Porous Open-Сell UHMWPE: Experimental Study of Structure and Mechanical Properties.

Ultra-high molecular weight polyethylene (UHMWPE) is a bioinert polymer that is widely used as bulk material in reconstructive surgery for structural replacements of bone and cartilage. Porous UHMWPE can be used for trabecular bone tissue replacement, and it can be used in living cell studies as bioinert 3D substrate permeable to physiological fluids. It is important to develop techniques to govern the morphology of open-cell porous UHMWPE structures (pore size, shape, and connectivity), since this allows control over proliferation and differentiation in living cell populations. We report experimental results on the mechanical behavior of porous open-cell UHMWPE obtained through sacrificial removal (desalination) of hot-molded UHMWPE-NaCl powder mixtures with pore sizes in the range 75 µm to 500 µm. The structures were characterized using SEM and mechanically tested under static compression and dynamic mechanical analysis (DMA), bending, and tensile tests. Apparent elastic modulus and complex modulus were in the range of 1.2 to 2.5 MPa showing a weak dependence on cell size. Densification under compression caused the apparent elastic modulus to increase to 130 MPa.

Probing the complex thermo-mechanical properties of a 3D-printed polylactide-hydroxyapatite composite using in situ synchrotron X-ray scattering.

Polylactide (PLA)-hydroxyapatite (HAp) composite components have attracted extensive attentions for a variety of biomedical applications. This study seeks to explore how the biocompatible PLA matrix and the bioactive HAp fillers respond to thermo-mechanical environment of a PLA-HAp composite manufactured by 3D printing using Fused Filament Fabrication (FFF). The insight is obtained by in situ synchrotron small- and wide- angle X-ray scattering (SAXS/WAXS) techniques. The thermo-mechanical cyclic loading tests (0-20 MPa, 22-56 °C) revealed strain softening (Mullins effect) of PLA-HAp composite at both room and elevated temperatures (<56 °C), which can be attributed primarily to the non-linear deformation of PLA nanometre-scale lamellar structure. In contrast, the strain softening of the PLA amorphous matrix appeared only at elevated temperatures (>50 °C) due to the increased chain mobility. Above this temperature the deformation behaviour of the soft PLA lamella changes drastically. The thermal test (0-110 °C) identified multiple crystallisation mechanisms of the PLA amorphous matrix, including reversible stress-induced large crystal formation at room temperature, reversible coupled stress-temperature-induced PLA crystal formation appearing at around 60 °C, as well as irreversible heating-induced crystallisation above 92 °C. The shape memory test (0-3.75 MPa, 0-70 °C) of the PLA-HAp composite demonstrates a fixing ratio (strain upon unloading/strain before unloading) of 65% and rather a ∼100% recovery ratio, showing an improved shape memory property. These findings provide a new framework for systematic characterisation of the thermo-mechanical response of composites, and open up ways towards improved material design and enhanced functionality for biomedical applications.