Patent Application
20230374330
The current invention relates to conductive electronic textiles, their formation and uses.
The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.
Electronic textile (E-textile) is a kind of smart textile integrated with sensors, transducers, energy devices, and wireless communications. It provides massive opportunities for healthcare and environment monitoring, human-machine interface, and defense. Human has a long history of using textile as clothing for body protection and fashion, and emerging flexible electronics impart new functions to traditional textiles. Textiles are in direct contact with human skin in daily wearing and thus, can serve both as substrates for the fabrications of emerged flexible devices and the platform for traditional rigid silicon circuits (Niu, S. et al., Nat. Electron. 2019, 2, 361-368; and Tian, X. et al., Nat. Electron. 2019, 2, 243-251). For E-textile, the most important part is the highly flexible and conductive electrodes and interconnections that can be durable to mechanical deformation, especially stretching, coming from the wearer's daily activities. Various technologies have been invented to pattern flexible conductive electrodes on textiles, from functionalizing the original fabrics to directly coating conductive materials on textiles (U.S. Pat. No. 8,945,328 B2; and Wang, L. et al., Adv. Mater. 2020, 32, 1901971).
Printing technologies, including screen printing, 3-D printing, inkjet printing, are highly compatible with textile technologies due to their low-temperature curing, ease of pattern design, scalable production, customer-design capability, and less material waste. The use of printing to produce conductive electrodes and interconnects on textiles has been widely adopted in wearable sensors, wireless communications, energy devices. The key components in wearable devices are stretchable ink, flexible and soft functional electrodes, and conductive wiring.
Silver (Ag)-based stretchable inks that include the elastomer at a high proportion as the binder, and Ag particles as the conductive fillers have been widely applied as inks for textile-based wiring or interconnect due to their superior conductivity, high stretchability and good stability compared with other inks, such as carbon inks and copper inks. The Ag-based inks can be printed directly on textiles or through a polymeric interlayer. During the stretching, the binder will prevent the separation of Ag conductive fillers and maintain the conductive percolation. However, most of the existing Ag-based textile still suffer from low initial resistance when compared with bulk Ag electrodes, and significant resistance increase during stretching. The high initial resistance comes from the surfactant on the surface of Ag flakes and a high proportion of elastomeric binder. The significant resistance increase during stretching cycling results from the separation of Ag flakes from adjacent flakes that will slide to accommodate the external strain.
Some methods have been invented to solve these problems. Takao Someya used low vapor pressure and high boiling point solvent to enable the deep permeation of ink into the textile and thus, the ink will cover each fiber bundle instead of filling up the vacancy (Jin, H. et al., Adv. Mater. 2017, 29, 1605848). Then, the wave structure of the textile instead of the Ag flakes ink was used to deform along with the strain. This method is limited by the use of a high boiling point solvent which means that the spreading of the ink after printing is very severe and thus, high precision is hard to reach. Also, the hot-press process (160° C.) in this method may induce the degradation of the textile substrate, considering most commodity textile degrades at temperatures between 125 and 180° C. Chung and coworkers used Ag powders to replace Ag flakes to improve the penetration of the ink into the textile (La, T.-G. et al., Adv. Healthc. Mater. 2018, 7, 1801033). However, the efforts only involve the enhancement of permeation depth and still can't reach both low initial resistance and high conductivity during cycling stretching.
The interaction between wearable electrodes and body motion, body heat, or biofluids can be applied to realize in-vivo physiological signal sensing (Kim, J. et al., Nat. Biotechnol. 2019, 37, 389-406), human-machine interactions (Wang, J. et al., Mater. Today 2018, 21, 508-526), drug delivery (Jin, H. et al., Adv. Mater. 2017, 29, 1605848), or bioenergy harvesting (La, T.-G. et al., Adv. Healthc. Mater. 2018, 7, 1801033; and Sun, S. et al., J. Mater. Sci. Mater. Electron. 2016, 27, 4363-4371). The materials design and interaction mechanism of wearable devices with the human body are vital for the successful realization of specific applications, which would open up a new era for the design of wearable devices. However, most of the research focuses on the versatile design of the functional electrodes, and the effect of biofluids on the conductive wiring has rarely been studied.
Human sweat is one of the most available biofluids that have been applied in the fabrication of non-invasive biosensors for continuous monitoring of physiological information (Kim, J. et al., Acc. Chem. Res. 2018, 51, 2820-2828). Also, biocompatible and safe wearable biofuel cells (Lv, J. et al., Energy Environ. Sci. 2018, 11, 3431-3442), supercapacitors (Manjakkal, L. et al., Adv. Mater. 2020, 32, 1907254), and batteries (Bandodkar, A. J. et al., Nat. Electron. 2020, 3, 554-562) that use human sweat as electrolytes have been fabricated as power sources for wearable sensors. Sweat containing lactic acid and glucose, and ions (such as Na+ and Cl−) enables the realization of wearable biofuel cells, supercapacitors, and batteries to function as well as their regular counterparts. Conductive and stretchable wiring plays a key role in the design and fabrication of stretchable devices. However, the effect of sweat on the Ag electrodes has always been considered a deteriorating effect for electrode's conductivity because of the enhanced corrosion behavior of bulk metals in the presence of saline solution (Zhou, W. et al., ACS Nano 2020, 14, 5798-5805).
Therefore, there exists a need to discover new wearable textile electronics that can overcome the limitations mentioned above.
1. A flexible textile-based silver electrode, comprising:
2. The flexible textile-based silver electrode according to claim 1, wherein the elastomeric material is selected from one or more of a silicone rubber, a styrenic elastomer, and a polyurethane-based elastomer.
3. The flexible textile-based silver electrode according to claim 2, wherein the elastomeric material is a hydrophilic polyurethane acrylate elastomer.
4. The flexible textile-based silver electrode according to claim 2 or claim 3, wherein the elastomeric material is cured.
5. The flexible textile-based silver electrode according to any one of claims 2 to 4, wherein the uncured elastomeric material has a formula I:
6. The flexible textile-based silver electrode according to any one of the preceding claims, wherein one or more of the following apply:
7. The flexible textile-based silver electrode according to any one of the preceding claims, wherein one or more of the following apply:
8. A sweat-activated battery comprising:
9. The battery according to claim 8, wherein the anode sweat-activated active material is selected from one or more of zinc powder (particles/flakes) and carbon particles (e.g. carbon black, graphite, carbon nanotube and graphene), optionally wherein the anode sweat-activated active material is a combination of zinc flakes and carbon black in a weight to weight ratio of from 80:20 to 95:5, such as 90:10.
10. The battery according to claim 8 or claim 9, wherein the weight to weight ratio of the anode sweat-activated active material to elastomeric material is from 1:1 to 1:3, such as 1:2.
11. The battery according to any one of claims 8 to 10, wherein the cathode sweat-activated active material is selected from one or more of Ag2O powder and carbon (e.g. carbon black, graphite, carbon nanotube and graphene), optionally wherein the cathode sweat-activated active material is a combination of Ag2O powder and carbon black in a weight to weight ratio of from 85:15 to 98:2, such as 95:5.
12. The battery according to any one of claims 8 to 11, wherein the weight to weight ratio of the cathode sweat-activated active material to elastomeric material is from 1:1 to 1:1.5, such as 1:1.2.
13. The battery according to any one of claims 8 to 12, wherein each of the first to third elastomeric materials are independently selected from one or more of a silicone rubber, a styrenic elastomer, and a polyurethane-based elastomer.
14. The battery according to claim 13, wherein each of the first to third elastomeric materials are a hydrophilic polyurethane acrylate elastomer.
15. The battery according to claim 13 or claim 14, wherein each of the first to third the elastomeric materials are cured.
16. The battery according to any one of claims 13 to 15, wherein for each of the first to third elastomeric materials the uncured elastomeric material has a formula I:
17. The battery according to any one of claims 8 to 16, wherein the weight to weight ratio of the silver flakes to the third elastomeric material is from 1:0.1 to 0.1:1, such as from 1:0.5 to 0.5:1 such as about 1:0.75.
18. The battery according to any one of claims 8 to 17, wherein one or more of the following apply:
19. A device comprising:
20. A method of making a flexible textile-based silver electrode as described in any one of claims 1 to 7, comprising the steps of:
21. The method according to claim 20, wherein the pH of the aqueous solution is from 2.5 to 4.0.
22. The method according to claim 20 or claim 21, wherein step (b) of claim 20 is conducted in a washing machine.
23. The method according to any one of claims 20 to 22, wherein the aqueous solution comprises from 0.1 to 1 wt/v %, such as from 0.4 to 0.6 wt/v %, such as about 0.5 wt/v % of an inorganic chloride salt and from 0.05 to 0.5 wt/v %, such as from 0.075 to 0.125 wt/v %, such as about 0.1 wt/v % of an organic acid.
24. The method according to any one of claims 20 to 23, wherein one or both of the following apply:
25. The method according to any one of claims 20 to 24, wherein the period of time in step (b) of claim 20 is at least 30 seconds to 24 hours.
26. The method according to any one of claims 20 to 25, wherein the flexible textile substrate is loaded with an organic acid, optionally wherein the organic acid is selected from one or more of the group consisting of citric acid, acetic acid, tartaric acid, malic acid and, more particularly, lactic acid.
It has been surprisingly found that a mild and non-harmful solution (e.g. artificial sweat, European Standard number EN1811: 2012) containing 0.5% of NaCl, 0.1% of KCl, 0.1% of lactic acid, and 0.1% of urea (weight/volume ratio) or analogues thereof) can be used to sinter and increase the conductivity of textile-based silver electrodes. This “sweat sintering” process is rapid and can take less than 5 minutes to provide a permanent change in the conductivity of textile-based silver electrodes. The stretchable electrodes contain elastomer binder and Ag-based conductive fillers. The original artificial sweat is slightly acidic with a pH of 2.7 and can be tuned with a higher pH to make it even less harmful to human skin and the textile substrate. Without wishing to be bound by theory, it is believed that in a mild acidic environment and in the presence of Cl−, the surfactant on the surface of the silver flakes can be partially removed and new silver nanoparticles can be formed that contact the adjacent silver, thus the initial resistance of the Ag-based electrode can be highly reduced. This results in a flexible textile-based silver electrode with enhanced properties.
Additionally, there remains a need for suitable electrodes that can be incorporated into wearable technologies in a manner that is unobtrusive and with may provide robust and secure connections even after being subjected to washing. It has been surprisingly found that such a material can be created using the method outlined above. Thus, in a first aspect of the invention there is provided a flexible textile-based silver electrode, comprising:
In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of” or synonyms thereof and vice versa.
The phrase, “consists essentially of” and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.
As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a composition” includes mixtures of two or more such compositions, and the like.
It is believed that the physical properties of the flexible textile-based silver electrode disclosed herein are achieved due to the treatment with sweat (or a sweat-like substance). Without wishing to be bound by theory, it is believed that silver flakes on at least the surface portion of the polymeric silver electrode wire react with the sweat (or sweat-like substance) and form particles with a coarser surface and/or adjacent silver flakes merge together. These physical changes may result in the enhanced physical properties obtained by the resulting product disclosed herein.
Textiles for the purposes of the present invention include, for example, woven and knitted fabrics, bonded and unbonded nonwovens and microfibre nonwovens. These may be made from synthetic, natural fibres and/or blends thereof. The flexible textile substrate may be stretchable or non-stretchable. When used herein, “stretchable” refers to a material that can be stretched to 120% of its original size in at least one dimension upon application of a strain and recovers to its original size (or at least 90% thereof) upon removal of said strain.
It will be appreciated that any suitable elastomeric material that can be applied in a pattern on a substrate surface, and which retains this pattern after it is laid thereon may be used herein. In embodiments that may be mentioned herein, the elastomeric material may be hydrophilic. Examples of suitable elastomeric materials include, but are not limited to a silicone rubber, a styrenic elastomer, a polyurethane-based elastomer and combinations thereof (e.g. blends thereof). In particular embodiments of the invention that may be mentioned herein, the elastomeric material may be a hydrophilic polyurethane acrylate elastomer.
The elastomeric material used in the polymeric silver electrode may be provided in an uncured or a cured form. When used herein, the term “uncured” is used to indicate a material that is capable of forming crosslinks, while the term “cured” is used to refer to a material that is capable of crosslinking that has been subjected to at least partial crosslinking (e.g. from 5 to 100% of the available crosslinking sites are crosslinked). The crosslinking may occur through the backbone of the polymer and/or through pendant groups on the polymer.
In embodiments that may be mentioned herein, the uncured elastomeric material may have a formula I:
Any suitable amount of silver flakes may be used in the polymeric silver electrode. For example, the weight to weight ratio of the silver flakes to elastomeric material may be from 1:0.1 to 0.1:1, such as from 1:0.5 to 0.5:1 such as about 1:0.75.
In particular embodiments of the invention, the resistance of the flexible textile-based silver electrode in a relaxed state may be from 0.1 to 1.5Ω. Additionally or alternatively, the resistance of the flexible textile-based silver electrode in certain embodiments may not exceed 7Ω when the flexible textile-based silver electrode is subjected to 500 cycles of being elongated by 30% of its original dimension in any direction. The above-mentioned parameters may apply to an electrode that has a length of 2 cm and a width of 3 cm, respectively (or vice versa).
It is noted that the flexible textile-based silver electrode disclosed herein may continue to benefit from an interaction with sweat during use. However, in order to give mechanical strength to the electrode wire it may benefit from an additional elastomeric coating layer that is applied on top of the polymeric silver electrode. That is in certain embodiments, a surface of the polymeric silver electrode wire that is not in direct contact with the textile substrate may be coated by a non-silver containing elastomeric material.
In certain embodiments, the polymeric silver electrode wire may be penetrated by parts of the textile substrate. This arrangement may be useful as it may allow the penetration of sweat into the interior portion of the polymeric silver electrode wire, thereby allowing the sintering process (described in more detail below and in the examples) to occur, which may help to provide the good properties reported herein. Thus, in certain embodiments, the flexible textile substrate may comprise a plurality of bundles of yarn on the surface of the textile substrate, where the plurality of bundles of yarn in contact with the polymeric silver electrode wire extend partly into the polymeric silver electrode wire.
The elastomeric material itself may be hydrophilic or hydrophobic in nature. However, in certain embodiments that may be mentioned herein the elastomeric material may have a water contact angle of from 10 to 25°, such as from 15 to 16°, after contact with a water droplet for 80 minutes. It is noted that the importance of contact angle depends on the application. For example, if there is a need for sweat to play a role in the use of the flexible textile-based silver electrodes, then one may prefer to use a hydrophilic material (e.g. one that has a water contact angle less than or equal to 90° after contact with a water droplet for 80 minutes (or one with the water contact angles described above). This is because it may not always be possible for the wearer to secrete a large amount of sweat during their daily activities. For applications where contact with sweat is not essential, then a hydrophobic material may be used as the elastomeric material instead (e.g. a material that has a water contact angle of greater than 90° after contact with a water droplet for 80 minutes). In such cases, if sufficient sweat or analogue solution is used for a sufficient amount of time, then the desirable properties mentioned herein may be achieved.
As will be appreciated, the flexible textile-based silver electrode may be particularly suited for use in wearable technologies. One example of a wearable technology that may be described herein is a sweat-activated battery. Thus, in a further aspect of the invention, there is provided a sweat-activated battery comprising:
The battery may make use of any suitable sweat-activated active material for the anode and cathode.
For example, the anode sweat-activated active material may be selected from one or more of zinc powder (particles/flakes) and carbon particles (e.g. carbon black, graphite, carbon nanotube and graphene). In particular embodiments that may be mentioned herein, the anode sweat-activated active material is a combination of zinc flakes and carbon black in a weight to weight ratio of from 80:20 to 95:5, such as 90:10. In more particular embodiments of the invention, the weight to weight ratio of the anode sweat-activated active material to elastomeric material may be from 1:1 to 1:3, such as 1:2.
For example, the cathode sweat-activated active material is selected from one or more of Ag2O powder and carbon (e.g. carbon black, graphite, carbon nanotube and graphene). In particular embodiments that may be mentioned herein, the cathode sweat-activated active material may be a combination of Ag2O powder and carbon black in a weight to weight ratio of from 85:15 to 98:2, such as 95:5. In yet more particular embodiments of the invention that may be mentioned herein, the weight to weight ratio of the anode sweat-activated active material to elastomeric material may be from 1:1 to 1:1.5, such as 1:1.2.
As will be appreciated, the first to third elastomeric materials may be the same or different and may be selected from any suitable material. The materials and properties for the first to third elastomeric materials in the battery may be similar to the elastomeric material mentioned hereinbefore in relation to the first aspect of the invention. As such, the first to third elastomeric materials may be independently selected from one or more of a silicone rubber, a styrenic elastomer, and a polyurethane-based elastomer. In more particular embodiments, each of the first to third elastomeric materials may be a hydrophilic polyurethane acrylate elastomer.
In keeping with the first aspect of the invention, each of the first to third the elastomeric materials may cured.
In particular embodiments that may be mentioned herein, each of the first to third elastomeric materials the uncured elastomeric material may have a formula I:
As will be appreciated, properties relating to the interplay if the silver flakes and the third elastomeric material may be the same as mentioned hereinbefore for the first aspect of the invention. Thus, the weight to weight ratio of the silver flakes to the third elastomeric material may be from 1:0.1 to 0.1:1, such as from 1:0.5 to 0.5:1 such as about 1:0.75. Additionally, one or more of the following may apply:
In a third aspect of the invention there is provided a device comprising:
As will be appreciated, additional components may be added to provide the device with any desired activity. Said device may, for example relate to a system comprising three sweat-activated batteries in series connected to a capacitor, which in turn drives a wireless temperature sensor (see the examples section below for further details).
As noted above, the production of the flexible textile-based silver electrode relies on a method of subjecting an original flexible textile-based silver electrode to a solution that is analogous to sweat. This method of production may comprise the steps of:
It is noted that the amount of sweat produced by a subject depends on many factors, such as gender, environment, age, the amount of physical exercise undertaken, and the location on the subject's body. Without wishing to be bound by theory, it is believed that the amount of sweat needed to obtain the desired properties described herein for the flexible textile-based silver electrode disclosed herein is not always possible to obtain from a subject. This is particularly the case when a hydrophobic elastomeric material is used, which is the case for conventional materials to date, as this minimises the ability of the sweat to contact the silver wire to cause any sintering reaction to occur—much less for there to be a uniform sintering reaction across the entirety of the flexible textile-based silver electrode.
The method described above only involves the use of a mild solution (e.g. artificial sweat or an analogue there) to increase the conductivity of the electrodes in relaxed and stretched states. This method can effectively increase the conductivity of the printed electrodes in less than 5 minutes. Additionally, this method can be used with almost any textile substrate and any suitable silver-based ink, while being non-harmful to the end user. Previous methods relying on the deep permeation of ink into the textile need a high-temperature (e.g. 160° C.), which sacrifices the precision of the originally printed traces. Some reported methods use either strong acid (HCl) or high concentration NaCl solution to treat unstretchable silver-based electrodes. However, when it comes to textile-based electrodes, the strong acid and high-concentration of NaCl may degrade the property of the textile substrate, especially for some sensitive textiles, like silks and cotton. Solely weak acid and low-concentration NaCl cannot sinter the Ag-based electrodes in an effective and fast manner. Artificial sweat contains weak acid-lactic acid and a low concentration of NaCl (and other chlorides salts), both are non-harmful to textile substrates and human skin. As described herein, the synergetic effect between these two components (and their replacements mentioned herein) enables the fast and effective sintering of silver-based electrodes. Our method can also be used to fabricate wearable silver-based textile electrodes that use human sweat to increase the conductivity during exercise activities because artificial sweat has similar components to that of real human sweat. The electrode may include the hydrophilic polymer HPUA as the binder.
Any suitable pH may be used in the method disclosed above. For example, the pH may be from pH 1 to 5. However, in order to prevent damage to the skin of a wearer and/or to the textile substrate, the pH of the aqueous solution may be from 2.5 to 4.0.
Step (b) of the process above may be conducted in a washing machine. As such, the resulting product may be machine washable. For example, the product may be able to withstand up to 100 machine washes, such as up to 50 machine washes, such as up to 25 machine washes, such as from 10 to 100 machine washes without losing its functionality.
While the product may make use of an artificial sweat solution, this can be replaced by an analogous solution. For example, the aqueous solution used in the process may comprise from 0.1 to 1 wt/v %, such as from 0.4 to 0.6 wt/v %, such as about 0.5 wt/v % of an inorganic chloride salt and from 0.05 to 0.5 wt/v %, such as from 0.075 to 0.125 wt/v %, such as about 0.1 wt/v % of an organic acid. Without wishing to be bound by theory, it is believed that it is the organic acid and the inorganic chlorides in sweat that provide the desirable results and that the other components present in sweat are not required.
In particular embodiments that may be mentioned herein, the non-toxic chloride salt may be selected from one or more of the group consisting of CaCl2), MgCl2 and, more particularly, NaCl and KCl. In additional or alternative embodiments that may be mentioned herein, the organic acid may be selected from one or more of the group consisting of citric acid, acetic acid, tartaric acid, malic acid and, more particularly, lactic acid.
Any suitable period of time may be used for step (b) of the process above. For example, the period of time in step (b) may be at least 30 seconds to 24 hours.
In particular embodiments of the invention, it may be desired to pre-load the flexible textile substrate with an organic acid before use in the method above. This may help ensure that the desired properties are obtained consistently along the length of the polymeric silver electrode wire. As will be appreciated, the organic acids discussed above may be used in this preloading. That is, the organic acid may be selected from one or more of the group consisting of citric acid, acetic acid, tartaric acid, malic acid and, more particularly, lactic acid.
Advantages associated with the current invention include the following.
Materials
Polytetrahydrofuran glycol (PTHF, Mw=1000 g/mol, 98%) and POE (Mw=1000 g/mol, 98%) were purchased from Sigma-Aldrich and degassed at 130° C. for 3 h before use. 1,1,1-tris(hydroxymethyl)propane (TMP, Mw=134.17 g/mol, 97%), dibutyltin dilaurate (DBTDL, 95%), HEMA (Mw=130.14 g/mol, 99%), dibutyltin dilaurate (DBTDL, Mw=631.56 g/mol, 95%), acetone (anhydrous 99.5%), sodium chloride, DL-lactic acid (˜90%), potassium chloride, sodium hydroxide, 2-butanone and urea were also received from Sigma-Aldrich and used without any purification. Isophorone diisocyanate (IPDI, a mixture of isomers, 98%), carbon black and silver oxide were purchased from Alfa Aesar. Methyl ethyl ketone (MEK, 99%), 1-hydroxycyclohexyl phenyl ketone (IRGACURE® 184, Mw=204.3 g/mol, 99%), Ag flakes (10 μm), SEBS (Tuftec™ H1052) with 20/80 S/EB weight ratio, and hydrophilic textile (100% polyester knitted fabric) were obtained from Fisher Chemical, Ciba Specialty Chemicals, Puwei Applied Materials Technology, Asahi Kasei Corporation, and MHTC Technology Company, respectively. Deuterated solvents for NMR characterization were obtained from Cambridge Isotope Laboratories, Inc. DI water was used throughout the study. Zn flakes were purchased from Hunan Jinhao New Material Technology Co., Ltd.
Analytical Techniques
Nuclear Magnetic Resonance (NMR) Spectroscopy
1H and 13C NMR were performed on 400 MHz Bruker DPX 400. 1H and 13C NMR were carried out at ambient temperature using deuterated solvents as lock and the residual solvent or tetramethylsilane (TMS) signal as the internal standard.
Fourier Transform Infrared Spectroscopy (FTIR)
FTIR was performed on Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR), Perkin Elmer, Frontier.
Thermogravimetric Analysis (TGA)
The thermal property of the hydrophilic poly(urethane-acrylate) (HPUA) film was characterized using TGA (TA Instruments Q500). 10-25 mg of HPUA samples were placed in a platinum pan and heated from room temperature (RT) to 700° C. under a nitrogen (N2) atmosphere at a heating rate of 10° C./min.
Dynamic Mechanical Analysis (DMA)
DMA analysis was conducted on a dynamic mechanical analyzer (TA Instruments, DMA Q800) and samples were measured in a temperature range of −80 to 170° C. using a heating rate of 10° C./min in a liquid N2 environment.
Ultraviolet-Visible (UV-Vis) Spectroscopy
UV-Vis absorption and transmission spectra were recorded with a UV-2501 PC spectrometer (Shimadzu, UV-2501 PC) in the range of 190-900 nm at ambient temperature.
Contact Angle Measurement
To measure solid-liquid contact angles between the glass and the testing artificial sweat liquids were applied on the dried HPUA films, while the contact angles were digitally measured by a contact angle meter (Data physics OCA 15 Pro).
Scanning Electron Microscopy (SEM)
SEM and high-magnification SEM characterization were performed on JEOL 7600.
Conductive Atomic Force Microscopy (C-AFM)
C-AFM was performed on Cypher S (Asylum Research). The samples were adhered on the glass substrate with a conductive wire connected to the AFM equipment.
NCO-Functionalized Urethane Prepolymers (OCN-PTHF-NCO)
PTHF (14.50 mmol) was charged in a dried three-necked glass vessel equipped with a thermometer, a mechanical stirrer, a condenser, and an inlet of dry N2 was degassed in a vacuum (<133 Pa) at 100° C. for 1 h to remove any moisture and then reduced to 85° C. Isophorone diisocyanate (44.95 mmol) and catalyst (DBTDL, 0.2 wt % of PTHF and IPDI, 2000 ppm) dissolved in MEK (5 mL) were added dropwise into the vessel and the resultant mixture was kept stirring at 85° C. for 6 h under an N2 atmosphere to yield OCN-PTHF-NCO. Afterward, 20 mL of dry MEK was added to dissolve OCN-PTHF-NCO.
HPUA
A typical procedure of the preparation of the HPUA and their chemical structures are shown in
1H-NMR (400 MHz, CDCl3, 25° C.) δ (ppm): 0.89 (s, 9H, —CH3), 0.91 (s, 3H, —CH3), 1.46 (d, 6H, —CH2), 1.67 (q, 4H, —CH2—), 2.86 (d, 2H, —CH2—NH—), 3.43 (d, 2H, —CH—O—), 3.98 (s, 4H, —C—CH2—O—), 3.39-3.42 (s, CH2O, PTHF), 4.05-4.07 (d, OCH2, PTHF) 5.98 (d, 2H, ═CH2, HEMA), 5.43 (d, 2H, ═CH2, HEMA), 7.96 (t, 1H, —NH—C═O—O—).
Results and Discussion
The photocurable HPUA elastomeric binder was synthesized by a one-pot step-growth polycondensation followed by radical polymerization from simply available PTHF as a diol, IPDI as a diisocyanate, hydrophilic POE as a macrodiol chain extender to yield covalently crosslinked networks, tri-functional TMP polyol as an internal crosslinker to yield covalent crosslinked PU networks, and hydrophilic HEMA as a reactive diluent, in the presence of catalyst DBTDL (
The successful synthesis of HPUA and its intermediates were verified by 1H and 130 NMR, FTIR (
The FTIR characteristic peaks at 3318 cm−1 (N—H) and 1713 cm−1 (C═O) stretching vibrations indicate the formation of —NH—C═O—O groups (
The freestanding film of HPUA was prepared as follows. The synthesized HPUA was diluted with anhydrous 2-butanone (85 wt %) stirred at 40° C. for 30 min. After perfect mixing, 1 wt % of 1-hydroxycyclohexyl phenyl ketone of the total weight of the HPUA resin was added. The reaction mixture was stirred for another 30 min at RT to obtain a homogeneous HPUA mixture. The resulting HPUA resin was then poured onto a glass petri dish and irradiated by a medium pressure UV lamp (365 nm) for 30 min, with a distance of 8-10 cm from the UV lamp to the focal point of the samples. The petri dish was coated with a releasing agent (WD-40 Spec Silicon Spray) to prevent adhesion of the HPUA resin to allow easy release from the petri dish. The resultant HPUA films were peeled off and placed into the desiccator at ambient condition for further analysis. The curing behavior of the HPUA film was analyzed by observing the changes in the absorption band of the acrylate group (C═C) at 1634 cm−1 and 810 cm−1.
Compared with SEBS which is one of the representatives of widely-used polystyrene-based block copolymers binder in printed stretchable conductors (You, I., Kong, M. & Jeong, U., Acc. Chem. Res. 2019, 52, 63-72; and Silva, C. A. et al., Adv. Funct. Mater. 2020, 30, 2002041), the synthesized HPUA demonstrated hydrophilic property to artificial sweat. The contact angle of the HPUA binder was 82.59°, which gradually decreased to 15.12° over a period from 0 to 80 min (
Tensile Tests
All tensile test samples were prepared according to ASTM D638-10 and mechanical stress-strain was obtained with MTS criterion model 43 (MTS Systems Corporation, Eden Prairie, MN, USA) static mechanical tester with a load cell of 500 kN at a strain rate of 100 mm/min at RT. The HPUA elastomer films were prepared with different molar ratios of POE, with a sample width of 5 mm, a thickness of 0.8-0.9 mm, and a length of 10 mm. To prepare the notched sample, a notch of 2.5 mm was made in the HPUA film with 1 mm thickness, 10 mm gauge length, and 5 mm width.
Results and Discussion
The tensile-test results show that HPUA-2 exhibited superior mechanical properties (
Formulation of Ag-HPUA Ink and Ag-HPUA Electrode Printing
Amber glass vials were used as containers to avoid the photo-curing of the ink before printing. 0.75 g HPUA was mixed with 1 g Ag flakes by shaking on a vortex mixer for 10 min. A stainless steel stencil (Micro Tech Technology, Shenzhen, China) fabricated by laser cutting with a thickness of 200 μm was applied to perform the screen printing. Electrode patterns were designed on Auto CAD software. The electrodes were directly printed on top of the hydrophilic textile in a lab-made black box and then cured in the ambient lab environment for 30 min. A 3D printer called System 30 M manufactured by Hyrel 3D (USA) with a 0.36 mm nozzle was used to perform the 3D printing on textile. The printing speed was 10 mm/s and the curing process was finished in a room environment without heating and UV lighting. The size of textiles was 20 mm*30 mm and Ag-HPUA electrodes with a size of 3 mm*30 mm were printed in the center of textiles.
Formulation of Ag-SEBS Ink and Ag-SEBS Electrode Printing
The Ag-SEBS ink was formulated by mixing Ag flakes with SEBS resin (3 g of SEBS/10 ml of toluene) at a weight ratio of 1:0.75 under shaking by a vortex mixer for 1 h (MX-S, DLAB Scientific Inc). The printing of Ag-SEBS electrode was performed by following the protocol above except Ag-SEBS ink was used instead of Ag-HPUA ink.
Results and Discussion
The sweat-enhanced conductivity was performed by using a multimeter (DAQ6510, Keithley) to test the resistance change of electrodes with one pass printing (length: 2 cm, width: 0.3 cm) after adding the artificial sweat. The artificial sweat was prepared based on the European standard. The composition of the original artificial sweat contained 87 mM NaCl, 13 mM KCl, 17 mM of lactic acid, and 16 mM of urea, and the original pH was 2.7 (Liu, G. et al., Sens. Actuators B Chem. 2016, 227, 35-42). pH was measured using a pH meter (HI 2020F edge, HANNA Instruments) and tuned by adding 0.1 M NaOH. The stretching test was performed on electrodes with 3 passes of printing using a motorized force test stand (ESM303, Mark-10) with a speed of 10 mm/min. The speed of cyclic stretching was 20 mm/min. The surface conductivity of the electrode was tested by conductive-atom force microscopy (Asylum Research Cypher S). XRD and XPS were characterized by a Shimadzu powder diffractometer (Cu Kα, λ=1.5406 Å) and a PHI Quantara II, respectively. FTIR and TGA were used to characterize the functional groups and weight percentage of surfactant on Ag flakes, respectively. The electrical hysteresis is defined as the resistance increase after the cycling stretching.
Results and Discussion
When the printed electrodes encounter human sweat, the acidic environment, and Cl− work together to partially remove the insulating lubricant layer and increase the contact among adjacent Ag flakes by redepositing Ag from dissolved Ag+, making the surface of Ag-HPUA electrodes rougher, as shown in
HPUA with ambient photo-curable property, hydrophilic nature, and high stretchability was synthesized to serve as the binder to increase the favorability of printing, accessibility of sweat, and endurability to mechanical deformation, respectively. The photoinduced HPUA elastic binder composed of hard-segments made up of carbamate groups (—NH—C═O—O—) and SS made up of aliphatic polyether (—O—) or polyester (—CO—O—) backbone is capped with acrylate (C═C) functionality at each end. Hydrogen bonds mainly form between N—H groups and C═O groups on the hard domains. In the HPUA synthesis, HEMA and POE are the key components in the design of photo-curable HPUA. Beyond acting as the reactive diluent to tune the viscosity of the prepolymer for the ease of printing, HEMA is crucial for the double bond of the acrylate (C═C) group on the covalent networks (Parida, K. et al., Nat. Commun. 2019, 10, 2158). The POE can enhance the hydrophilicity due to their ether oxygen bonds on SS (Kokkinis, D., Schaffner, M. & Studart, A. R., Nat. Commun. 2015, 6, 8643). The HPUA elastomer includes a —HN—(C═O)—NH-containing first structural unit capable of forming a strong hydrogen bond and a —HN—(C═O)—NH-tipped with acrylic units capable of forming radical polymerization with the carbon-carbon double bonds at the end of the chains which are bound to the polymer main chain. The strong hydrogen bond imparts flexibility and mechanical strength, while the hydrogen bond with the acrylic unit imparts UV-curability and hydrophilicity. The fast ambient photo-curable nature of HPUA polymer enables green printing without the massive use of organic solvents, and the potential for complex and high-precision structures design in 3D printing (Patel, D. K. et al., Adv. Mater. 2017, 29, 1606000). Considering the low sweat generation on some occasions, the hydrophilic HPUA binder favors the contact between human sweat and Ag-HPUA electrodes.
Comparing with commonly used thin polymer film substrates, the hydrophilic textile substrate used here facilitates in-situ sweat sampling, and acts as a reservoir. Sweat reservoiring capability and porous structure of textiles give prolonged reaction time and enhanced surface area for printed electrodes to contact with sweat. Besides, the component of human sweat varies with gender, secretion area, surrounding, time of day, age, and sweating rate, which will impede the completeness of sintering effects. The textile substrate can be used to pre-store active substances, such as lactic acid and Cl− salt, to weaken the effects of irregular sweating behavior of wearers. In our design, the whole electrode fabrication involves the direct printing on textiles, including screen printing and 3D printing (
The printed electrodes attached well on the top of the textile substrate after printing, as shown in the cross-sectional SEM images of the printed Ag-HPUA electrodes before and after soaking with sweat in
The sweat can instantly decrease the resistance of Ag-HPUA electrodes and the resistance loss induced by sweats with different pH is shown in
Therefore, the UV-curable inks that contain UV-curable hydrophilic binder and Ag flakes (
To understand the key components inside sweat that enhance the conductivity of Ag-HPUA electrodes, resistance change of the Ag-HPUA electrodes after soaking with 4 solutions including lactic acid/urea, NaCl/KCl/urea, lactic acid/NaCl/KCl, and original artificial sweat was studied by following the sweat-enhanced conductivity protocol in Example 7.
Soaking of Ag-HPUA Electrodes
After the curing of Ag-HPUA electrodes, they were soaked in 2 ml of one of lactic acid/urea, NaCl/KCl/urea, lactic acid/NaCl/KCl, and original artificial sweat solutions for 15 h. The resistance of the four electrodes was measured by a multimeter (Keithley DAQ6510).
Immersion of Ag Flake Powders
The immersing of Ag flake powders inside four solutions was performed by dropping Ag flakes (0.3 g) into glass vials with 2 ml of one of urea, lactic acid/urea, NaCl/KCl/urea, and artificial sweat solutions, followed by shaking on a vortex mixer (MX-S, DLAB Scientific Inc).
Results and Discussion
All the artificial sweats with different pH showed the capability to quickly reduce the resistance of Ag-HPUA electrodes. Different artificial sweats can be chosen based on the textile substrate's resistance to pH. 5 solutions containing urea, lactic acid, Cl−, lactic acid, and Cl−, and artificial sweat were explored, as shown in
The fast resistance drop likely comes from the reaction between the exposed Ag flakes of the Ag-HPUA electrode and sweat instead of the interaction/reaction between the inner Ag flakes. The change in resistance was faster than the drop in contact angle as a function of time (
To further unveil the reaction between Ag-HPUA and artificial sweat, Ag flake powders were immersed inside the urea, lactic acid/urea, and NaCl/KCl/urea solutions, and artificial sweat. The lubricant layer on the surface of Ag flakes is made from hydrophobic fatty acid. This is the reason why most Ag flakes in urea solution, lactic acid/urea, NaCl/KCl/urea solution stay on the top of the solutions, as shown in
It has been reported that both H+ and Cl− can remove the lubricant and increase the conductivity of the Ag-based electrodes (Sun, S. et al., J. Mater. Sci. Mater. Electron. 2016, 27, 4363-4371; Grouchko, M. et al., ACS Nano 2011, 5, 3354-3359; and Lee, S. J. et al., Nanoscale 2014, 6, 11828-11834). The long-term resistance change of Ag-HPUA electrodes after soaking in lactic acid/urea and NaCl/KCl/urea is shown in
Lactic acid plays a similar role as short-chain acids, such as malonic acid, adipic acid (Li, Y. et al., 9th International Symposium on Advanced Packaging Materials: Processes, Properties and Interfaces (IEEE Cat. No. 04TH8742). 2004 Proceedings., 2004, 1-6), acetic acid (Lu, D., Tong, Q. K. & Wong, C. P., IEEE Trans. Compon. Packag. Technol. 1999, 22, 365-371), and sulfuric acid (Tan, F., Qiao, X. & Chen, J., Appl. Surf. Sci. 2006, 253, 703-707), which could partially or fully remove or reduce the fatty acid-based lubricants. Besides, H+ is helpful in the dissolution of Ag through the following reaction (Li, X., Lenhart, J. J. & Walker, H. W., Langmuir 2010, 26, 16690-16698; and Peretyazhko, T. S., Zhang, Q. & Colvin, V. L., Environ. Sci. Technol. 2014, 48, 11954-11961):
4Ag+O2→2Ag2O (1)
Ag2O+2H+→2Ag++H2O (2)
However, the reaction between Ag-HPUA and urea/lactic acid solution is not obvious because the concentration of lactic acid is low at 0.1%, and lactic acid is a weak acid that is hard to dissociate completely to release H+ for lubricant removal and the release of Ag+. Beyond removing the lubricant, the Cl− ions have adverse effects on the dissolution and agglomeration of Ag flakes. On one hand, the Cl− ions have a high affinity with Ag+, which will accelerate the dissolution of Ag0 to Ag+. The Cl/Ag ratio decides the interaction between Ag and Cl− to form insoluble AgCl or soluble AgClx1-x species (Levard, C. et al., Environ. Sci. Technol. 2013, 47, 5738-5745). Low Cl/Ag ratio forms AgCl which impedes the further dissolution of Ag0, whereas high ratio induces the formation of soluble Ag—Cl species which increases the dissolution rate (Li, Y. et al., Environ. Sci. Technol. 2018, 52, 4842-4849). In our case, the ratio of Cl/Ag is relatively high and a negligible amount of insoluble AgCl was formed due to the lubricant layer which limited the exposed surface area of Ag flakes. The XRD in
In the dynamic equilibration of Ag0-Ag+ system, some Ag+ can redeposit to Ag0. The Cl− ions induce the aggregation of Ag0 by decreasing the surface double electrical layer of the Ag particles via elongated ionic length and partially destroying the absorbed lubricant surfactant at high concentration (Grouchko, M. et al., ACS Nano 2011, 5, 3354-3359; and Nehal, M. E. F. et al., Optik 2020, 224, 165568). As a result, the redeposited Ag (mostly) and precipitated AgCl make the Cl− treated Ag flakes coarser and more liable to aggregation compared with lactic acid/urea-treated Ag flakes. However, most NaCl/KCl/urea-treated Ag flakes still floated on top of the solution and Ag-HPUA electrodes experienced a minor resistance drop within 14 min, probably due to the limited capability of low-concentration Cl− ions to remove the lubricant and sinter the Ag flakes.
Synergistically, the combination of low-concentration lactic acid and Cl− ions induces the enhanced aggregation of silver flakes and fast resistance drop of Ag-HPUA electrodes because of the augmented capability to remove lubricant surfactant and accelerate dissolution/redeposition of Ag. TGA was applied to detect the weight percentage of lubricant in Ag flakes. The result in
During the reaction between artificial sweat and Ag-HPUA electrodes, certainly, some Ag+ ions redeposit on the exposed Ag flakes though the dissolution of Ag is also taking place. In the conjunction area where the surface potential is higher than the other surface areas, the dissolved Ag+ ions may be more prone to redeposit in the conjunction area, resulting in the fusion of the adjacent Ag flakes and enhanced electrical conductivity (Lee, S. J. et al., Nanoscale 2014, 6, 11828-11834). This surface change is probably due to the reaction between the surface layer and artificial sweat, and accounts for the sweat-induced conductivity increment. The C-AFM comparison shows that the surface of the electrodes became more conductive after reaction with artificial sweat (
To verify the stability of the produced electron transfer traces, the electrode was immersed in pure water with magnetic stirring (for washing) after the reaction with artificial sweat. The resistance remained stable at around 0.65Ω with minor fluctuations coming from the magnetic stirring (
The durability of Ag-HPUA electrodes was studied by following the sweat-enhanced conductivity protocol in Example 7.
Results and Discussion
The durability of the electrodes to mechanical deformations caused by the wearer's daily activities, especially stretching, can guarantee wearable devices to work properly under and after the strain. Due to the flowing property and fast curing of the Ag-HPUA ink, the formed electrodes cannot penetrate deeply and fully cover the surface of each fiber. Instead, the cavities of the textile substrate are filled and fiber bundles are bonded by printed inks. This form factor is not favorable for stretchable textile electrodes because the strain from the deformed textile substrates will transmit fully to the printed electrode by the movement of the fibers (Jin, H. et al., Adv. Mater. 2017, 29, 1605848; and Wang, L. et al., Adv. Mater. 2020, 32, 1901971). However, the synthesized highly stretchable and elastic HPUA binder can resist the formation of obvious cracks on the Ag-HPUA electrodes upon 50% strain, as shown in
During stretching in the presence of artificial sweat, the separation of Ag flakes and the sintering reaction take place simultaneously. Stretching makes Ag flakes inside the HPUA matrix slide with each other to accommodate the imparted strain and thus, the number of electrical conductive percolations is decreased, showing in the form of resistance increase. The reaction between artificial sweat and Ag-HPUA electrodes can create new electron transfer paths during the stretching and counteract stretching-induced conductivity loss. The SEM images of the dry and sweat-soaked electrodes under 50% stretching are shown in
The sweat-induced sintering reaction also significantly increased the electrode's durability to cyclic tensile deformation which is super catastrophic in real application scenarios, as shown in
The stretchable HPUA binder can hold the conductive fillers together and the sintering reaction is responsible for creating new conductive networks, making sweat (pH 4)-soaked Ag-HPUA electrode maintain low resistance at both released status (0.26Ω) and stretched status (5Ω) even after 500 cycles of 30% stretching (
Even after drying, the artificial sweat-soaked electrode still maintained low initial resistance and high stretchability. The single stretching and cycling stretching compared with the original Ag-HPUA electrode are shown in
In-Vivo Study
The in-vivo study was carried out by monitoring the resistance change of Ag-HPUA electrodes wrapped on the subject's arm when the subject was doing stationary cycling to initiate the sweating. The test was conducted under the approved IRB-2017-08-038-02.
Release of Ag+ into Sweat After soaking the electrode inside artificial sweat solution, the concentration of released Ag+ was determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Elan-DRC-e, Perkin Elmer).
Results and Discussion
The in-vivo study results are shown in
Washing Process
The washing process was performed by soaking a piece of textile with printed Ag-SEBS electrode on in 500 ml detergent/water (1 g/500 ml) solution with a stir bar at a speed of 500 rpm. The detergent (Top Concentrated Liquid Detergent) was purchased from Lion. The resistance of the electrode was monitored after every 10 cycles of washing and in total, 50 cycles of washing were finished.
Additional Procedure to Increase Stretchability of Electrodes
The additional procedure for applying the strain was done before encapsulating the electrode. During soaking of the electrode in the artificial sweat solution, the strain was utilized to further increase the stretchability of the electrode.
Results and Discussion
The resistance change of the encapsulated electrode during washing is shown in
The capability of sweat to enhance the conductivity of Ag-HPUA electrodes provides more guidance into the design of the wearable electronics that need to be in intimate contact with human skin as sweat is one of the most liable biofluids secreted by the human body. To demonstrate the application of sweat sintering reaction, we designed the first example of a textile-based stretchable Zn—Ag2O sweat battery that utilizes human sweat as the electrolyte and Ag-HPUA electrodes as stretchable current collectors to provide energy for the wearable electronics.
Printing of Stretchable Zn—Ag2O Sweat Battery
Zn flake powder and Ag2O powder were mixed with carbon black (CB) at the weight ratios of 90:10 and 95:5, respectively, by mortar grinding for 10 min. The Zn inks and Ag2O inks were formulated by mixing Zn/CB and Ag2O/CB with the HPUA binder in the ratio of 1:2 and 1:1.2, respectively, and then shaking for 10 min. The two formulated inks were printed on the textile substrate in an interdigit shape in a lab-made black box and cured at ambient condition for 10 min without any heating or specific UV lightning. Then, Ag-HPUA inks were printed on top of the two electrodes as the current collector (
Results and Discussion
The structure of the stretchable sweat-activated battery is shown in
Electrochemical Characterization of Stretchable Zn—Ag2O Sweat Battery
The artificial sweats (pH=4) with different NaCl/KCl concentrations were used as the electrolytes for in-vivo studies. The polarization curves were tested by an electrochemical potentiostat (Autolab) with a scan rate of 5 mV/s from open circuit potential to 0 V. The discharging curve was obtained from a battery-testing instrument (Newar) at a current density of 0.2 mA/cm2. The in-vivo energy generation was tested by anchoring the printed battery with a series-connected 1 KΩ resistor on a subject's arm and monitoring the current of the circuit continuously during stationary cycling. The application demonstration of the battery was performed by powering a wireless epidermal temperature sensor (MMC-T201-1, Miaomiaoce, China) with a 5.6 mF capacitor to modulate the energy generated by the 4 series-connected sweat batteries on the subject's arm. The voltage of the capacitor was measured by the DAQ 6510 multimeter and the temperature sensor sent in-vivo data continuously to the app in smartphone per 2 s.
Results and Discussion
The linear sweep voltammetry (LSV) curves and power curves of the stretchable sweat-activated battery using artificial sweat electrolyte with increased concentration of NaCl are shown in
The durability of mechanical deformation enables the printed batteries to generate energy properly on the subject's forearm during the secretion of sweat (
As the Covid-19 is plaguing the world, the continuous monitoring of the body temperature is pretty valuable in the prevention of infection. Printed stretchable sweat batteries can serve as the power source to power a commercial wireless temperature sensor to monitor the temperature on the hand of the subject, and to send the data to a smartphone, as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10202100851 | Jan 2021 | SG | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2022/050038 | 1/26/2021 | WO |
