Inkjet printing of growth factor concentration gradients and combinatorial arrays immobilized on biologically-relevant substrates.

Current methods for engineering immobilized, 'solid-phase' growth factor patterns have not addressed the need for presentation of the growth factors in a biologically-relevant context. We developed an inkjet printing methodology for creating solid-phase patterns of unmodified growth factors on native biological material substrates. We demonstrate this approach by printing gradients of fluorescently labeled bone morphogenetic protein-2 (BMP-2) and insulin-like growth factor-II (IGF-II) bio-inks on fibrin-coated surfaces. Concentration gradients were created by overprinting individual substrate locations using a dilute bio-ink to modulate the surface concentration of deposited growth factor. Persistence studies using fluorescently-labeled BMP-2 verified that the gradients retained their shape for up to 7 days. Desorption experiments performed with (125)I-BMP-2 and (125)I-IGF-II were used to quantify the surface concentration of growth factor retained on the substrate for up to 10 days in serum containing media after rinsing of the unbound growth factor. The inkjet method is programmable so the gradient shape can be easily modified as demonstrated by printed linear gradients with varying slopes and exponential gradients. In addition, the versatility of this method enabled combinatorial arrays of multiple growth factors to be created by printing overlapping patterns. The overlapping printing method was used to create a combinatorial square pattern array consisting of various surface concentrations of BMP-2 and fibroblast growth factor-2 (FGF-2). C2C12 myogenic precursor cells were seeded on the arrays and alkaline phosphatase staining was performed to determine the effect of FGF-2 and BMP-2 surface concentration on guiding C2C12 cells towards an osteogenic lineage. These results demonstrate the utility of inkjet printing for creating orthogonal growth factor gradients to investigate how combinations of immobilized growth factors influence cell fate.