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Wettability as determined from contact angle measurements is a suitable parameter for characterizing the physical bonding of a polymer matrix and reinforcing fibers, but it is very challenging to measure the capillary force exerted by a probe liquid on a fiber accurately for very fine fibers such as single carbon fibers. Herein, we propose an innovative method for measuring dynamic contact angles with a tensiometer, considering both the intrinsic variability of the carbon fiber diameter and the extremely small amplitude of the capillary forces, allowing the measurement of reliable dynamic contact angles over a large range of contact line velocities. The analysis of the contact angle dynamics by the molecular-kinetic theory permits us to check the relevancy of the measured contact angles and to obtain the static contact angle value, improving the prospect of employing tensiometry to better understand the wetting behavior of carbon fibers.

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Fused deposition modeling (FDM), a low cost and easy-to-use additive manufacturing technique, was pushed to its typical resolution limit to produce poly(lactic acid) (PLA) gyroid scaffolds. A gyroid morphology was selected as scaffold structure due to its spring shape architecture, high porosity, leading to good nutrient and waste diffusion, and favorable mechanical properties, such as isotropic resistance to pressure. Printing parameters were optimized and the need of a support material to improve printing quality was evidenced. The gyroid structure was compared with the more common strut-based structure. Scaffold porosity was measured by micro-CT, and mechanical properties were determined by compressive mechanical tests. The effect of mesh geometry, printing resolution, and PLA crystallinity on resistance against compression was evaluated. Moreover, the impact of PLA scaffold geometry and crystallinity on its degradation was studied in vitro. Porosity of the gyroid structure was 71%, close to the 74% expected from the model used for printing. The compression tests showed that the gyroid scaffold has an isotropic behavior, in contrast with the typical strut-based scaffold, which exhibits an orientation-dependent deformation. Upon aging in physiological conditions, gyroid scaffolds retained their integrity during 64 weeks, while control scaffolds lost struts one after the other starting from week 33, in a way that depended on crystallinity and printing resolution. For both geometries, the remaining mass started to decrease at week 52. Based on these results, the gyroid design is proposed as a suitable mesh architecture for tissue engineering scaffolds that can be elaborated using FDM techniques, to produce low cost and personalized implants.

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Physical adhesion was experimentally determined by measuring contact angles with different liquids on bamboo and glass fibers, using the Wilhelmy technique, and by applying the acid-base theory for calculating the surface energy components and the theoretical work of adhesion. The mechanical strength of the interfaces was assessed by single fibre pull-out tests. In order to consider the real mechanisms of interfacial failure of natural fiber composites, the fibre matrix interfacial bond strength was characterized by the critical local value of interfacial shear stress, , and the radial normal stress at the interface, σult, at the moment of crack initiation. Both interfacial parameters are used for correlating thermodynamic work of adhesion and practical adhesion. Pull-out tests (taking into account friction), XPS, and profilometry techniques were used to study the influence of rough natural fibre surfaces on the interface between the fibre and a thermoplastic matrix, by comparing the mechanical behaviour at the interface of a smooth optical glass fibre with that of rough natural fibres. The results suggest that the physical and chemical compatibility between the bamboo fibre and the matrix does not improve substantially the composite performance if compared with glass composites. The relatively low off-axis strength of the bamboo fibres is suggested as the main reason for the low stress transfer capability at the fibre-matrix interphase.Furthermore, the pull-out process may be friction-dominated in bamboo fibre systems.