(310e) Conformal 3D Printing and Laser Annealing of Strain Sensors

Strain sensors are essential for precisely monitoring materials and structures that experience dimensional variations due to mechanical stresses and external forces. These devices are widely used across various fields, including structural engineering, materials characterization, and structural integrity assessment. In the aerospace industry and for space applications, strain sensors are integral to structural health monitoring (SHM) systems, enabling real-time detection of structural fatigue, vibrations, deformations, and localized stresses to ensure the reliability and safety of aircraft and spacecraft operating under severe mechanical and thermal conditions. While conventional strain gauges are commonly implemented in SHM systems for detecting and evaluating localized strain behaviors, their effectiveness is often limited by inherently low sensitivity. This study proposes an alternative to commercial strain gauges, aiming to overcome limitations related to low sensitivity while enhancing mechanical performance. Direct Ink Writing (DIW), a high-resolution additive manufacturing technique, offers a strong platform for fabricating customizable, high-performance strain sensors. Additionally, the ability to print directly onto target substrates reduces interfacial issues and enables the integration of functional nanomaterials during ink formulation, thereby improving sensitivity, durability, and reliability. In this work, a series of inks were formulated from commercially available silver flake-based ink (CB028) by incorporating varying concentrations of ethyl cellulose (EC) and polyolefin (PO) as cohesion and adhesion modifiers, respectively. A systematic rheological characterization was conducted to better understand the viscoelastic properties of the different inks and aim for improved processing in applications such as high-sensitivity printed strain gauges. Using this method enables accurate extrusion and structural integrity across various geometries and substrates. In this study, the strain sensors were printed with a uniform line with approximately 120 μm in width and 25 μm in height on different substrates, such as flexible polyethylene terephthalate (PET) and high-pressure fiberglass laminate (G-10) cylinders. When printing in a complex (conformal) structure, an efficient alternative for improving the post-processability of these structures is the use of laser curing. This method overcomes physical limitations and reduces processing time and energy consumption compared to conventional thermal curing using an oven. To evaluate its effectiveness, parameters for laser ink curing of the strain sensors were carefully examined and compared to conventional thermal curing. Specifically, an infrared laser (1064 nm) was utilized, and factors such as laser power (0.2 – 1.5 W), number of passes (1 – 7 times), and speed (1 – 10 mm/s) were analyzed to assess their effects on the sensitivity and stability of the fabricated strain sensors. The results of laser annealing indicate that each tested parameter significantly affects ink resistivity and strain sensitivity, with laser power being the most influential, followed by the number of scans and scanning speed. After the laser annealing, the electrical stability studies showed a variation of less than 2% in the ink resistivity as a function of time (48h), temperature (25C) and humidity (50% RH). The proposed process allows for localized and controlled annealing, enabling precise adjustments to the electrical and mechanical properties of strain sensors to meet specific application requirements. Additionally, it establishes a scalable and adaptable manufacturing approach with significant potential for next-generation structural health monitoring systems, particularly in challenging environmental and mechanical conditions.