Hygroscopic Behaviour and Diffusion Characteristics of Flexible TPU Materials Fabricated by FDM for Potential Biomedical Applications

Flexible thermoplastic polyurethane (TPU) materials fabricated using fused deposition modeling (FDM) are increasingly used in engineering and biomedical applications where exposure to moisture is unavoidable. However, the relationship between material hardness, water absorption, diffusion behaviour, and dimensional stability remains insufficiently understood and investigated. In this study, the hygroscopic behaviour of eight commercially available TPU filaments (60A–98A Shore hardness) was systematically investigated. Specimens were produced using an FDM 3D printer under controlled processing conditions and immersed in physiological solution (0.9% NaCl) for up to 96 h. Water absorption, dimensional changes, and diffusion characteristics were analyzed. Diffusion coefficients were determined using the Fickian diffusion model based on the initial stage of water uptake. The results suggest a transition in behaviour between lower- and higher-hardness materials. Softer TPU materials (60A–85A) exhibited higher water absorption (up to ~1.80%) and an apparent linear trend between hardness and absorption within the investigated material group (R2 = 0.999). In contrast, higher-hardness materials (89A–98A) showed lower absorption (~1.18–1.42%) and a weaker apparent relationship with hardness (R2 = 0.4214). Diffusion coefficients ranged from 1.40 × 10−13 to 3.40 × 10−12 m2 s−1, with no monotonic dependence on hardness. Additionally, no clear correlation between diffusion kinetics and equilibrium absorption or volumetric expansion was observed. These findings indicate that hygroscopic behaviour of FDM-printed TPU materials cannot be reliably predicted based solely on hardness, and that diffusion, absorption, and swelling may be influenced by different mechanisms. The identified transition from hardness-dependent to behaviour potentially influenced by material structure provides new insight for the design and selection of flexible polymer components in moisture-exposed environments, particularly in biomedical applications.