Polytetrafluoroethylene (PTFE) is best known by the DuPont brand name Teflon. Whatever it is called, PTFE is the third slipperiest solid known – the poster child for non-stick, non-reactive, non-friction, non-conducting, high-temperature, and generally high-performing polymers. Silicone also has a nearly non-bondable surface – if you try to paint a silicone sealant, it simply pops off as the paint dries. In particular, creating a strong bond between PTFE and silicone has never been accomplished, even in the chemical laboratory. Until now.

The feat has been accomplished in Prof. Rainer Adelung's research group at Germany's Kiel Univesity (CAU), and offers great promise for a horde of industrial and medical applications. The trick is that the PTFE and silicone surfaces are not held together by a chemical adhesive.

One of the zinc oxide caltrops used as nano-staples (Image: Xin Jin, Copyright CAU)

Rather, zinc oxide nanocrystals (ZnONC) with the general aspect of a caltrop or tetrapod allow PTFE and silicone surfaces to be "stapled" together, forming a strong mechanical bond without any chemically-driven bond at all. A caltrop is an ancient antipersonnel weapon consisting of four spines so assembled together that, when thrown on the ground, at least one spike points up toward the feet of enemy soldiers or pack animals. Under certain growth conditions, ZnONC take the form of caltrops having four arms each having a hexagonal cross-section and a tetrahedral core.

Concept of the internal stapling process that allows PTFE and silicone to be solidly joined (Image: Jan Strüben, Copyright CAU)

To join PTFE and silicone, ZnONC are sprinkled evenly onto a heated layer of PTFE. In the title picture, this was done by using a heated non-stick frying pan. Then a layer of silicone is poured on top of the PTFE-nanocrystal surface. The materials are heated to 100° C (212° F) for less than an hour, during which time the silicone also cures. The peel strength of the silicone-PTFE interface is 200 Newtons/meter – similar to peeling plastic shipping tape off a clean glass surface.

Zinc oxide tetrapods sinking into a PTFE surface during the heating phase of PTFE-silicone bonding (Image: Xin Jin, Copyright CAU)

According to Xin Jin, a graduate student currently working on her Ph.D. thesis, “it’s like stapling two non-sticky materials from the inside with the crystals. When they are heated up, the nano tetrapods in between the polymer layers pierce the materials, sink into them, and get anchored.” Her colleague and supervisor, Dr. Yogendra Kumar Mishra, explains the adhesive principle: “If you try to pull out a tetrapod on one arm from a polymer layer, the shape of the tetrapod will simply cause three arms to dig in deeper and to hold on even firmer.”

The new process is of particular interest in medical engineering. The widespread use of silicone is often hampered by our current inability to make silicone stick firmly and reliably to other materials. Such applications require materials that are fully biocompatible, a requirement which is not fulfilled by many joining methods which involve chemical reactions. These can change the properties of the polymers, and the joining material can be harmful or toxic by itself or in combination with the chemically modified polymers. However, the Keil internal stapling process is purely mechanical, and zinc oxide is well known to be biocompatible. In vitro and clinical testing is required, but it is likely that internally stapled polymers will also be biocompatible.

One key to innovation is the appearance of new materials and new combinations of materials. It is rather easy to imagine certain applications for the internal stapling process, but it is likely that other important uses will come from a direction not even imagined at this point. A truly new process such as this is likely to surprise us all in the future.

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