Road Narrow‐Inspired Strain Concentration to Wide‐Range‐Tunable Gauge Factor of Ionic Hydrogel Strain Sensor

Abstract The application of stretchable strain sensors in human movement recognition, health monitoring, and soft robotics has attracted wide attention. Compared with traditional electronic conductors, stretchable ionic hydrogels are more attractive to organization‐like soft electronic devices yet suffer poor sensitivity due to limited ion conduction modulation caused by their intrinsic soft chain network. This paper proposes a strategy to modulate ion transport behavior by geometry‐induced strain concentration to adjust and improve the sensitivity of ionic hydrogel‐based strain sensors (IHSS). Inspired by the phenomenon of vehicles slowing down and changing lanes when the road narrows, the strain redistribution of ionic hydrogel is optimized by structural and mechanical parameters to produce a strain‐induced resistance boost. As a result, the gauge factor of the IHSS is continuously tunable from 1.31 to 9.21 in the strain range of 0–100%, which breaks through the theoretical limit of homogeneous strain‐distributed ionic hydrogels and ensures a linear electromechanical response simultaneously. Overall, this study offers a universal route to modulate the ion transport behavior of ionic hydrogels mechanically, resulting in a tunable sensitivity for IHSS to better serve different application scenarios, such as health monitoring and human–machine interface.

conductivity of an ionic conductor is given by the sum of all the carrier contributions: [1] ρ=ΣiZieniui (1)   Where Zi is the absolute value of the ion charge, e is the fundamental charge, ni is the charge carrier density, and µi is the mobility for each ion.It shows that increasing the ion concentration can only increase the initial conductivity, rather than Δρ under tension.
In addition, in hydrogel networks, the diffusion coefficient of ions is smaller than that in water, which needs to be adjusted according to pore size and topography: [2]  eff =  0   (2)   Where Deff is the effective diffusion coefficient in a porous network, D0 is the diffusion coefficient in liquid, ε is the porosity, and τ is the tortuosity.Therefore, the mobility of ionic species is greatly affected by the network topology of nanoscale ionic conductors.In nanofluids, τ describes the non-linear path from one side of the membrane to the other. [3]Highly ordered or longitudinally aligned ion-insulated nanostructures can provide low-bending paths to facilitate ion transport, resulting in significantly lower τ, and thus improved conductivity. [4]

Note S2. Limitation of ionic hydrogel's GF
External stimulus can change the cross-linking network configuration or pore shrinkage of the ionic hydrogel.However, these changes do not/hardly affect the modulation effect of ion transportation or conductive pathway as significantly as in the case of electronic conductive materials.Therefore, the ionic conductivity does not change or only slightly increases as stretched due to the preferential orientation of elastic chains. [5]The electrochemistry of hydrogels is similar to that of aqueous electrolytes, [6] and the response of ionic hydrogels to strain can be quantified by a piezoresistor which follows the equation: Where R is the bulk resistance, ρ is the conductivity, L and A are the length and cross-sectional area of the hydrogel.Therefore, the change in the resistance of the ionic gel during the stretching process mainly comes from the shape change L/A rather than the change in the conductivity.Thus, the GF of the ionic gel is confined to a small range and follows R/R0 = L/L0 = λ 2 . [7]

Note S3. Characterization and analysis
The content of PVA can directly affect the mechanical and electrical properties of hydrogels, as shown in Figure S3a and b.PVA10 (the number represents the mass percentage of the substance, same as follows) was decided to be used in this study due to its greatest elongation at break and sufficient water content.Glycerol can significantly improve the mechanical strength, extensibility, toughness, and stiffness of the PVA hydrogel and slightly affect its GF (Figure S3c and d).Because the glycerol-water binary system introduced more noncovalent interactions (such as hydrogen bonds, (Figure S4a) in the networks, which can act as sacrificial bonds to dissipate external energy during deformation efficiently. [8]In addition, the crystallization of PVA is promoted in the co-solvent environment and the crystallinity of PVA-G hydrogels is much higher than that of PVA hydrogel (Figure S4b).As physical cross-linking points, the PVA crystals can be rearranged or even ruptured to further dissipate energy at a high strain.Moreover, with the increase of glycerol, interconnected tiny pores can be formed and multiplied in the hierarchical porous architecture of PVA hydrogels (Figure S5, Supporting Information).This hierarchical porous architecture acts as a well-connected network of percolation paths, facilitating the diffusion and migration of ions. [8]However, the amount of glycerol is restricted to ensure adequate water content to support the movement of a large amount of ions.

Figure S1 .
Figure S1.The geometrical parameter of the funnel-shape structure.Scale: millimeter.

Figure
Figure S2.a) Strain distribution and b) diagram of heterogeneous structure designs for strain concentration.

Figure S3 .
Figure S3.Schematic of the fabrication process of porous TPU and TPU-reinforced funnel-shaped IHSS.

Figure S4 .
Figure S4.a) The stress-strain curve of PVAn hydrogels.b) The relative resistance change of PVAn hydrogels with the mass percentage of PVA varies from 8 wt% to 18 wt%.The strain range is 0-100 %. c) The stress-strain curve of PVA10-Gn hydrogels.d) The relative resistance change of PVA10-Gn hydrogels with mass percentage of G varies from 0 wt% to 40 wt%.

Figure S5 .
Figure S5.a) FT-IR curves of PVA powder, glycerol, and PVA10-Gn hydrogels.b) Typical XRD patterns of PVA hydrogel and PVA-Gn hydrogels in the dry state.

Figure S8 .
Figure S8.The maximum local strain (obtained by FEM) and GF of funnel-shaped ionic hydrogels under 10 % total strain.

Figure S9 .
Figure S9.a) Hydrogel solution can soak into the micropores on the surface of the TPU layer, resulting in a large interfacial area.After frozen cross-linking, the hydrogel was mechanically interlocked at the surface of the porous TPU layer.b) The schematic diagram of the 90°-peeling test.A stiff backing is introduced to prevent elongation of the hydrogel sheet along the peeling direction.

Figure S11 .
Figure S11.The stress-strain curve of a) pure PDMS and porous PDMS with different NaCl ratio and b) Nylon fabric.c) The GF of porous PDMS and Nylon fabric-reinforced funnel-shaped PVA10-G30-FeCl3 hydrogel at 50 % strain.100 % strain cannot be reached due to poor stretchability and weak interfacial bonding of hydrogel-PDMS hybrid.

Figure S12 .
Figure S12.Fruit recognition with four-finger hand (Allegro) grasping by attaching the sensor to the joints.