Polymers

Biodegradable synthetic polymers offer a number of advantages for developing scaffolds in tissue engineering [35, 37, 38]. The vital advantages include their great mechanical properties and degradation kinetics, which can be tailored by adjusting different ratios of monomers, concentrations and reaction conditions during the fabrication process. They can also be fabricated into various shapes with different surface topography and porosity to suit desired applications. Hence, biodegradable polymers are a popular choice as materials for tissue engineered scaffolds, and plenty of experiments and researches have been done on this field [39-41]. They are also promising matrix materials for the fiber or tube reinforced tissue scaffolds. Basically, there are two types of polymeric substrate materials, natural polymers and synthetic polymers, as shown in Table 2.3. Natural polymers, such as collagen, various proteoglycans, alginate-based substrates and chitosan, have all been used in the production of scaffolds for tissue engineering [41-43]. They have high degree of biocompatibility that can promote cell adhesion and proliferation. Moreover, natural polymers can be degraded gradually, and the adhered donor cells can produce their own ECM to replace the scaffolds finally. Malafaya et al. [44] found a chitosan scaffold produced by a particle aggregation method that allowed the ingrowth of the connective tissues and promoted the neo-vascularization even in the early stages of implantation. Additionally, chitosan films and collagen sponges are also the commercial product as the wound dressing. Chen et al. [45] reported the composite membranes, using type I collagen and chitosan fabricated by electrospinning technique, which exhibited better wound healing rate than the commercial collagen sponge. Although natural polymers have known biological activity, they are difficult to process into scaffolds and the lack of quantity limits their use. Furthermore, the natural polymers may also stimulate an immune response, which leads the concerns over antigenicity and delivery of diseases for allograft [46, 47].

A great number of synthetic polymers have been used as scaffold materials, including PLA, PGA, PLGA and PCL. These synthetic polymers have already passed FDA regulation and have been successfully used as degradable sutures for many years. And thus scaffolds made from these materials can provide a quick route to a commercial and clinical product [36]. The PLGA scaffolds have been used to repair defects in different tissues, such as bone, liver, nerve, skin and blood vessels. PLGA scaffolds loaded with neuron stem cells (NSC) permitted the NSCs to differentiate toward neurons, establish connections and exhibit synaptic activities [48]. Successful cartilage constructions were observed after 8 weeks in a joint cavity of a sheep in the PLGA scaffolds loaded with autologous mesenchymal stem cells [49]. Good shaping properties and controllable mechanical and degradation characteristics by altering chemical compositions and distributions of polymers are the additional advantages. Kim et al. [50] found that the PCL scaffolds with oriented nanofiber webs showed a mechanically anisotropic behavior and a higher hydrophilic property in comparison with the randomly distributed fibrous mats.

Additionally, most synthetic polymers are degraded by the hydrolysis process, and the degradation rate is not affected by the enzyme. Unlike the natural polymers, the degradation of the synthetic polymers scaffolds may not vary from patients to patients. Their major drawback is that the degradation products are not as compatible as those of the natural polymers. For example, CO2, the degradation product of PLLA and PLGA, lowers the local pH value and induces cell and tissue necrosis [51].

There are, however, some problems with using polymer scaffolds for bone regeneration. Although the mechanical properties of polymers can be matched with collage, their elastic modulus is much lower than that of bone, which limits their use in load-bearing anatomic sites undergoing compressive forces. Secondly, polymers exhibit fast strength degeneration during the degradation process, especially under load, which is too rapid for bone regeneration. On the base of these shortcomings, fiber- or tube- reinforced tissue scaffolds become a popular study point. For example, Jiao et al. [52] found that the mechanical properties of pure PLA scaffolds were much lower than those of natural bone, while chitosan was a unique cationic polysaccharide with satisfactory biocompatibility. By using chitosan fibers to reinforce PLA scaffold, the reinforced PLA scaffold could be used to repair larger bone defect. This kind of examples goes on and on. From years’ research reports, polymers have large scale use in fiber or tube reinforced tissue scaffolds, as shown in Table 2.4.

Table 2.4 Examples of polymer scaffolds employed in tissue engineering

Natural polymers

Synthetic polymers

Collagen (type I, II, III, IV)

Polylactic acid

Collagen-

glycosaminoglycan

copolymer

Polyglycolic acid

Fibrin

Poly-dl-lacic-co-glycolic acid

Poly(hydroxybutylate),

PHB

Polytrimethylene carbonate

Sodium alginate

Polycaprolactone

Chitin and chitosan

Poly(ortho esters)

 
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