For installing the electrode lead, two catheters are used, namely an inner catheter and an outer catheter. The outer catheter is used to blaze the trail towards the heart. In order to further adapt to the anatomic characteristics in proximity of the electrode placement, an inner catheter, due to its smaller diameter, can be driven through the outer catheter (see Figure 1). The radius of curvature of the inner catheter can be chosen specifically to fit the targeted place at the heart tissue. Once the catheters are placed, the electrode lead can be driven through the inner catheter. After the electrode lead has been placed, the catheters are slit away and removed from the patient whereas the electrode lead remains in the human body. This is known as the slit-away technique. The following two processes are discussed here from a tribological point of view: The first process is pushing the inner catheter through the outer catheter (see Figu- Aus der Praxis für die Praxis 36 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 1 Introduction Pacemakers are used to regulate the heart rate of patients. They are placed in proximity to the clavicle, whereas the electrodes are connected to their respective positions at the myocardium. The electrodes and the pacemaker are connected via an electrode lead which is placed in the Vena cava superior. Catheters are used to install the electrodes during surgery, when the pacemaker is inserted. Heart catheters are available in different diameters and radii of curvature to best adapt to the anatomy of the patient. Polyether block amide, known as Pebax, is one of the materials used to make these catheters. A Methodology for the Tribological Characterization of Hydrogel-Coated Catheters for Cardiovascular Applications F. Rummel, K. S. Pondicherry, M. Ruge, V. Fronk* Ein Verständnis des tribologischen Verhaltens ist wesentlich für die Funktionstüchtigkeit und Sicherheit von Medizinprodukten wie beispielsweise Kathetern, Implantaten oder anderer medizintechnischer Geräte. Eine steigende Nachfrage nach Medizinprodukten und deren zunehmende Vielfalt machen ein tiefgreifendes Verständnis der jeweiligen Applikationen erforderlich. In dieser Studie wird eine Methodik zur tribologischen Charakterisierung von mit Hydrogelen beschichteten Medizinprodukten wie beispielsweise Herzkathetern vorgestellt. Das Entwerfen von Modellsystemen für die experimentellen Untersuchungen von Folien mit Hydrogelbeschichtung und deren Charakterisierung auf einem Pin-on-Disk Tribometer werden vorgestellt. Ergebnisse aus Beispielmessungen mit unterschiedlichen beschichteten Folien werden gezeigt und durch Ergebnisse aus Zetapotentialmessungen ergänzt. Schlüsselwörter Medizintechnik, Katheter, Herzschrittmacher, Hydrogele, Biotribology, Zetapotential Understanding the tribological behavior of medical devices is crucial for functionality and safety of applications such as catheters, implants and other medically engineered devices. An increasing demand and an increasing variety in medical devices require deep understanding of the respective applications. Within this study, a methodology which allows for the tribological characterization of hydrogel-coated devices such as heart catheters is introduced. It is explained how model systems for experimental testing on hydrogel-coated foils can be designed and how they can be characterized on a modified pin-on-disc setup. Results from example measurements with different coated foils are presented and the tribological study is complemented by Zeta potential measurements. Keywords Medical Engineering, Catheters, Pacemakers, Hydrogels, Biotribology, Zeta Potential Kurzfassung Abstract * Florian Rummel, Dipl.-Ing. (TUM) Anton Paar Germany GmbH, 73760 Ostfildern, Germany Dr. Kartik S. Pondicherry Anton Paar GmbH, 8054 Graz, Austria Markus Ruge, Bachelor of Engineering Externer Berater, 10715 Berlin, Germany Vanessa Fronk, Master of Science Anton Paar Germany GmbH, 73760 Ostfildern, Germany T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 36 re 3). Here, the outer surface of the inner catheter is moved relatively to the coated inner surface of the outer catheter. This will be referred to as tribosystem B. The second process is to push the electrode lead through the inner catheter (see Figure 2). Here, the electrode lead is moved relatively to the coated inner surface of the inner catheter. This will be referred to as tribosystem A. An overview of the tribosystems is given in Table 1. In both scenarios, smooth sliding at the respective interfaces is required. This can be achieved by using hydrophilic coatings such as polyvinylpyrrolidone (PVP), in the form of a hydrogel. PVP coatings can be applied to a surface by sprayor dip-coating process and cured using ultraviolet (UV) light. The PVP coating needs to be activated prior to the medical intervention by applying water on the coating surface. There are two main goals in terms of product safety and usability that need to be achieved when developing and applying such coatings in heart catheters: • Low friction at the catheter interfaces • Reduced wear vulnerability The aim of this study is to present an experimental test methodology which enables characterization of the tribological behavior of hydrophilic-coated catheter interfaces at model scale using an MCR Tribometer. 2 Experimental Setup and Specimen The tribological tests were carried out on an MCR Tribometer equipped with a tribology cell with Peltier temperature control. The test setup and experimental conditions were set in an order to best simulate the real-world application. The inner surface of the catheter is represented by the PVP-coated side of a Pebax foil in the model systems. The foils being used differ slightly in their composition as well as in the process parameters applied during UV curing (see Table 2). The PVP-coated Pebax foil was cut into round sheets measuring approximately 60 mm in diameter and fixed on a 1.4301 steel disc using double-sided tape. The disc was Aus der Praxis für die Praxis 37 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 Figure 1: Tribosystems in heart catheter applications. Tribosystem A: electrode lead and inner catheter. Tribosystem B: inner catheter and outer catheter. Figure 2: Tribosystem A: The electrode lead is moved relatively to the inner catheter which is equipped with a hydrophilic coating. Figure 3: Tribosystem B: The inner catheter is moved relatively to the outer catheter which is equipped with a hydrophilic coating. Table 1: Counter-bodies in real-world tribosystems. Tribosystem A Tribosystem B Inner real-world Electrode lead Inner catheter counter-body outer surface Outer real-world Inner catheter Outer catheter counter-body inner surface inner surface T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 37 Pebax foil. The two coatings differ slightly in the curing time applied during the UV curing process. An additional test was carried out with an uncoated foil to assess the extent of the lubricating effects of hydrogel coatings. Aus der Praxis für die Praxis 38 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 then fixed in the lower sample holder (see Figure 4). This study is divided into two parts. The first part deals with tribosystem A and the second deals with tribosystem B. 2.1 Zeta Potential of Inner Catheter Surfaces Zeta potential represents the surface charge, which occurs in the presence of an aqueous solution when reactive (functional) groups dissociate on hydrophilic surfaces or water ions adsorb onto hydrophobic surfaces. Varying the pH values of the aqueous phase influences the equilibrium between dissociation and adsorption processes, giving insights into the chemical behaviour of the surface. The streaming potential is measured to determine the zeta potential of macroscopic solid surfaces when an aqueous solution is set to flow across the solid surface under defined pressure conditions. In the measuring cells for samples with planar surfaces, a defined gap is set between two opposing sample surfaces. During the measurement the liquid (1 mM potassium chloride solution) flows through this gap and produces a pressure gradient and a charge separation at the solid/ liquid interface. The streaming potential is the electrical response to the shift in the surface charge. The results in Figure 5 show that for the uncoated foil, the zeta potential is about -40 mV at the pH close to the physiological pH (7.4). This indicates poor wetting behavior and is typical for a hydrophobic surface. For both variants of the coated foil, the zeta potential is relatively higher, indicating a better wetting behavior. The isoelectric point is the pH value at the point where the zeta potential equals zero. The isoelectric point is at similar pH values for all three variants of foils. 2.2 Tribosystem A: Electrode Lead and Inner Catheter Tribosystem A consists of the outer surface of the electrode lead and the inner surface of the inner catheter, coated with the hydrogel. The electrode lead is usually made of silicone. In the model system, the electrode lead material is represented by a polydimethylsiloxane (PDMS) specimen. The PDMS specimen were made of Sylgard 184 (Dow Corning) with a PDMS to curing agent mixing ratio of 8: 1. The curing was carried out in a furnace for 1 hour at 70 °C. The cylindrical specimen measures 6 mm in diameter and length, with one rounded end having a radius of curvature of 3 mm (see Figure 6). The inner surface of the inner catheter is represented by a PVP-coated Table 2: Characteristics of the coatings. Coating Parameter characteristic Coating 1 shorter curing time Tribosystem A Coating 2 longer curing time Coating 3 thinner coating Tribosystem B Coating 4 thicker coating Figure 4: Picture of the coated Pebax foil in the lower T-PID44 sample holder. Figure 6: Schematic of the rounded PDMS pins (left) and picture of the rounded PDMS pins (right). Figure 5: Zeta potential as a function of pH for uncoated (red) and coated catheter foils: coating 1 (blue), coating 2 (green). T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 38 For simulating tribosystem A, the rounded PDMS pins were fixed in the T-PID44 pin holder, leading to a roundon-flat geometry (see Figure 7). New PDMS pins and new Pebax foils were used for every test. PDMS pins undergo running-in prior to the test on the Pebax foil. The running-in step was carried out on a 1.4301 steel disc with 3 N of normal force and a constant rotational speed of 50 rpm for a period of 3 minutes. This contact was lubricated with distilled water. tribosystem A, the inner surface of the outer catheter is represented by a PVP-coated Pebax foil. The foils differ slightly in the PVP concentration in the hydrogel. For simulating tribosystem B, the Pebax discs were fixed in the adapters with grub screws causing them to have a slightly convex shape. The adapters were then mounted into the T-PID44 holder to obtain a round-on-flat geometry (see Figure 9). The Pebax specimen were cleaned with isopropanol and reused for the tests, whereas a fresh Pebax foil was used for every test. Aus der Praxis für die Praxis 39 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 Figure 7: Schematic of the pins-on-disc setup with PDMS pins (left) and picture of the rounded PDMS pins mounted into the pin holder (right). Figure 9: Schematic of the three-discs-on-disc setup with Pebax discs (left) and picture of the Pebax specimen fixed in the holder with an adapter for disc-shaped specimen (right). Figure 8: Schematic of the Pebax specimen (left) and picture of the Pebax specimen (right). 2.3 Tribosystem B: Inner Catheter and Outer Catheter In tribosystem B, the outer surface of the inner catheter is moved against the inner surface of the outer catheter, coated with hydrogel. Within the model system, the outer surface of the inner catheter is represented by the disclike specimen made of Pebax. The specimen were punched from Pebax plates (1 mm in height and 6 mm in diameter). A schematic drawing and a picture of the Pebax specimen is shown in Figure 8. Like in the case of T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 39 3.3 Extended Stribeck Curve Measurements An extended Stribeck curve measurement consists of two steps: 1. In the first step, the test load of 3 N is applied and kept for five minutes This is done to enable the system to recover from stresses experienced in the previous steps of the test. 2. The sliding velocity is increased logarithmically from 10 -8 m/ s to 1 m/ s. Each curve consists of 81 data points with logarithmically decreasing measurement point duration ranging from 10 s to 4 s. 4 Results and Discussion The first part of this section deals with the results from tribosystem A, followed by those from tribosystem B. Finally, results from both model systems are compared. 4.1 Tribosystem A: Electrode Lead and Inner Catheter 4.1.1 Extended Stribeck Curve Measurements A comparison between coated and uncoated foils in the form of extended Stribeck curves is shown in Figure 11. It can be seen that the friction factor with coated foils is approximately 2 orders of magnitude lower than that of the uncoated foils. In addition, the extended Stribeck curve for the coated foils is smooth almost over the entire sliding velocity range. The uncoated foil, on the other hand, shows a hectic behavior at sliding velocity in the range of 0.001 m/ s to 0.1 m/ s. This could be explained by the poor wetting behavior of the uncoated foil leading to starvation at the PDMS/ foil interface. Aus der Praxis für die Praxis 40 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 3 Experimental Procedure Insertion of heart catheters can be described by the following characteristics and demands: • The procedure takes place with different speeds ranging from no relative motion between the catheter interfaces to sliding velocities of a few centimeters per second. • The PVP coating is supposed to stay intact at the foil surface during the intervention. Therefore, it is also of interest to observe how the tribological behavior of the tribosystems changes during the tests. Changes in the PVP coatings during tribological testing can be relevant indicators for robustness. Therefore, the running-in behavior of the tribosystems is discussed within this study. 3.1 Overview of the Test Procedure Prior to starting the measurement, the PVP coating is activated by adding 1 ml of 0.9 % physiological solution (B. Braun, Melsungen, Germany). The water is distributed evenly over the entire foil surface using the cleaned backside of a Pasteur pipette. Before starting the test, the measuring shaft is slowly lowered until the upper specimen touches the PVPcoated Pebax foil. The maximum normal force during this step is limited to 0.5 N to avoid shock-loading or sudden impacts on the specimen surfaces. Then the test load is applied, and the breakaway torque and extended Stribeck curve measurements are carried out. This procedure is repeated three times without breaking contact. Each repetition is called a run (see Figure 10). The breakaway torque measurements and the extended Stribeck curve measurements are explained in detail in section 3.2 and 3.3, respectively. 3.2 Breakaway Torque Measurements Measuring the transition from static to dynamic friction is realized by breakaway torque measurements. Here, the rotational torque is gradually increased until the surfaces start to slide against each other. The breakaway torque measurement consists of the following two steps: 1. The load is set to the test load of 3 N. The system is held at test load for five minutes to facilitate partial relaxation of the stresses caused by the freshly applied load. 2. The rotational torque is increased logarithmically from 0.2 mN·m. Event control is implemented to stop the measurement as soon as the system has been set into macroscopic motion. Figure 10: Schematic of the experimental procedure. The test consists of three runs. T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 40 The 1 st and the 3 rd runs from the extended Stribeck curve measurements with coating 1 are shown in Figure 12. It can be seen that in the 3 rd run (open symbols), the friction factor at small sliding velocities (8 ·10 -7 m/ s to 1 ·10 -4 m/ s) tends to higher values compared to the 1 st run (closed symbols). At sliding velocities in the range of 10 -4 m/ s to 10-2 m/ s, the frictional behavior shows somewhat hectic characteristics. Compared to coating 1, the frictional behavior for coating 2 is even smoother in the sliding velocity range of 10 -4 m/ s to 10 -2 m/ s (see Figure 13). Comparing the absolute friction factor values for coating 1 and coating 2, there is no significant difference in the 3 rd run, whereas in the 1 st run, coating 1 tends to have lower frictional resistance. For coating 2, there is no significant difference between the 1 st and the 3 rd run. This may indicate that coating 2 is less affected by tribological testing. This is in agreement with the curing process applied: The UV curing time for coating 2 was longer than that for coating 1, and hence, coating 2 is expected to be more stable. 4.1.2 Breakaway Torque Measurements The breakaway point is identified as the point of inflection of the curve. This point is marked with an arrow in the breakaway torque plots in Figure 14 and Figure 15. For coating 1, the breakaway point is clearly shifted to higher torques in the 3 rd run compared to the 1 st run (Figure 14). For coating 2, the1 st and 3 rd run breakaway torque measurements do not show any significant difference (see Figure 15). This is in accordance with observations of the extended Stribeck curve measurements. Aus der Praxis für die Praxis 41 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 Figure 11: Extended Stribeck curve measurements with coated and uncoated Pebax foils. Results from 3 rd runs are shown. Figure 12: Extended Stribeck curve measurements with coating 1. Results from 1 st and 3 rd runs are shown. Figure 13: Extended Stribeck curve measurements for with coating 2. Results from 1 st and 3 rd runs are shown. Figure 14: Breakaway torque measurements with coating 1. Results from 1 st and 3 rd runs are shown. Figure 15: Breakaway torque measurements with coating 2. Results from 1 st and 3 rd runs are shown. T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 41 4.3 Comparing Tribosystem A and Tribosystem B The contact geometries of the model system with rounded PDMS pins and the model system with Pebax discs Aus der Praxis für die Praxis 42 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 4.2 Tribosystem B: Inner Catheter and Outer Catheter 4.2.1 Extended Stribeck Curve Measurements The extended Stribeck curves for coating 3 show a distinct change in the slope at very small sliding velocities (10 -7 m/ s to 10 -6 m/ s). This change indicates the transition from static to dynamic friction. Once the limiting friction is overcome, the system is set into macroscopic motion. Comparing 1 st and 3 rd runs of measurements carried out with coating 3, it can be seen that there is a significant increase in friction over a broad range of sliding velocities, covering the entire dynamic regime of the extended Stribeck curve (see Figure 16). Also for coating 4, an increase in friction over several decades of sliding velocity between 1 st and 3 rd runs was observed (see Figure 17). However, the increase is less pronounced compared to that of coating 3. The PVP concentration in coating 4 was higher. This might have led to a greater degree of interaction between the PVP domains in the hydrogel and, hence, a more robust hydrogel coating. This could serve as an explanation for the less pronounced change in the frictional behavior during tribological testing. 4.2.2 Breakaway Torque Measurements The breakaway torque is shifted to higher values when one compares the 1 st and 3 rd breakaway torque measurements with coating 3 (see Figure 18). For coating 4, the same trend as for coating 3 was observed (see Figure 19), although slightly less pronounced. However, the trend towards higher values for breakaway torques is less pronounced compared to those observed in the extended Stribeck curve measurements. Figure 16: Extended Stribeck curve measurements with coating 3. Results from 1 st and 3 rd runs are shown. Figure 17: Extended Stribeck curve measurements with coating 4. Results from 1 st and 3 rd runs are shown. Figure 19: Breakaway torque measurements with coating 4. Results from 1 st and 3 rd runs are shown. Figure 18: Breakaway torque measurements with coating 3. Results from 1 st and 3 rd runs are shown. T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 42 are different in terms of different radii of curvature and different specimen fixation. This makes it hard to compare the two tribosystems directly. Differences in the specimen material (both upper and lower specimen) complicate direct comparison. Nevertheless, one does observe differences in the behavior of individual tribosystems with tiny changes in the properties of the hydrogel coatings. 5 Conclusion and Outlook This study presents a possibility for differentiating between slightly different hydrogel-coated Pebax foils, which can be used in cardiovascular applications by means of model scale tribological testing. The comparison here lies in the changes in the behavior of the tribosystems during the tests, and not in the absolute values of friction coefficients. The approach presented in this study could be extended to different types of hydrophilic coatings like polyurethane, hyaluronic acid, poly acrylate [1], etc., as well as for other biomedical applications. Additionally, understanding of the effects of bodily fluids such as blood, gastric juice, etc. may be helpful for a better understanding of the processes occurring during medical examinations and surgeries with regards to interactions between engineered equipment and biological matter. It should be made clear that this document only presents a methodology as to how one could run tribological tests on coated Pebax foils. The intention of this article is not to work out differences between different types of coated Pebax foils and how these differences can affect the tribological performance of tribosystems where these coatings are being used. Acknowledgements: The authors would like to thank Georg Krenn (Anton Paar GmbH) for engineering the adapter for disc-like specimen. References [1] J. Simon, Hydrophilic coatings: Considerations for product development and choice, Technical White Paper, Biocat Inc., 2011. Aus der Praxis für die Praxis 43 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 Aktuelle Informationen über die Fachbücher zum Thema „Tribologie“ und über das Gesamtprogramm des expert verlags finden Sie im Internet unter www.expertverlag.de Anzeigen Nutzen Sie auch unseren Internet-Novitäten-Service: www.expertverlag.de mit unserem kompletten Verlagsprogramm, über 800 lieferbare Titel aus Wirtschaft und Technik T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 43