Flexible wearable sensors based on lignin doped organohydrogels with multi-functionalities
Graphical abstract
Introduction
With the rapid development of high-speed network, intelligent wearable, biotechnology and microelectronics technology, electronic devices are gradually developing in the direction of miniaturization, integration, intelligence and wear-ability [1], [2], [3]. Among them, flexible strain sensor is widely used in the fields of biomedicine and health monitoring because of its advantages of flexibility fitting to different surfaces and low strain detection limit, which can realize the monitor of human motions and external stimulations [4]. The development of this kind of flexible sensor has stimulated innovation of materials and manufacturing technology. In order to prepare flexible strain sensors with good performance, it is important to develop materials with appropriate elastic modulus, excellent tensile strength and high conductivity. Conductive hydrogels have dual properties of electronic conductivity and flexibility, displaying great potentials in biological medicine, health monitoring and wearable devices [5], [6], [7]. Various conductive active materials, such as conductive nanoparticles, conductive polymers and conductive ions have been introduced to polymer matrix to obtain conductive hydrogels [8], [9], [10], [11]. Therefore, strain sensors composed of conductive hydrogels have been extensively studied. However, the hydrogel sensors still exhibit low mechanical properties, narrow application temperature range, irreparable damage and poor adhesiveness [12], [13], [14].
Recently, self-healing conductive hydrogels, on the basis of reversible covalent bonds (e.g. reversible ionic coordination, disulfide bonds and imine bonds) [15], [16], [17] and non-covalent interactions (hydrophobic interaction, hydrogen bonds and host–guest interaction) [12], [18] have been investigated. The introduction of self-healing property into hydrogels allows them to self-repair after being damaged, which can improve their stability and prolong service life. However, hydrogels with self-healing ability are usually accompanied by poor mechanical properties. The introduction of nanofillers into hydrogels to prepare nanocomposite hydrogels has been considered to be a good solution to obtain tough hydrogels [19]. Carbon nanotubes (CNT) and carbon black (CB) exhibiting a synergistic effect on the mechanical and electrical properties were introduced into polyvinyl alcohol hydrogels to fabricate a 3D structural conductive hydrogel PVA/Gly/CB/CNT [20]. The prepared hydrogel had a relatively high tensile strength of 4.8 MPa, a toughness of 15.93 MJ/m3 and a flexibility of up to 640% elongation at break.
Lignosulfonate (LGS) is a kind of cheap, nontoxic, ecofriendly natural biomass originating from sulfite pulping industry, which has a large number of aromatic rings and phenyl-propane units, and contains masses of hydroxyl and sulfonic groups [21], [22]. As a result, great interests were aroused to explore its applications. In sight of the high surface-to-volume ratio, LGS nanoparticles (nano-LGS) have promising surface characteristics that can lead to improved mechanical strength, adhesiveness and conductivity [23]. For instance, nano-LGS were applied to reinforce the mechanical properties of phenolic foams [24]. The incorporation of nano-LGS in phenolic foams resulted in a compressive modulus and strength that was up to 128% and 174%, respectively. The nano-LGS was doped into polyaniline, polypyrrole, and poly(3,4-ethylenedioxythiophene) to fabricate a series of conductive and long-term adhesive hydrogels [25].
Acrylic acid (AA) is an important anionic monomer, due to its easy polymerization and the presence of carboxyl groups, it has been widely used in construction of functional hydrogels. For instance, a hydrogel with semi-interpenetrating network was fabricated by polymerization of AA and BVIT monomers in PEO aqueous solution [26]. The as-obtained hydrogel exhibited high mechanical robustness, fatigue resistance and good ionic conductivity, which enable it to be ionic conductor for capacitive/resistive bimodal ionic sensor. A double-network (DN) hydrogel based on PAA with dynamic cross-links was fabricated by immersing into FeCl3 solution [27]. This DN hydrogel exhibited elastic, anti-fatigue, and anti-freezing performance, which showed great potential in wearable multifunctional sensors.
A good adhesion to the surface of the human body or other measured objects in order to achieve more accurate human–computer interaction is also demanded for hydrogel sensors [8]. A feasible strategy for obtaining such an adjustable adhesive hydrogel is to construct it with both positively and negatively charged monomers [14]. Both charges in the hydrogel are expected to interact arbitrary charged surfaces. However, two opposite charges easily lead to strong aggregates. Therefore, the application of copolymerization is a great way to prevent this circumstance. Methacryloxyethyltrimethyl ammonium chloride (DMC) as a functional cationic monomer, has been often used to prepare polyelectrolyte hydrogels. The combination of cationic DMC and anionic 4-styrenesulfonic acid sodium could endow the formed hydrogel with excellent self-healing and mechanical performances [28].
Herein, a facile one-pot strategy to prepare conductive organohydrogels composed of poly(acrylic acid-co-2-(methacryloyloxy)ethyl trimethylammonium chloride) (poly(AA-co-DMC) doped with nano-LGS was applied in this study. Glycerol is a kind of polyol and has been used as an effective aquasorb and inhibitor for water freezing. Glycerol/H2O binary solution was applied to expand available temperature range. AA and DMC monomers were dissolved in Glycerol/H2O binary solvent and polymerized in situ in the presence of chemical cross-linker and nano-LGS. The presence of dissociated ions and 3D network provides the resultant organohydrogels with excellent ionic conductivity. The abundant positive and negative charges can facilitate electrostatic interactions, enabling outstanding adhesiveness and self-healable performance after being damaged. Notably, the organohydrogels exhibited high sensitivity and excellent antibacterial activity. All the above mentioned features make these organohydrogels available for the application as flexible strain sensors.
Section snippets
Materials
Calcium lignosulfonate (93%, Mw = 800–10000 g/mol) and DMC were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. 1-Hydroxy-cyclohexyl phenylketone (Irg. 184), N’N-methylene-bis-acrylamide (MBA) and methanol were purchased from Shanghai Macklin Biochemical Co. Ltd. AA and ammonium persulfate (APS) were purchased from Sinopharm Chemical Reagent Co., Ltd. Glycerol was obtained from Nanjing Chemical Reagent Co., Ltd. All reagents were used as received.
Preparation of PADL organohydrogels
Nano-LGS was firstly prepared
Organohydrogel preparation and characterization
A series of PADL organohydrogels with suitable mechanical properties, conductivity, pH responsiveness, antibacterial activity, strong adhesiveness and self-healing activity was prepared by a one-step process. The overall process to fabricate PADL organohydrogel is shown in Fig. 1. Two oppositely charged monomers were selected to obtain polyampholyte hydrogels to achieve excellent adhesiveness [30]. This kind of hydrogel was copolymerized with negatively charged AA and positively charged DMC in
Conclusion
In summary, a facile strategy was constructed to fabricate highly stretchable and conductive PADL organohydrogels in this study. The PADL organohydrogels exhibited good mechanical properties, strong adhesiveness, antibacterial and anti-freezing properties, and good self-healing performance. The introduction of nano-LGS efficiently enhanced the antibacterial activity and mechanical properties. Benefiting from these characteristics, the PADL organohydrogels can be assembled into strain sensors to
CRediT authorship contribution statement
Lei Jiang: Conceptualization, Investigation, Writing - original draft. Jia Liu: Methodology, Investigation. Shu He: Methodology, Investigation. An Liu: Data curation. Jie Zhang: Data curation. Haijun Xu: Writing - review & editing, Supervision. Wei Shao: Conceptualization, Writing - review & editing, Supervision.
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.
Acknowledgements
The authors thank for Advanced Analysis and Testing Center of Nanjing Forestry University.
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