Fiber or Tube Reinforced Composites

Fibers or tubes are another kind of reinforcing materials, which have long been used in the repair or replacement of tissues ranging from their use as sutures to more complicated applications. Although particle reinforced composites with lower cost have been widely used in many industrial fields, in recent years there have been great achievements in fiber or tube reinforced composites, especially for being used as tissue engineering scaffolds [46, 47]. Furthermore, the addition of fibers or tubes is also useful for maintaining the original dimensions of the scaffolds. Gentleman et al. [48, 49] have characterized the mechanical performances of collagen fibers created from type I collagen. It seems that the addition of the fibers in a collagen gel significantly improved maintenance of the gel dimensions [49, 50]. On the other hand, technologically, the most important composites are exactly those, in which the dispersed phase is in the form of a fiber or tube. Fiber or tube reinforcement, as a controllable variable, can be manipulated when engineering the mechanical and physical properties of biomaterials to suit the applications of repairing specific tissues because the requirements for the mechanical properties of hard and soft tissue engineering scaffolds are really different. The reinforcing fibers or tubes can take on various configurations, such as woven, knitted, braided, continuous randomly aligned, staple randomly aligned, and continuous uniaxially aligned ones, etc., which have significant effects on the morphology, spatial orientation and mechanical properties of the scaffolds. Therefore, the structures of fibers or tubes reinforced composites represent a wide range of morphological and geometric possibilities that can be tailored for each specific tissue engineering applications. Technically, the continuous fiber systems normally have a preferred orientation, which can provide the ideal reinforcing effects [51]. A distinctive characteristic features of fibers or tubes reinforced composites is that they can be designed unrestrictedly by the selection of various reinforcements, and thus their mechanical, physical or biochemical properties can be tailored in a large range to satisfy the demands of the different host tissues. In particular, the mechanical properties of the scaffolds reinforced by fibers or tubes will largely depend on many factors, such as fiber or tube length, volume fraction and aspect ratio of the fibers or tubes, the organization of fibers or tubes, fiber-fiber or tube-tube contacts, impregnation of the fibers or tubes with the matrix, the interactions between the matrix and the fibers or tubes, the polymerization of the shrinkage of the matrix, and the individual properties of the fibers or tubes and the matrix, etc. [20, 52]. To date, it has been shown that many kinds of fibers and tubes can effectively reinforce scaffolds. These reinforced scaffolds have been largely applied to repair hard and soft tissues. They include ceramic fibers/tubes, bioglass fibers/tubes, carbon fibers/tubes, polymer fibers/tubes and so on [53] the diameter of which is normally smaller than 10 pm.

The main reinforcement mechanism in the fiber or tube reinforced composites is that the dispersed fibers or tubes can hinder or impede the motion of cracks in the matrix, which are caused by the applied load. In one situation, with the increase of the applied load, the restrain of the crack motion lead to the extraction of the fibers or tubes from the matrixes. In the other situation, with the increase of the applied load, the restrain of the crack motion lead to the deflection of the crack, which will be deflected again by another fiber or tube. In both cases, it is obvious that the fibers or tubes can significantly decrease the imposed destructive energy on the matrixes, thereby increasing their mechanical properties. For example, continuous fiber reinforced polymer scaffolds can offer significantly improvement in stiffness and strength over the parent matrix.

For the polymeric matrix, short fibers are the most commonly used reinforcements to enhance the mechanical properties while minimizing the addition of the reinforcements. Polylactic acid (PLA), a non-toxic, biocompatible, and biodegradable polymer, is currently widely used to fabricate tissue engineering scaffolds. However, the mechanical properties of pure PLA scaffold are much lower than those of natural bone. Furthermore, along with the degradation of the scaffold, the overall strength decreases too fast. So, it is very difficult to repair large defect using pure PLA scaffold. On the other hand, chitin is a unique cationic polysaccharide with satisfactory biocompatibility. It has been found that chitin fibers could effectively reinforce PLA scaffold, and that the reinforced scaffold could repair larger bone defect than pure PLA scaffold [34, 54]. In another recent study reported by Slivka et al. [55], the poly(glycolic acid) (PGA) fibers reinforced poly(lactic-co- glycolic acid) (PLGA) scaffolds had increased compressive modulus from 10 to 50 MPa, by the increasing the amount of the reinforcing fibers from 0 to 20 wt %. The resulted strength of the reinforced scaffolds was approximately fivefold higher than that of the cartilage, which might be beneficial for tissues to bear high loads.

Bioceramics are another kind of matrixes, which are often reinforced by fibers or tubes. Continuous fiber reinforced ceramic scaffolds can offer significantly enhanced toughness without compromising the high stiffness and wear resistance of the parent ceramic matrix. Calcium phosphate ceramics are commonly used materials for the restoration of bone defects with excellent biocompatibility and bioactivity. However, brittleness and low flexural/tensile strength so far limit their application to non-load bearing areas. It has been shown that reinforcement of calcium phosphate cements with collagen fibers can substantially improve its toughness and has been one major strategy to overcome the present mechanical limitations of calcium phosphate cements. Fiber reinforced calcium phosphate cements thus bear the potential to facilitate the use of degradable bone substitutes in load bearing applications [39]. The calcium phosphate cements (CPC) was approved by FDA for repairing craniofacial defects. However, the CPC applications were limited to the non-stress-bearing repair because of their brittle and weak nature. The resorbable fibers have been incorporated to provide the early-strength and then to create macropores after fiber dissolution. The bending strength was improved from 2.7 to 17.7 MPa with 45 vol % polyglactin fibers incorporation in the CPC. The strengthening effect can be further increased to 40.5 MPa by the simultaneously incorporation of chitosan lactate into the matrix [56, 57]. Zhao et al. [58] reported a significant increased fatigue resistance of CPC with the addition of 15% chitosan and 20% polyglactin fibers. The maximum stress of CPC composite scaffolds was 10 MPa at 2 x 106 cycles, compared to 5 MPa of CPC control. The mean stress-to-failure was 9 MPa for the reinforced composites, while 5.8 MPa for the CPC control. Besides by themselves, fibers can also reinforce tissue engineering scaffolds by the cooperation with particles. The synergic addition of PLA fibers and tricalcium phosphate (TCP) particle reinforcements in PCL scaffolds resulted in a high porosity (80%) structure and the elastic modulus was increased from about 0.4 MPa up to about 2.2 MPa [59]. In many cases, some fibers can improve the biocompatibility of the matrix at the time of serving as reinforcing materials. Guarino et al. [60] found that their prepared PLA fiber reinforced polycaprolactone (PCL) scaffolds could support higher activities of the human osteoblasts and human marrow stromal cells than pure PCL scaffold.

Similar to fibers, tubes are another kinds of reinforcing materials. Among all of the reinforcing materials, the nanostructured ones occupy significant position. Especially, nanotubes are a type of one-dimensional macromolecules with large surface area, and normally exhibit similar strengthening mechanism as nanofiber reinforced composites. For instance, carbon nanotubes (CNTs) are graphitic sheets grown as hollow cylinders, the diameter of which are at nanometer scale, and the length of which are at micrometer scale. They exist as either rolled-up single sheet of graphene, commonly called single-walled carbon nanotubes (SWNTs), or concentric rolled-up multiple sheets of graphene, commonly called multi-walled carbon nanotube (MWNTs). CNTs have exceptionally high strength and stiffness and thus are a kind of appropriate candidates to reinforce tissue engineering scaffolds. In addition, they possess superior electrical conductivities, which may aid in directing cellular functions, since the electrical stimulation has been shown to induce the cell differentiation, proliferation and dendritic extension, etc. Therefore, there is an increasing amount of publications investigating the fabrication and properties of the scaffolds reinforced by CNTs for tissue engineering [61, 62]. When CNTs are used to fabricate tissue engineering scaffolds, the major concern may be their compatibility

Table 1.1 In vitro mechanical performance of the different Hap-CNT composites [69]

Samples

Fracture

toughness (MPa m1/2)

Change in fracture toughness (%)

Flexural

strength

(MPa)

% change in flexural strength

Impact

strength

(J/m2)

Change in impact strength (%)

HAp

0.81

-

11.23

-

47.25

-

HAp + 1% CNTs

1.88

132.10

26.28

134.02

133.1

181.69

HAp + 2% CNTs

1.17

44.44

29.34

161.26

122.95

160.21

HAp + 1% f-CNTs

1.23

51.85

22.58

101.07

78.26

65.53

HAp + 2% f-CNTs

1.7

109.88

21.1

87.89

144.96

206.79

with cells and surrounding tissue [63, 64]. A great number of investigations have concentrated in the in vitro and in vivo biocompatibility of CNTs, which showed that CNTs had no toxicity and could support well cell adhesion, spreading and proliferation at low concentrations, and had high ability to absorb proteins [63, 65, 66], which guaranteed the satisfactory biocompatibility of CNT reinforced scaffolds. With the adsorption of the proteins and recombinant human bone morphogenetic protein-2 (rhBMP-2) in advance, MWNTs exhibited increased alkaline phosphatase (ALP)/DNA and protein/DNA and induced ectopic bone formation in the dorsal musculature of ddy mice [66]. However, most CNTs reinforced scaffolds contain up to 5 wt % CNTs because the hydrophobic CNTs tend to form agglomerates at high concentrations. Shin et al. [67] prepared the gelatin methacrylate (GelMA) hybrid reinforced by CNTs and a dose-dependent increase in mechanical properties was observed with increasing amount of the CNTs. The tensile modulus increased about 3 times and the compressive modulus increased about 2 times with the addition of 0.5 mg/ml CNTs in GelMA hybrids. The CNT reinforced scaffolds supported well the adhesion, spreading and proliferation of NIT-3 T3 cells and human mesenchymal stem cells. Mikael et al. found that the compressive strength and modulus of PLGA/MWNTs scaffolds were significantly increased (35 MPa, 510.99 MPa) compared to the pure PLGA scaffolds (19 MPa and 166.38 MPa) by adding only 3% MWNTs. Furthermore, their in vitro and in vivo study indicated that the PLGA scaffolds reinforced by MWNTs had satisfactory cellular and tissue compatibility [68]. Besides the random dispersion of CNTs into the matrixes, it may be possible to achieve high electrical conductivity and mechanical strength at their low concentrations by aligning CNTs to form a connective pathway throughout the structure and get efficient charge transfer. This kind of scaffolds may benefit the regeneration of neural tissues by promoting neurite outgrowth, orientation and branching, etc. [61]. Mukherjee et al. [69] used CNTs to reinforce hydroxyapatite (HAp) for bone regeneration. Their results showed that the mechanical properties, including fracture toughness, flexural strength and impact strength, have been greatly improved by the addition of CNTs and functionalised CNTs (f-CNTs) (Table 1.1). Moreover, it was observed that the bone defects with the CNTs reinforced scaffolds implanted contained greater extent of golden yellow zone than the defects with pure HAp scaffolds implanted (Fig. 1.1), indicating that the reinforced scaffolds promoted greater extent of new bone formation than the matrix scaffolds.

Fluorochrome labeling images of Hap; HAp + 1% CNTs

Fig. 1.1 Fluorochrome labeling images of Hap; HAp + 1% CNTs (HC1); HAp + 2% CNTs (HC2); HAp + 1% f-CNTs (HFC1); HAp + 2% f-CNTs (HFC2) at 2 and 4 month post-operatively showing the golden yellow new bone (NB) and the deep sea green old bone (OB) zones (Adapted with permission from Ref. [69]. Copyright 2016 Elsevier Ltd)

Titania (TiO2) nanotubes are another kind of satisfactory candidates to reinforce tissue engineering scaffolds. TiO2 is known as one kind of suitable biomaterials based on its excellent biocompatibility, especially for orthopedic applications, and has been widely used as coating for bone implants. Byrne et al. prepared polystyrene composites reinforced by TiO2 nanotubes, the Young’s modulus and tensile strength of which increased 18% and 30% respectively relative to the pristine polymer at extremely low nanotube loading levels (between 0 and 1 wt%) [70]. Another study shows that TiO2 nanotubes could reinforce acrylic cement resin [71]. A small amount of nanotube content (1 wt%) was shown to be able to provide a significant increase in fracture toughness (73%), flexural strength (42%) and flexural modulus (56%). One of the main reasons why the titania nanotubes can reinforce so significantly is that they can achieve satisfactory interactions with the matrix. Moreover, in many cases, TiO2 nanotubes can improve the biocompatibility and bioactivity of the matrix besides serving as reinforcing materials. To date, a great number of studies have demonstrated the TiO2 nanotubes exhibit positive effects biocompatibility and bioactivity, such as accelerated apatite formation, enhanced osteoblast adhesion, proliferation and differentiation, etc. [72, 73]. The in vivo studies demonstrated that the TiO2 nanotubular surfaces can significantly improve the bone bonding strength [74].

 
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