Medical Catheters and Plastics – Part V
In continuation of this series on the utilization of plastics in medical catheters, we will examine another crucial element of catheter design: the frictional and surface characteristics of catheters.
The coefficient of friction (COF) is the primary physical property used to indicate how easily a catheter can navigate through blood vessels. Additionally, the COF is relevant for assessing the ease of inserting other devices through the catheter shaft. A lower COF signifies enhanced lubricity in catheters. The study of friction, abrasion, and lubrication is referred to as tribology, and a tribometer is the device employed to measure friction.
Friction measurement is conducted through empirical methods; a tribometer assesses the frictional values of a reference surface against the surface requiring measurement. The sliding resistance between two materials is typically utilized to determine the coefficient of friction. The COF can be expressed as:
The coefficient of friction (COF) values are influenced by the materials utilized. In the context of catheter interactions with blood vessels, COF values around 0.04 suggest a low insertion force, while values exceeding 0.2 indicate a high insertion force.
Poly tetrafluoroethylene (PTFE) is well-known for having the lowest coefficient of friction among polymers typically employed in medical devices. Nevertheless, PTFE may lack other desirable characteristics such as elastic modulus and thermal processability that are advantageous for catheter design; therefore, it is often used as a liner in catheters rather than as the primary construction material.
The use of coatings on catheters is regarded as an additional method to enhance their surface properties. In addition to increasing lubrication, coatings can serve various other functions to improve the surfaces of catheters, thereby enhancing their interaction with the biological environment.
Coatings are frequently regarded as solutions to the inconsistency in the surface characteristics of catheters. The exploration of coating processes on polymers began in the 1950s, with the earliest patent for hydrophilic coating published in 1956. Although this patent did not specifically address applications in medical devices, it was significant as it outlined the fundamental chemistry of coating processes. This foundational chemistry was subsequently elaborated upon through various research efforts in the field of polymeric coatings, particularly hydrophilic coatings.
The polymeric systems utilized for catheter coatings, which were patented between the 1960s and 1980s, included polyvinylpyrrolidone (PVP), polyurethanes, polyacrylic acid (PAA), polyethylene oxide (PEO), and polysaccharides. Many of these systems remain in use today, albeit with minor modifications in their chemical composition. The original patent from DuPont introduced a two-layer system, where a bonding layer is initially applied to the substrate to ensure reliable adhesion for a topcoat. Subsequent research distinguished this fundamental technology into heat-cured and photo-cured coatings, and also examined the application of coating systems with either a single layer or a two-layer configuration consisting of a bonding layer and a topcoat.
In addition to lowering the coefficient of friction, the anti-fouling characteristics of the coating play a crucial role in its overall effectiveness. The adsorption of proteins is one of the initial reactions that occurs when a foreign object is introduced into the body. In many cases, protein adsorption is regarded as a precursor to blood clotting and thrombosis. Consequently, if the coating can effectively minimize protein adsorption, it will subsequently decrease the likelihood of thrombosis associated with the inserted catheter. Reducing the tendency for proteins to adhere to the surface is a fundamental strategy for developing a non-thrombogenic material and an effective anti-fouling coating. Initially, these coatings were applied to guidewires, but their use is expanding to include introducers and catheters. Some hydrophilic coatings also incorporate heparin, which possesses anti-thrombogenic properties by facilitating a reaction between antithrombin and thrombin, thereby influencing clotting by diminishing the formation of fibrin protein.
Current coating designs now incorporate drug delivery systems within the coatings. A widespread issue encountered in various medical procedures is the occurrence of operation-related bacterial infections. For instance, central venous catheters (CVCs) and peripherally inserted central catheters (PICCs) pose a significant risk of causing severe infections such as sepsis, with catheter infection rates reaching 5.3 per 1,000 catheter days. As a result, there is a strong initiative to integrate antimicrobial materials into hydrophilic coatings, which can introduce specific challenges based on the coating system employed. One successful approach involves embedding antimicrobial drugs within the coating. These anti-infection agents gradually release into the body, helping to avert the onset of infections post-procedure. Each combination of antimicrobial agents and catheter coatings has distinct requirements influenced by the chemical interactions among the components in the system. Therefore, it is essential to conduct comprehensive testing and validation of each system prior to clinical application. The antimicrobial agents utilized in contemporary catheter systems typically consist of silver compounds, chlorhexidine, and a range of antibiotics, including minocycline and rifampin in various combinations. Catheters infused with antimicrobial properties have demonstrated a reduction in infection rates. Nevertheless, the efficacy of this technology is contingent not only on the specific drug combinations employed but also on the conditions of application, the nature of tissue contact, and the duration of the implant.
An additional element influencing the efficacy of anti-infection medications and their release is the development of a biofilm on the implant's surface. A biofilm is characterized as a cluster of microorganisms where cells adhere to one another and to the surface. This biofilm acts as a barrier against the antimicrobial agents. Consequently, a contemporary strategy in hydrophilic coating technology involves creating surfaces that prevent biofilm formation and bacterial adhesion. By altering the surface with particular chemical species and charges, the adsorption of proteins can be postponed, which may directly or indirectly influence bacterial attachment to the protein layer on the surface.
This intervention interrupts the colonization process, and by maintaining low bacterial counts in the vicinity, the formation of biofilm can be minimized or postponed. The initial generation of coated medical devices, especially catheters, has successfully produced catheters with enhanced lubricity, improved biocompatibility due to their anti-fouling characteristics, and increased durability for their intended uses. The forthcoming generation of hydrophilic technologies is expected to provide improved functionalities, including drug-delivery systems and the inhibition of biofilm formation. As medical devices continue to evolve, there is a growing recognition of the challenges that future coating technologies must address. Future coatings will not only preserve the functionalities of earlier technologies but will also enhance the biomaterial surface to facilitate specific interactions between materials and tissues, enabling various cell types to colonize the device surface at different sites.
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