The Structure and Main Content of This Book
Above all, it has been well recognized that the use of fibers or tubes to reinforce desirable tissue engineering scaffolds is a very efficient means to further improve their mechanical properties so as to meet different needs of clinical applications. Since the fiber or tube reinforced scaffolds have been playing more and more important roles and gained substantial interest in the field of tissue engineering and regenerative medicine, we would like to launch this book. As for this kind of reinforced scaffolds, it has been well recognized that several important issues should be paid high attention to.
Firstly, the addition of the reinforcing fibers or tubes can obviously influence the structure of the scaffolds. It is impossible to enhance the porosity and mechanical properties of the scaffolds simultaneously. The addition of fibers or tubes normally decreases the porosity . However, the satisfactory mechanical properties, porosity and interconnected porous structures are all required to provide necessary support and space for the desired cell growthin and new tissue regeneration . Therefore, how to balance the two contradictory aspects needs to be concretely and correctly considered according to the practical application requirements.
Secondly, since the reinforced biomaterials are composed of two or more materials or phases that are different in composition, structure, and properties, etc., defining a continuous matrix phase and at least one reinforcing phase, the reinforcing fibers or tubes need to be homogeneously dispersed in the matrix so as to achieve satisfactory structures and properties. Therefore, how to utilize techniques and methods to achieve homogeneous composition and structure throughout the fiber or tube reinforced scaffolds as much as possible is another crucial point to consider comprehensively.
Thirdly, since the mechanical properties of the scaffolds reinforced by fibers or tubes are largely dependent on the bonding strength between the reinforcements and matrixes, it is a constant research hotspot to analyze the interactions between fibres or tubes and matrix with advanced instruments and technology, and enhance or improve further the interaction force between them by selecting or fabricating appropriate fibers or tubes and utilizing effective processing techniques. For example, Li et al.  crosslinked chitin fibers with nano-hydroxyapatite/collagen/PLLA (nHACP) by dicyclohexylcarbodimide (DCC) to enhance the bonding strength between the fibers and matrix (Fig. 1.2), thereby improving the reinforcing efficiency of the fibers. Their results showed that the chitin fibers with the crosslink treatment could enhance the compressive strength of the scaffold about three times while those without the crosslink treatment could only enhance the compressive strength of the scaffold about once. Moreover, it was shown that the crosslink was effective to improve the structural homogeneity of the scaffolds. In this case, on the other hand, the optimization of the matrix has been paid close attention to because fibers or tubes can only play its due role on the base of the matrix.
Fourthly, besides having the above physical and structure effects, the addition of the reinforcing materials actually influences the biocompatibility of the scaffolds. It has been showed that the mechanical properties of the scaffolds actually affect the cellular functions. For example, it has been shown that the substrate stiffnesss significantly promoted the osteogenic differentiation of the cultured rat bone marrow and adipose tissue derived mesenchymal stem cells (Fig. 1.3) . Moreover, it is obvious that as components in the reinforced scaffolds, the fibers or tubes themselves also have important effects on the biocompatibility and bioactivity of the scaffold. Therefore, during the design and fabrication of the scaffolds reinforced by
Fig. 1.2 Scanning electron microscopy of the fiber reinforced scaffolds without (a) and with (b) the crosslink treatment. The white arrow points to the fibers, indicating that the crosslink treatment has made the fibers bind to the matrix more tightly (Adapted with permission from Ref. . Copyright 2005 Elsevier Ltd)
Fig. 1.3 Results of the osteoblast-related gene expressions of rBMSCs and rAMSCs cultured on the different substrates for 7 days. Rat bone marrow derived mesenchymal stem cells (rBMSCs) and rat adipose tissue derived mesenchymal stem cells (rAMSCs) were divided, respectively, into six groups: the control, SOt (softest), SO (soft), M (middle), ST (stiff), and STt (stiffest) group. (a) Representative pictures of RT-PCR product bands. (b) Image analysis of (a). The expressions of osteoblast-related genes was detected in the M, ST and STt groups, especially in the STt group, which suggest the substrate stiffness was helpful for the osteogenic differentiation of rBMSCs and rAMSCs. rBMSCs expressed more osteoblast-related markers than rAMSCs cultured on the substrates with the same stiffness, which suggest that to start the osteogenic differentiation of rBMSCs needs the substrates to have higher stiffness than to start that of rAMSCs. Results are shown as the mean ± SD values (n=3). * # p < 0.05, compared with the control group; ?p < 0.05, rBMSCs group compared with rAMSCs (Adapted with permission from Ref.  Copyright 2013 Wiley Periodicals, Inc.) fibers or tubes, the efforts to maintain or even improve further the biocompatibility and bioactivity of the scaffold, meanwhile enhancing their mechanical properties, are also indispensable. And to investigate into the interactions between fibre or tube reinforced scaffolds and cell or tissues is necessary. Mryam et al. studied the biocompatibility and bioactivity of fiber reinforced hydrogel scaffolds for heart valve tissue engineering. Part of the results showed that the cells initially assumed round shape and then began to spread over time. By day 21, the cells had spread fully in the fiber reinforced scaffold The cell number on the fiber reinforced scaffold was significantly more than that on the matrix. Moreover, the cells on the reinforced scaffold also spread and distribute better than those on the matrix . The in vitro study of Li et al. showed that their prepared collagen based scaffold reinforced by chitin fiber could support better the attachment and proliferation of human bone marrow mesenchymal stem cells better than the matrix (Fig. 1.4), the main reason of which might be that the alkalescent degradation product of the chitin fibers neutralized the acid degradation product of PLLA in the matrix, thereby creating a more advantageous microenvironment for the cells to grow [23, 78, 79]. And their further in vivo study showed that the fiber reinforced scaffolds could achieve a significantly larger bone defect repair than the matrix scaffolds .
Fifthly, it is another agreed view that the behaviors of the fibers or tubes during the biodegradation of the whole reinforced scaffolds are also crucial for their final functions. To maintain or reduce the decrease rate of the mechanical properties of the fiber or tube reinforced scaffolds during the biodegradation has been another important research point in this field. For example, Li et al. studied the degradation in vitro of the porous nano-hydroxyapatite/collagen/PLLA scaffold reinforced by chitin fibers, the results of which showed that the reinforced scaffolds with the crosslink between the fibers and matrix could keep better compressive strength during degradation than those without the crosslink treatment (Fig. 1.5), the main reason of which is that the formed chemical bond by the crosslink can regenerated again after being destroyed during the degradation. Furthermore, along with the degradation, PLLA in the matrix is degraded into smaller moleculars, thereby providing more functional groups to restructure the chemical bonds with the fibers .
Finally, since requirements for soft tissue engineering scaffolds and those for hard tissue engineering scaffolds are certainly different, special design and fabrication of the novel fiber or tube reinforced scaffolds for each kind of tissue engineering have been always paid close attention to.
Therefore, this book include other 7 chapters besides the first one as follows, in which the above important issues are included:
Chapter 2: The potential matrixes and reinforcing materials for the preparation of the scaffolds reinforced by fibers or tubes for tissue repair will be introduced and discussed in this chapter. Also the general design and preparation methods of this kind of reinforced scaffolds will be included.
Chapter 3: The mechanical properties of the scaffolds reinforced by fibers or tubes for tissue repair will be discussed in this chapter, mainly to elaborate the reinforcement mechanisms of the fibers or tubes in the scaffold, present the efforts to get
Fig. 1.4 Scanning electron microscopy (upper two) and laser scanning confocal microscopy (lower two) of human bone marrow-derived mesenchymal stem cells cultured on the scaffolds, showing that the cells attached and proliferated better on the fiber reinforced scaffold than on the matrix scaffold (Adapted with permission from Ref. . Copyright 2005 Wiley Periodicals, Inc.)
the homogeneous structure and composition throughout the reinforced scaffolds, and introduce the processing techniques to improve adhesive strength between the matrix and the fibers or tubes, etc.
Chapter 4: The biodegradability of the scaffolds reinforced by fibers or tubes for tissue repair will be discussed in this chapter, mainly to expatiate how the addition of fibers or tubes affects the degradation rate, structure and mechanical property change of the scaffolds during the degradation in vitro and in vivo, and how the interactions between the matrix and the fibers or tubes affect the biodegradability of the scaffolds.
Chapter 5: The biocompatibility of the scaffolds reinforced by fibers or tubes for tissue repair will be discussed in this chapter, mainly to set forth how the addition
Fig. 1.5 Compressive strength of the scaffolds during the degradation in vitro, showing that the fiber reinforced scaffolds with crosslink treatment could keep better the mechanical property than those without crosslink treatment and matrix scaffolds (Adapted with permission from Ref. . Copyright 2005 Elsevier Ltd)
of fibers or tubes affects the interactions between the scaffolds with biomacromolecules, cells and tissues in vitro and in vivo, and how the interactions between the matrix and the fibers or tubes affect those performances of the scaffolds.
Chapter 6: The potential tissues and their properties will be introduced in this chapter, mainly to discuss about all the potential tissues and their structures and properties, which need to be repaired with the scaffolds reinforced by fibers or tubes.
Chapter 7: Scaffolds reinforced by fibers or tubes for hard tissue repair will be introduced and discussed in this chapter, mainly to state special design and fabrication of novel scaffolds reinforced by fibers or tubes for hard tissue repair, and how the addition of fibers or tubes affects the special functions in vitro and in vivo to repair hard tissues.
Chapter 8: Scaffolds reinforced by fibers or tubes for soft tissue repair will be introduced and discussed in this chapter, mainly to state special design and fabrication of novel scaffolds reinforcing by fibers or tubes for soft tissue repair, and how the addition of fibers or tubes affects the special functions in vitro and in vivo to repair soft tissues.
All in all, this book presents a review of the current understanding of the scaffolds reinforced by fibers or tubes for tissue repair, mainly including the selection of matrix and reinforcing materials, the reinforcement mechanism, properties affected by the addition of fibers or tubes, special design and fabrication of this kind of reinforced scaffolds for special tissue engineering application, etc., which provides special knowledge of materials for the persons with biomedical background, and special biomedical knowledge for the persons with the background of materials, which will hopefully become a valuable informative read for the researchers and students of biomedical engineering major. It gives a comprehensive and accurate summary of the recent related research progress besides basic knowledge and provides a valuable reference for the future development of this field. Moreover, it may provide a general guide or the design and fabrication of biomaterials and regenerative medicine.
Acknowledgements The authors acknowledge the financial supports from the National Natural Science Foundation of China (No. 31370959), Beijing Nova Programme Interdisciplinary Cooperation Project (No. xxjc201616), Fok Ying Tung Education Foundation (No. 141039), Key Laboratory of Advanced Materials of Ministry of Education of China (Tsinghua University), International Joint Research Center of Aerospace Biotechnology and Medical Engineering, Ministry of Science and Technology of China, and the 111 Project (No. B13003).