Strain sensors are essential devices for monitoring materials and structures that are subjected to dimensional variations due to different forces and mechanical stresses. They are widely used in structural engineering, materials testing, and structural integrity monitoring. In particular, in the aerospace sector, these sensors are extensively used for real-time monitoring of structural fatigue, vibrations, deformations, and localized mechanical stresses, ensuring the integrity of these aircraft, which are often subjected to extreme environmental conditions and variations. Among the numerous technologies employed in these devices, 3D printing, specifically Direct Ink Writing (DIW), has emerged as an effective alternative in the manufacture of strain sensors due to the possibility of customization, such as printing complex geometries, high precision, and simplicity of production. Furthermore, direct printing on target surfaces, which eliminates interface problems and the possibility of incorporating advanced nanomaterials during ink formulation, provides greater sensitivity, durability, and reliability of the devices. In this work we optimized a silver flake-based ink (CB028) through the addition of the polymers Polyolefin (adhesion modifier) and Ethyl Cellulose (cohesion modifier) in different concentrations. The optimal formulation was determined through rheological studies, including yield stress and viscoelastic recovery. In addition, the parameters of Flow curves and Shear thinning behavior were also evaluated. Subsequently, we printed the strain sensors with a serpentine pattern, utilizing flexible substrates made from polyethylene terephthalate (PET) using a high-resolution printer. and investigated the sensing performance of these devices through three-point bending tests. To evaluate their performance, we conducted a series of comprehensive three-point bending tests. Optimization studies were conducted on the laser annealing process to enhance manufacturing efficiency. In this study, an infrared laser with a wavelength of 1064 nm was used, and we investigated parameters related to power, number of passes, speed, and their influence on the sensitivity and stability of the fabricated strain sensors. The results showed that the formulation with the proportions of 90% CB028, 5% EC, and 5% PO (Ink5-5) by weight presented the best adhesion and tack of the ink to the substrate. With a viscosity value of 0.54 Pa.s, which denotes an ink with moderate shear thinning behavior, the printed devices presented a uniform line pattern with approximately 100 μm in width and 30 μm in height. The deformation tests corroborated what was observed in the rheology studies, showing that the best performance was achieved by devices manufactured with the Ink5-5 formulation. These devices presented an estimated Gauge Factor (GF) of 106, and remained stable for 300 consecutive cycles with a maximum deformation of 1.5%. Finally, laser annealing studies demonstrate that the ink resistivity values gradually decrease with increasing laser power (0.2 – 1.5 W), as well as with increasing number of passes (1 – 12 times) and decreasing laser incidence speed (15 – 2.5 mm/s). This demonstrates that targeted and punctual annealing is possible with the proposed process, and consequently, precise control of the electrical and mechanical properties of the ink is required according to the desired application. Our preliminary results suggest that adopting the methodologies and processes adopted both for the optimization of conductive inks and for the manufacture of strain sensors through high-precision direct 3D printing represents a promising alternative for the development of more effective, simple and low-cost sensors in additive manufacturing.
