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The development and implementation of successful transdermal devices for drug delivery requires an understanding of the adhesion occurring between the device and the soft dermal layer. As such, we are utilizing a mechanics approach to quantify the adhesive properties of representative pressure sensitive adhesives (PSAs) used as the adhesive layer in these systems. Debonding of PSAs is accompanied by cavitation in the PSA and the formation of an extensive cohesive zone behind the debond tip. The presence of such large-scale bridging provides significant energy dissipation and increased resistance to delamination. The strain energy release rate (G) during debonding of a cantilever-beam sample, containing at its midline a thin layer of PSA, is utilized to quantify the adhesion of the PSA. Stress-separation functions are also performed on these PSA systems in order to assist in developing predictive bridgining mechanics models of PSA debond behavior. This analysis accounts for both the work of adhesion as well as the viscoelastic constitutive behavior of the soft adhesive layer. Effects of strain rate, physiological environment, layer thickness, and permeation-enhancement additions are among those parameters whose effects are being investigated. The resistance of human stratum corneum to debonding between corneocyte layers is also being studied, as knowledge of this parameter is essential for developing techniques to test the fracture resistance of the PSA-stratum corneum interface present in the clinical use of these transdermal devices.
What is a transdermal drug delivery device? A transdermal drug delivery device, which may be of an active or a passive design, is a device which provides an alternative route for administering medication. These devices allow for pharmaceuticals to be delivered across the skin barrier. This approach to drug delivery offers many advantages over traditional methods. As a substitute for the oral route, transdermal drug delivery enables the avoidance of gastrointestinal absorption, with its associated pitfalls of enzymatic and pH associated deactivation. This method also allows for reduced pharmacological dosaging due to the shortened metabolization pathway of the transdermal route versus the gastrointestinal pathway. The patch also permits constant dosing rather than the peaks and valleys in medication level associated with orally administered medications. Multi-day therapy with a single application, rapid notification of medication in the event of emergency, as well as the capacity to terminate drug effects rapidly via patch removal, are all further advantages of this route.
Why should we use fracture mechanics to study transdermal devices? In order for a transdermal patch to be effective, satisfactory adhesion to the dermal layer must be maintained. This requires that there be adequate initial adhesion as well as appropriate long-term adhesion. A fracture mechanics approach to the study of the interface between the adhesive material and the dermal layer enables the quantification of the adhesive properties of the system and permits a means to determine the effects of such varied parameters as strain rate, temperature, humidity, physiological environment, and layer thickness on adhesion and device failure. Our research considers that the resistance to interface debonding may be quantified in terms of the interfacial fracture energy, G, and is a property of the system. This analysis accounts for both the work of adhesion as well as the viscoelastic constitutive behavior of the soft adhesive layer. Predictive bridging mechanics models are incorporated into this research in order to study cavitation in the PSA and the development of extensive cohesive zones associated with the debonding process. Despite the importance of G in providing a basis for understanding failure mechanisms associated with these transdermal devices, there have been limited attempts to accurately measure G using well defined fracture mechanics techniques. Accordingly, the intent of the present study is to characterize the interfacial fracture resistance of the adhesive-dermal layer interface in order to understand the underlying failure mechanisms. Additionally, a fracture mechanics approach can be extended directly to the stratum corneum, the top layer of skin. The study of the resistance of human stratum corneum to debonding between corneocyte layers has been undertaken, as knowledge of this parameter is essential for developing techniques to test the fracture resistance of the PSA-stratum corneum interface present in the clinical use of these transdermal devices.
One typically thinks that the strength of an interface is the energy required to separate two materials. In fact, you can think of this interface adhesion as the interface fracture resistance or energy required to propagate an interface crack. This interface fracture resistance is primarily determined by two different energy absorbing processes: the near-tip work of fracture and an energy dissipation zone surrounding the debond . In a region close to the debond crack tip, the intrinsic interface fracture resistance, G0, includes the thermodynamic work of adhesion, which provides a direct measure of the bonding forces at the interface. These may arise from actual chemical bonding. In a viscoelastic material such as the pressure-sensitive adhesives utilized in this study, energy dissipation through large-scale bridging effects as well as time-dependent strain-rate effects must be considered. How do we study interface failure? This research utilizes a double cantilever beam specimen configuration to evaluate interface fracture resistance. Resistance curve behavior is then measured using a screw-driven mechanical testing system accompanied by a variety of techniques that enable us to measure the in situ debond length.
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