Flexible wearable heart electrode for cardiovascular disease monitoring and preparation method thereofTechnical Field
The invention belongs to the technical field of MEMS sensors, and particularly relates to a flexible wearable heart electrode for cardiovascular disease monitoring and a preparation method thereof.
Background
Today, while wearable flexible ECG sensors have achieved miniaturization and wireless transmission, the use of everyday ECG recording systems is still limited by the discomfort and inconvenience of using wet electrodes to the patient, which puts new demands on ECG electrodes with high performance and good biocompatibility. Standard commercial Ag/AgCl electrodes for ECG signal detection maintain good conductivity between the electrode and the skin mainly by conductive gel. However, such conductive paste may become dry with time, resulting in a change in electrode-tissue interface impedance and a drastic decrease in signal quality. In addition, such conductive adhesives can also irritate the patient's skin, causing excessive discomfort to the patient. Furthermore, sweat is another factor that causes deterioration of wet electrode signals. These above problems make conventional Ag/AgCl electrodes unsuitable for routine and repeated ECG recordings in cardiovascular disease monitoring. Therefore, there is a need to develop a wearable flexible ECG dry electrode capable of 24-hour uninterrupted electrocardiographic monitoring, which addresses the need for high signal-to-noise ratio and high stability ECG signal acquisition in cardiovascular disease monitoring.
Yuki Yamamoto et al, university of Osaka, japan, in paper "Efficient Skin Temperature Sensor and Stable Gel-Less Sticky ECG Sensor for a Wearable Flexible Healthcare Patch" propose an adhesive ECG sensor based on Carbon Nanotubes (CNTs) and Polyethoxyethyleneimine (PEIE) doped PDMS composites. In the composite material, CNT mainly improves the conductive property of an ECG electrode, and PEIE mainly improves the adhesion property of the ECG electrode and skin. Although the quality of the recorded ECG signal can be improved by modulating the concentration of CNTs and PEIEs in the composite, the performance of such ECG electrodes is still affected by sweat and motion status. Afraiz Tariq Satti of korea university et al in paper Fabrication of Parylene-Coated Microneedle Array Electrode for Wearable ECG Device propose an ECG sensor based on SU-8 microneedle array, which realizes exercise electrocardiographic monitoring as well as long-term electrocardiographic monitoring by reducing electrode-tissue interface impedance by piercing the stratum corneum of the skin. However, the biocompatibility of such stainless steel-based SU-8 microneedle arrays has not been verified and thus has not been suitable for application on humans.
Disclosure of Invention
Aiming at the defects in the prior art, the invention utilizes MEMS micro-processing technology to form a hollow SU-8 micro-needle array on the SU-8 surface, adopts sputtering and electroplating to form a contact electrode point formed by metal and conductive polymer on the SU-8 micro-needle surface, and combines physical puncture, electrochemical modification and electrolyte release to realize the preparation of the low contact impedance ECG electrode.
The invention discloses a flexible wearable heart electrode for cardiovascular disease monitoring, which comprises an electrode part, a flexible substrate and a metal bonding pad. The electrode portion includes SU-8 microneedles and a support layer. The SU-8 micro-needle and the supporting layer are made of SU-8 or PI. An electrode position is arranged on the supporting layer; a plurality of SU-8 microneedles which are tapered are arranged on the electrode positions. The SU-8 microneedles are provided with fluid channels through the SU-8 microneedles and the support layer. The supporting layer is provided with a metal bonding pad. A metal layer is deposited on the support layer. The metal layer covers each SU-8 microneedle and forms a wire connecting the electrode position and the metal pad on the support layer. The surface of each SU-8 microneedle is provided with a conductive polymer. The supporting layer is attached to the PDMS flexible substrate. A fluid chamber is disposed in the PDMS flexible substrate. Electrolyte is stored in the fluid chamber and is communicated with the fluid pore canal on the SU-8 microneedle.
Preferably, the SU-8 microneedles are obtained by back-exposure on a support layer. The support layer and the PDMS flexible substrate were thermally bonded together by plasma treatment of the surface. Electroplating a layer of conductive polymer on the surface of the SU-8 microneedle by electrochemical deposition; the conductive polymer is PEDOT and PSS.
Preferably, the electrode position is in a circular shape with the diameter of 0.5-2 cm; the SU-8 microneedle is conical, the diameter of the bottom is 100-500 micrometers, and the height is 50-700 micrometers. The diameter of the fluidic channels in SU-8 microneedles is 10-100 microns. The thickness of the supporting layer is 5-100 micrometers, and the thickness of the PDMS flexible substrate is 0.1-2 millimeters.
Preferably, during use, the raised SU-8 microneedles penetrate the stratum corneum into the dermis, reducing the electrode-skin interface impedance by physical penetration. The fluid chamber inside the PDMS flexible substrate releases electrolyte to the skin through the fluid pore canal on the SU-8 micro needle, so that the conductive polymer on the SU-8 micro needle conducts electricity through ions and holes, and the interface impedance of the electrode and the skin is reduced.
Preferably, the conductive polymer is PEDOT: PSS, and is electroplated on the SU-8 micro-needle by electrochemical deposition.
Preferably, the material of the supporting layer is SU-8; the supporting layer and the PDMS flexible substrate are bonded together through hot pressing after plasma treatment on the surface.
Preferably, the material of the supporting layer is PI; the supporting layer and the PDMS flexible substrate are adhered together through silica gel.
Under the condition that the material of the supporting layer is SU-8, the preparation method of the flexible wearable heart electrode for monitoring cardiovascular diseases comprises the following steps:
1) PMMA was spin coated onto a quartz glass plate and cured to a sacrificial layer by heating.
2) And spin-coating SU-8 photoresist on the sacrificial layer and performing photoetching patterning to form a SU-8 supporting layer.
3) A layer of metal is deposited on the SU-8 support layer and patterned to form an opaque metal mask.
4) A layer of SU-8 photoresist was again spin coated on the metal deposited SU-8 support layer and back-exposed from the back side of the quartz glass plate.
5) And depositing a layer of metal on the SU-8 supporting layer and performing photoetching patterning to form SU-8 micro-needles, wires and metal pad structures.
6) A layer of water-soluble adhesive tape is stuck on the front surface of the quartz glass sheet, and the electrode part is released from the quartz glass sheet by the water-soluble adhesive tape.
7) And spin-coating a layer of SU-8 photoresist on another unused quartz glass sheet, and performing photoetching patterning to form a reverse mould structure of the PDMS.
8) And depositing a layer of parylene C on the surface of the reverse mould structure to serve as a release agent of the PDMS.
9) And spin-coating a layer of PDMS precursor on the front surface of the quartz glass, and heating and curing to form the fluid chamber structure.
10 The cured PDMS flexible substrate was released from the reverse mold structure and bonded with its opening facing up to another unused piece of quartz glass.
11 Oxygen plasma is used for respectively treating the surfaces of the PDMS flexible substrate and the SU-8 supporting layer, then the PDMS flexible substrate and the SU-8 supporting layer are attached together, and bonding is realized through pressurization and heating.
12 The bonded flexible wearable heart electrode is removed from the quartz glass sheet and then put into deionized water to dissolve out the water-soluble adhesive tape for release.
In the case that the material of the supporting layer is PI, the preparation method of the flexible wearable heart electrode for cardiovascular disease monitoring comprises the following steps:
1) PMMA was spin coated onto a quartz glass plate and cured to a sacrificial layer by heating.
2) And spin-coating PI photoresist on the sacrificial layer and performing photoetching patterning to form a PI supporting layer.
3) And depositing a layer of metal on the PI supporting layer and patterning to form an opaque metal mask.
4) A layer of SU-8 photoresist was again spin coated on the PI support layer of the deposited metal and back-exposed from the back side of the quartz glass plate.
5) And depositing a layer of metal on the PI supporting layer and carrying out photoetching patterning to form SU-8 micro-needles, wires and metal pad structures.
6) A layer of water-soluble adhesive tape is stuck on the front surface of the quartz glass sheet, and the electrode part is released from the quartz glass sheet by the water-soluble adhesive tape.
7) And spin-coating a layer of SU-8 photoresist on another unused quartz glass sheet, and performing photoetching patterning to form a reverse mould structure of the PDMS.
8) And depositing a layer of parylene C on the surface of the reverse mould structure to serve as a release agent of the PDMS.
9) And spin-coating a layer of PDMS precursor on the front surface of the quartz glass, and heating and curing to form the fluid chamber structure.
10 The cured PDMS flexible substrate was released from the reverse mold structure and bonded with its opening facing up to another unused piece of quartz glass.
11 Coating a layer of silica gel on the upper surface of the PDMS flexible substrate, and then adhering a PI supporting layer on the PDMS flexible substrate;
12 The bonded flexible wearable heart electrode is removed from the quartz glass sheet and then put into deionized water to dissolve out the water-soluble adhesive tape for release.
The invention has the beneficial effects that:
1. the preparation of the hollow SU-8 microneedle structure is realized by using a SU-8 or PI back exposure process, and the ECG electrode can pierce the stratum corneum and release electrolyte through the SU-8 microneedle, so that the contact impedance of the ECG electrode and the skin is reduced.
2. The invention uses electrochemical deposition to realize SU-8 microneedle surface modification, and can realize conduction between the ECG electrode and the skin through holes and ions through conductive polymer modification, thereby reducing the contact impedance of the ECG electrode and the skin.
3. According to the invention, PDMS is used as a supporting material of the ECG electrode, and the shape-preserving adhesiveness and biocompatibility of the ECG electrode can be improved by reducing Young modulus, so that the contact stability of the ECG electrode and skin is improved.
Drawings
FIG. 1 is a schematic diagram of the overall structure of an ECG electrode according to the present invention;
FIG. 2 is a schematic view of the structure of the ECG electrode portion and the PDMS flexible substrate according to the present invention;
FIG. 3 is a graph showing the effect of the ECG electrode portion surface modified conductive polymer according to the present invention;
FIG. 4 is a flow chart of the process for preparing ECG electrodes according to the present invention.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
Example 1
As shown in fig. 1, a flexible wearable heart electrode for cardiovascular disease monitoring comprises an electrode part 1, a PDMS flexible substrate 2 and a metal pad 3 which together form a heart electrode structure.
As shown in fig. 2, the electrode part 1 is composed of hollow SU-8 micro needles 1-1 and a SU-8 support layer 1-2 of the bottom layer. Three electrode positions are arranged on the SU-8 supporting layer 1-2; and each electrode position is provided with a plurality of tapered SU-8 micro-needles 1-1 which are uniformly distributed. Each SU-8 microneedle 1-1 is provided with a fluid channel extending through SU-8 microneedle 1-1 and SU-8 support layer 1-2.
Three metal pads 3 are provided on the SU-8 support layer 1-2. A metal layer is deposited on SU-8 support layer 1-2. The metal layer covers each SU-8 microneedle 1-1 and a plurality of wires are formed on SU-8 supporting layer 1-2. The metal layers of the SU-8 micro-needles 1-1 on the three electrode positions are respectively and electrically connected with the three metal bonding pads 3 through corresponding wires to form conductive paths.
As shown in FIG. 3, the surface of each SU-8 microneedle 1-1 is electroplated with a layer of conductive polymer (specifically PEDOT: PSS) by electrochemical deposition. PEDOT PSS is a conductive polymer with pseudocapacitive properties, which has both hole and ion conducting properties in the presence of an electrolyte.
The SU-8 supporting layer 1-2 is attached to the PDMS flexible substrate 2. The PDMS flexible substrate 2 is composed of a hollow fluid chamber 2-1 and a peripheral PDMS support layer 2-2. A plurality of fluid chambers 2-1 corresponding to the number of electrode sites are provided on the PDMS flexible substrate 2. The SU-8 supporting layer 1-2 and the PDMS flexible substrate 2 are bonded together by hot pressing after plasma treatment of the surface. The fluid chambers 2-1 are each aligned with a corresponding electrode site and communicate with each fluid channel on the corresponding electrode section 1. The electrolyte stored in the fluid chamber 2-1 can be released through the fluid channels on the SU-8 microneedles 1-1. Furthermore, only one fluid chamber 2-1 may be provided; and the fluid chamber 2-1 is connected to all three electrode sites.
The fluid chamber 2-1 contains an electrolyte. During the use process of the flexible wearable heart electrode, electrolyte can permeate out through a fluid pore canal on the SU-8 microneedle 1-1; the environment required by electrocardiosignal acquisition is provided, conductive adhesive does not need to be smeared, and a large-area wetting area cannot be generated, so that the electrocardiosignal acquisition process is more comfortable.
The specific preparation steps of the flexible wearable heart electrode for cardiovascular disease monitoring are as follows:
1) PMMA was spin coated on a quartz glass plate and heat cured under conditions of 110℃for 5 minutes, 150℃for 5 minutes, and 180℃for 10 minutes. This step process forms a sacrificial layer of ECG electrodes.
2) Spin-coating a 10 μm thick SU-8 (GM 1060) photoresist on the PMMA sacrificial layer, standing for 5 min, and pre-baking at 65deg.C for 3 min and 95 deg.C for 30 min. Exposing after the pre-baking is finished, wherein the exposure dose is 400mJ/cm2 . Then, post-baking is carried out, wherein the post-baking temperature is 65 ℃ for 5 minutes, and 95 ℃ for 20 minutes. After 10 minutes of standing, the mixture was put into PGMEA (propylene glycol methyl ether acetate) and developed for 4.5 minutes. After the development is completed, hard baking is carried out; the hard baking temperature is 135 ℃, and the hard baking time is 2 hours. This step process forms the SU-8 support layer 1-2 of the ECG electrode.
3) And sequentially depositing a layer of metal Cr and a layer of metal Au on the SU-8 supporting layer, wherein the thicknesses of the metal Cr and the metal Au are respectively 20 nanometers and 200 nanometers. And spin-coating positive photoresist with the thickness of 5 micrometers on the SU-8 supporting layer, and carrying out photoetching patterning to form a photoresist mask of the metal layer. Then, the metal Au and the metal Cr are respectively patterned by adopting a wet etching technology. This process forms a back exposure mask for the hollow SU-8 microneedles.
4) And spin-coating a 50-micrometer thick SU-8 (GM 1060) photoresist on the SU-8 supporting layer after metal deposition, standing for 15 minutes, and then performing pre-baking at 65 ℃ for 15 minutes and 95 ℃ for 60 minutes. Back exposure is carried out after the pre-baking is finished, and the exposure dose is 650mJ/cm2 . And then post-baking at 65 ℃ for 10 minutes and at 95 ℃ for 30 minutes. After 10 minutes of standing, the mixture was put into PGMEA and developed for 4 minutes. After the development is completed, hard baking is performed, the hard baking temperature is 135 ℃, and the hard baking time is 2 hours. This process forms SU-8 microneedle 1-1 with hollow microwells.
5) And sequentially depositing a layer of metal Cr and a layer of metal Au on the SU-8 supporting layer 1-2 with the SU-8 micro-needles, wherein the thicknesses of the metal Cr and the metal Au are respectively 20 nanometers and 200 nanometers. And spin-coating positive photoresist with the thickness of 5 micrometers on the metal layer, and carrying out photoetching patterning to form a photoresist mask of the metal layer. Then, the metal Au and the metal Cr are respectively patterned by adopting a wet etching technology. This step process forms the electrode portion 1, lead and pad structure of the ECG electrode.
6) A layer of water-soluble adhesive tape is stuck on the front surface of the quartz glass sheet, and the electrode part 1 of the ECG electrode is released from the quartz glass sheet by the water-soluble adhesive tape.
7) Another piece of quartz glass was spin coated with a 50 μm thick SU-8 (GM 1060) photoresist, and left to stand for 15 minutes, and then pre-baked at 65 ℃ for 15 minutes and 95 ℃ for 60 minutes. Back exposure is carried out after the pre-baking is finished, and the exposure dose is 650mJ/cm2 . And then post-baking at 65 ℃ for 10 minutes and at 95 ℃ for 30 minutes. After 10 minutes of standing, the mixture was put into PGMEA and developed for 4 minutes. After the development was completed, hard baking was performed at 135℃for 2 hours. This step process forms the SU-8 reverse structure of PDMS of the fluid chamber.
8) A layer of 5 μm thick parylene C was deposited on the surface of the SU-8 master using a chemical vapor deposition system as a release agent for PDMS.
9) A 500 μm thick layer of PDMS precursor was spin coated on the front side of a quartz glass plate (i.e. SU-8 master) and cured by heating in an oven at 60 c for 4 hours. This step process forms a PDMS flexible substrate 2 structure with a fluid chamber 2-1.
10 The cured PDMS flexible substrate 2 was released from SU-8 reverse mold, followed by passivation of the surface of the DMS flexible substrate 2 with oxygen plasma, and finally bonding it to another unused quartz glass plate in a state in which the fluid chamber 2-1 was opened upward.
11 The surface of the electrode part 1 formed by the PDMS flexible substrate 2 and the SU-8 is respectively treated by oxygen plasma, then the PDMS supporting layer of the PDMS flexible substrate 2 and the SU-8 supporting layer of the electrode part 1 are attached together, and bonding is realized by pressurizing and heating, so that the flexible wearable heart electrode for cardiovascular disease monitoring is formed.
12 The bonded flexible wearable heart electrode is removed from the quartz glass sheet and then put into deionized water to dissolve out the water-soluble adhesive tape for release.
Example 2
A flexible wearable heart electrode for cardiovascular disease monitoring, the difference between this embodiment and embodiment 1 is that: SU-8 support layer 1-2 is replaced with PI support layer.
The specific preparation steps of the flexible wearable heart electrode for cardiovascular disease monitoring are as follows:
1) PMMA was spin coated on a quartz glass plate and heat cured under conditions of 110℃for 5 minutes, 150℃for 5 minutes, and 180℃for 10 minutes. This step process forms a sacrificial layer of ECG electrodes.
2) A Polyimide (PI) photoresist with the thickness of 10 micrometers is coated on the PMMA sacrificial layer in a spin mode, and after standing for 5 minutes, pre-baking is carried out, wherein the pre-baking temperature is 80 ℃ for 10 minutes, 120 ℃ for 30 minutes, 150 ℃ for 10 minutes, 180 ℃ for 10 minutes and 220 ℃ for 40 minutes. This step process forms the PI support layer of the ECG electrode.
3) And sequentially depositing a layer of metal Cr and a layer of metal Au on the PI supporting layer, wherein the thicknesses of the metal Cr and the metal Au are respectively 20 nanometers and 200 nanometers. And spin-coating positive photoresist with the thickness of 5 micrometers on the PI supporting layer, and carrying out photoetching patterning to form a photoresist mask of the metal layer. Then, the metal Au and the metal Cr are respectively patterned by adopting a wet etching technology. This process forms a backside exposure mask for the hollow microporous structure of SU-8 microneedles.
4) And spin-coating a 50-micrometer thick SU-8 (GM 1060) photoresist on the PI supporting layer after metal deposition, standing for 15 minutes, and then performing pre-baking at 65 ℃ for 15 minutes and 95 ℃ for 60 minutes. Back exposure is carried out after the pre-baking is finished, and the exposure dose is 650mJ/cm2 . And then post-baking at 65 ℃ for 10 minutes and at 95 ℃ for 30 minutes. After 10 minutes of standing, the mixture was put into PGMEA and developed for 4 minutes. After the development is completed, hard baking is performed, the hard baking temperature is 135 ℃, and the hard baking time is 2 hours. This process forms SU-8 microneedle structures with hollow micropores.
5) A stainless steel hard mask prepared by laser cutting is placed on a PI supporting layer with SU-8 micro-needles, and then a layer of metal Cr and a layer of metal Au are sequentially deposited, wherein the thicknesses of the metal Cr and the metal Au are respectively 20 nanometers and 200 nanometers. The metal Au and metal Cr are then patterned after the stainless steel hard mask is removed from the substrate. This step process forms the electrode portions, leads and pad structures of the ECG electrode.
6) A layer of water-soluble adhesive tape is stuck on the front surface of the quartz glass sheet, and the electrode part is released from the quartz glass sheet by the water-soluble adhesive tape.
7) Another piece of quartz glass is taken, a layer of SU-8 (GM 1060) photoresist with the thickness of 50 microns is coated on the quartz glass in a spin mode, and after standing for 15 minutes, the quartz glass is subjected to pre-baking at the temperature of 65 ℃ for 15 minutes and at the temperature of 95 ℃ for 60 minutes. Back exposure is carried out after the pre-baking is finished, and the exposure dose is 650mJ/cm2 . And then post-baking at 65 ℃ for 10 minutes and at 95 ℃ for 30 minutes. After 10 minutes of standing, the mixture was put into PGMEA and developed for 4 minutes. After the development was completed, hard baking was performed at 135℃for 2 hours. This step process forms the SU-8 reverse structure of PDMS of the fluid chamber.
8) A layer of 5 μm thick parylene C was deposited on the surface of the SU-8 master using a chemical vapor deposition system as a release agent for PDMS.
9) A 500 μm thick layer of PDMS precursor was spin coated on the front side of the quartz glass plate (i.e. on SU-8 master) and cured by heating in an oven at 60 ℃ for 4 hours. This step process forms a PDMS flexible substrate 2 structure with a fluid chamber 2-1.
10 The cured PDMS flexible substrate 2 was released from SU-8 master and subsequently bonded to another unused piece of quartz glass with the fluid chamber 2-1 open upwards.
11 A layer of silica gel is coated on the upper surface of the PDMS flexible substrate 2, and then the electrode part 1 is stuck on the PDMS flexible substrate 2 in a state that the PI support layer is downward, and is left to stand for 24 hours for curing at normal temperature.
12 The bonded flexible wearable heart electrode is removed from the quartz glass sheet and then put into deionized water to dissolve out the water-soluble adhesive tape to realize release.