Process-Driven Mechanical Tuning of 3D Printed Foaming Thermoplastic Polyurethane for Age-Specific Cervical Intervertebral Disc Modelling

1. Introduction
Additive Manufacturing (AM) enables patient-specific cervical spinal models, however, while anatomical accuracy is derived from medical imaging, achieving physiologically relevant intervertebral discs (IVDs) mechanics that account for age and disease variations in mechanical behaviour remains challenging. Previous work demonstrated feasibility of modulating compressive properties using material selection and parameters inherent to AM [1], however limited understanding remains regarding how extrusion conditions govern microstructural foaming, and mechanical performance of foaming thermoplastic polyurethane (TPU). Consequently, this study investigates the process-structure-property relationship of 3D printed foaming TPU, enabling age-specific IVDs within anatomically accurate geometries.

2. Methods
Foaming TPU samples were manufactured with variations of extrusion temperature T (200-240°C), flow ratio f (0.60-0.90), layer height LH (0.12-0.28 mm), and print speed SP (40-60 mm s-1). Compression tests were conducted in accordance with ISO 604:2003, with compressive modulus EC determined using least-squares regression. For density and microstructural analysis, cubic samples were produced, with density used to characterise foaming behaviour and scanning electron microscopy (SEM) to analyse pore morphology.

3. Results
T and f were identified as the dominant variables governing foaming behaviour and compressive mechanical response. Increased T and lower f resulted in lower density and EC. Conversely, conditions that restricted foaming yielded stiffer samples, representative of younger IVD tissue [2] (Figure 1a). In contrast, changes in LH and SP produced no statistically significant changes in EC. Microstructural changes revealed that increasing T and decreasing f produced greater expansion of the foaming agent, while higher EC was attributed to denser filament packing with few pores and overlapping of filament tracks, evidenced thanks to SEM (Figure 1b).

4. Discussion
These findings demonstrate that foaming TPU enables controllable replication of age-specific IVDs through extrusion-driven changes of foaming behaviour. The relationship between processing parameters, pore morphology, density, and EC enables predictive control of mechanical behaviour. Importantly, stiffness tuning was achieved without changing geometry or material, enabling models to exhibit physiologically relevant behaviour (Figure 1c). This approach enhances the biomechanical accuracy of physical spinal models used for surgical simulation, thereby improving their value in clinical training and decision-making for personalised treatment strategies.