Hydrogels have emerged as highly attractive polymer-based materials owing to their unique solid–liquid rheological duality, which allows their use in 3D extrusion and inkjet printing, particularly in the field of biomedical applications. The dynamic moduli (storage modulus, G′, and loss modulus, G′′), relaxation modulus, shear-thinning behaviour, thixotropy, viscoelasticity, and yield stress are the most commonly employed rheological concepts for hydrogel applications in 3D/4D additive manufacturing. Extrusion, inkjet printing, and stereolithography are the most studied manufacturing technologies for hydrogel bioprinting applications. Moreover, hydrogels exhibit a combination of cohesive properties of solids and the transport characteristics of liquids. Their rheological behaviour, however, varies depending on whether they are chemically cross-linked, showing a pure solid elastic behaviour, or physically crosslinked, showing viscoelastic behaviour. While rheology reveals much information about the flow behaviour of liquids or deformation behaviour of solids, it is not as obvious as to anticipate the printability of hydrogels. Therefore, a deep understanding of rheological principles and their correlation with printability is essential. This review begins summarizing various polymer hydrogels. Subsequently, the definition and description of the most employed concepts, namely relaxation modulus, storage, and loss moduli, and many others, are necessary to understand and associate the feasibility of hydrogels for a particular bioprinting process. This review mainly addresses: (i) rheological determination of the processing window for direct ink writing (DIW), (ii) rheological restrictions for printing beyond direct ink writing (DIW), and (iii) vat photopolymerization bioprinting and the biological implications of bioprinting. Finally, all the above concepts are illustrated with a few examples of biomedical applications of 3D/4D printed hydrogels.

The recyclability of opaque PET, which contains TiO₂ nanoparticles, has not been as well-studied as that of transparent PET. The objective of this work is to recycle post-consumer opaque PET through reactive extrusion with Joncryl. The effect of the reactive extrusion process on the molecular structure and on the thermal/mechanical/rheological properties of recycling post-consumer opaque PET (r-PET) has been analyzed. A 1% w/w Joncryl addition caused a moderate increase in the molecular weight. A moderate increase in chain length could not explain a decrease in the overall crystallization rate. This result is probably due to the presence of branches interrupting the crystallizable sequences in reactive extruded r-PET (REX-r-PET). A rheological investigation performed by SAOS/LAOS/elongational studies detected important structural modifications in REX-r-PET with respect to linear r-PET or a reference virgin PET. REX-r-PET is characterized by a slow relaxation process with enlarged elastic behaviors that are characteristic of a long-chain branched material. The mechanical properties of REX-r-PET increased because of the addition of the chain extender without a significant loss of elongation at the break. The reactive extrusion process is a suitable way to recycle opaque PET into a material with enhanced rheological properties (thanks to the production of a chain extension and long-chain branches) with mechanical properties that are comparable to those of a typical virgin PET sample.

Direct Ink Writing (DIW) is an advanced additive manufacturing 3D-printing technique with significant potential for producing hydrogels in biomedical and engineering applications. This study presents a comprehensive rheological analysis of the yielding and recovery properties critical for ensuring the printability of carboxymethyl cellulose (CMC)-based hydrogels incorporating atenolol, an antihypertensive agent, as the active ingredient. The viscoelastic properties under shear conditions were examined using Large Amplitude Oscillatory Shear (LAOS) testing. To obtain both qualitative and quantitative insight into hydrogel dynamics, Lissajous-Bowditch plots and Fourier Transform (FT) coefficients were analyzed. The evaluation of stress signal anharmonicity and the decomposition of stress into its elastic and viscous components allowed for distinguishing structural evolution under flow among the tested hydrogels. Additionally, the analysis of the Sequence of Physical Processes (SPP) during each deformation cycle provided deeper insight into oscillatory yielding behavior, emphasizing the role of elastic strains in determining printability. Overall, the study offers valuable understanding of the nonlinear viscoelastic behavior of CMC-based hydrogels, providing a framework for optimizing hydrogel formulations in DIW applications.

Polyhydroxyalkanoates are natural polyesters synthesized by microorganisms and bacteria. Due to their properties, they have been proposed as substitutes for petroleum derivatives. This work studies how the printing conditions employed in fuse filament fabrication (FFF) affect the properties of poly(hydroxybutyrate-co-hydroxy hexanoate) or PHBH. Firstly, rheological results predicted the printability of PHBH, which was successfully realized. Unlike what usually happens in FFF manufacturing or several semi-crystalline polymers, it was observed that the crystallization of PHBH occurs isothermally after deposition on the bed and not during the non-isothermal cooling stage, according to calorimetric measurements. A computational simulation of the temperature profile during the printing process was conducted to confirm this behavior, and the results support this hypothesis. Through the analysis of mechanical properties, it was shown that the nozzle and bed temperature increase improved the mechanical properties, reducing the void formation and improving interlayer adhesion, as shown by SEM. Intermediate printing velocities produced the best mechanical properties.

The generation of long-chain branches (LCB) in biobased and biodegradable polylactide (PLA) by adding different amounts of a chain extender is studied. The rheological and calorimetric behavior have been used to determine the effect of LCB presence and their topology on PLA melt strength and crystallization behavior. Rheological modeling of linear and non-linear viscoelastic shear and extensional properties identified several possible branched structures. Moreover, remarkable differences were observed for the different topologies regarding the intrinsic non-linear parameters and the intra-cycle elastic and viscous non-linearities. Differential scanning calorimetry and polarized light optical microscopy measurements revealed a significant increase in the nucleation density and rate of PLA with increasing the amount of LCB, albeit they provoke a decrease in the growth rate due to a reduction in chain diffusion. Nevertheless, overall crystallization rate values revealed a predominant effect of nucleation over crystal growth. The introduction of LCB within the chains is highly beneficial as they increase nucleation, crystallinity, and elongational viscosity, thus improving the properties of biodegradable PLA.