In recent years, soft robotics has surged in applications like wearables, drones, smart fabrics, and medical instruments. Due to their compliance, these devices excel in tasks demanding dexterity and adaptability, such as manipulation, locomotion, crash resiliency, and surgical procedures. One intriguing design is the filiform structure, known as "smart fibers." They can be easily integrated into different objects and offer various functionalities such as actuation, sensing, energy generation, and adaptable stiffness. In medical devices, like catheters and endoscopes, smart fibers enhance dexterity and compliance through variable stiffness. Variable stiffness allows reversible changes from soft to rigid states triggered by factors like thermal, electrical, magnetic, or pneumatic. Incorporating variable stiffness does bring certain drawbacks into play. These include increased design complexity, a compromise in biocompatibility due to the use of toxic components, and performance limitations stemming from slower reaction times in stiffness changes. To overcome these limitations, we present in this thesis three filiform variable stiffness technologies that enable miniaturization, biocompatibility, and fast transition changes between soft and rigid states for medical devices used in minimally invasive surgeries. First, we propose an easily scalable filiform design made of a conductive phase-changing polymer, serving as a heater, temperature sensor, and variable stiffness substrate. We developed a new fabrication method using a dipping technique, enabling the creation of fibers with the desired thickness and electrical resistance. Then, we introduce two approaches to design filiform variable stiffness technologies with fast reaction times using phase-changing materials with active cooling and fiber jamming. Finally, we discuss their advantages and disadvantages in terms of scalability, stiffness change variation and speed. Thanks to variable stiffness fibers, the minimally invasive devices of the future will offer doctors increased dexterity and safety previously unimaginable.