Adhesive, stretchable and antibacterial hydrogel with external/self-power for flexible sensitive sensor used as human motion detection

https://doi.org/10.1016/j.compositesb.2021.108984Get rights and content

Highlights

  • PPS hydrogel was prepared by UV radiation and freeze-thaw cycle.

  • The hydrogel presented adhesive and stretchable properties.

  • The hydrogel showed excellent antibacterial property.

  • The hydrogel with external/self-power was used as flexible sensitive sensor.

Abstract

Conductive, flexible and stretchable hydrogel sensors have attracted a lot of interest in human-machine interfaces, medical monitoring, and soft robotics. However, there are still have great challenges in the preparation of conductive hydrogels with self-adhesive, self-power and antibacterial properties. In order to meet the requirements of high sensitivity, convenient carrying and anti-allergic, so as to overcome the problems of traditional sensors, such as non-stickiness, absolute dependence on external power supply and skin irritation, the prepared poly(vinylalcohol)/poly(acrylamide-co-[2-(methacryloyloxy) ethyl] dimethyl-(3-sulfo-propyl) ammonium hydroxide) (PVA/P(AM-co-SBMA), PPS) hydrogel was obtained by UV crosslinking and freeze-thaw cycles. The PPS hydrogel has good mechanical properties (the elongation was 700% and the breaking strength was 370 kPa), high adhesion to various substrates, antibacterial properties (~99%) and wide sensory sensitivity. And the self-power sensor can be achieved by using hydrogel as the conductive medium to assemble primary battery with zinc foil and copper foil, the open circuit voltage of this self-powered system can reach 0.86 V. Furthermore, the assembled strain sensor can also convert chemical energy into electrical energy, and transform the resistance changes caused by stretching or compression into voltage signals for output. It can be used as effective wearable power supply for human motion detection. This kind of self-adhesive and antibacterial self-powered strain sensor was expected to play a great role in flexible bioelectronics.

Introduction

In recent years, the demand for smart devices has increased, and the development of stretchable conductive elastomers with rich properties were play great importance to modern electronic products [[1], [2], [3], [4], [5]]. But most elastomers composed of semiconductors and metals would separate from each other under large strain, causing the conductive network to rupture, losing its conductivity stability and sensing performance [[6], [7], [8], [9]]. Therefore, many researchers use soft polymers as an alternative to new artificial skin. Hydrogel is a material with a large amount of water, which has a flexible 3D network structure [[10], [11], [12], [13], [14]]. High content of water can provide a transport path for conductive ions, and its mechanical properties are similar to human skin, so the hydrogel with good mechanical properties, biocompatibility and electrical conductivity were promising candidates for the manufacture of artificial skin [[15], [16], [17]]. At present, various hydrogel-based strain sensors have been manufactured. They play an important role in detecting human movements due to their high sensitivity, wearability and long-term usability [[18], [19], [20], [21], [22]].

However, many conductive hydrogels lack adhesion, resulting in high resistance and unstable electrical signals at the interface [23,24]. In fact, the skin strain sensor requires the hydrogel to have excellent adhesion properties. This ensures that the skin deformation can be converted into electrical signals to the greatest extent during the application process [25,26]. Interestingly, it has been noted that many plants exhibit adhesion spontaneously due to the presence of adhesion molecules in their extracellular matrix, such as aloe vera and maltose [27,28]. Inspired by this phenomenon, zwitterionic [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA) with both positive and negative charges on the polymer chain derived from natural polysaccharides has attracted people's attention [[29], [30], [31]]. Since the positive and negative charges in zwitterionic polymers can produce high dipole moments, hydrogels containing zwitterions can produce ideal adhesion to most surfaces through ion-dipole or dipole-dipole interactions force [[32], [33], [34]]. For example, Zhang and Fu et al. synthesized zwitterionic nanocomposite hydrogel by free radical polymerization of SBMA and HEMA, and the hydrogel had good mechanical properties, adhesion, self-healing and high conductivity sensitivity, and can be used to detect human movement [35]. Therefore, polyzwitterionic SBMA with good biocompatibility and inherent conductivity can make zwitterionic composite hydrogels have an excellent prospect in monitoring the complex dynamic movement of the human body surface [36,37].

In addition, most of the flexible electronic sensors prepared currently need external power supply to use [[38], [39], [40], [41]]. Once the external power supply meets problems, these sensors will not work, which greatly limits their applications [42,43]. Therefore, the development of a self-powered flexible strain sensor is of great significance in the manufacture of portable and wearable electronic skins, which can promote the further development of sensing devices [[44], [45], [46]]. Devices based on the galvanic battery structure are potential choices for self-powered sensing. For instance, Li et al. [47] used gelatin and tannic acid to prepare a hydrogel with rapid self-repair properties, which can be assembled into a self-powered strain sensor with the galvanic mechanism, and the self-powered sensor has good responsiveness and flexibility. Hydrogel with excellent ion transport capabilities is expected to an ideal electrolyte material for manufacturing “flexible batteries”.

Moreover, antibacterial properties are also essential for flexible bioelectronic skin. The antibacterial property can prevent allergic symptoms of the skin that was in direct contact with electronic skin devices, prolong the service life of the device and reduce the replaceable frequency of the device [[48], [49], [50], [51], [52]]. This is extremely important for flexible conductive sensors. Hu and Wang et al. synthesized polypyrrole or Zn-functionalized chitosan molecules and cross-linked with PVA to prepare a stretchable, conductive, self-repairing and antibacterial hydrogel, which can promote wound healing by electrical stimulation [53], but did not reflect the adhesiveness of the hydrogel and the characteristics of self-powered use, so it existed certain limitations on the high sensitivity and portability of the sensor.

In this work, we constructed a new type of external/self-powered hydrogel flexible sensor with good ion conductivity, excellent comprehensive mechanical properties, adhesion and antibacterial properties, as shown in Fig. 1. In particular, the PVA/P(AM-co-SBMA) (PPS) hydrogel prepared by the one-pot method and UV rapid cross-linking technology can detect human movement through ion signals and imitate the sensitivity of human skin. And the PPS hydrogel can achieve high adhesion to a variety of substrates, and still had satisfactory adhesion under sweat conditions. At the same time, the hydrogel was assembled with zinc (Zn) and copper (Cu) sheets to form a self-powered strain sensor with a galvanic cell structure, which was installed directly on the human body as a motion monitoring device to convert subtle motion stimuli into voltage signals without external power supply. Therefore, the hydrogel not only be used for sensing by external power supply, but also can be used to realize self-powered to real-time monitor the human body. More importantly, the prepared PPS hydrogel also has high-efficiency antibacterial properties and low toxicity, which can meet the needs of bioelectronic sensors. This research provides a new idea for non-power flexible strain sensors as the next generation of wearable and portable electronic devices.

Section snippets

Materials

Polyvinyl alcohol (PVA, type 1799) was purchased from Adamas. Acrylamide (Am, 99.0%), [2-(Methacryloyloxy) ethyl] dimethyl-(3-sulfo-propyl) ammonium hydroxide (SBMA), 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP) and N, N′-methylene-bis-AM (MBA) were purchased from Sigma-Aldrich. ZnCl2(≥98.0%) was bought from Greagent. Escherichia coli (ATCC25922), Staphylococcus aureus (ATCC6538), LB broth and LB agar were bought from Qingdao Haibo Biotechnology Co., Ltd. The mouse breast cancer

Formation and structure of the hydrogels

PPS hydrogel with good mechanical properties, electrical conductivity, adhesion and antibacterial properties was prepared successfully through UV radiation and freeze-thaw cycles. PVA and PAM were used to construct the hydrogel network system. Due to its good biodegradability, biocompatibility and has a large number of hydroxyl group, PVA can form the hydrogel system easily. PAM is non-toxic and has good mechanical properties and the crosslinking network was formed through copolymerization with

Conclusion

In summary, a facile method was designed to prepare the PPS conductive hydrogel and self-powered sensor. Surprisingly, the PPS hydrogel prepared by UV radiation and freeze-thaw cycle has good mechanical properties, adhesion, electrical conductivity and antibacterial properties. And the self-powered strain sensor prepared by combining the hydrogel sensor and the galvanic cell structure has satisfactory sensitivity, it can convert external stimuli into electrical signals effectively for stable

Author statement

Zixuan Zhou-Methodology, Validation, Investigation, Writing-Original Draft, Zhirui He-Investigation, Shiwu Yin-Supervision, Project administration, Xiaoyun Xie-Supervision, Project administration, Weizhong Yuan-Conceptualization, Resources, Writing-Review and Editing, Supervision, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (81771942 and 82072024).

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