The precise manipulation of particles and droplets is crucial to many microfluidic applications in engineering. The design of microfluidic devices is generally tailored to perform a specific task, with each specific application requiring a unique and fixed design. In this way, using a single device to perform multiple analyses of a wide range of specimens, from biological to chemical specimens, is unfeasible. Here, we address this issue and present a microfluidic approach that dynamically controls the hydrodynamic flow and the streamlines to realize complex multi-particle manipulations within a single device. Our approach combines the design of a flow-through microfluidic flow cell together with an optimization procedure to find a priori optimal particle path-lines, and a Proportion-Integral-Derivative-based (PID) feedback controller to provide real time control over the particle manipulations. In our device, particles are manipulated with hydrodynamic forces, by using a uniform flow through the flow cell and three inlets perpendicular to the flow cell. The streamlines within the device are manipulated by injecting or extracting fluid through the three inlets. We demonstrate the robustness of our approach by performing multiple functions within the device, including particle trapping, particle sorting, particle separation and assembly. We show that the real time control procedure affords accurate particle manipulation, with a maximum error on the order of the diameter of the particle. Our particle manipulation approach is particularly well suited to biological samples and living cells.@en

The durability of prosthetic arteriovenous (AV) grafts for hemodialysis access is low, predominantly due to stenotic lesions in the venous outflow tract and infectious complications. Tissue engineered blood vessels (TEBVs) might offer a tailor-made autologous alternative for prosthetic grafts. We have designed a method in which TEBVs are grown in vivo, by utilizing the foreign body response to subcutaneously implanted polymeric rods in goats, resulting in the formation of an autologous fibrocellular tissue capsule (TC). One month after implantation, the polymeric rod is extracted, whereupon TCs (length 6 cm, diameter 6.8 mm) were grafted as arteriovenous conduit between the carotid artery and jugular vein of the same goats. At time of grafting, the TCs were shown to have sufficient mechanical strength in terms of bursting pressure (2382 ± 129 mmHg), and suture retention strength (SRS: 1.97 ± 0.49 N). The AV grafts were harvested at 1 or 2 months after grafting. In an ex vivo whole blood perfusion system, the lumen of the vascular grafts was shown to be less thrombogenic compared to the initial TCs and ePTFE grafts. At 8 weeks after grafting, the entire graft was covered with an endothelial layer and abundant elastin expression was present throughout the graft. Patency at 1 and 2 months was comparable with ePTFE AV-grafts. In conclusion, we demonstrate the remodeling capacity of cellularized in vivo engineered TEBVs, and their potential as autologous alternative for prosthetic vascular grafts.

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Manipulating particles is of interest in diverse fields of engineering. Generally, manipulation activities carried out in micro-devices have a fixed design tailored to specific task. To address this issue, we designed a Hele-Shaw flow cell with "virtual" channels generated by uniform flow in transverse direction and three inlets in the longitudinal axis. These three inlets can inject or dispense fluid in the flow cell to deviate the streamlines. This device provides us the opportunity to integrate multiple functionalities such as particle trapping and separation onto a single device. Since the depth-averaged velocity over the channel in a Hele-Shaw cell is irrotational, we use potential flow theory to predict the flow field for manipulating particles.

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Abstract: In microscopic particle image velocimetry (micro-PIV), correlation averaging over multiple frames is often required, leading to a loss in temporal resolution, therefore limiting the measurement accuracy for unsteady flows. Here, we present a new PIV method suitable to study steady and unsteady laminar flows between parallel plates (i.e., Hele-Shaw flow), which is a common flow configuration in microfluidic applications. Our method reduces the effective seeding density and yields similar if not higher signal-to-noise ratio (SNR) compared to conventional micro-PIV. We call this algorithm Ψ -PIV. Ψ -PIV requires a much smaller number of frames to reach the same SNR compared to the widely used correlation averaging method. This leads to a significant improvement of the temporal resolution. The Ψ -PIV algorithm is used in an experimental investigation of steady and unsteady flows in a Hele-Shaw cell. Our experiment shows that Ψ -PIV reduces the number of required frames by 8 times and 30 times compared to the frames required by conventional PIV for steady and unsteady laminar flow, respectively. In this study, PIV and Ψ -PIV use a single-pass cross-correlation to present the underlying difference between the two approaches. Graphic abstract: [Figure not available: see fulltext.].

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The use of optical diffusers for the reduction of peak-locking errors in particle image velocimetry is investigated. The working principle of the optical diffusers is based on the concept of birefringence, where the incoming rays are subject to different deflections depending on the light direction and polarization. The performances of the diffusers are assessed via wind tunnel measurements in uniform flow and wall-bounded turbulence. Comparison with best-practice image defocusing is also conducted. It is found that the optical diffusers yield an increase of the particle image diameter up to 10m in the sensor plane. Comparison with reference measurements showed a reduction of both random and systematic errors by a factor of 3, even at low imaging signal-to-noise ratio.@en

In micro-PIV, reaching the optimum image density has always been difficult due to high displacement gradient, coagulation of particles at the inlet and due to particles adhering to the surface to name a few. The most widely used method is to take ensemble correlation average of multiple PIV images to extract the velocity field. However, this leads to low temporal resolution. Hence, a method with high temporal resolution is pivotal to study unsteady laminar flow in microfluidic application. This work aims at developing a new PIV algorithm which reduces the effective seeding density and at the same time yield similar if not higher SNR compared to conventional PIV. We call this algorithm Ψ-PIV. This method is suitable for steady and unsteady laminar flows which are generally found in microfluidic applications. The reliability and the precision of the new method as a function of particle image pairs is investigated by synthetic image analysis. The main advantage of Ψ-PIV is its ability to achieve higher temporal resolution from the typical micro-PIV raw images. The experimental investigation for the flow around a 2D cylinder in a Hele-Shaw cell showed that the Ψ-PIV results in a reduction by approximately a factor of 25 in the number of frames required compared to conventional PIV. @en