$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
Artificial lung fibers, also known as hollow fiber membranes, are essential materials for fabricating extracorporeal membrane oxygenation (ECMO) devices that provide respiratory support to critically ill patients. Multiple layers of these fibers make up a dense bundle that serves as the gas exchange unit. The polymeric fiber surface, however, activates the blood coagulation cascade -leading to clot formation (thrombosis). Thrombosis on artificial surfaces is primarily driven by the activation of the coagulation cascade, a complex series of enzymatic reactions that lead to the formation of a blood clot. When blood comes into contact with foreign materials, such as those in medical devices (e.g., artificial lungs, stents, catheters), the coagulation cascade is triggered1,2. This process begins with the exposure of blood to the surfaces of the artificial material, which activates the intrinsic pathway of the cascade. This activation leads to the generation of thrombin, a key enzyme that converts fibrinogen to fibrin, forming the structural basis of a clot. Simultaneously, platelets are activated and aggregate at the site, further reinforcing the clot. The result is thrombosis, which can obstruct blood flow and lead to serious complications such as stroke or myocardial infarction.
To prevent thrombosis on artificial surfaces, traditional anticoagulants, such as heparin, warfarin, and newer direct oral anticoagulants (DOACs), are commonly used3,4. These medications work by interfering with various steps of the coagulation cascade. For example, heparin enhances the activity of antithrombin III, a natural inhibitor of thrombin, while warfarin inhibits the synthesis of vitamin K-dependent clotting factors. However, the use of anticoagulants presents several challenges. First, they increase the risk of bleeding, which can be life-threatening in certain situations. Second, the effectiveness of anticoagulants can be variable, requiring regular monitoring and dose adjustments, particularly with warfarin. Additionally, long-term anticoagulant use is associated with adverse effects such as osteoporosis and skin necrosis. The need for systemic anticoagulation also limits the use of medical devices in patients who are at high risk for bleeding.
Because thrombosis can impede gas exchange across the hollow fiber membrane, antifouling coatings have been applied to the lung fibers using various methods, such as dip coating and electrospinning, to prevent biofouling5,6. Artificial lung manufacturers typically process hollow fibers that are commercially obtained from fiber manufacturers and assemble them into lungs through steps including bundling of the fibers around a solid core, potting (gluing) bundle ends, incorporating potted bundles in a housing capsule featuring gas and blood flow channels, and post assembly cleaning. While the coating of fibers that have not been implanted in the lung can be more flexible, a surface modification at the pre-bundling stage will be subjected to several manufacturing steps that necessitate mechanical and chemical interactions between the surface coating and the downstream process environments, which can lead to denuded fibers in a device where high coating coverage is essential for limiting thrombosis. Alternatively, the coating can be applied to the potted bundle. An advantage of the ability to coat finished lungs is that it is a practical and facile modification approach to surface engineering the artificial lung device and many other devices. But in general, the method of applying coatings, whether through spray or dip coating, is less critical to thrombosis prevention than the effectiveness of the coating itself. For instance, hollow fibers used in medical devices can be dip-coated during extrusion, then knitted into mats, wound into bundles, and incorporated into a finished artificial lung device. Alternatively, coatings can be applied after the device has been manufactured. The key factors, however, are the effective application, durability, and efficacy of the anti-thrombotic coating5. This is because, in the absence of systemic anticoagulation, the function of these coatings is an essential piece of the puzzle for preventing clot formation, necessitating the need for a highly efficient and long-lasting antifouling property to ensure effective thrombosis prevention.
Despite the application of antithrombogenic coatings and the simultaneous low-dose administration of anticoagulants to date, the artificial lung module must be replaced only after a relatively short period of use, ranging from days to 3 weeks7,8, because of thrombosis. The gas exchange efficiency of their fiber membranes deteriorates after a relatively short time because of fouling by a membranous blood clot structure (composed of fibrin, single cells, and cell clusters) that covers large areas of the fibers, increasing their gas diffusion barrier9. In general, the type of coating and application method10,11,12,13,14,15,16,17,18 used depends on the desired properties, such as biocompatibility and durability. Several examples of antifouling coatings have been used on artificial lung fibers. They include silicone which is widely used due to its biocompatibility19, durability, and resistance to biofouling; polyurethane (PU) due to its biocompatibility and resistance to biofouling20; chitosan due to its biocompatible and antimicrobial properties21,22, heparin that inactivates thrombin23,8, and hydrophilic24 polymer-based coatings including poly(ethylene glycol)25,26, poly (2-methoxyethyl acrylate)27, and phosphorylcholine28,29.
Zwitterionic coatings represent a promising strategy for reducing thrombosis on artificial surfaces without the need for systemic anticoagulation5,6. These coatings are composed of molecules with both positive and negative charges, which balance each other out and result in a highly hydrophilic, non-fouling surface. The zwitterionic nature of these coatings reduces protein adsorption and platelet adhesion, both of which are critical steps in the initiation of the coagulation cascade. By preventing the initial interaction between blood proteins and the artificial surface, zwitterionic coatings effectively inhibit the activation of the coagulation cascade and reduce the risk of thrombosis. This approach not only minimizes the need for systemic anticoagulants but also offers a more biocompatible solution for the long-term use of medical devices.
In this study, we evaluated the effectiveness of priming the artificial lung with an ultra-low fouling zwitterionic poly(sulfobetaine methacrylate) (pSBMA) coating combined with a surface adhesive polydopamine (pDOPA) layer. After priming the device, it was positioned side-to-side every 10 min for 2 h during the coating process. To assess potential variations in coating across the fiber bundle, we measured fibrinogen and platelet fouling on fibers located at the surface and within the bundle. Additionally, we analyzed the impact of flow on the antifouling performance by comparing fouling data from lungs before and after exposure to flow. For long-term antibiofouling applications involving complex media flow, zwitterionic coatings must not only inhibit fouling from whole blood -- a challenging task -- but also maintain their effectiveness under hemodynamic stress throughout the application period. These coatings need to provide strong steric repulsion against non-specific protein adsorption and achieve a suitable surface packing density to form a hydration film barrier between the substrate and the complex media. Moreover, they must remain securely attached to the surface without detachment of the linkers anchoring the coating to the substrate30. The protocol described here is designed to ensure the application of coatings that meet these critical requirements for effective and durable surface protection.