## 3 Results and Discussion
Theoretically, the amount of drug release profile for vancomycin with the degradation of PLGA is higher than that for ampicillin due to it having a lower PKA value. Therefore, the amount of drug release for sample (A) is the highest due to the maximum amount of vancomycin loaded on sample (A). However, they showed different results. Therefore, the short-term release profiles and the cumulative release profiles for total amounts of antibiotic agents (vancomycin + ampicillin salts) into the buffer solution for samples (A)-(E) are shown in Figure 3,II), respectively. Total amounts of drug released from the mixtures of ampicillin and vancomycin (samples (B)-(D)) in the buffer solution were higher than pure vancomycin (sample (A)) and ampicillin (sample (E)). From the results shown in Figure 3, the amount of drug in the buffer solution for sample (A) is higher than that for sample (E), which agrees well with the pKa values for vancomycin and ampicillin [25; 26]. A lower pKa value results in faster degradation of PLGA and releases more drug into the buffer solution. With the decrease in the loading amount of vancomycin in the samples (samples (B)-(D)), the released amount of drug in the buffer solution would also decrease if only the Pka factor influences the degradation of PLGA. However, sample (D) had the maximum released amount of antibiotic agent in the buffer solution, which indicated that other reasons affected the degradation of PLGA. The molecular weight for ampicillin (373.39 g/mol) is much smaller than that for vancomycin (1449.25 g/mol). Total moles of the antibiotic agents for sample (D) are larger than samples (C) and (B). Although the pKa value for vancomycin reported is lower than that for ampicillin, the difference between the values of pKa for vancomycin and ampicillin is not too much larger. Because sample (D) had the highest mole value for antibiotic agents compared to the other two samples (samples (B) and (C)), it indicated that the number of acidic groups for sample (D) was higher than other two samples, which resulted in the fast degradation of PLGA and resulted in larger amounts of drug released from sample (D) into the buffer solution compared with other samples. Figure 3 also showed cumulative release profiles for the total amount of antibiotic agents released from samples (A)-(E) into the buffer solution. Around 7-10% of the total amount of antibiotic agents released from samples (A)-(C) and (E) into the buffer solution was observed in the one-day test. Around 30% of the total amount of antibiotic agents released from sample (D) into the buffer solution was observed in the one-day test, which indicated that the long-term antibacterial performance for sample (D) may be worse than the other samples. In order to test long-term antibacterial performances of the samples, Figure 4,II shows the long-term release profiles and the cumulative release profiles for the total amount of antibiotic agents in the buffer solution for samples (A)-(E). Figure 3: _Cont._  
Figure 4: (**I**) Long-term release profiles and (**II**) the cumulative release profile for the total amount of antibiotic agents in the buffer solution for samples (A)–(E). Figure 3: (**I**) Short-term release profiles and (**II**) the cumulative release profiles for the total amount of antibiotic agents released from PEEK into the buffer solution for samples (A)–(E). Detailed drug release profiles for vancomycin and ampicillin salts in the buffer solution in the 30-day test are also given in Figure S4I,II, respectively. The values of MIC 50 and MIC 90 of _S. aureus_ for vancomycin and ampicillin reported in the literatures are also given in Figure 4I and Figure S4[27; 28; 29]. For the results shown in Figure S4I, the concentrations for vancomycin salts released from samples (A)-(D) in the 30-day test were almost higher than the value of MIC 90 for _S. aureus_. However, the concentration of vancomycin released from the samples (C) and (D) approached the value of MIC 90 for _S. aureus_ on the 30 th and 29 th day, respectively.