Material properties that allow for ease of dispensing (general printing logistics) should be considered, as well as properties allowing for the tissue to remain viable. That is, the material should be “printable” yet also provide structural integrity in a 3D form to maintain the physiological environment.
As such, many medical implant companies are having to tweak their formulations to allow for printing, rather than their traditional applications using molds to create implant shapes. Additionally, different printing technologies can handle different viscosities, so that plays a factor as well and should be considered in the overall design or choice of materials used. Having biocompatibility and providing tissue-specific cues is of course important, so finding ECM-like materials will give cells the most physiologically similar environment. Furthermore, the application might dictate some of these requirements; for example, if this is to be implanted in a body for regenerative therapies, the bioink should enable cell viability for some time until remodeling begins takes place and the biomaterial can be resorbed /replaced with the body’s tissue. FDA approvals will also be a consideration for these applications.
It should be noted that some bioinks must processed after printing (curing, controlling temperature, applying an additional agent) to allow for the kind of structural stability needed; some are based on photopolymers that use a UV (or other type) light to crosslink the polymers after they are printed. Special care must be taken that any post-processing does not damage any cells that are present, so a potentially easier alternative is to use biomaterial formulas that do not require this step. One bioink that I think shows promise is made by Xanofi; they are able to create nanofibers out of different starting materials (for a wide range of functional properties, depending on the application) to mimic the ECM of tissue. This has been shown to increase cell proliferation rates, while allowing the user control of the viscosity for the printing technique used (by varying the concentrations of the starting fibers and carrier gels).