Examining the processes in polymer laminate technology

Polymer laminate technology’s strengths include no tooling, complex closely spaced features, enclosed channels, and the use of a variety of materials.

By: Leanna Levine

Figure 1. The edge view of a fluidic card designed and built for autonomous cell culture experiments for NASA's GeneSat program.

The first part of this article (IVD Technology, June 2009, page 37) gave an overview of polymer laminate technology, a microfabrication platform for rapid prototyping of single use, lab-on-a-chip devices. This method offers the ability to form enclosed three-dimensional channels, complex and closely spaced features, and such functions as on-board valves, pumps, mixers, and porous membrane filters. The second part of this article discusses the processes involved in polymer laminate technology fabrication and covers two case studies of its application by users in the IVD field.

Polymer Laminate Fabrication Process

Twenty years ago, a group at the University of Washington (Seattle) first conceived of and developed polymer laminate technology for prototyping. With the development of specific applications and intellectual property that implemented their unique properties, polymer laminates have been used to explore the properties of microfluidic systems. The application of polymer laminate technology to low- to mid-volume manufacturing of fluidic components used in molecular diagnostics has only recently been explored and demonstrated.

The dogma in the IVD industry has been that polymer laminate technology cannot be used for manufacturing complex fluidic devices. However, as the lamination methods and processes improve to meet the demands of the LCD industry, for example, the cost of using these more-sophisticated laminate approaches can be expected to drop. Furthermore, using laminates reduces the cost of assembly by eliminating some of the special equipment and multiple assembly steps required to incorporate injection molded or embossed components, which are often assembled with adhesives.

Design and Cut. Polymer laminates are produced by a multistep process that begins with laying out a pattern using computer automated design software. A single instance plot designs a repeat pattern in the footprint of a laser table or sheet size. To make the cuts in the pattern, the laser direct writes it into the material. The power used and the cutting speed are controlled, producing a cut edge similar to machined or standard injection molded parts. A fabrication footprint can vary from 6 × 6 in. to 24 × 36 in., and has 0.002-in. repeatability across a diagonal, which allows extremely fine features to be cut.

The specific dimensions of the final design are achieved by adjusting the pattern for the laser kerf. The layer containing the channels often has the bonding adhesive on the top and bottom of the carrier or substrate that will form the channel height. Channel heights between 0.0005 in. (12.5 μm) and 0.080 in. (2 mm) can be produced using a typical laser system. The layers that form the top and bottom of the channel contain the vias, which are also laser cut, that lead to other layers of the polymer laminate.

Lamination. Once the polymer laminate parts are cut, they are cleaned to remove any debris and bonded to the adjacent layers in a Class 10,000 cleanroom on a laminator using hot or cold rollers. A common misconception has persisted regarding the quality and performance of the adhesives in polymer laminate technology. IVD manufacturers are sometimes concerned that the adhesives between the polymer substrates will lack structural integrity and interfere with the laminate's performance. However, strong, biocompatible silicone or acrylic-based pressure-sensitive adhesives that are used in the medical device market are also used in polymer laminate manufacturing. They are optically clear and provide a permanent bond to the substrate upon completion of curing in 12–24 hours. In fact, the bond can be so strong that the material that brings the parts together would break apart first before the adhesive would break down.

An alternative to pressure-sensitive adhesives is a 25-μm polyethylene thermal bond adhesive with a melt temperature of 160° F. This adhesive joins two substrates with the substrate surface through Van der Waals interactions as the polyethylene wets the surface. Once the part is laminated, it is returned to the laser table for final cutting into individual parts, cleaned, and inspected.

Connectors. The parts produced using polymer laminate techniques require fluid connections from the tops or bottoms of the parts. The insertion of tubing connections into the edge of a card is discouraged as it leads to more manual assembly. However, if such insertion is required, then the fluid assembly should include an injection molded port that is bonded to the polymer laminate (see Figure 1). The simplest and lowest dead-volume connections are formed with gaskets to a manifold for delivering air or fluid. As the polymer laminate is developed, on-board reagent reservoirs can be incorporated by using pneumatics or mechanical springs to drive fluid in a controlled fashion. Other options for interconnections include bonded-hose barb connections and tube stubs.

Quality Control. Inspection for quality control involves physical measurements of the polymer laminate features across the channel widths and depths, checking for variability of the dimensions along the length of the channels, and functional measurements to determine valve performance and fluid circuit flow. While fluid is more readily controlled in routine applications with pressures of 1-5 psi, the laminate parts can withstand pressures up to 70–80 psi. Failure occurs if a thin substrate material is able to flex and apply pressure that is perpendicular to the bonding area between the adhesive and the substrate. But pressure rarely exceeds a few psi in routine use, with 25 psi being the maximum when on-board pneumatic valves are used.

Feature Sizes. The polymer laminate parts have features with sizes ranging from 150 μm to 2 mm. The laminate's overall size can be as big as 12 × 12 in. for large manifolds and as small as 1 cm2. While the spacing between features is material-dependent, they can be as close as 0.5 mm on average.

Equipment for Producing Polymer Laminates

When creating polymer laminates, a cost-effective balance between capital equipment costs and ease of use in a production environment is achieved using CO2 lasers and standard laminators. Many small-footprint lasers or knife cutting systems are readily available for cutting layers that can then be bonded together with a tabletop laminator.

While such systems are good for occasional use, difficulties arise with a complex design consisting of more than a few layers. Other issues also arise when a design that was initially built to test a model needs to be scaled up. The designs created by inexperienced designers will not be amenable to scale-up since the issues regarding batch fabrication were not considered in the original design. Many sources of variability are eliminated when polymer laminates are produced in a manufacturing environment in which quality control is monitored throughout the process. Such elimination of variability is particularly evident if an IVD manufacturer wants to incorporate a serpentine channel in a thin, flexible layer. Without the specialized techniques that have been developed by polymer laminate technology to maintain dimensional stability, achieving reproducibility on such designs will be difficult.

Limitations of Polymer Laminates

While offering much in terms of flexibility of materials and functionality, polymer laminates, as with all microfabricated technologies, do not offer easy interconnects to outside components. They are flat and may require special equipment or custom manifolds to allow developers to test their performance under various conditions. While this requirement may also be the case with injection molded parts, connectors are built into such parts. Even though the polymer laminate technique is not suitable for producing features that are smaller than 150 μm in size, having smooth edges is not mandatory. But at dimensions smaller than 100 μm, smooth walls and channels become important for IVD performance, so embossed or injection molded parts are required.

Microfabrication and Multiplexing

Figure 2. Two examples of flowcells that attached directly to a microscope slide that were used to support Tetracore's early development.

The most important factor that distinguishes microfabrication platforms from older prototyping technologies is that they permit the development of more extensive and elaborate multiplexing. As more IVD tests move from single tests to panels, considering designs that utilize samples and reagents optimally and having instruments with small footprints that fit well into the clinical workflow becomes important.

Multiplexed analyses in microfabricated IVD devices offer low dead volume and high feature densities, resulting in less reagent usage. They also shorten assay time, a critically important factor for point-of-care (POC) testing. Polymer laminates offer a distinct advantage since they can have adjacent features that are separated by less than 1 mm and can contain complex fluid circuits with bifurcations to allow a distributed flow of samples or reagents to multiple zones. While developing such complexities is very expensive and difficult to accomplish in injection molded devices, they can be implemented in laminates. If designed with manufacturability in mind and by using the materials that comprise the final product, laminates are excellent final products for single-use disposables.

To implement multiplexing and handle multiple reagent streams in single-use disposable IVDs, on-board valves are built to provide a compact, low-dead-volume solution to manipulating such reagent streams that are necessary for sandwich immunoassays and wash steps to optimize assay sensitivity. Simple pneumatic valves that are fabricated using medical-grade flexible layers integrate with the polymer laminate fabrication process, offering a cost-effective means to incorporate valuable functionality.

Greater Flexibility with a Laminated Approach

Don VerLee, a microfluidics expert who helped to develop fluid-circuit technology at Abbott Laboratories (Abbott Park, IL), explained the flexibility of the laminated approach:

“It allows researchers one more degree of freedom to develop creative designs,” he said. “For example, laminated structures can enable easy access to fluid control structures for the purpose of adding reagents, adding test fluids, and removing waste products. Traditionally, connection to miniature fluidic systems have been made using tubing and point-to-point connectors, either screwed in or pressed on. With the laminated technique, you can create the equivalent of ribbon cable connectors, or multiple simultaneous connections of fluid channels. For example, one of my applications had multiple air control lines plumbed in one flexible circuit up to a point where they were connected directly to a more rigid fluid circuit.”

While at Abbott, the fluid circuit technology that VerLee's research team developed has enabled in miniature fluid control systems what printed circuit boards did for electronics. It allowed biochemical and mechanical engineers to solve the problem of interconnecting fluidic components once as a fluid circuit; thereafter, that integrated solution could be constructed as one component around which discrete microfluidic components could be mounted.

“Perhaps the greatest advantage of the laminating approach is the shortening of the design-build-test cycle,” said VerLee. “Microfluidic experiments are not just about determining optimum fluid metering or mixing, or incubation and/or detection of reaction results. They also require simultaneous application of well characterized and understood manufacturing processes that will affect the accuracy, repeatability, and performance of whatever fluidic structures you have conceived.”

Newer microfabrication technologies offer an alternative to making multiplexed, complex fluidic IVD devices inexpensively and within the timeframe of a development cycle. With polymer laminates, there is no need to wait around for tooling or embossing, which can draw out the development process to a year or longer and delay the next design iteration. Even though the new technologies may be less proven in the market today, they create more elegant, inexpensive, and robust device designs.

Prototyping a Low-Density Microarray

Tetracore Inc. (Rockville, MD) is prototyping a low-density, DNA- and/or antibody-based microarray that is intended to function as both an IVD POC device and a laboratory instrument for rapid detection in hazardous-material and food-safety environments. William Nelson, Tetracore's president, explained:

“A little more than a year ago, we purchased a start-up company, Seahawk Biosystems. It had a previous iteration of a device consisting of a plastic chamber with a silicon gasket that could be compressed on a microscope slide to create the channel. The design was very difficult to change because the gaskets were created in a mold, and the other parts were machined from plastic. With each iteration, a new gasket had to be created, and the machine shop had to make a whole new chamber as well. The microfluidic channels were sealed with tape, which was very unsatisfactory.

“The availability of pressure-sensitive adhesives and rapid laser cutting and lamination has made a big difference,” said Nelson. “After submitting our initial design, we were up and running within a few days, and since then, all changes to the design have been turned around very quickly. We have been testing different configurations for the fluidic channel and the chamber. For example, we needed to generate cartridges with a small flow cell to deliver materials to the low-density microarray in a process called fluid-forced discrimination, and we tested different numbers of features on the chip, from 16 to 64.

“We have also tested various types of onboard functionality such as membranes, wells to retain materials, and one-μm and 10-μm filters. Because this is a magnetic bead-based assay, we were examining different methods for trapping beads in the process and excluding beads from some functions. With polymer laminate fabrication, we can change the configuration of the fluidic channel and the chamber of the prototype, and have a new device to test whose parameters are close to the finished product within a few days after submitting a CAD drawing. There is no expensive tooling and no waiting around for injection molding.”

Tetracore's goal is to develop a device that combines a fixed-design cartridge with an internal fluidic pad that can be modified to perform six different assays. “The polymer laminate technology is very cost-effective and gives us the flexibility without the need to invest in expensive tooling,” said Nelson. “Our time to market will be much faster than it would have been with the original method used by Seahawk Biosystems. It's not a difficult process to scale up, and we will be able to mass-produce large numbers of devices in a very short period of time.”

Electrochemical Detection-Based DNA Microarrays

Figure 3. A five-layer laminate component for Osmetech's eSensor XT-8, a molecular diagnostic test for susceptibility to Warfarin that was recently approved by FDA. The laminate functions to bring together both an injection molded component for sample introduction and an array of electroactive sites. The laminate itself functions as a pneumatically driven recirculating pump.

Osmetech Molecular Diagnostics (Pasadena, CA) is in the final stages of obtaining FDA approval for a DNA microarray device that incorporates polymer laminate technology. The eSensor XT-8 is a DNA microarray device for warfarin sensitivity testing. Warfarin exhibits a very narrow therapeutic range, a wide inter-individual variation in dosage required to reach optimal therapeutic effect, and severe side effects from overdosing.

“The eSensor XT-8 cartridge device is a microfluidic-based IVD device that establishes an individual's genotype, on which proper dosing depends,” said Robin Liu, PhD, Osmetech's director of device technology. “It consists of a PCB chip, a cover, and a microfluidic component composed of a plate and a laser-cut multilayer laminate. The microfluidic component includes a diaphragm pump and check valves in line with a serpentine channel that forms the hybridization chamber above the array of electrodes.

“The PCB chip is prepared for an eSensor assay by depositing DNA capture probes and insulator molecules on the working electrodes. Each specific deposition solution is dispensed on the appropriate electrode using a robotic pipette system. The capture probe and insulator react with the gold surface to form an insulating self-assembled monolayer. After capture probe dispensing, the PCB chips are washed, dried, and assembled with the laminate, plate, and plastic cover into a cartridge to form a microfluidic circulating system that can hold approximately 140 μl.”

Osmetech is one of the few companies to develop an electrochemical-based detection technology. “Conventional microarray devices are very expensive because they rely on fluorescent detection-based technologies that require bulky optical detectors,” said Liu. “Our system not only is low cost but also has a small footprint that conserves bench space in a typical clinical laboratory.

“Polymer laminate technology has also been a helpful rapid prototyping tool,” he added. “Whenever we want to change or optimize the design, the laser's setting can be changed easily to provide us with redesigned laminate layers.

“Previous studies have demonstrated the feasibility of a single-use, sample-to-answer eSensor device, and recent developments in microfluidics provide additional tools to perform the required functions,” said Liu. “Point-of-care microarray systems using electrochemical detection of nucleic acids will meet critical healthcare needs, including rapid genotyping.”

Conclusion

Leanna Levine, PhD, is president and chief executive officer at ALine Inc. (Redondo Beach, CA). She can be reached at llevine@alineinc.com.

There is a misconception that polymer laminate technology is expensive. However, the ability to incorporate complex functionality has vast benefits compared with low-cost injection-molded parts that cannot perform the pumping nor contain such valves that can be built elegantly into a polymer laminate. Achieving the same function in an injection-molded system would require compromise in cost and disposability, and would produce a cumbersome and costly IVD device to manufacture. Polymer laminate technology is a low-risk alternative for prototyping multiplexed, complex fluidic devices for applications in molecular diagnostics and immunodiagnostics. It is inexpensive and shortens the development cycle.

Polymer laminate technology also adds value to the manufacture of up to one million IVD devices per year and facilitates the development of devices that might not otherwise reach the market due to tooling costs. It lowers the barrier to entry for IVD manufacturers with groundbreaking ideas for diagnostic tools and others that want to address smaller markets. By enabling a shorter development time and easing the production of multiple iterations inexpensively, polymer laminate technology speeds time to market and makes the microfabrication of IVD devices with complex features affordable.

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