PMC

SEM images of bio-inspired nanocomposite hydrogels (NC-10) fabricated by freeze-casting at a cooling rate of a) 10, b) 5, and c) 1°C/min, or d) random freezing. The upper and lower rows show the structures at the cross-section perpendicular and parallel to ΔT, respectively. By controlling the cooling rate during freeze-casting, the size of ice crystals and hence the pore size within the hydrogels could be tuned. The width (D) between two adjacent walls in a) to c) is 42.7 μm, 48.2 μm, and 107.7 μm, respectively. d) For the sample fabricated by random freezing, isotropic pores are 79 μm in size.

Mechanical properties of macroporous composite hydrogels from tensile tests. Representative stress-strain curves when stretching a) parallel and b) perpendicular to the freezing direction (inset), of hydrogels fabricated at different cooling rates or random freezing, as labeled. The stretch ratio (λ) is defined as the distance between the clamps during stretching divided by the distance before stretching. In the parallel direction, the hydrogels have an increasing tensile strength and fracture energy following the order of 10°C/min > 5°C/min > 1°C/min > random freezing. Freeze-cast samples show no obvious advantage over random freezing samples in perpendicular direction. c) Representative stress-strain curves of hydrogels with different amounts of clay platelets (NC-5, 10, and 15), all fabricated by freeze-casting at 5°C/min. Hydrogels with higher clay content have higher modulus and tensile strength.

a) Schematic illustration of the fabrication method of macroporous composite hydrogels by freeze-casting. A solution, composed of monomer (NIPAAm), initiator (DEAP), and crosslinker (clay platelet) at a given concentration was placed on a cold finger connected to a liquid nitrogen reservoir. During the cooling process, ice crystals grew from the cold finger and templated the assembly of monomer and clay platelets into a nacre-like layered nanocomposite. After freezing, the sample was placed under UV light to initiate cryo-polymerization. The as-prepared nanocomposite hydrogels have an anisotropically aligned structure at micrometer scale, as shown by the SEM images in both the b) parallel and c) perpendicular directions to the temperature gradient (ΔT). In the wall of the aligned structure, clay platelets and PNIPAAm are assembled into nacre-like layered nanocomposites, as shown by d) SEM and e) TEM images.

Thermoresponsive drug release and swelling properties of bio-inspired nanocomposite hydrogels. a) Drug release of the hydrogel is largely enhanced by increasing temperature from 23°C to 37°C, because of the coil-to-globule transition of PNIPAAm around its LCST (32°C). b) Visible absorption spectra showing the thermoresponsive release of rhodamine B from hydrogels fabricated by freeze-casting at 10, 5, and 1°C/min. For comparison, hydrogels loaded with rhodamine B were immersed in PBS at 23°C or 37°C for 24 hours. c) Absorption of PBS solution at a wavelength of 550 nm was showing to indicate the release profiles of NC-10 hydrogel (obtained by freeze-casting at 5°C/min) at different releasing temperatures. d) The hydrogels with different pores reach their equilib rium swelling state in PBS in about 20 hours. Hydrogels fabricated by freeze-casting at 1°C/min have a higher final degree of swelling (w_wet -w_dry)/w_dry, compared with those at 5 and 10°C/min, which is consistent with their pore size as shown in .