(181f) Ink Formulation and Direct Ink Writing of High Gauge Factor Strain Sensors

On-demand Structural Health Monitoring (SHM) systems are critical for NASA to ensure the safety of space exploration due to the unwanted stresses and resulting detrimental strains the spacecraft encounters in outer space. By deploying silver-based flexible strain gauges, SHM systems can promptly reveal and assess localized strain behaviors in structures. However, recent studies show that silver nanoparticle-based strain gauges have low sensitivity (gauge factor, GF), stretchability (stain %), and durability under repetitive strain, which limits their application. This study aims to fabricate a high-performing flexible strain gauge via Direct Ink Writing (DIW) using a commercial conductive silver ink CB028 (Dupont) and a flexible polyethylene terephthalate substrate. An adhesion-modifier (polyolefin – PO) and a binder (ethyl cellulose – EC) were used to modify the adhesion and elastic properties of the functional inks and tailor the final sensor performance.

To increase the performance of the sensors, a series of inks were prepared by varying the concentration between CB028, EC, and PO. The rheological properties (moduli, yield stress/strain, flow curves) of the formulated inks were studied and correlated with the printing performance. The results indicate (for the first time) that appropriate ink formulation facilitates printing thin traces (100 microns width) with enhanced adhesion, elastic, and resistance-change properties. The ink containing 5% PO, 2%EC, and 93% CB028 by weight had the most stable response with no noticeable cracks after repeated strain testing. The obtained strain gauge displays a GF as high as 40 at 1.5% strain (Figure 1), which is significantly higher (20x) than the GF of comparable and commercially available strain gauges. In addition, the resultant strain gauge exhibits a stable response for more than 300 cycles of repetitive strain. Overall, this research demonstrates the potential design window for silver ink formulation and fabrication of high-performing strain gauges using DIW for the manufacturing of SHM instrumentation.

Keywords: Structural health monitoring, silver ink, DIW, strain gauge, gauge factor