Self-Reinforcement

There is a growing interest for the development of self-reinforced materials, which are a substitute for traditional fiber-reinforced and nanotube-reinforced composites. This research topic has been focused because both the reinforcing and continuous phases involve the composites with the same chemical composition, which are easier to convert and facilitate mass production. Self-reinforced composite (SRC) is an interesting concept that a matrix is reinforced with oriented fibers and tapes, or particles, where both the reinforcing and matrix phases are given by the same suitable material but have different mechanical properties. Moreover, in the published literature, the matrix and reinforcing materials of the composites, which are different material but belong to the same family, are also referred to as SRCs [142].

SRCs are particularly important in biomaterials applications, since any additives composed of different chemicals can affect biocompatibility and biodegradability. As a new development field, the fundamental research and processing technologies are still far from mature, but the recent research results have demonstrated the great potential of SRCs as biomaterials in tissue engineering. More and more R&D works have been devoted to the field biomaterials [143, 144].

As a common phenomenon in nature, some natural materials are SRCs. For example, wood is formed by cellulose-based SRC, and muscle is composed of selfreinforced protein. In the recent, the development of SRCs has emphasized on natural materials. The SRCs of cellulose have also been widely used to reinforce other polymers of the same family. Li et al. [145] used regenerated silk fibroin SRC fabrics to enhance the interfacial adhesion between the silk fibers and the fibroin matrix, which significantly improved the overall mechanical properties and thermal stability of silk fiber/fibroin composites. Lu et al. [146] reported the development of the self-reinforced composite of sisal prepared by melt and hot pressing, which had inherent interfacial compatibility and full biodegradability. Similarly, Nishino et al. [40] reported that the whole cellulosic composite exhibited excellent mechanical, thermal and biodegradable properties in application.

Reinforcing a synthetic matrix by embedding the same fibers also provides a possible means of meeting the high strength and stiffness requirements for many tissue applications. Niiranen et al. [58] studied the properties and degradation behavior of self-reinforced bioabsorbable polymer/bioactive glass composites, which consisted of the same poly(L/DL)lactide (PLA) 70:30 and bioactive glass (BaG) 13-93 by conventional melt-extrusion [147]. The bioactive glass fillers increased the hydrophilic nature of the composite material at the interface, and they also played a role in providing the macropores at the die stretching. The structure and quantity of glass fillers on a macro-scale determined the hydrolysis degradation of the composite, which was more significant in vivo than in vitro. Thus, the selfreinforced composites with a porous structure can provide an advantageous substrate for creating the interlocking mechanism stability and guiding cell adhesion, proliferation and deposition to form bone tissue. In the other study, Majola et al. [148] indicated that the initial bending and shear strengths of PLLA SRCs at 250271 MPa and 94-98 MPa, respectively, were higher than those of non-reinforced PLLA with the same molecular mass. Similarly, Wrightcharlesworth et al. [149] also reported that a oriented PLA fiber SRCs prepared by the hot press method had higher initial mechanical properties. As for the study of other artificial polymers, Deng et al. [147] indicated that the ultra-high-molecular-weight polyethylene (UHMWPE) matrix reinforced with the same fibers was a good potential candidate of SRCs, which could be used in load-bearing biomedical applications. Compared with pure UHMWPE, the tensile modulus, double notch impact strength and creep resistance of the SRCs were increased, although there was no obvious difference in the wear properties of the two materials. In addition, Wright et al. [150-154] evaluated mechanical properties of three different types of wove PMMA SRCs, and found that each one had its own fracture processes different from others, which were caused by the different orientation of fibers and the fracture toughness. The best combined PMMA SRC, absorbing the same amount of water as bone cement, was equal to or better than the bone cement in the tests, which made the PMMA SRC worthy of further consideration as a candidate biomaterial in tissue engineering. Amer et al. [155] investigated optimum processing conditions of high-density PE (HDPE) SRCs, including processing temperature, pressure and duration, and found that the composite modulus increased with the decrease of the composite cooling rate. And the authors held that PE SRCs was promising candidate materials for the preparation of a number of scaffolds with wear-resistance in tissue engineering.

Self-reinforced materials are one of the most promising reinforcing biomaterials in tissue engineering, because they possess special characteristics that are different from conventional polymers. Thus, it is expected that the self-reinforced material as reinforcements will provide an interesting method to increase the strength, degradability and thermal stability of the composites for the future development in various biomedical applications. The growing interest in the self-reinforced material will continue to rise.

 
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