Digital Microfluidic Operations Research Articles

A-172 Feasibility of PT, aPTT, and Fibrinogen on a Multifunctional Digital Microfluidic Cartridge

Abstract Background Excessive bleeding is prevalent in severe trauma patients, and measurement of multiple coagulation parameters is vital to correcting coagulopathy and improving outcomes. There are no rapid, near-patient technologies that can perform multiple coagulation assays simultaneously on a single consumable. We are developing a point-of-care digital microfluidic (DMF) platform to test three critical coagulation markers: prothrombin time (PT), activated partial thromboplastin time (aPTT), and fibrinogen. Methods Five proficiency testing plasma samples were obtained from the College of American Pathologists (CAP) with normal, intermediate, and prolonged response of PT and aPTT, and normal and elevated fibrinogen levels. After loading 10 μL of sample onto the cartridge, autonomous digital microfluidic operations divided the sample into three droplets, which were then combined with reagents for PT, aPTT or fibrinogen. The viscosity of these droplets, measured from the electrical impedance on a toaster-sized instrument, changes at a rate proportional to the amount of coagulation precursors present from which time/activity was calculated. Results Classification of samples into normal, intermediate or prolonged categories along with the respective times obtained from the DMF cartridges matched those obtained on the Stago Compact Max instrument (Figure). We also demonstrated separation of plasma from whole blood within the DMF cartridge using agglutinating reagents, enabling an easy-to-use workflow without the need for centrifugation. Conclusions Our preliminary results demonstrate that PT, aPTT and fibrinogen can be measured on a single cartridge with performance similar to comparator methods, with a time to result of <15 minutes. To our knowledge, this is the first time these 3 assays have been performed simultaneously on a single consumable and furthermore, this is the first time viscoelastic tests have been performed on a multifunctional instrument that also performs chemistry and molecular tests. Studies are underway to evaluate analytical and clinical performance.

Low-Cost Graphene-Based Digital Microfluidic System.

In this work, the laser-scribing technique was used as a low-cost, rapid and facile method for fabricating digital microfluidic (DMF) systems. Laser-scribed graphene (LSG) electrodes are directly synthesized on flexible substrates to pattern the DMF electrode arrays. This facilitates the DMF electrodes’ fabrication process by eliminating many microfabrication steps. An electrowetting test was performed to investigate the effectiveness of the LSG DMF electrodes in changing the contact angles of droplets. Different DMF operations were successfully performed using the proposed LSG DMF chips in both open and closed DMF systems. The quality and output resolution were examined to assess the performance of such patterned electrodes in the DMF systems. To verify the efficacy of the LSG DMF chips, a one-step direct assay for the detection of Legionella pneumophila deoxyribonucleic acid (DNA) was performed on the chip without the need for any washing step. The high specificity in distinguishing a single-nucleotide mismatch was achieved by detecting target DNA concentrations as low as 1 nM. Our findings suggest that the proposed rapid and easy fabrication method for LSG DMF electrodes offers a great platform for low-cost and easily accessible point-of-care diagnostic devices.

Low-cost and low-topography fabrication of multilayer interconnections for microfluidic devices

Multilayer interconnections are needed for microdevices with a large number of independent electrodes. A multi-level photolithographic process is commonly employed to provide multilayer interconnections in integrated circuit devices, but it is often too expensive for large-area or disposable devices frequently needed for microfluidics. The printed circuit board (PCB) can provide multilayer interconnection at low cost, but its rough topography poses a challenge for small droplets to slide over. Here we report a low-cost fabrication of low-topography multilayer interconnects by selective and controlled anodization of thin-film metal layers. The process utilizes anodization of metal (tantalum in this paper) or, more specifically, repetitions of a partial anodization to form insulation layers between conductive layers and a full anodization to form isolating regions between electrodes, replacing the usual process of depositing, planarizing, and etching insulation layers. After verifying the electric connections and insulations as intended, the developed method is applied to electrowetting-on-dielectric (EWOD), whose complex microfluidic products are currently built on PCB or thin-film transistor substrates. To demonstrate the utility, we fabricate a three metal-layer EWOD device with steps (surface topography) less than 1 micrometer (vs. > 10 micrometers of PCB EWOD devices) and confirm basic digital microfluidic operations.

Ultra-Portable Smartphone Controlled Integrated Digital Microfluidic System in a 3D-Printed Modular Assembly

Portable sensors and biomedical devices are influenced by the recent advances in microfluidics technologies, compact fabrication techniques, improved detection limits and enhanced analysis capabilities. This paper reports the development of an integrated ultraportable, low-cost, and modular digital microfluidic (DMF) system and its successful integration with a smartphone used as a high-level controller and post processing station. Low power and cost effective electronic circuits are designed to generate the high voltages required for DMF operations in both open and closed configurations (from 100 to 800 V). The smartphone in turn commands a microcontroller that manipulate the voltage signals required for droplet actuation in the DMF chip and communicates wirelessly with the microcontroller via Bluetooth module. Moreover, the smartphone acts as a detection and image analysis station with an attached microscopic lens. The holder assembly is fabricated using three-dimensional (3D) printing technology to facilitate rapid prototyping. The holder features a modular design that enables convenient attachment/detachment of a variety of DMF chips to/from an electrical busbar. The electrical circuits, controller and communication system are designed to minimize the power consumption in order to run the device on small lithium ion batteries. Successful controlled DMF operations and a basic colorimetric assay using the smartphone are demonstrated.

High precision control of gap height for enhancing principal digital microfluidics operations

A variable gap size actuation (VGSA) mechanism is integrated into the digital microfluidic (DMF) system. The VGSA mechanism serves to optimize the aspect ratio during performing different microfluidic operations by changing the gap height between the top and bottom plates. This in effect will have a direct impact on the four main DMF operations including droplet transport, splitting, dispensing and merging as they are greatly affected by changing the aspect ratio. Experimental results demonstrate that the VGSA mechanism significantly enhances the principal DMF operations by retaining the appropriate gap height for each operation which is also dependent on droplets volumes when using fixed electrode size. Specifically, varying the gap height precisely between the two plates will enable us to transport the droplets more reliably, control the volume of the dispensed droplet, carry out splitting and merging more effectively, facilitate motion of residual droplets resulting from splitting or partial evaporation, enhance mixing at faster rates, achieve accurate positioning of droplets regardless of their volume, and minimize evaporation without complicating the DMF system with the use of a filler medium. The proposed mechanism is realized by accurate positioning of the top plate over the fixed bottom plate, instead of maintaining a fixed gap height during the operation. In this work, an experimental setup is constructed for the proof of concept to meet precise alignment requirements of the two parallel plates using a feedback-controlled positioning system. Three different methods viz., visual, capacitance-based and encoder values, are used to measure the gap height between the two plates precisely. For practical lab-on-chip devices, micro-actuators in conjunction with capacitance measurement feedback can be used to position the top plate during the operation. In this way, the proposed VGSA mechanism will introduce a mean for optimizing the parameters controlling the DMF operations.

Digital microfluidic operations on micro-electrode dot array architecture

As digital microfluidics-based biochips find more applications, their complexity is expected to increase significantly owing to the trend of multiple and concurrent assays on the chip. There is a pressing need to deliver a top-down design methodology that the biochip designer can leverage the same level of computer-aided design support as the semi-conductor industry now does. Moreover, as microelectronics fabrication technology is scaling up and integrated device performance is improving, it is expected that these microfluidic biochips will be integrated with microelectronic components in next-generation system-on-chip designs. This study presents the analysis and experiments of digital microfluidic operations on a novel electrowetting-on-dielectric-based 'micro-electrode dot array architecture' that fosters a development path for hierarchical top-down design approach for digital microfluidics. The proposed architecture allows dynamic configurations and activations of identical basic microfluidic unit called 'micro-electrode cells' to design microfluidic components, layouts, routing, microfluidic operations and applications of the biochip hierarchically. Fundamental microfluidic operations have been successfully performed by the architecture. In addition, this novel architecture demonstrates a number of advantages and flexibilities over the conventional digital microfluidics in performing advanced microfluidic operations.

Monolithic Fabrication of EWOD Chips for Picoliter Droplets

We report monolithic fabrication of parallel-plate electrowetting-on-dielectric (EWOD) chips for digital micro-fluidics of picoliter droplets. Instead of assembling a second substrate to form a top plate-the common practice with all previous parallel-plate EWOD chips-the top plate is surface micromachined as a transparent thin-film membrane that forms a monolithic cavity having a gap height on the order of micrometers with excellent accuracy and uniformity. The membrane is embedded with EWOD driving electrodes and confines droplets against the device substrate to perform digital microfluidic operations. Two main attributes of the monolithic architecture that distinguish it from tradition methods are: (i) it enables excellent control of droplet dimensions down to the micrometer scale, and (ii) it does not require the typical alignment and assembly steps. Basic device functions such as creation and splitting are verified by EWOD actuation of ~100 picoliter droplets surrounded by air or oil inside a 10 μm-high cavity. Additionally, flow focusing of droplets containing 5.3 μm beads demonstrates one example of the utilities afforded by monolithic fabrication.

Spectroscopic characterisation and manipulation of arrays of sub-picolitre aerosol droplets

Arrays of optically tweezed aerosol droplets, each of sub-picolitre volume, are manipulated by holographic optical tweezers and characterised by cavity enhanced Raman spectroscopy. A spatial light modulator is employed to generate arrays of optical traps from a single laser beam and to control the array dimensions and relative trap positions. Comparative hygroscopicity measurements are performed concurrently on five trapped droplets by monitoring the evolving size of each droplet. This is extended to the controlled coalescence of an array of droplets accompanied by spectroscopic measurements. These data represent the first ever simultaneous measurements of the evolving composition and size of an array of aerosol droplets. We consider the possibility of using aerosol arrays as a platform for studying chemical reactions in sub-picolitre volumes, exploiting the versatility of aerosol arrays for performing optical digital microfluidic operations accompanied by micro-total analysis.