Promising cellulose-based functional gels for advanced biomedical applications: A review

Novel biomedical materials provide a new horizon for the diagnosis/treatment of diseases and tissue repair in medical engineering. As the most abundant biomass polymer on earth, cellulose is characterized by natural biocompatibility, good mechanical properties, and structure-performance designability. Owing to these outstanding features, cellulose as a biomacromolecule can be designed as functional biomaterials via hydrogen bonding (H-bonding) interaction or chemical modification for human tissue repair, implantable tissue organs, and controlling drug release. Moreover, cellulose can also be used to construct medical sensors for monitoring human physiological signals. In this study, the structural characteristics, functionalization approaches, and advanced biomedical applications of cellulose are reviewed. The current status and application prospects of cellulose and its functional materials for wound dressings, drug delivery, tissue engineering, and electronic skin (e-skin) are discussed. Finally, the key technologies and methods used for designing cellulosic biomaterials and broadening their application prospects in biomedical fields are highlighted.

Improvement in the Thermomechanical Properties and Adhesion of Wood Fibers to the Polyamide 6 Matrix by Sequential Ball Milling Technique

The engineering thermoplastics industry has largely limited the use of natural fiber reinforcements due to their susceptibility to low-onset thermal degradation and water absorption. Therefore, in order to utilize these economically viable and environmentally friendly materials effectively through common composite fabrication methods such as hot pressing, safeguarding them from thermal degradation becomes essential. This work presents a viable industrially technique called sequential ball milling for processing unbleached softwood kraft pulp fibers (PF) with an engineering thermoplastics polyamide 6 (PA6) with high melting temperatures (>220 °C). An additional eco-friendly modification step that employs ball milling and cellulose nanocrystal (CNC) has been implemented in this study to enhance the mechanical properties of the composites. Special attention is given to fine-tuning key variables, such as milling duration and PF particle size, to produce optimal composites. Leveraging the ability of sequential ball milling to evenly distribute pulp fibers into PA6, a 160% increase in Young's modulus was achieved with the incorporation of 30 wt % PF. Importantly, the introduction of a 5 wt % CNC modifying agent elevated Young's modulus to 4.3 GPa, marking a 187% improvement over unmodified PA6. Diverse techniques, including rheological analyses, thermomechanical evaluations, morphological examinations, and assessments of moisture absorption, were utilized to validate the efficiency of the suggested processing approach and the modification phase.

Direct milling: Efficient, facile, and green method for processing fibrillated cellulose/polymeric nanocomposites with boosted thermomechanical and rheological performance

Bringing biobased nanomaterials into polymer manufacturing is essential to enhance polymers' properties and address the challenges posed by plastic waste. Using polymers such as polyamide 6 (PA6) in advanced industries, e.g., automotive sector, has been impeded as a direct consequence of their inability to meet the required mechanical properties. Herein, we utilize bio-based cellulose nanofibers (CNFs) to enhance the properties of PA6 by green processing, with no footprint on the environment. We address the issue of the dispersion of the nanofillers in polymeric matrices and demonstrate direct milling (cryo-milling and planetary ball milling) to facilitate a thorough integration of the components. Nanocomposites incorporating 1.0 wt% CNF, processed by pre-milling followed by compression molding, are shown to possess a storage modulus of 3.8 ± 0.2 GPa, Young's modulus of 2.9 ± 0.2 GPa, and ultimate tensile strength of 63 ± 3 MPa (all measured at room temperature). To show the superiority of direct milling in achieving these properties, other frequent approaches used to disperse CNF in polymers, such as solvent casting and hand mixing, are meticulously investigated and compared for the performance of their resulting specimens. The ball-milling method is demonstrated to provide PA6-CNF nanocomposites with excellent performance, better than solvent casting, with no associated environmental concerns.

Investigations of Thermal, Mechanical, and Gas Barrier Properties of PA11-SiO2 Nanocomposites for Flexible Riser Application

Acidic gas penetration through the internal pressure sheath of a flexible riser tends to cause a corrosive environment in the annulus, reducing the service life of the flexible riser. Nanoparticles can act as gas barriers in the polymer matrix to slow down the gas permeation. Herein, we prepared PA11/SiO2 composites by the melt blending method. The effect of adding different amounts of SiO2 to PA11 on its gas barrier properties was investigated by conducting CO2 permeation tests between 20 °C and 90 °C. As the temperature increased, the lowest value of the permeability coefficient that could be achieved for the PA11 with different contents of SiO2 increased. The composites PA/0.5% SiO2 and PA/1.5% SiO2 had the lowest permeation coefficients in the glassy state (20 °C) and rubbery state (≥50 °C). We believe that this easy-to-produce industrial PA/SiO2 composite can be used to develop high-performance flexible riser barrier layers. It is crucial for understanding riser permeation behavior and enhancing barrier qualities.

Dispersibility Characterization of Cellulose Nanocrystals in Polymeric-Based Composites

Cellulose nanocrystals (CNCs) are hydrophilic nanoparticles extracted from biomass with properties and functions different from cellulose and are being developed for property-oriented applications such as high stiffness, abundant active groups, and biocompatibility. It has broad application prospects in the field of composite materials, while the dispersibility of the CNC in polymers is the key to its application performance. Many reviews have discussed in-depth the modification strategies to improve the dispersibility of the CNC and summarized all characterization for the CNC, but there are no reviews on the in-depth exploration of dispersion characterization. This review is a comprehensive summary of the characterization of CNC dispersion in the matrix in terms of direct observation, indirect evaluation, and quantified evaluation, summarizing how and why different characterization tools reveal dispersibility. In addition, "decision tree" flowcharts are presented to provide the reader with a reference for selecting the appropriate characterization method for a specific composite.

3D Printing for Cartilage Replacement: A Preliminary Study to Explore New Polymers

The use of additive manufacturing technologies for biomedical applications must begin with the knowledge of the material to be used, by envisaging a very specific application rather than a more general aim. In this work, the preliminary study was focused on considering the cartilaginous tissue. This biological tissue exhibits different characteristics, such as thickness and mechanical properties, depending on its specific function in the body. Due to the lack of vascularization, cartilage is a supporting connective tissue with limited capacity for recovery and regeneration. For this reason, any approach, whether to repair/regenerate or as a total replacement, needs to fulfill the adequate mechanical and chemical properties of the surrounding native cartilage to be successful. This work aims to explore the possibility of using new polymers for cartilage total replacement approaches with polymeric materials processed with the specific 3D printing technique of fused filament fabrication (FFF). The materials studied were Nylon^® 12 (PA12), already described for this purpose, and LAY-FOMM^® 60 (FOMM). FOMM has not been described in the literature for biomedical purposes. Therefore, the chemical, thermal, swelling capacity, and mechanical properties of the filaments were thoroughly characterized to better understand the structure–properties–application relationships of this new polymer. In addition, as the FFF technology is temperature based, the properties were also evaluated in the printed specimens. Due to the envisaged application, the specimens were also characterized in the wet state. When comparing the obtained results with the properties of native cartilage, it was possible to conclude that: (i) PA12 exhibits low swelling capacity, while FOMM, in its dry and wet forms, has a higher swelling capacity, closer to that of native cartilage; (ii) the mechanical properties of the polymeric materials, especially PA12, are higher than those of native cartilage; and (iii) from the mechanical properties evaluated by ultra-micro hardness tests, the values for FOMM indicate that this material could be a good alternative for cartilage replacement in older patients. This preliminary study, essentially devoted to expanding the frontiers of the current state of the art of new polymeric materials, provides valuable indications for future work targeting the envisaged applications.

Comparing Percolation and Alignment of Cellulose Nanocrystals for the Reinforcement of Polyurethane Nanocomposites

The reinforcement of polymer nanocomposites can be achieved through alignment or percolation of cellulose nanocrystals (CNCs). Here, we compare the efficacy of these reinforcement mechanisms in thermoplastic polyurethane (PU) elastomer nanocomposites containing thermally stable cotton CNCs. CNC alignment was achieved by melt spinning nanocomposite fibers, while a percolating CNC network was generated by solvent casting nanocomposite films with CNC contents up to 20 wt %. While in films both the CNCs and the PU matrix were entirely isotropic at all concentrations as confirmed by wide-angle X-ray scattering and birefringence analysis, the CNCs in the fibers exhibited a preferential orientation, which improved with increasing CNC concentration. Increasing the CNC concentration in the fibers reduces, however, the alignment of the PU chains, resulting in an entirely isotropic PU matrix at high CNC contents. The mechanical properties of films and fibers were evaluated using stress-strain measurements. Nanocomposite fibers with low CNC content exhibited superior stiffness, extensibility, and strength compared to the films, while the films displayed superior mechanical properties at high CNC concentrations. These findings are rationalized using common semiempirical models describing the reinforcing effects of CNC alignment in fibers (Halpin-Tsai) and CNC percolation in films (percolation model). The formation of a percolating CNC network leads to a stronger reinforcement than CNC alignment, as the reinforcing effect of the latter is limited by the comparably low aspect ratio of CNCs extracted from cotton. As a consequence, above the percolation threshold for cotton CNCs, isotropic nanocomposite PU films show a higher stiffness than aligned nanocomposite PU fibers.

Recent Developments in Cellulose Nanomaterial Composites

Cellulose nanomaterials (CNMs) are a class of materials that have recently garnered attention in fields as varied as structural materials, biomaterials, rheology modifiers, construction, paper enhancement, and others. As the principal structural reinforcement of biomass giving wood its mechanical properties, CNM is strong and stiff, but also nontoxic, biodegradable, and sustainable with a very large (Gton yr^-1 ) source. Unfortunately, due to the relatively young nature of the field and inherent incompatibility of CNM with most man-made materials in use today, research has tended to be more basic-science oriented rather than commercially applicable, so there are few CNM-enabled products on the market today. Herein, efforts are presented for preparing and forming cellulose nanomaterial nanocomposites. The focus is on recent efforts attempting to mitigate common impediments to practical commercialization but is also placed in context with traditional efforts. The work is presented in terms of the progress made, and still to be made, on solving the most pressing challenges-getting properties that are competitive with currently used materials, removing organic solvent, solving the inherent incompatibility between CNM and polymers of interest, and incorporation into commonly used industrial processing techniques.

High Performance PA 6/Cellulose Nanocomposites in the Interest of Industrial Scale Melt Processing

On an industrial scale, it is a challenge to achieve cellulose based nanocomposites due to dispersion issues and high process temperatures sensitivity. The current study describes methods to develop mechanically strong and thermally stable polyamide 6 (PA 6) and cellulose nanofibers (CNF) composites capable of tolerating high processing temperatures. With PA 6 being a very technical polymer matrix to be reinforced with CNF, good dispersion can be achieved with a high speed kinetic mixer and also shield the CNF from excess thermal degradation by implementing extremely short processing time. This paper presents an industrially feasible method to produce PA 6/CNF nanocomposites with high CNF composition processed by a high speed kinetic mixer (GELIMAT^®) followed by compression molding to obtain a homogenous and thermally stable nanocomposites aimed at high performance applications. PA 6 was reinforced with three different wt % formulations (5, 15 and 25 wt %) of cellulose nanofibers. The resulting nanocomposites exhibited significant increase in Young’s modulus and ultimate strength with CNF content, owing to the effective melt processing and the surface charge density of the CNF, which necessitated the dispersion. The thermal stability and polymer crystallinity with respect to CNF composition for the PA 6/CNF nanocomposites were examined by TGA and DSC analysis. Rheology studies indicated that viscosity of the composites increased with increase in CNF composition. Overall, this work demonstrates industrially viable manufacturing processes to fabricate high performance PA 6/CNF nanocomposites.

Sustainable pathway towards large scale melt processing of the new generation of renewable cellulose-polyamide composites

Modern society's growing demands for accountable high-performance and more environmentally friendly materials is leading to increased interest and fast development of sustainable polymeric composite materials. New generations of "greener" products originating from renewable resources fulfil emerging requirements of low environmental and health & safety impacts and contribute to diminishing global dependence on fossil feedstock. The preparation of sustainable polymeric composites via reliable and reproducible melt-compounding methods is still challenging but has the potential to yield applicable and market competitive products. This literature survey reviews the current state of research involving the use of cellulosic materials, as bio-sourced and sustainable reinforcement in melt-processed polyamides and focuses on the main hurdles that prevent their successful large-scale melt-compounding. Particular emphasis is dedicated to emerging bio-sourced polyamides fitting the performance of engineering materials and at the same time offering additional interesting properties for advanced applications such as piezoelectricity for transducers, sensors, actuators and energy harvesters.

Towards the scalable isolation of cellulose nanocrystals from tunicates

In order for sustainable nanomaterials such as cellulose nanocrystals (CNCs) to be utilized in industrial applications, a large-scale production capacity for CNCs must exist. Currently the only CNCs available commercially in kilogram scale are obtained from wood pulp (W-CNCs). Scaling the production capacity of W-CNCs isolation has led to their use in broader applications and captured the interest of researchers, industries and governments alike. Another source of CNCs with potential for commercial scale production are tunicates, a species of marine animal. Tunicate derived CNCs (T-CNCs) are a high aspect ratio CNC, which can complement commercially available W-CNCs in the growing global CNC market. Herein we report the isolation and characterization of T-CNCs from the tunicate Styela clava, an invasive species currently causing significant harm to local aquaculture communities. The reported procedure utilizes scalable CNC processing techniques and is based on our experiences from laboratory scale T-CNC isolation and pilot scale W-CNC isolation. To our best knowledge, this study represents the largest scale where T-CNCs have been isolated from any tunicate species, under any reaction conditions. Demonstrating a significant step towards commercial scale isolation of T-CNCs, and offering a potential solution to the numerous challenges which invasive tunicates pose to global aquaculture communities.

Electrospinning of Cellulose Nanocrystal-Reinforced Polyurethane Fibrous Mats

We report the electrospinning of mechanically-tunable, cellulose nanocrystal (CNC)-reinforced polyurethanes (PUs). Using high-aspect ratio CNCs from tunicates, the stiffness and strength of electrospun PU/CNC mats are shown to generally increase. Furthermore, by tuning the electrospinning conditions, fibrous PU/CNC mats were created with either aligned or non-aligned fibers, as confirmed by scanning electron microscopy. PU/CNC mats having fibers aligned in the strain direction were stiffer and stronger compared to mats containing non-aligned fibers. Interestingly, fiber alignment was accompanied by an anisotropic orientation of the CNCs, as confirmed by wide-angle X-ray scattering, implying their alignment additionally benefits both stiffness and strength of fibrous PU/CNC nanocomposite mats. These findings suggest that CNC alignment could serve as an additional reinforcement mechanism in the design of stronger fibrous nanocomposite mats.

Melt-Spun Nanocomposite Fibers Reinforced with Aligned Tunicate Nanocrystals

The fabrication of nanocomposite films and fibers based on cellulose nanocrystals (P-tCNCs) and a thermoplastic polyurethane (PU) elastomer is reported. High-aspect-ratio P-tCNCs were isolated from tunicates using phosphoric acid hydrolysis, which is a process that affords nanocrystals displaying high thermal stability. Nanocomposites were produced by solvent casting (films) or melt-mixing in a twin-screw extruder and subsequent melt-spinning (fibers). The processing protocols were found to affect the orientation of both PU hard segments and the P-tCNCs within the PU matrix and therefore the mechanical properties. While the films were isotropic, both the polymer matrix and the P-tCNCs proved to be aligned along the fiber direction in the fibers, as shown using SAXS/WAXS, angle-dependent Raman spectroscopy, and birefringence analysis. Tensile tests reveal that fibers and films, at similar P-tCNC contents, display Young's moduli and strain-at-break that are within the same order of magnitude, but the stress-at-break was found to be ten-times higher for fibers, conferring them a superior toughness over films.

Stiffness-Changing of Polymer Nanocomposites with Cellulose Nanocrystals and Polymeric Dispersant

Bio-inspired, water-responsive, mechanically adaptive nanocomposites are reported based on cellulose nanocrystals (CNCs), poly(ethylene oxide-co-epichlorohydrin) (EO-EPI), and a small amount of poly(vinyl alcohol) (PVA), which is added to aid the dispersion of the CNCs. In the dry state, the CNCs form a reinforcing network within the polymer matrix, and the substantial stiffness increase relative to the neat polymer is thought to be the result of hydrogen-bonding interactions between the nanocrystals. Exposure to water, however, causes a large stiffness reduction, due to competitive hydrogen bonding of water molecules and the CNCs. It is shown here that the addition of PVA to the EO-EPI/CNC nanocomposite increases the modulus difference between the dry and the wet state by a factor of up to four compared to the nanocomposites without the PVA. The main reason is that the PVA leads to a substantial increase of the stiffness in the dry state; for example, the storage modulus E ' increased from 2.7 MPa (neat EO-EPI) to 50 MPa upon introduction of 10% CNCs, and to 200 MPa when additionally 5% of PVA was added. By contrast, the incorporation of PVA only led to moderate increases of the equilibrium water swelling and the E ' in the wet state.

Current characterization methods for cellulose nanomaterials

A new family of materials comprised of cellulose, cellulose nanomaterials (CNMs), having properties and functionalities distinct from molecular cellulose and wood pulp, is being developed for applications that were once thought impossible for cellulosic materials. Commercialization, paralleled by research in this field, is fueled by the unique combination of characteristics, such as high on-axis stiffness, sustainability, scalability, and mechanical reinforcement of a wide variety of materials, leading to their utility across a broad spectrum of high-performance material applications. However, with this exponential growth in interest/activity, the development of measurement protocols necessary for consistent, reliable and accurate materials characterization has been outpaced. These protocols, developed in the broader research community, are critical for the advancement in understanding, process optimization, and utilization of CNMs in materials development. This review establishes detailed best practices, methods and techniques for characterizing CNM particle morphology, surface chemistry, surface charge, purity, crystallinity, rheological properties, mechanical properties, and toxicity for two distinct forms of CNMs: cellulose nanocrystals and cellulose nanofibrils.

Rubber materials from elastomers and nanocellulose powders: filler dispersion and mechanical reinforcement

Rubber materials with well-dispersed fillers and large mechanical reinforcement have been obtained by melt-processing a diene elastomer matrix and tailored nanocellulose powders having both a high specific surface area and a modified interface. Such filler powders with a specific surface area of 180 m2 g-1 and 100 m2 g-1 have been obtained by freeze-drying suspensions of short needle-like cellulose nanocrystals (CNCs) and entangled networks of microfibrillated cellulose (MFC) in tert-butanol/water, respectively. A quantitative and toposelective filler surface esterification was performed using a gas-phase protocol either with palmitoyl chloride (PCl) to obtain a hydrophobic but non-reactive nanocellulose interface, or with 3,3'-dithiopropionic acid chloride (DTACl) to introduce reactive groups that can covalently bind the nanocellulose interface to the dienic matrix in a subsequent vulcanization process. A set of filled materials was prepared varying the filler morphology, interface and volume fraction. Transmission electron microscopy images of ultrathin cryo-sections showed that modified nanocellulose fillers presented a relatively homogeneous distribution up to a volume fraction of 20%. The materials also exhibited a significant modulus increase, while keeping an extensibility in the same range as that of the neat matrix. Strikingly, in the case of the reactive interface, a strong stress-stiffening behavior was evidenced from the upward curvature of the tensile curve, leading to a large increase of the ultimate stress (up to 7 times that of the neat matrix). Taken together, these properties, which have never been previously reported for nanocellulose-filled elastomers, match well the mechanical characteristics of industrial carbon black or silica-loaded elastomers.

Cellulose nanomaterials as green nanoreinforcements for polymer nanocomposites

Unexpected and attractive properties can be observed when decreasing the size of a material down to the nanoscale. Cellulose is no exception to the rule. In addition, the highly reactive surface of cellulose resulting from the high density of hydroxyl groups is exacerbated at this scale. Different forms of cellulose nanomaterials, resulting from a top-down deconstruction strategy (cellulose nanocrystals, cellulose nanofibrils) or bottom-up strategy (bacterial cellulose), are potentially useful for a large number of industrial applications. These include the paper and cardboard industry, use as reinforcing filler in polymer nanocomposites, the basis for low-density foams, additives in adhesives and paints, as well as a wide variety of filtration, electronic, food, hygiene, cosmetic and medical products. This paper focuses on the use of cellulose nanomaterials as a filler for the preparation of polymer nanocomposites. Impressive mechanical properties can be obtained for these materials. They obviously depend on the type of nanomaterial used, but the crucial point is the processing technique. The emphasis is on the melt processing of such nanocomposite materials, which has not yet been properly resolved and remains a challenge.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.

Simultaneously Tailoring Surface Energies and Thermal Stabilities of Cellulose Nanocrystals Using Ion Exchange: Effects on Polymer Composite Properties for Transportation, Infrastructure, and Renewable Energy Applications

Cellulose nanocrystals (CNCs) have great potential as sustainable reinforcing materials for polymers, but there are a number of obstacles to commercialization that must first be overcome. High levels of water absorption, low thermal stabilities, poor miscibility with nonpolar polymers, and irreversible aggregation of the dried CNCs are among the greatest challenges to producing cellulose nanocrystal-polymer nanocomposites. A simple, scalable technique to modify sulfated cellulose nanocrystals (Na-CNCs) has been developed to address all of these issues. By using an ion exchange process to replace Na⁺ with imidazolium or phosphonium cations, the surface energy is altered, the thermal stability is increased, and the miscibility of dried CNCs with a nonpolar polymer (epoxy and polystyrene) is enhanced. Characterization of the resulting ion exchanged CNCs (IE-CNCs) using potentiometry, inverse gas chromatography, dynamic vapor sorption, and laser scanning confocal microscopy reveals that the IE-CNCs have lower surface energies, adsorb less water, and have thermal stabilities of up to 100 °C higher than those of prepared protonated cellulose nanocrystals (H-CNCs) and 40 °C higher than that of neutralized Na-CNC. Methyl(triphenyl)phosphonium exchanged cellulose nanocrystals (MePh₃P-CNC) adsorbed 30% less water than Na-CNC, retained less water during desorption, and were used to prepare well-dispersed epoxy composites without the aid of a solvent and well-dispersed polystyrene nanocomposites using a melt blending technique at 195 °C. Predictions of dispersion quality and glass transition temperatures from molecular modeling experiments match experimental observations. These fiber-reinforced polymers can be used as lightweight composites in transportation, infrastructure, and renewable energy applications.