3D printing of hydrogel composite systems: Recent advances in technology for tissue engineering

Three-dimensional (3D) printing of hydrogels is now an attractive area of research due to its capability tofabricate intricate, complex and highly customizable scaffold structures that can support cell adhesion and promote cellinfiltration for tissue engineering. However, pure hydrogels alone lack the necessary mechanical stability and are too easilydegraded to be used as printing ink. To overcome this problem, significant progress has been made in the 3D printing ofhydrogel composites with improved mechanical performance and biofunctionality. Herein, we provide a brief overview ofexisting hydrogel composite 3D printing techniques including laser based-3D printing, nozzle based-3D printing, and inkjetprinter based-3D printing systems. Based on the type of additives, we will discuss four main hydrogel composite systems inthis review: polymer- or hydrogel-hydrogel composites, particle-reinforced hydrogel composites, fiber-reinforced hydrogelcomposites, and anisotropic filler-reinforced hydrogel composites. Additionally, several emerging potential applicationsof hydrogel composites in the field of tissue engineering and their accompanying challenges are discussed in parallel.