Recent developments have been made resulting in surfaces that can exhibit dynamic changes in interfacial properties, such as wettability, in response to external stimuli.¹ The type of stimulus used to change the surface may be done in several ways including temperature, electrical potential, pH, and solvent. The wetting characteristics of surfaces have always been important in the science of adhesion, and many prebond surface preparation processes have been designed to change the substrate surface to one which is more easily wet by the adhesive.

The unique fact about these more recent studies is that the surface can be changed after the adhesive is applied and cured. Depending on the surface chemistry and stimulus type, a substrate surface can be made "switchable". In fact it can be made to go from a super-hydrophobic state to a super-hydrophilic state or vice versa. These surfaces have many properties that are useful, including the ability to be self-cleaning, anti-bacterial, anti-reflecting, and anti-fogging.

One example of a switchable surface is based on temperature and the use of a thermo-sensitive polymer. These polymers undergo a phase transition at a temperature known as the lower critical solution temperature (LCST) where the behavior switches between hydrophobic and hydrophilic. Poly(N-isopropylacrylimide) is one such surface with a lower critical solution temperature of 32°-33°C. The switchable nature is enhanced by the incorporation of roughness of the surface due to the trapping of air in the cavities at the interface.

A switchable surface combines the attributes of both super-hydrophilic and super- hydrophobic surfaces and can potentially be used in a variety of applications. Such reversibly switching surfaces may open previously unknown opportunities in interfacial engineering including microfluidic pumps, drug delivery systems, separation, and protein concentrates.

Switchable surfaces could also have more practical and simple applications in adhesive bonding such as providing for release of a bonded joint on demand. The commercial possibilities for detachable adhesives are significant. The temporary bonding of graphic arts signage, packaging labels, and parts for machining or other finishing operations are just several examples. However, the most notable market drivers in the future will be the disposal and reclamation of automotive and consumer electronic components.

Detachable adhesives could be used for the purpose of recycling and reusing product parts. When an automobile is scrapped, for example, the plastics and metals that are bonded together can be separated for easier disposal. Parts or major sections of the automobile can even be reused if they are kept intact. Toward this end, the European Commissions End of Life Vehicles Directive has set a reuse and recovery target of 95% of materials from scrapped vehicles by 2016. Such controlled debonding may become a basic requisite for disassembly and recycling in the future.

Its funny how high tech advancements at the university level often find their way into mundane, commodity goods that are available to the general public. Some day switchable surfaces may rank with those achievements.