The market of programmable implantable pumps has bound to a monopolistic situation, inducing high device costs, thus making them inaccessible to most patients. Micro-mechanical and medical innovations allow improved performances by reducing the dimensions. This affects the consumption and weight, and, by reducing the number of parts, the cost is also affected. This paper presents the procedure followed to design an innovative implantable drug delivery system. This drug delivery system consists of a low flow pump which shall be implanted in the human body to relieve pain. In comparison to classical known solutions, this pump presents many advantages of high interest in both medical and mechanical terms. The first section of the article describes the specifications which would characterize a perfect delivery system from every points of view. This concerns shape, medication, flow, autonomy, biocompatibility, security and sterilization ability. Afterwards, an overview of existing systems is proposed in a decisional tree. Positive displacement motorized pumps are classified into three main groups: the continuous movement group, the fractioned translation group and the alternative movement group. These systems are described and the different problems which are specific to these mechanisms are presented. Since none of them fully satisfy the specifications, an innovation is justified.. The decisional tree is therefore extended by adding new principles: fractioned refilling and fractioned injection within the fractioned translation movement group, spider guiding system within the alternative translation movement group, rotational bearing guided device and notch hinge guided device in the alternative rotation movement group.