W. van der Wijngaart

The first self-priming and bi-directional valve-less diffuser micropump for both liquid and gas

A new micropump design was fabricated to make a qualitative study on the reliability of micro diffuser pumps. A novel two-level pump chamber geometry enables both gas and liquid pumping. The micropumps are fully self-priming and insensitive to cavitation and gas bubbles in the liquid. Changing the driving frequency enables bi-directional pumping for both liquid and gas, i.e. both forward and reverse pumping. The pumps consist of a silicon-glass stack and are fabricated with a new process involving three sequential deep RIE steps. A new and simple melt-on method for conveniently fixing external tubes to the device was developed. Design, fabrication and first experimental results are described and discussed.

Inertial microfluidics in parallel channels for high-throughput applications

Passive particle focusing based on inertial microfluidics was recently introduced as a high-throughput alternative to active focusing methods that require an external force-field to manipulate particles. In this study, we introduce inertial microfluidics in flows through straight, multiple parallel channels. The scalable, single inlet and two outlet, parallel channel system is enabled by a novel, high-density 3D PDMS microchannel manufacturing technology, mediated via a targeted inhibition of PDMS polymerization. Using single channels, we first demonstrate how randomly distributed particles can be focused into the centre position of the channel in flows through low aspect ratio channels and can be effectively fractionated. As a proof of principle, continuous focusing and filtration of 10 μm particles from a suspension mixture using 4- and 16-parallel-channel devices with a single inlet and two outlets are demonstrated. A filtration efficiency of 95-97% was achieved at throughputs several orders of magnitude higher than previously shown for flows through straight channels. The scalable and low-footprint focusing device requiring neither external force fields nor mechanical parts to operate is readily applicable for high-throughput focusing and filtration applications as a stand-alone device or integrated with lab-on-a-chip systems.

Modeling and simulation of electrostatically gated nanochannels

Today, despite the growing interest in nanofluidics, the descriptions of the many complex physical phenomena occurring at this scale remain scattered in the literature. Due to the additional complexity encountered when considering electrostatic nanofluidic gating, it is important to regroup several relevant theories and discuss them with regard to this application. In this work, we present a theoretical study of electrostatically gated phenomena and propose a model for the electrostatic gating of ion and molecular transport in nanochannels. In addition to the classical electrokinetic equations, that are reviewed in this work, several relevant phenomena are considered and combined to describe gating effects on nanofluidic properties more accurately. Dynamic surface charging is accounted for and is shown to be an essential element for electrostatic gating. The autoprotolysis of water is also considered to allow for accurate computing of the surface charge. Modifications of the Nernst-Planck equations are considered for more accurate computing of the concentration profiles at higher surface potentials by accounting for ion crowding near charge walls. The sensitivity of several parameters to the electric field and ion crowding is also studied. Each of these models is described separately before their implementation in a finite element model. The model is verified against previous experimental work. Finally, the model is used to simulate the tuning of the ionic current through the nanochannel via electrostatic gating. The influence of the additional models on these results is discussed. Guidelines for potentially better gating efficiencies are finally proposed.

A High-Yield Process for 3-D Large-Scale Integrated Microfluidic Networks in PDMS

This paper presents an uncomplicated high-yield fabrication process for creating large-scale integrated (LSI) 3-D microfluidic networks in poly(dimethylsiloxane) (PDMS). The key innovation lays in the robust definition of miniaturized out-of-plane fluidic interconnecting channels (=vias) between stacked layers of microfluidic channels in standard PDMS. Unblocked vias are essential for creating 3-D microfluidic networks. Previous methods either suffered from limited yield in achieving unblocked vias due to residual membranes obstructing the vias after polymerization, or required complicated and/or manual procedures to remove the blocking membranes. In contrast, our method prevents the formation of residual membranes by inhibiting the PDMS polymerization on top of the mold features that define the vias. In addition to providing unblocked vias, the inhibition process also leaves a partially cured, sticky flat-top surface that adheres well to other surfaces and that allows self-sealing stacking of several PDMS layers. We demonstrate the new method by manufacturing a densely perforated PDMS membrane and an LSI 3-D PDMS microfluidic channel network. We also characterize the inhibition mechanism and study the critical process parameters. We demonstrate that the method is suitable for structuring PDMS layers with a thickness down to 10 m.

A Low-Temperature Thermopneumatic Actuation Principle for Gas Bubble Microvalves

We introduce a novel type of microfluidic actuator in which a trapped air bubble functions as the thermally controlled volume displacing element. The pressure of the trapped air pocket is controlled by the changing equilibrium gas composition for static operation and by thermal expansion for dynamic operation. The volume displacement is determined by the liquid surface tension and the valve geometry. A fully functional demonstrator device was successfully fabricated and tested. The absence of moving mechanical parts, the electrical control of the valve, and the limited required actuation temperature make the actuator suitable for control of large-scale integration fluidic networking in biotechnical applications.

Wafer-Scale Manufacturing of Bulk Shape-Memory-Alloy Microactuators Based on Adhesive Bonding of Titanium–Nickel Sheets to Structured Silicon Wafers

This paper presents a concept for the wafer-scale manufacturing of microactuators based on the adhesive bonding of bulk shape-memory-alloy (SMA) sheets to silicon microstructures. Wafer-scale integration of a cold-state deformation mechanism is provided by the deposition of stressed films onto the SMA sheet. A concept for heating of the SMA by Joule heating through a resistive heater layer is presented. Critical fabrication issues were investigated, including the cold-state deformation, the bonding scheme and related stresses, and the titanium-nickel (TiNi) sheet patterning. Novel methods for the transfer stamping of adhesive and for the handling of the thin TiNi sheets were developed, based on the use of standard dicing blue tape. First demonstrator TiNi cantilevers, wafer-level adhesively bonded on a microstructured silicon substrate, were successfully fabricated and evaluated. Intrinsically stressed silicon dioxide and silicon nitride were deposited using plasma-enhanced chemical vapor deposition to deform the cantilevers in the cold state. Tip deflections for 2.5-mm-long cantilevers in cold/hot state of 250/70 and 125/28 mum were obtained using silicon dioxide and silicon nitride, respectively. The bond strength proved to be stronger than the force created by the 2.5-mm-long TiNi cantilever and showed no degradation after more than 700 temperature cycles. The shape-memory behavior of the TiNi is maintained during the integration process.

SMA Microvalves for Very Large Gas Flow Control Manufactured Using Wafer-Level Eutectic Bonding

This paper presents a novel gas microvalve design concept, in which a flow control gate is opened by a pneumatic pressure and closed by a shape memory alloy actuator, allowing large flow control. Two different design variations were fabricated using a novel wafer-level Au-Si eutectic bonding process for TiNi to silicon integration. The resulting microvalves demonstrate a record pneumatic performance per footprint area; a microvalve with a footprint of only 1 3.3 mm2 successfully controls a flow difference of 3100 sccm at a pressure drop of 70 kPa using a power of 0.35 W.

A compact, low-cost microliter-range liquid dispenser based on expandable microspheres

This work presents a new low-cost liquid dispenser for the dispensing of microliters to milliliter volumes. The dispensing mechanism is based on a thermal actuator where highly expandable microsphe ...

A seat microvalve nozzle for optimal gas-flow capacity at large-controlled pressure

Seat microvalves are the most common microvalve type for gas flow control. This paper presents a general method for optimising the flow capacity of a seat valve nozzle and diminishing the requirements on the valve actuators stroke-length. Geometrical analysis and finite element (FE) simulations show that for controlling large gas flow at elevated pressure, the optimal nozzle design in terms of flow capacity for a given actuator performance is a multiple-orifice arrangement with miniaturised circular nozzles. Experimental results support the design introduced in this paper.

A micromachined knife gate valve for high-flow pressure regulation applications

Cross-flow pressure regulating valve structures are attractive for high-flow pressure control applications due to the decreased actuation force required and the reduced device footprint area. A knife gate valve was fabricated, controlling a flow of 1.3 Nl/min at a supply pressure of 1.5 bar. The valve was microfabricated using deep reactive ion etching (DRIE) and silicon fusion bonding. The use of micromachined knife gate valves in pressure control systems enhances performance and cost savings can be realized.