| Matrix | Reinforcing agent | Compatibilization | Pros | Cons | Ref |
| PLA | Wood fiber (25 wt%) | - | Mechanical properties improved | Mechanical properties are dependent on the infill orientation of fiber; cohesion decreased, tensile strength decreased, and water absorption increased as print width increased. | [31] |
| PLA | Lignin (5 wt%) | - | Uniform dispersion of lignin | More brittle; break elongation decreased; tensile strength and Young’s modulus decreased by 18% and 6%, respectively, compared to pure PLA. | [115] |
| PLA | Lignin nanoparticles (0.5 wt%) | Ethyl acetate | Melt flow and mechanical properties improved; flexural, tensile, and impact strength increased by 130.8%, 56.1%, and 14.2%, respectively. | - | [116] |
| PLA | Carbon fiber (25 wt%) | - | 25% higher flexural strength compared to original printing process; good adhesion CF/PLA. | - | [117] |
| PLA | Carbon fiber | PVA | Tensile and bending strength increased by 35% and 108%, respectively, compared to pure PLA; good adhesion. | Delamination failure | [118] |
| PLA | Basalt fiber (20 wt%) | - | Lighter and better than conventional mold-pressed composites. | Voids (inter- and inner- filament voids) still exist; mechanical properties of composites depend on fiber length and fiber orientation | [119] |
| ABS | Short carbon fiber (5 wt%, length of 150 µm) | - | Tensile strength increased by 25% compared to pure ABS. | Porosity increased as fiber content increased | [120] |
| ABS | Carbon fiber (10 wt%) | - | Greater tensile and flexural strength compared to neat ABS; distortion and warping decreased. | Low interlaminar shear strength compared to injection parts; poor interface. | [121] [122] |
| ABS | Short carbon fiber (length of 0.2 - 0.4 mm) | - | Tensile and modulus increased by 115% and 700%, respectively, compared to neat ABS. | 20% void formation; poor adhesion. | [123] |
| ABS | Glass fiber | - | Stiffness increased by 84% compared to neat ABS; thermal stability unchanged. | - | [125] |
| ABS | Kevlar/carbon fibers | - | Rigidity and ductility increased. | - | [126] |
| ABS | Palm fiber (15 wt%) | - | Hydrogen bonding increased by 42%; Tg unchanged. | - | [127] |
| ABS | Bamboo fiber | Chemical treatment | Mechanical properties unchanged. | Density decreased. | [128] |
| ABS | Pine cone fiber (2 - 5 wt%) | Chemical treatment | Filament diameter and density unchanged. |
| [129] |
| ABS | Rice straw fiber (5 - 10 wt%) | - | - | Tensile and flexural strength decreased as rice straw fiber content increased; water absorption increased. | [130] |
| PET | Carbon fiber (15 wt%) | - | Elastic modulus, tensile, and shear strength increased by 180%, 230%, 40%, respectively, compared to neat PET. | - | [131] |
| PET-G | Carbon fiber (20 wt%) | - | Maximum 43.7% and 25% in tensile and flexural strength for honeycomb pattern; filament properties unchanged when replaced with recycled PET-G. | Viscosity increased; lower interlayer bonding; post-process treatment necessary. | [132] [133] [136] [137] |
| Recycled PET | Post-consumer textile (10 wt%) | Acid hydrolysis and silane functionali-zation | Impact resistance and the dampening characteristics improved; good adhesion; ductile failure. | High melt flow index (MFI). | [138] |
| HDPE | Birch fiber (10 - 30 wt%) | Maleic anhydride | Without significant warping, shrinkage, and other geometric deformation issues; deformation reduced up to 80%; Young’s modulus increased by 35%. | - | [140] |
| HDPE | Wood fiber (40 wt%) | MAPE | Strength of composite increased. | - | [141] |
| HDPE | Cardboard dust | - | Filaments could be printed with the cardboard content was up to 50 wt%. | High porosity; non uniformity of structure; Tg decreased; mechanical property, tenacity, and elastic modulus decreased. | [142] |
| PP | Harakeke fiber (30 wt%) | Maleated PP | Tensile and Young’s modulus increased by 74% and 214%, respectively, compared to neat PP; shrinkage decreased by 84%. | - | [143] |
| PP | Cellulose nanofibril (10 wt%) | MAPP | Flexural strength and modulus increased by 5.9% and 26.8%, respectively. | - | [144] |
| PP | Carbon fiber | MAPP | Uniform filler dispersion; good interface adhesion; mechanical properties improved. | Printing orientation affected the mechanical properties of composites. | [145] |
| PP | Glass fiber | POE-g-MA | Flexibility increased. | Modulus and strength decreased. | [146] |
| PP | Microcrystalline cellulose | n-octyltrieth- oxysilane | Good surface finish; easy printing; good mechanical properties. | - | [147] |
| Recycled PP | Rice husk fiber (5 - 10 wt%) |
| Crystallinity increased as rice husk fiber content increased; low warping. | Density decreased and water absorption increased as rice husk fiber content increased. | [148] |
| Recycled PS | Corn husk fiber (2.5 - 10 wt%) | Pre-process treatment with a layer of glue | Filament containing 2.5 - 7.5 wt% of fiber could be printable; slight decrease in thermal stability. | Filament containing 10 wt% of fiber failed to be printed; tensile strength and modulus decreased as fiber content increased; dull and rough surfaces. | [150] |
| PS | Cellulose nanocrystal | PEG/PEO | Mechanical properties improved. | - | [151] |