Diabete is a disease which is spreading faster than ever, impacting now a larger population. Patients suffering from this disease need the injection of insulin to control extracellular sugar levels. Pharmaceutical companies developed injector pens to allow patients to inject themself the right amount of drug. But the dosage depends on many parameters as the previous meal, current blood sugar level... and a wrong dosage can lead to health problem e.g., loss of conciousness at short terms and severe diseases like cardiac pathologies at long terms... To prevent these situations, a smart pen cap has been developed at Valtronic Technologies to follow and log the individual injections by measuring the level of drug in the pen, the temperature, and the relative humidity to allow the patient and the medical staff to have an history of the injections and the climatic conditions. Injector pen and smart pen cap have been firstly presented with a non-exhaustive list of different physics which can be used to measure a level of liquid. For need of precision, the capacitance technique is the best technology choice. Therefore, the smart pen cap is based on a capacitive sensor. Different capacitance converter topologies have been compared through a bibliographic study. It appears that the Lock-in converter gives the best sensitivity and Charge Sensitivity Amplifier has the lowest power consumption. A trade-of was to use a capacitance-to-digital-converter. A chip from Analog device has been selected due to its high resolution below the femto-farad level, its simple interface and its availability on the market which allows a fast development and industrialisation. Then, switch and electronic structure are presented. First tests with an empty device have been conducted to find the noise floor, which is below 0.1fF. Subsequently, a study of different electrodes configuration has been performed. Firstly, an analysis on two parallel flat electrodes has been conducted using theoretical and simulation approaches. Then, two semi-cylindrical electrodes have been analysed. A comparison has been made between two theoretical methods which are: considering an infinite number of parallel flat electrodes following the shape of the semi-cylindrical electrodes and the conformal mapping methods. This last method gives coherent results when comparing real measurements and simulation. The device is equipped with semi-cylindrical electrodes which allow for a small package and a good correlation between capacitance and drug level with a convertion slope of 0.5fF/µL. Tests have been performed which give an error of +-1 IU and +-4 IU for large injection (set point equal to 72 IU). Finally, the study of some parasitic effects is presented in the last section. An important effect is the climatic effect which affects the capacitance due to variation of electric permittivity linked to temperature change and water absorption of certain materials as plastics. To compensate for this effect, reference electrodes and a climatic sensor have been implemented. Another important effect comes from the cartridge position in the non-uniform electric field of the smart pen cap. A four electrodes device has been realized to allow a perpendicular rotation of the electric field to obtain a more constant averaged electric field. As a consequence, the position impact of the cartridge has been reduced. Other effects have been considered. The device can detect a minimum injection of 52uL.