###### Abstract  
The T1 DCoach mobile application is designed to educate patients--children with type 1 diabetes (T1D) and their caregivers and diabetes educators. The idea behind the mobile application is that its users perform actions that the patient needs to perform in real life. These include measuring blood glucose levels, operating the insulin pump, meal calculation, bolus administration, etc. These in-application activities are performed on the patient's digital twin. To increase user engagement, gamification elements have been implemented in the application. An important element of the T1 DCoach mobile application is its interface, which should be adapted to very different groups of users: children, their caregivers and educators. In addition to presenting the T1 DCoach application, the paper presents the stage examining the quality of the interface using three research groups: children, their caregivers and educators. The research was conducted using the scenario method, using eye-tracking, recording activities and thinking aloud. After the application testing sessions, surveys were carried out using the System Usability Scale method and focus group interviews were conducted. The research results are presented in the article along with the most important recommendations for improving the application interface.  
type 1 diabetes education; mobile application; interface quality; eye tracking; thinking aloud; System Usability Scale +
Footnote †: journal: Medical  
factors, including the patient's parameters and the dynamics of his/her disease, as well as lifestyle, including food intake. T1D disease manifests itself among young people and they are faced with the need to control many aspects of their lives.  
Medical-related applications are becoming more and more popular, and therefore their success on the market is conditioned by their appropriate quality and usability [5; 6]. In such a case, the quality of the Graphical User Interface (GUI) plays a very important role in their evaluation by potential users. These applications should be ergonomic, efficient and it should be easy to use and understand the interface, even by users who have no experience with this type of program. Applications of this type can be classified as broad-use applications, i.e., they are used by very different groups of users. This means that the graphical user interface must be adapted to the needs of a wide audience, including children [7]. Therefore, it is important to be guided by existing criteria and rules for designing user interfaces [8] when designing and creating this type of software, which will ensure a positive reception of the application by a potential user [9]. It is also necessary to introduce into the application development process activities related to testing its GUI with the participation of its potential users and using the research results to improve its quality.  
Apart from the Introduction, the paper presents a short review of the literature on software interface quality research, with particular emphasis on research on mobile applications with user participation. Then, the T1 DCoach application and its interface as well as the initial research plan are presented. The methods used in the study and the principles of selecting research groups are also indicated. The research implementation process and its results are considered in detail. The final part of the article presents a discussion of the study results along with the main recommendations for improving the T1 DCoach application interface. The article ends with conclusions.