Liquid biopsy, based on the analysis of blood constituents, and more in particular, the study of circulating nucleic acids (CNAs) in plasma, is a less-invasive alternative to tissue biopsy which is showing great potential in the diagnosis and monitoring of cancer and other pathologies. Sample preparation is one of the bottlenecks that prevents the wider applicability of CNAs based technologies in clinical settings. Current methods for CNAs sample preparation involve bulky equipment and trained staff, non-compatible with near patient blood processing. This thesis describes the development of a microfluidic platform for the fast, user-friendly and reproducible sample preparation of CNAs from whole blood in a single step, deployable near patient. A fast-prototyping method developed to ease the iterative design of the disposable cartridges is described together with the design process that led to the final device. Finally, the characterisation of clinical samples within the microfluidic platform, on either microRNA or cell-free DNA extraction, is benchmarked against the gold standard benchtop protocol, currently used in molecular biology laboratories. The device is deemed to reduce sample preparation cost and allow analyses of samples before aging phenomena occur, thus showing the true state of disease.