The electrohydrodynamic stabilization of direct-written fluid jets is explored to design and manufacture tissue engineering scaffolds based on their desired fiber dimensions. It is demonstrated that melt electrowriting can fabricate a full spectrum of various fibers with discrete diameters (2-50 µm) using a single nozzle. This change in fiber diameter is digitally controlled by combining the mass flow rate to the nozzle with collector speed variations without changing the applied voltage. The greatest spectrum of fiber diameters was achieved by the simultaneous alteration of those parameters during printing. The highest placement accuracy could be achieved when maintaining the collector speed slightly above the critical translation speed. This permits the fabrication of medical-grade poly(ε-caprolactone) into complex multimodal and multiphasic scaffolds, using a single nozzle in a single print. This ability to control fiber diameter during printing opens new design opportunities for accurate scaffold fabrication for biomedical applications.

Additive manufacturing with electrohydrodynamic direct writing is a promising approach for the production of polymeric microscale objects. In this study we investigate the stability of one such process, melt electrospinning writing, to maintain accurate placement of the deposited fibre throughout the entire print. The influence of acceleration voltage and feeding pressure on the deposited poly(ε-caprolactone) fibre homogeneity is described, and how this affects the variable lag of the jet drawn by the collector movement. Three classes of diameter instabilities were observed that led to poor printing quality: (1) temporary pulsing, (2) continuous pulsing, and (3) regular long bead defects. No breakup of the electrified jet was observed for any of the experiments. A simple approach is presented for the melt electrospinning user to evaluate fibre writing integrity, and adjust the processing parameters accordingly to achieve reproducible and constant diameter fibres.

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Melt electrowriting (MEW) is an additive manufacturing technique that directwrites ultrafine fibers onto a surface using molten fluid columns that are stabilized with an applied voltage. [1][2][3] The process is different to polymer melt, [4] hydrogel, [5] and colloidal ink [6,7] extrusion through nozzles which all operate with direct-contact deposition for each layer. In this study, the electrified molten jet is periodically written back and forth across a wall-like structure with remarkable consistency, with minimal variation in structural dimensions. When embedded within a hydrogel, these "out-of-plane fibers" distinctly increase the shear modulus of the composite, even though they partake in a small fraction of the total composite volume.Previously MEW has been used for "inplane" printing, meaning that the fiber is aligned along a single plane for cartesian coordinates, [1] or a single curvature for rotating collectors. [8] The technique is capable of producing micrometer-scale dia meter fibers, ranging from 45 µm [1] to as small as 820 nm. [9] In addition, MEW results in a narrow fiber diameter distribution (3-5% coefficient of variation), emphasizing the reproducibility of this technique. [10] The accurate and reproducible fiber deposition is a crucial characteristic for the use of such a technology in biomedicine, filtration, and energy applications. [11][12][13][14][15] The mechanical advantage of accurate control over fiber placement was shown in a recent study, where a weak hydrogel matrix was reinforced with either small-diameter MEW (2-7 vol%) fibers or with thicker (16 vol%) fused deposition modeling (FDM) fibers. [16] The MEW-reinforced constructs were able to recapitulate the compressive properties of native articular cartilage, whereas the FDM fibers-containing structures were significantly stiffer than the native tissue and failed at comparatively low deformations (less than 10% strain). [16] The implications for tissue engineering (TE) applications is that such fiber/hydrogel composites enable the use of a mechanically weak hydrogel for cell differentiation and matrix formation, while still providing a structural support required for high compressive loading conditions. [17] Other methods to reinforce hydrogels include using random solution electrospun meshes, [18] interpenetrating polymer networks, [19] or the inclusion of carbon nanofiber tubes. [20] However, the restricted control over the fiber meshes architectures limits their reinforcing potential of soft hydrogels by One challenge in biofabrication is to fabricate a matrix that is soft enough to elicit optimal cell behavior while possessing the strength required to withstand the mechanical load that the matrix is subjected to once implanted in the body. Here, melt electrowriting (MEW) is used to direct-write poly(ε-caprolactone) fibers "outof-plane" by design. These out-of-plane fibers are specifically intended to stabilize an existing structure and subsequently improve the shear modulus of hydrogelfiber composites. The stabilizing fiber...

A new method for fabricating hydrogels with intricate control over hierarchical 3D porosity using microfiber porogens is presented. Melt electrospinning writing of poly(ε-caprolactone) is used to create the sacrificial template leading to hierarchical structuring consisting of pores inside the denser poly(2-oxazoline) hydrogel mesh. This versatile approach provides new opportunities to create well-defined multilevel control over interconnected pores with diameters in the lower micrometer range inside hydrogels with potential applications as cell scaffolds with tunable diffusion and transport of, e.g., nutrients, growth factors or therapeutics.

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