Have you ever day-dreamed of the science-fiction idea of treating a patient’s injuries in a floating rejuvenation tank? The phenomenon that allows for this suspension, neutral buoyancy, seems to have inspired researchers to discover a solution for 3D printing materials with soft and liquid-like inks that would have great difficulty maintaining their print fidelity in air, with gravity forces collapsing it. Despite the fact that water-based support fluids (as the ones used in the floatation tanks) would never work with the water-based hydrogels widely used in tissue engineering applications, this idea of 3D printing in a support bath evolved to minimize the effects of gravity by using gel-like materials that could reversibly show solid and liquid-like behaviors, allowing for hydrogel deposition within it.

This month of November, we explored Freeform Reversible Embedding of Suspended Hydrogels (FRESH) bioprinting, an embedded bioprinting approach specifically optimized to support the printing of soft and low viscosity liquid-like bioinks, which are widely used for tissue engineering applications. Generally speaking, one can think about embedded 3D bioprinting as a build chamber filled with a support material within which biomaterials and cells are deposited in the 3D space using a syringe-based extruder. The FRESH method then allows the printing of unmodified biological hydrogels such as collagen type I and decellularized ECM with high density cell-laden bioinks for fabrication of functional engineered tissues.

The key engineering challenge in this approach is to develop a support bath material able to hold the soft printed structures as they are extruded and cured while still allowing for the easy movement of the extruder needle through the support bath during printing. Specifically, the rheological requirement for the support bath material is that it must act as a solid below a threshold of applied shear stress, i.e. the yield stress, at which it transitions from solid to liquid-like behavior. Around the needle, the liquid-like behavior of the support bath (applied shear stress higher than the yield stress) allows the easy movement of the needle. Once the needle departs, the support bath resolidifies (applied shear stress lower than the yield stress). The optimal yield stress for the support bath is dependent on the materials being printed, the needle diameter and length, and process parameters such as the print speed [1].

The FRESH printing method presents several outstanding advantages. First, its versatility. Although FRESH was originally described using a gelatin-based support bath [2], the development of different baths has enabled the printing of soft materials with the broadest range of gelation mechanisms (Figure 1). Moreover, since the cross-linking agents are mixed in the bath, the extruded bioink filament is immediately exposed to the liquid on all sides, which rapidly initiates a concurrent gelation process. This is in contrast to printing in air, where there are limited ways to initiate gelation. Second, the support bath provides an environment that prevents cell death and maintains high cell viability during the printing process. In addition to temperature control, the aqueous part of the support bath can be composed of cell culture media, growth factors or any other biomolecule intended to protect cell survival. As a consequence, the third main advantage of the technique is the size of the constructs that could be printed. To date, most 3D bioprinted tissue constructs have been relatively small when compared to the tissues or organs they are intended to replace. FRESH has already been used to create a neonatal-scale physical model of the human heart out of collagen type I based on MRI imaging data [3]. And researchers affirm that this technique has the potential to be a scalable additive manufacturing technology that can produce full-scale tissues based on patient-specific anatomic data.