The Mechanical Properties of the Scaffold Reinforced by Fibers or Tubes for Hard Tissue Repair
In order to provide a suitable biomimetic extracellular matrix for the growth of osteoblast-related cells, the mechanical properties of scaffolds should be close to the human hard tissues. Generally, the Young’s modulus value of natural bone varies from 0.1 to 27 GPa, which exhibits hyperelastic performance and high mechanical properties. In particular, human cancellous bone with an elastic modulus in the range of 0.1-0.5 GPa, and compressive strength in the range of 4-12 MPa. However, the elastic modulus value of human compact bone is between 12 and 18 GPa, and its compressive strength is between 130 and 180 MPa. Once the mechanical properties of the scaffold materials cannot meet the requirements, it could lead to bone resorption and even implant failure . Therefore, the desired scaffold should have appropriate strength, stiffness and mechanical behaviors. In addition, the physical properties of scaffolds can affect their mechanical properties, such as the porosity, pore interconnectivity, pore size. Generally, if biological scaffolds possess porous structure, they can offer large, interconnected pore spaces for cell adhesion and the nutrients interchange to supply the cells or metabolic waste to be removed, respectively. However, this structure tends to reduce the mechanical properties of the scaffold. Because of the good mechanical properties of the fiber and tubular materials, they are usually used in the preparation of scaffold materials in the field of hard tissue to overcome the shortcoming. As long as the design and preparation conditions are there, fiber or tube reinforced scaffold materials can meet the requirements of mechanical properties. Meanwhile, the scaffolds may show different structural properties, functional characteristics through appropriate preparation methods, which may be more suitable for hard tissue repair [24, 25]. To our knowledge, many kinds of fibers and tubes have already served as reinforcing component in scaffolds based on their excellent mechanical properties. Moreover, they can provide large specific surface areas to facilitate the improvement in the biological functions of the scaffolds .
Carbon fibers have been utilized as biological scaffold mechanical support materials due to their nontoxic property, good biocompatibility, high strength and modulus . Moreover, they can guide the proliferation of fibrous tissue by making the tissue cells follow the orientation of their filaments; therefore, they can promote the tissue regeneration process . Shi et al. prepared a novel scaffold for hard tissues by introducing activated carbon fibers (ACF) with high adsorption capacity into poly(lactic-co-glycolic) acid (PLGA) . The ACF and PLGA all were the materials with excellent bioactivity and biocompatibility. They obtained the ACF by high- temperature processing of carbon fibers to get richly distributed pore structures. In addition, the ACF/PLGA composite scaffolds were prepared by switched capacitor/ piecewise-linea (SC/PL) method with sodium chloride particles as a leachable component. SC/PL method can change the porosity and pore size of the scaffolds by respectively controlling the initial salt weight fraction and sieving salt particle size . Their results showed that the scaffolds had good mechanical properties.
J. Wei et al.
Fig. 7.1 Scanning electron microscope images of pure PLGA (a and b) and ACF/PLGA scaffolds(c and d), showing that both ACF/PLGA scaffolds and pure PLGA were porous, and the ACF distributed randomly inside scaffolds (Adapted with permission from Ref. . Copyright 2012 Elsevier Ltd)
The scanning electron microscope images of the scaffold were Fig. 7.1. Meanwhile, the cell experiments showed that the ACF/PLGA scaffolds had higher cell adhesion and proliferation, which demonstrated that the ACF/PLGA scaffolds had great potential to be applied in hard tissue repair.
The low Yang’s moduli of dental composites restrict their application in the dental research field. Sabbagh et al. investigated the dynamic elastic modulus of the scaffold containing biological molecule and nanotubes. They found that the 3% biofunctionalized nanotubes composite possessed the greatest Young’s modulus (16.8 GPa) compare to control composite. The result likely contributed to the reinforcement of the composite by nanotube bundles and strong adhesion between n-TiO2 and the resin matrix . Therefore, these reinforced composites, serving as restorative materials, could provide an excellent mechanical match with tooth structure. In addition, the fibers or tubes also can be used for coating materials to reinforce the mechanical properties of scaffolds . Collagen is one of the main components of the extracellular matrix [33, 34]. To make use of it, the chitosan scaffolds can be dip coated in an ultrasonic collagen bath. Collagen in the bath can fibrillate on the fiber surfaces and cause a coating with nanofibrous extensions. The addition of nanofiber and collagen enlarged the inner surfaces, and collagen provided good conditions for cell adhesion and growth.
To date, many fabrication techniques are used in the manufacture of hard tissue scaffolds, which can fabricate into meet the needs of hard tissue scaffold materials. Among these preparation methods, meltspinning, which is based on the extrusion process of a polymer melt, is regarded as a suitable method for the production of fiber or tube reinforced scaffold used in hard tissue scaffolds . This method does not require the use of volatile organic solvents, which could possibly affect cell viability or a removal process for the remnant solvent. Park et al. prepared melt- spun shaped fibers with enhanced surface effect. The report showed the woven scaffolds composed of the shaped fibers had good stiffness and increased significantly the proliferation of human osteosarcoma MG63 cells . However, conventional fabrication techniques are not sufficiently suitable to control scaffold structure to modulate mechanical properties, which put forward the urgent need for the new preparation process. Within novel scaffold fabrication processes, rapid prototyping technique has already attracted great attention as powerful tools for the fabrication of hard tissue scaffolds through a stage material deposition, these scaffolds can be built layer by layer in two different circumstances—a molten phase (referred to as fused deposition modeling (FDM)) [37-39] or droplets with a binding agent (known as three-dimensional (3D) printing) [40-42]. During this fabrication, the layers are deposited as interpenetrating networks, leading to the 3D outcomes of 100% interconnected porous scaffolds every definition. Scaffolds can be built in a customized shape through computer-aided design techniques, achieving a defined structure and architecture. Among these devices, 3D fiber deposition (3DF) [44, 45] and 3D plotting  have been recently developed and used for tissue engineering. 3DF is a fused deposition technique, during the processes of which extrudated molten polymer is deposited from the XYZ motor drive syringe on a stationary stage under the applying pressure. 3D fiber deposition is characterized by 100% interconnected pores with various sizes and shapes, which evidently affects mechanical properties, performing great potential for tissue engineering applications because of the precision in making reproducible 3D scaffolds. Furthermore, the versatility and flexibility of scaffolds allow us to use rapid prototyping devices to generate improved scaffolds, as well as to study the effects of different structural phenomena on tissue reconstruction. In this respect, FDM techniques offer appealing solutions for scaffold fabrication, having been recently used for tissue engineering purposes.
Electrospinning technique has also been widely used in the fabrication of fiber scaffolds for hard tissue, which can enlarge the functional surface of the scaffolds and get nanofiber scaffolds that have better mechanical strength . The nanofibrous networks interweave in the total inner structure of the scaffolds, and then constitute a uniform hierarchical structure so that the microfibers form an interconnected pore space for cell adhesion, migration, growth, and proliferation [47, 48]. Lee and his group designed the biointerface of biopolymer electrospun fibrous matrices with a biofunctional protein including fibronectin 9-10 domain (FNIII9- 10) and osteocalcin (OCN), where FNIII9-10, the 9-to-10 sequence of FN domain III, was used with OCN protein by vector expressions. They found the biopolymer fiber scaffolds have a strong affinity, binding to the mineralized surface in large quantities while preserving stability over a long period, which was not readily achievable in the bare polymer fiber surface .
Horner et al. fabricated electrospun core-shell scaffolds by biocompatible polymer systems, the core materials of which are polyetherketoneketone (PEKK) and gelatin, maintaining the shell polymer with polycaprolactone (PCL). In their study, they found the mechanical properties of individual core-shell fibers were decided by core-shell composition and structure. As their study showed, the modulus of individual fiber is related to the increase core size, which rangs from 0.55 ± 0.10 GPa to 1.74 ± 0.22 GPa and 0.48 ± 0.12 GPa to 0.53 ± 0.12 GPa for the PEKK-PCL and gelatin-PCL fibers, respectively . In addition, Liu et al. developed a novel approach to encapsulate and distribute the cells in the nano-fibrillated cellulose based matrices during the swelling process. The nanocellulose hydrogels made through this way with a resultant aerogels, with porosity up to 99.7% and specific surface area up to 308 m2/g, and the various swelling degree, which up to 500 times, by tuning the material processing approach, the swelling media conditions, and nanocellulose charge density .
From a mechanical perspective, mechanical properties are intimately correlated with the porosity of porous structures. Specifically, a stiffer and less porous scaffold shows a better integration with the surrounding natural tissue, while a flexible and more porous one allows cells to attach and proliferate more efficiently. It is rapid prototyping that offers probabilities to compromise such varying requirements into one scaffold, because it adds the freedom of different structural parameters to the non-variable bulk mechanical properties of the material used.