Embodiments of the present invention provide drug-delivery devices comprising a drug reservoir chamber containing a substance to be delivered, in fluid connection with a drug administration means, and at least one displacement-generating battery cell coupled to said drug reservoir chamber by a coupling means, the at least one displacement-generating battery cell comprising an element that changes shape as a result of discharge of the battery cell so as to cause a displacement within the battery unit, the arrangement being such that the displacement derived from said battery unit is conveyed by said coupling means to cause displacement of at least a portion of a wall of said drug reservoir chamber reducing the volume of said drug reservoir chamber such that said substance is expelled from said drug reservoir chamber towards said drug administration means upon discharge, thereby being a self-powered drug delivery device.