Today’s porous adhesives can add functional capabilities beyond bonding to diagnostic and wound-care devices.
Their properties and potential applications are described.
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Delivering unencumbered flow
As medical device developers consider ways to improve comfort and patient compliance with regard to wound-care products, or increase assay sensitivity while reducing device complexity and response time of diagnostic devices, material suppliers are embracing opportunities to develop the enabling technologies that will fuel the next generation of these devices. A porous pressure-sensitive adhesive (PSA) is described here as an example of how a conventional component is now being formulated to deliver increased functionality. Not only does this adhesive provide reliable bonding capabilities, but it also facilitates the unencumbered flow of fluids and gases.Conventional PSAs are impermeable films that have been used in the medical field for decades to attach devices to skin or for bonding membranes and other in vitro diagnostic (IVD) test components. Although a number of porous polymers, foam and membranes are used in medical devices, the concept of a porous adhesive serving a dual role for bonding components while increasing device functionality is relatively new to the industry.
Porous adhesives demonstrate all the desirable properties of a conventional PSA such as quick adhesion and the capacity to bear load, while allowing the free transport of fluids and gases through the adhesive. The adhesive forms instant bonds to join film substrates, foams, membranes, pads, filter element or plastic parts without the need for curing or clamping during production of the finished product. This technology platform provides medical device manufacturers with the same manufacturing efficiencies as conventional PSAs, including continuous roll-to-roll manufacturing and simplified manipulation of small die-cut multi-laminate structures, with added functional capabilities for the advancement of a number of diagnostic and wound-care device designs.
Porous structures and chemistries
| Figure 1: The porous pressure-sensitive adhesive forms isolated channels to control flow and movement of fluids or gases. |
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The porous adhesive technology described here is a PSA that offers hundreds of micron-sized open pores or cells in a low-density, highly permeable structure. The pores range in diameter from approximately 200–500 µm. The distribution of pores throughout the matrix results in 30–50% porosity and a finished film thickness of 51–203 µm. The porous adhesive formulation is customisable for each application. The pores in the adhesive form isolated channels (Figure 1). These channels enable flow and movement of aqueous-based fluids and/or gases from one substrate to the next through the z direction of the adhesive, while acting as a gasket seal in the x–y direction.
There is no one adhesive that meets every need of a medical device designer and adhesive chemistry must be carefully selected to match the specifics of the end application. For this reason, the porous adhesive technology is available in several chemistries as transfer adhesives or upported film constructions that are:
- inherently hydrophilic to promotemoisture absorption and aqueous flow; this property can be further enhanced with the addition of wetting compounds to facilitate rapid transmission of aqueous fluids from one membrane to the next
- hydrophobic, thereby slowing the exchange of polar fluids while facilitating good vapour transmission; these adhesives are recommended for bonding to non-polar substrates such as polyolefins and when laminates are expected to maintain bonds in the presence of acids, polar organic solvents, alcohol or water
- a balance of adhesion and flow properties that maintains bonding capabilities and porosity while withstanding a variety of environments, temperature cycling and solvents.
The stable pore structure of this technology prevents collapse during handling and ageing that would affect functionality. The pores retain their physical dimension and resist crushing and closing when exposed to normal handling and pressure during lamination processes. Table I compares the porous structure before and after accelerated ageing.
The porous adhesive’s ability to transport fluids freely may enable its use in a number of application areas where PSAs have not been previously used in in vitro diagnostic devices or advanced wound-care products.
Membrane-based immunoassay systems
| Table I: Porous structure comparison before and after accelerated ageing. | ||||||||||||||||||||||||
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These tapes can be used for bonding, laminating and assembling of test kits. Many characteristics of the porous technology platform such as thickness, pore size, chemistry and peel strength are customised for a specific application. Chemistries can be chosen with resistance to certain solvents, pHs and buffers. Materials that are suitable for laminating to the porous PSA include cellulose; nitrocellulose; polyethersulphone; polyvinylidene fluoride (PVDF), micro-porous polyethylene and nylon membranes; and polyurethane foams. Nitrocellulose membranes are delicate and are characterised as having low tensile strength and tear resistance. However, laminating these membranes to the porous adhesive provides additional reinforcement to help prevent breakage during handling and processing.
The porous adhesive’s isolated pore structure provides an alternative way to couple multiple layers of materials for rapid and even flow of analytes and other fluids in vertical or flow-through devices. The adhesive guides fluid flow from one layer to the next; this prevents the fluid from spreading to the device’s edges, which could result in potential false readings. The adhesive’s columnar pore design can also enable the cost-effective design of multiple parallel flow paths within a vertical-flow device for incorporating separate fluid streams to react with multiple reagents.
Bonding filters and advanced separation materials
As more IVD assays are developed to measure lower concentration analytes and biomarkers from complex clinical samples, cost-effective sample preparation methods utilising filters to isolate target compounds will become more important. A porous PSA can contribute to the advancement of these devices by bonding multiple layers in a thin profile to facilitate rapid flow and more timely results. The use of one or a combination stack of filters for advanced separation materials can separate target sample components or remove unwanted impurities for test efficiency and a streamlined product design.
Microplates
A number of molecular biology and cell culture applications require the sealing of microplates with porous materials to allow gas permeation for respiration while preventing cross-contamination. Porous adhesives laminated to porous papers or membrane such as porous polyethylene or PVDF provide a good sealing option for covering wells while enabling oxygen intake and carbon dioxide outgassing. Cross-contamination is controlled by the adhesive’s hydrophobic characteristics and isolated columnar structure.
Microfluidic devices
These devices require adequate ventilation to prevent the entrapment of air or gas bubbles in flow channels. Often, the vents are plugged or covered with a porous film to prevent liquid leakage. Laminating a porous adhesive to a porous membrane can provide an alternative method for sealing air vents and enabling air to escape while preventing fluid spillage.
Advanced wound-care devices and dressings
Advanced wound-care products include biological dressings, surgical sealants, synthetic dressings, anti-adhesion and other related wound-healing products. Dressings that feature a porous adhesive may provide a dynamic exudate handling system with an internal wicking layer to regulate wound moisture.
There is a growing demand for adhesives that offer high moisture vapour transmission rates (MVTR) to promote breathability in extended-wear applications. The porous adhesive’s MVTR of approximately 9000 g/m2/day compares well with conventional solid film PSAs that typically monstrate MVTR of tens to several hundred g/m2/day. The porous PSA’s MVTR promotes breathability and may be used with a combination of different substrates to provide desired MVTR performance.
Flexibility
The patent-pending porous PSA technology delivers the same bonding and manufacturing efficiencies of conventional PSAs while offering a stable, porous format to enable new capabilities and simplify device designs with increased functionalities. Compatible with most materials, the different adhesive chemistries of the porous adhesive format address the varying environmental conditions and chemical requirements of lateral and vertical flow IVDs, microplate, and microfluidic and wound-care device applications.
Ranjit Malik, PhD, is Group Leader, Core Technology for the ARcare Division at Adhesives Research Inc., 400 Seaks Run Road Glen Rock Pennsylvania, USA
tel. +1 717 227 3294
tel. +1 717 227 3294
e-mail: rmalik@arglobal.com [5]
European headquarters: Adhesives Research Ireland Ltd Raheen Business Park, Limerick, Ireland, tel. +353 61 300 300
www.adhesivesresearch.com [6]
European headquarters: Adhesives Research Ireland Ltd Raheen Business Park, Limerick, Ireland, tel. +353 61 300 300
www.adhesivesresearch.com [6]