This PhD research investigates the application and potential of 3D and 4D printing within fashion and technical outerwear design, with a particular focus on enhancing garment breathability through responsive, self-actuating components. The study aims to examine the extent to which additive manufacturing technologies are currently integrated into fashion design practice; to explore and critically evaluate existing methods of garment breathability and innovation; to combine interdisciplinary expertise from fashion, additive manufacturing, product design, and textile engineering; and to design and develop a self-actuating component that actively supports thermal regulation in an outdoor jacket. Adopting a mixed, practice-based research methodology informed by design thinking and design science principles, the research integrates qualitative analysis, technical investigation, and iterative design development. The methodology includes a critical review of smart textiles and ventilation strategies, analysis of outdoor garment brands and patented vent systems, user-centred data collection through questionnaires, and material-led experimentation using 3D and 4D printing technologies. Biomimicry is employed as a key design influence, informing the development of responsive structures that emulate natural systems of adaptation and regulation. Design development is supported through iterative testing and evaluation, enabling the translation of analytical and theoretical insights into functional design outcomes. The primary outcome of the research is the development of a 4D printed, self-actuating ventilation component for outdoor clothing. The component responds dynamically to environmental and bodily conditions to regulate airflow without requiring manual intervention, addressing limitations identified in static or wearer-controlled ventilation systems. The contribution to knowledge lies in the development and documentation of what is proposed as the first fully integrated 4D printed smart textile component incorporated into a wearable fashion and outerwear context. By situating 4D printing within textile engineering and smart textiles research, the study advances understanding of how additive manufacturing can move beyond prototyping and aesthetics to enable adaptive, performance-driven garment systems, offering new directions for innovation in technical apparel and responsive textile design.