An elongated device includes an optical fiber arranged to allow transmission of light to phase change material arranged long the elongated device, for optically heating the phase change material to change its stiffness from one stiffness value to a different stiffness value. Using distributed tilted or blazed Bragg gratings with light wavelength dependent unique grating periods along the optical fiber, a guide wire or catheter is provided which can be stiffness controlled at selected longitudinal portions. Behavior of the tip of a guide wire or catheter is controlled for optimal navigation. Portions of phase change material arranged inside a tube material can be activated at selected longitudinal parts of the elongated device.