Nanocellulose: Source, Structure, Production, Properties, and Applications in the Biomedical and Dental Fields

Cellulose nanocrystals (CNCs)

As previously mentioned, hydrolysis of cellulose using sulfuric acid results in the separation of nanocrystals (CNCs), which are cylindrical elongated rod-shaped nanoparticles measuring 100–6,000 nm in length and 4–70 nm in width, with a crystallinity index ranging from 54% to 88%.¹³ Previously, nanocrystalline cellulose was referred to by various names, including cellulose nanowhiskers or rod-like cellulose crystals. However, in recent years, the terminology has largely converged to terms such as nanocrystalline cellulose, cellulose nanowhiskers, and cellulose nanocrystals.²⁷

Cellulose nanofibrils (CNF)

Cellulose nanofibrils (CNF), alternatively named nanofibrillar cellulose and nanofibrous cellulose, typically produced through mechanical processes, form an interconnected network of wider and longer nanofibers with lengths over 10,000 nm and widths ranging from 20 to 100 nm. Compared to CNCs, CNFs exhibit lower crystallinity, greater flexibility, biodegradability, superior mechanical strength, high surface area, and light weight. Cellulose nanofibrils (CNF) have been utilized as reinforcing agents in a variety of composites, including hybrid plastics, paper products, and thin packaging films. For example, the incorporation of CNF during the papermaking wet-end process enhances the strength of the resulting paper.^1,2,28

Nanocellulose for medical applications

Scientific research has broadened globally to explore the applications of nanocellulose, including cellulose nanocrystals (CNCs), in various medical applications such as implants, tissue regeneration, drug delivery, and antibacterial agents.^1,8,16,29 Cellulose nanocrystals (CNCs) demonstrate significant potential due to their biocompatibility, non-toxic characteristics, wound-healing capabilities, high surface area-to-volume ratio, and antimicrobial characteristics. Additionally, their negative interfacial charges and abundant hydroxyl groups enable effective electrostatic adsorption onto the tissues. The key biomedical applications of CNCs include the development of antimicrobial materials, wound dressings, bone regeneration scaffolds, and drug delivery systems.^16,30

a. Antimicrobial materials

Recently, renewable and biodegradable cellulose nanomaterials have emerged as excellent candidates for antimicrobial applications in healthcare. They are used in wound dressings because of their antimicrobial properties. Although native cellulose nanomaterials lack inherent antimicrobial properties, they can be tailored for such applications via two primary strategies. The first involves combining nanosized particles of antimicrobial agents, such as chitosan, antibiotics, silver, gold, and copper, with cellulose nanomaterials to create hybrid nanocomposites. The second strategy involves modifying the surfaces with quaternary ammonium compounds or aldehyde groups, which act as antimicrobial functional groups. Because viruses and many microbial species possess negatively charged surfaces, cellulose nanomaterials linked with quaternary groups enhance electrostatic interactions, thereby boosting antimicrobial effectiveness.²⁴

b. Regeneration of bone

Promoting the deposition of phosphate and calcium is key for effective bone regeneration. A nanocomposite containing collagen and bacterial cellulose was developed and utilized for bone regeneration applications. This combination demonstrated excellent binding properties, making it an effective medium for joining apatite with collagen for bone repair. The study found that the bacterial cellulose-collagen-apatite nanocomposite supported the regeneration of bone without exhibiting any cytotoxic, genotoxic, or mutagenic effects. Incorporating cellulose nanocrystals into collagen-based composites has improved biocompatibility and contributes to stable mechanical performance and its ability to enhance the tensile strength of scaffolds.³¹

c. Delivery of drugs

Research has highlighted the potential of cellulose nanofibril (CNF) hydrogels as drug carriers owing to their unique drug release kinetics. Cellulose nanocrystals (CNC) can deliver small interfering RNAs (siRNAs) with therapeutic potential when their surface charges are modified. Naked RNA is unable to reach cancer cells effectively and is prone to rapid degradation. However, CNC, a non-toxic and biodegradable material, can bind to RNA and act as a nanocarrier, facilitating its intracellular delivery and enhancing its therapeutic efficacy.³²

d. Tissue engineering (TE), a rapidly expanding field in biomedical science, has become a leading strategy for creating biological substitutes to regenerate, treat, or repair badly affected or lost organs and tissues. This approach integrates the principles and techniques of medicine, engineering, chemistry, and biology.^20,33 Materials based on cellulose nanocrystals (CNCs) have gained attention in TE research because of their alignment with the key concepts of Tissue Engineering, including biocompatibility, good mechanical properties, sustainability, and ability to promote cell differentiation and growth.^21,34 Manufacturing tissue engineering (TE) scaffolds have been extensively researched in the past two decades, including numerous techniques such as electrospinning, 3D printing, solvent casting, and freeze-drying.^17,29 Cellulose nanocrystals have consistently demonstrated their potential in various TE formulations, particularly for tissue repair, especially after modifying their physical and chemical properties. In recent years, numerous formulations of biological materials loaded with cellulose nanocrystals have been developed, yielding significant results in various applications.^{4,13,15,20,29,35} For example, Shaheen et al. created a scaffold composed of chitosan, alginate, hydroxyapatite, and cellulose nanocrystals (CNCs) for bone tissue engineering using a freeze-drying method.³⁶ Their findings demonstrated the enhancement of the physical and mechanical characteristics of the scaffold that included CNCs, in addition to improving cell adhesion and proliferation. It has also been found that CNC aerogels cross-linked with hydrazones are suitable as bone tissue scaffolds.³⁷ In addition, 3D and 4D printing has supported the development of custom bone and cartilage grafting.³⁸

Nanocellulose for dental applications

With increasing environmental awareness, both patients and dental practitioners have shown a preference for eco-friendly alternatives. Sustainable biomaterials address this need by providing environmentally conscious choices for dental treatments and enhancing the patient experience while supporting eco-friendly dental practices instead of conventional dental materials such as amalgam fillings and non-biodegradable polymers. Their use helps to minimize waste generation and lowers the environmental impact of dental practices. Derived from renewable resources, these materials are typically biodegradable from a health standpoint, and the biocompatibility of dental materials is crucial, as it directly impacts tissue response and long-term clinical success. Cellulose-based sustainable biomaterials are recognized for their excellent biocompatibility. They promote favorable cellular interactions and tissue integration, thereby reducing the likelihood of adverse reactions and complications. These materials have led to considerable advancements in regenerative dentistry. Thus, they offer innovative possibilities for oral tissue repair and regeneration.³⁹ Cellulose-based biomaterials, including hydrogels, composites, and nanofibers, exhibit distinct properties that can be customized for various uses in dentistry, such as periodontal and bone regeneration.⁴⁰ Cellulose derivatives such as carboxymethylcellulose (CMC), hydroxyethyl cellulose (HEC), methylcellulose (MC), and bacterial cellulose (BC) possess distinct properties that make them suitable for specific dental applications. Owing to their enhanced mechanical properties, biocompatibility, and antibacterial activity, cellulose composites and nanomaterials including cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) have become essential materials in dentistry.⁴¹ Durability is a critical factor in the dental field, and nanocellulose shows promise as a reinforcing agent. A study of restorative dental materials investigated the incorporation of cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) into a glass ionomer cement matrix to assess their effects at varying concentrations. The results showed that cellulose nanocrystals integrated seamlessly into the structure, improving the mechanical properties of the glass ionomer material and confirming its potential as an additive for tooth restoration. In contrast, the cellulose nanofibers were not effectively incorporated into the structure.⁴² A study revealed that modifying flowable dental composite resins with nanocellulose significantly enhances their compressive strength. The greatest improvement (approximately 40%) was observed at 2% concentration of nanocellulose. However, increasing the concentration beyond 2% did not result in proportional gains, indicating a saturation limit.⁴³ Another study demonstrated a significant increase in the elastic modulus and flexural strength of polymethyl methacrylate (PMMA) denture base materials reinforced with cellulose nanocrystals and produced via injection molding. (p < 0.05).⁴⁴ A study examined the effect of incorporating nanocellulose fibers into polymethyl methacrylate (PMMA) denture base materials. The results indicated that adding cellulose nanofibers at concentrations of 0.5–1% by weight significantly improved both transverse and impact strength, with the greatest enhancement achieved at 0.5% concentration. Cellulose nanofibers also enhanced the hardness and surface roughness of the resin in a concentration-dependent manner, remaining within clinically acceptable limits without impacting other material properties. At 0.5% concentration, the translucency of the acrylic material remained unaffected, whereas a 1% concentration led to a reduction in translucency. Furthermore, cellulose nanofibers at concentrations of 0.5% and 1% significantly increased the color hue (a), whereas a 0.5% concentration notably improved the color chroma (b). Scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) analyses confirmed the effective dispersion and excellent integration of cellulose nanofibers within the acrylic resin material.⁴⁵ Another study investigated the effect of adding silica-based fillers and nanocrystalline cellulose to newly developed dental adhesives. Silica particles with a micron-sized spherical shape were synthesized using the sol-gel method, while nanocrystalline cellulose, measuring 40–80 nm in diameter, was obtained via acid hydrolysis. Fillers with higher levels of nanocrystalline cellulose (NCC) exhibit a higher degradation temperature than silica-based fillers. Moreover, they demonstrated excellent biocompatibility and enhanced proliferation of fibroblasts in vitro. Increased NCC content also resulted in significant improvements in the flexural strength, shear bond strength, Vickers hardness, and compression strength.⁴⁶ Another study showed enhanced flexural and compressive strengths in nanohybrid dental composite resin reinforced with nano-silica derived from rice husks.⁴⁷ A recent study found that incorporating 1% by weight of bacterial cellulose nanocrystals significantly enhanced the micro shear bond strength of glass ionomer cement in the dentin structure of primary teeth. Although no statistically significant difference was observed in the microleakage score of the modified glass ionomer cement, its bond strength to dentin tissue showed notable improvement.⁴⁸ In another study, cellulose nanowhiskers were integrated into gelatine-based hydrogels synthesized using the thawing-freezing method. Three concentrations of cellulose nano whiskers-1%, 2%, and 5% by weight) were evaluated to assess their impact on the solubility/degradation rate, swelling properties, and capacity for anesthetic drug loading. The hydrogels were evaluated for their potential applications in lidocaine delivery for dental procedures. The kinetics and mechanisms of lidocaine release were analyzed using established mathematical models. These loaded hydrogels show promise as raw materials for developing buccal patches, offering a potential alternative to reduce or minimize reliance on injected anesthesia during dental treatments.⁴⁹ One study focused on the development and evaluation of a novel remineralizing oral film composed of a hydroxyethylcellulose (HEC) and cellulose nanofiber (CNF) blend infused with nepheline fluorapatite glass powder. The film was assessed in vitro for its thickness, folding endurance, disintegration time, surface pH, ion release, and remineralizing effect on demineralized teeth using SEM for detailed analysis. The results indicated that the films had consistent thickness, folding endurance exceeding 300 cycles, and disintegration times lasting over 24 h. The pH values were nearly neutral and significant amounts of calcium and fluoride ions were released. SEM analysis demonstrated the uniform distribution of glass powder particles within the films. The enamel microhardness (VHN) and ultra-morphology results showed a significant increase in the mean VHN after 15 and 30 days of remineralization compared to the demineralized specimens. Thus, the newly developed films demonstrated effectiveness as a potential approach for remineralizing early demineralized tooth lesions.⁵⁰