Regenerative medicine is a branch of translational research in tissue engineering and molecular biology which deals with the “process of replacing, engineering or regenerating human cells, tissues or organs to restore or establish normal function”. 3D-printing in general is considered an advanced manufacturing technique and 3D-printing of non-viable biomaterials to serve e.g. as scaffold for cell growth or as structure for medical devices is already broadly used.

However, bio-printing technology involving living cells is still in early stages of development, but has a huge potential for tissue engineering, drug testing and other biomedical applications. Tissue-specific functional 3D bio-printing is a new approach for transplantation applications in regenerative medicine, relying on the fabrication of tissues and organs with respect to the desired shape and function and their delivery and application in vivo. “In-situ bio-printing” known as printing cells and biomaterials directly onto or in a patient, or 4D bio-printing, which introduces a “time” variable that allows 3D printed materials to change shape or function when external stimulus is applied, are recent developments facing multiple additional challenges.

Despite some success of 3D bio-printing with thin tissue, thick tissue and complex organs remain a bottleneck because it is difficult to sufficiently mimic their metabolic needs, and the scientific knowledge about their intimate architecture and interplay with other tissues are not sufficiently elucidated. Next to these limitations are a lack of standardised manufacturing protocols and standardised bio-ink formulations with tuneable properties, unstable cellular behaviour, material biocompatibility and printability, etc. Taken together, 3D bio-printing is confronted with several challenges that currently hamper its large-scale deployment.