The two selected PDMS sources were then formulated into printable inks by blending the selected PDMS source with a photoinitiator and a co-initiator. It is known that the choice of photoinitiating system can significantly affect the final mechanical properties of the printed structures due to increased conversion of the acrylate functional groups during printing. Thus, two types of photoinitiators, 2-isopropylthioxanthone (2-ITX) and 4,4'-diethylaminobenzophenone (DEABP), with different initiation efficiencies were selected and were mixed with the selected acrylated PDMS materials (A and B) and TOA as co-initiator (Figure ). The expectation is that the inks prepared using the more efficient photoinitiator (DEABP, inks 1-2) will generate stiffer materials compared to inks prepared using the less efficient 2-ITX (inks 3-4). ). Another important criterion for the photoinitiator/material choice is cytocompatibility, since a notable application of 3D printed soft elastomeric microstructures is to mimic the extracellular matrix of cells.