In this work, we developed a numerical approach based on an experimental platform to determine the working conditions on a cryoplatform and to predict and evaluate the cryogenic printing of hydrogels. Although hydrogels have good biocompatibility, their material properties make it difficult to print them with high precision and shape fidelity. To overcome these problems, a cryogenic cooling platform was introduced to accelerate the physical stabilisation of each deposited layer during the printing process. By precisely controlling solidification (crystallisation), each printed material can withstand its own weight to maintain shape fidelity, and the porosity of the scaffolds can also be controlled more selectively. The thermophysical properties of gelatine hydrogels were investigated to gain a better understanding of the phase change upon freezing. The corresponding material properties and experimental observations of gelatine solidification served as the basis for developing a computational fluid model (CFD) to mimic the solidification of gelatine hydrogels using a cryoplatform at different process conditions and extruder speeds. The goal was to develop a tool simple enough to predict acceptable process conditions for printing gelatine hydrogels using a cryoplatform.

Modeling lyophilization in a vial is frequently done on a single vial level. When setting up a numerical model, the main focus is on heat and mass transfer inside the lyophilizate, whereas the vapor dynamics in the headspace of the vial is taken into account simply through imposing the system pressure as a pressure boundary condition. The present paper offers a deeper insight into the interaction of the sublimated vapor flow and the corresponding vapor pressure conditions inside the headspace of a partially stoppered vial. This is achieved through a coupled numerical solution of the heat and mass transfer inside the product by means of a 0D model describing the frozen domain (ice) and the 3D fluid flow inside the vial geometry with the partially opened stopper, computed by means of Computational Fluid Dynamics. Due to low pressures, the slip flow regime within the continuum hypothesis has to be considered, leading to imposing velocity slip conditions at the solid walls. The 0D model is used for the computation of sublimation mass flow rate as well as heat transfer rate to the vial, with the results of the water vapor mass flow rate and the temperature communicated to the 3D CFD model as a new inlet boundary conditions for computation of compressible fluid flow dynamics inside the vial. The obtained CFD pressure field solution allows derivation of a pressure resistance model for a targeted vial stopper combination, which is then used in calculating the corresponding pressure drop in the headspace of the partially stoppered vial. The coupled CFD-0D model results are validated based on the results of dedicated experimental water runs on several vial and stopper geometries and show, that the vial geometry, but especially the installed stopper, alter the pressure field conditions inside the vial. The increased in-vial local vapor pressure values lead to a decrease of the mass flow rates and an increase of temperatures at the bottom of the product, which range from 0.6 K for the highest system pressure and up to 5.4 K for the lowest system pressure tested. The presented coupled model is suitable for the use in further studies of the impact of various vial forms as well as stoppers on the lyophilization dynamics in a vial.

This study demonstrates the successful synthesis of Ni/Y₂O₃ nanocomposite particles through the application of ultrasound-assisted precipitation using the ultrasonic spray pyrolysis technique. They were collected in a water suspension with polyvinylpyrrolidone (PVP) as the stabiliser. The presence of the Y₂O₃ core and Ni shell was confirmed with transmission electron microscopy (TEM) and with electron diffraction. The TEM observations revealed the formation of round particles with an average diameter of 466 nm, while the lattice parameter on the Ni particle’s surface was measured to be 0.343 nm. The Ni/Y₂O₃ nanocomposite particle suspensions were lyophilized, to obtain a dried material that was suitable for embedding into a polylactic acid (PLA) matrix. The resulting PLA/Ni/Y₂O₃ composite material was extruded, and the injection was moulded successfully. Flexural testing of PLA/Ni/Y₂O₃ showed a slight average decrease (8.55%) in flexural strength and a small decrease from 3.7 to 3.3% strain at the break, when compared to the base PLA. These findings demonstrate the potential for utilising Ni/Y₂O₃ nanocomposite particles in injection moulding applications and warrant further exploration of their properties and new applications in various fields.