Advances and Trends of Photoresponsive Hydrogels for Bone Tissue Engineering

The development of static hydrogels as an optimal choice for bone tissue engineering (BTE) remains a difficult challenge primarily due to the intricate nature of bone healing processes, continuous physiological functions, and pathological changes. Hence, there is an urgent need to exploit smart hydrogels with programmable properties that can effectively enhance bone regeneration. Increasing evidence suggests that photoresponsive hydrogels are promising bioscaffolds for BTE due to their advantages such as controlled drug release, cell fate modulation, and the photothermal effect. Here, we review the current advances in photoresponsive hydrogels. The mechanism of photoresponsiveness and its advanced applications in bone repair are also elucidated. Future research would focus on the development of more efficient, safer, and smarter photoresponsive hydrogels for BTE. This review is aimed at offering comprehensive guidance on the trends of photoresponsive hydrogels and shedding light on their potential clinical application in BTE.

4D Printable Tough and Thermoresponsive Hydrogels

Hydrogels with attractive stimuli-responsive volume changing abilities are seeing emerging applications as soft actuators and robots. However, many hydrogels are intrinsically soft and fragile for tolerating mechanical damage in real world applications and could not deliver high actuation force because of the mechanical weakness of the porous polymer network. Conventional tough hydrogels, fabricated by forming double networks, dual cross-linking, and compositing, could not satisfy both high toughness and high stimuli responsiveness. Herein, we present a material design of combining responsive and tough components in a single hydrogel network, which enables the synergistic realization of high toughness and actuation performance. We showcased this material design in an exemplary tough and thermally responsive hydrogel based on PVA/(PVA-MA)-g-PNIPAM, which achieved 100 times higher toughness (∼10 MJ/m³) and 20 times higher actuation stress (∼10 kPa) compared to conventional PNIPAM hydrogels, and a contraction ratio of up to 50% simultaneously. The effects of salt concentration, polymer ratio, and structural design on the mechanical and actuation properties have been systematically investigated. Utilizing 4D printing, actuators of various geometries were fabricated, as well as lattice-architected hydrogels with macro-voids, presenting 4 times faster actuation speed compared to bulk hydrogel, in addition to the high toughness, actuation force, and contraction ratio.

Enhanced Release of Molecules upon Ultraviolet (UV) Light Irradiation from Photoresponsive Hydrogels Prepared from Bifunctional Azobenzene and Four-Arm Poly(ethylene glycol)

Advances in biosensors and drug delivery are dependent on hydrogels that respond to external stimuli. In this work, we describe the preparation and characterization of photoresponsive hydrogels prepared by cross-linking of di-NHS ester of azobenzoic acid and four-armed, amine-terminated poly(ethylene glycol). The porous structure and composition of the hydrogels were confirmed by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The reversible photoisomerization of the azobenzene-containing hydrogel cross-linkers in the gels was confirmed by absorption spectroscopy. Specifically, the photoisomerization of the cross-linkers between their trans and cis configurations was observed by monitoring the absorbance of the hydrogels at the two characteristic peaks of azobenzene (π-π* at 330 nm and n-π* at 435 nm). The effect of photoisomerization on the hydrogel structure was investigated by microscopy. Ultraviolet (UV) irradiation-induced reduction in hydrogel size was observed, which may be a result of the inherently smaller footprint of the cis azobenzene conformation, as well as dipole-dipole interactions between the polar cis azobenzene and the polymer network. The UV-triggered reduction in hydrogel size was accompanied by enhanced release of the near-infrared fluorescent dye Alexa Fluor 750 (AF₇₅₀). Enhanced release of AF₇₅₀ was observed in samples irradiated with UV versus dark control. Together, these data demonstrate the potential of these systems as reversible photoresponsive biomaterials.