Table 3.

Characteristics and challenges of filament extrusion

	Characteristics	Challenges
Method	The filament guided by gears is moved, then it is melted and pushed forward through the nozzle orifice[10]. HME is the main method for creating good quality filaments containing APIs[10]. Another method of filament preparation is the incorporation of model drugs by filament swelling in volatile solvent solution of API and drying[10,12,16].	For complex geometries, it requires printing support structures, which must be removed during post-processing[16]. The preparation of the filament is tedious because the quality of the final piece depends on this[10,12].
Material	The filaments are made of thermoplastic polymers, such as acrylonitrile butadiene styrene (ABS), poly lactic acid (PLA), high-impact polystyrene (HIPS), and nylon[10]. This process can produce drug delivery systems with multiple APIs[16].	Thermoplastic polymers are only used due to the heating step[12,16]. Filament extrusion process is not suitable for the thermolabile APIs[12,16].
Quality	The diameter of the nozzle orifice has an impact on the resolutionA. The filament has a big impact in the quality by its attributes, such as constant dimension, elasticity stiffness, and homogenous drug distribution[10]. As mentioned previously, the resolution depends on different factors. For example, Stratasys Company (US) has the Fortus Printer that works with a layer thickness of 178 or 254 mm, which can achieve a resolution of 250 mm[18].	The rheology of raw materials can produce inconsistent extrusion patterns[10,11].