**1. Introduction**

The biocontamination of both urinary and central line catheters is one of the principal causes of nosocomial infections, mainly in patients who are in intensive care units [1,2]. One of the reasons for this is that these devices are made of polymeric materials that have a tendency towards microorganism contamination in biological environments. According to surveys carried out in different countries, it is estimated that one in seven hospitalizations presents an incidence of nosocomial infection, of which approximately 25% are associated with the use of medical devices [3–5]. The National Healthcare Safety Network (NHSN) reports eight types of microorganisms that cause the most nosocomial infections, among which, the following are prominent: *Staphylococcus aureus*, *Escherichia coli*, and coagulasenegative *staphylococci* [6].

Therefore, searching for materials that are resistant to bacterial contamination is relevant to the medical field [7–9]. A material can present resistance to contamination by microorganisms through two mechanisms [10]. The first mechanism consists of the incorporation of active agents into the material; it can be in its internal structure as groups of quaternary amines [11,12] or stored to be released at a site of interest. Among the most widely used active agents for release are antibiotics and silver or zinc metallic nanoparticles with antibacterial properties [13–15]. The second mechanism is based on the generation of materials whose surface prevents the adhesion of the microorganism and its proliferation. These materials generally owe their antifouling capacity to the formation of superficial hydration layers stabilized by van der Waals interactions or electrostatic interactions, as in the case of zwitterionic polymers. The development of materials with dual antimicrobial capacity, that is, materials capable of both preventing adhesion and releasing an active agent, is a challenge for materials science [16,17].

**Citation:** Duarte-Peña, L.; Magaña, H.; Bucio, E. Catheters with Dual-Antimicrobial Properties by Gamma Radiation-Induced Grafting. *Pharmaceutics* **2023**, *15*, 960. https://doi.org/10.3390/ pharmaceutics15030960

Academic Editor: Ana Isabel Fernandes

Received: 14 February 2023 Revised: 10 March 2023 Accepted: 14 March 2023 Published: 16 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Modified systems for the release of active agents are a point of interest in biomedicine because they can provide the optimal amount of drug at the right time and place, giving rise to a continuous release of therapeutic dose without reaching maximum levels, thus avoiding side effects caused by large drug discharges and concentrating the drug in the affected area [18,19]. Within these systems are smart polymers, that is, polymers that respond to external stimuli, such as temperature, pH, ionic strength, or light, by changing their structure, which allows more control over the load and release of active agents depending on the environmental conditions [20]. The poly 4-vinylpyridine (P4VP) is pH-sensitive polymer that undergoes protonation at a pH below its pKa. This leads to the formation of cations that repel each other thus increasing the distance between the chains of the material and changing its structure.

Zwitterionic polymers have a high antifouling capacity because their ion distribution allows them to create an electrostatically stabilized surface hydration layer, which significantly reduces bacterial adhesion to these surfaces, in addition to being highly hydrophilic systems [21–23]. However, the synthesis of these materials is limited by the low solubility of the polymer in the solvents commonly used for polymerization. Due to this, alternative techniques are used to obtain these materials, such as the functionalization of ionic polymers with an ion of opposite charge, to form the zwitterion in situ [24]. Hydrogels developed with this type of polymer have shown relevant antifouling properties and good biocompatibility [25–28].

This work presents the development of PVC catheters modified with a 4-vinylpyrinidine and zwitterionic polymer to provide their surface with antifouling capacity and pH sensitivity. This material constitutes a dual antimicrobial system that has the ability to load and release ciprofloxacin. This system allows the localized release of the antibiotic because the drug is stored in the device, which can help improve drug efficiency. The modification was carried out by graft polymerization of 4VP using gamma radiation as the initiator and subsequently a zwitterion was formed by the functionalizing of the grafted 4VP with 1,3-propane sultone (PS). The synthesized materials were characterized to determine their antimicrobial capacity. Materials with dual antimicrobial capacity have potential applications in the manufacturing of medical devices that can reinforce their prophylactic potential or even help treat infections. In this case, modified catheters represent an alternative device which can reduce the nosocomial infections associated with traditional catheter use.

### **2. Materials and Methods**

### *2.1. Materials*

PVC catheters (outer diameter 3 mm and thickness 0.5 mm) were from Biçakcilar (Istanbul, Turkey). 4VP (95%), 1,3-propane sultone (PS), and dimethylformamide anhydrous were purchased from Aldrich Chemical, Saint Louis, MO, USA. 4VP was purified by vacuum distillation to remove the inhibitor. Chloride (NaCl), potassium chloride (KCl), sodium phosphate dibasic (NaH2PO4), and potassium phosphate monobasic (KHPO4) were also purchased from Aldrich Chemical, Saint Louis, MO, USA; these materials were used as received. Ciprofloxacin was from Sigma Aldrich. Distillate water was used for all the assays. Software DDSolver from Excel was used for modeling drug delivery. The gamma-ray source was a 60Co Gammabeam 651-PT of Nordion International Inc from Ottawa, ON, Canada proportioned by the Nuclear Science Institute at Universidad Nacional Autónoma de México (ICN-UNAM).

### *2.2. Synthesis of PVC-g-4VP*

The 4VP graft on PVC was performed using the direct irradiation method, following the parameters used in previous studies to obtain graft percentages of 12 and 23%. A sample of PVC approximately 6 cm in length was placed in a glass ampoule, a solution of 4VP in H2O/MeOH was added, and oxygen was removed by air displacement with argon bubbling for 15 min. The sealed ampoule was kept at 5 ◦C for 4 h and irradiated using

gamma radiation. The grafted catheters were removed and cleaned with methanol. Finally, the samples were dried for 12 h at 30 ◦C in a vacuum oven, and the percentage of grafting was calculated by the difference in weight using Equation (1), where Wf is the weight of the grafted sample (g) and Wi is the weight of the sample without modification (g).

$$\text{Grafting (\%)} = (\text{W}\_{\text{f}} - \text{W}\_{\text{i}}) \times 100 / \text{W}\_{\text{i}} \tag{1}$$

### *2.3. Formation of PVC-g-4VP/4VPPS Graft by Functionalization*

A dry and weighed sample of PVC-*g*-4VP was placed in a glass ampoule and left under vacuum for 20 min. Then, a solution of PS in dimethylformamide anhydrous was added, the ampoule was sealed, and the solution was heated for a certain period of time. Finally, the modified material was removed, washed with methanol and water for 12 h, and dried at 30 ◦C under a vacuum for 12 h. PS reacts quickly with water, hydrolyzing to hydroxysulfonic acid, so the reaction must be carried out under anhydrous conditions. The reaction yield was calculated using Equation (2), where Mf is the final weight of the material, Mi is the initial weight of the material, and 4VP (%) is the percentage of 4VP grafting in the initial material.

$$\text{Reaction yield (\%)} = (\text{M}\_{\text{f}} - \text{M}\_{\text{i}}) \times 8607.7 \text{/(M}\_{\text{i}} \times 4 \text{VP(\%)}) \tag{2}$$

The effect of the different reaction conditions was studied, varying the temperature (50, 60, and 70 ◦C), the reaction time (30, 45, 60, and 75 min), and the concentration of PS (0.35, 0.5, 0.65, 0.8, 0.8, and 1 M).

### *2.4. Infrared Spectroscopy and Thermal Analysis*

Infrared spectroscopy was performed on a Perkin Elmer Spectrum 100 spectrophotometer (Perkin Elmer Cetus Instruments, Norwalk, CT, USA) with 16 scans, in the ATR module, in the range of 4000 to 650 cm−1. On the other hand, the thermal behavior was monitored by TGA under a nitrogen atmosphere from 30 to 700 ◦C at a heating rate of 10 ◦C/min using a TGA Q50 (TA Instruments, New Castle, DE, USA).

### *2.5. Swelling and Contact Angle*

For the swelling tests, a dry sample was weighed and placed in a glass with distilled water at 25 ◦C. Once removed from the beaker, excess solvent was removed from the sample and it was weighed every 5 min for the first 15 min and then at 0.5, 1, 2, 4, 6, and 12 h. The swelling percentage was determined using Equation (3), where W2 is the weight of the swollen sample and W1 is the dry sample weight.

$$\text{Swelling (\%)} = (\text{W}\_2 - \text{W}\_1) \times 100 / \text{W}\_1 \tag{3}$$

The contact angle provided information on the degree of wettability; this determination was measured using a DSA 100 Krüss GmbH, German goniometer from Hamburg, using the sessile drop method with water. The samples were split, flattened, using glass plates, and dried at 40 ◦C in a vacuum oven for 4 h. For the determination, a drop of distilled water was deposited on the flat surface, and the angle formed between the surface and the liquid was measured. All of the measurements were carried out six times.

### *2.6. pH-Responsiveness*

To determine the pH response of the samples, phosphate buffer solutions of pH 2, 3, 4, 5, 6, 8, 10, and 12 were prepared. A dry sample was weighed, and the solution with pH 2 was added, maintaining a controlled temperature at 25 ◦C for 2 h. Later the sample was removed, and the swelling percentage was calculated. The same procedure was used with the other solutions.
