A method for manufacturing a microelectrode array structure by a fused filament fabrication process, a microelectrode array structure and a microelectrode array

Abstract

A method for manufacturing a microelectrode array structure (A) by a fused filament fabrication process, wherein the microelectrode array structure (A) comprises a base plate (1) extending in an xz-plane, wherein an x-direction, a y-direction and a z-direction are perpendicular to each other, a support plate (2) extending in an xz-plane, wherein a first surface of the base plate (1) and a second surface of the support plate (2) face to each other in the y-direction, wherein the microelectrode array structure (A) further comprises a plurality of pillar portions (3) extending from the first surface of the base plate (1) to the second surface of the support plate (2) in the y-direction, the plurality of pillar portions (3) being electrically conductive, wherein the microelectrode array structure (A) is a stack of m layers being stacked in the z-direction, wherein m is equal to or greater than 3, each of the layers extending in an xy-plane, wherein the method comprises forming a first layer of the microelectrode array structure (A), forming a second layer of the microelectrode array structure (A) on the first layer, forming a nth layer of the microelectrode array structure (A) on a (n-1)th layer, wherein n = 3, 4, … m.

Description

DESCRIPTIONTITLEA method for manufacturing a microelectrode array structure by a fused filament fabrication process, a microelectrode array structure and a microelectrode arrayTECHNICAL FIELD

[0001] The present disclosure relates to a method for manufacturing a microelectrode array structure by a fused filament fabrication process, a microelectrode array structure and a microelectrode array, in particular a microelectrode array for being brought into contact with a skin or cells or for being implanted into tissues such as a nerve, in order to apply and / or record an electrical signal to and / or from the skin or the tissues. More specifically, the microelectrode array according to the present disclosure is for clinical and / or biosensing applications.BACKGROUND

[0002] In current practice, microelectrode arrays (MEAs) are commonly used for measuring electro-encephalography (EEG) signals. The microelectrode arrays are brought into contact with a skin surface, typically a scalp surface, of a user, and such microelectrodes are for a non-invasive usage. Another application of microelectrode arrays is implantable microelectrodes, i.e. for an invasive usage, which are brought into the body of the patient via surgery and are used for recording from or stimulating the neural target tissue and nerves.

[0003] For measuring EEG signals, flexible electrodes may be especially advantageous, which would allow a dry-contact measurement i.e. without any medium such as gels between the electrodes and a skin and establish a stable contact on the skin. A document “Comparison of dry electrodes for mobile EEG system” (Kocturova et al., Conference on Theory and Practice of Information Technologies, 2019) discloses soft electrodes for measuring EEG signals. These electrodes are fabricated typically by an injection molding, and a diameter of one electrode (i.e. one pillar or needle electrode) is in a millimeter range. An electrode with a diameter in a micrometer range in accurate geometries cannot be obtained via this method due to the scaffold of the injection molding and / or a high visocity of the material.

[0004] A document “Clinical applications of penetrating neural interfaces and Utah Electrode Array technologies” (Normann et. al, Journal of Neural Engineering 13, 2016) discloses asilicon-based three-dimensional MEA having a plurality of microelectrode elements being microneedles. This silicon-based MEA however requires a rigid silicon substrate, and the height of the microelectrode element is restricted to the thickness of the substrate. Additionally, the complex manufacturing process and high costs of silicon-based 3D MEAs can be prohibitive for some applications.SUMMARY OF THE INVENTION

[0005] The technical problem to be solved can be formulated to provide a microelectrode array with microstructures and to provide its manufacturing method, which can more efficiently manufacture a microelectrode array with microstructures in low costs.

[0006] According to the present disclosure, a method for manufacturing a microelectrode array structure (A) is carried out by or through a fused filament fabrication process. The fused filament fabrication process is a three-dimensional (3D) printing process, in which typically a thermoplastic filament is extruded through a heating element and / or a nozzle and is deposited in layers on e.g. a printer stage (or a built plate). The nozzle maybe moved in an xy-plane (i.e. in two-dimensions) or only in an xy-plane during one layer is being formed. The nozzle may be moved in any directions within the xy-plane. When the one layer has been formed, the nozzle or the printer stage may be moved in the z-direction and a subsequent layer is formed on the one layer by moving the nozzle in an xy-plane.

[0007] The microelectrode array structure (A) comprises a base plate (1) extending in and / or along an xz-plane being perpendicular to a y-direction. An x-direction, the y-direction and a z-direction are of the three-dimensional rectangular coordinate system, which are orthogonal to each other. The microelectrode array structure (A) further comprises a support plate (2) a support plate (2) extending in and / or along an xz-plane being perpendicular to the y-direction. Each of the base plate (1) and the support plate (2) has a first surface and a second surface, wherein each of the first surface and the second surface extends in and / or along an xz-plane being perpendicular to the y-direction. Each of the base plate (1) and the support plate (2) has a thickness in the y-direction and the thickness is defined between the first and second surfaces. The second surface of the base plate (1), the first surface of the base plate (1), the first surface of the support plate (2) and the second surface of the support plate (2) are arranged in this order in and / or along the y-direction. The first surface of the base plate (1) and the second surface of the support plate (2) face to each other in the y-direction. The base plate (1) and the support plate (2) preferably overlap in the y-direction and are preferably parallel to each other.[ooo8]The microelectrode array structure (A) further comprises a plurality of pillar portions (3) (or pillars or needle portions) extending from the first surface of the base plate (1) to the second surface of the support plate (2) in and / or along the y-direction. Each of the plurality of pillar portions (3) has a length and an axis of the length is in and / or along the y-direction or essentially only the y-direction. Each of the plurality of pillar portions (3) may have a cylinder form or a prism form, for example a quadratic prism. A cross-section of each of the pillar portions in an xz-plane perpendicular to the y-direction may be a circle or an oval. The maximum thickness of the cross-section, for example a diameter of the cross-section, maybe in a range of 10 pm to 500 pm, more preferably in a range of 50 pm to 400 pm, more preferably in a range of too pm to 300 pm. The cross-sectional area of each of the pillar portions may be substantially same or same along its entire length. The length axis of each of the pillar portions may be perpendicular or substantially perpendicular to the first surface of the base plate (1). The plurality of pillar portions (3) is electrically conductive. A distal end of each of the plurality of pillar portions may penetrate into at least a part of the support plate (2) or may penetrate through the support plate (2).

[0009] The microelectrode array structure (A) is a stack of m layers being stacked in the z- direction, wherein m is an integer, which is equal to or greater than 3. Each of the m layers extends in an xy-plane being perpendicular to the z-direction. Each of the m layers includes a cross-section of the microelectrode array structure (A) in an xy-plane and may be a slice or a segment of the microelectrode array structure (A) having a thickness in the z-direction. The method for manufacturing the microelectrode array structure (A) comprises forming a first layer of the microelectrode array structure (A), and subsequently forming a second layer of the microelectrode array structure (A) on or directly on the first layer. The first layer and the second layer are stacked in the z-direction. The method further comprises forming a nthlayer of the microelectrode array structure (A) on a (n-i)thlayer , wherein n = 3, 4, ... m, and n is an integer. The (n-i)thlayer and the nthlayer are stacked in the z- direction. The microelectrode array structure (A) is manufactured by depositing material(s) layer by layer and / or in a layered manner. The total number of layers (i.e. the number of m) may be up to too per centimeter in the z-direction. The total number of the pillar portions may be also up to too centimeter in the z-direction.

[0010] The method according to the present disclosure may have one of more of following technical advantages: The plurality of pillar portions (3) can be formed by moving a nozzle in the length direction (i.e. the length axis direction and / or the y-direction) of the pillar portion, which is substantially parallel or parallel to the surface of the printer stage, while extruding melted or softened filament(s) from the nozzle. The microelectrode arraystructure (a) can thus be printed such that the base plate (i), the pillar portions (3) and the support plate (2) are simultaneously formed layer by layer from bottom to top in the z- direction, so that each of the pillar portions can be always fixed to (or supported by) the base plate (1) and the support plate (2) at its both ends. This would be especially advantageous to effectively prevent deformations of the pillar portions during the printing process (i.e. the manufacturing process), while the pillar portions can be formed in to a long and thin microstructure. The pillar portions can be accordingly formed in a stable and efficient manner using 3D printing technology. In particular, forming the pillar portions in the y-direction by the fused filament fabrication process can be more accurately performed in a shorter time than a method of forming pillar portions by injecting materials in a form of beads only in the z-direction (i.e. perpendicular to the surface of the printer stage) and building up plastic beads in the z-direction.

[0011] The method according to the present disclosure may further have one or more of the following advantages, which have not been achieved by conventional technologies such as injection moulding: The method according to the present disclosure would provide the pillar portions with diameters in the order of 50 to 500 pm and lengths in the order of centimetres, such as 1 to 5 cm. The support plate (2) can be removed from the microelectrode array structure (A) when the microelectrode array structure (or microelectrode array) is to be used, for example by cutting off a part of each of the pillar portions. The support plate (2) would be further advantageous to protect the thin pillar portions before the use, for example during transport. Such thin pillar portions, which would serve as electrodes to be brought into direct contact with a targeted sample, may be further advantageous in on-skin measurements, where they reduce a burden on the skin. Also, in case of an invasive measurement, thin pillar portions can be more smoothly inserted into e.g. targeted tissues without damaging tissues. Additionally, it is possible to provide more numbers of pillar portions on the base plate, due to its thin structure, and this may be further advantageous to perform measurements more accurately, as the total contacting area of a microelectrode array will be increased. The length and thickness of the pillar electrodes can be easily adjusted by changing the parameters of the printer, so that different designs of MEAs can be produced to suit different applications.

[0012] Preferably, the base plate (2) comprises a plurality of feedlines (4) and an insulating part. Each of the plurality of feedlines (4) is connected to a corresponding one of the plurality of pillar portions (3). One of the feedlines (4) may be connected to only one of the pillar portions (3), so that each pillar portion can be individually controlled or measured by its corresponding feedline. The plurality of feedlines (4) is separated from each other by the insulating part. The method preferably comprises forming a part of the microelectrodearray structure corresponding to the plurality of feedlines (4) by extruding a first polymer- based material being electrically conductive from a first nozzle (Ni). When one of the m layers includes a part of the feedline(s), this part of the feedline(s) is formed by the first polymer-based material with the first nozzle. The method further comprises forming a part of the microelectrode structure corresponding to the insulating part by extruding a second polymer-based material being electrically insulative from a second nozzle (N2), the second nozzle being different from the first nozzle (Ni). Accordingly, when one of the m layers includes a part of the insulating part, this part of the insulating part is formed by the second polymer-based material with the second nozzle. These steps may be further advantageous in that the microelectrode array structure including at least two different materials (or composites) can be formed by the printing process.

[0013] Alternatively, the method may comprise forming the microelectrode array structure only with the first polymer-based material being electrically conductive. For an on-skin use, it would not be necessary to electrically isolate each of the pillar portions, and therefore the microelectrode array structure can be more efficiently manufactured.

[0014] Preferably, the method comprises forming a passivation layer (5) on at least a part of the base plate (1) and the plurality of pillar portions (3). More specifically, the passivation layer (5) may be formed on a surface or an entire surface of the base plate (1) and on a surface or an entire surface of the plurality of pillar portions (3). The passivation layer (5) may be formed also on a surface or an entire surface of the support plate (2). The passivation layer (5) may be electrically insulative and preferably a polymer layer, for example parylene.

[0015] Preferably, the first polymer-based material and / or the second polymer-based material comprises thermoplastic polyurethane (TPU) and / or polylactic acid (PLA) and / or polycarbonate, polypropylene (PP) and / or acrylonitrile butadiene styrene (ABS) and / or acrylonitrile styrene acrylate (ASA) and / or butenediol vinyl alcohol co-polymer (BVOH) and / or carbon and / or chlorinated polyethylene (CPE) and / or glassfiber and / or high impact polystyrene (HIPS) and / or polyvinyl chloride (PVC) and / or polyvinylidene fluoride or polyvinylidene difluoride (PVDF) and / or polyethylenimine (PEI) and / or polyetherketoneketone (PEKK) and / or polyethylene terephthalate (PETG), wherein preferably the first polymer-based material is a composite of polymers and a conductive material.

[0016] Preferably, the first polymer-based material comprises carbon nanoparticles such as carbon blacks and / or graphites and / or carbon nanotubes. Additionally or alternatively, thefirst polymer-based material comprises metals such as Au and / or Ag. Additionally or alternatively, the first polymer-based material comprises conductive polymers such as PEDOT:PSS.

[0017] Preferably, the method further comprises forming an adhesive layer (S) on a printer stage (or a built plate) and forming the microelectrode array structure (A) directly on the adhesive layer (S), and subsequently removing the adhesive layer (S) from the microelectrode array structure (A). The adhesive layer (S) may be formed with the first polymer-based material and / or the second polymer-based material. The adhesive layer (S) may be advantageous to stably print the microelectrode array structure (A) on the printer stage.

[0018] A method for manufacturing a microelectrode array (B) comprises a method according to any one of the above methods for manufacturing the microelectrode array structure (A). The method further comprises removing the support plate (2) from the microelectrode array structure (A). Preferably, the removing of the support plate (2) comprises cutting a tip portion of each of the plurality of pillar portions (3) preferably by a laser cutter. Each of the plurality of pillar portions may be cut at a position, which is nearer to the second surface of the support plate (2) than the first surface of the base plate (1). The distal end of each of the plurality of pillar portions would be thereby exposed, so that the pillar portions may be used to establish (direct) contacts with targeted samples.

[0019] Preferably, the step of removing the support plate (2) is performed after the step of forming the passivation layer (5). The passivation layer (5) maybe for example formed on an entire surface of the microelectrode array structure (A), and the support plate (2) may be thereafter removed from the microelectrode electrode array structure (A). A top end surface of each of the pillar portions is thereby exposed, which is electrically conductive, while the other part of the pillar portions is covered by the passivation layer (5).

[0020] A microelectrode array structure (A) according to the present disclosure, which can be manufactured by any one of the above-mentioned methods, comprises a base plate (1) extending in an xz-plane being perpendicular to the y-direction, a support plate (2) extending in an xz-plane being perpendicular to the y-direction, wherein a first surface of the base plate (1) and a second surface of the support plate (2) face to each other in the y- direction, wherein the microelectrode array structure (A) further comprises a plurality of pillar portions (3) extending from the first surface of the base plate (1) to the second surface of the support plate (2) in the y-direction, the plurality of pillar portions (3) beingelectrically conductive. An x-direction, a y-direction and a z-direction are perpendicular to each other.

[0021] The microelectrode array structure (A) according to the present disclosure may have one of more of following technical advantages: Each of the pillar portions may have a diameter in the order of 50 to 500 pm and a length in the order of centimeters, such as 1 to 3 cm. The support plate (2) can be removed from the microelectrode array structure (A) when the microelectrode array structure (or microelectrode array) is to be used, for example by cutting off a part of the pillar portions. The support plate (2) would be advantageous to protect the thin pillar portions before the use, for example during transport.

[0022] A microelectrode array (B) according to the present disclosure, which can be manufactured by any one of the above-mentioned methods, is for being brought into contact with a skin or cells or for being implanted into tissues such as a nerve, in order to apply and / or record an electrical signal to and / or from the skin or the tissues. The microelectrode array (B) maybe for invasive and / or non-invasive clinical applications. In particular, one of microelectrodes of the microelectrode array is capable of measuring and / or recording voltage changes or electrical currents at an interface of a skin surface and / or within tissue and / or a single biological cell. The microelectrode array (B) is preferably used for delivering an electrical stimulus to a tissue and / or cell e.g. in a (human) brain. A targeted tissue and / or cell may have typically a diameter and / or size in a range of one to several tens of micrometres. Such a tissue and / or a single biological cell maybe for example a neuron or a nerve fibre having a dimeter in a range of several hundred nanometres to ten micrometres. The microelectrode element (A) is preferably capable of measuring 1 pV to several mV, and / or the electrode element (A) is preferably capable of applying stimulation currents to a biological sample in the regime of 10 pA to several mA, and / or the electrode element (A) is preferably capable of measuring currents from a biological sample, e.g. for measuring neurotransmitter release, from 500 fA to too pA.

[0023] The microelectrode array (B) comprises a base plate (1) extending in an xz-plane being perpendicular to the y-direction, a plurality of pillar portions (3) extending from a first surface of the base plate (1) in the y-direction, the plurality of pillar portions (3) being electrically conductive, wherein each of the plurality of pillar portions (3) has a thickness in the z-direction in a range of 10 pm to 500 pm, more preferably in a range of 50 pm to 400 pm, more preferably in a range of too pm to 300 pm. An x-direction, a y-direction and a z-direction are perpendicular to each other.

[0024] The microelectrode array (B) according to the present disclosure may have one of more of following technical advantages: The microelectrode array (B) comprises thin pillar portions in the micrometer range, which may be further advantageous in on-skin measurements, where they reduce a burden on the skin. Also, in case of an invasive measurement, thin pillar portions can be more smoothly inserted into e.g. targeted tissues without damaging tissues. Additionally, it is possible to provide more numbers of the pillar portions on the base plate, due to its thin structure, and this maybe further advantageous to perform measurements more accurately.

[0025] According to the microelectrode array structure (A) and / or the microelectrode array (B), the base plate (1) preferably comprises a plurality of feedlines (4) and an insulating part, each of the plurality of feedlines (4) being connected to a corresponding one of the plurality of pillar portions (3). The plurality of feedlines (4) preferably comprises a first polymer-based material being electrically conductive, and the insulating part comprises a second polymer-based material being electrically insulative.

[0026] Preferably, the microelectrode array structure (A) and / or the microelectrode array (B) further comprises a passivation layer (5) formed on at least a part of the base plate and the plurality of pillar portions. The passivation layer (5) may be for example parylene.

[0027] Preferably, the first polymer-based material comprises thermoplastic polyurethane (TPU) and / or polylactic acid (PLA) ) and / or polycarbonate, polypropylene (PP) and / or acrylonitrile butadiene styrene (ABS) and / or acrylonitrile styrene acrylate (ASA) and / or butenediol vinyl alcohol co-polymer (BVOH) and / or carbon and / or chlorinated polyethylene (CPE) and / or glassfiber and / or high impact polystyrene (HIPS) and / or polyvinyl chloride (PVC) and / or polyvinylidene fluoride or polyvinylidene difluoride (PVDF) and / or polyethylenimine (PEI) and / or polyetherketoneketone (PEKK) and / or polyethylene terephthalate (PETG). Preferably, the first polymer-based material is a composite of polymers and a conductive material.

[0028] Preferably, the first polymer-based material comprises carbon nanoparticles such as carbon blacks and / or graphites and / or carbon nanotubes. Additionally or alternatively, the first polymer-based material comprises metals such as Au and / or Ag. Additionally or alternatively, the first polymer-based material comprises conductive polymers such as PEDOT:PSS.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figs. 1(a) to (d) schematically illustrate manufacturing steps of a microelectrode array structure according to a first embodiment.Figs. 2(a) to (c) schematically illustrate a perspective view, a side view, a top view of the microelectrode array structure according to the first embodiment, respectively.Fig. 3 schematically illustrates a perspective view of the microelectrode array according to the first embodiment.Fig. 4 shows a photo of Sample i according to a first example.Figs. 5(a) and (b) show photos of Samples i and 2 according to the first example.Figs. 6(a) and (b) show results of electrochemical experiments performed with the first example.Figs. 7(a) to (c) schematically illustrate manufacturing steps of a microelectrode array structure according to a second embodiment.Fig. 8 schematically illustrates a top view of the first surface of the base plate with feedlines according to the second embodiment.Fig. 9 schematically illustrates a perspective view of the microelectrode array structure according to the second embodiment.Fig. 10 schematically illustrates a side view of the microelectrode array structure according to the second embodiment.Figs. 11(a) and (b) shows photos of Sample 2 according to a second example.Fig. 12 shows results of electrochemical experiments performed with the second example.DETAILED DESCRIPTION OF EMBODIMENTSFirst Embodiment

[0030] Figs. 1(a) to (d) schematically illustrate steps manufacturing process of a microelectrode array structure (A) according to a first embodiment. The fused filament fabrication process (hereinafter FFF) is used to manufacture the microelectrode array structure (A). The FFF is a three-dimensional (3D) printing process, in which typically a thermoplastic filament is softened by heating and is extruded through a nozzle (N). The softened filament is deposited in layers on a printer stage (not illustrated) while moving the nozzle in an xy-plane. In this embodiment, a single composite material (for example a composite conductive TPU) is used as the thermoplastic filament, which is an example of the first polymer-based material. Typically, a nozzle having an outlet diameter of in a range of 1 pm to 1000 pm, for example in a range of 200 pm to 400 pm, is used.

[0031] As depicted in Fig. 1(a), an adhesive layer (S) is at first printed (directly) on the printer stage and the microelectrode array structure is then printed on the adhesive layer (S) in a layered manner. The microelectrode array structure (A) to be printed (i.e. the finished (i.e. final) product of the FFF process) comprises a base plate (i) and a support plate (2), which face to each other in the y-direction and may be parallel to each other. Between the base plate (1) and the support plate (2), a plurality of pillar portions (3) is disposed and is connected to the base plate (1) and the support plate (2) at its distal ends. The length axis of each of the pillar portions and the surface of the base plate (1) may be perpendicular to each other. Each of the pillar portions extends in the y-direction and has a homogeneous thickness along its entire length.

[0032] The microelectrode array structure (A) is printed to a stack of m layers being stacked in the z-direction. In this embodiment, m is 65, i.e. the final layer being on the top in the z- direction of the microelectrode array structure is the 65thlayer. Each of the 65 layers extends in an xy-plane having a thickness in the z-direction. The thickness of one layer is typically determined by the nozzle diameter, and in this embodiment, one layer is in the range of 200 to 400 pm. One layer is therefore a so-called a slice of the microelectrode array structure (A), which includes a cross-section of the microelectrode array structure (A) in an xy-plane. The layers are printed by forming a nthlayer of the microelectrode array structure (A) directly on a (n-i)thlayer (n = 1, 2, 3, 4, ... m), by extruding the first polymer- based material from the nozzle. Fig. 1 illustrates (a) where the 5thlayer is finished (i.e. the first to fifth layers are stacked), (b) where the 25thlayer is finished (i.e. the first to 25thlayers are stacked), (c) where the 45thlayer is finished (i.e. the first to 45thlayers are stacked), and (d) where the 65thlayer (final layer) is finished (i.e. the first to 65thlayers are stacked). A single one layer of the microelectrode array structure (A) (i.e. among the m layers) may therefore include each of a part of the base plate (1), a part (or entire) of one (or more) pillar portion(s) (3) and a part of the support plate (2), and the plurality of pillar portions (3) is formed by moving a nozzle in the length axis direction (i.e. in the y-direction) of the pillar portion. The pillar portions may be thus printed such that the length axis direction is substantially parallel or parallel to the surface of the printer stage. The pillar portions are accordingly fixed to the base plate (1) and the support plate (2) at its both ends, which can stably hold the thin pillar microstructure during the printing process. By controlling a printing speed, for example by printing at a higher speed than a printing speed at the base plate to elongate the extruded material, it would be further possible to form the pillar portion (for example a diameter of the pillar portion) with a thickness (a diameter) being thinner than the nozzle diameter.

[0033] Figs. 2(a) to (c) schematically illustrate a perspective view, a side view, a top view of the microelectrode array structure (A) according to the first embodiment, which has been printed according to the manufacturing steps as explained above (i.e. the adhesive layer (S) has been removed from the microelectrode array structure (A)). In this embodiment, the number of the pillar portions (3) is 140, which is only an example and the number is not limited thereto. According to this embodiment, the microelectrode array structure (A) consists of the first polymer-based material being conductive. Optionally, the microelectrode structure (A) may be entirely covered by a passivation layer, such as parylene.

[0034] In order to obtain a microelectrode array (B) as illustrated in Fig. 3, the support plate (2) is removed from the microelectrode array structure (A) to expose tip portions of the pillar portions (3), which are to be brought in contact with targeted sample (e.g. a skin surface) to be measured. The support plate (2) can be removed by cutting off a part of each of the pillar portions (3). In case the passivation layer is formed on the entire surface of the microelectrode array structure (A), a top end of the pillar portion is exposed by the cutting process, i.e. a part being uncovered by the passivation layer and electrically conductive.Second Embodiment

[0035] Figs. 7(a) to (c) schematically illustrate manufacturing steps of a microelectrode array structure (A) according to the second embodiment. Similar to the first embodiment, the microelectrode array structure according to the second embodiment is manufactured layer by layer by the FFF process as explained in the first embodiment. The second embodiment is different from the first embodiment in that the layers of the microelectrode array structure (A) are formed by using two different nozzles, the first nozzle (Ni) and the second nozzle (N2). The first nozzle (Ni) is used to extrude a conductive material (first polymer- based material, for example a composite PLA) and the second nozzle (N2) is used to extrude a non-conductive material (second polymer-based material, for example a standard, non-conductive PLA).

[0036] Similar to the first embodiment, the microelectrode array structure (A) to be printed (i.e. the finished (final) product of the FFF process) comprises a base plate (1) and a support plate (2), which face to each other in the y-direction and may be parallel to each other according to this embodiment. Between the base plate (1) and the support plate (2), a plurality of pillar portions (3) is disposed and is connected to the base plate (1) and the support plate (2) at its distal ends. The pillar portions (3) are formed by the conductive first polymer-based material. The length axis of each of the pillar portions and the surface ofthe base plate (i) may be perpendicular to each other. Each of the pillar portions extends in the y-direction and has a homogeneous thickness along its entire length.

[0037] As depicted in Fig. 7(a), the base plate (2) according to the second embodiment comprises a plurality of feedlines (4), which is formed by the first polymer-based material. Each of the plurality of feedlines (4) is connected to (only) a corresponding one of the plurality of pillar portions (3), so that each of the pillar portions can be individually and electrically accessible. Each of the feedlines (4) extends from the pillar portion to an edge (i.e. an end side) of the first surface of the based plate (1) and has a thickness in the y- direction, which is smaller than the thickness of the base plate (1) in the y-direction. Each of the feedlines (4) therefore extends in the y-direction from the first surface of the base plate (1) inside of the base plate (1). The other part of the base plate (1) is formed by the non-conductive second polymer-based material and is an insulating part. The feedlines are therefore isolated from each other by the insulating part. The support plate (2) is formed by the second polymer-based material.

[0038] When the m layers including the base plate (1), the plurality of pillar portions (3) and the base plate (2) has been printed (cf. Fig. 7(a)), a part of the feedlines are covered by a cover (C), for example a tape, and the entire part of the microelectrode array structure except the part covered by the cover (C) is passivated by a passivation layer (5), for example parylene. To obtain a microelectrode array (B), the base plate (2) is removed by cutting off a part of each of the pillar portions by using e.g. a laser cutter (cf. Fig. 7(b)), and finally removing the cover (C), so that the feedlines is (physically and / or electrically) contactable.

[0039] Fig. 8 schematically illustrates a top view (viewing from the y-direction perpendicular to the first surface) of the first surface of the base plate (1) with the feedlines (4) according to the second embodiment. The support plate (2) is not illustrated in Fig. 8. The base plate (1) may have a squared shape, and in this embodiment, 60 feedlines are arranged for 60 pillar portions. To each side of the square-shaped first surface, 15 of the feedlines are extended. Another end of each of the feedlines extends to the center portion of the base plate (1), from which a pillar portion extends in the y-direction. Fig. 9 schematically illustrates a perspective view of the microelectrode array structure (A) according to the second embodiment, and Fig. 10 schematically illustrates a side view (viewing from the x- direction) of the microelectrode array structure according to the second embodiment. The pillar portions are arranged such that a first group of the pillar portions is aligned along a diagonal line of the square-shaped first surface, and a second group of the pillar portions is aligned along another diagonal line of the first surface.

[0040] In order to obtain a microelectrode array (B) as illustrated in Fig. 7(c), the support plate (2) is removed from the microelectrode array structure (A) to expose a top end of the pillar portions (3), which are to be brought in contact with a targeted sample (e.g. a skin surface) to be measured. The support plate (2) can be removed by cutting off a part of each of the pillar portions (3). The obtained microelectrode array (B) would be especially advantageous for in vitro measurements: Cells and / tissues can be incubated directly on the pillar portions, for example by forming a incubation well around the pillar portions, and signals from the incubated cells / tissues can be directly measured through the conductive pillar portions by applying and / or recording electrical signals via the feedlines. The microelectrode array (B) according to the second embodiment may be also used for electrochemistry experiments such as a nano particle detection.EXAMPLES

[0041] Certain aspects and embodiments of the present disclosure will be illustrated by way of examples. Such examples of the present disclosure are representative only and should not be taken to limit the scope of the present disclosure to only such representative examples.

[0042] First ExampleSample 1 and Sample 2, which are examples of the microelectrode array structure according to the first embodiment, were manufactured. Microelectrode array samples were further manufactured from Sample 1 and their electrochemical characteristics were analyzed:

[0043] <Manufacturing>The microelectrode array structure was at first designed using software programs, Autodesk AutoCAD 2023 and Ultimaker Cura 5.3.1. Sample 1 has 140 pillar portions, each of which has a diameter of 200 pm and a length of 3 mm, a base plate having circular surfaces with a diameter of 13 mm (i.e. a diameter of a first surface extending in an xz- plane) and a thickness of 4 mm (i.e. a thickness in the y-direction), and a support plate circular surfaces with a diameter of 13 mm (i.e. a diameter of a second surface extending in an xz-plane) and a thickness of 0.5 mm (i.e. a thickness in the y-direction). Sample 2 is different from Sample 1 in the diameter of the pillar portion, which is 400 pm, and all other parameters are same to those of Sample 1.

[0044] For the fabrication, Ultimaker S3 FDM printer from Ultimaker BV was used. A conductive carbon-based TPU filament (Recreus Industries S.L.), which is an example of a first polymer-based material, was extruded from a steel nozzle (with a diameter 200 pmfor Sample i and with a 400 pm for Sample 2). The extruded material was deposited in layers onto a build plate. The built plate was kept at 6o°C. The steel nozzle was moved in an xy-plane to print one layer. When one layer was completed, the build plate was moved along the z-axis with respect to the steel nozzle and a subsequent layer was printed by the steel nozzle. The print speed for Sample 1 was 65 mm / s for printing the pillar portions, and the other parts were printed in a range 40 to 55 mm / s. For Sample 2, the print speed was in a range 40 to 55 mm / s. The material flow was kept at 87 %. The print temperature (i.e. the temperature at the steel nozzle and / or the temperature in the extrusion chamber or core) was 255 °C for Sample 1 and 245 °C for Sample 2. A brass nozzle may be used instead of the steel nozzle.

[0045] The photo of Sample 2 is shown in Fig. 4, which shows a perspective view of Sample 2 with an adhesive layer being still attached to the bottom part of Sample 2. The photos of a part of the pillar portions of Sample 1 and Sample 2 are shown in Figs. 5(a) and 5(b), respectively. Each of the pillar portions with a diameter of 200 pm (Fig. 5(a)) and 400 pm (Fig. 5(b)) was successfully obtained in high qualities.

[0046] The adhesive layer was then manually removed from each of the samples. The support plate was substantially removed from each of the samples by cutting joint portions between the pillar portions and the support plate (i.e. distal ends of the pillar portions being connected to the support plate) by a laser cutter (Keyence 3-Axis CO2 Laser Marker with power utilization at 100%, a scanning speed of 200 mm / s, a frequency of 80 Hz, and a repetition count of 50 cycles), so that tip portions of the pillar portions are exposed.

[0047] <Impedance Evaluation>On-skin electrode impedance for a microelectrode array sample obtained from Sample 1 was evaluated by (skin) Electrochemical Impedance Spectroscopy performed using the PalmSens4 potentiostat. An ac sinusoidal excitation signal was applied with 40 frequency steps, from 10kHz to 1Hz. Prior to the experiment, the excitation potential was evaluated for linearity, by applying three test potentials, 10 mV, 50 mV and too mV. Only at too mV was a smooth excitation response obtained. The impedance and phase shift are shown in Figs. 6 (a) and (b), respectively. Fig. 6(a) shows the impedance (log scale) as a function of frequency and Fig. 6(b) shows the phase angle (in degrees) versus frequency. The mean values are represented by the solid lines and the region between the dashed lines indicating the standard deviation. The impedance spectroscopy curve matches impedance values of commercially available electrodes. Commercially available electrodes were tested with respect to an electrogeled reference and a ground electrode, while a tip part of the electrode was plated with silver. On the other hand, the microelectrode array according to the presentexample was measured using a dry reference and ground electrode, and no additional metal layer was formed on the pillar portions and the impedance across frequency was below 1 M(1.

[0048] Second ExampleSample 3, which is an example of a microelectrode array structure according to the second embodiment, was manufactured. A microelectrode array sample was further manufactured from Sample 3 and its electrochemical characteristics were analyzed:

[0049] < Manufacturing >The microelectrode array structure was at first designed using software programs, Autodesk AutoCAD 2023 and Ultimaker Cura 5.3.1. Sample 3 has a square-shaped base plate (5 mm x 5 mm in an xz-plane, 5 mm thickness in the y-direction) with 60 feedlines. Each of the feedlines extends from the first surface of the base plate into the base plate and has a thickness (or a depth) of 3 mm in the y-direction (i.e. the depth direction of the base plate). Fifteen feedline contact pads are exposed on each of four sides of the square-shaped base plate. The other side of the feedline, which is an opposite side of the contact pad extend toward the center portion of the base plate, a pillar portion is formed at each of the end portion of the feedlines being disposed on the center portion of the base plate, and the pillar portions extend in the y-direction. The number of the pillar portions is therefore 60. The contact portion of the feedline has a width of 0.68 mm. Each of the distal ends of the pillar portions penetrates into a support plate (5 mm x 5 mm in an xz-plane, 5 mm thickness in the y-direction). In this Sample 3, the pillar portions penetrate through the support plate. Each of the pillar portions has a diameter of approximately 200 pm. The distance between the first surface of the base plate and the second surface of the support plate is 3.5 mm.

[0050] For the fabrication, Ultimaker S3 FDM printer from Ultimaker BV was used. This printer features a dual extrusion print head, and the electrically conductive polylactic acid (Protoplant, Inc.), which is an example of the first polymer-based material, and a standard polyactic acid, which is an example of the second polymer-based material, were separately extruded from two steel nozzles. The built plate of the printer was kept at 6o°C. Each of the steel nozzles was moved in an xy-plane to print one layer. When one layer was completed, the build plate was moved along the z-axis with respect to the steel nozzles and a subsequent layer was printed by the steel nozzles. At first, a printer stage (i.e. a build plate) was coated with PVA (glue stick) and then an adhesive layer was printed with the standard (non-conductive) polylactic acid directly on the built plate of the printer, and Sample 3 was printed directly on the adhesive layer. The print speed was 45 mm / s and printingtemperature is at 250 to 300 °C °C for the conductive polylactic acid and at 200 °C for the standard (non-conductive) polylactic acid. The material flow was kept at 87 %. The feedlines and the pillar portions were printed with the conductive polylactic acid and the other part of Sample 3 were printed with the standard (non-conductive) polylactic acid.

[0051] The adhesive layer was then manually removed from Sample 3. The entire surface of Sample 3 was then coated with parylene-C with a thickness of 5 to 10 pm by PDS 2010 LABCOTER™ 2 Parylene Deposition System. Fig. 11(a) shows a photo of a part of Sample 3 after the parylene coating.

[0052] The support plate was subsequently removed from Sample 3 by cutting off a part of each of the pillar portions by a laser cutter (Keyence 3-Axis CO2 Laser Marker with power utilization at 100%, a scanning speed of 200 mm / s, a frequency of 80 Hz, and a repetition count of 50 cycles), so that tip portions of the pillar portions are exposed. The length of the pillar portions from the first surface of the base plate is 3 mm. Fig. 11(b) shows a photo of a part of Sample 3 after the removal of the support plate.

[0053] <Impedance Evaluation>Electrical Impedance Spectroscopy (EIS) and Chronoamperometry (CA) measurements were conducted across Sample 3 with Potentiostat BioLogic VSP-300. The experimental setup entailed a 3-electrode cell configuration, with a distinct bristle serving as the working electrode. Initially, Chronoamperometry (CA) was performed with a lOOmV applied voltage over a 6o-second duration, utilizing a i-second timestep for each pillar portion. Subsequently, Electrical Impedance Spectroscopy (EIS) was conducted within a frequency range spanning from lOOmHz to 100kHz, with an applied voltage of -lOOmV.

[0054] Fig. 12 shows a result of the electrical impedance spectroscopy for the microelectrode array sample obtained from Sample 3. The top plot shows impedance (log scale) as a function of frequency and the bottom plot displays phase angle (in degrees) versus frequency. The mean values are represented by the solid lines and the region between the dashed lines indicating the standard deviation. Impedance decreases as frequency increases, which is typical behaviour for the electrodes. The phase angle appears to increase with frequency.< Reference numbers >A: Microelectrode array structure, B: Microelectrode array, C: Cover, N: Nozzle, Ni: First nozzle, N2: Second nozzle, S: Adhesive layer, 1: Base plate, 2: Support plate, 3: Pillar portion, 4: Feedlines, 5: Passivation layer

Claims

Claims i. A method for manufacturing a microelectrode array structure (A) by a fused filament fabrication process, wherein the microelectrode array structure (A) comprises a base plate (i) extending in an xz-plane, wherein an x-direction, a y-direction and a z-direction are perpendicular to each other, a support plate (2) extending in an xz-plane, wherein a first surface of the base plate (1) and a second surface of the support plate (2) face to each other in the y-direction, wherein the microelectrode array structure (A) further comprises a plurality of pillar portions (3) extending from the first surface of the base plate (1) to the second surface of the support plate (2) in the y-direction, the plurality of pillar portions (3) being electrically conductive, wherein the microelectrode array structure (A) is a stack of m layers being stacked in the z-direction, wherein m is equal to or greater than 3, each of the layers extending in an xy-plane, wherein the method comprises forming a first layer of the microelectrode array structure (A), forming a second layer of the microelectrode array structure (A) on the first layer, forming a nthlayer of the microelectrode array structure (A) on a (n-i)thlayer , wherein n = 3, 4, ... m.

2. The method according to claim 1, wherein the base plate (2) comprises a plurality of feedlines (4) and an insulating part, each of the plurality of feedlines (4) being connected to a corresponding one of the plurality of pillar portions (3),wherein the method comprises forming a part of the microelectrode array structure corresponding to the plurality of feedlines (4) by extruding a first polymer-based material being electrically conductive from a first nozzle (Ni), and forming a part of the microelectrode array structure corresponding to the insulating part by extruding a second polymer-based material being electrically insulative from a second nozzle (N2), the second nozzle being different from the first nozzle (Ni).

3. The method according to any one of claims 1 to 2 further comprises forming a passivation layer (5) on at least a part of the base plate (1) and the plurality of pillar portions (3).

4. The method according to any one of claims 1 to 3, wherein the first polymer-based material and / or the second polymer-based material comprises thermoplastic polyurethane (TPU) and / or polylactic acid (PLA), wherein preferably the first polymer-based material is a composite of polymers and a conductive material.

5. The method according to any one of claims 1 to 4, wherein the first polymer-based material comprises carbon nanoparticles such as carbon blacks and / or graphites and / or carbon nanotubes, and / or metals such as Au and / or Ag, and / or conductive polymers such as PEDOT:PSS.

6. The method for manufacturing a microelectrode array structure (A) according to any one of claims 1 to 5 comprisingforming an adhesive layer (S) on a stage and forming the microelectrode array structure (A) directly on the adhesive layer (S), and subsequently removing the adhesive layer (S) from the microelectrode array structure (A).

7. A method for manufacturing a microelectrode array (B) comprising a method any one of claims 1 to 6, the method further comprising removing the support plate (2) from the microelectrode array structure (A), wherein preferably the removing of the support plate (2) comprises cutting a tip portion of each of the plurality of pillar portions (3) preferably by a laser cutter.

8. The method according to claim 7, wherein the step of removing the support plate (2) is performed after the step of forming the passivation layer (5).

9. A microelectrode array structure (A) comprising a base plate (1) extending in an xz-plane, wherein an x-direction, a y-direction and a z-direction are perpendicular to each other, a support plate (2) extending in an xz-plane, wherein a first surface of the base plate (1) and a second surface of the support plate (2) face to each other in the y-direction, wherein the microelectrode array structure (A) further comprises a plurality of pillar portions (3) extending from the first surface of the base plate (1) to the second surface of the support plate (2) in the y-direction, the plurality of pillar portions (3) being electrically conductive.

10. A microelectrode array (B) for being brought into contact with a skin or cells or for being implanted into tissues such as a nerve, in order to apply and / or record an electrical signal to and / or from the skin or the tissues, the microelectrode array (B) comprising a base plate (i) extending in an xz-plane, wherein an x-direction, a y-direction and a z-direction are perpendicular to each other, a plurality of pillar portions (3) extending from a first surface of the base plate (1) in the y-direction, the plurality of pillar portions (3) being electrically conductive, wherein each of the plurality of pillar portions (3) has a thickness in the z-direction in a range of 10 pm to 500 pm, more preferably in a range of 50 pm to 400 pm, more preferably in a range of too pm to 300 pm.

11. The microelectrode array structure (A) according to claim 9 or the microelectrode array (B) according to claim 10, wherein the base plate (1) comprises a plurality of feedlines (4) and an insulating part, each of the plurality of feedlines (4) being connected to a corresponding one of the plurality of pillar portions (3), wherein the plurality of feedlines (4) comprises a first polymer-based material being electrically conductive, and the insulating part comprises a second polymer-based material being electrically insulative.

12. The microelectrode array structure (A) according to claim 9 or 11 or the microelectrode array (B) according to any one of claims 10 or 11 further comprises a passivation layer (5) formed on at least a part of the base plate and the plurality of pillar portions.

13. The microelectrode array structure (A) according to any one of claim 9, 11 or 12, or the microelectrode array (B) according to any one of claims 10 to 12,wherein the first polymer-based material comprises thermoplastic polyurethane (TPU) and / or polylactic acid (PLA), wherein preferably the first polymer-based material is a composite of polymers and a conductive material.

14. The microelectrode array structure and / or the microelectrode array according to any one of claims 11 to 13, wherein the conductive material of the first polymer-based material comprises carbon nanoparticles such as carbon blacks and / or graphites and / or carbon nanotubes, and / or metals such as Au and / or Ag, and / or conductive polymers such as PEDOT:PSS.

15. A microelectrode array structure (A) being obtained by a method according to any one of claims 1 to 6.

16. A microelectrode array (B) being obtained by a method according to claim 7 or 8.

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