Lauren D. Zarzar

Synthetic homeostatic materials with chemo-mechano-chemical self-regulation.

Living organisms have unique homeostatic abilities, maintaining tight control of their local environment through interconversions of chemical and mechanical energy and self-regulating feedback loops organized hierarchically across many length scales. In contrast, most synthetic materials are incapable of continuous self-monitoring and self-regulating behaviour owing to their limited single-directional chemomechanical or mechanochemical modes. Applying the concept of homeostasis to the design of autonomous materials would have substantial impacts in areas ranging from medical implants that help stabilize bodily functions to ‘smart’ materials that regulate energy usage. Here we present a versatile strategy for creating self-regulating, self-powered, homeostatic materials capable of precisely tailored chemo-mechano-chemical feedback loops on the nano- or microscale. We design a bilayer system with hydrogel-supported, catalyst-bearing microstructures, which are separated from a reactant-containing ‘nutrient’ layer. Reconfiguration of the gel in response to a stimulus induces the reversible actuation of the microstructures into and out of the nutrient layer, and serves as a highly precise ‘on/off’ switch for chemical reactions. We apply this design to trigger organic, inorganic and biochemical reactions that undergo reversible, repeatable cycles synchronized with the motion of the microstructures and the driving external chemical stimulus. By exploiting a continuous feedback loop between various exothermic catalytic reactions in the nutrient layer and the mechanical action of the temperature-responsive gel, we then create exemplary autonomous, self-sustained homeostatic systems that maintain a user-defined parameter—temperature—in a narrow range. The experimental results are validated using computational modelling that qualitatively captures the essential features of the self-regulating behaviour and provides additional criteria for the optimization of the homeostatic function, subsequently confirmed experimentally. This design is highly customizable owing to the broad choice of chemistries, tunable mechanics and its physical simplicity, and may lead to a variety of applications in autonomous systems with chemo-mechano-chemical transduction at their core.

Bio-inspired Design of Submerged Hydrogel-Actuated Polymer Microstructures Operating in Response to pH

IO N Responsive and reversibly actuating surfaces have attracted signifi cant attention recently due to their promising applications as dynamic materials [ 1 ] that may enable microfl uidic mixing, [ 2 ] particle propulsion and fl uid transport, [ 3 ] capture and release systems, [ 4 ] and antifouling. [ 5 ] Analogs in nature serve as inspiration for the design of such advanced adaptive materials systems—microorganisms use fl agella for propulsion, [ 6 ] cilia line the human respiratory tract to sweep mucus from the lungs and prevent bacterial accumulation, [ 7 ] and echinoderms use pedicellariae for body cleaning and food capture. [ 8 ] Signifi cant characteristics of these biological systems include functionality in a fl uidic environment, controllable actuation direction or pattern, and the ability to translate chemical signals or stimulus into mechanical motion. Researchers have taken various approaches to fabricating biomimetic actuators, among which are biomorph actuators made using microelectromechanical systems (MEMS) technology, [ 9 ] magnetically actuated polydimethylsiloxane (PDMS) structures, [ 10 ] and artifi cial cilia or actuators made from responsive gel. [ 11 , 12 ] However, most fabricated actuators, such as MEMS or magnetically actuated PDMS posts, must be driven by an external force or fi eld and are not responsive to chemical stimuli. Actuating structures that have been made from responsive hydrogel are either low aspect ratio and their motion is not patternable, [ 11 ] or the movement is irreversible. [ 12 ] Microscale actuation systems which exhibit reversible chemo-mechanical response and control of actuation direction or pattern have proven diffi cult to achieve. Inspired by biological actuators, which can be broadly interpreted as composites consisting of an active “muscle” component coupled with a passive “bone” structure, we recently developed a hybrid actuation system in which a crosslinked polyacrylamide-based hydrogel, acting as an analog to muscle, drives the movement of embedded silicon [ 13 , 14 ] or polymer [ 15 ]

Structural Transformation by Electrodeposition on Patterned Substrates (STEPS): A New Versatile Nanofabrication Method

Arrays of high-aspect-ratio (HAR) nano- and microstructures are of great interest for designing surfaces for applications in optics, bio-nano interfaces, microelectromechanical systems, and microfluidics, but the difficulty of systematically and conveniently varying the geometries of these structures significantly limits their design and optimization for a specific function. This paper demonstrates 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. We demonstrate 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, we 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 research 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.

Microbristle in gels: Toward all-polymer reconfigurable hybrid surfaces

We report on the fabrication of biologically-inspired “smart” surfaces using hybrid architectures comprising polymer microbristle embedded in a hydrogel layer. The dynamic bending of the microposts—the passive structural element in the design—and their return to the upright orientation are achieved during the volume-phase transition of the hydrogel layer—the active element of the structure—upon hydration/dehydration. We compare the performance of the hybrid architectures bearing soft and stiff microposts and show that the use of soft polymeric materials results in bending actuation of the posts in cases where actuation of identically-sized posts of stiffer materials, such as silicon, would not have been possible. Modeling of the actuation process and the supporting experimental results confirm that the bending orientation of the microposts can be individually controlled by modulating the thickness gradients in the active hydrogel layer achieved by transferring micropatterns to the liquid-phase hydrogel precursor. Such procedures orchestrate coordinated actuation of the microbristle and make it possible to create elaborate reconfigurable micropatterns, such as opening/closing microflorets and microtraps. In combination with diverse hydrogel systems exhibiting response to various stimuli, these “smart” hybrid all-polymer architectures open a new avenue in advanced functional materials that harness the adaptive nature of these structures for various applications.

Multiphoton Lithography of Nanocrystalline Platinum and Palladium for Site-Specific Catalysis in 3D Microenvironments.

Integration of catalytic nanostructured platinum and palladium within 3D microscale structures or fluidic environments is important for systems ranging from micropumps to microfluidic chemical reactors and energy converters. We report a straightforward procedure to fabricate microscale patterns of nanocrystalline platinum and palladium using multiphoton lithography. These materials display excellent catalytic, electrical, and electrochemical properties, and we demonstrate high-resolution integration of catalysts within 3D defined microenvironments to generate directed autonomous particle and fluid transport.

Direct Writing and Actuation of Three‐Dimensionally Patterned Hydrogel Pads on Micropillar Supports

Many biological organisms employ microand nanoscale systems to actuate structural components with a high degree of spatial control. The resulting patterned or predetermined movement of the components gives rise to versatile biological materials with locally reconfigurable features and regionspecific dynamic properties. On the molecular level, biological systems may regulate the availability of catalytic sites on enzymes by local reconfiguration of the protein structure, such as in allosteric modulation. On the microscale, echinoderms use actuating pedicellariae for particle capture and release, and body cleaning, and bacteria employ the movement of flagella to generate directional locomotion. Squid use the mechanical expansion and contraction of chromatophores to reversibly change color and pattern for camouflage and communication. These systems provide inspiration for the development of artificial “smart” materials and surfaces with similar properties that respond autonomously and reversibly to environmental cues. Recently, such reversibly responsive materials, particularly those patterned or manipulated on the nanoand microscale, have been the subject of intense research because of their promising impact in areas including sensors and actuators, microfluidic systems, microelectromechanical systems, and switchable surfaces with adaptive wettability, optical, mechanical, or adhesive properties. In particular, hydrogels can be tailored to respond volumetrically to a wide variety of stimuli including temperature, pH, light, and biomolecules (e.g., glucose), and there has been a significant amount of research and applications devised for this class of materials in areas ranging from tissue engineering to responsive photonics. We recently described a responsive and reversibly actuating surface based on a hybrid architecture consisting of passive polymeric structural (“skeletal”) elements embedded in and under the control of a responsive hydrogel layer (“muscle”) attached to a solid support. While the volume change of the polymer muscle enables large-area, directional movement of skeletal elements, anchoring to a solid support imposes a serious constraint on the capacity for hydrogel expansion or contraction, thus limiting the extent of induced actuation of the structural elements. Moreover, this approach does not allow the formation of hydrogel islands that would induce localized actuation of selected areas and the associated regional changes in surface properties. To expand the opportunities for integration of hydrogels in such composite systems, it would be advantageous to tailor not only the chemistry and swelling properties of the hydrogels but also the size, shape, and placement of the gel in relation to other system components. For example, welldefined, three dimensionally patterned, responsive hydrogel pads placed at the tips of micropillars with microscale control would enable nearly unrestricted gel swelling, both in and out of plane, which would locally actuate the pillars with more precise control over the movement of individual elements. While extensive research has been devoted to tailoring the swelling, chemical properties, and responsive behavior of hydrogels, less attention has been paid to the development of patterning protocols that would offer area-specific synthesis and 3D control over the microor nanoscale features of the gel. Many routes to defining hydrogel patterns have been explored including photolithography, soft lithography, and masking techniques, but these 2D approaches lack [*] Prof. C. J. Brinker, Dr. B. Kaehr Advanced Materials Laboratory, Sandia National Laboratories 1001 University Blvd. SE, Albuquerque, NM 87106 (USA) and Department of Chemical and Nuclear Engineering and Center for Micro-engineered Materials, University of New Mexico Albuquerque, NM 87106 (USA) E-mail: bjkaehr@sandia.gov

Photothermally triggered actuation of hybrid materials as a new platform for in vitro cell manipulation

Mechanical forces in the cell’s natural environment have a crucial impact on growth, differentiation and behaviour. Few areas of biology can be understood without taking into account how both individual cells and cell networks sense and transduce physical stresses. However, the field is currently held back by the limitations of the available methods to apply physiologically relevant stress profiles on cells, particularly with sub-cellular resolution, in controlled in vitro experiments. Here we report a new type of active cell culture material that allows highly localized, directional and reversible deformation of the cell growth substrate, with control at scales ranging from the entire surface to the subcellular, and response times on the order of seconds. These capabilities are not matched by any other method, and this versatile material has the potential to bridge the performance gap between the existing single cell micro-manipulation and 2D cell sheet mechanical stimulation techniques.

Developmentally-inspired shrink-wrap polymers for mechanical induction of tissue differentiation.

A biologically inspired thermoresponsive polymer has been developed that mechanically induces tooth differentiation in vitro and in vivo by promoting mesenchymal cell compaction as seen in each pore of the scaffold. This normally occurs during the physiological mesenchymal condensation response that triggers tooth formation in the embryo.

Environmentally responsive active optics based on hydrogel-actuated deformable mirror arrays

We report hybrid polymer actuator arrays based on environmentally responsive hydrogel and actuatable optical microstructures that are designed to reversibly switch optical properties in response to the environment. Arrays of micrometer scale plates were patterned by deep reactive ion etching of silicon which served as master structures for replica molding in polydimethylsiloxane (PDMS). UV-curable epoxy was cast in a metal-sputtered PDMS mold to transfer a thin metal film onto each microplate to form a micromirror array. Polyelectrolyte hydrogel, such as poly(acrylamide-co-acrylic acid), was patterned on the micromirror array and acted as an artificial muscle, bending the micromirrors in response to the change in humidity or pH. Such hybrid systems showed reversible switching between high transmittance (low reflectivity) and low transmittance (high reflectivity) without the aid of external power. Our design of hybrid actuated optics opens a broad avenue for developing environmentally responsive adaptive and active optics.

Using Laser-Induced Thermal Voxels to Pattern Diverse Materials at the Solid-Liquid Interface.

We describe a high-resolution patterning approach that combines the spatial control inherent to laser direct writing with the versatility of benchtop chemical synthesis. By taking advantage of the steep thermal gradient that occurs while laser heating a metal edge in contact with solution, diverse materials comprising transition metals are patterned with feature size resolution nearing 1 μm. We demonstrate fabrication of reduced metallic nickel in one step and examine electrical properties and air stability through direct-write integration onto a device platform. This strategy expands the chemistries and materials that can be used in combination with laser direct writing.