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Liquid handling in 3D printed autonomous microfluidics
Book - Dissertation
Rapid diagnostic testing at a patient's location, so-called 'point-of-care' (POC) testing, is essential to provide healthcare when a fully equipped laboratory is not accessible. In developing countries, suitable POC diagnostics could save millions of lives yearly via the early diagnosis of a small number of treatable conditions identified by the World Health Organization (WHO). As a result of the inverse correlation between the number of lives saved and the level of diagnostic infrastructure required, the WHO underscores the need for low-cost, disposable assays that require minimal user-dependent steps. Lateral flow tests (e.g., home pregnancy tests) are one of the few technologies that meet these criteria. In these tests, the liquid sample wicks through a porous paper-like membrane driven by capillarity. Notwithstanding their success, lateral flow tests are typically not quantitative, and their sensitivity is limited since chemical signal amplification is not possible in this one-dimensional format. More sensitive amplification-based analyses (e.g., enzyme-linked immunosorbent assays, ELISAs) require a precisely timed sequence of steps.We developed a novel powder bed 3D printing approach to fabricate porous objects with different internal surface chemistries and functionalised volumes. Furthermore, reactants can be inkjetted during the printing process, so pre-loaded printouts can be produced. We applied this 3D printing approach to fabricate passive microfluidic devices by locally controlling the surface chemistry in porous printed parts. This way, 3D networks of hydrophilic channels surrounded by hydrophobic walls can be produced through which aqueous solutions can be driven by capillary forces alone. The channel wicking rate was highly reproducible, with a standard deviation of less than 4%, and the relation between the wicking distance and the time were modelled. Channels of specific lengths were designed to time the fluid delivery within the microfluidic platform structures. To achieve further control over the capillary flow, printable fluidic triggers were developed. These triggers can be activated by a timed flow, thus opening new flow directions within the channel network at a specific time. By combining these triggers and the control over wicking speed, timed reactant delivery sequences can be programmed in the 3D microfluidic platform to enable running a multistep bioassay in an autonomous fashion. As a proof of concept, a complete ELISA for IgE was successfully integrated on the platform. This device, in combination with a camera and automated image analysis, enabled to determine IgE concentrations as low as 200 ng.mL-1 while keeping the manual steps in the assay limited to sample and buffer deposition.The 3D printing method developed in this thesis represents a radically new concept for POC diagnostics capable of chemical signal amplification and compliant with the WHO criteria for low-resource settings. This 3DP platform brings new opportunities to the domain of microfluidics, since complex 3D channel networks can be easily created. More generally, this approach opens perspectives for other fields where porous materials with functionalised surfaces are required, including heterogeneous catalysis, fuel cells, etc.