Sabrina Kartmann

Additive manufacturing (AM), so‐called 3D printing, has reshaped manufacturing technologies with attractive alternatives to the traditional centralized, subtractive‐based mass production. Further functionalities like multimaterial AM (MMAM) are essential for mainstream manufacturing. With MMAM of electrical traces, the traditional printed circuit board can be transformed to freeform 3D electronics using the entire volumetric space. Although research groups attempt MMAM for freeform 3D electronics, achieving easy integration of the electrical traces with high conductance into dielectric substrates in a streamlined and simplified process remains challenging. Herein, an in‐house developed printer, named Synkròtima, is demonstrated for one‐stop printing of 3D dielectric parts with fully embedded and structured conductive features via drop‐on‐demand printing of molten metal for the first time. Printed traces show 12 times lower electrical resistance of at 230 μm linewidth than the lines from conductive inks. Printed traces with 100 μm dot spacing reveal exemplary shape fidelity with a standard deviation of only 7 μm. Quantitative estimation of adhesion of metal traces to dielectric reveals a mean shear‐off force of 7.20 N which is half of what is obtained for reflow soldered electronics. Demonstrators with embedded functional electrical circuits, directly interconnected components, and 3D out‐of‐plane metal structures are printed via Synkròtima.

Spheroids, organoids, or cell-laden droplets are often used as building blocks for bioprinting, but so far little is known about the spatio-temporal cellular interactions subsequent to printing. We used a drop-on-demand bioprinting approach to study the biological interactions of such building blocks in dimensions of micrometers. Highly-density droplets (approximately 700 cells in 10 nL) of multiple cell types were patterned in a 3D hydrogel matrix with a precision of up to 70 μm. The patterns were used to investigate interactions of endothelial cells (HUVECs) and adipose-derived mesenchymal stem cells (ASCs), which are related to vascularization. We demonstrated that a gap of 200 μm between HUVEC and ASC aggregates led to decreased sprouting of HUVECs towards ASCs and increased growth from ASCs towards HUVECs. For mixed aggregates containing both cell types, cellular interconnections of ASCs with lengths of up to approximately 800 µm and inhibition of HUVEC sprouting were observed. When ASCs were differentiated into smooth muscle cells (dASCs), separate HUVEC aggregates displayed decreased sprouting towards dASCs, whereas no cellular interconnections nor inhibition of HUVEC sprouting were detected for mixed dASCs/HUVEC aggregates. These findings demonstrate that our approach could be applied to investigate cell–cell interactions of different cell types in 3D co-cultures.