Just microns in length, the bio-engineered cell rover swims through the human body performing useful medical tasks such as drug delivery, waste product or toxin removal, cell targeting, intracellular transport and cellular repair. This de novo protein device has a modular design with an internal frame of keratin, chitin or calcium carbonate. Skin panels are made of lipid bilayers to permit biomolecular inclusions such as "self" molecules. There are one or more internal organelles for mission-specific applications. Various propulsion schemes are possible, including bacterial cilia, bacterial flagella, systolic means, pilin harpoons and a novel external traction system utilizing kinesin or its analogs, with power derived from natural in vivo resources. One method of communicating (perhaps also supplying electric power) with its onboard control system might utilize a deployable, submillimeter band single-molecule radio antenna. The above device can be built without the use of atomic assemblers. Biocompatibility requires engineering the machine to appear as a native cell.