A semi-interpenetrating network ionic composite hydrogel with low modulus, fast self-recoverability and high conductivity as flexible sensor
Introduction
Flexible sensors based on the soft conductive materials have attracted a great attention in recently due to their plentiful potential applications in wearable devices, robotics, actuators and electronic skins (Kaltenbrunner et al., 2013; Liao et al., 2017; Sun, Keplinger, Whitesides, & Suo, 2014; Tian et al., 2017). The composites of elastomers (such as polydimethylsiloxane and polyurethane) (Chowdhury, Saha, Patterson, Robison, & Liu, 2019; Kim et al., 2013) and conductive fillers (liquid metals, metal nanowires, metal particles, carbon materials, and conductive polymers) (Chun et al., 2010; Park et al., 2012) are commonly used to fabricate flexible sensors. However, it is remained a challenge to solve the compatibility between the elastomers and fillers, and most of these devices exhibit opaque, low elongation and incompatible with biological tissues, which greatly limits their applications. Conductive hydrogels have become the promising candidate for flexible sensor in recent years due to their similarity to natural soft tissues and better performance (Hou, Wang, Zhang, Li, & Zhu, 2014; Sun et al., 2014). Generally, conductive hydrogels can be mainly divided into two categories. The first one is the electronic conductive hydrogel, which mainly contains the conductive filler (e.g. carbon nanotubes, graphene/graphene oxide, polypyrrole and polyaniline) and the polymeric network (Cong, Chen, & Yu, 2014; Pan et al., 2012; Wang, Li et al., 2019). The conductivity mainly originates from the conductive network formed by the conductive fillers. However, the electronic conductive hydrogels have poor mechanical properties, low compatibility and the issue of filler agglomeration (Gan et al., 2019). The other is the ionic conductive hydrogel containing a large number of free ions from the polyelectrolyte or additional salts (Sun et al., 2014; Zhou et al., 2019). The conductivity of this type of hydrogel comes from the directional transportation of the free ions similar to the biosystems, which makes these gels suitable for the soft wearable or implantable sensors. Fruitful achievements of the flexible wearable devices with satisfactory conductivity along this line have been published recent years (Ge et al., 2018; Li, Pan, Wang, & Sun, 2020; Sun et al., 2014; Zhou et al., 2019). However, many of them show poor performance in strength, elongation, and recoverability that restricts their application. Therefore, there is a huge room to develop a new class of flexible devises with a combination of excellent mechanical properties and high conductivity.
Many innovative strategies have been employed to improve the mechanical properties of hydrogel, including the double-network (DN) (Ge et al., 2018; Gong, Katsuyama, Kurokawa, & Osada, 2003; Yang & Yuan, 2019), triple-network (TN) (Argun, Can, Altun, & Okay, 2014; Dai, Qing, Lu, & Xia, 2009), nanocomposite (NC) (Li et al., 2015), topological slide-ring (Sugihara et al., 2017), semi-interpenetrating network (semi-IPN) (Martínez et al., 2015; Zhao, Kang, & Tan, 2006), and so forth (Dai et al., 2015; Liang, Ding, Wang, & Sun, 2019; Wang, Zhang et al., 2019; Zheng et al., 2016). For example, Gong et al. have devised many tough DN hydrogels behaving excellent mechanical properties which can be comparable to cartilage and rubber (Gong et al., 2003; Gong, 2010). He et al. have prepared the physical gelatin hydrogels by simply soaking an original gelatin gel in an ammonium sulfate solution, showing tensile stress and strains of 3 MPa and 500 %, respectively (He, Huang, & Wang, 2018). However, the fracture of the brittle network in DN gels and dynamic bonds in physical gels gives them poor or slower recoverability. What’s more, some of these gels possess high modulus (beyond 500 kPa), and are not suitable for the wearable electronics that prefers a Young’s modulus lower than that of the human skin (25−220 kPa) (Chen et al., 2018). Among these tactics, the hydrogels with semi-IPN structure have aroused tremendous attentions in improving mechanical properties due to their easy feasibility. The obtained hydrogels mainly contain three-dimensional crosslinked polymeric network and linear chain polymers (Chen, Tian, & Du, 2004; Chen, Wu et al., 2004; Mandal, Kapoor, & Kundu, 2009; Zhao et al., 2006). The linear polymers mainly include natural polymers (e.g. cellulose (Rokhade et al., 2006), chitosan (Chen, Tian et al., 2004; Chen, Wu et al., 2004) and protein (Mandal et al., 2009)) and synthetic polymers (e.g. polyvinyl alcohol (Zhu, Ma, Wang, & Zhang, 2015), polyvinylpyrrolidone (Jin, Liu, Zhang, Chen, & Niu, 2006), and polyaniline (Uluturk & Alemdar, 2018)). As a common water-soluble chitosan derivative, carboxymethyl chitosan (CMC) possessing a large number of hydrophilic groups is widely used to prepare semi-IPN hydrogels due to its non-toxicity, biocompatibility and biodegradability (Chen, Tian et al., 2004; Chen, Wu et al., 2004; Guo & Gao, 2007; Mohamed, Seoudi, & Sabaa, 2012; Wei, Luo, Fu, Zhang, & Ma, 2013). However, the studies on CMC-based hydrogel reported before mainly focus on the properties of pH/temperature-responsiveness, drug delivery and biodegradability, rarely on the improvement of mechanical and electrical conductive properties.
Herein, it is aimed at constructing a new class of semi-IPN CMC/PAM/sodium chloride (NaCl) hydrogel integrating good mechanical and high ionic conductivity using a facile one-step method. The incorporation of NaCl regulated the conformation of the CMC chains, promoting the formation of more hydrogen bonds between the CMC and PAM chains, which enhanced the mechanical properties of hydrogels. The mechanical performances of the hydrogels can also be regulated by changing the content of the components. The composite CMC/PAM/NaCl hydrogel with some transparency has sufficient tensile strength, elongation, toughness, but low modulus comparable to human skin. The hydrogel exhibits fast self-recovery ability and satisfactory self-healing capability due to the dynamic nature of hydrogen bonds. In addition, the hydrogel possesses high conductivity, good water retention and anti-freezing capacities due to the existence of NaCl. A strain sensor was fabricated with the aforementioned characters and showed good sensitivity in monitoring human motions including the bending of a finger, a wrist and an elbow, as well as the continuous swallowing. Moreover, the sensing property of the conductor can be well resumed after self-healing. The newly developed PAM/CMC/NaCl composite hydrogel has great potential in applications such as soft wearable device, artificial electronic skin and intelligent robot.
Section snippets
Materials
Acrylamide (AM, Aladdin), N,N,N',N'-Tetramethylethylenediamine (TEMED, 99 %), and potassium bromide (KBr, 99 %) were received from Shanghai Aladdin Chemistry Co., LTD. Potassium persulfate (KPS, 98 %) and N,N'-Methylenebisacrylamide (MBAA, 99 %) were provided by Sigma-Aldrich. Hydrochloric acid (HCl, 38 %), sodium hydroxide (NaOH, 99 %) and sodium chloride (NaCl, 99.5 %) were supplied by Sinopharm Chemical Reagent Co. LTD. Lithium chloride (LiCl, 99 %), ammonium chloride (NH4Cl, 99.5 %), urea
Synthesis of hydrogels
The carboxymethyl chitosan/polyacrylamide/sodium chloride (CMC/PAM/NaCl) composite hydrogels were synthesized by the free-radical polymerization using one-step method with different composition (Fig. 1a). Briefly, a certain concentration of CMC solution should be prepared first of all, and other reactants including AM (monomer), MBAA (cross-linking agent), CMC, NaCl and KPS (initiator) were then add to this solution. After the dissolution of all chemicals, the solution was transferred into a
Conclusions
In summary, ionic conductive PAM/CMC/NaCl hydrogels were developed using a facile one-step method. The hydrogels are composed with semi-IPN structure containing the chemically cross-linked PAM network and interspersed chains of CMC. The introduction of sodium chloride adjusts the conformation of the CMC chains, promoting the formation of more hydrogen bonds between the CMC and PAM chains, which enhances the mechanical properties. The mechanical performances of the hydrogels can also be
CRediT authorship contribution statement
Hongyao Ding: Conceptualization, Investigation, Formal analysis, Writing - original draft. Xiaoxu Liang: Formal analysis. Qiao Wang: Formal analysis. Miaomiao Wang: Formal analysis. Zongjin Li: Supervision, Writing - review & editing. Guoxing Sun: Supervision, Writing - review & editing.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgments
This work was funded by The Science and Technology Development Fund, Macau SAR (File no. 0083/2018/A2); Multi-Year Research from University of Macau, Macau SAR (File no. MYRG2019-00135-IAPME), and Research & Development Grant for Chair Professor (File no. CPG2020-00002-IAPME) from University of Macau, Macau SAR.
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