3D bioprinting of tissue units with mesenchymal stem cells, retaining their proliferative and differentiating potential, in polyphosphate-containing bio-ink.

The three-dimensional (3D)-printing processes reach increasing recognition as important fabrication techniques to meet the growing demands in tissue engineering. However, it is imperative to fabricate 3D tissue units, which contain cells that have the property to be regeneratively active. In most bio-inks, a metabolic energy-providing component is missing. Here a formulation of a bio-ink is described, which is enriched with polyphosphate (polyP), a metabolic energy providing physiological polymer. The bio-ink composed of a scaffold (-carboxymethyl chitosan), a hydrogel (alginate) and a cell adhesion matrix (gelatin) as well as polyP substantially increases the viability and the migration propensity of mesenchymal stem cells (MSC). In addition, this ink stimulates not only the growth but also the differentiation of MSC to mineral depositing osteoblasts. Furthermore, the growth/aggregate pattern of MSC changes from isolated cells to globular spheres, if embedded in the polyP bio-ink. The morphogenetic activity of the MSC exposed to polyP in the bio-ink is corroborated by qRT-PCR data, which show a strong induction of the steady-state-expression of alkaline phosphatase, connected with a distinct increase in the expression ratio between RUNX2 and Sox2. We propose that polyP should become an essential component in bio-inks for the printing of cells that retain their regenerative activity.