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Skin bioprinting has the potential to revolutionize treatment approaches for injuries and surgical procedures, while also providing a valuable platform for assessing and screening cosmetic and pharmaceutical products. This technology offers key advantages, including flexibility and reproducibility, which enable the creation of complex, multilayered scaffolds that closely mimic the intricate microenvironment of native skin tissue. The development of an ideal hydrogel is critical for the successful bioprinting of these scaffolds with incorporated cells. In this study, we used a hydrogel formulation developed in our laboratory to fabricate a 3D-bioprinted skin model. The hydrogel composition was carefully selected based on its high compatibility with human skin cells, incorporating alginate, methyl cellulose, and nanofibrillated cellulose. One of the critical challenges in this process, particularly for its commercialization and large-scale production, is ensuring consistency with minimal batch-to-batch variations. To address this, we explored methods with which to preserve the physicochemical properties of the hydrogels, with a focus on freezing techniques. We validated the pre-frozen hydrogels' printability, rheology, and mechanical and surface properties. Our results revealed that extended freezing times significantly reduced the viscosity of the formulations due to ice crystal formation, leading to a redistribution of the polymer chains. This reduction in viscosity resulted in a more challenging extrusion and increased macro- and microporosity of the hydrogels, as confirmed by nanoCT imaging. The increased porosity led to greater water uptake, swelling, compromised scaffold integrity, and altered degradation kinetics. The insights gained from this study lay a solid foundation for advancing the development of an in vitro skin model with promising applications in preclinical and clinical research. [ABSTRACT FROM AUTHOR]

Delayed ettringite formation (DEF) is a chemical reaction with proven damaging effects on the mechanical properties of hydrated cementitious composite (concrete). Ettringite crystals can cause cracks and the widening of cracks due to pressure on the crack walls caused by the positive volume difference in the reaction. In this paper, we investigated the potential to utilise the positive volume difference in DEF in order to improve the mechanical properties of hydrated fine grained aerated concrete. Fine dispersed crystallisation nuclei, achieved by adding air-entraining agent (AEA) and by short vibration of specimens, are presented as the main requirement for such improvements. Control tests of expansion and mechanical properties were performed on samples of concrete with and without AEA by inducing DEF. The microstructure of fine grained aerated concrete was examined with an optical microscope and scanning electron microscope. We found that the controlled DEF, which is guaranteed by adding AEA and with the formation of uniformly dispersed air bubbles, which are crystallisation sites for ettringite crystals, improves the mechanical properties. The specimens with induced DEF were measured and found to have a 6.8% increase of compressive strength.

This paper deals with the influence of admixtures – silica fume, superplasticizer and air entraining agent – on the improvement of the bond strength between bundled carbon filaments and cement mortars. Pull-out tests were carried out to examine the bond between carbon-fiber yarn and various cement matrices. An overall microstructural analysis was conducted and correlated with pull-out data. Experimental results showed that all the applied admixtures contributed to improved bonding. The air entraining agent was found to be the most effective in improving the bond strength of the studied carbon-fiber reinforced cement mortar. Air bubbles formed inside the mortar mix as a result of added air entraining agent and the ones formed in the vicinity of the outer filaments of the yarn may provide a mechanical barrier at the interface, leading to a significant increase in interface friction.

Introduction: The most commonly used materials for prosthetic denture base are acrylic resins - polymethyl methacrylate, which has all the important physical and chemical properties. The bond between a denture base and an acrylic tooth has a significant functional value. Aim: To evaluate the influence of various procedures on bonding quality between the denture base and the artificial tooth, the measurement of gaps when connecting the tooth and the denture base using an optical or electron microscope, and evaluating the mechanical properties by compressive testing. Materials and methods: Forty eight acrylic models with bonds between acrylic teeth and prosthetic denturebases, and 12 models for compressive testing were created as the materials in this in vitro study. The acrylic models were allocated to four groups (with 12 samples) for microscopy, and three groups for compressive testing. The samples for microscopy contained denture bases with fixed, unprocessed acrylic teeth, mechanically processed acrylic teeth, chemically processed acrylic teeth, and mechanically and chemically processed acrylic teeth, respectively. The acrylic models had the same dimensions, and the surface of each model was prepared using a slow speed saw, abrasive diamond paste, and special paper grinders. A special metal cast was created for compressive testing with internal dimensions of 20 x 9 mm. The size of a gap was measured on the light inverse microscope (Nikon Epiphot 300) and pressure testing was performed on a Zwick/Roell Z010 pressure machine. Results: The average value of the gap on mechanically and chemically treated samples was 68.250 μm, which was significantly lower than the gap on the untreated samples with the average value of 103.75 μm. The compressive strength was 3200 N/mm2 on untreated samples and above 6000 N/mm2 on the mechanically and chemically treated ones. Conclusions: It can be concluded that surfaces which come into contact must be mechanically and chemically processed. Although in practice prostheses are made using mechanical tooth-base retention, larger gaps occur when compared to mechanically and chemically processed surfaces.