Nanocellulose-Based Hydrogel for Strain Sensor
The wearable strain sensor with self-healing property has the ability to restore its structure and sensing function from damage. Therefore, this kind of self-healing strain sensor exhibits excellent safety, durability, and reliability, which have raised more attention of researchers (Li et al. 2017). Recently, the self-healing conductive gels have been extensively investigated and applied as wearable strain sensors in various fields. However, the conventional self-healing conductive gels always have a relatively low mechanical strength due to the dynamic cross-links, which has limited their application in strain sensing with stable performances (Chen et al. 2012). To endow the conductive gels with both great self-healing and mechanical properties, incorporating nanoscale fillers (e.g., CNCs) into the gel matrix that could dissipate energy is a promising approach, because the added fillers would tune the interactions between themselves and the surrounding polymer phase in conductive gels (De France et al. 2017).
In addition, self-adhesive function is also significant for the practical application of conductive gels as wearable strain sensors. The self-adhesive strain sensors can be directly attached onto the surface of the object to be tested (e.g., skin of human body) without the assistance of additional adhesive tapes and/or gels. Compared to the adhesive polydopamine (PDA), the colourless tannic acid (ТА) with the merits of low- cost and biocompatibility is regarded as a better material for the preparation of ‘green’ and safe self-adhesive hydrogels (Sileika et al. 2013, Krogsgaard et al. 2016). In this chapter, novel, tough, self-healing, and self-adhesive CNC-based hydrogels were designed and fabricated; their potential applications as wearable strain sensors were also demonstrated.
Preparation of the Nanocellulose-based Hydrogel
The CNCs coated with ТА (TA@CNC) were firstly fabricated. As shown in Figure 3.1 la, the CNCs colloidal suspension w'as stable with light blue colour. After mixing with ТА. the obtained TA@CNC solution remained stable and with the colloidal dispersion, but the colour w'as changed into slightly yellow. Figure 3.11 b presents the atomic force microscope (AFM) images of CNC and TA@CNC. As mentioned before, CNCs were rod-like crystals with low aspect ratio, while the TA@ CNCs had dramatically thickened walls due to the deposition of ТА onto the CNCs surface. The introduction of ТА into the TA@CNC can be further proved by the dimension distribution and FTIR spectra.
The fabrication process of CNC-based hydrogel is illustrated in Figure 3.1 lc. The acrylic acid (AA, monomer), ammonium persulfate (APS, initiator), N,N'- methylenebis(acrylamide) (MBA. chemical cross-linker), and TA@CNC were added together in aqueous solution to prepare cross-linked composite gels by free radical polymerization. Then, the as-fabricated composite gels were soaked in 0.1 mol L_l AlCl, aqueous solution to generate ionically cross-linked domains. Finally, the gels were soaked in DI water for more than 24 h to remove superfluous cations. The obtained CNC-based hydrogels had a hierarchically porous structure and coordination bonds among TA@CNC, PAA, and Al,+. The possible coordination modes w-ere
FIGURE 3.11 (a) Photographs of the CNC (1.02 wt%) and TA@CNC (1.2 wt%) suspension. (b) AFM images of CNC and TA@CNC. (c) Schematic illustration of the fabrication of the CNC-based hydrogel.
metal-phenolic coordination between TA@CNCs, metal-carboxylate coordination between PAA chains, as well as hybrid bridging between TA@CNCs and PAA chains.
Characterization of the Nanocellulose-based Hydrogel
The mechanical properties of CNC-based hydrogels were demonstrated by the tensile measurement. Figure 3.12a presents the stress-strain curves of the as-prepared hydrogels with different TA@CNC contents. When compared to the pure PAA, the hydrogels with TA@CNC exhibited significantly enhanced mechanical properties. In particular, the hydrogel with 0.6 wt% TA@CNC has the highest elasticity (strain up to 2900%), which is also illustrated by the photograph in Figure 3.12a. The hydrogel with 0.8 wt% TA@CNC exhibits the highest toughness (5.60 MJ nr3), which is extremely higher than that for the pure PAA. The enhancement of mechanical properties by TA@CNC for hydrogels may be contributed to the resistance against crack propagation in bunting and energy dissipation. The hydrogels have great self-recovery ability and could dissipate energy with pronounced hysteresis loops. The highest recovery ratio for the CNC-based hydrogels was 92.5% with 12% residual strain, indicating their excellent resilience and fatigue resistance. Furthermore, the hydrogels also exhibited brilliant recovery properties against compressing; the great compressive toughness endowed the hydrogel with the ability to withstand 85% deformation and almost full recovery.
Self-healing function is critical to the CNC-based hydrogels with a stable performance of strain sensing in practical application. Figure 3.12b illustrates the
FIGURE 3.12 (a) Stress-strain curves for the hydrogels with different TA@CNC contents
and the photograph of the hydrogel under stretching state, (b) Photograph of the hydrogels before and after self-healing, (c) Photograph showing the excellent adhesive properties of the hydrogels, (d) Adhesion strength of the hydrogels with different substrates tested by tensile adhesion tests.
self-healing behaviour of rectangle-shaped hydrogel specimens. The specimen was cut into two parts and one of them was stained by methylene blue to make it better visible. As can be seen, two parts of the hydrogel specimen could be recombined together when they were placed in contact without any additional stress. Furthermore, the self-healed hydrogel still exhibited great mechanical strength and toughness during stretching. Besides, the effect of time on the self-healing efficiency of hydrogels was also investigated. By increasing the self-healing time, the fracture stress of the healed hydrogels dramatically increased, which indicates that enlarging processing time would improve the self-healing properties of CNC-based hydrogels. In addition, increasing the TA@CNC contents in hydrogels was also beneficial for their self- healing behaviours, because the dynamic TA@CNC motifs play a crucial role for the reversible rearrangement in the self-healing process.
The CNC-based hydrogels exhibited an excellent self-adhesive property by mimicking the mussel adhesion principle through the catechol groups of oxidized polyphenols. The hydrogels have strong adhesion to the surfaces of various substrates, such as glass, poly(tetrafluoroethylene) (PTFE), rubbers, wood, and carnelian. As can be seen in Figure 3.12c, the hydrogel can be stably adhered on the fingers to achieve up to 200% recoverable stretching, which indicated the excellent self-adhesiveness of CNC-based hydrogels with skin. Furthermore, the already adhered hydrogel can also be easily peeled off and removed from the skin without any residues or the irritation of skin. Importantly, the stripping lag at the gel-skin interface was crucial to the immediate detachment between CNC-based hydrogels and skin; however, no stripping lag was observed for the pure PAA hydrogels, indicating that the TA@CNC could improve the self-adhesive properties of the hydrogels. Figure 3.12d shows the adhesion strength of the CNC-based hydrogels to the surfaces of different substrates. The aluminium had the highest adhesion strength among them, which may be contributed to the synergy of metal complexation and hydrogen bonding between hydrogel and aluminium. The PTFE also exhibited relative high adhesion strength due to both the hydrophobic interaction and hydrogen bonding. The aforementioned excellent features (mechanical, self-healing, and self-adhesive) of CNC-based hydrogels are potentially attractive for practical applications as wearable strain sensors.
Application of the Nanocellulose-based Hydrogel for Wearable Strain Sensor
The CNC-based hydrogel had a good electric conductivity; it could work as one part of the circuit to lighten the light-emitting diode (LED). Furthermore, the brightness of this LED changed when the hydrogel was starched or compressed, indicating the piezoresistant feature of CNC-based hydrogel (ionic gel). The relative resistance change of the hydrogel under different strains was further tested; the relative resistance change is defined as AR/R„ = (R - R0)/R(), where R„ is the resistance of the hydrogel in releasing state and R is the resistance of the hydrogel after stretching. As can be seen in Figure 3.13a, the relative resistance change is increased linearly by increasing strain. The relative resistance-change curve of the hydrogel was divided into three linear parts and the hydrogel sensor’s sensitivity was evaluated by gauge factor (S), which is defined as S = (AR/R0)/e, where e is the applied strain. In the strain range of 0-40%, the gauge factor of the hydrogel sensor was 0.23, while it increased to 4.90 under 65-75% strain. The inset of Figure 3.13a shows the gauge factor changes of hydrogel sensor under different strain; the gauge factor increased from 5.5 to 7.8 by increasing the strain from 100% to 2000%. Besides, the hydrogel sensor also exhibited excellent stability and durability for strain sensing.
The electrical self-healing property of the CNC-based hydrogel was also demonstrated by connecting in series with an LED in the circuit. The LED could be relighted
FIGURE 3.13 (a) Relative resistance changes of the hydrogel sensor as a function of the
applied strain, (b) Relative resistance changes of the hydrogel-based strain sensor to monitor finger bending with different angles, (c) Relative resistance changes of the sensor for wrist bending.
when two separated hydrogels were recombined after self-healing; the electrical- conductive ability of the hydrogel was almost recovered. This reveals that the hydro- gel-based strain sensor had great reliability against physical damage and can be applied in various fields.
Taking advantages of great self-adhesiveness, high sensitivity for strain sensing, excellent electrical stability, and fast self-healing ability, the CNC-based hydrogel was used as wearable strain sensor to detect human motions by attaching it on body skin. As can be seen in Figure 3.13b, the hydrogel-based strain sensor was used to detect the forefinger under bending with different angles. As the bending angles increased from 0° to 90°, the relative resistance changes of the sensor also exhibited monotonical increment and it had the stepwise feature. As expected, the signal intensity was decreased by decreasing the finger bending angles. Importantly, the signal intensity under the same bending angle was consistent in case of both stepwise increased and decreased bending angels. This indicates the great reliability and reproducibility of the hydrogel-based strain sensor, which is vital to the sensor’s practical applications. Figure 3.13c illustrates the application of the hydrogel-based sensor to detect elbow’s motions, such as repeated bending and releasing. The obtained signals w'ere dramatic and stable, which could demonstrate the bending state of elbow. Besides, the hydrogel-based sensor was able to accurately detect subtle motions, such as pulse and breath. Therefore, it could be used for real-time monitoring of personal healthcare. Furthermore, the obtained signals of strain sensing can be transmitted wirelessly to the smartphones through blue-tooth module, enabling users to analyse the sensing data and interact with the sensor.
