D-shaped optical fibers (D-fibers) have been proven as platforms on which in-fiber sensors can be created. In-fiber devices avoid the problems of high insertion loss, power reflection, and poor mechanical integrity that accompany the interfacing of bulk optical components with single-mode fibers. In-fiber devices also replace the costly fiber pigtailing process with automated fusion splicing. In prior work the proximity of the core to the flat surface of the D-fiber has been exploited in various kinds of evanescent sensors. In this presentation we show how to remove a portion of the core and replace it with a sensing agent. Replacement of the core allows for greater interaction strength than that allowed by evanescent sensors. The differential etch rates of the core and cladding materials of the D-fiber in hydrofluoric acid (HF) allow the core to be etched while the cladding remains mostly intact. The amount of core removed is controlled by monitoring laser power transmission through the fiber while it is immersed in a HF bath. The etching process leaves an approximately 2 cm long groove along the fiber. Sensing material is then spun into this groove. The etching and deposition processes result in a low-loss in-fiber waveguide that can be used as the basis for a variety of in-fiber sensors. This core-replacement technique is flexible because many different types of materials can be deposited into the core and it also allows for gratings to be written into the sensing material.