The potential of electrospinning process to fabricate ultrafine fibers as building blocks for tissue engineering scaffolds is well recognized. The scaffold construct produced by electrospinning process depends on the quality of the fibers. In electrospinning, material selection and parameter setting are among many factors that contribute to the quality of the ultrafine fibers, which eventually determine the performance of the tissue engineering scaffolds. The major challenge of conventional electrospun scaffolds is the nature of electrospinning process which can only produce two-dimensional electrospun mats, hence limiting their applications. Researchers have started to focus on overcoming this limitation by combining electrospinning with other techniques to fabricate three-dimensional scaffold constructs. This article reviews various polymeric materials and their composites/blends that have been successfully electrospun for tissue engineering scaffolds, their mechanical properties, and the various parameters settings that influence the fiber morphology. This review also highlights the secondary processes to electrospinning that have been used to develop three-dimensional tissue engineering scaffolds as well as the steps undertaken to overcome electrospinning limitations.

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The current developments in three-dimensional printing also referred as “additive manufacturing” have transformed the scenarios for modern manufacturing and engineering design processes which show greatest advantages for the fabrication of complex structures such as scaffold for tissue engineering. This review aims to introduce additive manufacturing techniques in tissue engineering, types of biomaterials used in scaffold fabrication, as well as in vitro and in vivo evaluations. Biomaterials and fabrication methods could critically affect the outcomes of scaffold mechanical properties, design architectures, and cell proliferations. In addition, an ideal scaffold aids the efficiency of cell proliferation and allows the movements of cell nutrient inside the human body with their specific material properties. This article provides comprehensive review that covers broad range of all the biomaterial types using various additive manufacturing technologies. The data were extracted from 2008 to 2018 mostly from Google Scholar, ScienceDirect, and Scopus using keywords such as “Additive Manufacturing,” “3D Printing,” “Tissue Engineering,” “Biomaterial” and “Scaffold.” A 10 years research in this area was found to be mostly focused toward obtaining an ideal scaffold by investigating the fabrication strategies, biomaterials compatibility, scaffold design effectiveness through computer-aided design modeling, and optimum printing machine parameters identification. As a conclusion, this ideal scaffold fabrication can be obtained with the combination of different materials that could enhance the material properties which performed well in optimum additive manufacturing condition. Yet, there are still many challenges from the printing methods, bioprinting and cell culturing that needs to be discovered and investigated in the future.

Fabrication of three dimensional (3D) tissue engineering scaffolds, particularly for hard tissues remains a challenge. Electrospinning has been used to fabricate scaffolds made from polymeric materials which are suitable for hard tissues. The electrospun scaffolds also have structural arrangement that mimics the natural extracellular matrix. However, electrospinning has a limitation in terms of scaffold layer thickness that it can fabricate. Combining electrospinning with other processes is the way forward, and in this proposed technique, the basic shape of the scaffold is obtained by a fused deposition modelling (FDM) three dimensional (3D) printing machine using the partially hydrolysed polyvinyl alcohol (PVA) as the filament material. The 3D printed PVA becomes a template to be placed inside a mould which is then filled with the fully hydrolysed PVA/maghemite (γ-Fe 2 O 3 ) solution. After the content in the mould solidified, the mould is opened and the content is freeze dried and immersed in water to dissolve the template. The 3D structure made of PVA/maghemite is then layered by electrospun PVA/maghemite fibers, resulting in 3D tissue engineering scaffold made from PVA/maghemite. The morphology and mechanical properties (strength and stiffness) were analysed and in vitro tests by degradation test and cell penetration were also performed. It was revealed that internally, the 3D scaffold has milli-and microporous structures whilst externally; it has a nanoporous structure as a result of the electrospun layer. The 3D scaffold has a compressive strength of 78.7 ± 0.6 MPa and a Young's modulus of 1.43 ± 0.82 GPa, which are within the expected range for hard tissue engineering scaffolds. Initial biocompatibility tests on cell penetration revealed that the scaffold can support growth of human fibroblast cells. Overall, the proposed processing technique which combines 3D printing process, thermal inversion phase separation (TIPS) method and electrospinning process has the potential for producing hard tissue engineering 3D scaffolds.

Nanocrystalline cellulose is an abundant and inexhaustible organic material on Earth. It can be derived from many lignocellulosic plants and also from agricultural residues. They endowed exceptional physicochemical properties, which have promoted their intensive exploration in biomedical application, especially for tissue engineering scaffolds. Nanocrystalline cellulose has been acknowledged due to its low toxicity and low ecotoxicological risks towards living cells. To explore this field, this review provides an overview of nanocrystalline cellulose in designing materials of bone scaffolds. An introduction to nanocrystalline cellulose and its isolation method of acid hydrolysis are discussed following by the application of nanocrystalline cellulose in bone tissue engineering scaffolds. This review also provides comprehensive knowledge and highlights the contribution of nanocrystalline cellulose in terms of mechanical properties, biocompatibility and biodegradability of bone tissue engineering scaffolds. Lastly, the challenges for future scaffold development using nanocrystalline cellulose are also included.

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An electronic nose (E-nose), comprising eight metal oxide semiconductor (MOS) gas sensors, was used in situ for real-time classification of black tea according to its quality level. Principal component analysis (PCA) coupled with signal preprocessing techniques (i.e., time set value preprocessing, F1; area under curve preprocessing, F2; and maximum value preprocessing, F3), allowed grouping the samples from seven brands according to the quality level. The E-nose performance was further checked using multivariate supervised statistical methods, namely, the linear and quadratic discriminant analysis, support vector machine together with linear or radial kernels (SVM-linear and SVM-radial, respectively). For this purpose, the experimental dataset was split into two subsets, one used for model training and internal validation using a repeated K-fold cross-validation procedure (containing the samples collected during the first three days of tea production); and the other, for external validation purpose (i.e., test dataset, containing the samples collected during the 4th and 5th production days). The results pointed out that the E-nose-SVM-linear model together with the F3 signal preprocessing method was the most accurate, allowing 100% of correct predictive classifications (external-validation data subset) of the samples according to their quality levels. So, the E-nose-chemometric approach could be foreseen has a practical and feasible classification tool for assessing the black tea quality level, even when applied in-situ, at the harsh industrial environment, requiring a minimum and simple sample preparation. The proposed approach is a cost-effective and fast, green procedure that could be implemented in the near future by the tea industry. Author Contributions: Conceptualization, K.T. and A.M.P.; Data curation, I.F., D.L., and N.N.; Formal analysis, S.N.H., K.T., A.C.A.V. and A.M.P.; Funding acquisition, K.T.; Investigation, I.F.; Methodology, K.T., T.J., A.C.A.V.; Project administration, T.J.; Resources, Y.Y. and N.N.; Software, S.N.H., A.C.A.V. and A.M.P.; Supervision, K.T. and N.N.; Validation, K.T., T.J., Y.Y.; Visualization, D.L. and A.M.P.; Writing-original draft, S.N.H., D.L.; Writing-review & editing, K.T., A.C.A.V., A.M.P.

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