The study aimed to investigate the dynamics of e-textile-supported STEAM activities, specifically focusing on the design and development process of e-textile projects. A professional development (PD) program with seven hands-on and design-based activities was developed and implemented in two camps involving 20 and 19 in-service science teachers. Design-based research was adopted, and interviews were triangulated with multiple forms of data from observations, camp evaluation forms, surveys, and design artifacts. Results indicated that the hands-on activities incorporate key design components with pros and cons, offering engaging, interactive, and collaborative learning experiences for science teachers. However, they involve significant challenges in coding, aesthetics, circuitry, sewing, design sketching, and materials. The study also offers ten design principles for individuals to make informed decisions about the design and development of STEAM projects using e-textile technology. This study has significant implications for educational designers who consider designing e-textile-supported STEAM activities for K-12 teachers/students and for teachers who are interested in integrating these activities into their science courses.