Reconfiguring Gaussian Curvature of Hydrogel Sheets with Photoswitchable Host–Guest Interactions

Photoinduced shape morphing has implications in fields ranging from soft robotics to biomedical devices. Despite considerable effort in this area, it remains a challenge to design materials that can be both rapidly deployed and reconfigured into multiple different three-dimensional forms, particularly in aqueous environments. In this work, we present a simple method to program and rewrite spatial variations in swelling and, therefore, Gaussian curvature in thin sheets of hydrogels using photoswitchable supramolecular complexation of azobenzene pendent groups with dissolved α-cyclodextrin. We show that the extent of swelling can be programmed via the proportion of azobenzene isomers, with a 60% decrease in areal swelling from the all trans to the predominantly cis state near room temperature. The use of thin gel sheets provides fast response times in the range of a few tens of seconds, while the shape change is persistent in the absence of light thanks to the slow rate of thermal cis–trans isomerization. Finally, we demonstrate that a single gel sheet can be programmed with a first swelling pattern via spatially defined illumination with ultraviolet light, then erased with white light, and finally redeployed with a different swelling pattern.


Preparation of Gels
N-isopropylacrylamide (177 mg, 1.0 mmol), N,N'-methylenebisacrylamide (5.5 mg, 0.036 mmol), 4-acrylamidoazobenzene (10.4 mg, 0.041 mmol), and AIBN (5.2 mg, 0.032 mmol) were dissolved in 0.4 mL of 5:1 (v:v) of THF:H 2 O in a 2 mL vial capped with a rubber septum. The solution was de-gassed with nitrogen for 30 min at room temperature. Following de-gassing, the vial was placed in an oil bath at 65 °C and monitored for several minutes as the viscosity increased. After ≈ 10 min, the pre-gel solution was transferred via capillary into cells composed of two clean glass slides glued together with a separation of 25 µm as set by Kapton spacers. The samples were heated overnight at 65 °C on a hot plate in the dark. Following gelation, the gels were harvested by gently opening the cells with a razor blade. Gels were left attached to one of the glass slides and sequentially swelled in 5:1 THF:H 2 O and DI H 2 O. Gels were stored at room temperature in the dark in DI H 2 O until use. For experiments, gels were cut into 1 mm disks using a biopsy punch and transferred into 15 mg mL -1 -CD in DI H 2 O and stored in a refrigerator overnight to equilibrate.

Optical Properties
UV-vis spectra were recorded using a fiber optic spectrometer (Ocean Optics Flame) equipped with a deuterium-tungsten halogen light source. Extinction spectra of 4acrylamidoazobenzene were measured in EtOH using 1-cm pathlength quartz cuvettes (ThorLabs). Photo-switching properties of gels were characterized by placing gels swollen in aqueous solutions containing 15 mg mL -1 -CD on quartz substrates and spectra were measured using a custom-built film set-up. Spectra were thermally equilibrated prior to measurement and subsequently exposed to light via a 365 nm LED (ThorLabs).
Thermal relaxation rates of the cis-isomer at different temperatures were measured using a double-beam UV-Vis spectrometer (Hitachi-U3010). Gels were placed in quartz cuvette and filled with water. The temperature was allowed to equilibrate for 15 min in a thermallyregulated cell holder. After reaching thermal steady state, gels were irradiated with 365 nm LED and absorbance was probed at 354 nm over the course of several hours.

Temperature-Dependent Swelling
Gel disks of 1 mm in diameter were first equilibrated in the dark overnight. After equilibration, gels were place in glass-bottomed petri dishes and immersed in a minimal amount of -CD solution (15 mg mL -1 ) and illuminated for 5 min at 50 mW cm -1 using an LED (ThorLabs). Gels were then heated stepwise from 4 -42 °C using a heat stage (INSTEC HCS621V) and equilibrated at each temperature for 5 min prior to imaging. Gels were imaged using an upright microscope (Zeiss Aziotech Vario) equipped with a camera (Pixel Link). Areal changes in gel swelling were measured using image analysis (ImageJ).

Photo-patterning
Swelling metrics were patterned using an inverted optical microscope (Nikon ECLIPSE Ti) equipped with a digital micromirror device (DLP Discovery 4100, 0.7 XGA, Texas Instruments) and a UV LED (CoolLED). Photomasks were drawn in Illustrator (Adobe) and projected onto the sample plane using custom-built Matlab code (Mathworks). All exposures were performed at room temperature and images were procured by using a needle to gently manipulate the exposed gel to the appropriate angle and recording the image using an attached camera. Figure S2. Extinction spectra of 4-azobenzeneacrylamide in EtOH in the trans state following thermal equilibration (black) and after exposure to 365 nm UV light (blue). In the UV region, the extinction coefficient decreases by an order of magnitude allowing for uniform light penetration through the thickness of the gel sheets.  Figure S5. A) Thermal cis  trans relaxation kinetics at different temperatures for azobenzenecontaining hydrogel films in DI water, determined from the normalized UV-vis absorbance at 354 nm, along with single exponential fits. B) Corresponding characteristic lifetimes of cis isomers as a function of temperature. At room temperature, the thermal relaxation time is several hours, thus allowing shape change to be persistent in the absence of illumination. Figure S4. UV-vis spectra of azo gels in DI water. Figure S3. Time-lapse images of a gel under flood UV illumination. Transient bending is observed as light penetrates through the disk. The final disk is de-swollen uniformly, indicating uniform through-thickness photoisomerization. Scale bar = 1 mm.