279250 Bio-Inspired Design of Adaptive, Dynamic, and Multi-Functional Materials and Architectures

Sunday, October 28, 2012
Hall B (Convention Center )
Philseok Kim, Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA

Can we develop new materials and structures that can sense and respond to the environment to optimize, reconfigure, self-heal to maintain sustainability just like a living organism? My research theme is focused on “bio-inspired design, synthesis, and characterization of adaptive and dynamic materials and architectures” to develop energy-efficient adaptive building materials, nanostructured materials for electrochemical energy conversion and storage, and dynamic surfaces for interfacing biological systems and nanosystems. Keeping this big picture in mind, I have been always looking at the surface of a material or the interfaces of dissimilar materials. It would be impossible to discuss all the details of the projects I have been studying during this short meeting. However, I would be happy to discuss any of the following selected topics at your request at the AIChE meeting:

1. Hydrogel-Actuated Integrated Responsive Systems (HAIRS): Interfacing ‘dynamic’ materials with ‘passive’ structures to create adaptive materials

Taking the dynamic and adaptive nature of biological systems as a source of inspiration, I seek to develop sustainable, dynamic, and multi-functional materials that are able to adapt to the changing environment by sensing and responding to stimuli, reconfiguring their structures, and thereby generating new functions. One approach is to combine responsive materials, such as hydrogels, with various nano/microstructures in which the responsive materials harvest energy from the environment and transduce it into mechanical energy to reconfigure and reassemble the structures into a wide range of reversible surface patterns.  These hybrid actuation systems generate a new class of adaptive materials that can respond to various environmental cues exhibiting reversibly switching functions important for applications in wetting, adhesive, color change, optics, anti-fouling, catalysis, capture and release, propulsion and transportation, and bio-nano interface. The versatility of this strategy is huge as any given stimulus-response pair can be put together in a variety of configurations to program how forces are synchronized and transferred between them, orchestrating responses that vary over a wide range of patterns on micro and macro scales. The design, synthesis, fabrication, and characterization of such new generation of multi-functional materials based on this strategy, hydrogel-actuated integrated response systems (HAIRS), along with potential applications as adaptive light redirecting and thermal gain regulating building envelopes (e.g. façades, glazings, and roofing systems) for energy-efficient buildings will be presented.

2. Bio-inspired Slippery Ice-repellent Coatings on Aluminum (SLIPS-Al): Wetting and adhesion of ice on a surface

Ice repellent coatings on aluminum can have significant impact on the global energy saving and improving safety in many infrastructures, transportation, and cooling systems. Recent efforts for developing ice-phobic surfaces have been mostly devoted to utilize lotus leaf-inspired superhydrophobic surfaces, yet these surfaces fail in high humidity conditions due to water condensation and frost formation and even lead to increased ice adhesion due to a large surface area. I have developed a radically different type of ice repellent coating material based on slippery, liquid-infused porous surfaces, SLIPS, that was previously developed in my group inspired by pitcher plant. SLIPS maintains an ultrasmooth and slippery liquid interface on a porous surface by infusing a water-immiscible liquid into it. I will discuss special advantages of SLIPS-coated aluminum as ice-repellent surfaces: extremely low contact angle hysteresis and ice adhesion strength, highly reduced sliding droplet sizes. I will also present results from icing/deicing experiments demonstrating the SLIPS coating on aluminum (SLIPS-Al) remains essentially ‘ice-free’ while conventional materials accumulate ice. These results indicate that SLIPS-Al is a promising candidate for developing robust anti-icing materials for broad applications even in high humidity, such as refrigeration, aviation, roofs, wires, outdoor signs, wind turbines, where ice can be easily removed by tilting, agitation (e.g. wind or vibrations), or even by a weak shear force.

3. Nanostructure Fabrication Using Conductive Polymers (STEPS and SCREEN): Electrodeposition of materials from solution onto a surface, Nucleation & Growth

Sophisticated 3D structures such as arrays of high-aspect-ratio (HAR) nano- and microstructures are of great interest for designing surfaces for applications in energy harvesting and storage, optics, and bio-nano interfaces. However, the difficulty of systematically and conveniently tuning the geometries of these structures significantly limits their design and optimization for specific applications. I have developed a low cost, high-throughput benchtop method that enables a HAR array to be reshaped with nanoscale precision by electrodeposition of conductive polymers. The method—named STEPS (structural transformation by electrodeposition on patterned substrates)—makes it possible to create patterns with proportionally increasing size of original features, to convert isolated HAR features into a closed-cell substrate with a continuous HAR wall, and to transform a simple parent two dimensional HAR array into new three-dimensional patterned structures with tapered, tilted, anisotropic, or overhanging geometries by controlling the deposition conditions. STEPS allowed for the fabrication of substrates with continuous or discrete gradients of nanostructure features, as well as libraries of various patterns, starting from a single master structure. By providing exemplary applications in plasmonics, bacterial patterning, and formation of mechanically reinforced structures, I will show that STEPS enables a wide range of studies of the effect of substrate topography on surface properties leading to optimization of the structures for a specific application. This work identifies solution-based deposition of conductive polymers as a new tool in nanofabrication and allows access to 3D architectures that were previously difficult to fabricate.

4. Mechano-responsive Optical Materials (“Stretch Me”): Mechanics of thin film under small deformation conditions

A stiff, thin membrane bonded to a compliant substrate undergoes  surface buckling (i.e. wrinkles) when the membrane encounters a compressive load that exceeds the critical strain. The wrinkled patterns arising from these mechanically mismatched interfaces have been studied for mechanically tunable gratings, as well as a metrological tool for measuring the modulus of a thin film. Nevertheless, the dynamic changes in the optical transmittance of these wrinkled materials have not been studied in depth. I have investigated the mechano-responsive optical behavior of uni-axially pre-stretched and oxidized Sylgard 184 membranes and found that these materials undergo reversible transition between a transparent state and two opaque states. I will present detailed correlations between the optical transmittance spectra and the changes in the wrinkle patterns such as pitches, amplitudes, and orientations under given mechanical load, along with an analytical model to describe the observed mechanical behaviors and optical effects. The mechano-responsive and dynamically tunable optical properties of wrinkled elastomers offer numerous potential applications, including instantaneous privacy screens, reversible switching of microstructured patterns, data encryption, shadings and windows for buildings, and mechano-optical sensors, all from a low cost material and inexpensive processing method.

5. Academic Outreach: Teaching the concept of hydrophilicity and hydrophobicity of a surface

I have been closely working with local high school teachers for developing ideas for demonstrations that help K-12 and undergraduate students to understand important scientific concepts. I will discuss a recently developed demonstration entitled “Hydroglyphics” that visualizes the difference between hydrophilic and hydrophobic surfaces in a very exciting and approachable way. It involves placing a shadow mask on an optically clear hydrophobic plastic dish, corona treating the surface with a modified Tesla coil, removing the shadow mask, and visualizing otherwise invisible message or pattern by applying water, thus entitled as hydroglyphics. This demonstration was carried out in public at the Boston Museum of Science during the Nanodays event funded by National Science Foundation (March 31, 2012) and at the USAEF event funded by National Science Foundation (April 27-29, 2012).

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