Facile Synthesis of Highly Stretchable, Tough, and Photodegradable Hydrogels

Abstract

Recently, highly stretchable and tough hydrogels that are photodegradable on‐demand have been reported. Unfortunately, the preparation procedure is complex due to the hydrophobic nature of the photocrosslinkers. Herein, a simple method is reported to prepare photodegradable double‐network (DN) hydrogels that exhibit high stretchability, toughness, and biocompatibility. Hydrophilic ortho‐nitrobenzyl (ONB) crosslinkers incorporating different poly(ethylene glycol) (PEG) backbones (600, 1000, and 2000 g mol−1) are synthesized. These photodegradable DN hydrogels are prepared by the irreversible crosslinking of chains by using such ONB crosslinkers, and the reversible ionic crosslinking between sodium alginate and divalent cations (Ca2+). Remarkable mechanical properties are obtained by combining ionic and covalent crosslinking and their synergistic effect, and by reducing the length of the PEG backbone. The rapid on‐demand degradation of these hydrogels is also demonstrated by using cytocompatible light wavelength (λ = 365 nm) that degrades the photosensitive ONB units. The authors have successfully used these hydrogels as skin‐worn sensors for monitoring human respiration and physical activities. A combination of excellent mechanical properties, facile fabrication, and on‐demand degradation holds promise for their application as the next generation of substrates or active sensors eco‐friendly for bioelectronics, biosensors, wearable computing, and stretchable electronics.

Photodegradable Non‐Drying Hydrogel Substrates for Liquid Metal Based Sustainable Soft‐Matter Electronics

“…[41,44] To prove the concept, glycerolbased hydrogels were irradiated for 3 h, since the greatest loss of properties occurred after this time, as shown in our previous report. [37] As expected, the tensile strength and elastic modulus values increased dramatically after UV exposure (120 kPa and 41 kPa, respectively). This result shows that the solvent displacement method has no effect on the cleavage of the ONB-PEG 600 crosslinker and does not lead to the failure of the PAAm network.…”

“…First, to irreversibly destroy the covalent network, the samples were irradiated with UV light to induce the photocleavage of the photodegradable crosslinker. [37] The photodegradation of PAAm was confirmed by the liquid appearance of the sample after light exposure (Figure 5A). Then, to dissolve the ionic network, the irradiated samples were then immersed in a solution of a strong calcium chelator such as EDTA (Figure 5B).…”

“…At the same time, the PEG backbone with an Mw (weight-average molecular weight) of 600 g mol −1 provides the required hydrophilicity of the crosslinker, while enabling the proper mechanical properties and degradability of the hydrogel, as described in our previous report. [37] The DN hydrogels were prepared in a one-step process in an aqueous solution. The thermal initiation process, using ammonium persulfate (APS) as the photoinitiator and N,N,N′,N′tetramethylethylenediamine (TEMED) as the catalyst, triggers radical polymerization to form long-chain PAAm, which is subsequently crosslinked by ONB-PEG 600 .…”

“…Synthesis of Photodegradable DN Hydrogels: Photodegradable DN hydrogels were prepared according to our previous work. [37] Alginate and Aam (the total amount of alginate and Aam was 14 wt.%, and the weight ratio of alginate to AAm was 1:8) were dissolved in deionized water. The solvent content was fixed at 86 wt.%.…”

“…The photodegradable crosslinker (ONB-PEG 600 ) was synthesized according to our previously reported procedure. [37] Deionized water was purified in an 18 MΩcm MILIQ Plus water system. To prepare the desired polymer-filler-LM composites, the following materials were provided and used without further purifications: Silver (Ag) flakes (Particle size distribution (PSD): ≈ 5 μ, Technique), Polystyrene-block-polyisoprene-block-polystyrene styrene (SIS) (Sigma Aldrich), and toluene (José Manuel Gomes dos Santos, Lda).…”