Matthew L. Smith

Contactless, photoinitiated snap-through in azobenzene-functionalized polymers

Significance Photomechanical effects in polymers are distinguished by the ease with which actinic light can be regulated to contactlessly trigger the magnitude and directionality of mechanical adaptivity with spatio-temporal control. The materials examined to date have not demonstrated power densities or actuation speeds necessary for applications seeking to exploit the promise of wirelessly triggered actuation. Using mechanical design, we employ two classes of azobenzene-functionalized polymers and demonstrate contactless snap-through of bistable arches realizing orders-of-magnitude enhancement in the actuation rates (∼102 mm/s) and powers (∼1 kW/m3) under moderate irradiation intensities (<<100 mW/cm2). The experimental characterization of the snap-through is supported with modeling that elucidates the effect of geometry, mechanical properties, and photogenerated strain on the actuation rate and energy output. Photomechanical effects in polymeric materials and composites transduce light into mechanical work. The ability to control the intensity, polarization, placement, and duration of light irradiation is a distinctive and potentially useful tool to tailor the location, magnitude, and directionality of photogenerated mechanical work. Unfortunately, the work generated from photoresponsive materials is often slow and yields very small power densities, which diminish their potential use in applications. 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. A model is developed to elucidate the underlying mechanics of the snap-through, specifically focusing on isolating the role of sample geometry, mechanical properties of the materials, and photomechanical strain. 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.

Torsional mechanical responses in azobenzene functionalized liquid crystalline polymer networks

Soft materials capable of both planar and flexural–torsional responses could enable the development of soft robotic elements that emulate the dexterity and functionality of a multitude of creatures in the animal kingdom. Here, we examine the response of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs) specifically focusing on realizing large magnitude flexural–torsional responses observed as out-of-plane twisting or coiling. Towards this end, azo-LCNs were prepared in either the twisted nematic (TN) or hybrid orientations. The characterization of the flexural–torsional photomechanical responses is complimented with examination of thermomechanical properties. The diverse range of tailorable photomechanical responses is shown to be strongly dependent on the alignment of the nematic director to the film geometry and the actinic light intensity.

Designing light responsive bistable arches for rapid, remotely triggered actuation

Light responsive azobenzene functionalized polymer networks enjoy several advantages as actuator candidates including the ability to be remotely triggered and the capacity for highly tunable control via light intensity, polarization, wavelength and material alignments. One signi cant challenge hindering these materials from being employed in applications is their often relatively slow actuation rates and low power densities, especially in the absence of photo-thermal e ects. One well known strategy employed in nature for increasing actuation rate and power output is the storage and quick release of elastic energy (e.g., the Venus ytrap). Using nature as inspiration we have conducted a series of experiments and developed an equilibrium mechanics model for investigating remotely triggered snap-through of bistable light responsive arches made from glassy azobenzene functionalized polymers. After brie y discussing experimental observations we consider in detail a geometrically exact, planar rod model of photomechanical snap-through. Theoretical energy release characteristics and unique strain eld pro les provide insight toward design strategies for improved actuator performance. The bistable light responsive arches presented here are potentially a powerful option for remotely triggered, rapid motion from apparently passive structures in applications such as binary optical switches and positioners, surfaces with morphing topologies, and impulse locomotion in micro or millimeter scale robotics.

Chemical wave characterization of self-oscillating gelatin and polyacrylamide gels

Self-oscillating hydrogels driven by the Belousov-Zhabotinsky (BZ) reaction provide a unique foundation for the mimicry of autonomic biological systems. One of the key challenges for assessing practical performance limits of these materials is detailed knowledge of the chemical and mechanical characteristics of the BZ gels at various states of autonomic behavior. Recently we developed two BZ gel systems based on gelatin and polyacrylamide. The desired chemical response for effective swelling-deswelling oscillation and mechanical force production involves a delicate balance of chemical wave period, amplitude, and gel swelling properties. The chemical performance of gelatin and polyacrylamide BZ gels according to this criteria is discussed.

New biomimetic gel chemistries may refine device designs

Scientists want to develop biomimetic materials both for practical design purposes and as simple model systems to improve their understanding of more complex biological phenomena. Promising stimuli-responsive materials have been synthesized with entangled gels or cross-linked polymer networks swollen with varying degrees of solvent. These responsive gels react to external environmental changes such as in temperature, pH (a measure of the acidity or basicity of a solution), or electric field.1, 2 Typically, the response involves swelling, collapse, or reorientation of the polymer network, leading to notable deformation or release of particles originally trapped inside. Applications range from drug delivery to mechanical actuators in microfluidic devices. If the gel’s response could be autonomous, it will be spontaneous as a result of internal stimuli. Classical examples of automatic behavior are the visceral nervous system that affects our internal organs and the beating heart. Responsive gels with this characteristic are beginning to become a reality. For example, such gels driven by the Belousov-Zhabotinsky (BZ) reaction3 display autonomous, rhythmic chemical and mechanical swelldeswell oscillations without needing an external stimulus.4 In the BZ gel reaction, a metal catalyst immobilized in the network is involved in both rapid autocatalytic and reset steps. In the former, the metal loses an electron (oxidized) and during the reset step it gains an electron (reduced). The primary effect is that the gel oscillates between less hydrophilic (reduced) and more hydrophilic (oxidized) states. As a result, it deswells and swells when the metal is reduced and oxidized, respectively. A color change also accompanies the catalyst reductionoxidation, providing an easy way to see the chemical changes (see Figure 1). We can easily observe traveling waves associated with millimeter-scale gel systems. Uniform swell-deswell oscillations occur for samples smaller than the chemical wavelength (hundreds of micrometers).4 Figure 1. Color change in a Belousov-Zhabotinsky reaction gelatin film (color enhanced).