Photo-degradable, tough and highly stretchable hydrogels

Abstract

We present for the first time highly stretchable and tough hydrogels with controlled light-triggered photodegradation. A double-network of alginate/polyacrylamide (PAAm) is formed by using covalently and ionically crosslinked subnetworks. The ionic Ca²⁺ alginate interpenetrates a PAAm network covalently crosslinked by a bifunctional acrylic crosslinker containing the photodegradable o-nitrobenzyl (ONB) core instead of the commonly used methylene bisacrylamide (MBAA). Remarkably, due to the developed protocol, the change of the crosslinker did not affect the hydrogel's mechanical properties. The incorporation of photosensitive components in hydrogels allows external temporal control of their properties and tuneable degradation. Cell viability and cell proliferation assays revealed that hydrogels and their photodegradation products are not cytotoxic to the NIH3T3 cell line. In one example of application, we used these hydrogels for bio-potential acquisition in wearable electrocardiography. Surprisingly, these hydrogels showed a lower skin-electrode impedance, compared to the common medical grade Ag/AgCl electrodes. This work lays the foundation for the next generation of tough and highly stretchable hydrogels that are environmentally friendly and can find applications in a variety of fields such as health, electronics, and energy, as they combine excellent mechanical properties with controlled degradation.

Keywords: Double-network; Hydrogel; Stretchable light-degradable; Tough; o-nitrobenzyl.

Graphical abstract

Fig. 1

Overview of the approach: A) Synthesis of two o-nitrobenzyl (ONB) crosslinkers with different labile bonds. B) Photocleavage of ONB crosslinkers upon UV irradiation, forming an aromatic nitroso compound and a carboxylic acid. The R1 group determines the rate of photodegradation and the nature of resulting cleavage products. C) Structure of the alginate/PAAm hydrogels. In a polyacrylamide hydrogel, the polymer chains form covalent crosslinks through prepared ONB crosslinkers. The alginate hydrogel is formed by the ionic crosslinks through Ca²⁺ and. the G-blocks. D) After UV irradiation of the DN hydrogel, the ONB crosslinkers undergo an irreversible photocleavage, breaking the polyacrylamide hydrogel crosslinks. When stretched the PAAm network cannot stabilize the deformation, while the alginate network unzips progressively, resulting in a high loss of the material toughness.

Fig. 2

Mechanical properties of photodegradable DN hydrogels determined by tensile tests: A) Stress-strain curves of gels with different percentages of DMSO (25–30 ​vol%) and amounts of covalent photodegradable crosslinker, ONB-1 (0.06%, 0.3%, 0.6% and 0.12% of ONB-1 is represented by 1, 2, 3 and 4, respectively). Each test was performed by pulling the samples at a constant rate of 20 ​mm min⁻¹. B) Tensile strength; C) Elongation at break; D) Elastic modulus calculated from the initial slopes of the stress-strain curves. The error associated with the values presented is the standard deviation of the five valid tests.

Fig. 3

Mechanical properties of DN hydrogels determined by tensile tests: A) Stress-strain curves of the control hydrogel and the photodegradable hydrogel (H25_2). Each test was performed by pulling the samples at a constant rate of 100 ​mm·min⁻¹ until rupture; B) Photodegradable hydrogel.

Fig. 4

Rheological analyses of photodegradable hydrogels (H25_2) before and after UV irradiation, at different irradiation times.

Fig. 5

Stress-strain curves of photodegradable hydrogel (H25_2) without irradiation and after 6 and 24 ​h of UV irradiation. Each test was performed by pulling the samples to rupture at a constant velocity of 100 ​mm ​min⁻¹.

Fig. 6

Representative SEM images of the cross-section morphologies of control hydrogels before (A and B) and after 24 ​h of irradiation (C and D), and photodegradable hydrogels before (E and F) and after 24 ​h of irradiation (G and H).

Fig. 7

Results of in vitro cell viability when cells are exposed to gel conditioned medium. Data are represented as means ​± ​standard deviation of three independent experiments with n ​= ​3 samples for each treatment. The statistical significance was indicated as ∗∗∗∗p ​< ​0.0001 by two-way ANOVA with a Tukey post-hoc test.

Fig. 8

Representative Z-stack images captured by confocal microscopy that were reconstructed into a 3D image using ZEN 2.3 Imaging Software. Green cells indicate live cell colonizing the surface and interior of the hydrogels, while dead cells appear in red colour, after 48 ​h of incubation. The white bar shown the scale is 100 pixel.

Fig. 9

Microscopic phase contrast images of NIH3T3 cells grown on the surface of the hydrogels at different degradation time intervals - after 24 ​h incubation (top) and 72 ​h incubation (bottom). The magnification of all the photographs is the same with the scale bar equals to 200 ​μm.

Fig. 10

Case study of application in bioelectronics: A) Skin-electrode impedance in function of the frequency, for three different electrodes: Ag/AgCl, stainless steel and hydrogel electrodes. B) Cd and Rd values for all electrodes after fitting to the electrical equivalent circuit, during the impedance analysis. C) Portion of the ECG signal obtained with the hydrogel electrodes. D) Setup used for the acquisition of the ECG signal: monitorization chest band with integrated hydrogel electrodes.