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The development of conductive, soft, ultra-stretchable, and asymmetrically adhesive hydrogels is difficult and essential for both wearable electronics and anti-adhesion tissue dressings. In particular, there is still no simple, effective and universal approach to construct an asymmetrically adhesive multifunctional hydrogel. Here, we first synthesized lignosulfonate sodium (LS)-doped PAA hydrogels with uniform adhesion (adhesive strength: ~30.5 kPa), conductivity (~0.45 S/m), stretchability (up to ~2250%), and low compressive modulus (~20 kPa). In the second step, an oriented soaking of Fe3+ onto the upper surface of the resultant composite hydrogel renders the upper surface non-adhesive. This novel strategy masterfully delivers asymmetric adhesion behavior to the upper and bottom surfaces of the same hydrogel (~0 kPa adhesive strength for the upper surface; strong adhesive strength of ~27 kPa for the bottom surface). The asymmetric adhesive hydrogel has proven to adhere well onto the human skin and achieve waste-barrier. Importantly, this hydrogel assembled pressure sensor demonstrates excellent anti-interference and wearable comfort. [ABSTRACT FROM AUTHOR]

In recent years, various biomacromolecule-based hydrogels have been extensively and deeply studied in the field of wearable electronics. However, the application of lignin-based hydrogels in flexible devices is still in its infancy. This is mainly due to the significant differences in physical and chemical properties of industrially extracted lignin. In order to seek the universal applicability of diversified lignin in the preparation of hydrogel electronics, we mainly paid attention to the natural physical and chemical properties of lignin to discuss feasible solutions for functional gel design. These properties include chemical reactivity, UV shielding, antibacterial, bio-degradability, anti-oxidation, etc. Finally, in view of lignin's unique properties and the demand for high-quality flexible electronics, some insights are proposed regarding the future research and development directions of lignin-based hydrogel electronics. [ABSTRACT FROM AUTHOR]

• The asymmetric hydrogel is adjusted and assembled by lignin and cellulose derivatives. • The hydrogel exhibits completely asymmetric adhesion, strength, and electriferous. • The hydrogel-based sensor has excellent skin adaptability and application stability. • The hydrogel can be applied to self-adhesive bioelectrodes and self-powered sensors. Mechanical adaptability, great wearability, application stability, and self-powered sensing characteristics are important requirements for hydrogel-based strain sensors. In this study, a novel double-layer hydrogel was fabricated with asymmetrical adhesion, strength, and electriferous properties. Wherein, the lignosulfonate sodium (LS)-borax mediated bottom hydrogel layer exhibits excellent softness (Young's modulus: ~14.2 kPa) and skin-adhesion (Adhesive strength: ~18.7 kPa) while the quaternary hydroxyethyl cellulose (QHEC) mediated top hydrogel layer demonstrates great mechanical strength (Young's modulus: ~101.3 kPa) and non-adhesive (Adhesive strength: ~2.2 kPa) properties. These complementary asymmetrical adhesion and strength properties endow the hydrogel-based sensor with exceptionally stable sensing performance and adaptive wearability; moreover, the lignocellulosic materials utilization plays a significant role in the designability, antibacterial and biodegradable properties. In addition, the synergy of negative LS (-) and positive QHEC (+) particles enables the double-layer hydrogel great self-powered sensing because of the directional movement of free ions initiated by the external mechanical stimulus. This study presents a hierarchical design idea of wearable electronics, which will have potential applications in many fields from wearable bioelectrodes to self-powered sensors. [ABSTRACT FROM AUTHOR]