Designing a robot with soft materials that are lightweight and compact allows it to enable safe human-robot interaction and portable design. While most research has been concentrated on various external stimuli to trigger soft actuators and sensors, less work has been conducted on stimuli for soft brakes that lock and release the motion. Brakes are essential components for robotic systems to achieve stable positioning and increase safety against unintended movements. In this study, a tubular brake is proposed that utilized the hygromechanical behavior of polymers for compact soft wearable robots. The inner diameter of the tube changes quantitatively with varying moisture content in the tube. This change adjusts the contact pressure between the tube and its inner wire, to control the friction applied to the force transmission wire. The brake can generate high frictional braking force per unit mass (similar to 774 N g(-1)) and unit area (similar to 0.5 MPa), which is 2.5 times higher than other soft brakes. It is also lightweight, flexible, and easily adaptable to the existing wire-driven mechanisms by replacing the conventional wire sheath with the hygroscopic polymer tube. The effectiveness of the brake was demonstrated by implementing it into an electrical- power-free wearable rehabilitation glove for a stroke survivor.