Hydrogel coating as a method for solid substrate surface modification, beside advantages like improving material biocompatibility, hydrophilization and lubrication, brings the additional possibility of the active agent incorporation. Combination of the hydrophilic matrix and the hydrophobic drug seems to be especially promissing.

We have developed a method for polymer coating by water insoluble hydrogel, based on polyurethane (PUR) and polyvinylpyrrolidone (PVP), designed for medical polymeric devices. This hydrogel layer was characterized by the means of the Fourier Transform Infra-Red Attenuated Total Reflection (FTIR-ATR) spectroscopy, static and kinematic friction factor relative to the uncoated backbone material and against porcine tissue counter-face, water wetting angle and microscopic observations.

Our tests confirmed changes in surface composition, super-hydrophilicity and enormous lubricity in hydrated state (even 10-fold friction factor reduction). In case of urethral poly(vinyl chloride) catheters with hydrogel coated inner surface the capillary action phenomenon was observed, proving high affinity between coating and water molecules. Experiments with Escherichia coli biofilm growth on unmodified backbone material and hydrogel coated one revealed a significant decrease in a number of adhered bacterial colonies.

Hydrogel modified surface have another advantage over unmodified one, while it can serve as a drug reservoir for a local drug delivery. There are cases when drug dosage time should last at least few hours, but no longer than 3 days. It can be desirable in case of implantation of devices like tracheotomy tubes, when anti-inflammatory active substance should be released at the very beginning to prevent later side effects like tracheal stenosis,  but later can not interrupt normal cell divisions in subsequent levels of healing process and epithelium formation. In case of hydrophilic matrix with hydrophobic drug these profile can be easy obtained due to its behavior as the swelling controlled drug release system.

In further investigations we used antibacterial triclosan and anti-inflammatory dexamethasone as model hydrophobic drugs, which were incorporated in two modes: as a component of the solution in any step of the coating formation or through the additional impregnation bath. The recent mode allows also for modification of devices like silicone catheters or polyglycolic acid resorbable sutures without polymeric coating step.

The effects of active agent incorporation were then verified through drug dissolution tests to the phosphate buffered saline (PBS) with 20% methanol or ethanol, extraction, and, in case of germicidal drug, by the inhibited growth zones method. The dependency of the rate of drug dissolution and the load capacity, as well as coating stability on the process parameters was investigated.

We also observed an interesting “spraying effect” during hydrogel swelling, when solid microparticles of the drug were precipitaded out of the coating layer to the surrounding solution. This phenomenon can be utilized in design of drug release systems, reacting on water content increase as the start signal.

Fig. 1. The capilary action: left - hydrogel coated inner surface, right - unmodified (PVC catheters, outer diameter 5 mm; distilled H₂O).

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