BACKGROUND: The biological properties of silicone elastomers such as polydimethylsiloxane (PDMS) have widespread use in biomedicine for soft tissue implants, contact lenses, soft robots, and many other small medical devices, due to its exceptional biocompatibility. Additive manufacturing of soft materials still has significant challenges even with major advancements that have occurred in development of these technologies for customized medical devices and tissue engineering. OBJECTIVE: The aim of this study was to develop a mathematical model of tangential stress in relation to shear stress, shear rate, 3D printing pressure and velocity, for non-Newtonian gels and fluids that are used as materials for 3D printing. METHOD: This study used FENE (finitely extensible nonlinear elastic model) model, for non-Newtonian gels and fluids to define the dependences between tangential stress, velocity, and pressure, considering viscosity, shear stress and shear rates as governing factors in soft materials friction and adhesion. Experimental samples were fabricated as showcases, by SLA and FDM 3D printing technologies: elastic polymer samples with properties resembling elastic properties of PDMS and thermoplastic polyurethane (TPU) samples. Experimental 3D printing parameters were used in the developed analytical solution to analyse the relationships between governing influential factors (tangential stress, printing pressure, printing speed, shear rate and friction coefficient). Maple software was used for numerical modelling. RESULTS: Analytical model applied on a printed elastic polymer, at low shear rates, exhibited numerical values of tangential stress of 0.208–0.216 N m - 2 at printing velocities of 0.9 to 1.2 mm s - 1, while the coefficient of friction was as low as 0.09–0.16. These values were in accordance with experimental data in literature. Printing pressure did not significantly influence tangential stress, whereas it was slightly influenced by shear rate changes. Friction coefficient linearly increased with tangential stress. CONCLUSION: Simple analytical model of friction for elastic polymer in SLA 3D printing showed good correspondence with experimental literature data for low shear rates, thus indicating possibility to use it for prediction of printing parameters towards desired dimensional accuracy of printed objects. Further development of this analytical model should enable other shear rate regimes, as well as additional soft materials and printing parameters.

The principles of the friction welding (FW) process of the two different non-ferrous metals, aluminum and copper, are presented in this paper. Considering that the bimetallic Al-Cu joints find applications in electrical engineering, as well as in other industrial fields, the basic characteristics and compatibility of these metals are discussed, along with the influence of various parameters on the properties of their friction welded joints. The experimental study involved rotation friction welding (RFW), which was used to weld aluminum and copper samples. The samples were monitored for shortening due to the applied deformation, as well as the size of the formed mushroom. Then, the central part of the welded joint was cut from the welded samples to determine the hardness and microstructure of the joint. At the end of the research, the possibility of applying software for the design of a numerical model for analysis of the possibility of joining aluminum and copper, with the same input parameters as those used in the experiment, was considered. The numerical simulation exhibited a high agreement with the experimental results.

This study investigates the influence of physiological body fluids on the mass stability and tribological performance of polylactic acid (PLA) samples produced by Fused Deposition Modeling (FDM) 3D printing. Body fluid exposure was simulated using Dulbecco’s Modified Eagle Medium (DMEM) under controlled conditions. Black PLA filament was printed with three infill densities (15%, 20%, and 90%) and immersed in DMEM for 7 days at 37 ± 1 °C. Mass measurements revealed that lower infill densities resulted in significantly higher mass loss, with the 15% infill samples exhibiting the greatest reduction (5.07%), while the 90% infill samples showed negligible change (0.17%). Tribological testing using a CSM nanotribometer under loads of 5 mN, 500 mN, and 1000 mN demonstrated that infill density critically affects friction and wear behavior. The 90% infill samples exhibited the lowest wear volumes and the most stable tribological response, while the 15% infill samples showed degradation-dominated contact behavior. Although the friction measurements for the 15% infill samples were consistent, their interpretation should be approached with caution due to pronounced surface deterioration and debris-mediated sliding. This behavior is attributed to structural weakening caused by immersion in DMEM, which promoted material degradation and influenced the tribological response. These findings confirm the critical role of structural density in wear resistance. To the best of our knowledge, this is the first study to systematically investigate the combined effect of hydrolytic degradation and tribological behavior of FDM-printed PLA as a function of infill density under simulated physiological conditions. These findings provide a scientific basis for optimizing infill density in the design of PLA-based surgical instrument guides, where both degradation resistance and tribological performance under body fluid exposure are essential. The findings should be interpreted within the limitations of the experimental design.

Deterministic and stochastic models of Brownian motion in ferrofluids are of interest to researchers, especially those related to drug delivery systems. The Brownian motion of nanoparticles in a ferrofluid environment was theoretically analyzed in this research. The state of the art in clinical drug delivery systems using ferromagnetic particles is briefly presented. The motion of the nanoparticles in an external field and as a random variable is elaborated by presenting a theoretical model. We analyzed the theoretical model and performed computer simulation by using Maple software. We used simple low-dimensional deterministic systems that can exhibit diffusive behavior. The ferrofluid in the gravitational field without the presence of an external magnetic field in the xy plane was observed. Control parameter p was mapped as related to the fluid viscosity. Computer simulation showed that nanoparticles can exhibit deterministic patterns in a chaotic model for certain values of the control parameter p. Linear motion of the particles was observed for certain values of the parameter p, and for other values of p, the particles move randomly without any rule. Based on our numerical simulation, it can be concluded that the motion of nanoparticles could be controlled by inherent material properties and properties of the surrounding media, meaning that the delivery of drugs could possibly be executed by a ferrofluid without an exogenous power propulsion strategy. However, further studies are still needed.