Melt electro writing (MEW) has emerged as a precision additive manufacturing technique capable of producing microscale fibers for tissue engineering scaffolds with exceptional control over geometry and architecture. By combining principles of electrohydrodynamic fiber attraction and melt extrusion, MEW enables the direct writing of thermoplastic polymer filaments in diameters ranging from 800 nanometers to 40 micrometers. This capability allows replication of extracellular matrix (ECM) fiber dimensions, such as collagen fibrils or elastin fibers, critical for guiding cell ingrowth, alignment, and differentiation.
The process begins with molten polymer extruded through a nozzle under pneumatic or volumetric feed. A high-voltage electric field between nozzle and collector forms a Taylor cone at the nozzle tip, drawing a stable polymer jet toward the collector. Control over parameters such as melt temperature, feed pressure, collector speed, and nozzle-to-collector distance determines fiber diameter, deposition accuracy, and layer stacking. Hochleitner et al. demonstrated submicrometer filaments using a 0.108 mm nozzle, while Hrynevich et al. provided predictive formulas linking fiber diameter to changes in pressure or collector speed.
Precise deposition is challenged by phenomena like “fiber pulsing,” caused by mismatches between mass flow and electrostatic draw, and “jet lag,” the offset between fiber release and deposition at speeds above the critical translation speed (CTS). Adjusting toolpath speed profiles, as Castilho et al. did for hexagonal pore scaffolds, can minimize defects such as coiling at angled segments. Conversely, operating below CTS can deliberately induce sinusoidal fiber patterns to mimic tendon biomechanics.
Layer-by-layer fabrication demands maintaining consistent electrostatic field strength as scaffold height increases. Voltage and collector spacing adjustments counteract changes in fiber flight path and charge accumulation on deposited layers, enabling structures up to 9 mm high with hundreds of layers. Scaffold architecture—fiber diameter, density, and alignment—directly influences cell behavior. Jenkins and Little observed that smaller fibers promote orthogonal cell alignment, while larger fibers encourage elongation along their length. Denser fiber networks can enhance proliferation but reduce migration speed.
MEW’s geometric versatility includes microscale layer shifting, enabling overhangs, branching walls, and tilted structures without supports, and out-of-plane fiber deposition to tailor mechanical properties like shear modulus in hydrogel composites. Electrostatic interactions between deposited fibers impose limits on minimum inter-fiber spacing, with attraction or repulsion effects varying by substrate conductivity and fiber diameter.
Integration with other fabrication methods expands MEW’s utility. Embedding MEW frames in hydrogels can increase elastic modulus by orders of magnitude, as Bas et al. showed, while maintaining cell motility through reduced crosslinking density. Combining MEW with fused deposition modeling (FDM) yields multi-scale scaffolds: MEW’s fine fibers support cell adhesion, and FDM’s thicker struts provide structural strength. MEW can also complement solution electrospinning (SES) to produce bilayered scaffolds with controlled fiber orientation and varied diameters, supporting distinct cell types in vascular grafts.
Adaptations to collectors and nozzles further broaden MEW’s scope. Printing on non-planar or tubular collectors enables anatomically relevant shapes, such as patient-specific aortic root scaffolds fabricated by Saidy et al. Rotating mandrels demand dynamic speed adjustments to maintain deposition accuracy. Nozzle innovations include coaxial designs for hollow fibers, as Eberle et al. achieved, and exit-channel shape modifications to reduce fiber diameter.
Material selection remains critical. Polycaprolactone (PCL) dominates due to its biocompatibility and low melting point, but blends like poly(hydroxymethylglycolide-co-ε-caprolactone) offer tunable degradation and biofunctionalisation potential. Elastic polymers such as poly(urea-siloxane) and poly(L-lactide-co-ε-caprolactone) extend MEW’s reach to mechanically dynamic tissues, while hydrogels like poly(2-ethyl-2-oxazine) enable chemically crosslinked fiber networks with high water content.
Through meticulous control of multi-parametric printing conditions, MEW delivers scaffold architectures with precise microstructural features tailored to specific tissue engineering applications. By leveraging its compatibility with complementary manufacturing methods, adaptable hardware configurations, and evolving material portfolio, MEW stands as a transformative platform for producing high-resolution, application-specific scaffolds.