【Scientific Abstract】
Deformed hydrogels have a wide range of applications in soft robotics, flexible electronics and biomedical devices. Controlling the composition distribution and internal stresses in hydrogels is critical to changing shape. However, existing hydrogel gradient structures, once encoded by chemical reactions and covalent bonds, are often non-reconfigurable. For each new configuration, it is inevitable to manufacture a hydrogel with a unique gradient structure, resulting in poor reusability, adaptability and sustainability, which is detrimental to various applications. Recently, the team of Professor Wu Ziliang of Zhejiang University reported a hydrogel containing reversible light crosslinking that reprograms gradient structures and 3D deformations into various configurations. Hydrogels are prepared by micellar polymerization of hydrophobic coumarin monomers and hydrophilic acrylic acid. The presence of cetyltrimethylammonium chloride micelles increases the local concentration of coumarin units and improves the mechanical properties of the hydrogel by forming a robust polyelectrolyte/surfactant complex, which is used as a physical crosslinker. The efficient photodimerization and photolysis reactions of coumarin are achieved under light irradiation of 365 and 254 nm, respectively, thus reversibly adjusting the network structure of the hydrogel. By lithography, different gradient structures are composed sequentially in a hydrogel, thereby orienting the deformation to different configurations. This strategy should be applied to other photo-unstable hydrogels to enable reprogrammable control of the network structure and common functions. The paper was published in Advanced Materials under the title Ofconstructable Gradient Structures and Reprogrammable 3D Deformations of Hydrogels with Coumarin Units as the Photolabile Crosslinks.
【Graphic analysis】
As shown in Figure 1, a small portion of hydrophobic MAEMC is incorporated into the hydrophilic PAAc network by heat-induced micellar copolymerization and then irradiated at 365 nm light to trigger the [2+2] ring addition of coumarin in CTAC micelles.
Figure 1 a) Under 365/254 nm light irradiation, the coumarin unit is reversible photodimerization/photolysis. b) Schematic diagram of synthesis of photo-unstable hydrogels with reversible regulation of networks and properties. Hydrogels are prepared by micellar copolymerization of AAc and coumarin monomers in the presence of CTAC micelles. After 365 nm light irradiation, the coumarin motif forms dimers that act as chemical crosslinking bonds for the gel. After the gel is swelled in water, the oppositely charged PAAc and CTAC form a polyelectrolyte/surfactant complex (PESC) that acts as a physical crosslinking bond for the gel. Photolysis and photodimerization of coumarin in PESC reversibly regulate the network structure and physical properties of hydrogels.
After micellar copolymerization, the photodimerization and photolysis of coumarin in hydrogel films were monitored by absorption spectroscopy. As shown in Figure 2a, the characteristic absorption peak of the coumarin double bond at 320 nm is gradually reduced under 365 nm light irradiation, indicating that dimerization forms a cyclobutane ring. After subsequent irradiation at 254 nm, the absorption peak gradually increases, indicating that a photo-fracture reaction occurs in coumarin dimer (Figure 2b). Dimer D can be calculated by D = (A0 – At/A0× 100%, where A0 and At are the absorbance of the original gel at 320 nm, as well as one irradiated at different times. Even under the longer conditions of 254, D does not decrease to zero because photolysis and photodimerization reactions are in dynamic equilibrium under light exposure at 254 nm. Under alternating light irradiation at 365 and 254 nm, photodimerization and photolysis of coumarin units are reversibly triggered. Such a process can be repeated for at least four cycles (Figure 2c). It is worth noting that after the periodic photolysis reaction, the absorbance of the gel at 320 nm gradually decreases.
Figure 2 a, b) Absorption spectra of the hydrogel produced by micellar polymerization under 365 nm irradiation (a) and subsequently under 254 nm irradiation (b). The power intensities of 365 nm light and 254 nm light are 5 and 1 mW cm-2, respectively. c) Absorbance of the gel at 320 nm during cyclic irradiation at different times under 365 and 254 nm light. The illustrations in (a) and (b) show the degree of dimerization as a function of irradiation time.
The mechanical properties of the hydrogel are tested by tensile testing. After water equilibration, the photocrossed hydrogel became stronger, with σb being 200 kPa, εb being 220%, and E being 160 kPa (Figure 3a, c), although the water content (q) and length swelling rate (S) increased from 62 wt% and 1.0 to 80 wt% and 1.2, respectively. As shown in Figures 3c, d, the fluorescence spectrum of the photolytic crosslinked gel shows that compared to the photocrossed gel, the blue shift is about 20 nm, the fluorescence intensity is significantly increased, the changing fluorescence is synchronized with the swelling behavior of the hydrogel, attributed to the quenching effect caused by aggregation and the different states of coumarin.
Figure 3 a, b) Tensile stress-strain curve (a), Young's modulus and length swelling ratio (b) of equilibrium photocrossed and photolytic crosslinked hydrogels. The error bars represent the standard deviation of the mean (N = 3). c,d) Fluorescence spectra of equilibrium photocrossing and photolytic crosslinking hydrogels (c) and corresponding peak intensities and peak positions (d). The excitation wavelength is 254 nm.
Taking into account the absorbance of the hydrogel containing coumarin (Figures 2a, b), the penetration thickness of the 254 nm light depends on the feed concentration (Cm) and irradiation time of the coumarin unit (ti, 254). For thicker hydrogels, the entire thickness gradient can be constructed using a network of partially penetrating light and therefore partially de-crosslinked. A photocrossed gel strip with a thickness of 600 μm was selected to study the full thickness gradient and folding deformation of the light regulation. As shown in Figure 4a, the central region of the gel strip is exposed to light at 254 nm by a metal mask with a striped pore (width of 1 mm) perpendicular to the rectangular gel long axis. With ti, 254 increased from 0 min to 10 min and the angle θ increased from 0 ° to 40 °. A further increase in ti,254 will only result in a slight increase in θ, which becomes 55° at ti,254 = 30 min (Figure 4b). Figure 5 discusses programmable patterning to achieve 3D deformation.
Figure 4 a) Scheme and photograph showing the folding and recovery process of hydrogel strips in a full thickness gradient of photo-conditioning. By site-specific cleavage of coumarin dimers by photomask at 254 nm light, a complete thickness gradient (i) is formed in the gel strip. After equilibration in water (ii), the hydrogel shows folding deformation. After incubation in acidic solution, the deformed gel returns to flat. The photolytic cross-linked network is then reduced to the original network (iii) by the photodimerization reaction of the previously lysed coumarin. The patched gel strips swell slightly after balancing in water, but still maintain a flat shape (iv). Scale bars: 500 μm. b) The folding angle θ of the gel strip in water changes with the 254 nm light irradiation time ti, 254. c) The θ of the gel strip changes with the change of the pH of the culture bath. d) The change of the θ of the rubber strip in the water after the cycle folding and recycling process. Photolytic crosslinking is acted on the same location to study the reversible adaptability of the gel network. The thickness of the gel is 0.6 mm and the width of the stripes in the photomask is 1 mm. For gels in (a), (c), and (d), ti,254 for 10 min. The error bars represent the standard deviation of the mean (N = 3).
Figure 5 Reprogrammable deformation of hydrogel strips based on reconfigurable gradient structures. Patterned gradients are lithographically repeatedly encoded to selectively de-crosslink coumarin dimer using phototochemin at 254 nm and eliminate by dimerization of coumarin using light at 365 nm. Gel strips with a uniform network structure are flat (central). After constructing and erasing the gradient structure sequentially, the same hydrogel deforms into four different configurations (corners). i–iii) photograph of the gel in daylight (i); photograph of the strip under ultraviolet light (ii); this schematic diagram shows the configuration of the gel (iii). The thickness of the gel is 0.6 mm. Scale: 1 cm.
【Summary】
Based on reversible crosslinking and reconfigurable networks, reprogrammable deformations are achieved in a single hydrogel. The efficient photodimerization and photolysis reaction of coumarin is achieved by increasing the local concentration of coumarin in CTAC micelles that form PESC with a partially ionized PAAc matrix. Swelling and mechanical properties are regulated by light-unstable crosslinking, which facilitates reprogramming of the hydrogel's structural gradients and 3D deformations. The presence of PESCs also gives hydrogels good mechanical properties and a unique but controllable swelling capacity between photorelamination and photolysis crosslinking states. In addition to coumarin, other dynamic covalent bonds,including trithiocarbonate and anthracene—can also be used to reprogram the network structure of gels and elastomers to enable specific functions and applications, including soft robotics, flexible electronics, and tissue engineering.
Reference: doi.org/10.1002/adma.202008057
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