Electrochemistry‐Induced Improvements of Mechanical Strength, Self‐Healing, and Interfacial Adhesion of Hydrogels

Abstract

physical/chemical functionalization is capable of optimization of mechanical properties, [7] interfacial adhesion, [8] and self-healing ability of hydrogels. [9] These protocols normally introduced functional elements or structures that elicited physical/chemical interactions for the performance improvements of hydrogels. For example, polymeric nanoparticles from crystallization-driven self-assembly enabled mechanical improvements on hydrogel by physical hybridization with matrix. [10] Grafting silica nanoparticles to a double-network polymer also improved the hydrogel mechanical performances. [11] Construction of electrostatic interaction between carboxyl groups and various surfaces resulted in a catechol-chemistrybased hydrogel for the long-term adhesiveness. [12] Treatment by metal ions resulted in a hydrophobic surface that promoted formation of a water-resistant molecular bridge between the hydrogel surface and hydrophobic domains on the substrates, endowing the hydrogels prominent underwater-adhesion capability. [13] Similar adhesive hydrogel could also be achieved by dopamine-modified clay nanosheets. [14] An effective molecular structure design based on acid-ether hydrogen bonding and imine bonds was capable of accelerating hydrogel healing time to 30 min. [15] A dynamic borate bond in network enabled 100% cure of hydrogel in air. [16] Reversible metal-ligand coordination bonding interaction could also be used to construct self-healing hydrogel. [17] These protocols are efficient for the construction of functional hydrogels, yet still challenging to synchronously improve the mechanical strength, self-healing, and interfacial adhesion of a hydrogel, and especially, hard to endow the hydrogel with modular sensitivity to external pressure. Therefore, the development of a new class of protocol is still essential.Herein, we proposed an electrochemistry functionalization protocol, in which the functional improvements on hydrogels were achieved by the electrode reactions, and electrochemistrytriggered ionic and molecular migration. This protocol was capable of enabling the function improvements of the hydrogel in mechanical strength, interfacial adhesion, and self-healing. In the meantime, it allowed generation of various patterns on the hydrogel surface, and endowed the hydrogel modular sensitivity to external pressure. The functional improvements Hydrogels have demonstrated great potential in biomedical and engineering areas. To improve the physical performance, development of efficient physical/chemical protocols is essential. Herein, an electrochemistry functionalization strategy that is capable of enabling the functional improvements of hydrogel is reported. The electrochemistry functionalization is demonstrated on a hydrogel model of polyacrylamide (PAAm)@κ-carrageenan. The electrochemistry reaction generates metal ions (Fe 3+ ) that migrate and coordinate with the sulfate groups of κ-carrageenan resulting in the prominent function improvements. In comparison with untreated PAAm@κcarrageenan hydrogel, it c...

Patterning meets gels: Advances in engineering functional gels at micro/nanoscales for soft devices

“…Alternatively, Fe 3+ generated by local electrochemical oxidation can serve as versatile pathways to functionalization of the κ-carrageenan/PAM hydrogel, which not only improved the mechanical strength, but permitted negative-pressure-induced adhesion and self-healing capability. 108 Moreover, with the patterned stiffness on the gels, when the whole gel was stretched to 100% strain, the stiffer region would undergo minimal strain down to 12% (Figure 11B). This endowed stretchable electronics with robust integration under large strains, where the F I G U R E 1 0 (A) Dynamic multimodal holograms of organogels with microscale photo-crosslinked conjugated chains for tunable structural color and holographic modes in various solvents.…”

“…Inkjet printing 105 and transfer printing 106,107 of Fe 3+ have been demonstrated for post‐gelation engineering of tough hydrogels with locally enhanced stiffness through Fe 3+ ‐carboxyl complexation crosslinking (Figure 11A). Alternatively, Fe 3+ generated by local electrochemical oxidation can serve as versatile pathways to functionalization of the κ‐carrageenan/PAM hydrogel, which not only improved the mechanical strength, but permitted negative‐pressure‐induced adhesion and self‐healing capability 108 . Moreover, with the patterned stiffness on the gels, when the whole gel was stretched to 100% strain, the stiffer region would undergo minimal strain down to 12% (Figure 11B).…”

Highly Conformal Polymers for Ambulatory Electrophysiological Sensing

“…[125,178,179] As is well-known, most of existing electrodes have gaps with the curved skin during muscle contraction, impairing the capacitive coupling of ionic and electronic current. [180,181] Besides, the PAAm @𝜅-carrageenan hydrogel Electrochemistry functionalization =4800% Adhesion energy ≈ 1400 J m −2 to glass [183] induced charge density at the interface is also reduced, leading to inferior compliance and high interfacial impedance with skin (Figure 12a). [182] To overcome this challenge, Chen's group constructed a highly compliant electrode based on adhesive alginatepolyacrylamide (Alg-PAAm) hydrogel, which tightly adhered to stratum corneum of skin via affluent hydrogen bonds and strong electrostatic interaction (Figure 12b).…”

“…Additionally, Zhang's group recently proposed an electrochemistry functionalization strategy to effectively improved the interfacial adhesion and mechanical performance of the synthesized polyacrylamide @ κ ‐carrageenan hydrogel. [ 183 ] In detail, the hydrogel was prepared via a photocrosslinking reaction, in which the network of crosslinked PAAm offered an elastic substrate, whereas the κ ‐carrageenan component enabled the hydrogel conductive. Remarkably, the electrochemistry reaction produced a succession of metal ions (Fe 3+ ).…”

Electrically Programmable Interfacial Adhesion for Ultrastrong Hydrogel Bonding

“…[22] Polyacrylamide (PAAm)/carrageenan hydrogels and iron electrodes could form ionic bonds and chain entanglement via electricalstimulation-induced Fe 3+ migration, displaying an adhesive toughness of 1400 J m −2 to glass. [23] Electrical stimulation made a borate ester polymer hydrogel quickly switch between the adhesive and nonadhesive states, which provided an alternative approach for robot climbing. [24] Electroadhesion could also be applied to tissue engineering.…”