Self-healing hydrogel

Abstract

A self-healing hydrogel which includes a network of poly(3,4- ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), polyvinyl alcohol (PVA), sodium tetraborate, glycerol, and water wherein the network is bonded together by intermolecular interactions between the network of components. Methods of making the self-healing hydrogel and medical device applications for the hydrogel are also disclosed.

Description

SELF-HEALING HYDROGELFIELD OF THE DISCLOSURE

[0001] The present disclosure pertains to hydrogels, and, in particular, to disposable self- healing hydrogels which may be used for wearable sensing applications.BACKGROUND

[0002] Self-healing hydrogels are a unique class of materials that possess the ability to autonomously repair damage and restore their structural and functional integrity. These properties make self-healing hydrogels suitable for applications ranging from drug delivery, tissue engineering, soft robotics, coatings, to wearable devices.

[0003] For wearable sensing applications, self-healing hydrogels are in high demand due to their exceptional deformability, adhesion properties, and remarkable resilience to both mechanical and electrical damage. Nonetheless, hydrogels face challenges, including delayed healing time and unavoidable electrical hysteresis, which undermines their practical effectiveness. The present disclosure addresses these needs.SUMMARY

[0004] The present disclosure provides a self-healing hydrogel which comprises a network of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), polyvinyl alcohol (PVA), sodium tetraborate, and water. The hydrogel’s network is bound together by intermolecular interactions between these components which contribute to its advantageous self- healing properties.

[0005] In one aspect thereof, the present disclosure provides a self-healing hydrogel comprising a network of: (i) poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS); (ii) polyvinyl alcohol (PVA); (iii) sodium tetraborate; (iv) glycerol; and (v) water wherein the network is bonded together by intermolecular interactions between components (i) through (v).

[0006] In another aspect thereof, the present disclosure provides a method of making a self-healing hydrogel comprising the steps: (i) preparing a first solution comprising water, polyvinyl alcohol (PVA), poly(3,4-ethylenedioxythiophene):polystyrene sulfonate(PEDOT:PSS), and glycerol; (ii) preparing a second solution comprising water and sodium tetraborate; and (iii) mixing the first solution and second solution in a 1:4 volume ratio to produce the self-healing hydrogel.

[0007] In another aspect thereof, the present disclosure provides an electronic skin (e- skin) device comprising: a self-healing hydrogel comprising a network of: (i) poly(3,4- ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS); (ii) polyvinyl alcohol (PVA); (iii) borax; (iv) and glycerol wherein the network is bonded together by intermolecular interactions between components (i) through (iv); and b) a sensing component.

[0008] In various aspects, the hydrogel may comprise hydrogen bonds between glycerol and sodium tetraborate.

[0009] In various aspects, the hydrogel may comprise hydrogen bonds between poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and sodium tetraborate.

[0010] In various aspects, the hydrogel may comprise hydrogen bonds between polyvinyl alcohol and poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS).

[0011] In various aspects, the hydrogel may have a hysteresis of less than 0.65 under a cyclic strain of from 1% to 500%.

[0012] In various aspects, the hydrogel may have a recovery time of less than 0.12 seconds.

[0013] In various aspects, the hydrogel may comprise a network of poly (3,4- ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS); polyvinyl alcohol (PVA); borax; and glycerol. The network may be bonded together by intermolecular interactions between the components of the hydrogel.

[0014] In various aspects, the first solution may be prepared at 95°C.

[0015] In various aspects, the hydrogel may comprise hydrogen bonds between glycerol and water.

[0016] In various aspects, the hydrogel may comprise hydrogen bonds between polyvinyl alcohol (PVA) and glycerol.

[0017] In various aspects, the sensing component may be capable of monitoring electrophysiological signals selected from the group consisting of electrocardiograms (ECGs), electromyograms (EMGs), and electrooculograms (EOGs).

[0018] In various aspects, the device may have at least one of the following features: stretchability of greater than 800%; electrical conductivity of greater than 7 S cm1; self-adhesion to skin of greater than 0.3 N cm'2; and / or reliable cyclic sensing performance when exposed to an applied strain of from 1% to 200%.

[0019] In various aspects, the device does not comprise a conductive ion gel.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above-mentioned and other advantages and objects of the aspects of this disclosure, and the manner of attaining them, will become more apparent, and the disclosure itself will be better understood, by reference to the following description of embodiments of the disclosure taken in conjunction with the accompanying drawings, wherein:

[0021] FIG. 1 A provides a photograph and schematic illustration of a self-healing hydrogel of the present disclosure.

[0022] FIG. IB provides photographs of the self-healing hydrogel highlighting its features.

[0023] FIG. 2 provides a schematic diagram showing self-healing mechanisms of the self-healing hydrogel.

[0024] FIG. 3A is a graph that shows resistance variation of the self-healing hydrogel throughout a cutting and healing process over time.

[0025] FIG. 3B is a graph showing comparison of self-healing performance for the self- healing hydrogel and comparative examples.

[0026] FIG. 3C shows AR / Ro of the strain gauges according to Examples 3 and 4 under loading-unloading cycles over a wide strain range of up to 500%.

[0027] FIG. 3D shows a comparison of the electrical hysteresis and strain performance of the self-healing hydrogel and comparative examples.

[0028] FIG. 4A is a schematic illustration of the strain gauge according to Example 3.

[0029] FIG. 4B is an image showing the strain gauge according to Example 3 under stretching, compression, folding, and twisting.

[0030] FIG. 5 shows the AR / Ro of the strain gauges according to Examples 3 and 4 under loading-unloading cycles over a wide strain range of up to 500% over 7 days.

[0031] FIG. 6A shows AR / Ro of the strain gauges under loading-unloading cycles at low strains of 1%, 3%, and 5% according to Example 4.

[0032] FIG. 6B shows AR / Ro of the strain gauges under loading-unloading cycles at high strains of 100%, 300%, and 500% according to Example 4.

[0033] FIG. 6C shows AR / Ro versus time for the step-strain experiment according to Example 4.

[0034] FIG. 6D shows response / recovery time of the hydrogel according to Example 2.

[0035] FIG. 6E shows AR / Ro of the hydrogel throughout 1,000 stretching-releasing cycles with a strain of 100% according to Example 4.

[0036] FIG. 6F shows the electrical hysteresis of the hydrogel according to Example 4.

[0037] FIG. 6G shows the selective response of the hydrogel to stretching compared to other deformation modes, with insets showcasing its behavior under stretching, bending, hammering, and twisting.

[0038] FIG. 7 shows the response / recovery time of the strain gauges according to Example 4.

[0039] FIG. 8 shows the AR / Ro of the strain gauges according to Example 4 throughout 1,000 stretching-releasing cycles with a strain of 100%.

[0040] FIG. 9 shows the AR / Ro of the hydrogel according to Example 4 throughout 1,000 stretching-releasing cycles with a strain of 200%.

[0041] FIG. 10 shows the detection of skin deformations or vibrations from the hydrogel attached to body parts according to Example 5.

[0042] FIG. 11 A shows images of forearm skin before application of the hydrogel compared to skin after applying 3M tape as described in Example 5. The right column shows the hemoglobin map measured using a hyperspectral line-scanning system.

[0043] FIG. 1 IB shows a bar graph of the average hemoglobin content for a sampled area of forearm skin (n=5) from Example 5.

[0044] Corresponding reference characters indicate corresponding parts throughout the figures. Although the drawings represent embodiments of the present disclosure, the drawings are not necessarily to scale, and certain features may be exaggerated or omitted in some of the drawings in order to better illustrate and explain the present disclosure.DETAILED DESCRIPTIONI. Definitions

[0045] For purposes of the following detailed description, it is to be understood that the disclosure may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term “about”.Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0046] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0047] Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0048] In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and / or” unless specifically stated otherwise, even though “and / or” may be explicitly used in certain instances. Further, in this application, the use of “a” or “an” means “at least one” unless specifically stated otherwise.

[0049] As used herein, the term “self-healing” in the context of hydrogels refers to the ability of the hydrogel material to autonomously repair or regenerate itself after experiencing damage or deformation.II. Self-Healing Hydrogels

[0050] The present disclosure provides a hydrogel which is capable of self-healing after experiencing damage or deformation. This characteristic is desirable in a variety of applications as it mimics the natural healing process of living tissues.

[0051] The self-healing capability of the hydrogels described herein may be the result of a network of intramolecular and intermolecular interactions which contribute to the material’s ability to repair and regenerate. The molecular interactions may involve hydrogen bonding, dipole-dipole forces, induced dipole forces, pi-pi stacking, van der Waals forces, covalent crosslinking, or ionic interactions. In some embodiments, increasing the number of intermolecular interactions or healing sites is favorable for improving regenerative properties.

[0052] In one embodiment, the self-healing hydrogel may comprise a network of:(i) poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS);(ii) polyvinyl alcohol (PVA);(iii) sodium tetraborate;(iv) glycerol; and(v) water wherein the network is bonded together by intermolecular interactions between components (i) through (v).

[0053] FIG. la provides a photograph and schematic illustration of one embodiment of the self-healing hydrogel, which is made up of a dynamic and highly interconnected network of PEDOT:PSS, PVA-borax, and glycerol. FIG. 2 provides a schematic of the types of dynamic covalent bonds and non-covalent bonds which are present in the hydrogels of the present disclosure. Referring to the illustration shown therein, glycerol, with its multiple hydroxyl (-OH) groups, reacts with borax to form glycerol-borax and glycerol-water networks through hydrogen bonding with water. Meanwhile, the sulfonate groups of PEDOT:PSS may also create hydrogen bonds with borate to form the PEDOT:PSS-borax network. In addition to this hydrogen bonding, the -OH groups on PVA may form hydrogen bonds with both the -OH groups of glycerol and thesulfonate groups on PEDOT:PSS, which generates PEDOT:PSS-PVA and glycerol -PVA networks. Without being bound to theory, it is believed that the weakening of the PVA-borax crosslinked network leads to greater rearrangement of PVA chains during gelation, increasing its deformability.

[0054] The relatively high number of healing sites in the hydrogel allows it to flex and conform to a variety of different surface textures. FIG. IB provides photographs of one embodiment of the self-healing hydrogel demonstrating its features including deformability, reshaping compliance, fingerprintability, and adaptability to conform to intricate contours or uneven surfaces, such as wrinkles on the wrist.III. Synthesis of Self-Healing Hydrogels

[0055] The present disclosure also provides synthetic methods for producing the self- healing hydrogels described in Section II above.

[0056] Generally, the synthesis involves preparing a solution of PVA, PEDOT:PSS, and glycerol in water. Then, a solution of sodium tetraborate in water is prepared, and added to the first solution to produce the hydrogel.

[0057] In one embodiment, the method comprises the following steps:(i) preparing a first solution comprising water, polyvinyl alcohol (PVA), poly(3,4- ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and glycerol;(ii) preparing a second solution comprising water and sodium tetraborate; and(iii) mixing the first solution and second solution in a 1 :4 volume ratio to produce the self-healing hydrogel.

[0058] In step (i), the first solution may be prepared by mixing the three components in a stepwise manner, or they may be mixed together simultaneously.

[0059] In one embodiment, the PVA may be first dissolved into distilled water to produce a solution. This dissolution may be carried in a vessel with vigorous mechanical stirring. The dissolution process may take 1 hour, 2 hours, 3 hours, 4 hours, or 5 hours, and may be done at a temperature of at least 80°C, at least 85°C, at least 90°C, at least 95°C, or at least 100°C. Subsequently, PEDOT:PSS may be added to the PVA solution and stirred for 1 hour, 2 hours, 3 hours, 4 hours, or 5 hours until it is completely dissolved. After the PEDOT:PSS is dissolved, glycerol may be added to the PVA and PEDOT:PSS solution and stirred for 1 hour, 2 hours, 3hours, 4 hours, or 5 hours. Separately, sodium tetraborate may be dissolved in distilled water to produce a homogenous solution. Once dissolved, the sodium tetraborate solution may be added to the solution of glycerol, PVA, and PEDOT:PSS and stirred vigorously.

[0060] The volume ratio of the first solution comprising water, polyvinyl alcohol (PVA), poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and glycerol to the second solution comprising water and sodium tetraborate may be 1 :1, 1:2, 1 :3, 1 :4, 1 :5, 1 :6, or 1 :7.IV. Properties of Self-Healing Hydrogels

[0061] The self-healing hydrogels of the present disclosure may have a number of properties which make them desirable for incorporation into medical devices and wearable sensing applications.

[0062] In some embodiments, the self-healing hydrogels may have a low electrical hysteresis. When exposed to cyclic strains of 100%, 200%, 300%, 400%, or 500%, the electrical hysteresis of the self-healing hydrogels of the present disclosure may be lower than 0.70%, lower than 0.65%, lower than 0.60%, lower than 0.55%, lower than 0.50%, lower than 0.45%, or lower than 0.40%. To assess the electrical hysteresis, the self-healing hydrogels may be subjected to a series of loading and unloading cycles at the specified strain levels and the corresponding electrical output recorded for each cycle. The electrical hysteresis value is measured as the difference in electrical output between the loading and unloading phases of the cycle.

[0063] In some embodiments, the self-healing hydrogels may have a recovery time of less than 0.20 seconds, less than 0.19 seconds, less than 0.18 seconds, less than 0.17 seconds, less than 0.16 seconds, less than 0.15 seconds, less than 0.14 seconds, less than 0.13 seconds, less than 0.12 seconds, less than 0.11 seconds, less than 0.10 seconds, less than 0.09 seconds, less than 0.08 seconds, less than 0.07 seconds, less than 0.06 seconds, or less than 0.05 seconds. The recovery time of the self-healing hydrogels may be measured by utilizing a loading and unloading rate of 500 mm min'1under specified strains (i.e., 5%, 10%, 100%, etc.). The recovery time is measured as the time it takes for the sensors to react to changes in strain.

[0064] In some embodiments, the self-healing hydrogels may have high deformability when exposed to tensile testing. Referring to FIG. IB, the hydrogels may be able to stretch many times their original length when exposed to a tensile force. For example, the self-healinghydrogels may have a deformability of 100% or greater, 500% or greater, 1000% or greater, 2000% or greater, 3000% or greater, 4000% or greater, 5000% or greater, 6000% or greater, 7000% or greater, 8000% or greater, 9000% or greater, 10,000% or greater, 15,000% or greater, or 20,000% or greater, as measured according to a comparison of its original length before application of a stretching force.

[0065] In some embodiments, the self-healing hydrogels may have good reshaping compliance and the ability to mold into different shapes. Referring to Fig. IB, the hydrogels may hold the shapes of different letters, or conform to the intricate contours of uneven surface such as wrinkles on skin. The deformability may also allow the hydrogels to capture and retain very intricate features, such as the details of a fingerprint. In some embodiments, the hydrogel may adhere to human skin easily, while simultaneously adapting to the movements of fingers and joints without becoming loose.

[0066] In some embodiments, the self-healing hydrogels may have a self-adhesion to plastic of 0.35 ± 0.08 N cm’2, a self-adhesion to glass of 0.18 ± 0.002 N cm’2, a self-adhesion to wood of 0.17 ± 0.01 N cm’2, or a self-adhesion to pigskin of 0.10 ± 0.005 N cm’2.V. Applications of Self-Healing Hydrogels

[0067] The present disclosure also provides apparatuses which comprise the self-healing hydrogels as described herein. As noted in section IV above, the self-healing hydrogels have rapid healing and low electrical hysteresis which make them particularly suitable for a variety of wearable apparatuses.

[0068] In one embodiment, the self-healing hydrogels of the present disclosure may be incorporated into an electronic skin device. The electronic skin device may comprise: a) A self-healing hydrogel comprising a network of:(i) poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS);(ii) polyvinyl alcohol (PVA);(iii) borax;(iv) and glycerol wherein the network is bonded together by intermolecular interactions between components (i) through (iv); and b) a sensing component.

[0069] In some embodiments, the sensing components may be capable of monitoring electrophysiological signals selected from the group consisting of electrocardiograms (ECGs), electromyograms (EMGs), and electrooculograms (EOGs).

[0070] In some embodiments, the electronic skin device may be attached to various positions on the human body without adhesives. The soft, sticky, and self-healing qualities of the hydrogel may allow the electronic skin device to detect human motion and move with the body part without becoming detached.

[0071] In some embodiments, the electronic skin device may not comprise or contain a conductive ion gel.

[0072] The electronic skin devices contemplated by the present disclosure may have certain advantageous features or characteristics which make them particularly suitable for applications in healthcare, robotics, and prosthetics. In some embodiments, the electronic skin device may have at least one of the following features: a stretchability of greater than 800%, an electrical conductivity of greater than 7 S cm'1, self-adhesion to skin of greater than 0.3 N cm'2; and / or reliable cyclic sensing performance when exposed to an applied strain of from 1% to 200%.

[0073] The hydrogels of the present disclosure may also be incorporated into a variety of other consumer or health devices including but not limited to wound dressings, drug delivery systems, biosensors, implants, implant coatings, smart watches, and fitness trackers.

[0074] It should be understood that the foregoing description is only illustrative of the present disclosure. Various alternatives and modifications can be devised by those skilled in the art without departing from the disclosure. Accordingly, the present disclosure is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.

[0075] The following non-limiting examples are provided to illustrate various embodiments of the present disclosure.EXAMPLES

[0076] The materials used for the Examples were purchased from commercially available sources. PVA (Mw: 146,000-186,000), PEDOT:PSS, glycerol, sodium tetraborate were purchased from Sigma-Aldrich. Encapsulating tape (VHB 4905) was purchased from 3M. All thereagents were used as received without further purification. Cu tapes (CST5) were purchased from Digi-Key.Example 1 : Fabrication of the Hydrogel

[0077] One gram (g) of PVA was added to 9 g DI water and stirred at 95 °C until completely dissolved. The PVA solution and 1 g of PEDOT:PSS solution were thoroughly stirred for over 2 hours. Two grams (g) glycerol was added to the above PVA-PEDOT:PSS solution and stirred for over 2 hours until a homogeneous solution was formed. Meanwhile, 0.4 g sodium tetraborate was dissolved in 10 mL deionized water to form a homogeneous solution.The borax aqueous solution and PVA-PEDOT:PSS-GL solution were mixed in a 1 :4 volumetric ratio by vigorous stirring, until the PVA-PEDOT:PSS-GL hydrogel was obtained. PVA- PEDOT:PSS hydrogel was prepared in a similar way.Example 2: Healing Time of Hydrogel

[0078] A piece of hydrogel was placed on a glass slide and a razor blade was used to cut it. Electrical self-healing experiments were carried out on the hydrogel samples as described further herein. Two Cu tapes were placed at bottom ends of the hydrogel and a source meter (Keithley 2400; Tektronix, Inc.) was used to measure the resistance of the hydrogel in a two-wire configuration.

[0079] The electrical healing efficiency was calculated using the following equation (1): r|=R r / R i

[0080] where Rr is the recovered conductivity and Ri is the initial conductivity.

[0081] FIG. 3A shows the resistance variation of the hydrogel throughout the cutting and healing process over time. Upon being fully cut into two pieces, the resistance of the hydrogel increased to infinity but resumed its initial value after only 0.12 s of the healing process.

[0082] The self-healing performance of the hydrogel was also tested according to several comparative examples shown in FIG. 3B. The hydrogel demonstrated the fastest recovery time (0.12 s) and higher self-healing efficiency (approximately 100%) compared to the comparative examples.Example 3 : Preparation of Strain Gauge

[0083] The hydrogel strain gauge was prepared by sandwiching the hydrogel between encapsulation tapes. The assembly setup involved three encapsulation tapes (VHB-4905; 3M Inc.), positioned at the bottom, middle, and top. The middle encapsulation tape featured a specific groove. The bottom encapsulation tape was aligned to support the middle tape, which contained the groove configured to receive the hydrogel. Once the hydrogel was carefully filled into the grove, the copper electrodes (CST5; Digi-Key, Inc.) were positioned at the hydrogel ends within the groove, establishing the required connections. The assembly was finalized by placing the top encapsulation tape on the setup, thus creating a seal. This top encapsulation tape is subsequently secured with glue to ensure all components were properly enclosed and safeguarded.

[0084] FIG. 4A shows a schematic illustration of the strain gauges with the stackup of the hydrogel with the sealing tapes and the conductive film.

[0085] FIG. 4B shows the resulting strain gauges demonstrated exceptional mechanical flexibility, being stretchable up to 860%, compressible, foldable, and twistable up to 360° without compromising the inherent properties of the hydrogel, up to the point of rupture of the encapsulation layer.Example 4: Characterization of Strain Gauges

[0086] In this Example, the strain gauges prepared in Example 3 were tested to determine some of their properties.

[0087] To evaluate the electrical hysteresis of the strain sensor, the resistance of each cycle was set as an initial value when plotting the curves of AR / R0 versus strain. R0 signifies the resistance measured with no strain applied. The degree of hysteresis was set from the width of the hysteresis loop. The areal variation between the loading and unloading states of relative electrical signal change versus strain was quantitatively defined as electrical hysteresis and by the following equation (2):H=(lA_L-A_Ul) / A_U x l00 %

[0088] where AL and AU are the areas of relative electrical signal change versus strain in the loading and unloading states, respectively.

[0089] FIG. 3C shows the relative resistance change (AR / RO) of the strain gauges under loading-unloading cycles over a wide strain range of up to 500%. The results indicate negligible electrical hysteresis, with values of 0.61 ± 0.45%, 0.76 ± 0.18%, and 0.64 ± 0.58% at strains of 100%, 300%, and 500%, respectively (n = 3 each). Consequently, the strain gauges exhibited full-scale, ultralow electrical hysteresis across a broad strain range, in contrast to previously reported PEDOT:PSS-based materials that are limited by rigid conducting polymers and irreversible energy dissipation (FIG. 3D).

[0090] Importantly, these observations regarding rapid healing time and ultralow electrical hysteresis remained consistent even after more than a week. FIG. 5 shows AR / Ro of the strain gauges under loading-unloading cycles over a wide strain range of up to 500% over 7 days.

[0091] FIG. 6A displays the AR / RO of the strain gauges under loading-unloading cycles at low strains of 1%, 3%, and 5%. Figure 6B shows the consistent results at high strains of 100%, 300%, and 500%. These identical response patterns indicate the strain gauges exhibited excellent stable and continuous response to the various loading-unloading cycles for both low and high strains. FIG. 6C shows that the AR / RO of the strain gauges quickly increased when strains were applied and then remained at a stable level, with the first step being 10% strain and the second step being 5%, with no sharp peaks. This result indicates the strain gauges show the steady signal output and rapid detecting ability at different strains.

[0092] FIG. 6D presents the response and recovery time of the hydrogel under a 5% strain with a loading and unloading rate of 500 mm / min1, which were measured as -239 ms and -238 ms, respectively. Unlike others made of elastomers, the strain gauges exhibited negligible overshoot or relaxation behavior. At a higher strain of 10%, the strain gauges maintained fast response (-441 ms) and recovery (-423 ms) time with no visible overshoot behavior (FIG. 7). FIG. 6E shows a long-term durability with > 1,000 cycles of stretching at an applied strain of 100%. The results indicate that the strain gauges show good stability of the electrical signal with only slight signal fluctuation over 1,000 cycles of stretching. FIG. 6F shows electrical hysteresis of strain gauges with a maximum strain of 100% for the first, 500th, 1000th cycle of applied strain. The AR / RO versus strain of the strain gauges at first, 500th, 1000th cycles has been shown in FIG. 8 to obtain electrical hysteresis. The results indicate that strain gauges have maintained ultra-low electrical hysteresis of < 1% against > 1,000 cycles of stretching at the applied strain of100% as shown in FIG 6F. The results remained consistent at the applied strain up to 200% (FIG. 9). FIG. 6G shows that the strain gauges were only sensitive to axial stretching and remained largely unchanged upon hammering and bending, as evidenced by the barely changed AR / RO values.Example 5: Hydrogel in Wearable Sensing Applications

[0093] To demonstrate the utility of the hydrogel in the context of human motion monitoring and electrophysiology monitoring, several specimens were tailored in size and shape for application to human skin. To showcase an example of the hydrogel for motion tracking, multiple specimens were designed to measure strain changes ranging from minor (0-5%) to substantial (5-50%) in various body parts, including the facial muscles (i.e., smile and frown), joints during bending (i.e., wrist, elbow, knee, finger, and ankle), and skin deformation (i.e., skin stretching and squeezing). These motions were tested on a human subject, as shown in FIG. 10A. The hydrogel can be easily attached to various positions on the human body without adhesives and detect diverse human motion even after self-healed process due to its uniquely soft, sticky and self-healable feature (FIG. 10A). The skin deformation or joint bending by the various body motions led to a detectable shift of resistance in the hydrogel, which could be clinically useful for managing health and diseases.

[0094] The hydrogel is usable independently or integrated with various sensor types such as electrocardiograph (ECG), electromyography (EMG) and electrooculogram (EOG). Measurements are recorded in high-quality bio-signals while the hydrogel remains in direct contact on the forearm without using adhesives. The ECGs, EMGs, and EOG demonstrate clear detection of the P, Q, R, S, and T waves; electrical currents generated in the muscles during contraction (neuromuscular activities); and electrical signals produced by eye movements, respectively. These recordings are qualitatively comparable with those obtained using commercial EP recording electrodes (RedDotTM, 3M, USA).

[0095] Because of the rapid self-healing properties of the hydrogel, the used hydrogel may be recycled into any shape or size, which increases its utilization. Importantly, there was no sign of skin irritation observed during these demonstrations (FIG. 11) since the hydrogel showed excellent biocompatibility and is made of all biocompatible materials (i.e., PVA-borax, PEDOT:PSS, and glycerol).

[0096] Erythema-induced inflammation, which alters the concentrations of hemoglobin, is a prevalent response to irritation of human skin. Hyperspectral line-scan images (hypercube) of human skin were captured to investigate inflammation, typically marked by erythema and shifts in hemoglobin concentrations, ensuing from skin irritation. A monochrome camera (GS3- U3-120S6M-C; FLIR), equipped with a 23 pm slit width and 150 mm-1 groove density, was utilized for imaging. An LED light source with a 6,500K color temperature (D65) was employed for illumination. The spectrograph underwent spectral calibration using a xenon calibration light source that radiated numerous narrow peaks at specific wavelengths. For skin imaging, a fixed focal length lens (MVL25M1; Navitar) was primarily used, enabling a field of view as miniscule as 10 mm x 10 mm RGB images of the same area were taken with a smartphone camera (iPhone 11 Pro; Apple). The hydrogel was applied to the medial antebrachial cutaneous area of the forearm for approximately 10 minutes, while a 3M tape was adhered to the same area for an equal duration as a positive control. Images were snapped pre- and post-experiment to contrast hemoglobin content. A custom MATLAB interface was harnessed for data acquisition. A control group included human subject or human subject skin that was not provided with hydrogel or 3M tapes (i.e., left with bare skin). While there is no significant difference between the hydrogel and control group (p=0.81454), there are significant differences between the 3M tape and control group (p= 0.00609) or the 3M tape and hydrogel (p= 0.02682). The hydrogel did not cause any irritation of the skin and the volunteer did not feel pain during peel off, while 3M tapes did cause skin irritation and the volunteer felt pain during peel off.

[0097] To further investigate effects of the hydrogel on the skin, the inflammation response was evaluated after the hydrogel was applied to the skin of volunteers. Inflammation accompanied by erythema, which changes the concentrations of hemoglobin, is a common response after irritation of the human skin. Line-scan hyperspectral images (hypercube) of the human skin were obtained using a monochrome camera (GS3-U3-120S6M-C; FLIR), with a slit width of 23 pm and groove density of 150 mm'1. An LED light source with a color temperature of 6,500K (D65) was used as the illumination source. Spectral calibration of the spectrograph was performed using a xenon calibration light source that emitted multiple narrow peaks at specific wavelengths. A fixed focal length lens (MVL25M1; Navitar) was mainly used to image the skin, with a field of view as small as 10 mm x 10 mm. The same area was imaged with a smartphone camera (iPhone 11 Pro; Apple) to capture RGB images. The hydrogel was appliedonto the medial antebrachial cutaneous of the forearm for around 10 minutes. As a positive control, a 3M tape was attached to the same area for 10 minutes. Images were acquired before and after the experiment for hemoglobin content comparison. A mechanical linear scan step was performed at 0.25 mm. The data was acquired using a custom MATLAB interface. A tissue reflectance spectral model was used to extract key hemodynamic parameters from the groundtruth hyperspectral image. The theory of radiative transport and robust approximations (e.g., diffusion, Born, and empirical modeling) was used to model light propagation in tissue. The intensity reflected from a biological sample can be expressed as a function of X in the visible range:

[0098] where bi, b2, and bs are associated with the scattering (Mie or Rayleigh) contributions at Xo = 800 nm, 8nbo2 (X) denotes the absorption coefficient of oxygenated hemoglobin (HbCh), sub (X) denotes the absorption coefficient of deoxygenated hemoglobin (Hb), b4 is the hemoglobin concentration multiplied by the optical pathlength, and bs is the blood oxygen saturation (sPCh). In the study, the hemoglobin contents multiplied by the optical pathlength (b4) was used to indicate the level of skin irritation by equation (1). All fitting parameters were computed using the simplex search (Nelder-Mead) algorithm. The hemoglobin content of the samples of human skin exposed to nothing (control group), the skin exposed to the hydrogel, and the skin exposed to 3M tape are shown in FIGS. 11 A and 1 IB. As shown, skin exposed to the hydrogel only showed very minor irritation compared to skin compared to 3M tape.

[0099] While the embodiments of this disclosure have been shown and described as having preferred designs, the described embodiments may be modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses or adaptations of the embodiments using general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this disclosure pertains.

Claims

CLAIMS:What is claimed is:

1. A self-healing hydrogel comprising a network of:(i) poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS);(ii) polyvinyl alcohol (PVA);(iii) sodium tetraborate;(iv) glycerol; and(v) water wherein the network is bonded together by intermolecular interactions between components (i) through (v).

2. The self-healing hydrogel of claim 1, wherein the hydrogel comprises hydrogen bonds between glycerol and sodium tetraborate.

3. The self-healing hydrogel of claim 1, wherein the hydrogel comprises hydrogen bonds between glycerol and water.

4. The self-healing hydrogel of claim 1, wherein the hydrogel comprises hydrogen bonds between poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and sodium tetraborate.

5. The self-healing hydrogel of claim 1, wherein the hydrogel comprises hydrogen bonds between polyvinyl alcohol (PVA) and glycerol.

6. The self-healing hydrogel of claim 1, wherein the hydrogel comprises hydrogen bonds between polyvinyl alcohol and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS).

7. The self-healing hydrogel of claim 1, wherein the hydrogel has a hysteresis of less than 0.65 under a cyclic strain of from 1% to 500%.

8. The self-healing hydrogel of claim 1, wherein the hydrogel has a recovery time of less than 0.12 seconds.

9. A method of making a self-healing hydrogel comprising the steps:(i) preparing a first solution comprising water, polyvinyl alcohol (PVA), poly(3,4- ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), and glycerol;(ii) preparing a second solution comprising water and sodium tetraborate; and(iii) mixing the first solution and second solution in a 1 :4 volume ratio to produce the self-healing hydrogel.

10. The method of claim 9, wherein the hydrogel comprises a network of(i) poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS);(ii) polyvinyl alcohol (PVA);(iii) borax;(iv) and glycerol wherein the network is bonded together by intermolecular interactions between components (i) through (iv).

11. The method of claim 9, wherein the first solution prepared at 95°C.

12. The method of claim 9, wherein the hydrogel comprises hydrogen bonds between glycerol and sodium tetraborate.

13. The method of claim 9, wherein the hydrogel comprises hydrogen bonds between glycerol and water.

14. The method of claim 9, wherein the hydrogel comprises hydrogen bonds between poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and sodium tetraborate.

15. The method of claim 9, wherein the hydrogel comprises hydrogen bonds between polyvinyl alcohol (PVA) and glycerol.

16. The method of claim 9, wherein the hydrogel has a hysteresis of less than 0.65 under a cyclic strain of from 1% to 500%.

17. The method of claim 9, wherein the hydrogel has a recovery time of less than 0.12 seconds.

18. The method of claim 9, wherein the hydrogel comprises hydrogen bonds between polyvinyl alcohol and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS).

19. An electronic skin (e-skin) device comprising: a) a self-healing hydrogel comprising a network of:(i) poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS);(ii) polyvinyl alcohol (PVA);(iii) borax;(iv) and glycerol wherein the network is bonded together by intermolecular interactions between components (i) through (iv); and b) a sensing component.

20. The electronic skin device of claim 19, wherein the sensing component is capable of monitoring electrophysiological signals selected from the group consisting of electrocardiograms (ECGs), electromyograms (EMGs), and electrooculograms (EOGs).

21. The electronic skin device of claim 19, wherein the device has at least one of the following features: stretchability of greater than 800%; electrical conductivity of greater than 7 S cm’1; self-adhesion to skin of greater than 0.3 N cm’2; and / orreliable cyclic sensing performance when exposed to an applied strain of from 1% to 200%.

22. The electronic skin device of claim 19, wherein the device does not comprise a conductive ion gel.

Images