Melt electrowriting (MEW) is an advanced additive manufacturing technique that enables the fabrication of highly ordered polymeric scaffolds with precise micro-architectures. This method is particularly valuable in tissue engineering, where controlled cell alignment and extracellular matrix deposition are critical for functional tissue regeneration. Polycaprolactone (PCL) is a widely used biodegradable polymer in MEW due to its favorable mechanical properties, slow degradation rate, and biocompatibility. By programming specific micro-patterns such as chevrons or lattices, MEW can direct cell migration and tissue organization, making it suitable for applications like tendon repair and corneal stroma regeneration.

The MEW process involves the extrusion of molten polymer through a nozzle under an applied electric field. Unlike solution electrospinning, which relies on polymer solutions and produces randomly oriented nanofibers, MEW uses a melt-based approach to deposit microfibers with controlled alignment and spacing. The key parameters influencing fiber deposition include nozzle design, applied voltage, collector speed, and temperature. Nozzle diameter typically ranges between 100 and 500 micrometers, allowing for the precise extrusion of fibers with diameters from 5 to 50 micrometers. The electric field, usually set between 5 and 15 kV, stabilizes the polymer jet and reduces instabilities that could lead to fiber irregularities.

Nozzle design is critical in MEW to ensure consistent fiber deposition. Tapered nozzles with a small inner diameter minimize polymer clogging and improve resolution. The distance between the nozzle and the collector, typically 5 to 20 mm, must be optimized to balance fiber stretching and solidification. A shorter distance results in thicker fibers, while a longer distance allows for greater stretching and thinner fibers. The collector speed, often between 100 and 1000 mm/s, determines fiber alignment, with higher speeds promoting unidirectional orientation.

Voltage parameters play a significant role in fiber morphology. Lower voltages (5-8 kV) produce straighter fibers, while higher voltages (10-15 kV) introduce slight bending due to increased electrostatic forces. However, excessive voltage can lead to jet instability and irregular deposition. Temperature control is equally important, as PCL must remain molten but not degrade. Processing temperatures between 70 and 100°C are common, ensuring proper flow while preventing thermal degradation.

MEW's ability to create programmable micro-patterns makes it advantageous for guiding cell behavior. Chevron patterns, for instance, promote directional cell migration by providing contact guidance cues. Studies have shown that fibroblasts and tenocytes align along chevron structures, mimicking the native organization of tendon fibers. Similarly, lattice patterns with controlled pore sizes support corneal stromal cell growth and collagen deposition, essential for transparent tissue regeneration. The precision of MEW allows for scaffold designs that replicate the anisotropic properties of native tissues, enhancing mechanical strength and functional outcomes.

In tendon tissue engineering, MEW scaffolds with aligned microfibers improve tensile strength and promote tenogenic differentiation. The controlled architecture ensures that cells deposit collagen in an organized manner, closely resembling natural tendon structure. For corneal stroma applications, layered lattices with alternating fiber orientations mimic the lamellar arrangement of collagen fibrils, critical for maintaining transparency and mechanical integrity.

Contrasting MEW with solution electrospinning highlights distinct advantages and limitations. Solution electrospinning produces nanofibers with diameters below 1 micrometer, offering high surface area for cell attachment but limited control over fiber placement. The random orientation of electrospun fibers can hinder directional tissue growth, whereas MEW's precision enables tailored architectures. Additionally, solution electrospinning requires toxic solvents, complicating post-processing and raising biocompatibility concerns. MEW eliminates solvent use, simplifying fabrication and improving scaffold safety. However, MEW's resolution is lower than electrospinning, making it less suitable for applications requiring nanoscale features.

The mechanical properties of MEW scaffolds are superior to those of electrospun mats due to the larger fiber diameters and controlled alignment. Tensile testing reveals that MEW-produced PCL scaffolds exhibit higher Young's modulus and ultimate tensile strength, closely matching native tendon and corneal stroma requirements. Degradation rates can also be tuned by adjusting fiber diameter and spacing, ensuring scaffold stability during tissue maturation.

Future developments in MEW may focus on multi-material printing and dynamic patterning to further enhance tissue regeneration. Integrating conductive polymers or growth factors into MEW scaffolds could enable electrically stimulated tissue growth or localized drug delivery. Advances in nozzle design may also improve resolution, bridging the gap between melt-based and solution-based techniques.

In summary, MEW of PCL scaffolds offers unparalleled control over microfiber architecture, enabling precise guidance of cell behavior for tendon and corneal applications. Its advantages over solution electrospinning include solvent-free processing, superior mechanical properties, and programmable patterning. By optimizing nozzle design, voltage parameters, and collector dynamics, MEW can produce scaffolds that closely mimic native tissue structures, advancing the field of regenerative medicine.