*3.2. Anti-Fouling Capability of PEDOT Derivative Nanohybrid Coatings*

smooth morphology.

smooth morphology.

**Figure 7.** Atomic force microscopy (AFM) images of (**a**) the SUS316L substrate, and (**b**) PEDOT/PSS, (**c**) PEDOT/GO, and (**d**) PEDOT/GO/PDDA nanohybrid coatings. To evaluate the anti-fouling capability of the PEDOT derivative nanohybrid coated on the surface of SUS316L stainless steel (Figure 8), 10<sup>5</sup> CFU/mL of S. aureus was chosen as the model bacteria. Green dots of the fluorescence microscope images represent live *S. aureus* bacteria adhering to the surface of the SUS316L substrate and PEDOT derivative nanohybrid coatings. A dense distribution of *S. aureus* (approximately 10<sup>8</sup> /cm<sup>2</sup> ) adhered to the surface of the pristine SUS316L substrate, as shown in Figure 8a. Compared with the pristine SUS316 substrate (Ra = 76.2 nm), a lower adhesion density of bacteria was obtained on the PEDOT derivative nanohybrid coatings (Figure 8b–d), which was due to the lower surface roughness. Therefore, the numbers of adhered bacteria were decreased to

approximately 10<sup>6</sup> /cm<sup>2</sup> for PEDOT/PSS, approximately 10<sup>6</sup> /cm<sup>2</sup> for PEDOT/GO, and approximately 10<sup>7</sup> /cm<sup>2</sup> for PEDOT/GO/PDDA substrates. In particular, the negative charge of PEDOT/PSS (Figure 8b) and PEDOT/GO (Figure 8c) nanohybrid coatings can further inhibit and reduce the adhesion of the negative charge of *S. aureus*, demonstrating the anti-fouling capability. On the other hand, the numbers of *S. aureus* decrease from 10<sup>5</sup> CFU/mL (SUS316L substrate) to approximately 10<sup>2</sup> CFU/mL (PEDOT/PSS and PEDOT/GO nanohybrid coating), and only 0.1% of bacteria can be adhered on the surface. However, the numbers of adhering bacteria (approximately 10<sup>3</sup> CFU/mL) on the PEDOT/GO/PDDA nanohybrid coating were slightly higher than that of the PEDOT/PSS and PEDOT/GO nanohybrid coating. The results indicate that the bacteria seemed to slightly prefer to adhere on the positive charge of the PEDOT/GO/PDDA substrate (Figure 8d). The negative cell wall of the bacteria would be locked by the positive charge of PEDOT/GO/PDDA substrates to inhibit the growth; thus, the bacteria are going to die [6]. The substrate exhibited excellent anti-bacterial capability, as discussed in the next section. decreased to approximately 106/cm2 for PEDOT/PSS, approximately 106/cm2 for PEDOT/GO, and approximately 107/cm2 for PEDOT/GO/PDDA substrates. In particular, the negative charge of PEDOT/PSS (Figure 8b) and PEDOT/GO (Figure 8c) nanohybrid coatings can further inhibit and reduce the adhesion of the negative charge of *S. aureus*, demonstrating the anti-fouling capability. On the other hand, the numbers of *S. aureus* decrease from 105 CFU/mL (SUS316L substrate) to approximately 102 CFU/mL (PEDOT/PSS and PEDOT/GO nanohybrid coating), and only 0.1% of bacteria can be adhered on the surface. However, the numbers of adhering bacteria (approximately 103 CFU/mL) on the PEDOT/GO/PDDA nanohybrid coating were slightly higher than that of the PEDOT/PSS and PEDOT/GO nanohybrid coating. The results indicate that the bacteria seemed to slightly prefer to adhere on the positive charge of the PEDOT/GO/PDDA substrate (Figure 8d). The negative cell wall of the bacteria would be locked by the positive charge of PEDOT/GO/PDDA substrates to inhibit the growth; thus, the bacteria are going to die [6]. The substrate exhibited excellent anti-bacterial capability, as discussed in the next section.

d), which was due to the lower surface roughness. Therefore, the numbers of adhered bacteria were

*Polymers* **2020**, *12*, 1467 9 of 12

To evaluate the anti-fouling capability of the PEDOT derivative nanohybrid coated on the surface of SUS316L stainless steel (Figure 8), 105 CFU/mL of S. aureus was chosen as the model bacteria. Green dots of the fluorescence microscope images represent live *S. aureus* bacteria adhering to the surface of the SUS316L substrate and PEDOT derivative nanohybrid coatings. A dense distribution of *S. aureus* (approximately 108/cm2) adhered to the surface of the pristine SUS316L substrate, as shown in Figure 8a. Compared with the pristine SUS316 substrate (Ra = 76.2 nm), a lower

*3.2. Anti-Fouling Capability of PEDOT Derivative Nanohybrid Coatings* 

**Figure 8.** The number of bacteria adhering to (**a**) the SUS316L substrate, and (**b**) PEDOT/PSS, (**c**) PEDOT/GO, and (**d**) PEDOT/GO/PDDA nanohybrid coatings. **Figure 8.** The number of bacteria adhering to (**a**) the SUS316L substrate, and (**b**) PEDOT/PSS, (**c**) PEDOT/GO, and (**d**) PEDOT/GO/PDDA nanohybrid coatings.

#### *3.3. Anti-Bacterial Capability of PEDOT Derivative Nanohybrid Coatings 3.3. Anti-Bacterial Capability of PEDOT Derivative Nanohybrid Coatings*

The anti-bacterial capability of PEDOT derivative nanohybrid coatings was evaluated using *S. aureus* as a model bacterium. As compared with other anti-bacterial tests, the zone of inhibition testing is a rapid and inexpensive test for anti-bacterial activity. Nevertheless, the zone of inhibition testing is a qualitative test to inhibit the growth of tested bacteria. In this study, the anti-bacterial capability was investigated by measuring the inhibition zone incubated on a nutrient agar plate, as shown in Figure 9. When the zone of inhibition shows on the nutrient agar plate, the results displayed that the tested bacteria were susceptible to the PEDOT derivative nanohybrid coatings. The pristine The anti-bacterial capability of PEDOT derivative nanohybrid coatings was evaluated using *S. aureus* as a model bacterium. As compared with other anti-bacterial tests, the zone of inhibition testing is a rapid and inexpensive test for anti-bacterial activity. Nevertheless, the zone of inhibition testing is a qualitative test to inhibit the growth of tested bacteria. In this study, the anti-bacterial capability was investigated by measuring the inhibition zone incubated on a nutrient agar plate, as shown in Figure 9. When the zone of inhibition shows on the nutrient agar plate, the results displayed that the tested bacteria were susceptible to the PEDOT derivative nanohybrid coatings. The pristine SUS316L substrate, and the PEDOT/PSS and PEDOT/GO nanohybrid coatings, showed the absence of an inhibition zone. However, a significant inhibition zone (7 mm) on the PEDOT/GO/PDDA nanohybrid coating was observed. This is due to the positive charge of PDDA absorbed on the PEDOT/GO substrate to create the positive charge surface of the PEDOT/GO/PDDA substrate, which can inhibit the growth of the bacteria. Compared with our previous studies [24,33], the negative charge of PEDOT derivatives (PEDOT/PSS, PEDOT/GO, PEDOT/Heparin, PEDOT/ chondroitin sulfate, and PEDOT/carboxymethyl-hexanoyl chitosan) can suppress the adhesion of the negative charge of proteins, platelets, and bacteria to form the anti-fouling surface. However, it would be an anti-bacterial surface

after immobilizing the positive charge of PDDA on the PEDOT/GO substrate. Therefore, the inclusion of PDDA in the PEDOT/GO/PDDA substrate can effectively diffuse and then kill bacteria, as shown as the larger inhibition zone [6]. charge of proteins, platelets, and bacteria to form the anti-fouling surface. However, it would be an anti-bacterial surface after immobilizing the positive charge of PDDA on the PEDOT/GO substrate. Therefore, the inclusion of PDDA in the PEDOT/GO/PDDA substrate can effectively diffuse and then kill bacteria, as shown as the larger inhibition zone [6].

sulfate, and PEDOT/carboxymethyl-hexanoyl chitosan) can suppress the adhesion of the negative

*Polymers* **2020**, *12*, 1467 10 of 12

SUS316L substrate, and the PEDOT/PSS and PEDOT/GO nanohybrid coatings, showed the absence of an inhibition zone. However, a significant inhibition zone (7 mm) on the PEDOT/GO/PDDA nanohybrid coating was observed. This is due to the positive charge of PDDA absorbed on the PEDOT/GO substrate to create the positive charge surface of the PEDOT/GO/PDDA substrate, which can inhibit the growth of the bacteria. Compared with our previous studies [24,33], the negative

**Figure 9.** Anti-bacterial capability (inhibition zone) of (**a**) the SUS316L substrate, and (**b**) PEDOT/PSS, (**c**) PEDOT/GO, and (**d**) PEDOT/GO/PDDA substrates. **Figure 9.** Anti-bacterial capability (inhibition zone) of (**a**) the SUS316L substrate, and (**b**) PEDOT/PSS, (**c**) PEDOT/GO, and (**d**) PEDOT/GO/PDDA substrates.

#### **4. Conclusions 4. Conclusions**

This study successfully fabricated PEDOT derivative nanohybrid coatings on a flexible SUS316L stainless steel substrate by electrochemical polymerization for improving the anti-bacterial and antifouling capabilities. Via the addition of hydrophilic derivatives, these PEDOT derivative nanohybrid coatings decrease the water contact angle and surface roughness to form a hydrophilic surface. In addition, the negatively charged surface of the PEDOT/GO nanohybrid coating can be further modified by the electrostatically absorbed process of the positively charged PDDA. PEDOT/PSS and PEDOT/GO nanohybrid coatings with lower surface roughness and negative charge led to less adsorption of bacteria (*S. aureus*). By comparison, the positively charged PEDOT/GO/PDDA nanohybrid coating inhibited the growth of bacteria, resulting in excellent anti-bacterial capability. Therefore, the electrochemically polymerized PEDOT derivative nanohybrid coatings can provide an anti-fouling and anti-bacterial surface, which offers a straightforward and rapid method to develop anti-bacterial and anti-fouling coatings for medical devices. This study successfully fabricated PEDOT derivative nanohybrid coatings on a flexible SUS316L stainless steel substrate by electrochemical polymerization for improving the anti-bacterial and anti-fouling capabilities. Via the addition of hydrophilic derivatives, these PEDOT derivative nanohybrid coatings decrease the water contact angle and surface roughness to form a hydrophilic surface. In addition, the negatively charged surface of the PEDOT/GO nanohybrid coating can be further modified by the electrostatically absorbed process of the positively charged PDDA. PEDOT/PSS and PEDOT/GO nanohybrid coatings with lower surface roughness and negative charge led to less adsorption of bacteria (*S. aureus*). By comparison, the positively charged PEDOT/GO/PDDA nanohybrid coating inhibited the growth of bacteria, resulting in excellent anti-bacterial capability. Therefore, the electrochemically polymerized PEDOT derivative nanohybrid coatings can provide an anti-fouling and anti-bacterial surface, which offers a straightforward and rapid method to develop anti-bacterial and anti-fouling coatings for medical devices.

**Author Contributions:** Conceptualization, C.-C.H. and T.-Y.L.; Data curation, C.-C.H., C.-C.L. and X.-Y.P.; Funding acquisition, T.-Y.L. and M.-C.Y.; Investigation, Y.-W.C., C.-C.L. and X.-Y.P.; Methodology, Y.-W.C. and T.-Y.L.; Validation, C.-C.H. and M.-C.Y.; Formal analysis, C.-C.L. and Y.-W.C., Visualization, C.-C.H.; Project administration, T.-Y.L.; Supervision, T.-Y.L., C.-C.H. and M.-C.Y.; Resources, T.-Y.L. and M.-C.Y.; Writing original draft, C.-C.H., Y.-W.C., C.-C.L., M.-C.Y. and T.-Y.L. Writing—revised manuscript, C.-C.H., Y.-W.C., M.- **Author Contributions:** Conceptualization, C.-C.H. and T.-Y.L.; Data curation, C.-C.H., C.-C.L. and X.-Y.P.; Funding acquisition, T.-Y.L. and M.-C.Y.; Investigation, Y.-W.C., C.-C.L. and X.-Y.P.; Methodology, Y.-W.C. and T.-Y.L.; Validation, C.-C.H. and M.-C.Y.; Formal analysis, C.-C.L. and Y.-W.C., Visualization, C.-C.H.; Project administration, T.-Y.L.; Supervision, T.-Y.L., C.-C.H. and M.-C.Y.; Resources, T.-Y.L. and M.-C.Y.; Writing—original draft, C.-C.H., Y.-W.C., C.-C.L., M.-C.Y. and T.-Y.L. Writing—revised manuscript, C.-C.H., Y.-W.C., M.-C.Y. and T.-Y.L. All authors have read and agreed to the published version of the manuscript.

C.Y. and T.-Y.L. All authors have read and agreed to the published version of the manuscript. **Funding:** This study was financially supported by the Taiwan Association of Cardiovascular Surgery Research, Research Center for Intelligent Medical Devices of Ming Chi University of Technology and Ministry of Science and Technology of Taiwan (MOST 108-2622-E-131-002-CC3; MOST 108-2218-E-002-010).

**Conflicts of Interest:** The authors declare no conflict of interest.
