Expanding Geometries Available for Melt Electrowritten Scaffolds Using Microscale Layer Shifting

Melt electrowriting (MEW) is a high-resolution additive manufacturing technology that fabricates biomaterials or scaffolds for tissue engineering. It is based on direct writing of polymer melts, however the nozzle is raised few millimeters above the collector, while the applied electric field prevents Plateau-Raleigh instabilities and therefore sustains a thin molten jet at low flow rates. The advantage of MEW from a fabrication perspective is that the resulting fiber diameters are substantially smaller than for melt extrusion techniques. Conveniently, the fiber diameter can be altered during the print, from 2 to 50 μm; using a single nozzle.<br/>Most MEW research involves repeatedly stacking straight individual fibers directly upon each other, or using fiber suspension to induce a more porous morphology. An important approach to achieve straight fibers with MEW is the jet speed, which should be slower than the collector speed. The jet speed is measurable using the critical translation speed (CTS); the speed of the collector where the fiber transitions from non-linear to linear. When collector moves faster than the CTS, the jet is stretched and lands a slight distance behind the nozzle position, thus notably affecting the fiber placement when printing non-linear patterns. Substantial printing defects arising due to such a jet lag have been observed when printing sinusoidal patterns. In this study we report how these defects can be completely eliminated through deliberately offsetting printing trajectory for each printed layer, and how same approach can dramatically expand the geometrical complexity of MEW scaffolds.<br/>We refer to “microscale layer shifting” as deliberately offsetting the printing trajectory for each printed layer using micrometer-level adjustments. Typical inaccuracies during the printing of sinusoidal walls can be corrected via layer shifting, resulting in accurate control of their geometry. Such fiber path adjustment can be used to arbitrarily control not only the tilt of sinusoidal walls but also the mechanical properties since upon stretching each fiber experiences a different stress depending on its position within the fiber wall. Substantial layer shifting allows horizontal stacking of fiber layers, overcoming the electrostatic autofocusing effect that favors vertical layer stacking. As a result, a handful of novel nonlinear geometries were achieved, such as overhangs, wall texturing and branching, as well as smooth and abrupt changes between fiber layers, thus demonstrating the flexibility of the layer shifting approach beyond the state-of-the-art. Importantly, we detail the printing parameters controlling the fiber placement, which minimizes trial and error and enables predictable printing of fiber walls with custom-designed cross-sections. Finally, microscale layer shifting unlocks the capability to fabricate new complex designs of MEW scaffolds for tissue engineering applications, allowing to independently control the mechanical properties of the scaffold in different directions.<br/>In summary, we demonstrate how layer positioning inaccuracies that occur within MEW due to jet lag can be corrected by updating the printing path for each layer. Such microscale layer shifting can be used to layer fibers in a non-vertical manner to create overhangs without support structures, wall branching and texturing, as well as nature-inspired designs. As sinusoidal MEW printing substantially affects the mechanical properties of soft network composites, microscale layer shifting can substantially alter their mechanical properties. “Primitive” lines and boxed-shaped structures have been widely used in MEW since it was not previously known how to deliberately tilt walls to obtain more complex features. Microscale layer shifting is therefore a powerful design approach within MEW to enable the fabrication of highly resolved and tailor-designed MEW products that have utility in a spectrum of value-added applications.