# Impact of Nitric Oxide on the Surface Properties of Selected Polymers
## 1 Introduction  
Extracorporeal membrane oxygenation (ECMO) is a clinical technique used to support patients in critical conditions such as heart and lung transplantations or acute respiratory distress syndrome (ARDS). Blood is taken from the patient's venous system, pumped through a so-called oxygenator, and then infused back into the patient's venous or arterial system [1, 2]. One of the common issues of such a system, particularly in cases of long-term patient support, is the formation of thrombi in the ECMO system [3, 4]. Progression in the size of the formed thrombi can ultimately lead to a failure of the ECMO circuit [5, 6]. Replacing the ECMO circuit causes a gap in patient support, blood loss, and blood dilution, increasing the risk for already compromised patients [3, 7]. To suppress the thrombus formation and increase the life span of the ECMO circuit, the addition of nitric oxide (NO) could be a promising pathway. NO is a reactive molecule that occurs naturally in the human body and is part of the blood clotting mechanism as it is released by aggregated blood platelets to limit further aggregation [8, 9].  
Research investigating the chemical impact of NO treatment on the polymers used in the oxygenator was previously published by some of the authors of this paper [10].  
Using Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopy, the formation of several functional groups was elucidated. NO reacted with the surrounding air to form NO\({}_{2}\), which reacted with the air humidity to form nitric acid (HNO\({}_{3}\)). Both of these compounds reacted with the polymers to form nitro and nitrate ester groups. HNO\({}_{3}\) was also adsorbed on the polymer surfaces [10].  
This communication is meant to further expand the understanding of the interaction between NO and the polymer surfaces investigated in the previous paper. Zeta-potential and dynamic contact-angle measurements were conducted to elucidate the surface properties of the materials and the impact of NO on them. Polymethyl pentene (PMP), methyl methacrylate acrylonitrile butadiene styrene (MABS), and polyurethane (PU) were chosen because all three materials are in contact with blood during the application of the oxygen-guerator (Figure 1). The FTIR-ATR experiments discussed in this communication aimed to expand on the stability measurements under dry conditions discussed in the previous paper [10]. The experiments discussed here were performed to quantify the influence of liquid contact on the effects of NO treatment. The possibility of analyzing both the hydrophilicity and the zeta-potential is of practical interest in this context, as the surface properties of the polymers could influence their blood contact properties [1; 11]. The overall goal of the research presented in this and the previous paper [10] is to enable the use of NO as an additive to the sweep gas of an ECMO circuit to suppress thrombus formation and increase its longevity. These increases have historically been achieved through substantial modifications to the oxygen- and the pump both in terms of materials and geometry [12; 13; 14]. The addition of coagulation suppressants such as heparin or argatroban has been another method of achieving this increased longevity [12; 15]. The addition of NO to the sweep gas would, in contrast, represent a simpler way to increase the longevity of the ECMO circuit, requiring little to no modification to the circuit itself [16]. At the same time, NO would allow for a more continuous addition of the coagulation suppressant than compounds like heparin. NO has been trialed in ECMO circuits and other implants, being released from specialized materials. The results with these materials were positive overall with significant improvements to the thrombotic properties [17; 18]. The use of NO as a sweep gas additive has so far produced inconclusive results that warrant further research [19; 20].  
Figure 1: _Cont._