Tuning Printability and Adhesion of a Silver-Based Ink for High-Performance Strain Gauges Manufactured via Direct Ink Writing.

Structural health monitoring (SHM) systems are critical in ensuring the safety of space exploration, as spacecraft and structures can experience detrimental stresses and strains. By deploying conventional strain gauges, SHM systems can promptly detect and assess localized strain behaviors in structures; however, these strain gauges are limited by low sensitivity (gauge factor, GF ∼ 2). This study introduces an approach to printing strain gauges with high sensitivity, while also considering stretchability and long-term durability. Through direct ink writing (DIW), these devices can be produced by the extrusion of a wide range of viscoelastic inks. The viscoelastic properties of the ink can be tuned with the help of additives to aid in the processing for a desired application. In this work, a series of inks were prepared from commercially available CB028 (silver ink used in screen printing) by adding a combination of ethyl cellulose (EC) and polyolefin (PO) (additives). With the goal of optimizing the long-term sensing response of the printed strain gauges, a systematic study of the rheological properties (frequency sweep analyses, yield stress, viscoelastic recovery, viscosity measurements, and tack tests) was conducted. A viscoelastic window approach was used to predict the optimal properties of the formulated inks. Using this approach, it was determined that 90% CB028, 5% EC, and 5% PO provided enhanced elastic properties, adhesion, and peel strength compared to commercial CB028. The formulated ink has enhanced tack (129 mN/mm) and peel strength (23.3 kJ/mm), which led to a viscoelastic window ideal for direct ink writing of the strain gauges. Printed structures were tested in a three-point bending configuration to record the piezoresistive responses that were correlated to the formulated rheological properties and underlying microstructure. The results revealed gauge factors as high as 106 with stable sensing responses for more than 300 cycles of strain. Scanning electron microscopy analysis also revealed minimal crack formation, which resulted in a stable response. The research demonstrated the feasibility of developing high-performance inks for potential printed strain gauge applications.