Optimization of Balloon Surface Morphology Using Breath Figure Method to Enhance Stent Retention Force

Recently, the need for multifunctional medical balloons is increasing for stent delivery, drug delivery, and biosensing applications. To meet this demand, researches were conducted to apply coronary intervention by increasing the drug loading efficiency, which was done by increasing the specific surface area through texturing on the surface of the balloon. In addition, studies have been conducted to apply micro/nanoscale structures to enteroscopy procedures by increasing the frictional force in the human body. Among the various applications of medical balloons, stent delivery is the most commonly used balloon worldwide. Recently, bioabsorbable vascular stents (BVS) have been continuously developed as an improvement of the drug-eluting stent (DES) and bare-metal stents (BMS).<br/>However, despite these advantages, the BVS sometimes dislodges from the balloon in an undesirable place during its delivery to the lesion. This is owing to the phenomenon of using slippery materials, such as Nylon12 and PEBA, to withstand the high-pressure expansion force (more than 20 ATM) of the medical balloon. Despite the issue of stent delivery, research and development related to this issue are insufficient.<br/>In this study, we present a balloon with the porous surface that can increase stent retention strength. A porous film was formed using a thermoplastic polyurethane with high adhesion, and this was applied to the balloon surface. In particular, we focused on increasing the contact area with the stent strut through the 3D porous structure when the stent is crimped on the balloon.<br/>To form a 3D-porous structure, the breath figure method (BFM), a type of self-assembly, is used along with the dip-coating method. Our system is templateless, porogenless, and simple to configure. BFM is a process that self-arranges the condensed droplets on the triple interface of air, water, and solvent using the Marangoni and capillary effects as driving forces.<br/>When the film is in liquid state, that is, before being solidified, the droplets condense on the interface and either float or sink depending on Υ_w(surface tension of water), Υ_s(surface tension of solution), Υ_w/s(interfacial tension between water and solution). Accordingly, monolayer or multilayer coating structures can be formed. <b>I</b>n this study, we control Υ_swith the concentration of the polymer solution and mixing ratio (chloroform + pentane) to control the cross-sectional structure of the coating layer according to the variable surface tension over a wide range. In addition, Υ_wis controlled as the concentration of the aqueous solution containing the surfactant F-127 or NaCl. As a result, the decrease in Υ_wshows a tendency to create a hemispherical monolayer structure on the surface of the coating layer, and we can observed that this phenomenon is caused by water droplets floating on the liquid interface. On the other hand<b>, </b>when<b> </b>Υ_wincreases, spherical pores are generated in a multilayered structure, because water droplets sink into the polymer solution before the film solidifies. When only Υ_sdecreases, spherical pores are created, and it is confirmed that they are formed as monolayers. However, as Υ_sincreases, hemispherical pores are formed as monolayers. These results indicate that the viscosity of the polymer solution during the BFM process is related to the shape of the structure. Consequently, when the stent dislodgement test is performed, the maximum <b>retention</b> force is observed as the porosity of the structure increases.