Meso/macro-scale ultra-soft materials' mechanical property evaluation device and testbed.

Abstract

Ultra-soft materials find applications in biomedical devices, sensors and actuators, robotics, and wearable electronics. The mechanical properties of soft materials are often determined using nanoindentation and atomic force microscope techniques, which provide localized properties at a small-scale length. There is a need to evaluate the meso/macro-scale properties of ultra-soft materials to develop integrated devices made of the same. Metallic probes in the existing macroscale equipment cannot be used as they can pierce through the soft materials and fail to capture the associated adhesion forces, resulting in inaccurate values. This study has developed a meso/macro-scale mechanical testing platform to characterize ultra-soft materials accurately. This probe can be adapted to any mechanical testing load frame with a small load cell to capture the adhesion forces during the approach and detachment segments of soft materials' indentation. The integrated camera with the probe enables overcoming the challenge of surface detection and capturing the pull-on and pull-off events. Indentation tests on soft materials with varying stiffness (e.g., high-fat yogurt, chicken breast, aloe Vera, toothpaste, gelatin, and a chocolate bar) were conducted using a 10mm stiff flat-end polymer probe. A variation of the Johnson-Kendall-Roberts technique was adopted to account for adhesion forces and compute stiffness. Our results suggest that the novel device and methodology can measure mechanical stiffness in the extensive range of 0.5kPa to a few MPa with high reproducibility at the macro-scale length. The validation was carried out using a commercially available nanoindenter for soft materials.