Advances in adhesives and ease-of-use make pressure-sensitive adhesives (PSAs) the number one choice for medical devices that must be temporarily attached to a patient's skin, such as transdermal patches and ECG electrodes. But PSAs can also use multi-lamination materials to simplify assembly of medical devices.

The large roll of single coated plastic film uses a specially formulated adhesive for IVD or in vitro diagnostics. Peeling off the liner exposes the adhesive. This roll of stock would typically be used for in-line machine laminations by IVD manufacturers.

Unlike liquid adhesives, PSAs are typically supplied in ready to use sheets, rolls, or die cut forms, and can be supplied in precise, uniform coating thicknesses adjusted to the application. Another plus is that PSAs require no activation by water, solvent, or heat to bond with a substrate.

Bonding assemblies

Pressure-sensitive adhesives, as the name implies, adhere to most materials immediately when applied with light to moderate pressure. Bonds build to predesigned levels over short periods. PSAs make it relatively fast and easy to bond several materials into a laminate, and these can be similar or dissimilar materials, such as those that are stiff and flexible, hard and soft, and thick and thin. Today's medical market provides a wide variety of ready-to-use PSA coated films, foams, and fabrics.

PSAs balance adhesive flow and resistance to flow. A bond forms because the adhesive is soft enough to flow, or wet out, onto another substrate. But the bond is strong because the adhesive is hard enough to resist flow with applied stress. Furthermore, PSAs are said to be in dry form, meaning they have little or no solvent or water retention and are tacky at room temperature. (The tackiness allows for a quick initial stick.)

A PSA with a blue liner comes out of a rotary die cutter

The bond strengthens over time and generally reaches maximum strength about 24 hours after lamination. So users should allow time for proper bonding, especially when dealing with materials that are difficult to bond together, such as low-surface energy substrates. These include polyethylene, polypropylenes and PTFE films, or silicones, polyurethanes, fiberglass pads, and ether foams.

While some applications require internal or functional bonds, others have more specialized requirements. Take skin contact and in-vitro diagnostics, for example. Tapes for use on the skin in devices such as electrodes, ostomy bags, and surgical grounding pads must meet all FDA or local government regulations, and should be tested to government standards as well as thoroughly tested in the actual application.

For in-vitro diagnostics (IVD), pressure-sensitive adhesives should be formulated to produce a chemically inert adhesive system that will not migrate into sample pads or membranes. The PSA should be an acrylic biocompatible with components of the test with no affect on the stability of reagents or membranes.

Building a PSA tape

Pressure-sensitive adhesives often come in the form of tapes and generally in three options:

• Double-coated tapes come on a carrier, such as polyester or other film, coated on both sides with a PSA and protected by one or two release liners.

• Single-coated tapes come on a carrier, such as a film, foam, or woven material, which is coated on one side with a PSA and protected by one release liner.

• Transfer (unsupported) tapes use a layer of PSA coated onto a protective release liner without additional support or backing.

To protect the adhesive before and during assembly, a protective release liner must be placed over the adhesive. The liner can be paper or film treated for release characteristics and moisture resistance. The release agent is typically a silicone coating that makes it easy to remove the liner from the adhesive.