*3.3. Anti-Microbial Effect*

Biomaterials and surgical implants based on the sc-PLA material are increasingly being applied for the above-mentioned biomedical applications, including tumor treatment and tissue engineering. However, preventing contamination from foreign microorganisms, such as bacteria and viruses is essential for the application of sc-PLA as a biomaterial in the human body [87–89]. Sterilization is necessary to prevent contamination before medical surgery, albeit it has been shown that contamination from bacteria such as *Staphylococcus aureus* (*S. aureus*) most frequently occurs during surgery [90–92]. Therefore, biomaterials based on sc-PLA are critical for securing anti-microbial effects to prevent bacterial proliferation to decrease adverse events and maximize clinical efficacy. Normally, biomaterials are sterilized by ethylene oxide gas, gamma ray irradiation, dry-heat sterilizer, microwave, autoclave, and supercritical carbon dioxide (CO2) [93–97]. Unfortunately, these methods have remaining concerns not only regarding changes in the inherent properties of the material during the sterilization process, but also the difficulty in preventing secondary contamination. Thus, biomaterials must possess sustained anti-microbial effects to inhibit external microorganisms before and after implantation in the body. Spasova et al. (2010) fabricated electrospun sc-PLA fibers with antibacterial and hemostatic effects using diblock copolymers composed of poly(N,N-dimethylamino-2-ehtylmethacrylate) (PDMAEMA) [98]. After the incubation of mats composed of these fibers with *S. aureus*

and *Escherichia coli*, the adhesion of these bacteria was observed. Consequently, sc-PLA mats containing PDMAEMA significantly inhibited bacterial adhesion and proliferation on the surface and exhibited effective antibacterial effects, while the control group showed bacterial adhesion and biofilm formation on the surface. This was attributed to the surface of tertiary amino groups from PDMAEMA blocks [99–101]. This surface modification strategy for antibacterial effects could remove concerns about contamination in surgical procedures and sustain its efficacy in the human body. Ajiro et al. (2016) polymerized PLLA and PDLA using catechin (CT) as an initiator precursor, which is an antibacterial compound [102]. Figure 19a shows the polymerization of CT-conjugated PLLA and PDLA at the chain end groups. Lactide was polymerized with benzyl catechin (BnCT), and then CT was chemically combined with PLAs to protect the phenolic hydroxyl groups. To assess antibacterial properties of the polymerized products, the ratio of total viable bacteria was calculated after 24 h according to the JIS Z 2801 test, as shown in Figure 19b. The CT-absorbed substrate reduced the ratio of total viable counts by 20% compared to the control. In contrast, both PLLA-CT and PLA-CT-SC substrates had approximately 50% significantly lower values than those of the control group. As shown in Figure 19c, the counts of killed bacteria per CT unit were compared with those of the CT-absorbed substrate, PLLA-CT, and PLA-CT-SC. Both PLLA-CT and PLA-CT-SC groups showed antibacterial properties mainly induced by the phenolic hydroxyl groups of CT. The antibacterial effect was dependent on the amount of CT, and it could be maintained for long-term use.

**Figure 19.** (**a**) Synthesis process schematic of chain-end modified PLLA and PDLA. (**b**) Comparison of antibacterial properties through ratio of live bacteria counts of (**i**) control, (**ii**) PLLA-Bn, (**iii**) CT, (iv) PLLA-CT, and (**v**) PLA-CT-SC against the control group. Top scheme indicates a method for measurement of antibacterial properties. The graphs were represented as mean values ± SD (*n* = 3). (**c**) Comparison of the counts of killed bacteria per CT unit of (**i**) CT, (ii) PLLA-CT, and (**iii**) PLA-CT-SC. The graphs were represented as mean values ± SD (*n* = 3) [102], Copyright 2016. Reproduced with permission from WILEY-VCH Verlag GmbH & Co.

Y. Li et al. (2013) suggested a novel approach for synthesizing charged hydrogels based on non-covalent interactions for disrupting biofilms and microbes [103]. They fabricated the antimicrobial gel by stereocomplexation between PLLA-PEG-PLLA and a charged PDLA-polycarbonate-PDLA (PDLA-CPC-PDLA) triblock polymer in aqueous solution. At physiological temperature (37 ◦C), the physical properties of the stereocomplex hydrogel were transformed into shear thinning behavior with supramolecular fiber and ribbon-like structures. This improved antimicrobial activity of the cationic hydrogel against diverse pathogenic microorganisms, such as fungi and both Gram-positive and Gram-negative bacteria. Up to 60% film biomass of *S. aureus*, *E. coli*, *Candida albicans* (*C. albicans*), and Methicillin-resistant *S. aureus* (MRSA) were eliminated after hydrogel treatment. In the safety test for skin sensitization, acute dermal toxicity, and skin irritation, the stereocomplex hydrogels appeared to provoke no adverse events in animal models of rats, guinea pigs, and rabbits. L. Mei et al. (2018) fabricated hybrid nanofibers by electrospinning in addition to PLA stereoisomers for inducing stereocomplexation between the stereoisomer chains and addition of chlorogenic acid (CA) of an antibacterial agen<sup>t</sup> [104]. To prevent damage of the agent, the stereocomplex nanofibers were electrospun at relatively lower temperature (65 ◦C). The antibacterial fibers could effectively remove both gram-positive and gramnegative bacteria by quickly released CA within a few hours. The fibers based on sc-PLA have a potential to be applied in various fields such as filter, masks, and packages, owing to enhanced mechanical and thermal properties as well as full biodegradability. Recently, Y. Ren et al. (2019) reported successful fabrication of a novel eco-friendly sc-PLA nanofiber by electrospinning, with both functions of adsorption of heavy metal ions and inhibition of bacterial growth [105]. In the fabrication process, an antibacterial agen<sup>t</sup> called HTA, which could be synthesized from tannic acid and hexamethylenediamine, was loaded into the products to provide an antibacterial effect. Furthermore, the electrospun nanofibers had improved tensile strength and Young's modulus, as well as thermal resistance owing to the formation of stereocomplex crystallites. The nanofiber mats based on sc-PLA exhibited excellent abilities for adsorption of Cr(VI) and capture of *E. coli* and *S. aureus*. Cr(VI) was converted to less toxic Cr(III) after its adsorption. This indicates that the heavy metal pollutant can be changed into eco-friendly and stable compounds in nature.
