Adaptive Hybrid Architectures

Dynamic structures that respond reversibly to changes in their environment are central to self-regulating thermal and lighting systems, targeted drug delivery, sensors, and self-propelled locomotion.

Since an adaptive change requires energy input, an ideal strategy would be to design materials that harvest energy directly from the changing condition itself and use it to drive an appropriate response. 

Liu Y, Yong X, McFarlin IV G, Kuksenok O, Aizenberg J, Balazs AC. Designing a gel–fiber composite to extract nanoparticles from solution. Soft Matter. 2015;11 (44) :8692-8700.Abstract

The extraction of nanoscopic particulates from flowing fluids is a vital step in filtration processes, as well as the fabrication of nanocomposites. Inspired by the ability of carnivorous plants to use hair-like filaments to entrap species, we use computational modeling to design a multi-component system that integrates compliant fibers and thermo-responsive gels to extract particles from the surrounding solution. In particular, hydrophobic fibers are embedded in a gel that exhibits a lower critical solution temperature (LCST). With an increase in temperature, the gel collapses to expose fibers that self assemble into bundles, which act as nanoscale ‘‘grippers’’ that bind the particles and draw them into the underlying gel. By varying the relative stiffness of the fibers, the fiber–particle interaction strength and the shear rate in the solution, we identify optimal parameters where the particles are effectively drawn from the solution and remain firmly bound within the gel layer. Hence, the system can be harnessed in purifying fluids and creating novel hybrid materials that integrate nanoparticles with polymer gels.

Liu Y, Bhattacharya A, Kuksenok O, He X, Aizenberg M, Aizenberg J, Balazs AC. Computational modeling of oscillating fins that “catch and release“ targeted nanoparticles in bilayer flows. Soft Matter. 2016;12 (5) :1374-1384.Abstract

A number of physiological processes in living organisms involve the selective ‘‘catch and release’’ of biomolecules. Inspired by these biological processes, we use computational modeling to design synthetic systems that can controllably catch, transport, and release specific molecules within the surrounding solution, and, thus, could be harnessed for effective separation processes within microfluidic devices. Our system consists of an array of oscillating, microscopic fins that are anchored onto the floor of a microchannel and immersed in a flowing bilayer fluid. The oscillations drive the fins to repeatedly extend into the upper fluid and then tilt into the lower stream. The fins exhibit a specified wetting interaction with the fluids and specific adhesive interactions with nanoparticles in the solution. With this setup, we determine conditions where the oscillating fins can selectively bind, and thus, ‘‘catch’’ target nanoparticles within the upper fluid stream and
then release these particles into the lower stream. We isolate the effects of varying the wetting interaction and the fins’ oscillation modes on the effective extraction of target species from the upper stream. Our findings provide fundamental insights into the system’s complex dynamics and yield guidelines for fabricating devices for the detection and separation of target molecules from complex fluids.

Jeon I, Cui J, Illeperuma WRK, Aizenberg J, Vlassak JJ. Extremely Stretchable and Fast Self-Healing Hydrogels. Adv. Mater. 2016;28 (23) :4678-4683.Abstract
Dynamic crosslinking of extremely stretchable hydrogels with rapid self-healing ability is described. Using this new strategy, the obtained hydrogels are able to elongate 100 times compared to their initial length and to completely self-heal within 30 s without external energy input.
Hu Y, You J-O, Aizenberg J. Micropatterned Hydrogel Surface with High-Aspect-Ratio Features for Cell Guidance and Tissue Growth. ACS Appl. Mater. Interfaces. 2016;8 (34) :21939-21945.Abstract

Surface topography has been introduced as a new tool to coordinate cell selection, growth, morphology, and differentiation. The materials explored so far for making such
structural surfaces are mostly rigid and impermeable. Hydrogel, on the other hand, was proved a better synthetic media for cell culture because of its biocompatibility, softness, and high permeability. Herein, we fabricated a poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel substrate with high-aspectratio surface microfeatures. Such structural surface could effectively guide the orientation and shape of human mesenchymal stem cells (HMSCs). Notably, on the flat hydrogel surface, cells rounded up, whereas on the microplate patterned hydrogel surface, cells elongated and aligned along the direction parallel to the plates. The microplates were 2 μm thick, 20 μm tall, and 10−50 μm wide. The interplate spacing was 5−15 μm, and the intercolumn spacing was 5 μm. The elongation of cell body was more pronounced on the patterns with narrower interplate spacing and wider plates. The cells behaved like soft solid. The competition between surface energy and elastic energy defined the shape of the cells on the structured surfaces. The soft permeable hydrogel scaffold with surface structures was also demonstrated as being viable for longterm cell culture, and could be used to generate interconnected tissues with finely tuned cell morphology and alignment across a few centimeter sizes.

Liu Y, Kuksenok O, He X, Aizenberg M, Aizenberg J, Balazs AC. Harnessing Cooperative Interactions between Thermoresponsive Aptamers and Gels To Trap and Release Nanoparticles. ACS Appl. Mater. Interfaces. 2016;8 (44) :30475-30483.Abstract

We use computational modeling to design a device that can controllably trap and release particles in solution in response to variations in temperature. The system exploits the thermoresponsive properties of end-grafted fibers and the underlying gel substrate. The fibers mimic the temperature-dependent behavior of biological aptamers, which form a hairpin structure at low temperatures (T) and unfold at higher T, consequently losing their binding affinity. The gel substrate exhibits a lower critical solution temperature and thus, expands at low temperatures and contracts at higher T. By developing a new dissipative particle dynamics simulation, we examine the behavior of this hybrid system in a flowing fluid that contains buoyant nanoparticles. At low T, the expansion of the gel causes the hairpin-shaped fibers to extend into the path of the fluid-driven particle. Exhibiting a high binding affinity for these particles at low temperature, the fibers effectively trap and extract the particles from the surrounding solution. When the temperature is increased, the unfolding of the fiber and collapse of the supporting gel layer cause the particles to be released and transported away from the layer by the applied shear flow. Since the temperature-induced conformational changes of the fiber and polymer gel are reversible, the system can be used repeatedly to “catch and release” particles in solution. Our findings provide guidelines for creating fluidic devices that are effective at purifying contaminated solutions or trapping cells for biological assays.

Shillingford C, Russell CW, Burgess IB, Aizenberg J. Bioinspired Artificial Melanosomes As Colorimetric Indicators of Oxygen Exposure. ACS Appl. Mater. Interfaces. 2016;8 (7) :4314-4317.Abstract

Many industries require irreversibly responsive materials for use as sensors or detectors of environmental exposure. We describe the synthesis and fabrication of a nontoxic surface coating that reports oxygen exposure of the substrate material through irreversible formation of colored spots. The coating consists of a selectively permeable rubber film that contains the colorless organic precursors to darkly pigmented synthetic melanin. Melanin synthesis within the film is triggered by exposure to molecular oxygen. The selectively permeable rubber film regulates the rate of oxygen diffusion, enabling independent control of the sensitivity and response time of the artificial melanosome, while preventing leaching of melanin or its precursors.

Based on this concept, we have developed a class of adaptive materials that, similar to skeletomuscular systems, use a hybrid architecture to interconvert energy between different forms and scales. To specify the materials’ functions, we use surfaces bearing arrays of nanostructures.

Their unique topography can be designed to confer a wide range of optical, wetting, adhesive, anti-bacterial, motion-generating, and other behaviors, similar to their natural counterparts used by lotus leaves to shed water, geckos to stick to surfaces, echinoderms to keep their skin clean, and fish to sense flow. An additional beauty of these nanostructured surfaces is that any of these properties can be switched or fine-tuned just by bending or tilting the structures to alter the patterns.

To harvest and channel energy to drive such reconfigurations, we embed the nanostructures in hydrogels that can be chemically tailored to sense a wide selection of chemical, mechanical, humidity, temperature, light, biochemical, and other environmental conditions.

Changes in these conditions cause the gel to swell or contract, generating not only the mechanical energy of bulk size change but an entire multiscale, 3D cooperative network of mechanical forces within the gel that provide the work for reconfiguring the nanostructures.

Using both experimental and modeling approaches as well as new fabrication methods, we are developing our ability to take full advantage of the immense potential for energy coupling within these hybrids to create a generation of sustainable, self-reporting, self-adapting materials.

Biomolecules Sorted with Catch-and-Release System, Genetic Engineering & Biotechnology News, March 24, 2015. 

Catching and releasing tiny molecules, Harvard press release, March 23, 2015. 

Lifelike cooling for sunbaked windows, Wyss Institute press release, July 30, 2013. 

Hydrogels: The catalytic curtsey, Nature Materials, July 24, 2012. 

Materials with SMARTS, Chemical & Engineering News, July 16, 2012. 

Nanomaterial duplicates self-regulation of living organisms, IEEE Spectrum, July 13, 2012. 

New SMART materials regulate, respond to their environment, Txchnologist, July 12, 2012. 

Homeostatic hydrogels to help heat the home, Chemistry World, July 12, 2012. 

Smart buildings, Nature's weekly podcast (at 06:49), July 12, 2012. 

Smart materials get SMARTer, Harvard press release, July 11, 2012.