An Easy-to-Prepare Conductive Hydrogel for Smart Wearable Materials Based on Acrylic Derivatives and Acrylamide

Abstract

Electrically conductive hydrogel materials can be used as materials for wearable sensors, which can quickly and accurately detect the activities of human joints and convert these movements into electrical signals. More specifically, they have potential for application in wearable electronic devices and electronic skins. However, a number of challenges remain regarding the preparation of conductive hydrogel materials. In this study, we synthesized the P(BHMP-AM)-Zn²⁺ hydrogel material in situ via a one-pot method using acrylic monomer derivatives, acrylamide, and zinc ions. The resulting hydrogel exhibited a high cytocompatibility (89%), excellent cyclic tensile properties, satisfactory adhesion properties (3.4 kPa), and good electrical conductivity. Furthermore, the addition of Zn²⁺ ions imparted antibacterial properties to the hydrogel, with sterilization rates of 65.9 and 10.9% being determined against Escherichia coli and Staphylococcus aureus, respectively. The hydrogel was able to sense the activities of joints or other parts of the human body when attached to the skin, converting these movements into electrical signals to allow the real-time monitoring of motion. This conductive hydrogel material, therefore, exhibits potential for use in wearable electronic devices and electronic skins, among other devices.

1. Introduction

In recent years, wearable electronic sensing devices have attracted much attention because they can easily monitor human joint movement and transfer it into electronic signals [1]. The former modality of wearable electronic sensing devices was more conductive materials combined with a flexible substrate by an external auxiliary attached to the surface of the body [2]. Then, the devices transferred the motion into electronic signals via the changes in relative resistance. Traditional wearable electronic sensing devices, however, still have some disadvantages. They are often less resilient and less suitable for complex shapes such as skin, which lead to poor user experience [3].

Hydrogels are composed of strong three-dimensional network structures. As a result of this structure, hydrogels exhibit ultra-high hydrophilicity, resulting in the absorption of large amounts of water and rendering these materials flexible and ductile due to their ability to swell rapidly without undergoing dissolution in an aqueous environment [4,5,6,7]. A large number of crosslinked networks exist in the hydrogel structure, and as the degree of crosslinking increases, the water absorption properties decrease. Due to these variable properties, hydrogels have broad application prospects in the fields of clinical medicine, human–machine interfaces, and smart sensing; however, their long-term stabilities and service lives remain inadequate [8,9,10,11]. Although significant progress has been made in the area of hydrogel materials, the reported hydrogel-based smart materials, and in particular, those based on polyacrylamide hydrogels, still exhibit some obvious and fatal defects [12,13], such as a low adaptive adhesion on the hydrogel surface, poor self-healing and shape memory properties, and insufficient smart responsiveness.

An increasing number of studies have recently focused on the application of hydrogel materials in wearable smart materials. In such applications, the hydrogel material must exhibit good adhesion to the skin, in addition to being able to flex with the movement of joints when used for human motion detection. In addition, hydrogels are able to detect the activities of human joints and other parts efficiently, accurately, and quickly while also exhibiting good tensile and anti-fatigue properties. The development of a simple route to hydrogel materials that possess the above-mentioned properties is therefore desirable.

In this context, electrically conductive hydrogels (ECHs), as a new type of hydrogel material, possess the characteristics of traditional hydrogels while also exhibiting electrical conductivity, and as such, they are considered suitable candidates for wearable smart materials. Such conductivity is achieved in ECHs through the addition of conductive polymers [14,15,16], electrolytes [17], or inorganic conductive fillers, such as carbon nanotubes [18] and graphene [19]. These properties have rendered conductive hydrogels suitable for use in flexible electronics [20,21], tissue engineering [22], and wearable devices [23,24,25,26]. Conductive hydrogels can be divided into three types, namely, nanocomposite conductive hydrogels, ionic conductive hydrogels, and conductive polymer-based hydrogels, depending on their conductive components.

In terms of the preparation of such materials, a graphene-based conductive hydrogel was obtained by means of a one-step hydrothermal method. More specifically, graphene oxide was added to a high-pressure reactor together with the uniformly dispersed monomer; following a 12 h reaction at 180 °C, the corresponding conductive hydrogel was obtained, which exhibited a conductivity of 5 × 10⁻³ S·cm⁻¹ [15].

Despite these advances, a number of deficiencies must be addressed. For example, conductive fillers, such as graphene, carbon nanotubes, metal nanoparticles, and nanowires, are commonly introduced into the hydrogel network to construct nanocomposite conductive materials [18,27,28,29]. However, since these fillers tend to be inorganic components, they exhibit poor interfacial compatibilities with the organic components present in the system, ultimately leading to agglomeration of the inorganic components during hydrogel application and resulting in a decreased electrical conductivity and poor mechanical properties [30,31]. In addition, conductive hydrogels possess various shortcomings in terms of their mechanical strength, antibacterial properties, opacity, and tissue adhesion, which seriously hinder the further development of conductive hydrogel sensors [32,33].

Inspired by mussels, hydrogels containing catechol groups, such as dopamine and tannins, have recently received increasing research attention [34,35,36,37,38] since these hydrogels are constructed from various types of interactions, including π–π stacking, cation–π interactions, hydrogen bonds, hydrophobic interactions, and Michael addition reaction. These interactions endow the hydrogel with strong adhesion properties [39], even underwater. However, owing to the facile aging, darker color, and poor biocompatibility of such materials, they require further research and development to enhance their properties and expand their potential application.

Thus, we herein report the preparation of hydrogels that exhibit good adhesion, tensile, and fatigue resistance properties. More specifically, we prepare P(BHMP-AM)-Zn²⁺ via a one-pot in situ polymerization of BHMP (3-[bis(carboxymethyl)amino]-2-hydroxypropyl-2-methyl-2-propenoate), AM (acrylamide), and Zn²⁺, using MBA (N,N′-methylenebisacrylamide) as an initiator. Following its preparation, the hydrogel properties are investigated during joint movement, and the hydrogel biocompatibility is evaluated. In addition, we considered that the addition of zinc ions could impart the hydrogel with antibacterial and bactericidal properties, and this assumption was evaluated against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus).

2. Materials and Methods

2.1. Materials

Zinc acetate dihydrate (C₄H₆O₄Zn·2H₂O, Aladdin Chemical Co., Ltd., Shanghai, China) (99%) was used as the source of Zn²⁺. BHMP was synthesized according to a previously reported synthetic route [40]. AM (99%) and MBA (99%) were provided by J&K. All chemicals were used as received without further purification.

2.2. Preparation of the Hydrogel

Using the one-pot radical polymerization approach, the P(BHMP-AM)-Zn²⁺ hydrogel was synthesized from AM, BHMP, and zinc acetate dihydrate. More specifically, BHMP (100 mg) and zinc acetate dihydrate (100 mg) were dissolved in distilled water (5 mL) and stirred vigorously for 30 min until a homogeneous solution was obtained. In addition, a 10 mg/mL solution of MBA was prepared and added to the reaction system (50 μL) along with AM (150 mg), and stirring was continued for 15 min. After this time, the system was purged with nitrogen for 30 min prior to injection into a mold. The solution was injected into a self-designed mold which consisted of a pair of parallel glasses and a 1 mm thick silicone gasket [41]. This mold was sonicated for 15 min to expel any gas from the solution, and radical polymerization was performed under 50 W UV light for 5 h to obtain the desired P(BHMP-AM)-Zn²⁺ hydrogel.

BHMP was prepared in the following manner.

Iminodiacetic acid (IDA) was added to the potassium hydroxide (KOH) solution, then glycidyl methacrylate (GMA) was added to the solution. After the solution was stirred at 65 ℃ for 1 h, the solution changed from a mixture of oil and water to a transparent water phase. The pH was adjusted to 7. Functional monomers were extracted in acetone. The product was verified by FT-IR and 1H-NMR.

FT-IR (KBr): = 3419 (OH), 3070, 2964, 2875, 1714, 1631, (C=O), 1440 (C=C), 1172 cm⁻¹. 1H NMR (500 MHz, Deuterium Oxide) δ 6.09 (d, J = 1.3 Hz, 1H), 5.69–5.66 (m, 1H), 4.39–4.33 (m, 1H), 4.20 (d, J = 4.1 Hz, 1H), 4.14 (d, J = 5.3 Hz, 1H), 3.82 (d, J = 6.1 Hz, 5H), 3.45 (dd, J = 13.3, 2.9 Hz, 1H), 3.32 (dd, J = 13.3, 10.4 Hz, 1H), 1.86 (d, J = 1.3 Hz, 3H).

2.3. Characterization

The samples were analyzed by Fourier transform infrared (FTIR) spectroscopy using a Shimadzu FTIR-8100 spectrometer with a spectral range of 500–4500 cm⁻¹. After dialysis and freeze-drying of the hydrogel samples, the microstructure was observed by scanning electron microscopy (SEM, FEI Nova Nano 450). In addition, the hydrogel was cut into 10 mm × 50 mm × 1 mm samples and the tensile properties were tested at room temperature using an Instron 3300 tensile testing machine at a tensile speed of 50 mm∙min⁻¹. For the compression experiments, the hydrogel material was cut into cylindrical samples with a diameter of 15 mm and a height of 12 mm, and a compression speed of 10 mm/min was employed when carrying out the measurements. Furthermore, during the loading/unloading test, a layer of silicone oil was evenly coated on the surface of the hydrogel sample to reduce water evaporation.

2.4. Electrical Measurements

The changes in the hydrogel resistance were tested by using an electrochemical workstation (Shanghai Chenhua electrochemical workstation chi660e). The hydrogel was attached to the human joint skin and connected to an electrochemical device. When the joint moves, the resistance of the hydrogel changes accordingly, and the device converts the hydrogel resistance changes into an electrical signal to monitor the changes in joint movement [42].

The conductivity (P, S/m) of the hydrogel was tested as the following equation:

$$P=L/\left(R\times A\right)$$

where L is the length of the hydrogel material, A is area of the cross-sectional, and R is the resistance of the hydrogel. Moreover, the gauge factor (GF) is defined as follows:

$$GF=\frac{\left(R-R_0\right)/R_0}{\mathsf{\epsilon }}$$

where ε is the tensile strain (%). Thus, the relative change of resistance was calculated by using the following formula:

$$\Delta R/R_0=\left(R-R_0\right)/R_0$$

where R₀ and R were defined as the initial resistance and the real-time resistance of the hydrogel, respectively.

2.5. Adhesion Measurements

By filming the photo of the objects which adhered to the hydrogels, the adhesive properties of the hydrogels were verified, and the tissue adhesion was tested. When the hydrogel could maintain adhesion with the object for 10 min, it was considered that the hydrogel had passed the adhesion test with the object of this weight.

The hydrogel could adhere to porcine skin, liver, lung intestines, and pork firmly both in the air and underwater at room temperature. The adhesion strength of hydrogel was tested using an Instron 3300 tensile tester with a separation rate of 15 mm/min. The hydrogel samples were cut into rectangles with a size of 10 mm × 10 mm × 1 mm and sandwiched between two sheets of glass, iron, aluminum, wood, and porcine skin.

2.6. Biocompatibility Measurements

The cytocompatibility of the hydrogel was evaluated using a cell counting kit-8 (CCK-8) test and a live/dead test using mouse fibroblast L929 as a model cell [43]. The hydrogels were soaked in ethanol for half a day and in deionized water for one day. Then, the hydrogels were sterilized for 1 h under violet light, and the sterilized hydrogels were immersed in DMEM medium for 24 h. L929 cells were seeded in 96-well plates and incubated for 24 h. The above clear liquid was used as the medium, and the medium without hydrogel was used as the control group. P(BHMP-AM)-Zn²⁺ were used as experimental groups. After incubation in 96-well plates for a specified period of time, 100 μL CKK-8 was added to each well and cells were incubated for 2 h. Absorbance measurements were performed on the cells using a microplate reader with a wavelength of 450 nm. Similarly, after 24 h of incubation, the hydrogel-soaked supernatant was extracted and a 96-well plate was added in as medium for 20 min. Cell morphology was observed by laser scanning confocal microscopy [44,45].

2.7. Antimicrobial Measurements

With the adoption of the disk diffusion method, hydrogels’ surface antibacterial properties were tested using E. coli and S. aureus. A round sterile sample of 12 mm diameter was placed in a tube containing 5 mL of sterile LB liquid medium. Then, 100 mL 106 CFU/mL bacterial suspension was inoculated into LB liquid medium. After culturing for 24 h at 37 ℃, the bacterial solution was diluted 104-fold, and 100 uL of the bacterial suspension obtained above was inoculated on LB AGAR plates and incubated for another 24 h. The control bacterial suspension was diluted 104-fold on LB AGAR plates and incubated at the same time [44,45].

3. Results and Discussion

3.1. Structural Characterization

The P(BHMP-AM)-Zn²⁺ hydrogel was prepared via the one-pot in situ radical polymerization of BHMP with AM and Zn²⁺. As shown in Figure 1, after the addition of MBA, BHMP and AM were copolymerized to form a polymer network and generated a P(BHMP-AM) hydrogel. Subsequently, the P(BHMP-AM)-Zn²⁺ hydrogel was obtained following the complexation of the P(BHMP-AM) hydrogel with Zn²⁺.

The molecular structure of the P(BHMP-AM)-Zn²⁺ hydrogel was confirmed using FTIR spectroscopy (Figure 2). More specifically, upon the comparison of the FTIR spectra of BHMP and P(BHMP-AM)-Zn²⁺, it was found that the characteristic C=C bond peaks observed in the spectrum of BHMP at 1612 and 1616 cm⁻¹ were absent from the spectrum of P(BHMP-AM)-Zn²⁺, indicating that polymerization of the monomers had been successful. At the same time, the C–C peaks characteristic of tertiary alcohols (i.e., the carboxyl groups of BHMP, 1410–1310 cm⁻¹) were significantly weakened after polymerization, indicating that the carboxyl groups coordinated with the metal ions.

3.2. Image and Conductivity

Figure 3a shows the image of P(BHMP-AM)−Zn²⁺ hydrogel. Figure 3b,c show that the hydrogel was connected in the circuit. As shown in Figure 3b, when the wire was not connected to the hydrogel, the small lamp bead would not light up. As shown in Figure 3c, when the wire was connected with the hydrogel, that is, when the hydrogel was connected in the circuit, the small lamp bead could glow.

3.3. Tensile Stress–Strain Curves

To determine the mechanical properties of the prepared P(BHMP-AM)-Zn²⁺ hydrogel, the tensile stress–strain curves were recorded for both the P(BHMP-AM)-Zn²⁺ and PAM hydrogels. As shown in Figure 4a, when the tensile stress exceeded 150 kPa for the P(BHMP-AM)-Zn²⁺ hydrogel, the tensile strain reached 2000%, which was significantly improved compared to that of PAM. This was attributed to the fact that additional hydrogen bonds and metal coordination interactions formed in the hydrogel after the addition of BHMP and Zn²⁺, which greatly increased the crosslinking density of the hydrogel and enhanced its mechanical properties. The compressive stress–strain curve of the P(BHMP-AM)-Zn²⁺ hydrogel is shown in Figure 4b, wherein it can be seen that when the compressive stress exceeded 17 MPa, the compressive deformation rate was 85%. Cyclic tensile tests were also performed on the P(BHMP-AM)-Zn²⁺ hydrogel to evaluate its durability and recovery. As presented in Figure 4c, it was found that at a fixed strain of 400%, the mechanical strength of hydrogels after ten loading/unloading cyclic tensile tests did not vary much.

3.4. Adhesion Properties

When the hydrogel could maintain adhesion with the object for 10 min, it was considered that the hydrogel had passed the adhesion test with an object of this weight. As shown in Figure 5a, when adhering different weights to the P(BHMP-AM)-Zn²⁺ hydrogel, the hydrogel was able to bear a weight of 30 g in the air. Subsequently, the adhesion properties of the hydrogel toward different materials were investigated using the lap shear test. As shown in Figure 5b, the hydrogel exhibited bonding strengths of 6.7, 7.3, 8.2, and 3.4 kPa to the glass, wood, steel, and porcine skin specimens, respectively.

3.5. SEM Imaging and Motion Detection Performance

The SEM images of the PAM and P(BHMP-AM)-Zn²⁺ hydrogels were then recorded. More specifically, as shown in Figure 6a, in the absence of BHMP and Zn²⁺, the surface of the PAM hydrogel exhibited a porous structure, with pores measuring ~100 μm in diameter. However, as shown in Figure 6b, the pores of the P(BHMP-AM)-Zn²⁺ hydrogel were significantly smaller, measuring 10–30 μm in diameter, thereby indicating that the addition of BHMP and Zn²⁺ greatly improved the crosslinking density. Because of its superior stretchability, toughness, rapid recovery, self-adhesion properties, and biocompatibility, the P(BHMP-AM)-Zn²⁺ hydrogel is expected to be a candidate material for use in flexible wearable devices. As shown in Figure 6c,d, the hydrogel can adhere to the elbow joint and deform as the joint moves, which causes a change in the resistance of the hydrogel. As shown in Figure 6e,f, when the stress–strain of the hydrogel was increased from 0 to 200%, the relative resistance of the hydrogel also changed. Thus, the hydrogel was attached to the joint of a human arm, and when the arm carried out a repeated straightening/bending action, the relative resistance of the hydrogel changed rapidly, thereby indicating its sensitivity to motion.

3.6. Antimicrobial Properties and Biocompatibility of the Hydrogel

As a biomaterial, the biocompatibility and antibacterial properties of a hydrogel are crucial, wherein a cytocompatibility >70% is preferable [46]. Thus, we tested the cytotoxicity of the P(BHMP-AM)-Zn²⁺ hydrogel using the CKK-8 reagent, and the cell viability was determined to be 89.16%, as shown in Figure 7a. In addition, the cell morphology can be seen in Figure 7b, wherein it is apparent that the majority of L929 cells were still alive (green fluorescence), and only a small number of dead cells were observed (red color) [44,45]. The cells were spindle-shaped, indicating that the growth conditions were favorable and that the P(BHMP-AM)-Zn²⁺ hydrogel exhibited a low cytotoxicity along with a high biocompatibility, thereby rendering it a suitable candidate material for use in wearable devices.

Additionally, we evaluated the antibacterial activity of the P(BHMP-AM)-Zn²⁺ hydrogel against E. coli and S. aureus. In Figure 7c, the two Petri dishes on the left-hand side of the figure represent the control group, while the right-hand images represent the E. coli and S. aureus colonies on the agar plate after contact with the P(BHMP-AM)-Zn²⁺ hydrogel on the agar plate. Compared with the blank group, the colony counts of the two bacteria were greatly reduced in the hydrogel-treated group. As a result, the antibacterial activities of the hydrogel against E. coli and S. aureus were determined to be 10.92 and 65.92%, respectively, and this was attributed to the coordination complexation between Zn²⁺ and the BHMP carboxyl groups enhancing the antibacterial properties of the hydrogel. This also led to a bactericidal effect, indicating that the P(BHMP-AM)-Zn²⁺ hydrogel exhibited good bactericidal properties against both bacteria [40]. Therefore, the P(BHMP-AM)-Zn²⁺ hydrogel exhibited excellent antibacterial properties against E. coli.

3.7. Testing of the Rheological Properties

To further compare the mechanical properties of PAM and P(BHMP-AM)-Zn²⁺ hydrogels, their dynamic storage moduli (G′) and loss moduli (G″) were scanned at frequencies ranging from 0.1 to 100 rad/s. As shown in Figure 8, The G′ and G″ values of the P(BHMP-AM)−Zn²⁺ hydrogel were consistently higher than those of the PAM hydrogel, indicating that the mechanical properties of the P(BHMP-AM)-Zn²⁺ hydrogel were superior to those of the PAM hydrogel.

4. Conclusions

We herein reported the preparation of a P(BHMP-AM)-Zn²⁺ hydrogel (BHMP = 3-[bis(carboxymethyl)amino]-2-hydroxypropyl-2-methyl-2-propenoate, AM = acrylamide) as a simple and low-cost conductive hydrogel that was obtained via a facile polymerization and metal complexation approach, as confirmed by Fourier transform infrared spectroscopy. This hydrogel exhibited excellent adhesion to the skin, good tensile properties, and suitable biocompatibility. In addition, the tensile strain exceeded 2000% when the tensile stress was set at 150 kPa, and the compressive rate was 17 MPa at a compressive stress of 85%. Moreover, the P(BHMP-AM)-Zn²⁺ hydrogel was found to exhibit antibacterial properties, leading to antibacterial activities of 10.92 and 65.92% against Escherichia coli and Staphylococcus aureus, respectively. Furthermore, when the hydrogel was attached to a joint of the human arm, it was able to sensitively convert the movement of the joint into an electrical signal, thereby indicating the potential of this hydrogel for use as a wearable sensor material. Additionally, when P(BHMP-AM)-Zn²⁺ was prepared as an adhesive material and attached to a joint, the hydrogel underwent deformation upon movement of the joint. This resulted in a change in resistance, ultimately indicating that it was possible to monitor joint movement by detecting changes in the resistance value. Overall, the device prepared using this hydrogel was able to rapidly and accurately detect the activity of a human joint, in addition to transferring the movement signals into electrical signals. The above results, therefore, demonstrate that the P(BHMP-AM)-Zn²⁺ hydrogel appears to be a suitable candidate material for use in the low-cost and simple preparation of wearable devices.