Microfluidic devices powered by integrated elasto-magnetic pumps

Abstract

We show how an asymmetric elasto-magnetic system provides a novel integrated pumping solution for lab-on-a-chip and point of care devices. This monolithic pumping solution, inspired by Purcell's 3-link swimmer, is integrated within a simple microfluidic device, bypassing the requirement of external connections. We experimentally prove that this system can provide tuneable fluid flow with a flow rate of up to 600 μL h-1. This fluid flow is achieved by actuating the pump using a weak, uniform, uniaxial, oscillating magnetic field, with field amplitudes in the range of 3-6 mT. Crucially, the fluid flow can be reversed by adjusting the driving frequency. We experimentally prove that this device can successfully operate on fluids with a range of viscosities, where pumping at higher viscosity correlates with a decreasing optimal driving frequency. The fluid flow produced by this device is understood here by examining the non-reciprocal motion of the elasto-magnetic component. This device has the capability to replace external pumping systems with a simple, integrated, lab-on-a-chip component.

Fig. 1. The geometry of the pump. A diagram of the system explored in this study, depicting the pump, channels and magnetic fields employed. The grey region signifies PDMS, the gold region signifies the NdFeB magnet of side length 250 μm and the blue region signifies fluid. a) Depicts the integrated pump. The pump consists of three elastic links, labelled L1 to L3. The width of the links is 150 μm and the thickness is 300 μm. The blue arrow shows the direction of the uniaxial oscillating magnetic driving field, B[combining right harpoon above], and the red arrow shows the direction of magnetisation, m[combining right harpoon above], of the NdFeB magnet. b) Depicts the device geometry. The device consists of a fully enclosed pumping module with the labelled inlet and outlet being square channels, 1000 μm in width and 900 μm in depth. The pump module is formed of three layers of PDMS, each with a feature size of 300 μm, the middle layer contains the integrated pump. The pump module is connected to an open reservoir module via PTFE tubing with length of 4 cm and internal diameter of 860 μm. The reservoir module allows for easy introduction of fluid and tracer particles during experiments. c) Depicts the geometry of the pumping chamber with the dimensions displayed.

Fig. 2. A graphical representation of various sequences of motion incorporating the Purcell 3-link swimmer. The links are represented in grey, while the hinges and their directions of rotation are represented in blue and red. a) Represents a non-reciprocal sequence of motions which is not time-reversible. By breaking time-reversal symmetry, when the swimmer has completed the sequence of motions and returns to its original configuration, it has undergone a net spatial translation. b) Represents the two hinges rotating in phase with each other. All sequences of motion are time-reversible because any motion can only be followed immediately by its inverse. This swimmer is incapable of self-propulsion and is analogous to a single hinged system.

Fig. 3. A 3D rendering of the device. a) Is an exploded projection of the pump module. We see the pump module is made from three distinct layers. The middle layer contains the integrated pump (Fig. 1), the upper and lower layers act to extend the height of the microfluidic channels by 300 μm each side, while also acting as capping layers. b) Shows that the pump module and reservoir module are assembled onto a microscope slide and connected via PTFE tubing as in the experiments.

Fig. 4. The hysteretic pump motion. Four non-sequential frames are displayed from high speed video recordings as described in section 2.5 and are labelled here 1–4. The pump head follows the red line, taking a different path during the pump stroke and the recovery stroke. This non-reciprocal motion is the source of pumping and the sequence of motions is described in the text. This image is adapted from data taken in these experiments and is used to represent the typical motion of the pump. The viscosity of the fluid is 0.001 kg ms^–1 and the driving field amplitude and frequency are 6 mT and 50 Hz respectively.

Fig. 5. The pump motion as a function of frequency. a) Depicts the dependence on fluid viscosity with a constant field amplitude of 6 mT and b) depicts how the motion depends on field amplitude with a constant dynamic viscosity of 0.001 kg ms^–1. The blue curve is duplicated between both a) and b) for reference. The median area contained within the closed loop path traced by the non-reciprocal motion of the magnetic head of the pump, as suggested in Fig. 4, is shown. This is recorded as a function of driving frequency and repeated for a range of dynamic viscosities and driving field amplitudes. The plotted line is a simple smoothing spline between the data points to act as a visual guide.

Fig. 6. The net volume of fluid displaced per pump cycle. This consists of both the pump and recovery stroke. a) Depicts the displacement when varying the fluid viscosity at constant field amplitude of 6 mT. b) Depicts the displacement when varying the amplitude of the applied field at a constant fluid viscosity of 0.001 kg ms^–1. This is measured as described in section 2.4. The plotted line is a simple smoothing spline between the data points to act as a visual guide.

Fig. 7. The average volumetric flow rates produced by this pumping system. a) Shows the volumetric flow rate when varying the fluid viscosity at constant field amplitude of 6 mT. b) Shows the volumetric flow rate when varying the amplitude of the applied field at a constant fluid viscosity of 0.001 kg ms^–1. The plotted line is a simple smoothing spline between the data points to act as a visual guide.