Supramolecular nanofibrillar hydrogels as highly stretchable, elastic and sensitive ionic sensors

Author(s): Xiaohui Zhang ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Nannan Sheng ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Linan Wang ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Yeqiang Tan ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Chunzhao Liu ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Yanzhi Xia ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Zhihong Nie ⁶ ^, ⁷ ^, ⁸ ^, ⁹ , Kunyan Sui ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵

Publication date (Electronic): 2019

Journal: Materials Horizons

Publisher: Royal Society of Chemistry (RSC)

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Abstract

Nature-inspired ionic conductors based on supramolecular nanofibril are developed for flexible and stretchable strain/pressure sensors.

Abstract

There is a growing demand for flexible and stretchable strain/pressure sensors for different applications. However, existing conductors usually cannot meet all the requirements for use in next-generation wearable sensors. In this work, we demonstrate a new class of nature-inspired ionic conductors based on supramolecular sodium alginate (SA) nanofibrillar double network (DN) hydrogels with complex shapes by injection. Owing to their dermis-mimicking structures, these hydrogels exhibit unique features, such as high transparency (99.6%), high tension/compression strength (0.750 MPa/4 MPa), high stretchability (3120%), high toughness (4.77 MJ m ⁻³) and superior elasticity (100%) at high strain (1000%). In particular, the use of salts ( e.g., NaCl) as triggers in supramolecular assembly combining SA makes the hydrogels ideal ionic conductors. The ionic conductors were demonstrated as strain sensors with high sensitivity to an extremely broad strain window (0.3–1800%) and a low applied voltage (down to 0.04 V), as well as with high pressure sensitivity (1.45 kPa ⁻¹). These hydrogel-based ionic sensors may find applications in sports monitoring, human/machine interfaces and soft robotics.

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Highly stretchable and tough hydrogels.

Jeong-Yun Sun, Xuanhe Zhao, Widusha R K Illeperuma … (2012)

Hydrogels are used as scaffolds for tissue engineering, vehicles for drug delivery, actuators for optics and fluidics, and model extracellular matrices for biological studies. The scope of hydrogel applications, however, is often severely limited by their mechanical behaviour. Most hydrogels do not exhibit high stretchability; for example, an alginate hydrogel ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels have achieved stretches in the range 10-20, but these values are markedly reduced in samples containing notches. Most hydrogels are brittle, with fracture energies of about 10 J m(-2) (ref. 8), as compared with ∼1,000 J m(-2) for cartilage and ∼10,000 J m(-2) for natural rubbers. Intense efforts are devoted to synthesizing hydrogels with improved mechanical properties; certain synthetic gels have reached fracture energies of 100-1,000 J m(-2) (refs 11, 14, 17). Here we report the synthesis of hydrogels from polymers forming ionically and covalently crosslinked networks. Although such gels contain ∼90% water, they can be stretched beyond 20 times their initial length, and have fracture energies of ∼9,000 J m(-2). Even for samples containing notches, a stretch of 17 is demonstrated. We attribute the gels' toughness to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed on unloading. The unzipped ionic crosslinks cause internal damage, which heals by re-zipping. These gels may serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications.