersive X-ray spectroscopy was also employed to detect the Pt and Carbon elements distribution around the through (FIG. 3A and 3B).

[0098] Table 1. Electrical performance and geometry parameters of Pt flexible electrode on PI (by using four-probe station)

[0099] Results and Discussions

[0100] By utilizing a precision punching tool to create through-holes in the PI substrate, conductive tapered through-holes were created. The current invention employs a unique through-hole design wherein each hole tapers from a wider entrance on the top side of the PI substrate to a narrower exit at the bottom (FIG. 4A). This geometric alteration significantly increases the surface area accessible for Pt deposition / fabrication at critical junctures, ensuring a more uniform and thorough coverage, particularly around the perimeter of each through-hole (FIG. 4B). With this feature, the through-holes allow the formation of a continuous coating that connects both sides of the PI substrate.

[0101] To achieve enhanced biocompatibility and electrical performance, platinum is selected for its excellent biocompatibility and superior electrical conductivity. The deposition of Pt is conducted through methods such as thermal evaporation, E-beam and sputtering (PVD), tailored to achieve deep penetration and complete coverage within the uniquely structured through-holes and across the porous substrate. The deposition parameters are meticulously optimized to ensure that Pt forms a continuous conductive film that effectively bridges the top and bottom electrode layers across the PI substrate. In addition, due to this deposition process, the Pt film has achieved a porous structure that enables a progressive wetting feature (FIG. 5A). Compared to the contact angle of water on PI (76.4°) , the contact angle on Pt was 106.9° at the very beginning and decreased to 72.1 ° in 30 min (FIG. 5B). This would benefit a low impedance interface between electrode and tissues (by reducing air gap and enhancing the wetting of body fluid) when these electrodes were applied as implants.

[0102] Example 2. Engineering of PI Substrate Surfaces

[0103] Before the sputtering process described in Step 2 of Example 1 , the surface of the PI substrate used can be engineered to exhibit a porous morphology. This can be achieved through several approaches, including laser drilling or etching, chemical etching, plasma etching.

[0104] Chemical Etching

[0105] For example, in chemical etching, the mixture of CaCOs particles (20 nm - 2 pm) and polyimide solution was plated on PI substrate. After completing thermal imidization process at 360 °C, the as-prepared CaCOs-PI composited film was immersed in dilute hydrochloric acid solution to remove CaCOs. After cleaning by deionized (DI) water, the porous structure of PI was fabricated, and the pore size are dependent on the size of the CaCOs particles.

[0106] Results and Discussions

[0107] The porous surface structure enhances the substrate's effective surface area, which further promotes an incased mechanical interlocking between PI and Pt (FIG. 6A). In this Example, with a thickness of -1.5 pm Pt on PI substrate, the interface in between presented a strong adhesion that can withstand scraping of tweezers (FIG. 6B and 6C). Even though the color of Pt coating changed to a more metallic color after scraping (may refer to porous structure of Pt was destructed), the Pt coating maintained conductive in-plane and out-of-plane connected by stable conductive through-hole in the middle of the plate.

Claims

Claims1 . A flexible electrode comprising : a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface; at least one through hole in the porous flexible substrate material, where the at least one through hole extends from the first surface to the second surface, the at least one through hole having an internal surface; a conductive material attached to the first and second surfaces of the porous flexible substrate material, wherein: the at least one through hole is formed such that a first entry hole, having a first entry hole diameter, in the first surface has a larger diameter than a corresponding second entry hole in the second surface, or vice versa, such that the internal surface of the through hole has a tapered configuration; the conductive material is also attached to the internal surface of the at least one through hole to provide a continuous conductive film attached to the first and second surfaces, where the first and second surfaces are conductively connected by the conductive material in the at least one through hole; and the conductive material penetrates into the porous flexible substrate to form a mechanical interlock between the conductive material and the porous flexible substrate.

2. The flexible electrode according to Claim 1 , wherein the porous flexible substrate material has a plurality of pores having a diameter of from 5 nm to 5 pm, such as from 10 nm to 3 pm, such as from 20 nm to 2 pm.

3. The flexible electrode according to Claim 1 or Claim 2, wherein the continuous conductive film has a thickness of from 0.1 to 3 pm, such as from 1 to 2 pm, such as about 1.5 pm.

4. The flexible electrode according to any one of the preceding claims, wherein the continuous conductive film can maintain conductive in-plane and out of plane conductivity after being subjected to scraping by a suitable implement (e.g. metallic tweezers).

5. The flexible electrode according to any one of the preceding claims, wherein the flexible electrode exhibits a water contact angle after 30 minutes that is less than or equal to the water contact angle of the porous flexible substrate material immediately following contact with water.

6. The flexible electrode according to Claim 5, wherein the water contact angle after 30 minutes is from 30 to 90s, such as from 65 to 85s, such as from 70 to 80s, such as about 72.1s.

7. The flexible electrode according to any one of the preceding claims, wherein the first entry hole and second entry hole of the at least one through hole have a first edge and a second edge respectively, where the conductive material is distributed substantially homogeneously over: the first surface and the second surface of the porous flexible substrate; and the first edge, the second edge, and the internal surface of the at least one through hole, thereby providing a continuous conductive film.

8. The flexible electrode according to any one of the preceding claims, wherein the conductive material is a metal, optionally wherein the metal is selected from one or more of the group consisting of tungsten, molybdenum, magnesium, titanium, and, more particularly, gold and platinum.

9. The flexible electrode according to Claim 8, wherein the conductive material is gold or, more particularly, platinum.

10. The flexible electrode according to any one of the preceding claims, wherein porous flexible substrate material is selected from one or more of the group consisting of a silicone rubber (e.g. a polydimethylsiloxane (PDMS)), a styrene-ethylene-butylene-styrene (SEBS) copolymer, a styrene-butadiene-styrene (SBS) copolymer, a styrene-isoprene-styrene (SIS) copolymer, a polyurethane (PU), a polyamide (e.g. nylon), a polyvinyl alcohol (PVA), a polyethylene terephthalate (PET), and a polyimide (PI).

11. The flexible electrode according to Claim 8, wherein the porous flexible substrate material is a polyimide (PI).

12. The flexible electrode according to any one of the preceding claims, wherein the first entry hole has a diameter of from greater than 50 to 300 pm and the second entry hole has a diameter of from 100 to less than 300 pm, or vice versa, provided that the first entry hole diameter is larger than the second entry hole diameter or vice versa.

13. The flexible electrode according to any one of the preceding claims, wherein the internal surface of the at least one through hole defines a truncated conical shape.

14. The flexible electrode according to any one of the preceding claims, wherein the flexible electrode has one or more of the following properties:(i) a sheet resistance of from 0.1 to 10 Q / square, such as from 0.5 to 5 10 Q / square, such as about 1 Q / square;(ii) a resistivity value of from 0.01 to 0.5 mQ / cm, such as from 0.05 to 0.3 mfl / cm, such as from 0.1 to 0.2 mQ / cm, such as about 0.15 mQ / cm; and(iii) a conductivity of from 3 to 10 x105S / m, such as from 4 to 8 x105S / m, such as from 5 to 7 x105S / m, such as about 6.7 x105S / m.

15. A method of forming a flexible electrode as described in any one of Claims 1 to 14, the method comprising:(a) providing a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface, the porous flexible substrate further comprising at least one through hole in the porous flexible substrate material, where the at least one through hole extends from the first surface to the second surface, the at least one through hole having an internal surface; and(b) depositing a conductive material onto the first and second surfaces of the porous flexible substrate and onto the internal surface of the at least one through hole to provide a continuous conductive film.

16. The method according to Claim 15, wherein the porous flexible substrate material of step (a) in Claim 15 is provided by punching at least one through hole into a porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface.

17. The method according to Claim 17, wherein the porous flexible substrate material having a first surface and a second surface diametrically opposed to the first surface is obtained using one or more of mechanical punching, E-beam etching, and more particularly, laser drilling, laser etching, chemical etching, and plasma etching, optionally wherein chemical etching is used (e.g. removal of CaCOs from a flexible substrate material where CaCOs is homogeneously distributed throughout the flexible substrate material).

18. The method according to any one of Claims 15 to 17, wherein step (b) is conducted using one or more of chemical vapour deposition, and, more particularly, thermal evaporation, and physical vapour deposition (e.g. using an electron beam and sputtering).

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