**Background:** Heart failure (HF) affects an estimated 64.34 million people worldwide and is associated with low quality of life, high morbidity and mortality, and frequent hospitalizations. Self-care behaviors are critical for maintaining compensation, yet poor adherence is a global problem. Mobile health (mHealth) technologies offer a promising strategy to improve self-management, but no Portuguese-language apps for HF self-care were found in Brazilian app stores. This study aimed to develop and validate a mobile application prototype to promote self-care in people with HF.
**Methods:** This technological development study was based on Contextualized Instructional Design (CID) and carried out at a federal teaching hospital in Paraíba, Brazil, a reference in cardiology care. The study proceeded in three steps: (1) Analysis — a narrative literature review using PubMed/MEDLINE, CINAHL, and Scopus to identify educational content for HF self-care; (2) Design/Development — instructional content was created using accessible language, with interface design in Figma® and hybrid development using React Native for Android® and iOS® platforms; (3) Validation — content validation by six cardiology specialist nurses (mean age 38.67±6.62 years, 66.7% female, all PhDs in Nursing) using the Suitability Assessment of Materials (SAM) instrument adapted for Brazilian Portuguese (22 items, six categories; maximum 42 points after excluding one item). Face validation was conducted with 13 HF patients (mean age 55.92±12.95 years, 53.8% female, 100% completed high school, mean diagnosis duration 7±6.25 years) at the cardiology outpatient clinic using a four-point Likert scale and Content Validity Index (CVI). Data were collected from October 2021 to February 2022.
**Key Results:** The "Tum Tum" application prototype features a home screen with registration or guest mode, three tabs (Mosaic with educational content on HF causes, symptoms, diagnosis, treatment, and nine self-care topics; My Health with medication/exam/appointment reminders, daily weight tracking, symptom checkboxes with alerts for ≥3 clinical manifestations indicating decompensation, and orthopnea assessment via pillow count; and Settings). Content validation by specialists yielded an overall SAM score of 88.4%, classifying the app as superior material (≥70%). Among individual items, most were rated "Adequate" by all six specialists for graphic illustrations, layout, and cultural adequacy. One expert rated content for promoting self-care as "Inadequate," suggesting more educational information could be added. Face validation by the target audience showed a total CVI of 0.98. Only item 6 (font size) scored below the 0.78 threshold (CVI=0.69), leading to adjustments. All other items achieved CVI≥0.92, including organization (1.0), language ease (0.92), illustration appropriateness (1.0), content sufficiency (0.92), and motivation to use (1.0). Participants reported positive feedback, with comments such as "I believe that the use of the application will help the treatment" (P1) and "I will definitely use it, it will be like a companion" (P3). No participant reported disliking the application.
**Clinical Implications:** The "Tum Tum" application represents an innovative, nurse-led mHealth technology developed in Northeast Brazil that provides accessible educational content and self-monitoring tools for HF patients. The app includes features for early recognition of clinical decompensation (red flags for ≥3 symptoms), weight monitoring with alerts for sudden gain, medication reminders, and consultation scheduling — all aligned with Brazilian and European HF guideline recommendations. The high content and face validity scores suggest the app is suitable for its target audience, though limitations include a small specialist sample, patients from a single service with higher-than-average education levels, and the need for future usability and effectiveness testing. The authors note that the high financial cost of mHealth development is a limiting factor and call for interdisciplinary collaboration to create low-cost solutions. Future studies will evaluate usability, efficiency, satisfaction, and clinical effectiveness before public sector deployment.