Manufacturers across the pharmaceutical industry employ a variety of techniques to create safe, efficacious products. Frequently,
processing aids are employed to help maintain the integrity of the dosage form during manufacturing.
Release liners, also referred to as simply liners, are one such processing aid that have been an integral part of transdermal
product design since the first commercially available transdermal patch product was approved by the FDA in 1979. These liners
frequently serve as both processing aids and final components in transdermal patch systems.
Typically, release liners are a siliconecoated polyester or paper product, and their value and use for transdermal patch systems
are well understood. However, the use, chemistry and benefits of release liners as a processing aid and/or component for new,
liquid cast dosage forms — particularly oral thin films (OTFs) and buccal mucosal systems — is growing in importance as the
use of these applications become more widespread.
More than just a film or paper
A release liner is a web of sheet material that covers the adhesive side of a pressuresensitive adhesive (PSA) tape product
to provide a protective surface during storage and transit, as well as to provide functional support during manufacturing
and converting.1 Developed originally for industrial PSA tape and label applications, liners quickly became an essential processing aid in
the adhesive casting process. In this scenario, a bulk liquid adhesive formulation is coated or cast onto a continuous web
of liner material that is then run through a drying tunnel until the desired dryness and adhesive tack characteristics are
achieved (Figure 1). In label applications, the web is then laminated to a label "face stock" material, typically a paper or film, and then
wound into rolls for transport and conversion. The successful conversion of a PSA product is fully dependent upon the integrity
of the release liner because it must resist the force of the cutting die, while withstanding the speed and rigour of processing.
Depending upon the needs of the end application, the liner may remain intact with the rolled adhesive tape product until the
point of application, or it could be removed during finishing and converting. Most PSA labels leave the release liner component
intact to aid in application.
The key performance criteria for release liners in any application are:
- the liner must have consistent contact with the product until it is deliberately removed
- upon removal, the adhesive should remain intact, with no residual adhesive or polymer remaining on the liner
- the liner should not change the chemical nature of the adhesive.
The use of release liners, and their associated safety and performance criteria, grew as adhesive products entered the medical
arena with the introduction of more sophisticated wound dressings, EKG monitors and device applications.
Figure 1: Casting onto release liner.
With the advancement of transdermal drug delivery in the 1980s, the use and requirements of release liners took on a new dimension.
In addition to the standard technical performance requirements, new safety and regulatory considerations for release liner
components quickly became paramount. Adhesives and film manufacturers dedicated extensive development resources to identify
and characterise materials that would be acceptable for use in pharmaceutical applications to ensure that the drug product
remained stable, the delivery system easytouse, and the chemistry safe for use on or around humans. Developers adopted liner
technologies from the electronics industry where ultraclean and low or nooutgassing liners were vital for maintaining electronic
component integrity. In addition, liners and liner components were evaluated against the standard tests for adhesives such
as cytotoxicity, skin irritation and so on. New companies emerged with chemistries and technologies wellsuited for providing
an ultraclean, consistent product comprised of pharmaceutically acceptable components.
The author says…
As even more advanced and active transdermal delivery technologies enter the market, the utilisation and function of release
liners in pharma applications has evolved in response to the manufacturing demands of these technologies. Whether or not release
liners remain a component of the final assembled system, the critical nature of their function remains the same. They provide
integrity and strength to the multitude of rolled goods that are manufactured, repeatedly handled, converted, and finally
integrated and assembled into a sophisticated delivery device. Liners can also serve as a layertolayer protective barrier
to prevent materials from intermingling or interfering with one another.
Aside from their many process benefits when manufacturing delicate or tacky materials, release liners are emerging as a critical
component in OTFs and buccal delivery systems. The launch of the Novartis Thin Strips in 2004 was due, in part, to developing
sophisticated liners that could enable a robust, costeffective process for producing and packaging dissolvable films.
This next generation of liner development has focused on creating materials that are safe as processing aids for oral delivery
forms and that are also able to maintain the perfect level of adhesion with an inherently tackless product through the stresses
and rigours of the manufacturing process.