Optimization and Validation of a Hydrogel-Based Wearable Cortisol Sensor for Continuous Soldier Load Monitoring

The Army’s greatest asset is the Soldier and ensuring peak physical and cognitive readiness is essential to maintaining overmatch across operational domains. While physical performance is routinely measured via wearables and fitness assessments, no field-deployable system currently exists to quantify a Soldier’s internal biochemical load. This capability gap impedes efforts to optimize readiness, prevent overuse injuries, and improve recovery strategies. Overuse musculoskeletal injuries, which are often invisible until they result in mission-limiting damage, cost the Army over $2 billion annually and comprise over 80% of all injury-related expenses. A key biomarker linked to both cognitive strain and musculoskeletal breakdown is cortisol, the gold-standard for measuring stress. Fluctuations in cortisol levels reflect the body’s total load and, if monitored in real-time, could inform evidence-based training cycles, injury prevention, and warfighter recovery. E-Sentience, Inc. (DBA: DIA) has developed a wearable electrochemical sensor that non-invasively detects cortisol in sweat and transmits real-time data to an edge device. Feasibility was demonstrated in our Phase II SBIR effort through successful detection of cortisol in biologically relevant concentrations and the development of a fully functional wearable prototype. However, to transition this technology to the Army, additional R&D is required to improve performance across variable sweat rates, enhance the device’s wearability, and validate field utility in operationally relevant environments. Under this Sequential Phase II SBIR effort, DIA will optimize and validate a hydrogel-based microfluidic interface that enables volume-independent cortisol sensing across a range of sweat rates (1-30 µL/min), from rest to high-exertion conditions. The hydrogel layer will prevent electrode fouling and ensure stable measurements by transporting sweat to a controlled-volume reservoir. Concurrently, the wearable housing and attachment strap will be redesigned to ensure constant tensile pressure on the skin, ease of donning/doffing, and enable on-body recharging. Form, fit, and function testing will be conducted in collaboration with the DEVCOM Soldier Center. Initial testing will evaluate comfort and usability during Army physical training protocols, followed by obstacle course testing with concurrent salivary sampling to validate cortisol trends via third-party immunoassay. This work will elevate the system to TRL 6 and directly support Army modernization priorities in human performance monitoring, injury prevention, and predictive health. Transition pathways include PEO Soldier, the Army’s H2F program, and integration with commercial platforms through established relationships with wearables companies such as OURA. By enabling real-time biochemical monitoring, this sensor system has the potential to revolutionize how the Army quantifies, manages, and optimizes Soldier readiness.