Particulate systems have dimensions that are approximately equal in all directions, so they remain predominantly isotropic in their physical behavior. Technically, the most important composites are those in which the dispersed phase is in the form of fibers. Fiber-based structures represent a wide range of morphological and geometric possibilities that can be customized for each specific tissue engineering application [69, 70]. Materials with fiber reinforcemens have long been used in the repair or replacement of tissues, ranging from their use to more complicated applications such as vascular grafts, hernia replacement tissue and musculoskeletal tissues such as bone or cartilage [9, 71].
Fibers are a continuous material structure with an extremely high aspect ratio that can be supplied directly from nature or produced from synthetic polymers [72, 73]. The fibers are fairly adequate materials as reinforcements, which can be utilized as controllable elements to make the mechanical and physical properties of biomaterials suitable for a particular application. Natural polymers as fiber reinforcements are fibrous proteins, including collagen, such as gelatin, elastin, silk fibroin, and fibrinogen, and carbohydratebased polymers, such as chitin, chitosan, and hyaluronic acid [74-76], and most of them have been successfully fabricated and evaluated for various tissue applications. As for synthetic fiber reinforcement, the polymers include but not limited to poly(a-hydroxy esters) , such as PLA, PGA, PCL, poly(hydroxyalkenoates) (PHAs), poly(propylene fumarate) (PPF) and poly(3-hydroxybutyrate-co-3 -hydroxyvalerate) (PHBV) .
In addition to conventional textile techniques, such as knitting, weaving, or braiding, various methods and strategies, such as electrospinning, wetspinning, meltspinning, interfacial complexation, direct writing and 3D printing, have been implemented to fabricate fiber reinforced materials with varying geometries, mechanical properties, porosity, anisotropy. The four main factors that govern the contribution of fibers are the basic mechanical properties, the surface interaction, the amount and the orientation of those fibers in the composite. The resulting properties of fiber-reinforced composites depend on the properties of the fiber and matrix as well as the fiber-matrix interactions . Technically, the fiber reinforced systems typically have a preferred orientation to provide optimum mechanical properties . As shown in Fig. 2.4, CaP biocomposites can be enhanced with different fiber dispositions, and the result biocomposites present different mechanical properties. For different types of matrix materials, mechanical properties are considered to be different.
This option for the reinforcement of the scaffold in tissue engineering should be determined primarily by the type of host tissue as well as its mechanical properties and anatomical structure. Suitable reinforcing materials generally have harder and stronger mechanical properties than those of the substrate, and their desired surface properties, porosity, morphology and biocompatibility are also important factors for their successful applications . In fact, the mechanical properties of the reinforced composites can be affected by a large number of material parameters, as shown in
Fig. 2.4 Fibre-reinforced CPCs: different fiber dispositions
Fig. 2.5. In addition to the complex interaction between the above factors, scaffolds in clinical applications require biodegradation over time, making the scaffold behavior time-dependent .
Fiber-based materials or structures, having been widely used as reinforcing additives in tissue engineering, are be designed to have high porosity, good mechanical properties based on the arrangement of the fibers and suitable conformability for various tissues and organs [1, 75]. There are a number of studies conducting on the mechanical properties of the polymeric matrix polymers reinforced by fibers. Caves and cooperators  combined EMC protein analogs to produce multilamellar, elastin-like protein sheets reinforced with collagen microfibers. The manufacturing process of an angle-ply composite was suitable for varying fiber orientation and
Fig. 2.5 Material parameters which influence the time dependent mechanical behavior of the FRCPC composite.  (Adapted with permission from Ref. . Copyright 2012 Elsevier Ltd)
Fig. 2.6 Fabrication of a collagen microfiber reinforced elastin-like protein sheet. (a) Collagen microfiber is wound about rectangular frames to obtain the desired orientation and average spacing. (b) A cooled protein polymer solution is distributed over the microfiber layout and molded into a thin membrane through a temperature-driven solegel process. (c) Stacked membranes are laminated by a temperature transition to yield multilamellar, angle-ply composites (d, e).  (Adapted with permission from Ref. . Copyright 2011 Elsevier Ltd)
volume fraction within and between individual sheets of a planar sheet, leading to an adjustable biomechanical property similar to that of the natural tissue (Fig. 2.6). The addition of reinforced collagen fibers extends the range of mechanical properties of elastomeric protein polymer composites to carry a wide range of suitability for the load bearing soft tissue. Significantly, these composites provided excellent mechanical support for a full thickness abdominal wall defect (Fig. 2.7). In the other study reported by Slivka et al. , the PGA fiber-reinforced PLGA scaffold had an increasing compressive modulus from 10 to 50 MPa with the amount PGA fibers from 0 to 20 wt.%. The resulted strength was approximately five fold higher than the cartilage, which might be beneficial for tissues with high loads. Guarino et al.  prepared the PLA fiber-reinforced PCL scaffolds and revealed that the composite scaffolds exhibited a higher activity of the human osteoblasts and human marrow stromal cells. The synergic addition of PLA fiber and TCP particle reinforcements in PCL scaffolds resulted in a high porosity (80%) structure, and the elastic modulus increased up to a maximum value of 2.21 ± 0.24 MPa . Cont et al.  designed a novel composite scaffolds, consisting of long continuous bidirectional fibers embedded in an electrospinning matrix. The long bidirectional continuous PDO fibers were embedded in a PLA matrix to induce anisotropic behavior and increase the mechanical strength of the scaffolds along the direction(s) of the PDO fibers. The highest ultimate tensile load applied to PLA fibers (17.9 N) was almost one
Fig. 2.7 Abdominal defect repair. Appearance of the multilamellar elastin-composite immediately following implant (a) and at 8 weeks (b). None of the repaired defects demonstrated hernia formation for the duration of the study (c), as compared to unrepaired defects (d). Scale 10 mm.  (Adapted with permission from Ref. . Copyright 2011 Elsevier Ltd)
quarter of the lowest ultimate load applied on the PLA-PDO composite (70.6 N) along the fibers.
Many research studies have used strong fibers to enhance hydrogels, which are highly desirable but difficult for medical applications. This reinforced composite will incorporate sufficiently robust properties to provide the strength and transport of the liquid phase inside the hydrogel. Illeperuma et al.  successfully fabricated a composite using an alginate-polyacrylamide hydrogel reinforced with a random network of stainless steel fibers. The tests showed that both stiffness and strength could be significantly increased by adding the fibers to the hydrogel. Yodmuang et al.  investigated a silk-based hydrogel composite made from silk microfibers and silk hydrogels, having the potential to be used as a support material for engineered cartilage. Calvert et al.  described a hydrogel scaffold reinforced with elastic fibers to achieve high strength and high fracture strain. Agrawal et al.  demonstrated a new class of hydrogel composites reinforced with elastic fibers had a cartilage-like structure by using a 3D rapid prototyping technique. In addition, Visser et al.  also reinforced soft hydrogels with highly organized, high-porosity microfiber networks to fabricate a high-porosity scaffold by combining the 3D-printing and melt-electrospinning writing technology.
The fiber reinforcement with its unique characteristics and functions has played a critical role in improving biomechanics, biocompatibility, bioactivity, integration, and degradation of artificial composite scaffolds in tissue repair and regeneration. The properties of fiber-reinforced systems are influenced by the fiber’s organization, volume fraction, mechanical properties, structure etc. Despite the current studies with significant achievements, there is still a need to develop advanced theories and satisfactory processing techniques of fiber-reinforced scaffolds to get the homogeneous structure, properties and composition similar to native ECM.