Bonnie L. Gray

PDMS as a sacrificial substrate for SU-8-based biomedical and microfluidic applications

We describe a new fabrication process utilizing polydimethylesiloxane (PDMS) as a sacrificial substrate layer for fabricating free-standing SU-8-based biomedical and microfluidic devices. The PDMS-on-glass substrate permits SU-8 photo patterning and layer-to-layer bonding. We have developed a novel PDMS-based process which allows the SU-8 structures to be easily peeled off from the substrate after complete fabrication. As an example, a fully enclosed microfluidic chip has been successfully fabricated utilizing the presented new process. The enclosed microfluidic chip uses adhesive bonding technology and the SU-8 layers from 10 µm to 450 µm thick for fully enclosed microchannels. SU-8 layers as large as the glass substrate are successfully fabricated and peeled off from the PDMS layer as single continuous sheets. The fabrication results are supported by optical microscopy and profilometry. The peel-off force for the 120 µm thick SU-8-based chips is measured using a voice coil actuator (VCA). As an additional benefit the release step leaves the input and the output of the microchannels accessible to the outside world facilitating interconnecting to the external devices. (Some figures in this article are in colour only in the electronic version)

A sacrificial SU-8 mask for direct metallization on PDMS

A new fabrication technology utilizing SU-8 as a sacrificial mask for metallization of the PDMS surface is presented. The sacrificial SU-8 layer process offers superior performance for reliable and repeatable metallization on the PDMS layer. Sacrificial SU-8 masks from 45 µm to 250 µm thickness are successfully fabricated on the PDMS layer to pattern gold on the PDMS surface. These layers are successfully peeled off from the PDMS surface after a metal deposition step. Metal lines from 10 µm to 500 µm wide and 1 mm to 50 mm long are successfully patterned and tested. Furthermore, the sacrificial SU-8 mask can be removed within minutes to realize metal patterns on the PDMS surface and does not leave any residue after removal of the SU-8 layer. As this new process is intended for use in fabrication of microfluidic and biomedical microdevices, electrodes of an electro-enzymatic glucose sensor are presented to demonstrate the technology.

Enclosed SU-8 and PDMS microchannels with integrated interconnects for chip-to-chip and world-to-chip connections

This paper presents the simulation, fabrication and testing of fluid-tight enclosed microchannels with integrated interconnects for modular snap-together chip-to-chip and world-to-chip connections. Components consisting of microchannels with integrated interconnect were fabricated in SU-8 photopolymer and polydimethylsiloxane (PDMS). Simulations were performed in ANSYS for component microchannels with chip-to-chip and world-to-chip interconnects in order to determine the expected flow profile and pressure drop. Components with world-to-chip structures were then experimentally pressurized to 252 kPa and 148 kPa for SU-8 and PDMS respectively, without leakage. Components with chip-to-chip interconnects were assembled in a hybrid configuration utilizing SU-8 and PDMS for individual components assembled in a chain, and pressurized to 124 kPa before leakage. The duality of these structures as both world-to-chip and chip-to-chip connections increases their usability since they can provide micro- to macro-connections, as well as chip-to-chip connections without needing to redesign the interconnection scheme.

Fabrication Process for Electromagnetic Actuators Compatible with Polymer Based Microfluidic Devices

We demonstrate a new hybrid-soft-lithography micromolding process that results in a mechanically-compliant, magnetically actuated membrane. The new microactuator consists of a thin undoped polydimethylsiloxane (PDMS) membrane with a central magnet feature, which is also micromolded using soft lithography. The central magnet is composed of a magnetic nanocomposite polymer (M-NCP) material, which is achieved by uniformly dispersing rare earth magnetic powder (MQP-12-5) in the PDMS polymer matrix. The hybrid fabrication technique is capable of realizing a highly flexible membrane with the ability of providing bidirectional deflection without sacrificing the transparency of the device, which may be required for many biomedical applications. Furthermore, we show that the hybrid process also yields improved deflection of the membrane using lower magnetic fields than an opaque membrane fabricated entirely in nanocomposite polymer. These lower fields are more suitable to on-chip production of microactuators excited via electronic signals in microcoils.

Bidirectional magnetic microactuators for uTAS

We present the design, fabrication and characterization of a novel bidirectional magnetic microactuator. The actuator has a planar structure and is easily fabricated using processes based on laser micromachining and soft lithography, allowing it to be readily integrated into microfluidic, microelectromechanical systems (MEMS) and lab-on-a-chip (LOC) designs. The new microactuator is a thin magnetic membrane with a central magnet feature. The membrane and magnet are both composed of a magnetic nanocomposite polymer (M-NCP) material that is fabricated by embedding magnetic powder in a polydimethysiloxane (PDMS) polymer matrix. The magnetic powder (MQP-12-5) has the chemical composition of (Nd0.7Ce0.3)10.5Fe83.9B5.6, and contains grains that are 5-6 microns in size. The powder is uniformly dispersed at a weight percentage of 75 wt-% in the PDMS matrix, and micropatterned using soft lithography micromolding to realize magnetic microstructures, which sit on a thinner magnetic PDMS membrane of the same material. The molds are fabricated by laser-etching into Poly (methyl methacrylate) (PMMA) using a Universal Laser Systems VersaLASER© laser ablation system. The PDMS-based M-NCP is then poured and spun over the mold patterns, producing a thin polymer membrane to which the polymer micromagnets are attached, forming a one-piece actuator. The M-NCP is initially un-magnetized, but is then magnetized by placing it in a 2.5T magnetic field to produce permanent bidirectional magnetization that is polarized in the specified direction. To characterize the bidirectional actuators, a uniform magnetic field is established via a Helmholtz coil pair, and is characterized by applying varying currents. The magnetic field (and thus the actuator deflection) is controlled by regulating the current in the Helmholtz pair. Using this apparatus, deflection versus field characteristics are obtained, with maximum deflections varying as a function of actuator dimensions and the applied magnetic field. Permanent rare earth magnets are used to produce supplemental fields for higher magnetic fields and higher deflections. Deflections of 100 micrometers and more are observed for 3 to 8 mm square membranes with central magnetic features ranging from 0.8 to 3.6 mm squares, in magnetic fields ranging from 52 to 6.2 mT. In addition, smaller membranes (1 mm and 2 mm with 0.4 mm and 0.6 mm central magnets, respectively) also deflect 20 and 50 microns, respectively, under 72 mT fields.

Fabrication and Testing of Integrated Permanent Micromagnets for Microfluidic Systems

We present fabrication of a novel (Nd0.7Ce0.3)10.5Fe83.9B5.6 magnetic powder and polydimethysiloxane bonded material that can be micropatterned into micromagnets. The magnetic powder, with an average particle size of 5μm-6μm, has been prepared from an alloy ingot of raw materials which are put in a vacuum induction furnace and melt spun to obtain ribbons with nanocrystalline microstructure. The ribbons are crushed using vibrating ball milling under inert atmosphere to obtain coarse powder (average particle size of 200μm). In order to obtain 5μm fine powder the course powder is jet milled at 6000rpm under inert atmosphere. The fine magnetic powder (referred to as MQFP-15) is ultrasonically uniformly dispersed in a polydimethylsiloxane matrix (PDMS) using a horn tip probe operating at a frequency of 42 kHz. Micromagnets (diameter of 50μm, height 30μm) are fabricated from the prepared composite via soft lithography and are tested using a SQUID magnetometer, showing a remanent magnetization (Mr) of 60.10 emu/g and coercivity (Hc) of 5260 G at 75 weight percentage of magnetic powder in the PDMS matrix.

Creation of embedded structures in SU-8

Two methods were investigated for the creation of encapsulated micro-fluidic channels and bridges in negative tone SU-8 photoresist. The first uses two exposures at different wavelengths to create the channel sidewalls and microchannel encapsulation layer; the other method creates both using a single I-line (365 nm) exposure and a grayscale photomask. These methods can define structures with vertical dimensions ranging to hundreds of microns and introduces very little extra processing complexity. For the dual wavelength method, an I-line light source is used to define the channel walls while a non-collimated deep-UV (254 nm) light source provides a large energy dose to the top surface of the SU-8 to produce a membrane over all the channels. Using the dual wavelength method allows SU-8 to be used as the material for the channels and the encapsulation method is self-limiting avoiding the requirement for precise control over the exposure dose. The rate of UV dose and the post-exposure baking parameters are critical to the quality and strength of the micro-channels. Properly designed channels have been successfully developed in lengths up to 1 cm. Alternatively using a grayscale Zn/Al bimetallic photomask and a single I-line exposure, 3D bridge micro-structures were successfully made on SU-8. The use of grayscale masks for both techniques also provides the possibility of shaping the channel. With the ability to create micro-bridges, further research will be performed to investigate how well the single exposure technique can be used to produce micro-channels of various sizes and dimensions.

Flexible Three-Dimensional Electrochemical Glucose Sensor with Improved Sensitivity Realized in Hybrid Polymer Microelectromechanical Systems Technique

Background: Continuous glucose monitoring for patients with diabetes is of paramount importance to avoid severe health conditions resulting from hypoglycemia or hyperglycemia. Most available methods require an invasive setup and a health care professional. Handheld devices available on the market also require finger pricking for every measurement and do not provide continuous monitoring. Hence, continuous glucose monitoring from human tears using a glucose sensor embedded in a contact lens has been considered as a suitable option. However, the glucose concentration in human tears is very low in comparison with the blood glucose level (1/10–1/40 concentration). We propose a sensor that solves the sensitivity problem in a new way, is flexible, and is constructed onto the oxygen permeable contact lens material. Methods: To achieve such sensitivity while maintaining a small sensor footprint suitable for placement in a contact lens, we increased the active electrode area by using three-dimensional (3-D) electrode micropatterning. Fully flexible 3-D electrodes were realized utilizing ordered arrays of pillars with different shapes and heights. Results: We successfully fabricated square and cylindrical pillars with different height (50, 100, and 200 μm) and uniform metal coverage to realize sensor electrodes. The increased surface area produces high amperometric current that increases sensor sensitivity up to 300% using 200 μ tall square pillars. The sensitivity improvement closely follows the improvement in the surface area of the electrode. Conclusions: The proposed flexible glucose sensors with 3-D microstructure electrodes are more sensitive to lower glucose concentrations and generate higher current signal than conventional glucose sensors.

Design, fabrication and characterization of an arrayable all-polymer microfluidic valve employing highly magnetic rare-earth composite polymer

We present a new magnetically actuated microfluidic valve that employs a highly magnetic composite polymer (M-CP) containing rare-earth hard-magnetic powder for its actuating element and for its valve seat. The M-CP offers much higher magnetization compared to the soft-magnetic, ferrite-based composite polymers typically used in microfluidic applications. Each valve consists of a permanently magnetized M-CP flap and valve seat mounted on a microfluidic channel system fabricated in poly(dimethylsiloxane) (PDMS). Each valve is actuated under a relatively small external magnetic field of 80 mT provided by a small permanent magnet mounted on a miniature linear actuator. The performance of the valve with different flap thicknesses is characterized. In addition, the effect of the magnetic valve seat on the valves performance is also characterized. It is experimentally shown that a valve with a 2.3 mm flap thickness, actuated under an 80 mT magnetic field, is capable of completely blocking liquid flow at a flow rate of 1 ml min−1 for pressures up to 9.65 kPa in microfluidic channels 200 μm wide and 200 μm deep. The valve can also be fabricated into an array for flow switching between multiple microfluidic channels under continuous flow conditions. The performance of arrays of valves for flow routing is demonstrated for flow rates up to 5 ml min−1 with larger microfluidic channels of up to 1 mm wide and 500 μm deep. The design of the valves is compatible with other commonly used polymeric microfluidic components, as well as other components that use the same novel permanently magnetic composite polymer, such as our previously reported cilia-based mixing devices.

Fabrication and testing of thermally responsive hydrogel-based actuators using polymer heater elements for flexible microvalves

We present the design, fabrication and characterization of a mechanically flexible diaphragm-based microvalve actuator employing a reservoir of the thermally responsive hydrogel PNIPAAm and a conductive nanocomposite polymer (C-NCP) heater element. The microvalve actuator can be fabricated employing traditional soft lithography processes for fabrication of all components, including the tungsten-based C-NCP heater element, the hydrogel reservoir, and the deflecting polymer membrane. Shrinking of the hydrogel under the application of heat supplied by the flexible heater, or the removal of this thermal energy by turning off the heater, forces the diaphragm to move. The silicone diaphragm actuator is compatible with a normally-closed polymer microvalve design where-by the fluidic channel can be opened and closed via the hydrogel diaphragm actuator, in which the hydrogel is normally swollen and heating opens the valve via membrane deflection. Our prototype hydrogel actuator diaphragms are between 100-200 micrometers in diameter, and experimentally deflect approximately 100 micrometers under heating to 32 degrees ºC or above, which is sufficient to theoretically open a microvalve to allow flow to pass through a 100 micrometer deep channel. We characterize the flexible tungsten C-NCP heaters for voltage versus temperature and show that the flexible heaters can reach the hydrogel transition temperature of 32 degrees °C at approximately 13-15 V. We further characterize the hydrogel response to heat, and diaphragm deflection using both hot plate and flexible C-NCP heater elements. While our results show diaphragm deflection adequate for microvalves at a reasonable voltage, the speed of deflection is currently very slow and would result in slow microvalve response speed (30 seconds to open the valve, and 120 seconds to reclose it).