3D/4D printing of cellulose nanocrystals-based biomaterials: Additives for sustainable applications.

Cellulose nanocrystals (CNCs) have gained significant attraction from both industrial and academic sectors, thanks to their biodegradability, non-toxicity, and renewability with remarkable mechanical characteristics. Desirable mechanical characteristics of CNCs include high stiffness, high strength, excellent flexibility, and large surface-to-volume ratio. Additionally, the mechanical properties of CNCs can be tailored through chemical modifications for high-end applications including tissue engineering, actuating, and biomedical. Modern manufacturing methods including 3D/4D printing are highly advantageous for developing sophisticated and intricate geometries. This review highlights the major developments of additive manufactured CNCs, which promote sustainable solutions across a wide range of applications. Additionally, this contribution also presents current challenges and future research directions of CNC-based composites developed through 3D/4D printing techniques for myriad engineering sectors including tissue engineering, wound healing, wearable electronics, robotics, and anti-counterfeiting applications. Overall, this review will greatly help research scientists from chemistry, materials, biomedicine, and other disciplines to comprehend the underlying principles, mechanical properties, and applications of additively manufactured CNC-based structures.

Additive manufacturing of sustainable biomaterials for biomedical applications.

Biopolymers are promising environmentally benign materials applicable in multifarious applications. They are especially favorable in implantable biomedical devices thanks to their excellent unique properties, including bioactivity, renewability, bioresorbability, biocompatibility, biodegradability and hydrophilicity. Additive manufacturing (AM) is a flexible and intricate manufacturing technology, which is widely used to fabricate biopolymer-based customized products and structures for advanced healthcare systems. Three-dimensional (3D) printing of these sustainable materials is applied in functional clinical settings including wound dressing, drug delivery systems, medical implants and tissue engineering. The present review highlights recent advancements in different types of biopolymers, such as proteins and polysaccharides, which are employed to develop different biomedical products by using extrusion, vat polymerization, laser and inkjet 3D printing techniques in addition to normal bioprinting and four-dimensional (4D) bioprinting techniques. This review also incorporates the influence of nanoparticles on the biological and mechanical performances of 3D-printed tissue scaffolds. This work also addresses current challenges as well as future developments of environmentally friendly polymeric materials manufactured through the AM techniques. Ideally, there is a need for more focused research on the adequate blending of these biodegradable biopolymers for achieving useful results in targeted biomedical areas. We envision that biopolymer-based 3D-printed composites have the potential to revolutionize the biomedical sector in the near future.

Graphene nanoparticles as data generating digital materials in industry 4.0.

One of the potential applications of 2D materials is to enhance multi-functionality of structures and components used in aerospace, automotive, civil and defense industries. These multi-functional attributes include sensing, energy storage, EMI shielding and property enhancement. In this article, we have explored the potential of using graphene and its variants as data generating sensory elements in Industry 4.0. We have presented a complete roadmap to cover three emerging technologies i.e. advance materials, artificial intelligence and block-chain technology. The utility of 2D materials such as graphene nanoparticles is yet to be explored as an interface for digitalization of a modern smart factory i.e. "factory-of-the-future". In this article, we have explored how 2D material enhanced composites can act as an interface between physical and cyber spaces. An overview of employing graphene-based smart embedded sensors at various stages of composites manufacturing processes and their application in real-time structural health monitoring is presented. The technical challenges associated with interfacing graphene-based sensing networks with digital space are discussed. Additionally, an overview of the integration of associated tools such as artificial intelligence, machine learning and block-chain technology with graphene-based devices and structures is also presented.

Repair of Aerospace Composite Structures Using Liquid Thermoplastic Resin.

In this study, two types of carbon-fiber-reinforced plastic (CFRP) composite scarf geometries were created using two scarf angles, i.e., 1.43° and 5.71°. The scarf joints were adhesively bonded using a novel liquid thermoplastic resin at two different temperatures. The performance of the repaired laminates was compared with pristine samples in terms of residual flexural strength using four-point bending tests. The repair quality of the laminates was examined by optical micrographs, and the failure modes after flexural tests were analyzed using a scanning electron microscope. The thermal stability of the resin was evaluated by thermogravimetric analysis (TGA), whereas the stiffness of the pristine samples was determined using dynamic mechanical analysis (DMA). The results showed that the laminates were not fully repaired under ambient conditions, and the highest recovery strength at room temperature was only 57% of the total strength exhibited by pristine laminates. Increasing the bonding temperature to an optimal repair temperature of 210 °C resulted in a significant improvement in the recovery strength. The best results were achieved for laminates with a higher scarf angle (5.71°). The highest residual flexural strength was recorded as 97% that of the pristine sample repaired at 210 °C with a scarf angle of 5.71°. The SEM micrographs showed that all the repaired samples exhibited delamination as the dominant failure mode, whereas the pristine samples exhibited dominant fiber fracture and fiber pullout failure modes. The residual strength recovered using liquid thermoplastic resin was found to be much higher than that reported for conventional epoxy adhesives.

Experimental Characterization and Modeling Multifunctional Properties of Epoxy/Graphene Oxide Nanocomposites.

Thermomechanical modeling of epoxy/graphene oxide under quasi-static and dynamic loading requires thermo-mechanical properties such as Young's modulus, Poisson's ratio, thermal conductivity, and frequency-temperature dependent viscoelastic properties. In this study, the effects of different graphene oxide (GO) concentrations (0.05, 0.1, and 0.2 wt%) within an epoxy matrix on several mechanical and thermal properties were investigated. The distribution of GO fillers in the epoxy was investigated using transmission electron microscopy (TEM). The digital image correlation (DIC) technique was employed during the tensile testing to determine Young's modulus and Poisson's ratio. Analytical models were used to predict Young's modulus and thermal conductivity, with an error of less than 13% and 9%, respectively. Frequency-temperature dependent phenomenological models were proposed to predict the storage moduli and loss tangent, with a reasonable agreement with experimental data. A relatively high storage modulus, heat-resistance index (THRI), and thermal conductivity were observed in 0.2 wt% nanocomposite samples compared with pure epoxy and other lower concentration GO nanocomposites. A high THRI and derivative of thermogravimetric analysis peak temperatures (Tm1 and Tm2) were exhibited by adding nano-fillers in the epoxy, which confirms higher thermal stability of nanocomposites than that of pristine epoxy.

Potential applications of polycarbohydrates, lignin, proteins, polyacids, and other renewable materials for the formulation of green elastomers.

Renewable resources including polycarbohydrates, lignin, proteins, and polyacids are the intrinsically valuable class of materials that are naturally available in great quantities. Their utilization as green additives and reinforcing bio-fillers, in substitution of environmentally perilous petroleum-based fillers, for developing high-performance green rubber blends and composites is presently a highly tempting option. Blending of these renewable materials with elastomers is not straight-forward and research needs to exploit the high functionality of carbohydrates and other natural materials as proper physicochemical interactions are essential. Correlating and understanding the structural properties of lignin, carbohydrates, polyacids, and other biopolymers, before their incorporation in elastomers, is a potential approach towards the development of green elastomers for value-added applications. Promising properties i.e., biodegradability, biocompatibility, morphological characteristics, high mechanical properties, thermal stability, sustainability, and various other characteristics along with recent advancements in the development of green elastomers are reviewed in this paper. Structures, viability, interactions, properties, and use of most common natural polycarbohydrates (chitosan and starch), lignin, and proteins (collagen and gelatin) for elastomer modification are extensively reviewed. Challenges in commercialization, applications, and future perspectives of green elastomers are also discussed. Sustainability analysis of green elastomers is accomplished to elaborate their cost-effectiveness and environmental friendliness.

Temperature-Frequency-Dependent Viscoelastic Properties of Neat Epoxy and Fiber Reinforced Polymer Composites: Experimental Characterization and Theoretical Predictions.

In general, aerospace structures manufactured using fiber reinforced polymer composites are exposed to fluctuating temperatures and subjected to cyclic loading during their service life. Therefore, studying the temperature-frequency dependent properties of composites for different fiber orientations is essential. However, such experiments are expensive, time-consuming and labor-intensive while theoretical models minimize these issues, but temperature-frequency-dependent viscoelastic models for predicting the full-range of the storage and loss moduli curves of composites are limited. In this study, the dynamic mechanical properties of a neat epoxy resin, unidirectional ([0°]6, [45°]6 and [90°]6), symmetric angle-ply [+45°/-45°/+45°]s and quasi-isotropic [±45°/0°/90°]s carbon/epoxy and glass/epoxy composite panels were investigated. Experiments were performed from room temperature (approximately 35 °C) to 160 °C at five different frequencies (1, 10, 20, 33 and 50 Hz). Two parameter viscoelastic models as function of temperature and frequency were used, and their applicability in predicting the storage and loss moduli for the entire region of the temperature curve is shown. The storage modulus values were compared and validated against the static flexural modulus values coupled with scanning electron microscopy analysis. The flexural and storage moduli values were found to be higher for [0°]6 carbon/epoxy composites, while the activation energy values were found to be higher in the case of [+45°/-45°/+45°]s carbon/epoxy composites compared with epoxy resin and other laminates in different orientations. The predicted results were in reasonably good agreement with the experiments. Both experimental and modeling approaches used in this study are highly valuable for designing aerospace composites for harsh in-service loading conditions.

Investigation of Impact Strength and Hardness of UHMW Polyethylene Composites Reinforced with Nano-Hydroxyapatite Particles Fabricated by Friction Stir Processing.

The impact strength and surface properties of polymeric materials are of critical importance in various engineering applications. Friction stir processing (FSP) is a novel method for the fabrication of composite materials with superior mechanical properties. The main objective of this study is to investigate the impact strength and Rockwell hardness of UHMW polyethylene composites reinforced with nano-hydroxyapatite particles fabricated through FSP. The spindle speed (ω), tool traverse speed (f), volume fraction (v) of strengthening material and shoulder temperature (T) were key processing parameters. The analysis of variance (ANOVA) indicated that the selected processing parameters were significant. Microscopic investigations unveiled that high levels of (v, f) and low levels of (T, ω) caused agglomeration of the reinforcing particles and induced voids and channels, which consequently reduced the impact strength and hardness of the manufactured composite. However, medium conditions of processing parameters exhibited better distribution of particles with minimum defects, and hence resulted in better mechanical properties. Finally, the models to predict the impact strength and hardness are proposed and verified. Sets of process parameters favorable to maximize the impact strength and Rockwell hardness were worked out, which were believed to increase the impact strength, Rockwell hardness number, and ultimate tensile strength by 27.3%, 5.7%, and 11.2%, respectively.