*3.2. Tissue Engineering*

Tissue engineering is an increasingly popular next-generation biomedical technology to treat defects and malfunctions in human organs. This technology has the potential to expand medical coverage and resolve problems, such as a lack of organ donation and transplant rejection [65–69]. Scaffolds that are suitable for tissue engineering require high biocompatibility, biodegradability, good processability, mechanical properties similar to those of native tissue, and proper flexibility [70–77]. sc-PLA has been extensively applied in tissue engineering owing to its excellent biocompatibility, full biodegradability, improved mechanical properties, and thermal stability. In particular, it is suitable for scaffolds that require robust properties for bone, cartilage, and orthopedic implants. V. Katiyar et al. (2017) fabricated orthopedic implants based on nano-hydroxyapatite (n-HAP)-grafted sc-PLA composites using 3D printing [78]. As shown in Figure 14a, n-HAP-grafted PDLA was polymerized by in situ ring-opening polymerization, and the sc-PLA/n-HAP filament with a diameter of approximately 1.6 mm for 3D printing was fabricated by melt mixing with PLLA in a twin-screw extruder. A middle phalanx bone composed of filaments was successfully manufactured using 3D printing. As shown in FE-SEM images, the fractured Sc-PLA/n-HAP nanocomposites exhibited a smooth surface and uniform dispersion of n-HAP of 60 nm size (Figure 14b). The n-HAP provided a reinforcement effect and expansion of the surface area as a filler in the sc-PLA matrix. As shown in Figure 14c, sc-PLA/n-HAP of 2.5% increased the ultimate tensile strength up to a maximum of 16% above that of neat sc-PLA (40.2 vs. 33.8%, respectively). This is because an increase in intermolecular bonding and cross-linking in stereocomplex crystals, together with strong interfacial bonding between the polymer matrix and the filler, could increase its crystallinities. Furthermore, the ductility of sc-PLA increased by the addition of n-HAP fillers, which increased the elongation at the break of sc-PLA/n-HAP up to a maximum of 131.6 %. Improving ductility could enhance the durability of biocomposites resulting from the prevention of fracture and abruption, thus expanding its application for implants that require robust resistance for high loads.

Subsequently, V. Katiyar et al. (2019) focused on the synthesis of linear block copolymers composed of hard and soft segments of PLLA/PDLA and PCL [79]. Diblock and stereotriblock copolymers were successfully polymerized with PCL as a macroinitiator using sequential ring-opening polymerization, as shown in Figure 15a. Values of the tensile strength and elongation at the break for diblock copolymers were improved from 14.8 to 28.9 MPa and from 6.4 to 17.8%, respectively, with an increase of the block length of PDLA (Figure 15b). The synthesized materials were thermally processed based on injection molding to manufacture cancellous and cortical bone screws, which are considered as orthopedic fixation devices, as shown in Figure 16a. In a study on thermo-mechanical stability, cancellous bone screws consisting of sc-PLA/PCL blends could stably maintain their shape and structure at 121 ◦C for 60 min, more than those of commercial homo PLA (Figure 16b). This is because the stereocomplex crystallites of the hard segmen<sup>t</sup> in the blend copolymer improved the thermal resistance of the scaffold. Consequently, the sc-PLA and PCL in the sc-PLA block copolymer enabled the scaffold to increase mechanical and thermal stability, and to reduce brittleness of PLA by its plasticization effect; thus, scaffolds composed of these biomaterials are considered suitable for biomedical implants with good clinical outcomes.

**Figure 14.** (**a**) A schematic representation for synthesis, processing, and application of sc-PLA/n-HAP. (**b**) Field emission scanning electron microscopy (FESEM) images of fractured surface of (**i**) a neat sc-PLA and (**ii**) a sc-PLA/n-HAP biocomposite. Pointed arrows depict n-HAP particles of approximately 60 nm diameter. (**c**) (**i**) Load–elongation curves of sc-PLA and sc-PLA/n-HAP. (**ii**) Comparison of ultimate tensile strength and elongation at break of sc-PLA and sc-PLA/n-HAP with diverse HAP contents [78], Copyright 2017. Reproduced with permission from the American Chemical Society.

**Figure 15.** (**a**) Polymerization process of (**i**) diblock and (**ii**) stereotriblock copolymers. (**b**) Stress– strain (SS) curves of homopolymers and diblock copolymers [79], Copyright 2019. Reproduced with permission from the American Chemical Society.

**Figure 16.** (**a**) Schematic illustration for thermal processing of diblock copolymer and enantiomeric diblock blend for fabrication of cortical and cancellous bone screws as orthopedic fixation devices. (**b**) Comparison of thermo-mechanical stability of cancellous bone screw comprising commercial PLA 2003D (Natureworks) and enantiomeric blend at 121 ◦C at intervals up to 60 min [79], Copyright 2019. Reproduced with permission from the American Chemical Society.

Enhancing mechanical properties of biodegradable polymers is critical to biomedical fields, such as bone fixation. Numerous processing methods have been developed to improve the strength of the polymers. Many studies have demonstrated that solid-state drawing (SSD) can induce self-reinforcement through the maximization of macromolecular chain orientation in polymeric materials [80–82]. Im et al. (2016, 2017) determined that the tensile strengths of PLLA monofilaments and films could be increased up to two- and ninefold, respectively, by increasing the draw ratio using a directly designed processing machine for the SSD method (Figure 17a) [83,84]. Furthermore, this study showed that solid-state drawn PLLA enhanced blood compatibility and cell adhesion. Recently, Li et al. (2021) successfully oriented shish-kebab crystals in sc-PLA using the SSD method, as shown in Figure 17b [85]. The oriented sc-PLA scaffold had a tensile strength of 373 MPa and an elongation of 9%. This processing could lead to fibrous crystals of shish and kebabs with parallel lamellar microstructures along the direction of the drawing. This shishkebab microstructure with a specific topography could provide a self-reinforcing effect and prevent cracking and collapse of aligned kebabs into biomaterials based on sc-PLA.

**Figure 17.** (**a**) Schematic of processing machine for SSD method with PLLA films or filaments [83], Copyright 2016. Reproduced with permission from IOP Publishing, Ltd. (**b**) Schematic representation for preparation process of oriented sc-PLA using SSD method [85], Copyright 2021. Reproduced with permission from the American Chemical Society.

C. Wang et al. (2019) synthesized injectable thermogels based on the sterocomplex 4-arm poly(ethylene glycol)-polylactide (PEG-PLA) and cholesterol-modified 4-arm PEG-PLA for optimized cartilage regeneration (Figure 18) [86]. The cholesterol-modified sc-PLA gels exhibited improved mechanical strength, lower critical gelation temperature, higher chondrocyte proliferation, and slower degradation than unmodified specimens. Moreover, cholesterol-modified sc-PLA gel-loaded chondrocytes showed considerably more cartilage-like tissues than fibrous- and bone-like tissues. This is attributed to the improved mechanical properties and microstructure induced by cholesterol modification of the sc-PLA gel.

**Figure 18.** Schematic illustrations for (**i**) copolymer synthesis of 4-armPEG-PLL(D)A-cholesterol and (**ii**) in vivo cartilage regeneration of thermogels entrapped chondrocytes via subcutaneous injection into nude mice [86], Copyright 2019. Reproduced with permission from WILEY-VCH Verlag GmbH & Co.
