Figure 1.

Fabrication of the CES and COM grafts. (a) In the power-down technique, unidirectional crystal growth in a chitosan solution is achieved by freezing at a defined cooling rate with a maintained constant local temperature gradient. The temperature is controlled by two heating plates, which are in contact with liquid nitrogen (LN₂). The crystals exclude chitosan and, after freeze drying, leave behind longitudinal pores with intact chitosan walls. (b) Experimental chitosan cores were produced in a three-part copper mold with Teflon insulation. The mold produces 36 chitosan cores at the same time. For open, longitudinal pores, upper and lower parts were sheared off after freezing. (c) Principle of electrospinning: a polymer solution is slowly pumped through a nozzle and gets charged into the high voltage electrical field. Due to the positive charge accumulated in the polymer drop, a Taylor cone forms with an emerging polymer jet at the tip. With continuous evaporation of the solvent, the jet solidifies into a polymer fiber that is randomly deposited on a grounded collector. This collector can have any shape, determining the shape of the product. (d) Rotator for assembly of the composite graft (COM) sheath. By grounding the shaft, electrospun fibers are deposited on chitosan cores. A PCL polymer solution was spun at 24–28 kV with a flow rate of 0.5 mL/h at a distance of 21 cm to the collector. After initial spinning for 30 s on two opposite sides, the carrier was removed and the chitosan core was spun in a rotational manner for 4 min.