Author	G. Alkhaldi
Author	E. Murray
Author	F. Hamilton
Author	S. Michie
Abstract	Background Digital interventions (DIs) provide effective and potentially cost-effective models for improving health behaviour outcomes as they deliver health information and services that are widely disseminated, confidential, and can be tailored to needs of the individual user. DIs have been used successfully for different health behaviours such as smoking cessation and increasing physical activity. However, their effectiveness is limited by low usage rates, with non-engagement a major challenge. Engagement can potentially be optimised through the use of context-specific prompts such as emails and text messages. Aim To inform, develop and assess the potential of using email and text message as prompts to optimise users' engagement with a DI called HeLP-Diabetes (Healthy Living for People with Diabetes). Context HeLP-Diabetes is a DI targeting self-management for people living with Type 2 diabetes. It aims to provide its users with the tools and support to improve and maintain their health and well-being. Methods The following series of studies with their corresponding objectives were conducted: • A systematic review and meta-analyses to determine the range and effectiveness of technology based engagement prompts, and to identify characteristics of prompts to test in subsequent studies. • Developing, piloting and testing prompts, within the context of HeLP-Diabetes using the Medical Research Council guidance for complex intervention, to identify the methodological and practical challenges of developing engagement prompts and integrating them with HeLP-Diabetes. • A mixed method study to identify the characteristics of prompts, specifically the content and delivery mode, that have the potential to promote engagement with HeLP-Diabetes. The mixed method study used quantitative data from HeLP-Diabetes usage data, and qualitative data from think aloud interviews exploring prompt content with a selected group of users. • Two consecutive randomised controlled trials comparing different prompt content and delivery modes in order to assess the potential impact of prompts on engagement with HeLP-Diabetes and to test the procedures of the pilot trial to identify any potential challenges for future trials.