1.

INTRODUCTION

With the advancement of healthcare, people’s demand for biomedicine, health detection, and non-invasive medical treatment is getting higher and higher. Traditional flexible sensors are made by applying conductive materials to the surface of a flexible substrate for sensing. Although such sensors have a certain degree of bendability, due to biocompatibility and sensor materials, there are problems such as not being able to meet the user’s comfort requirements and not being able to achieve a good fit with the skin. Hydrogel is a polymer with a three-dimensional mesh structure, which is water-absorbent and water-retentive, and does not dissolve after absorbing water, so it can maintain its original structure. Due to its good electrical conductivity, better biocompatibility and other properties, hydrogel has great potential in the fields of biomedical materials and flexible sensing [1-3].

Hydrogels can be categorized into synthetic hydrogels [4-6] and natural polymer hydrogels [7-9]. Synthetic hydrogels have good mechanical properties and electrical conductivity, but are usually less biocompatible. Natural polymer hydrogels, on the other hand, have good biocompatibility, but their mechanical properties are usually low, making it more difficult to meet the mechanical requirements of the application. Therefore, how to combine the advantages of the two kinds of hydrogels has become a hot research topic. Based on the amphiphilic ion-conducting hydrogel consists of a three-dimensional polymer network and free ions in it, which has a certain degree of water absorption and retention and ductility, and its conductivity comes from the migration of ions. The three-dimensional porous structure of amphiphilic ion-conducting hydrogel provides freely movable channels for the conductive ions in the hydrogel, due to the fact that the principle of conductivity of ion-conducting hydrogel is similar to that of the human skin, and it has good mechanical and electrochemical properties [10-12]. Therefore, there is an urgent need to develop an amphiphilic ion-conducting hydrogel sensor with stable sensing performance, good mechanical properties and self-healing.

In this paper, zwitterionic hydrogel sensor with excellent adhesion, self-healing, mechanical properties and electrical conductivity was successfully prepared based on SBMA, AM and NaCl. The introduction of SBMA provides good self-healing properties and excellent adhesion properties, enabling the hydrogel to adhere closely to different surfaces, while its ionic properties help to enhance the conductivity of the hydrogel. The introduction of AM improves the tensile and shear resistance of the hydrogel, which enhances the mechanical properties of the hydrogel. The addition of NaCl greatly improves the electrical conductivity and sensing properties of the hydrogel. Relying on the self-adhesive property of the hydrogel, the hydrogel can be tightly adhered to the skin and will not fall off during the detection process. In the experiments, the hydrogel can not only accurately convert the wide range of human wrist, elbow and knee movements into electrical signals, but also recognize small movements such as human fingers. We believe that this hydrogel will provide a new option for the design and preparation of multifunctional adhesive hydrogel sensors.

2.

EXPERIMENT

3.

RESULT AND DISCUSSION

3.1

Synthetic analysis

The internal structure of the hydrogel and the composition of the functional groups were analyzed using FTIR spectroscopy. As shown in Figure 1, the absorption peaks at 3341 cm^-1 and 3168 cm^-1 for the AM monomer correspond to the antisymmetric and symmetric stretching vibrations of the amino-NH2, in addition to the absorption peak at 1611 cm^-1 resulting from the stretching vibration of the conjugated olefin C=C. In the SBMA/AM hydrogel the stretching vibration peak at 1170 cm^-1 is the SBMA sulfonic acid group, the free -NH2 stretching vibration peak at 3402 cm^-1 and the stretching vibration peak of the amide in AM at 1665 cm^-1. This data indicates that the SBMA/AM hydrogel contains two different chain units corresponding to the two monomers, SBMA and AM, and the amphiphilic ion-based hydrogel preparation was realized.

Figure 1.

Fourier transform infrared (FTIR) spectrogram

3.2

Mechanical performance

The SBMA/AM/NaCl hydrogel was made into strips of 30 mm×10 mm×2 mm, the effective tensile length was set to 20 mm, and the tensile rate was set to 50 mm/min. 3 samples were prepared at the same time, and in order to ensure the accuracy of the experiments, the average value of the measurement results was taken to reduce the experimental error. The fracture stress σ=F/A, where σ is the fracture stress, F is the force of the hydrogel at fracture, A is the cross-sectional area of the sample. The fracture strain ε=L₀/L×100%, where L₀ is the initial length of the sample clamped on the electrode; L is the length of the sample at fracture. As shown in Figure 2, the maximum tensile stress of the hydrogel can reach 180 kPa, and the maximum strain at break of the hydrogel can reach 1200%. The mechanical properties of the hydrogel are excellent, which can meet the needs of subsequent hydrogel sensing experiments.

Figure 2.

SBMA/AM/NaCl hydrogel tensile stress-strain curve

3.3

Adhesion

The adhesion of SBMA/AM/NaCl hydrogels is mainly attributed to non-covalent interactions in the interface between the hydrogel and the substrate material, such as hydrogen bonding, hydrophobic interactions, electrostatic interactions, metal-ligand interactions as well as adhesion through covalent bonds. Ideal adhesive hydrogels must have good cohesive strength and mechanical strength in addition to strong adhesion to the matrix material. When subjected to external forces, the hydrogel may be damaged not only on the surface, but also in its internal structure. The hydrogel was prepared as a 30 mm×10 mm×2 mm sheet and fully adhered to the finger, and then the adhesion effect was tested with the four matrix materials shown in Figure 3, including: metal, rubber, glass and plastic, respectively, to observe the adhesion effect of the hydrogel. During the experiment, the hydrogel was able to realize a better adhesion effect on these four base materials.

Figure 3.

SBMA/AM/NaCl hydrogel adhesion test. Hydrogel adhesion to (a) metal, (b) rubber, (c) glass, and (d) plastic.

3.4

Self-healing

The SBMA/AM/NaCl hydrogel was cut into blocks with specifications of 2 cm×2 cm×0.5 cm, and two hydrogel blocks with the same specifications were tightly affixed. Since the hydrogel loses water quickly, it is not possible to carry out long time experiments, in order to prevent the effect of water evaporation on the self-healing properties of the hydrogel, we will be affixed to the hydrogel wrapped with plastic wrap, and then every 1h to detect whether the hydrogel realizes self-healing. As shown in Figure 4a, the prepared hydrogels are shown. Figure 4b shows two pieces of hydrogels being tightly attached to each other. Figure 4c shows that the hydrogels were pulled at both ends of the hydrogel after 3h, and it can be found that the hydrogel has completed self-healing.

Figure 4.

SBMA/AM/NaCl hydrogel self-healing test. (a) Schematic diagram of hydrogel cut. (b) Close fitting of hydrogel. (c) Tensile testing of the healing hydrogel.

3.5

Conductivity

The SBMA/AM/NaCl hydrogel will be prepared as a 30 mm×10 mm×2 mm sheet, using electrochemical workstation through the impedance time method, as shown in Figure 5, the resistance value of the hydrogel without the addition of NaCl is around 110,000 Ω, but due to the existence of anion and cation in the amphoteric ionic monomer SBMA, the conductivity is poor at this time, but the calculated conductivity can reach 0.0057 S/m. The resistance value of the hydrogel with NaCl is around 445 Ω. The resistance value of the hydrogel with the addition of NaCl is about 445 Ω, and the calculated conductivity can reach 2.1 S/m. It can be seen that the addition of the conductive ion NaCl in the hydrogel can significantly reduce the resistance value of the hydrogel and improve the electrical conductivity of the hydrogel due to the increase of the concentration of free ions in the hydrogel, and more ions can be free to migrate under the action of the voltage.

Figure 5.

SBMA/AM/NaCl hydrogel conductivity test. (a) Hydrogel with NaCl added. (b) Hydrogel without NaCl added.

3.6

Motion detection

The SBMA/AM/NaCl hydrogel can be used as a flexible strain sensor directly and tightly adhered to the surface of the human skin to monitor various movements of the human body in real time through resistance changes. As shown in Figure 6, the hydrogel’s sensitivity to small strains enables it to distinguish the tensile changes during different movements of the finger and express them through the resistance change signals. The hydrogel sensor is able to detect small strain changes when the finger performs different movements and recognize the movement through the relative resistance change. These results show that the hydrogel strain sensor is able to accurately sense subtle strain processes in the human body. Test results show that the hydrogel adheres tightly to the finger without the use of traditional bandages or tapes and does not come off. During testing, the hydrogel sensor was able to differentiate between changes in the finger at different bending angles, such as 0°, 30°, 60°and 90°. What’s more, the electrical signal also remained stable and unchanged when the finger was kept at a certain angle, proving the reliability and accuracy of the hydrogel sensor in the signal transmission process. As shown in Figure 6a, the hydrogel sensor is attached to the wrist, and at different bending angles, the hydrogel sensor can clearly sense the bending changes of the wrist and convert them into electrical signals. The hydrogel sensor is able to accurately detect the movement of the elbow joint and record the associated resistance changes. As shown in Figure 6b, the hydrogel sensor can acutely capture the bending of the elbow joint and convert it into an electrical signal. The hydrogel sensor accurately detects knee joint motion and records the corresponding resistance changes. As shown in Figure 6c, the hydrogel sensor can acutely sense the flexion changes in the knee joint and convert them into electrical signals. Test results show that the hydrogel is characterized by fast response time and high strain sensitivity, and can be tightly adhered to the skin without additional auxiliary materials. In summary, hydrogel has a wide range of applications in human motion monitoring.

Figure 6.

Continuous human motion detection in SBMA/AM/NaCl hydrogels.

4.

CONCLUSION

In conclusion, amphoteric SBMA and nonionic AM were used as monomers, NaCl was added to enhance the conductivity of hydrogel, and the polymerization of hydrogel monomers was initiated by UV irradiation, and hydrogels based on amphoteric ions were successfully prepared. The content of SBMA can improve the self-healing, adhesion and softness of the hydrogel, and the content of AM affects the mechanical properties of the hydrogel, and the best properties ratio was determined through performance optimization. By adding NaCl, the conductivity of the hydrogel can be significantly improved to meet the required conductivity of the sensor. Importantly, the hydrogel has good adhesion to various materials and self-healing properties, and is able to accurately sense the subtle motion strain process of the human body. In the field of flexible electronics, hydrogel’s excellent electrical conductivity and mechanical properties make it an ideal material for flexible strain sensors, flexible displays and electronic skins. These applications enable real-time monitoring and data transmission with high sensitivity and reliability, driving the development of wearable technology.