In some examples, a medical device may include circuitry on a circuit board, a casing, and a fill material at least partially within the casing. The casing may receive the circuit board and surround at least a portion of the circuit board. The casing may define an aperture configured to allow one or more electrical connectors of the circuit board to extend through the aperture for one or more electrical connections with one or more components outside of the casing. An outer surface of the casing may define a first portion of an outer surface of the medical device. The casing may comprise a material having a first melting temperature. The fill material may define a second portion of the outer surface of the medical device and may have a second melting temperature that is lower than the first melting temperature.