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ASSAY DEVELOPMENT

Adhesives Research’s porous pressure-sensitive adhesive forms isolated channels to control flow and movement of fluids or gases.

Diagnostic assays are indispensable for detecting such compounds as drugs, hormones, enzymes, proteins, antibodies, and infectious agents in biological fluids and tissue samples. Diagnostic device developers are currently considering ways for increasing assay sensitivity while reducing device complexity, reaction time, and costs. Under such circumstances, IVD material suppliers have an opportunity to create the next wave of component technologies to support assay development.

Since the 1980s, IVD manufacturers have used conventional pressure- sensitive adhesives (PSA) in lateral-flow devices to form an impermeable bonding layer between two substrates, such as nitrocellulose membranes and polyester films. As IVD device designs have become more focused on reducing test time and increasing sensitivity, new opportunities are emerging for PSA tapes. One developmental area is in constructions to facilitate the free and unencumbered exchange of gases and fluids between multiple substrates.

For many decades, porous polymers and membranes have been available on the market and have been used in IVD devices. However, the concept of a porous adhesive serving a dual role in bonding components and increasing device functionality is new to the IVD industry.¹

Formulating Porous PSAs

Over the years, various techniques have been used to generate pores in conventional PSA films. Some methods include physical techniques such as laser drilling and biaxially or uniaxially stretching the adhesive film to produce perforated, straight-through, nontortuous holes.² Other methods entail evaporating dissolved gases or incorporating soluble particles, followed by dissolution to form pores. Another technique involves evaporating a volatile solvent that is dispersed in a conventional emulsion or solvent-borne PSA.

Another method for making porous adhesives is casting a thermoset adhesive mixture onto a perforated web. After doing so, jets of cold air are released through the perforation on the underside of the web. This process clears the liquid formula immediately above the perforations, thus forming holes in the coating. Simultaneous curing of the adhesive film solidifies the liquid mixture, thereby permanently locking the holes in place.³ Other techniques for modifying conventional PSAs presented limitations, including difficulties with reproducibility, coat weight, uniformity of pore size and porosity, and pore stability over time.⁴

Noncontinuous adhesive coatings that are made through various printing processes have also been used to create the perception of a porous adhesive. This technique involves spray, pattern, or slot-die printing of a substrate with discrete dots of adhesive while leaving substantial interspaced areas uncovered to function as pores. Screen printing is another example of a way to produce a patterned, noncontinuous adhesive coating.

This article presents a porous adhesive technology that does not rely on modifying a conventional PSA but rather on formulating a novel product to overcome the challenges of past formulas and methods. The pores in the porous PSA by Adhesives Research (Glen Rock, PA) are formed by a gasification process after coating a specialized adhesive formula onto a substrate. The gas is trapped within the adhesive matrix in the form of bubbles that create open pores on the top and bottom surfaces of the adhesive film. PSAs are viscoelastic materials. Because of that, controlling cold flow, or the process in which a material spontaneously flows under its own weight, is one challenge adhesive formulators often encounter to avoid issues with handling or converting that could lead to compromised product functionality over time.

A Twist to a Conventional Technology

Adhesives Research’s porous adhesive technology exhibits all the desirable properties of a conventional PSA (e.g., quick stick and capacity to bear load) while facilitating uninhibited passage of fluids and gases across the film’s two surfaces. The adhesive instantly bonds film substrates, membranes, pads, filter elements, or plastic parts without any curing or clamping during the manufacturing of the finished product. As with traditional PSAs, the porous PSA provides IVD companies the same production efficiencies, such as continuous roll-to-roll manufacturing and simplified handling of small die-cut, multilaminate structures.

The porous adhesive formulation can be customized for each diagnostic application in order to bond similar or dissimilar substrates, membranes, and plastic components. Because porous membranes used in IVD devices are nontacky, many device designs rely on either plastic housings to clamp components together or an impervious PSA for bonding. In this example, the porous PSA can provide both a physical separation and a bonding layer between the substrate materials while allowing for the rapid passage of fluids through the adhesive. This article discusses several other IVD applications for porous PSAs.

Porosity and Structure. The porous adhesive technology offers open pores or cells that are uniform in size and distribution to create a low-density, highly permeable structure. The pores are approximately 200-500 µm in diameter and have 30–50% porosity. The finished film thickness is 2–8 mil. The adhesive’s pore structure can assume two different forms, isolated channels or a network of interconnected pores, which allow cross-talk or lateral flow between the pores’ contents. The primary focus for near-term commercialization is developing the ability of PSAs to feature isolated channels that control flow and movement of aqueous-based fluids and/or gases. The channels enable such flow between substrates through the z direction of the adhesive, while acting as a gasket seal in the x–y direction.

An adhesive’s pores must not close or collapse during the downstream processing steps or normal aging, since the result would be the loss of porous functionality. An advancement of Adhesive Research’s porous adhesive technology is the stable pore structure. The pores retain their physical dimension and resist crushing, closing, and collapsing when they are exposed to normal handling and pressure during lamination processes.

Table I. (click to enlarge) Typical characteristics of the porous adhesive.

Chemistry. Because no universal adhesive exists that can meet the myriad needs of IVD designers, the adhesive chemistry must be modified for each specific membrane and substrate. For this reason, the porous adhesive is available in the following three chemistries, with a choice of either a transfer adhesive or a supported film format (see Table I):

• Inherently hydrophilic constructions can be further enhanced by adding wetting compounds to facilitate rapid transmission of aqueous fluids.
• Hydrophobic constructions slow down the exchange of polar fluids while facilitating good vapor transmission. Such adhesives are recommended for bonding to nonpolar substrates, like polyolefins. They are the first choice when laminates are expected to maintain bonds in the presence of acids, polar organic solvents, alcohol, or water. Laminates produced with this adhesive can separate two chambers for the retarded exchange of fluid flow between the chambers. This adhesive is ideal for applications that require increased residence time for a reaction to take place.
• The final construction provides a good balance of adhesion and flow properties. This chemistry maintains its bonding capabilities and porosity while withstanding various environments, temperature cycling, and solvents that may be present in polymerase chain reaction (PCR) and cell-culturing applications.

Design Latitude. Many characteristics of the porous adhesive technology (e.g., thickness, pore size, chemistry, and peel strength) may be customized as needed for each diagnostic application. The adhesive can be supplied as a free standing transfer film or supported on a membrane. Chemistries can be chosen with resistance to specific solvents, pH, and buffers. The materials that are ideally suited for laminating to the porous PSA include the following: cellulose, nitrocellulose, polyethersulfone, polyvinylidene fluoride, microporous polyethylene and nylon membranes, and polyurethane foams. The porous adhesive technology also allows the formulation of thin, nontacky (no adhesive) porous films.

Diagnostic Applications

The porous adhesive’s ability to facilitate the free exchange of fluids can support many applications in IVD devices in which conventional PSAs have not previously been used. The typical pore size of 200 µm is large enough to allow the passage of a whole blood sample. Alternatively, the adhesive can be laminated to a porous membrane to filter red blood cells. Flexible and conformable, these adhesive tapes can be used for bonding, laminating, and assembling test kits. For example, the porous PSA can be applied to produce a stack of filtration membranes for cost-effective sample preparation. In vertical-flow or combination lateral-flow IVDs, the adhesive layer can provide a physical separation between substrate materials while allowing for the rapid passage of fluids through the adhesive.

Many applications in molecular diagnostics, including high-throughput screening, reverse-transcription PCR, cell culture, and compound storage, require high-performance bonding tapes. Porous adhesives were developed to address the growing needs of such devices as microtiter plates, microfluidic chips, and microarrays in which liquids need to be contained while simultaneously providing ventilation for gas exchange.

In addition, membranes such as nitrocellulose are delicate, and are characterized by low tensile strength and tear resistance. When laminated to a porous adhesive, such membranes are reinforced, thereby resisting damage during handling and processing.

Bonding Filters and Advanced Separation Materials. As more IVD assays are developed to measure low-concentration analytes and biomarkers from complex clinical samples, cost-effective sample preparation methods that include filters to isolate target compounds will become more important.

Many sample preparation techniques for cell separation, including centrifugation, sedimentation, and differential extraction, require costly machinery, trained technicians, and lengthy reaction times.⁵ In many applications, using one filter or a combination stack of filters or advanced separation materials can either selectively separate target sample components or remove unwanted impurities with a high degree of efficiency while streamlining product design.⁶ A porous PSA can contribute to such devices by bonding multiple filter layers in a thin profile to facilitate more timely results through the rapid flow of fluids. Filtration concepts that could be enhanced by using the porous adhesive technology can be envisioned for vertical-flow devices and in filtration plates for multiwell-plate formats.

Membrane-Based Immunoassay Systems. Two popular designs exist for membrane-based immunoassay systems. One design is a lateral-flow assay in which the liquid flow is parallel to the membrane’s plane. The second design is a vertical-flow, or flow-through, device in which the liquid travels through layers of membranes stacked vertically relative to the device. While each device design has certain advantages and disadvantages, the porous adhesive can offer potential applications in both designs.

Current lateral-flow devices use conventional PSAs to laminate the membrane to a plastic backing. Migration of the adhesive into the pores of the membrane leads to clogging of the pores and is a common problem that reduces the flow of the analyte-labeled receptor complex along the membrane. Reduced flow rates may create a leading or lagging-edge effect, resulting in an uneven distribution of the complex across the detection zone, which makes data difficult to read. Using a porous adhesive will help to mitigate this issue because its high porosity reduces the surface contact points with the membrane.

On the other hand, vertical-flow devices were historically designed with a single membrane placed on an absorbent pad. Bonding a membrane to the underlying pad with conventional adhesives was not practical in a flow-through application due to the adhesive’s impervious nature. As the sample fluid flows through the membrane, a weak link in the chain of communication between the membranes and the absorbent pad leads to issues with repeatability, consistency, and rapid fluid flow. If irregular flow patterns develop in the membrane or if excessive flow time is experienced, false positives or false negatives can occur.

Although improved material technology is broadening the application landscape for vertical-flow devices, sample flow consistency continues to be an issue that affects the accuracy of test results.

Figure 1. (click to enlarge) Porous pressure-sensitive adhesives (PSA) can bond multiple layers within vertical-flow devices to facilitate rapid and even fluid flow.7

The porous adhesive’s isolated pore structure provides an alternative for coupling multiple layers of materials, including filters, sample pads, absorbent pads, and membranes, to improve the flow of analytes and other fluids (see Figures 1 and 2). One primary challenge is that liquid has a natural tendency to spread, which often results in the fluid bypassing intermediate layers and potentially compromising test accuracy. Bonding layers with a porous adhesive reduces fluid spread to device edges because liquids are guided by isolated pores to control flow from one layer to the next.

Figure 2. (click to enlarge) Porous adhesives are an alternative for coupling multiple layers of filters, sample pads, absorbent pads, and membranes for improved flow of analytes and other fluids.

The isolated pore structure can also support the cost-effective design of multiple, parallel flow paths in a vertical-flow device in order to incorporate separate liquid streams and allow a reaction with several reagents. The widespread use of multimembrane systems may become a reality with this technology. For example, a patent describes a flow-through device consisting of a filter stack that can benefit from using a porous adhesive.⁸

Figure 3. (click to enlarge) Diagnostic devices that combine the benefits of lateral and vertical flow can be envisioned by using porous adhesives.

Combining Vertical and Lateral Flow. IVD manufacturers are considering developing hybrid systems that combine the ease-of-use of lateral-flow devices with the speed and sensitivity of vertical-flow devices (see Figure 3).

For example, a multilayer vertical-flow device detects HDL and LDL in such a way that the first filter layer removes the red blood cells.⁹ This device is another avenue for the porous adhesive technology. With this technology, filter laminates that separate plasma from blood cells can be produced, thereby enabling the development of devices for multiple disease diagnosis from a single whole blood sample. Furthermore, porous adhesives can be incorporated into fabricated devices for applications such as drug, agriculture, and veterinary testing.

Microplates. Many molecular biology and cell culture applications use microplates that need to be sealed after they are filled. Some applications require an airtight seal, while others need to be gas-permeable for cultivating microorganisms such as bacteria, yeast, or cells. While traditional PSAs have been used in many sealing applications for protection from cross-contamination in cell culturing and PCR applications, they do not facilitate vapor transmission.

Cell culturing depends on a sufficient supply of oxygen and the removal of carbon dioxide to support cell growth. In conventional microtiter plates, the supply of oxygen for cell respiration is pulled from the header space in each well, which limits the rate of gas exchange and affects the overall cell growth.¹⁰ Porous adhesives laminated to porous papers or membranes, such as porous polyethylene or PVDF, provide a good sealing option.

Figure 4. (click to enlarge) Porous adhesives laminated to porous membranes provide a good gas permeable sealing option while preventing cross-contamination in microtiter plates. Traditional PSAs may be used for impermeable seals in the same device.

Figure 4 illustrates a multiwell cell culture plate design in which an apertured plate is sandwiched between two membranes: the top aperture is sealed using a porous adhesive, and the bottom aperture is sealed with a conventional PSA. Each aperture functions as a microchamber. The result is a liquid-tight seal for each microchamber, and, at the same time, uniform gas exchange and equilibrium to the cells in the microchambers.

Microfluidic Devices. At a minimum, microfluidic devices consist of three regions: fluid input, sample containment, and venting. All three regions communicate through a network of capillary channels. Microfluidic devices are useful for detection, separation, and cell-sorting operations in immunoassays, capillary electrophoresis, and lab-on-a-chip. PSAs are used to bond molded plastic parts to enclose the fluidic channels.

Microfluidic devices require adequate ventilation to prevent the entrapment of air or gas bubbles in their flow channels. The device’s vents are plugged or covered with a porous film to prevent liquid from leaking. However, microfluidic devices that exhibit less back-pressure and trapped air when filled with a liquid sample are in demand, since the trapped air bubbles impede the desired flow of fluids in the channels. The porous adhesive can be laminated to a porous membrane, providing a new means of sealing the vents and enabling air to escape while preventing fluid spillage.

Ranjit Malik, PhD, is group leader, core technology for the ARcare division at Adhesives Research Inc. (Glen Rock, PA). The author can be reached at rmalik@arglobal.com.

Continuous Monitoring and Skin Contact Diagnostics. For skin contact devices, the open pores on the adhesive’s surface translate into less surface area for the adhesive to bond to the skin, resulting in low-trauma skin removal following extended wear. When the device must be worn long-term, the high moisture-vapor transmission rate of the porous PSA promotes breathability to prevent skin damage. The porous adhesive is ideal for devices such as continuous glucose monitoring systems since it increases functionality and performance.

Conclusion

Adhesives Research’s patent-pending porous PSA technology delivers the same bonding and manufacturing efficiencies of conventional PSAs, while offering a stable, porous format that supports new diagnostic capabilities, simplifies IVD device designs, and increases functionality. Compatible with most materials, the porous adhesive format has been developed to be available in several different adhesive chemistries to address the varying environmental conditions and chemical requirements of lateral- and vertical-flow IVDs, microplates, and microfluidic devices. As development continues to further the advancement of porous PSAs, reduced porosity and forms with smaller and larger pore structures will be evaluated so that they can function optimally in specific IVD applications.