# Cellulose Nanofibers and Other Biopolymers for Biomedical Applications. A Review
## 4 Outline of Biomedical Applications  
### Tissue Engineering  
Through tissue engineering, synthetic substitutes can be created which act to replace or repair damaged organs and tissues. The biocompatible and biodegradable nature of biopolymers makes them ideal materials for development in this field. Many studies have been undertaken in developing biopolymer composites for tissue scaffolds [115]. 3D Scaffolds are used in tissue engineering as an artificial ECM for cell seeding, proliferation, and new tissue formation. One of the advantages of biopolymers (nanocellulose, collagen, xylans, etc.) for scaffolds is the biological recognition that may positively support cell adhesion and function. The scaffold should biodegrade naturally in the body at the same rate as new tissue grows, and subsequently, be safely eliminated [62]. The chemical structure of nanocellulose allows it to easily form a hydrogen bond network which has high mechanical strength, good hydrophilicity and excellent biocompatibility. These properties make nanocellulose a perfect candidate for being used in tissue engineering applications. The nanocellulose scaffolds present all the requirements of tissue engineering technology, such as superior water absorption, water retention, biocompatibility and mechanical properties. The scaffolds for tissue engineering can be prepared in several ways, such as electrospinning, freeze-drying, 3D printing, solvent casting, etc. Nanocellulose has been used as a major component or as reinforcement in many formulations for  
Figure 11: The proposed structure of lignin. Reproduced with permission of [PERSON] et al. [114]. tissue engineering that covers its application in the repair of skin, vascular, neural, muscular, hepatic and ophthalmological tissue [116]. For skin tissue engineering CNC and CNF have been combined with several polymers to produce structures to obtain features similar to native skin improving the biocompatibility, mechanical, thermal and swelling properties. CNC was incorporated to polylactide-polyglycolide (PLGA) to produce PLGA/CNC membranes by electrospinning improving the cell compatibility, adhesion, spreading and proliferation of fibroblast in comparison with neat PLGA to skin tissue engineering [117]. PVA, GelMa, collagen, gelatin and chitosan has been used in combination with nanocellulose to produce structures to create the optimum microenvironment for cells to adhere, grow and metabolise in a similar way that natural skin does [118; 119]. CNC and CNF improve the fiber morphology, tensile strength, elastic modulus, indentation modulus and biocompatibility of the scaffolds created for vascular tissue engineering. The endothelialisation is a key parameter in the production of artificial vessels in this application. CNC and CNF have been combined with poly(lactic acid) (PLA) or poly(butylene succinate) (PBS) to create scaffolds with multi-scaled structure with interconnected porous morphology and high surface area that are essential to cell growth and nutrient exchanges [120]. Nanocellulose can act as control elements for the porosity, mechanical strength, orientation and flexibility of scaffolds for neural tissue engineering. Neural tissue presents a limited capacity to be self-repaired and regenerated after some damage. Nanocellulose scaffolds alone or in combination with carbon nanotubes guide and enhance the development of neural cells, maintaining the axon channel and promoting the neural stimulation and activity [121]. For skeletal muscle, CNC was used to produce tissue engineering scaffolds with the ability to contract and posses' similar functionalities to native muscle. CNC was used to produce spin-coated structures with oriented direction. The oriented surface of CNC promotes the fusion degree of myoblasts being capable of ameliorating the myotube orientation [122]. Scaffolds for cardiac tissue engineering should have stability, stretchability, flexibility, along with cytocompatibility.