Differential Stretchable Strain Sensor System for Monitoring Penile Axial Rigidity in Erectile Dysfunction Assessment

Abstract

Erectile dysfunction (ED) is a highly prevalent condition, projected to affect over 320 million men worldwide by 2025, as reported in the Massachusetts Male Aging Study (MMAS). Among the biomechanical indicators of erectile function, axial penile rigidity, defined as the penis’s ability to resist buckling during penetration, is considered the most functionally relevant for successful intercourse. However, existing diagnostic tools such as RigiScan assess only radial rigidity and rely on rigid loop-based hardware that may compromise comfort and fail to capture axial stiffness. These limitations highlight the need for a noninvasive, physiologically accurate, and continuous method to monitor axial rigidity.

To address this gap, a soft, stretchable strain sensor system was developed to assess axial rigidity through differential mechanical response. The system consists of two screen-printed sensors fabricated by depositing stretchable silver-based conductive ink onto a medical-grade thermoplastic elastomer substrate (styrene - ethylene - butylene - styrene, SEBS). These sensors, engineered with different stiffnesses, are designed to be placed longitudinally on either side of the penile shaft. During erection, the penis itself provides the elongation that passively stretches the sensors. The softer sensor deforms more and exhibits a greater relative change in resistance, while the stiffer sensor deforms less. Because both sensors require far less force to stretch than the tissue itself, they do not interfere with the natural erection process.

Bench-top testing involved elongating both sensors to 100 percent under axial forces up to 2.5 N, demonstrating clear and repeatable mechanical differentiation. While these tests do not replicate full physiological rigidity, they validate the principle of using resistance ratios to infer stiffness. In clinical application, the deforming force comes from the penis, and the resulting resistance ratio can be calibrated against known clinical thresholds. Earlier studies defined rigidity as insufficient below 3.9 N, borderline between 3.9 and 4.9 N, sufficient between 4.9 and 9.8 N, and optimal above 9.8 N. More recent cavernosometric data suggest that the ability to resist at least 14.7 N, equivalent to 1.5 kg, corresponds to optimal rigidity and is associated with intracavernosal pressures above 50 mmHg. The proposed system will be calibrated against both traditional and modern standards to allow clinically meaningful classification of erectile rigidity.

Future validation will be carried out in a controlled clinical setting in collaboration with a sexual health research laboratory. By relying on physiological elongation and passive sensor deformation, the system avoids mechanical interference and enables the potential for remote, wireless monitoring of erectile function. Additionally, the platform is designed to incorporate a printed temperature sensor that is currently being developed and characterized, supporting future multimodal assessment that includes both mechanical and thermal parameters. This differential stiffness strain sensor system offers a novel, non-invasive, and user-friendly solution for assessing axial penile rigidity.