Polymers Convert Light to Mechanical Movement

We are not surprised when fPlanten groeien met kunstlicht, fotosynthese, Nederland 1970-1980.  .lowers open at sunrise and close at sunset. But seeing a polymer behave like that is inspiring. A light-responsive polymer actuator has been designed using photosensitive polymer by a collaborative team of scientists from the University of Pittsburgh, Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio, and Hope College in Holland, Mich.

Photosensitive polymers respond to ultraviolet or visible light by exhibiting a change in physical properties or chemical constitution. That includes changes in molecular shape (photo-responsive polymers), constitution (photo-reactive polymers), or color (photochromic polymers).

Now researchers have combined the photo-responsive properties of azobenzene-functionalized polymers, which undergo light-induced  isomerization with elastic instability, converting conformational changes of polymer molecules into a snapping action, or, in other words, efficiently converting light into mechanical movement (download to see the video). Here is how M.R. Shankar and others explain the process in their recent publication in PNAS:

 Here, we investigate photoinitiated snap-through in bistable arches formed from samples composed of azobenzene-functionalized polymers (both amorphous polyimides and liquid crystal polymer networks) and report orders-of-magnitude enhancement in actuation rates (approaching 102 mm/s) and powers (as much as 1 kW/m3). The contactless, ultra-fast actuation is observed at irradiation intensities <<100 mW/cm2. Due to the bistability and symmetry of the snap-through, reversible and bidirectional actuation is demonstrated.

Using light to trigger contactless, ultrafast actuation in an otherwise passive structure is a potentially versatile tool to use in mechanical design at the micro-, meso-, and millimeter scales as actuators, as well as switches that can be triggered from large standoff distances, impulse generators for microvehicles, microfluidic valves and mixers in laboratory-on-chip devices, and adaptive optical elements.

Polymer actuators have been previously associated with electrically active polymers (EAP) and artificial muscles. In fact, an arm-wrestling match between EAP-actuated robot and a human was held first during the SPIE Annual International Conference in 2005 and again over the years.

As light is often easier to deliver than electricity, light-induced polymer actuators can open new, literally wireless possibilities in medical devices and electronics. The research is featured in Science Daily.

Source: Contactless, photoinitiated snap-through in azobenzene-functionalized polymers. M.R. Shankar, M L. Smith, V.P. Tondiglia, K.M. Lee, M.E. McConney, D.H. Wang, L.-S. Tan, T.J. White. Proceedings of the National Academy of Sciences, 2013; DOI: 10.1073/pnas.1313195110
Source: WorldWide Electroactive Polymer Actuators Webhub, NASA.gov
Source: Snap to Attention: Polymers That React and Move to Light, Science Daily, Nov. 9, 2013
Source: video download from PNAS
Image by Nationaal Archief