The Development and Protocol for Testing a Co-Created Digital Intervention (Sentinel) to Improve Mental Well-Being and Help Manage and Prevent Trauma in First Responders.

Background: First responders (FRs) and frontline workers are frequently exposed to traumatic events within their professional roles. This exposure places them at risk of experiencing acute stress, posttraumatic stress disorder, burnout, and other adverse mental health outcomes. Despite growing awareness of these risks, there remains a lack of evidence-based digital interventions (DIs) tailored to meet their unique mental health needs. Objective: This study aims to address this gap by developing and testing Sentinel, an evidence-based, co-created DI to promote mental well-being, build resilience, and help manage and prevent trauma among FRs and frontline workers. The objectives include exploring their experiences of occupational trauma, identifying their preferences for digital mental health tools, and evaluating the feasibility of the Sentinel intervention. Methods: The development of Sentinel followed a rigorous, 4-phase approach. In phase 1, we conducted market analysis and in-depth qualitative interviews with 54 FRs from fire services, police, and emergency health sectors. The aim was to explore their mental health needs, barriers to accessing support, and views on the potential role of DIs in addressing these needs. In phase 2, we developed the content for Sentinel by integrating findings from phase 1 with existing evidence, policies, and theoretical frameworks, ensuring the intervention reflected the lived experiences of FRs. In phase 3, a high-fidelity clickable prototype of Sentinel will be tested through co-design workshops, iterative development sprints, and usability evaluations. Feedback from FRs and frontline workers during this phase will refine the app's design and functionality. Phase 4 involves a mixed methods, nonrandomized feasibility study to evaluate Sentinel's acceptability, usability, safety, and implementation potential. Quantitative data will be collected from up to 100 FRs, complemented by qualitative interviews with 30 FR participants and 20 health and social care professionals who refer FRs to the intervention. Results: The co-creation process has proven essential in ensuring Sentinel meets the specific needs of FRs and frontline workers. Preliminary feedback highlights the app's relevance and usability. Our pilot testing began in August 2025 and is planned for completion by August 2026. As of January 16, 2026, a total of 119 participants have completed the initial survey, and 59 have downloaded the Sentinel app, with 24 having used the app for 6 weeks and completed the follow-up survey. Of this group, 13 participants have consented to an interview. We are on track to meet our target recruitment sample, and these data will provide detailed insights to inform refinements and determine readiness for a larger efficacy trial. Conclusions: Sentinel represents a novel, personalized digital solution designed to address the unmet needs of FRs and frontline workers exposed to occupational trauma. Future work will evaluate its capacity to improve mental well-being, support trauma recovery, and build resilience.