Magneto-capillary valve for integrated biological sample preparation (2011)

A major technological trend in in-vitro diagnostics is the integration and miniaturization of laboratory procedures into so-called ‘lab-on-chip’ devices. The aim is to achieve better integration of diagnostics into the medical workflow by providing compact devices that can analyze patient samples at the point of care, close to the patient. Ease-of-use is an important characteristic of point-of-care diagnostics. One specific feature that enables such easy-to-use devices is a ‘sample in – result out’ type of performance. However, in many cases a raw body fluid is not directly suitable for analysis. Therefore, an elaborate multi-step process of sample preparation is required before actual analysis of the sample can take place. While many detection technologies have been fully automated and successfully miniaturized and integrated into a lab-on-chip format, sample preparation has been staying behind on this trend. As a result, sample preparation requires a substantial amount of manual handling by a trained operator and is often the bottleneck in the process from sample to result. Integration and miniaturization of automated sample preparation is thus required to provide the ease-of-use and portability that is needed to bring diagnostics closer to the patient. This thesis aims at advancing the level of integration and miniaturization of automated biological sample preparation to enable point-of-care applications with a ‘sample in – result out’ type of performance. For this purpose, a novel microfluidic actuation concept is proposed: the magneto-capillary valve (MCV). The MCV technology is based on stationary microfluidics, in which discrete units of liquid are present at fixed positions in a microfluidic device. The MCV cartridge is a capillary device, in which aqueous liquids are confined by capillary forces resulting from specific features of the cartridge. Magnetic particles are transported through a hydrophobic valve medium from one stationary liquid to another by externally applied magnetic forces. The MCV technology provides a means for solid phase extraction, which is a common type of sample preparation. Analytes are coupled to magnetic particles in the sample matrix and are transported through one or more washing buffers to be finally eluted from the particles in a buffer that is appropriate for detection of the analyte. A key advantage of the MCV is its high valving efficiency due to the minimal quantity of liquid that is co-transported with the particles carrying the analyte. Moreover, by choosing a large sample volume and a small elution volume, the sample can be enriched and its volume matches the sample volume requirements for lab-on-chip devices. The envisioned system consists of a low-cost disposable cartridge that is driven by an instrument containing a magnetic actuation system like, for example, a movable permanent magnet. Many cartridges of different designs and various architectures were fabricated with a lead time of less than a week, due to a well-defined and yet very flexible fabrication process. In total, almost 1000 cartridges were fabricated over a period of about 2 years. This large number of cartridges was necessary to investigate the principles of magneto-capillary valving, to create options and define limitations of the MCV concept, and to test the performance of the MCV concept in biological sample preparation. Several MCV instruments were built as experimental setup to investigate the behavior of the valve and as instrument to enable experiments of biological sample preparation. The setup allows for quantification of the magnetic force that is applied to the particles. This quantification is realized by combining recorded images of the magnetic particle cloud with the measured susceptibility of the particles and the calculated magnetic field of the magnet. The behavior of the valve is described by a model that balances magnetic forces, capillary forces and friction forces. The performance of the MCV was evaluated by investigating the physics of magneto-capillary valving. The valving efficiency, the transport of magnetic particles, and pinch-off were investigated experimentally to characterize the valve operation. The conditions for successful operation of the valve were defined as a function of several design parameters. Investigation of the friction forces resulted in understanding of the intra-chamber dynamics, leading to the concept of force gradient mixing. Experimental results of DNA purification from spiked water and plasma samples demonstrate the feasibility of integrated biological sample preparation using the MCV technology. The performance of DNA purification in MCV cartridges was comparable to the performance of common commercially available solutions, while the MCV cartridge technology is much less complex. Integrated enrichment of DNA from 800 µl water samples showed an effective enrichment of 40 times, thus providing a substantial increase in detection sensitivity. DNA was also extracted successfully from samples with THP1 cells, which is a step further towards the total integration of a molecular test. The enrichment of proteins that was demonstrated in the MCV technology enables a whole new range of applications based on immunocapture of biomolecules. The approach of stationary microfluidics provides a strong reduction in complexity of the system, which is particularly valuable for point-of-care devices. From evaluation of the various valve architectures, the geometrical air valve appears to be the most suitable magneto-capillary valve architecture for integrated biological sample preparation. It features at the same time the best performance for a wide range of biochemical assays, as well as simplicity, which is essential for integration and for the concept of low-cost disposable cartridges. With that, the MCV technology has the potential to open new opportunities for integration and miniaturization of automated biological sample preparation.