Three-dimensional (3D) printing incorporated with controlled delivery is an effective tool for complex tissue regeneration. Here, we explored a new strategy for spatiotemporal delivery of bioactive cues by establishing a precise-controlled micro-thin coating of hydrogel carriers on 3D-printed scaffolds. We optimized the printing parameters for three hydrogel carriers, fibrin cross-linked with genipin, methacrylate hyaluronic acid, and multidomain peptides, resulting in homogenous micro-coating on desired locations in 3D printed polycaprolactone microfibers at each layer. Using the optimized multi-head printing technique, we successfully established spatial-controlled micro-thin coating of hydrogel layers containing profibrogenic small molecules (SMs), Oxotremorine M and PPBP maleate, and a chondrogenic cue, Kartogenin. The delivered SMs showed sustained releases up to 28 d and guided regional differentiation of mesenchymal stem cells, thus leading to fibrous and cartilaginous tissue matrix formation at designated scaffold regions in vitro and in vivo. Our micro-coating of hydrogel carriers may serve as an efficient approach to achieve spatiotemporal delivery of various bioactive cues through 3D printed scaffolds for engineering complex tissues.

Microwave irradiation, being an electromagnetic wave, facilitates volumetric heating through various dielectric heating modes such as dipolar polarization and ionic conduction. In this review, an attempt has been made to critically discuss various principles associated with microwave–material interactions. The review has given particular importance to recent developments in microwave-assisted material synthesis and processing of various materials such as metals, ceramics, polymers, nanoparticles, and food materials. The microwave method has various advantages over conventional heating methods in terms of reaction kinetics, product uniformity, and energy efficiency. The review also critically discusses some of the challenges faced by microwave–material interactions and how they can be addressed by adopting new strategies, such as hybrid heating and reactor innovations. In addition, future research directions have also been outlined to take microwave technologies to new heights in material processing.