WO2011070403A1

WO2011070403A1 - A dry active bio signal electrode with an hybrid organic-inorganic interface material

Info

Publication number: WO2011070403A1
Application number: PCT/IB2009/055758
Authority: WO
Inventors: João Paulo TRIGUEIROS DA SILVA CUNHA; Luís António FERREIRA MARTINS DAIS CARLOS; David Manuel Dieteren Ribeiro
Original assignee: Universidade De Aveiro; Ieeta - Instituto De Engenharia Electrónica E Telemática De Aveiro
Priority date: 2009-12-11
Filing date: 2009-12-15
Publication date: 2011-06-16
Also published as: PT104882A

Abstract

The invention presented here is an bio signal electrode that is characterized by the use of an organic-inorganic sol gel derived hybrid skin to electrode interface material. Furthermore the incorporation of a buffering and signal processing circuit on a flexible PCB makes this electrode less prone to fluctuating skin to electrode interfacial impedance and thus reducing noise artifacts. The organic-inorganic hybrid interface material has never been used before in this type of application, and by incorporating all the parts described here we invented a novel flexible dry active electrode. In summary this invention consist on a novel dry active electrode that in combination with the electronics described uses an organic-inorganic hybrid as a skin to electrode interface material.

Description

Title of Invention: A DRY ACTIVE BIO SIGNAL ELECTRODE

WITH AN HYBRID ORGANIC-INORGANIC INTERFACE

MATERIAL FIELD OF THE INVENTION

[1] The invention presented here is an bio signal electrode that is characterized by the use of an organic-inorganic sol gel derived hybrid skin to electrode interface material. Furthermore the incorporation of a buffering and signal processing circuit on a flexible PCB makes this electrode less prone to fluctuating skin to electrode interfacial impedance and thus reducing noise artifacts.

BACKGROUND OF THE INVENTION

[2] The most popular electrode used today for measuring bioelectric signals is the silver/ silver chloride electrode. Although some good results could be obtained, this type of material brings several problems like deteriorating electrical properties and allergic reactions. Although this electrode also has its advantages it also brings with it some major disadvantages. Some of these advantages and disadvantages are summarized below.

[3] Advantages

- Low cost of the electrodes gives physicians the possibility to dispose of the electrodes after use

- A simple design ensures reliability when properly attached to the subject.

- Good signal quality when the signal is used for a short period of time.

[4] Disadvantages

- The electrode suffers from relative large skin electrode interfacial impedance and therefore making its very sensitive to electromagnetic interference and movement.

- A difference in electrode skin interfacial impedance caused by dehydration of the electrolyte introduces noise into the system. This means that in prolonged measurements the electrolyte needs to be replaced regularly.

- The skin preparation needed in order to lower the skin electrode interfacial impedance, are time consuming and inconvenient especially in prolonged measurements.

- There are some toxicological concerns with the use of electrolytic gel. Although rare some cases of dermatitis have been reported.

[5] We can see in the summary above that the wet electrode suffers from 3 major disadvantages which are:

- Noise introduced into the system due to differences in skin electrode interfacial impedance, noise induced due to electromagnetic interference.

- Time consuming and uncomfortable skin preparation

- In prolonged recordings electrolyte needs to be replaced regularly.

[6] The first is especially important when measuring very low amplitudes of bioelectrical signals. In the literature dry active electrodes have been pointed out as the solution for these problems. Active electrodes distinguish themselves from standard electrodes due to the fact that they use an impedance converting device at the sensing site. As mentioned above the difference in skin impedance is one of the main sources of noise. Due to the large input impedance of the operational amplifier, the measuring system becomes less sensitive to changes in skin electrode interface impedance. Active electrodes can generally be categorized into two main groups; wet active electrodes and dry active electrodes. Wet active electrodes are usually the electrodes which use silver/silver chloride as an interface material. These electrodes are similar to the standard electrodes except for the fact that the active ones incorporate electronics at the sensing site.

[7] Dry active electrodes may consist of various types of materials such as inert;

materials like platinum, or a metal with a ceramic outer layer, or a polymer. Furthermore some authors refer to adhesives loaded with conductive particles such as graphite or silver, but it was concluded that these materials degrade the adhesive properties. Also one author refers the use of CNT or carbon nanotubes which penetrate the outer layer of the skin. Although good results can be achieved the penetration of the skin brings with it some toxicological concerns. One of the main reasons for research in active electrodes is the choice of electrode interface material. This part of the electrode is in direct contact with the skin and therefore is one of the first stages of noise introduction. The material of choice should be a good conductor, should also be flexible and have a long life span. The flexibility is important so that the electrode can adapt to the contours of the human body.

[8]

[Table 1]

[Table ]

properties is

something we could

adapt to our

advantage.

[Table 2]

[Table ]

Title: Skin Impedance matched Publication number: US 2005177038 Biopotential electrode

Features Evaluated Important design features for our product

Substrate Non flexible insulating capsule

Electronics Buffering circuit for impedance transformation

Interface material Moulded rubber sheet, carbon suspension

[Table 3] [Table ]

[10] [Table 4]

[Table ]

Title: Biopotential sensor electrode Publication number: US 6434421

Features Evaluated Important design features for our product

Substrate A non flexible closed body

Electronics An impedance transformation circuit and wireless system for signal transmission without cable

Interface material A hybrid solution of dry and insulating materials

[Table 5] [Table ]

[Table 6] [Table ]

[12] [Table 8]

[Table ]

Impact on our design thinking

This product uses micro needles to

penetrate the top layer of the skin. This gives some tox- icological concerns. Similar to our design is the shape of the housing and the integration of the electronics o the back of the carrier.

[13] [Table 10] [Table ]

[Table 11]

[14] [Table 12]

[Table ]

[Table 13]

[15] Although many of the patents have the same object, which is sensing low amplitude bioelectrical signals such as EEG, EMG or EKG), the inventions differ in the use of material regarding the skin to electrode interface. Furthermore many of the patents adopt a similar approach when it comes to lowering the skin to electrode interface, by using electronics with particular operational amplifiers and the combination with a good interface material the SNR ration can be dramatically improved. Also we have encountered several patents, which have some similarities regarding flexibility, shape and the use of electronics. Nevertheless we can conclude that none of the existing patents found use the organic-inorganic hybrid material called diureasils solely as an interface material or in combination with electronics as an dry active electrode.

SUMMARY OF THE INVENTION

[16] The Approach presented in this patent proposal describes a dry active electrode, in combination with the use of a new interface material with innovative characteristics, being this one, of the issues claimed.

[17] The invention we present here is an electrode that can be attached to outer skin in order to capture electrophysiological signals caused by heart, brain, muscle activity or other sources. The electrode is flexible and therefore can adapt to the contours of the human body. The electrode presented here is especially suitable for the detection of signals such as the Electrocardiogram (ECG), but can easily be tuned for any other bio- electrical signal, such as Electroencephalogram signals ( EEG) or Electromyography signals (EMG) . Depending on the electrophysiological signal we could change the shape, size and electrical properties of our electrode, to better suit each application mentioned above. As an example, in EEG a large quantity of electrode is present in a small surface, therefore a smaller shaped electrode would be preferable. Also as EEG is much smaller in amplitude we could enhance the conductivity of the interface material and thus changing the electrical properties. As for EMG studies muscle movement could interfere with skin to electrode interfacial contact. We could therefore enlarge the electrode and add some adhesive properties in order to maintain a high level of signal quality. Furthermore the electrode can be used in wearable electronics or even integrated into fabrics. The electrode consists of the following elements: the sensor consisting of a flexible PCB substrate with the interface material deposited onto its surface. On top of the small flexible PCB is the electronic circuit and a metal button or conductive cloth for easy connection to a measuring device and integration into clothing and other wearable intelligent textiles as for example into vital jacket . The sensor is made from a flexible substrate and coated with an organic-inorganic sol gel derived hybrid, which interfaces with the outer layer of the skin. The sensor captures the signal and conducts it to the electronic circuit. The electrical contact between organic-inorganic hybrid and flexible PCB substrate is made but not limited by conductive textile, yarn or copper wire. The interface material - an organic-inorganic sol-gel derived hybrid - was never used before as a biosensor and therefore constitutes one of the innovative novelties of this product. The organic-inorganic hybrid used as interface material performs similar as the standard wet-electrodes. Furthermore the material can also be attached to fabrics and so enabling the use in wearable electronic sensors. The sensor incorporates a circuit for impedance adaptation and signal processing. As in all active electrodes the circuit has very high input impedance and very low output impedance, but also incorporates a high pass filter for signal conditioning.

[18] The circuit can be powered by a small single 3.6V lithium battery or isolated power supply to prevent the risk of electric shock. The voltage can drop as low as 3.3V, 3.0V to 2.7V until the circuit stops working.

BRIEF DESCRIPTION OF THE DRAWINGS [19] FIG 1: Electrode top view

1 - Flexible PCB substrate

2 - Operational amplifier (op-amp)

3 - Mounting button

4 - Cable connector

5 - Voltage inverter

[20] FIG 2: Electrode bottom view

1 - Flexible PCB substrate

6 - Organic -inorganic dry active electrode attached to the PCB

[21] FIG 3: Mechanical drawing of the electrode as shown in FIG 2, 3

7 - Electrode to view

8 - Electrode side view

9 - Material deposition

10 - Button mounted on top

[22] FIG 4: Block diagram and electrical schematic of the organic-inorganic dry active electrode

[23] 4a) Block schematic

11- Organic-inorganic hybrid material

12- High pass filter

13- High impedance buffer

14- Voltage inverter

15- Output to bio signal recorder

[24] 4b) Buffer circuit

[25] 4c) Voltage inverter

[26] 4d) Power and signal connector

[27] FIG 5: Experimental setup

16 - Dry active electrode RA (right arm)

17 - Dry active electrode LA (left arm)

18 - Wet silver chloride electrode RA (right arm)

19 - Wet silver chloride electrode LA (left arm)

20 - Wet silver chloride electrode reference

21 - Bio signal recorder

22 - Computer

[28] FIG 6: ECG segments for

6 a) Dry active electrode

6 b) Ag/AgCl electrodes

6 c) and 6 d) Power spectral density estimate (PSD) calculated for the complete 5 hour recording. [29] Sampling frequency is 150Hz.

DETAILED DESCRIPTION

[30] Figures 1 to 3 show the mechanical properties of the electrode. As we can see in figure 1 the electrode is build from a flexible substrate (1), which gives the electrode the mechanical strength. Mounted on the top of the substrate are the electronic components (2, 4). We have chosen for a flexible substrate so that the electrode can adapt itself around the contours of the human body. Figure 1 also shows us a mounting button (3) so that the electrode can be attached to textile. A small connector (5) connects the electrode to its supply voltage and signal recorder. In figure 2 we can see the organic-inorganic hybrid (6) attached to the bottom of the flexible substrate (1). The material is first poured into a mould, which is mounted on the electrode and is 2mm to 5mm thick. After drying 24 to 72 hours, the mould is removed and a solid but flexible hybrid is obtained.

[31] Figure 3 shows us the mechanical dimensions of the electrode. The electrode has a diameter between 10mm to 50mm and 2mm to 5mm in height. The material is 10mm to 60 mm in diameter and has a height of about 2mm to 5mm. The button is between 10mm to 20mm in diameter and 5mm to 10mm in height. All of these dimensions can be changed for different purposes in the future.

[32] Illustrated in figure 4 is the block and electrical schematic of the electrode. In 4A we can see the block schematic, which globally describes the dry active electrode. In 4B we see the buffering circuit, which consists of an op-amp, resistor and a capacitor. The Resistor and the capacitor form a high pass filter cut-off at 0.03Hz and thus eliminating DC offsets. The setup tested for ECG signals used a 5G-ohm resistor and a lOnF capacitor, but can easily be modified according to application requirements. The op- amp was chosen for its low power consumption and small size. The power supply used can drop from 5V to 2.7V and the current consumption is 25uA at 5V. Furthermore this op-amp has a very large input impedance which is typically >10 ΤΩ. In order to make the device more universal we have used a voltage inverter 4C in order to generate a negative voltage supply for the op-amp. The dual supply setup gives the op- amp more precision and a greater dynamic range. We have chosen a voltage inverter for our invention. The inverter comes in a very small package and is perfectly suited for battery-powered applications. Because of the low power consumption in this application the efficiency of the voltage inverter is >90 . The resistor 50 ohm and the capacitor lOOnF form a low pass filter with a cut-off frequency of 31 kHz in order to filter switching frequency from the inverter.

[33] In order to test our novel dry active electrodes and compare them to regular Ag/AgCl electrodes we used a wireless wearable two-lead bio-potential recorder (21) see Figure 5. The recorder can accept and power the novel dry active electrodes (16, 17) as use the standard Ag/AgCl electrodes (18, 19, 20) for comparison. For each channel an analogue low pass filter was applied with a cut-off frequency of 100Hz to eliminate high frequency noise. Also a high pass filter of 0.03Hz was applied to eliminate DC offsets. Both channels are sampled at 150Hz and the ADC has a resolution of 8 bits. As can be seen in figure 5 we have applied both electrode types simultaneously on the volunteer's body. Furthermore before attaching the Ag/AgCl electrodes skin preparation some cardiac gel was applied. The dry active electrodes were used without any cardiac gel or skin preparation. We have recorded both signals for 5 hours under laboratory conditions. Presented in figure 6a and 6b we can see the results for both types of electrodes. Figure 6a displays a 10 sec segment of the data recorded regarding the dry active electrodes and 6b the data regarding the Ag/AgCl electrodes. If we compare both data we can see that both electrode types suffer from minor power line interference, which is visible in figures 6c and 6d at the 50Hz peak. Although a minor difference, we can conclude that the dry active electrodes present similar characteristics regarding power line interference when compared to the Ag/AgCl electrodes. Furthermore we look at the 0 to 10 Hz frequency region in figure 6c and 6d we can see that the dry active electrode has less amplitude than the Ag/AgCl electrode. This can be related to the electrode location, which can present differences in signal amplitude and shape. Also we have some movement artifact present in figure 6b but with less intensity as in figure 6a. The results presented here clearly prove that we have a clear working concept for practical applications.

[34] Described below is the synthesis for obtaining the interface material. The raw

materials used in this particular synthesis could be changed according to application.

[35] The material used as interface material is an organic-inorganic hybrid, and is

prepared through the sol-gel process. This process involves the transition of a material from a liquid (colloidal 'sol') state into a solid (gel) state. Sols are a dispersion of particles in liquid called colloids. These colloids are solid particles with a diameter of 1 - 100 nm integrated in a solvent. A gel is an interconnected, rigid network with pores of sub micrometer dimensions and polymeric chains whose average length is greater than a micrometer. A simple example for the sol-gel process is gelatine. Gelatine is at first a powder that is poured in a solvent, which in its case is water. The mixture is then heated at moderate temperatures (<100°C) and the solvent evaporates from the pores and becomes (through hydrolysis and poly condensation) gelatine. Although the proposed materials are manufacture through the sol-gel process, the final product is a solid flexible material that can be made in any shape and size. The organic-inorganic hybrids described in this patent are called diureasils. Diureasils consist of polyethylene oxide (POE) chains of variable length grafted, on both ends, to siloxane domains by means of urea cross linkages. This means that the material has an organic part, which in our case consists of a carbon-based chain, and a surrounding inorganic part, which consists of an organosilica network. These two materials are connected to each other through covalent bonds. The most obvious advantage of organic-inorganic hybrids is that they can favourably combine the often-dissimilar properties of organic and inorganic components in one material. Furthermore in this case the material was doped with iron, but it could also be an alkali metal (e.g. Li, K) or any other transition metal for example Fe, Au or Ag.

[36] A typical procedure for the synthesis of the diureasil host (in this case one with

longer polymer chains, d-U(2000)) doped with Fe²⁺ includes two steps: In the first step, 1.5 g (0.75 mmol) of the diamine precursor ED-2001 was dissolved in 5 mL of tetrahy- drofuran (THF) under ultrasonic conditions. Then 0.39 ml (1.5 mmol) of

3-isocyanatepropyltriethoxysilane (ICPTES) was added under stirring with the molar ratio of ICPTES to ED-2001 to be 2.0. The mixtures were further stirred for 24 hours at room temperature in a nitrogen (N₂) atmosphere. In the second step, 0.5956 g (1.52 mmol) of ((NH₄)₂Fe(S0₄)₂) was dissolved in 1.0 mL of 2.0 mol/L HCI with the molar ratio of n=[O]/[Fe²⁺]=20. Here the [O] represents the number of OCH₂CH₂ units in the polymer chain of ED-2001 while the [Fe2+] stands for the OCH₂CH₂ molar number of per Fe2+ cation. When ((NH₄)₂Fe(S0₄)₂) was dissolved totally, a clear green solution was obtained, and added to the sol made from the first step dropwise under stirring. After a short time the sol changed to a viscous solution, which was poured in a mould. Gelification occurred from 2 to 5 minutes and the resulting organic-inorganic hybrid can be used as an electrode to skin interface material for our novel dry active electrode.

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Claims

A flexible organic-inorganic dry active electrode for bioelectrical signal detection characterized by the following elements:

an active electrode, which comprises a flexible organic-inorganic hybrid material(6) attached but not limited to a flexible electronics substrate(l) and electrically connected to a buffering circuit(2, 5), which is located on the flexible PCB substrate.

A flexible organic-inorganic dry active electrode according to claim 1 characterized by the dry active electrode uses a flexible organic-inorganic hybrid material(6) connected to electronics which are flexible electronics embedded on the organic-inorganic hybrid material, offering a bendable dry active electrode enabling better contact with the skin

A flexible organic-inorganic dry active electrode according to claims 1 or 2 characterized by the buffering circuit(4b) in combination with an high pass filter comprising of a resistor and a capacitor(4b), and which power supply is a positive and a negative voltage(4c).

A flexible organic-inorganic dry active electrode according to claim 3 characterized by a metal button(3) or conductive textile, which allows simple connection to a measuring device and integration into clothing and other wearable intelligent textiles for long term monitoring of bioelectrical signals.

A flexible organic-inorganic dry active electrode according to claim 3 characterized by an electrode with the flexible PCB substrate completely embedded into the organic-inorganic hybrid material creating so an electrode which is completely surrounded by the organic-inorganic hybrid material.

A flexible organic-inorganic dry active electrode according to claims 1, 2, 3, 4 or 5 characterized by the electrical contact between organic-inorganic hybrid and flexible PCB substrate be made but not limited by conductive textile or copper wire.

A flexible organic-inorganic dry active electrode according to claim 1, 2, 3, 4, 5 or 6 characterized by the flexible organic- inorganic hybrid material be doped with transition metals such as Fe, Au or Ag.

A flexible organic-inorganic dry active electrode according to claim 1, 2, 3, 4, 5 or 6 characterized by the flexible organic- inorganic hybrid material be doped with alkali such as K or Li.