Particle Reinforced Composites (PRCs)
Currently, particle reinforced composites have been widely used for industrial applications. Particulate systems have the dimensions that are approximately equal in all directions and thus they remain predominantly isotropic in their physical behaviors. According to the particle size, they fall into two categories: large particle reinforced and small particle reinforced composites. For the large particle reinforcement, the sizes of the particles are between 1 and 50 pm in diameter. The interactions between the particles and matrix cannot be brought about at the atomic or molecular level. The particulate phase is generally hard and stiff. The reinforcement mechanism in the large particle reinforced composites is that the matrix can transfer some of the loaded stress to the particles and the reinforcing particles tend to restrain the movement of the matrix phase in the vicinity of each of them. The degree of reinforcement depends on the bonding interactions between the particles and matrix. In the case of small particle reinforced composites, the sizes of the particles are between 10 and 100 nm in diameter.
The reinforcement mechanism in the small particle reinforced composites is that the dispersed small particles can hinder or impede the motion of dislocations and thus the plastic deformation is restricted . In one situation, with the increase of the applied load, the dislocations inside the matrix are impeded by the small particles and finally form dislocation loops. In the other situation, with the increase of the applied load, the dislocations cut the small particles. In both cases, it is obvious that the particles can significantly decrease the imposed destructive energy on the matrixes, thereby increasing their mechanical properties.
The matrixes of particle reinforced composites, which are use as tissue engineering scaffolds, are usually polymers. And the reinforcing particles are normally polymers, ceramics or bioglasses. For example, 3D porous silk scaffolds have good biocompatibility and biodegradability as tissue engineered scaffolds. However, their mechanical properties are still too weak for musculoskeletal tissue repair. The study of Kim et al.  showed that the strength of the silk scaffolds remained weaker than that of natural bone (wet compressive modulus < 200 kPa and yield strength <40 kPa). Rajkhowa et al.  used crystalline silk particles to reinforce the silk scaffolds. With the addition of 200% particle reinforcement, the specific compressive modulus and the yield strength of the reinforced biocomposites increased about 40 times, compared to the matrix. The excellent reinforcement achievement was based on not only the satisfactory interfacial compatibility between the particles and the matrix, but also the high interfacial cohesion between them because of the partial solubility of the particles in the matrix. Furthermore, some drug encapsulated polymeric microparticles could not only be used to reinforce tissue engineering scaffolds but also perform as growth factor or drug carriers for their controlled delivery in vivo . Besides polymer particles, some bioactive glass and bioceramic ones have also been used to reinforce polymer matrix to prepare tissue engineering scaffolds because of their high stiffness. Meanwhile, in many cases, based on their satisfactory biocompatibility, bioactivity, osteoconductivity and even osteoinductivity, they can also improve the biofunctions of the polymer matrixes, especially for bone repair. For example, the study of Niu et al.  showed that the biocomposites of PLLA and mesoporous bioglass (m-BG) particles possessed significantly higher compressive strength and compressive modulus than pure PLLA scaffolds. Moreover, it was indicated that the incorporation of the m-BG particles into the PLLA scaffolds significantly improved their water absorption, degradability and bioactivities, which was dependent on the content of the particles. Furthermore, their data demonstrated that the addition of the particles could enhance the attachment, proliferation and ALP activity of MC3T3-E1 cells on the scaffolds in vitro, and osteogenesis and bone repair ability of the scaffolds in vivo. Another study example is about the use of bioceramic particles to reinforce polymer matrixes. Lou et al.  prepared PLLA scaffolds reinforced by beta-tricalcium phosphate (P-TCP) particles. Their study results showed that the particles not only significantly enhanced the mechanical properties of the scaffolds, but also improve the proliferation, penetration, and ECM deposition of the osteoblasts cultured on the scaffolds.