Vascular sheaths are medical devices used in vascular interventional diagnosis and treatment. They establish an access channel from the skin to blood vessels via percutaneous puncture, enabling the insertion and withdrawal of interventional instruments. Meanwhile, they protect the punctured vascular wall during repeated instrument passage and reduce vascular injury, thereby realizing the diagnosis or treatment of vascular diseases.

A vascular sheath is assembled by bonding two high-performance biomedical materials: thermoplastic polyurethane (TPU) for the extension tube and polypropylene (PP) for the sheath hub. The two components are bonded with UV-curable adhesive.
TPU is an (AB)ₙ-type multi-block copolymer consisting of flexible soft segments and rigid hard segments, classified as a special synthetic polymer elastomer. PP has a simple molecular structure of –CH₂–CH(CH₃)– without polar groups featuring uneven electron distribution, resulting in weak surface adsorption capacity. Both TPU and PP are polymers with high surface chemical inertness and poor inherent bonding performance, which impairs their clinical application.

Adhesive curing refers to the process in which liquid adhesive transforms into a solid state and adheres to substrates through complex mechanisms including diffusion, adsorption and mechanical interlocking. Bonding involves three sequential stages: substrate surface pretreatment, interfacial adhesion, and interfacial energy absorption. To improve bonding performance, plasma treatment that does not damage base materials is commonly applied to low-polarity substrates.

Plasma treatment is a mainstream surface activation method for polymers. Its mechanism lies in high-energy particles in plasma bombarding the substrate surface to enhance polymer surface activity. Plasma activation is only one factor governing bonding performance; multiple physical parameters also exert influences, such as adhesives of different viscosities used during interfacial adhesion, and absorbed energy regulated by curing distance, curing time and irradiation intensity ratio during UV curing.

Effects of Plasma Treatment on Bonding Performance of TPU Extension Tubes and PP Vascular Sheath Hubs

According to adsorption theory, the wettability (characterized by contact angle and surface energy) of substrates to adhesives exerts a significant impact on intermolecular forces. Static water contact angles of TPU and PP surfaces under different plasma treatment durations were measured by an optical contact angle goniometer, and the corresponding surface free energy was calculated. The test results are shown in Figure 1.

As illustrated in Figure 1a, plasma activation delivers a remarkable effect when treatment time is less than 90 s, accompanied by a sharp drop in static water contact angle. The contact angle of PP decreases from 85.887° to 53.552°, while that of TPU falls from 70.049° to 21.328°.

When the treatment duration ranges from 90 s to 180 s, the static water contact angle changes slightly, and the plasma surface modification effect reaches saturation after 120 s. Under plasma action, the density of active particles rises with extended treatment time. The energy of active particles ranges from several to tens of electron volts, generally exceeding the carbon-carbon bond energy (2.6–5.2 eV) and other carbon-containing bond energies on material surfaces. This provides sufficient energy to activate substrate surfaces, improve wettability and render substrates hydrophilic.

The hydrophilization of substrates essentially originates from the introduction of active groups. Polar oxygen-containing functional groups such as hydroxyl (–OH) and unsaturated carbonyl groups (–C=O) are generated on the low-energy surfaces of TPU and PP. The measured contact angle tends to stabilize with prolonged treatment, because the newly formed surface functional groups are continuously destroyed and reconstructed by high-energy particles, reaching a saturated wettability state.

Wettability enables continuous contact between adhesive and substrate. In line with diffusion theory, improved wettability facilitates the diffusion of adhesives or coatings, expands the adsorption area, and thus promotes bonding performance.

Figure 1b indicates that after plasma treatment, the surface free energy of PP (a low-surface-energy plastic) calculated via the Fowkes method rises from 61.0961 mJ/m² to 120.5973 mJ/m², a 1.97-fold increase; the surface free energy of TPU increases from 73.9959 mJ/m² to 169.6084 mJ/m², a 2.29-fold improvement.

This demonstrates that reduced contact angle corresponds to elevated surface free energy. Low-surface-energy plastics are modified into high-surface-energy materials with drastically enhanced hydrophilicity, which strengthens intermolecular forces at the substrate-adhesive interface and improves overall bonding performance.

Figure 1 Static water contact angle (a) and surface free energy (b) of TPU and PP under different plasma treatment durations

Activating the surface of vascular sheath substrates via plasma treatment reduces contact angle, raises the surface free energy of chemically inert plastics, strengthens adhesive adhesion on substrate surfaces, and ultimately optimizes bonding performance.