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:
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.