Symposium SM2 : Bioinspired Dynamic Materials---Synthesis, Engineering and Applications

Elastic instabilities enable systems to store energy, and when appropriately triggered, release that energy quickly. Elastic instabilities for example are the reason the venus flytrap is able to snap close fast enough to capture an insect. While elastic instabilities are commonly expoited for large-scale systems, they have not been sigificantly considered for small systems and structures. In one example, we created elastically instable pH-responsive colloidal particles, which change shape rapidly (less than 200 ms), even when the stimuli (pH) is changing slowly. Through both finite element analysis and experiment, we show that the mechanical hysteresis and bistability derives from the colloids’ spherical curvature. Eergy landscapes obtained from simulations suggest that by tuning the elastic moduli and thicknesses of the constituent polymer layers, few micrometer-sized microparticles such as presented here may be designed to actuate on timescales as fast as 1 µs. Elastic instabilities have also been expoited to control the folding of mesoscale sysems and drive the transformation of 2D patterned mesoscale materials into 3D forms.

Structural color, which arises from highly controlled arrangement of micro- and nano-structures, widely exists in nature. It has been found that structural color plays a crucial role in camouflage, communication, and temperature control. For example, dermal cells (iridocytes) of squid in the Loliginidae family contain alternating layers of cell membrane-enclosed platelets, which function as modular Bragg reflectors that selectively reflect light of certain wavelengths. The spacing of lamellar structures can be tuned through biological pathways, leading to modulation of skin coloration across the entire visible spectrum.
Inspired by this, it should be possible to fabricate “smart” reflectors that are able to tell environmental parameters by using lamellar design of responsive materials. As stimuli-responsive crosslinked polymers, hydrogels can change their volume significantly in response to small alterations of certain environmental parameters. In addition, hydrogel can be chemically adjusted to provide a large assortment of sensitivities as diverse as humidity, temperature, light, mechanical stress, magnetic or electric field, pH, glucose and other molecular species. As a result, we consider hydrogel as an ideal candidate for bio-mimetic “smart” reflectors.
Herein, we fabricated a bio-inspired hydrogel interferometer as a novel type of gas sensors. Ultra-thin hydrogel features were deposited onto a reflective substrate (e.g. silicon wafer). Due to light interference from top and bottom interfaces of the gel feature, a certain color would be observed. When the thickness of gel feature changed under external stimuli, its apparent color would also change, resulting in a measurable signal. The responsiveness of hydrogel interferometer could be tuned by varying its chemical composition. Compared to previously established hydrogel based optical sensors, this design (1) didn’t require complex or expensive fabrication procedure, (2) allowed detection of multiple types of analytes and (3) responded rapidly to analyte at all concentrations within its dynamic range. We believe this technique is a critical alternative to current sensing systems and will greatly facilitate the development of the next generation hydrogel sensors.

As a specialized division of robotics, the field of soft robotics has shown a unique ability to mimic biological motions. Arguably, the most successful soft robots are those driven by fluidic elastomer actuators (FEAs); however, their fabrication techniques limit them to prismatic shapes and not the 3D forms observed in animal physiology. In order to realize the potential of soft robotics in biomimicry, new materials and processing techniques must be developed for producing true replicas of natural mechanisms. As simple fabrication is a hallmark of soft robots, any new manufacturing technique must not be overly laborious.

We introduce the first use of foamed materials as the basis for pneumatically powered soft machines. We easily form these open-celled, silicone elastomer foams into a variety of shapes all containing an embedded pneumatic network. We have shown the ability to tailor the foam’s mechanical properties and actuation behavior by controlling porosity via formulation. Additionally, we characterized the airflow rate through foam samples, and compared to a theoretical model. Extracting all the parameters for the model from flow studies and X-ray computed tomography (CT), we found excellent fits to our experimental data for rigid foams, using no free parameters. We also discovered exagerated flow from the predicted model for our highly extensible elastomeric foams. Using these foam actuators, we molded an entirely soft water pump having the external shape of the human heart that is capable of pumping at physiologically relevant frequencies and pressures. Furthermore, to the best of our knowledge, this pump attains a flow rate higher than all previously reported soft pumps.

We are currently building upon this work to generate customized cardiac assist devices. We are developing a process to convert patient-specific CT scans into 3D-printed molds which we will use to fabricate custom-fit assistive devices.

Portions of this work have appeared in Advanced Materials (Mac Murray, BC et al.; DOI: 10.1002/adma.201503464)

Approaches for regulated fluid secretion, which typically rely on fluid encapsulation and release from a shelled compartment, do not usually allow a fine continuous modulation of secretion, and can be difficult to adapt for monitoring or function-integration purposes. I will describe a self-regulated, self-reporting secretion systems consisting of liquid-storage compartments in a supramolecular polymer-gel matrix with a thin liquid layer on top, and demonstrate that dynamic liquid exchange between the compartments, matrix and surface layer allows repeated, responsive self-lubrication of the surface and cooperative healing of the matrix. Depletion of the surface liquid or local material damage induces secretion of the stored liquid via a dynamic feedback between polymer crosslinking, droplet shrinkage and liquid transport that can be read out through changes in the system’s optical transparency. We foresee diverse applications in fluid delivery, wetting and adhesion control, and material self-repair.

The skin structure of cephalopods endows them with remarkable dynamic camouflage capabilities. Consequently, much research effort has focused on understanding and emulating these animals’ color changing abilities in the visible region of the electromagnetic spectrum. In contrast, despite the importance of infrared signaling and detection for many industrial and military applications, few studies have attempted to translate the principles underlying cephalopod adaptive coloration to infrared camouflage. We have drawn inspiration from nanostructures implicated in cephalopods’ camouflage abilities and developed strategies for the self-assembly of unique cephalopod structural proteins into dynamically tunable biomimetic camouflage coatings on both transparent and flexible substrates.^1,2 Our substrates can adhere to arbitrary surfaces, and their reflectance can be reversibly modulated from the visible to the near-infrared regions of the electromagnetic spectrum with both chemical and mechanical stimuli.^1,2 Thus, we can endow common objects with any shape or form factor with tunable camouflage capabilities.^1,2 Our work represents a key step toward the development of wearable biomimetic color and shapeshifting technologies for stealth applications.

1. Phan, L.; Walkup IV, W. G.; Ordinario, D. O.; Karshalev, E.; Jocson, J.-M.; Burke, A. M.; Gorodetsky, A. A. Adv. Mater. 2013, 25, 5621-5625.
2. Phan, L.; Ordinario, D. D.; Karshalev, E.; Walkup IV, W. G.; Shenk, M. A.; Gorodetsky, A. A. J. Mater. Chem. C. 2015, 3, 6493-6498.

Mammalian skeletal muscle is an intriguing hierarchical material with obvious beneficial mechanical properties. Materials that seek to imitate muscle typically focus on matching its modulus, elasticity and strength. In addition, functional devices targeted for implementation within the human body must also be biocompatible. While hydrogels are commonly considered as polymeric candidates that satisfy these requirements, organogels have also been shown to mimic the electromechanical properties of muscle and typically possess higher elasticity and achievable strain than most hydrogels. The organic liquids employed in organogels must likewise be sufficiently biocompatible and include many natural oils. Recent efforts have replaced these oils with crystallizable components to impart shape-memory characteristics. Judicious selection of crystallizable additives on the basis of their melting temperature can create tunable shape-memory materials that actuate at any target temperature, which can be quite useful when brought in direct contact with the human body. The present work demonstrates that a thermoplastic elastomer (TPE) modified with a crystallizable hydrocarbon yields versatile materials that closely resemble the electromechanical properties of skeletal muscle while in the gel state but remain rigid and unyielding in the solid state. Unlike traditional, covalently cross-linked elastomers, the unique ability to (re)process such TPE-based materials makes them suitable for the controlled development of nanostructure-induced mechanical properties that regulate, in highly tunable fashion, both shape memory and functionality. Here, we present several applications that could benefit from the design of these unique materials: multi-shape memory from a single material, multi-shape memory based on micro/macrofabrication, actuation rate control in shape memory, and shape-memory conductors containing a nontoxic liquid metal.

One major barrier that restricts the application of conducting polymers is their poor mechanical properties, which are mainly due to the conjugated structure, making the main chain rigid. Inspired by the structure of bones, where rigid hydroxyapatite crystals and soft collagen fibers cooperate to form strong and tough materials, we proposed to incorporate 'soft' polymers into rigid conducting polymers to improve the mechanical properties. The soft polymer are designed to interact with rigid conducting polymers through directional supramolecular interactions, such as hydrogen bonds, dynamic covalent bonds and so on. These interactions will help the soft polymers to fit between rigid conducting polymer chains, which would improve the mechanical strength. The soft polymer can change its conformation more readily to allow deformation. Upon a large stress, the weak supramolecular interactions between soft polymers and rigid conjugated polymers would break and reform to help dissipate the destructive energy, therefore improve the toughness of the materials. In addition, the dynamic nature of supramolecular interactions could also bring new properties to conducting polymers. Herein, we report three examples of utilizing the supramolecular approach to make conducting polymers strong, tough and stimuli-responsive.
In the first case, we incorporated branched polyethylene glycol (PEG) into polypyrrole to form dynamic hydrogen bonding network. The resulted polypyrrole displays remarkable mechanical properties: tensile strength 130 MPa, tensile stain at break 100%, while it retains a good conductivity (100 S/cm). The dynamic hydrogen bonding network enables the polypyrrole a high sensitivity to moisture. Based on this material, we developed an actuator and a generator driven solely by low temperature water vapor.
In the second case, we incorporated a linear polyethylene glycol (PEG) to polythiophenes, which greatly improved the mechanical properties of polythiophenes: tensile strength 150-170 MPa, tensile strain at break 120-150%. The breaking energy of such polythiophene film reach 160 MJ/m³, which is similar to spider silk, the toughest materials ever reported.
In the third case, we incorporated polyvinyl alcohol (PVA) into polyaniline. A small amount of 3-aminophenylboronic acid was copolymerized with aniline. The dynamic covalent bond between PVA and boronic acid helps to interconnect PVA and polyaniline chains. The resulted materials are hydrogels with outstanding mechanical properties: tensile strength 25-30 MPa, tensile strain at break > 300%. The hydrogel displays a very high electrochemical activity: capacitance 1000 F/g.
With these examples, we demonstrate that the supramolecular approach could be a general method to improve the mechanical properties of rigid conducting polymers. Meanwhile, the dynamic nature of supramolecular interactions could bring more beneficial properties to the materials.

We have designed a series of responsive nanocomposites that all take advantage of a single temperature responsive surfactant, an N-isopropyl acrylamide (NIPAM) oligomer terminated with a long alkyl chain. NIPAM polymers are well known for their lower critical solution temperature (LCST) above which they lose their water solubility. This molecule has interesting phase behaviors that vary dramatically depending upon the details of its deployment. By itself in solution, it has a critical micellar concentration (CMC) that varies significantly with temperature, and counterintuitively, the CMC increases above the LCST where the solubility would be expected to be lower. The large change in CMC with temperature allows the controlled release and re-uptake of micellar contents with swithching temperatures. When placed on the surface of a nanoparticle, the NIPAM surfactant’s response depends strongly upon the mobility of the construct in which it is imbedded. Owing to the long alkyl chain, the molecule readily inserts into a number of nanoparticle coatings. In nanoparticles that have a relatively immobile layer of surfactants, such as alkanethiols on metals, the nanoparticles demonstrate a modest shift in solubility with temperature. When the NIPAM surfactant is imbedded in a mobile surface layer, such as a lipid bilayer, the change in agglomeration state with temperature is dramatic and immediate. Nearly instantaneous agglomeration and precipitation can occur in particles with as little as a few percent of the responsive NIPAM surfactant. In mobile coatings, the NIPAM surfactants are able to move to maximize their favorable interactions with the NIPAM surfactants on neighboring particles. This mechanism allows a minor component of the surface coating to dominate the solution behavior of particles. This is analogous to cell surfaces where a minority of species are highly active and have an outsized role in the cell’s interaction with the outside world. Experiments and explanatory simulations, as well as future directions for this and related responsive surface coatings will be discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

“Natural biological structures, in particular, moth’s eye and lotus leaf were the inspirations for the formation of low-refractive index antireflective glass film that embody omni-directional optical properties over a wide range of wavelengths, while also possessing water-repelling, or superhydrophobic, capability that holds significant potential for solar panels, lenses, detectors, architectural windows, optical components used in weapons systems and in many other products. The coatings comprise an interconnected network of nanoscale pores surrounded by a nanostructured silica framework. These structures result from a novel fabrication method that utilizes metastable spinodal phase separation in low-alkali borosilicate glass materials. The approach not only enables design of surface microstructures with graded-index antireflection characteristics, where the surface reflection is suppressed through optical impedance matching between interfaces, but also facilitates self-cleaning ability through modification of the surface chemistry. Based on near complete elimination of Fresnel reflections through a single-side coated glass and corresponding increase in broadband transmission, the fabricated nanostructured surfaces are found to promote a general and an invaluable ~ 3-7% relative increase in current output of multiple direct/indirect bandgap photovoltaic cells, while preventing dust/pollution accumulation. Moreover, these antireflective, self-cleaning surfaces demonstrate superior resistance against mechanical wear and abrasion and can be engineered to block ultraviolet radiation, provide antifogging as well as omniphobic functionalities. With demonstrated scalable and manufacturable formulations, providing an all-in-one combination of multiple salient and unique performance enhancers, our approach represents a conceptually fundamental basis to be developed for leading edge coated optical quality products.

Introduction
Approximately 260,000 tons of scallop shells become industrial waste every year in Japan. So far, attempts to reuse the shells have been mainly limited to the commercial production of CaCO₃. Currently, almost all CaCO₃ in Japan is produced from limestone. Each year in Japan, 200 million tons of limestone is mined. CaCO₃ produced from limestone is less expensive than that produced from scallop shells. There are no obvious economical benefits to using scallop shells as a source of CaCO₃. Therefore, we are attempting to find a new value added use for scallop shells as an advanced functional material. In previous experiments, we have been trying to use calcined shell powder to extract alkaline metal and alkaline-earth metal from seawater.
The tsunami of March 11, 2011, caused severe damage to the Fukushima nuclear power plant. Today, many problems still exist, but a particularly serious problem is the leakage of highly toxic contaminated water. Within the storage tanks, there remains a large quantity of contaminated water. This water contains radioactive Sr and Cs. Sr tends to accumulate in bones and is believed to be a cause of bone cancer. Therefore, it is highly desirable to develop a method for the removal of Sr from the contaminated water. Here we will show the potential of scallop shells to adsorb Sr from the solution.

Experimental
First, we crushed the shells using a stamp mill and an automatic grinder. Next, we classified the shell powder by sieves according to their diameter. Then, we put the shell powder into a beaker with a Sr solution and stirred it. Finally, we measured the Sr concentration by Atomic Adsorption Spectrometer (AAS) (Thermo, iCE3300). We compared the Sr concentration in the solution both prior to and then again after it was stirred.
We analyzed the crystal structure and the morphology of both the shell and CaCO3. The crystal structure of the sample was examined by using X-ray Diffractometer. The surface morphology of samples were analyzed by Scanning Electron Microscope.

Results and discussion
The morphology of the scallop shell was unique. A micro column structure was only observed on the inner surface of the shell. All parts of the inner surface of the scallop shells does not have the same morphology, as far as we observed by SEM. We thought this difference of the morphology would affect Sr adsorption performance. Then we put the shell powder into a beaker with a Sr solution and stirred it. We measured the Sr concentration by AAS. Sr adsorptivity is entirely dependent on which part of the scallop shell is used. Therefore, it is possible that structure affects the adsorptivity. There is a possibility of coprecipitation.

Although continuous subcutaneous insulin infusion (CSII) using an external pump prevails as a cost-effective solution for diabetic patients, a number of complications restrain its use to only 72 hours of continuous wear. It is therefore important to devise strategies for mitigating the failure modes. For example, an alternative solution is to employ a fully implantable subcutaneous pump to extend operational longevity and for attractive appearance. However, traditional fully implanted insulin pumps are large and heavy, significantly precluding their widespread adoption. Thus, there is a critical need to develop fully implantable pumps that are lightweight, have small dimension and low-power consumption, and which can significantly supersede the current CSII performance. To address this need, we developed a mechanical-motor-free insulin pump using a bioinspired polypyrrole (PPy)/polyol-borate composite that we had invented earlier as the electroactuators. In the current work, our goal is to improve the lifetime and performance of this PPy electroactuator by incorporating a flexible skeleton of stainless steel mesh in it. We electrochemically synthesize the PPy/polyol-borate composite as before but using a stainless steel mesh with 150-micron pore size as a substrate to produce the skeleton-reinforced PPy (srPPy). PPy of 10-micron thickness is deposited on the mesh (5×20 mm). We measure the electrical conductivity of this srPPy using a four-point probe. Using an electrochemical workstation, we characterize the electroactuation by measuring its maximum bending angle under a 2 V, 60 s bias in phosphate-buffered saline. The lifetime of the material is evaluated by recording the degradation of the maximum bending angle over successive electroactuation cycles. We further assemble two srPPy electroactuators in our pump and compare the new pump’s performance to that of our previous version with the PPy/polyol-borate electroactuators. Comparing to our previous PPy/polyol-borate composite, this srPPy has a much higher conductivity due to the stainless steel skeleton. Under the same driving voltage, this srPPy electroactuator generates larger stress, suggesting that a smaller voltage is needed to produce a certain stress, which is particularly beneficial to reducing electrodegradation of the PPy. This srPPy also mitigates the delamination problem frequently encountered in metal-film reinforced PPys and is more durable for handling and cutting during device fabrication. Combining with its excellent biocompatibility and flexibility, our new srPPy promises as a high-performance electroactuator for various biomedical applications.

Biological self-healing involves the autonomous localization of healing agents at the site of damage. Herein, we design and characterize a synthetic repair system where self-propelled nanomotors autonomously seek and localize at microscopic cracks and thus mimic salient features of biological wound healing. We demonstrate that these chemically powered catalytic nanomotors, composed of conductive Au/Pt spherical Janus particles, can autonomously detect and repair microscopic mechanical defects to restore the electrical conductivity of broken electronic pathways. This repair mechanism capitalizes on energetic wells and obstacles formed by surface cracks, which dramatically alter the nanomotor dynamics and trigger their localization at the defects. By developing models for self-propelled Janus nanomotors on a cracked surface, we simulate the systems’ dynamics over a range of particle speeds and densities to verify the process by which the nanomotors autonomously localize and accumulate at the cracks. We take advantage of this localization to demonstrate that the nanomotors can form conductive “patches” to repair scratched electrodes and restore the conductive pathway. Such a nanomotor-based repair system represents an important step toward the realization of biomimetic nanosystems that can autonomously sense and respond to environmental changes, a development that potentially can be expanded to a wide range of applications, from self-healing electronics to targeted drug delivery.

Organic electrochemical transistors (OECTs) have recently gained great interest due to their advantages such as ease of fabrication and biocompatibility. PEDOT:PSS is a conducting polymer with unique properties arising mainly from its mixed ionic and electronic conductivity, making it an ideal material for interfacing electronics with biology. Recently, inkjet printing has emerged as a versatile and low-cost technique for the deposition of functional inks. Moreover, its compatibility with a wide range of substrates such as paper, an ecofriendly and inexpensive alternative for plastics, makes it an ideal method to fabricate disposable biosensors. In this work, we present an inkjet printed OECT-paper based biosensor for the detection of glucose based on its enzymatic catalyzed reaction, in the presence of glucose oxidase (GOx) using an electron transfer mediator, ferrocene. The PEDOT:PSS ink used for the deposition of the electrodes, was adjusted in order to meet the inkjet rheological requirements, resulting thus in the formation of conductive layers with electrical conductivity up to 420 S/cm. Following, the gate was further functionalized by printing an additional layer of chitosan chemically branched with ferrocene, to act as the electron transfer mediator. GOx was immobilized in the electrolyte gel of phosphate buffered saline. The chronoamperometric response on the OECTs for successive additions of glucose indicated a detection range between 2.70 and 10.00 mM which is consistent with its concentration in blood. These results underline the potentiality of inkjet printing as a facile and inexpensive technique to fabricate high quality OECTs onto ecofriendly substrates paving the way for the development of the future biosensing platforms for point of care diagnostics.

The ability to get adapted to mechanical deformation without physical damage is highly desirable for electronics and energy applications. Our work has successfully introduced mechanically adaptive properties into functional electronic polymers and realized electronic materials with high performance and long lifetime. By introducing weak interactions into conductive polymers, which is used as polymer binders for electrode materials, we successfully realized the stable cycling of high capacity electrode materials. The polymer can get adapted to the volumetric changes of the electrode and maintain the strong interactions between the polymer and the electrode materials.

Rapid prototyping has recently grown in popularity and migrated into the homes of consumers. However, these solid models demonstrate limited dynamic actuation. A large range of soft materials are readily available for sensing and structural applications, but soft actuators are still relatively weak and inefficient. One method of introducing strong, controllable actuation to devices with gentle stimulation is through hygroscopic actuators. Here, we present a CNC ink-jet platform with the ability to produce printable actuators for soft robotic applications. We develop an improved spore ink suspension that resists sedimentation and improves ink jet performance and surface wetting characteristics. We demonstrate that the directionality of printing controls the direction, speed, and magnitude of actuation. Extended work demonstrates potential for both contractile and expansive actuation.

The surface of a cicada’s wing is a nanostructured, inherently antibacterial surface (Ivanova, et al., Small, 2012). Using state of the art nanofabrication tools, researchers have mimicked the nanospiked surface of the cicada’s wing and shown that synthetic surfaces including silicon (Ivanova, et al. Nature, 2013) and poly(methyl methacrylate) (Dickson, Liang, et al., BioInterphases, 2015), can be made inherently antibacterial. However, a key challenge is the removal of adhered dead bacteria. In nature, dynamic systems such as microvilli in the lung facilitate self-cleaning, inspiring our design of a voltage-activated, controlled micro-buckling surface. We describe in this paper a PDMS microwell array which supports a piezoelectric polymer film displaying the bactericidal nanopatterns. Upon application of a voltage pulse, the piezoelectric film exhibits buckling defined by the pattern of the array, which mechanically removes adhered bacteria.

Early detection of mechanical damage in polymer materials has the potential to avoid catastrophic failure, enable more reliable operation in the field, and increase the lifetime of materials. In this work, we investigate mechanically-induced fluorescence signaling to achieve autonomous indication of damage in a variety of polymer materials. We successfully microencapsulate fluorogenic molecules dissolved in a solvent and disperse the microcapsules in a polymer coating. Initially, the capsules are non-emissive; however, when mechanical damage ruptures the microcapsules, release of the payload triggers a rapid fluorescence response. As a result, the damaged regions become locally fluorescent. We explore the effects of capsule concentration and damage size on the fluorescent intensity. Significantly, this fluorogenic response is independent of the polymer matrix and other intermolecular interaction. Thus, this new microcapsule-based system is highly robust and establishes a versatile platform for autonomous indication in a wide range of materials and applications.

Azobenzene dyes represent an interesting class of molecules for their potential use as photo-switches due to their fast, powerful and reversible trans-cis isomerization. Due to these properties, azo dyes have been incorporated in a wide variety of thin films to create photo-responsive bio-compatible surfaces. Previous efforts in creating bio-compatible surfaces usually covalently bind the azo moiety to the polymer backbone. However, covalent binding of the azo dye to the polymer backbone often requires long synthesis steps and intricate purification procedures, and then resulting biocompatibility of the new material is uncertain. Herein, we explore new avenues to produce new systems that bypass these hindrances using non-covalent interactions, such as ionic or hydrogen bonding. For example, FDA-approved food dyes containing azo moieties can be non-covalently bound to a variety of biocompatible polymers, such as cellulose or hyaluronic acid. The physical characterization of model films, such as their optical properties, stability and strength of binding between dye and polymer, is carried out by different spectroscopic, DSC and optical measurements. Thin films of these complexes and other azo dyes are prepared using layer-by-layer assembly, spin-coating or dip-coating methods. Films of polyvinylpyrrolidone (pVP) and disperse red 1 (DR1) or methyl red (MR), have been prepared as successful azo-polymer complexes. Other polymers such as polyacrylic acid are also investigated to pair with these azos. The extent of dye loading and nature of the binding in these films is monitored by DSC, IR and UV-Vis spectroscopy. The interaction between dye and polymer show up to a 15 cm^-1 shift in the IR νC=O band of pVP for both DR1 and MR. Broadening of the band by about the same wavenumber indicates a mixture of bound and unbound species. Further evidence of binding is shown in DSC profiles by the absence of dye melting at lower dye ratios. A depression of glass transition temperature in both systems is also observed. A blue shift is observed in the UV-Vis spectra of DR1 and MR complexes upon exposure to pure water vapour or high humidity air. DR1 complexes change readily upon exposure to humid air and exhibit reversibility upon heating, whereas the effect is much slower and irreversible with MR complexes. Preliminary optical studies to measure birefringence as a probe of optically-inducible molecular orientation on MR complexes, suggest that no stable birefringence remains in the dark, for any of the various polymer backbones used for complexation. New photo-responsive materials can be easily prepared and successful complexation readily confirmed spectroscopically and via calorimitry. Understanding the nature and strength of the interaction between the backbone and dye could lead to a better prediction and fine-tuning of the optical properties of new materials, and allow the preparation of others by exploiting similar structural features.

Unlike traditional reactions triggered by light, electricity, or heat, selective bond scission of molecules can be induced by mechanical stress. Spiropyran (SP) undergoes a force-induced reversible ring-opening reaction from colorless, non-fluorescent SP form to colored, red-fluorescent merocyanine (MC) state when it is integrated into a polymer with proper attachments. These unique polymers have potential application for damage-detection or stress-sensing coatings, which can enhance the lifetime of a material by indicating the damaged region with bright colors. To exploit this beneficial reaction for damage sensing, we need a deeper understanding of the kinetics of SP—MC transition at different stress levels.
In this study, we synthesized SP-linked polyurethanes (SP-PU) and measured the fluorescence intensity change at different values of a macroscopic stress. First, SP-PU dog-bone specimens were elongated at a desired stretch value and bleached to revert any activated MCs to inactivated SPs. The change in fluorescence was monitored at discrete time intervals and the forward activation rate constants were extracted under dark conditions at different stress levels. After reaching to a plateau value of fluorescence intensity, the reverse (deactivation) rate constants were determined under exposure the green light (λ=532 nm). Below a minimal level of a stress, there was no change in reaction kinetics. After the threshold limit, the forward rate constant increased and the reverse rate constant decreased proportionally to the applied stress. We also estimated the change of activation energy as a function of the stress for both reactions and quantified the effect on the activation energy barrier for the transition of SP—MC.

Osmotic pressure drives flow movement in plant cells for the purposes of respiration and dynamic motions. Technologically, this simple mechanism has been successfully adapted for osmotic actuators, e.g., implantable macroscale units capable of dispensing microliter drugs. The advantage of this power source over typical metal-based MEMS devices is it can be made out of less toxic, biocompatible polymer that have the potential for large deformations when actuated. An additional benefit of this chemical potential-based energy source arises from its distributed nature, requiring only a membrane and aqueous environment in which to function similar to swelling-driven motion. Our end-goal is to leverage this power source to enable compact, directed motion on environmentally sensitive areas, such as for the medically-relevant soft tissues and marine applications.

However, before we can realize this goal, several key behaviors of these micro-osmotic actuators must be understood. While continuum transport theory for osmotic devices predicts behavior at the mm to cm length scale and is used to infer behavior in plant tissue, it has not been demonstrated at the cellular scale for a simple osmotic actuator. Further, the relationship between osmotic driving force and mechanical response at a tissue level is unclear. Here, we begin to understand these chemo-mechanical systems through a focus on a single, osmotically driven micro actuator. To do so, we built a single plant cell-like structure in which the semi-permeable membrane acts as both water transport and actuation membrane in contrast to previous devices that employ two separate membranes. These micro osmotic devices are fabricated using PDMS (Sylgard 184). PDMS is known for its flexibility and biocompatibility, however another key feature is its negligible salt (e.g. NaCl) permeability in comparison to water. Devices are comprised of 50-200 μm diameter cylindrical wells that we bond to a PDMS film (5 - 10 μm) while submerged in an aqueous environment. Thus, the salt concentration within the compartment can be controlled (0.1 M, ~6ng NaCl). We submerge these devices in DI water allowing the osmotic driving force to flow water into the compartment, bulging the thin membrane, which can be measured optically. This dynamic response is compared with existing theory, which assumes a linear elastic membrane response and a spherical cap membrane geometry. Analysis shows that non-linearity in the material response (modeled via ABAQUS), however, is key to replicating our experimental observations. With this correction, we can therefore observe that continuum transport theory applies even at these cellular length scales.

Cells have unique abilities to sense, process, and actuate based on environmental stimuli: their molecular components are constantly running many parallel programs that ensure correct growth, motion, reshaping, and repair in response to external inputs. How can modern engineers harness such powerful toolkit of DNA, RNA, and proteins to create the next generation of smart materials? I will describe our efforts in this area, which are centered on the combination of nucleic acids nanotechnology and dynamical systems theory. I will summarize our efforts in the design and synthesis of reconfigurable biomaterials built with DNA and RNA, where the material growth is directed by autonomous molecular signals such as transcriptional networks and oscillators.

Using genetic modification of a host cell to produce complex biomolecules is a critical route to new pharmaceutical production. However the separation of the desired protein from the myriad of required chemicals in living systems is the most costly and complex step. Typical separations use a genetically modified marker, such as histidine, on the protein to bind to a nickel complex on a chromatography column. This bound protein is later released into a collection stream using a release agent such as low pH or imidizol. The problems with the conventional techniques are non-specific binding, inefficient mass transport to binding sites, and inefficiencies in purge cycling. Nature on the other hand is able to separate proteins by pumping them across cellular membranes. Needed for efficient bio-chemical production is a separation system that can selectively pump proteins across a membrane barrier in a continuous process. Here we show a membrane system with nm-scale thick electrodes that is able to selectively bind genetically modified proteins and pump them across the membrane with sequential voltage pulses. The electrodes are located at the first 5nm of pore entrances and bound proteins block non-specific protein transport through the pores. During the release cycle, concentration of imidizol is controlled to keep the pore blocked while release proteins at the bottom edge of electrode. A separation factor for GFP and BSA of 16 was achieved with observed GFP electrophoretic mobility of 7x10^-6 cm²/v-S. This non-optimized membrane system with an area of 0.9 cm² has the same throughput as 1ml of commercially available chromotagraphy columns showing viability as a continuous industrial process. This system will enable continuous separation of expressed proteins directly from fermentation broths dramatically simplifying the separation process as well as reducing biomolecule pharmaceutical production costs. Addionally further advances in biochemical kinetics and process modeling are discussed. [Chen Z, Chen T, Su, Sun X, Hinds BJ, Adv. Func. Mater, 24, 4317 2014]

Protein hydrogels have attracted considerable interest due to their potential applications in biomedical engineering as well as basic biological studies. Amongst these biomaterials, most are engineered from non-globular elastomeric proteins that behave like entropic springs, such as those based on elastin, resilin, collagen and abductin. A large number of proteins, such as the giant muscle protein titin and many extracellular matrix proteins, are tandem modular proteins that consist of many individually folded functional domains to confer required biological functionalities and mechanical properties for various tissues. Experimental efforts to incorporate such tandem modular proteins into functional biomaterials have just begun. Here I report our latest progress in this exciting new area. Using proteins with defined mechanical properties as building blocks, we have engineered highly elastic and tough protein hydrogels, as well as protein hydrogels with dynamically tunable mechanical properties. The macroscopic mechanical properties of these hydrogels correlate well with the nanomechanical properties of protein building blocks, bridging the gap between protein mechanics at the single molecule level and mechanics of biomaterials at the macroscopic level. The insights revealed from these experiments will help tailor design protein biomaterials for specific biomedical applications.

Motivated by the increasing demand of fresh pure water, particularly in inhabited deserts and arid regions, our study focuses on the condensation of atmospheric vapor by radiative cooling without any energy source on inclined biomimetic surfaces. These surfaces are made of micropatterns of hydrophilic polymer brushes.
Usual microfabrication techniques were employed to produce the substrates (silicium wafer) with grooves of 30-500 micrometers in spacing and 50–150 micrometers in depth. Such patterns induce a faster growth of the drops by coalescence, leading to earlier drainage and collection of water at the bottom of the plate. We proved that an additional grafting of hydrophilic and/or thermosensible polymers such as poly(oligo-ethylene glycol methacrylate) and (poly(N-isopropyl acrylamide) by surface-initiated controlled radical polymerization can even increase the efficiency of such condensation devices. It was shown that water droplet nucleation is faster on the functionalized substrates than on the bare ones, with higher water collection yields. These results are very promising for the harvesting of atmospheric water by biomimetic surfaces.

A significant goal of materials science is to develop polymeric biomaterials that mimic the structure and properties of biological tissues, presenting the need for new approaches to achieve unique combinations of anisotropy, responsiveness, and multifunctionality. Tissues of interest to many biomaterials researchers to be replicated or mimicked are those of the vasculature, with success holding the promise of repair and replacement of vessels and arteries rendered dysfunctional by rampant cardiovascular disease. Our group has recently undertaken the development of elastomeric composites with shape memory and drug delivery characteristics in an attempt to mimic the properties of blood vessels, particularly targeting achievement of small-diameter vascular grafts. Here, we present a novel elastomeric polymeric construction capable of localized and sustained long-term tunable drug release while also exhibiting one-way shape memory properties of use for facile surgical manipulation or device deployment. The composites are fabricated using an electrospinning process to produce a micro-fibrous framework that is subsequently imbibed with an elastomeric matrix or co-deposited with interwoven elastomeric fibers. Incorporation of drug molecules into the primary fiber phase enables good distribution of the drug, with control of drub release being manipulated by the transport properties of the elastomeric matrix. We will present in vitro drug release studies for a model drug and for nitric oxide, both conducted in PBS under physiological conditions, to evaluate the effects of the elastomeric matrix and varying the drug concentration. Shape memory properties of the materials in isotropic, anisotropic, and laminated forms will be presented and discussed in the context of natural tissues that continuously serve as design inspiration.

Nature has mastered mechanotransduction as a mechanism to create dynamic, responsive materials for use in a range of functions, from biochemical signaling to tissue remodeling to camouflage. This talk will present examples of synthetic materials inspired by these biological examples, whose mechanotransduction is mediated by coupling covalent chemical reactions to mechanical stress gradients. Opportunities, limitations, and current challenges in the field will be discussed.

3D Printing has become commonplace for the manufacturing of objects with unusual geometries. Recent developments that enabled printing of multiple materials indicate that the technology can potentially offer a much wider design space beyond unusual shaping. Here, we show that a new dimension in this design space can be exploited through the control of the orientation of anisotropic particles used as building blocks during a direct ink writing process. Particle orientation control is demonstrated by applying low magnetic fields on deposited inks pre-loaded with magnetized stiff platelets. Multi-material dispensers and a two-component mixing unit provide additional control over the local composition of the printed material. Combining inks of sufficiently low viscosity to enable particle orientation with viscoelastic inks that allow for the formation of distortion-free geometries is essential for the proposed multi-material magnetically-assisted 3D printing platform (MM-3D Printing). The potential of this approach is demonstrated by fabricating heterogeneous composites with exquisite microstructural features. The importance of the particle orientation is proven by mechanical testing of MM-3D printed samples. The wide design space offered by the proposed printing platform greatly expands the current set of toolboxes available for the design and fabrication of functional parts through additive manufacturing technology. MM-3D printing opens the way towards the manufacturing of functional heterogeneous materials with microstructural features thus far only accessible by biological materials grown in nature.

Reference: Kokkinis, D. et al. Multimaterial magnetically assisted 3D printing of composite materials. Nat. Commun. 6:8643 doi: 10.1038/ncomms9643 (2015).

Hydrogels are intriguing materials with high potential for applications in smart structures and biomedical engineering. However, they have seen few real-world applications as structural materials because there is currently no method for producing hydrogels with multiple well-balanced mechanical properties, including the proper modulus, high toughness, self-resilience, self-healing, and damping. In addition, hydrogels usually have poor adhesion to other hydrogels or solid materials, which makes it difficult to develop hydrogel composites or integrate hydrogels with different materials for various applications. In this work, we develop novel hydrogels that integrate all of these functions simultaneously by using the reversible sacrificial bonds[1-3]. As an example, we use the ionic bonds of polyampholytes that are dynamic and mechanically weak, as the reversible sacrificial bonds. We will show that the polyampholyte hydrogels exhibit 100% self-resilient, high fatigue resistance, self-healing, quick and strong adhesion to bio-tissues and other materials, along with a high toughness[1,2]. Taking advantages of the high toughness and strong adhesion of the polyampholyte hydrogels, for the first time, we succeeded in developing extra-ordinary tough ligament-like hydrogel composites [3,4]. The new ligament-like composites, consisting of polyampholyte hydrogels and glass fiber woven fabrics, exhibit extremely high toughness (250,000 J/m2), high tear strength(~65 N/mm), high tensile modulus(606 MPa), and low bending modulus(4.7 MPa), while being composed of water-containing biocompatible materials. These excellent mechanical performances are obtained first time for water-containing materials, comparable to high toughness Kevlar/polyurethane blends and fiber-reinforced polymers. Importantly, the mechanical properties of these composites greatly outperform the properties of either individual component. A mechanism is proposed based on established fabric tearing theory, which will enable the development of a new generation of mechanically robust hydrogel composites based on fabric. These results will be important in the field of soft biological prosthetics, and more generally for commercial applications such as tear-resistant gloves and bullet-proof vests. This sacrificial bond concept may be applied to other dynamic physical bonds and thus substantially broaden the range of real-world applications of hydrogels.
References
1) Gong, J. P. “Materials both Tough and Soft”, Science 344, 161(2014).
2) Sun, T. L., et al., Physical Hydrogels Composed of Polyampholytes Demonstrate High Toughness and Viscoelasticity, Nature Materials, 12, 932 (2013).
3) Roy, C. K., et al., Self-Adjustable Adhesion of Polyampholyte Hydrogels," Advanced Materials, published online (2015)
4) King, D. R., et al., "Extremely tough composites from fabric reinforced polyampholyte hydrogels," Materials Horizons, 2(6), 584(2015).

The ability to spontaneously repair damage, which is termed as self-healing, is an important survival feature in nature because it increases the lifetime of most living creatures. This feature is highly desirable for synthetic materials for their wide applications in packaging, electronics and energy storage^[1-3]. Though many synthetic self-healing chemistries have been reported, the development of next generation self-healing polymers still have many challenges. Among them, self-healing polymers with water tolerant self-healing and good mechanical performances are of key importance. Herein, we report a water tolerant self-healing polymer by using a CO₂-grafting strategy^[4]. The key to the strategy is the development of a highly efficient methodology for the reduction of CO₂ to formoxylated silanes by hydrosiloxanes using an earth-abundant copper(I) catalyst, which enables rapid reductive grafting of CO₂ onto the side chains of polymethylhydrosiloxane, an inexpensive and abundant industrial side product, to give pure polyformoxymethylsiloxane as the crosslinker. The development of a highly efficient methodology for CO₂ hydrosilylation using an earth-abundant catalyst allows for efficient synthesis of PFMS from two inexpensive and abundant waste products—PMHS and CO₂—in 1 hour. Further explorations of PFMS as a crosslinker to make silicone rubbers may provide a greener alternative for acetoxy silanes used in commercial products, in which the carbons are derived exclusively from fossil fuels. The remarkably high mass ratio of CO₂ in PFMS (ca. 40%), as well as that in CO₂/epoxide copolymers (up to 50%), unequivocally shows the great potential of CO₂ as a useful carbon source to make polymers. In addition, we also successfully realized a self-healing elastomer with ultra-high stretchabilities.

[1] B. C-K. Tee*, C. Wang*, R. Allen, Z. Bao Nat. Nanotechnol., 2012, 7, 825-832.
[2] C. Wang, H. Wu, Z. Chen, M. T. McDowell, Y. Cui, Z. Bao, Nat. Chem. 2013, 5, 1042.
[3] C. Wang, N. Liu, R. Allen, J. B. Tok, Y. Wu, F. Zhang, Y. Chen, Z. Bao Adv. Mater. 2013, 25, 5785-5790.
[4] B. Lin, C. Wang, A. Thomas, Z. Bao, T. D. P. Stack, 2014, submitted.

Self-healing materials can undergo either autonomic healing or induced healing in response to a specific stimulus that triggers damage repair, thereby restoring their strength and function. However, self-healing materials responding to multiple types of stimuli while exhibiting superior mechanical properties are still rare. Dynamic covalent chemistry and non-covalent interactions, due to their reversible nature, have been successfully employedto develop self-healing materials. Metal–ligand interactions, whichare not only thermodynamically stable but also kinetically labile, are of interest for self-healing materials. Carbon nanotubes (CNTs) have been widely used as reinforcing fillers for polymers due to their outstanding mechanical properties. Furthermore, they also exhibit superior chemical stability, excellent electrical and thermal conductivity, and the ability to absorb infrared (IR) light. In this study, terpyridine-ligand-terminated polyurethane (PU) was prepared by in situ polymerization at the surface of multi-walled carbon nanotubes (CNT). The terpyridine-terminated PU/CNT ligand macromolecules were dynamically crosslinked with the metal ion Zn²⁺ to obtain metallo-supramolecular polymer nanocomposites. The tensile strength and tensile strain-at-break of the PU/CNT nanocomposites increased from 14.22 MPa and 620% to 23.23 MPa and 1076%, respectively, with the addition of Zn²⁺. The Zn²⁺-coordinated materials showed a rare combination of strong, tough, and elastic mechanical properties and were able to self-healvia multiple stimuli, including remotely controlled near infrared (NIR) light (4.2 mW/mm², 30 min), relatively low temperatures (90 ^oC, 1 hr), and/or solvents, all with excellent healing efficiencies (i.e., higher than 95%) and short healing times. The metallo-supramolecular polymer nanocomposite films were extensively studied using various techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), small angle X-ray diffraction, thermogravimetric analyzer (TGA), differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA), tensile tests, and surface profilometry.

Carbon–carbon bonds are one of the most important covalent bonds in organic and polymeric materials. Although the bond dissociation energy of carbon–carbon bond in ethane is high, that of the central carbon–carbon bond in tetraphenylethane is much lower. In the present paper, we report self-healing and mechano-responsive behavior of polymeric materials with tetraarylethane derivatives.
We demonstrated self-healing of cross-linked polymeric materials with diarylbibenzofuranone (DABBF)-based dynamic covalent linkages at ambient temperature, and investigated the healing behavior from both macroscopic and microscopic viewpoints. The macroscopic behavior was inspected by mechanical tests, and the linkage reaction (equilibrium) was evaluated by electron paramagnetic resonance (EPR) measurements. These assessments revealed that the healing is strongly dependent on temperature, which is attributable to synergism between changes in the chain mobility and in the equilibrium of the incorporated linkages.
In addition, mechano-responsive behavior of polymers with tetraarylethane derivatives was investigated. Linear, star-shaped, and cross-linked polymeric samples showed mechano-responsive behavior by grinding, stretching, and freezing. In particular, the central the carbon–carbon bonds of DABBF units embedded in polymer skeletons can afford the corresponding arylbenzofuranone radicals with blue color by homolytic cleavage of the covalent bonds by the mechanical stress, and the regeneration of the carbon–carbon bonds was also confirmed. The mechano-responsive behavior was also investigated by EPR measurements. The relationship between mechano-responsive behavior and polymer architectures will be discussed.

Hierarchical assembly of self-healing adhesive proteins creates strong and robust structural and interfacial materials, but understanding of the molecular design and structure–property relationships of structural proteins remains unclear. Elucidating this relationship would allow rational design of next generation genetically engineered self-healing structural proteins. Here we report a general self-healing and -assembly strategy based on a multiphase recombinant protein based material. Segmented structure of the protein shows soft glycine- and tyrosine-rich segments with self-healing capability and hard beta-sheet segments. The soft segments are strongly plasticized by water, lowering the self-healing temperature close to body temperature. The hard segments self-assemble into nanoconfined domains to reinforce the material (∼1 GPa). The healing strength scales sublinearly with contact time, which associates with diffusion and wetting of autohesion. The finding suggests that recombinant structural proteins from heterologous expression have potential as strong and repairable engineering materials.

Many natural biopolymers not only have advanced mechanical properties such as high modulus, toughness, and elasticity, but also exhibit dynamic properties. Inspired by Nature, we have designed a series of biomimetic modular polymers with folded nano-domains as the repeat units. These new material manifest an exciting combination of key mechanical, as well as adaptive, properties that have until now proven difficult to achieve in man-made systems (Nature Materials 2014, 13, 1055). Another important feature of natural materials is their capability to self-heal after mechanical damage. Inspired by nature, our laboratory has recently developed several self-healing polymers having strong mechanical properties and autonomous self-healing capability. For noncovalent healing mechanism, we developed multiphase supramolecular thermoplastic elastomers that combine high modulus and toughness with spontaneous healing capability by using hydrogen-bonding (Nature Chemistry 2012, 4, 467) and metal-ligand interactions (J. Am. Chem. Soc. 2014, 136, 16128). For using dynamic covalent interactions, my laboratory has demonstrated both Ru-catalyzed olefin metathesis (J. Am. Chem. Soc. 2012, 134, 14226) and boronic ester exchange (J. Am. Chem. Soc. 2015, 137, 6492) reactions can afford efficient self-healing polymers. In this talk, I will discuss the most recent development from our lab on dynamic and self-healing materials. In this presentation, I will discuss the design, synthesis, and single molecule and macroscopic property studies of our biomimetic dynamic polymers, including stiff and autonomic self-healing systems.

Two-dimensional materials are of increasing interest due to their unusual properties for use in filtration, sensing, nanoelectronics and biomedical devices. Using a class of biomimetic polymers called peptoids, we have succeeded in making defect-free 2D crystalline materials that exhibit the ability to self-repair on a range of solid substrates under suitable pH conditions, regardless of whether the substrates is negatively or positively charged. Moreover, we have shown that we can utilize this property to create nanoscale patterns of one peptoid within 2D sheets assembled from a different peptoid. To do so, we use an AFM-based method called nano-shaving to produce peptoid-free patterns within a pre-assembled sheet. Upon introduction of a second peptoid solution, the peptoids begin assembling at the active edges generated by nanoshaving. During the repair process, we find that the speed of the advancing edge depends on the direction of edge relative to the outer edge of the sheet and correlates with the aspect ratio of the as-grown sheets. This in turn reflects the two-fold symmetry of the underlying peptoid lattice. The mechanism of self-repair will be discussed. Finally, because these sheets are stabilized by hydrophobic interactions, if the solution contains peptoids having an identical sequence in the hydrophobic block, they will fill in the defects to form the nanopatterns even if the hydrophilic regions are distinct. Consequently, we can assembly sheets exhibiting multiple functional groups on their surfaces for a range of potential applications.

Two-dimensional (2D) materials have attracted intense interest due to their novel properties and potential for applications in molecular separation, electronics, catalysis, optics, energy storage, and biomedicine. An ability to develop sequence-defined synthetic polymers that mimic lipid amphiphilicity for assembly of self-repairable 2D membrane mimetics and exhibit protein-like, sequence-specific molecular recognition would significantly advance the development of functional 2D materials including artificial membranes.

Here we report assembly of diblock-like peptoids into extremely stable, crystalline, free-standing and self-repairable 2D membrane materials through a facile crystallization process, in which inter-peptoid interactions drive the anisotropic packing of peptoids to form a highly-ordered structure containing well-aligned strips of hydrophilic domains. Molecular dynamics simulations confirm that the membrane structure deduced from experimental data is energetically favored; moreover, they provide more detailed (atomic level) structural information to show the anisotropic packing of peptoids within the membrane structure.

These peptoid membranes exhibit a number of properties associated with cell membranes, including thicknesses in the 3.5 - 5.6 nm range, spontaneous assembly at interfaces, thickness variations in response to external stimuli, such as changes in Na⁺, K⁺ and Ca²⁺ concentrations. Because these membranes are built by the reversible supramolecular interactions, such as hydrogen bonds and π-stacking hydrophobic interactions, they exhibit unique ability to self-repair. The quantified repair rates revealed by in situ AFM imaging show that peptoid membranes exhibit much faster repair along x-direction (parallel to the straight edges of membranes) than along y-direction, which is consistent with results observed in the time-dependent membrane crystallization and molecular dynamics simulations.

We further demonstrated that these membranes are superior to lipid bilayers and other assembled 2D materials because: 1) they are free-standing, atomically ordered, and highly stable in pure organic solvents (e.g. acetonitrile and ethanol) as well as high temperature; and 2) attaching a broad range of functional groups at arbitrary locations in the peptoid sequence leaves this basic membrane structure intact. Given that peptoids are sequence-specific, highly stable, biocompatible, and exhibit protein-like molecular recognition, we anticipate this new class of 2D materials will provide a robust platform for development of self-repairable membranes tailored to specific applications.

Understanding shockwave-induced physical and chemical changes of impact-absorbing materials is an important step toward the rational design of materials that mitigate the damage. We have recently reported a series of network-forming ionic liquids (NILs) that possess an intriguing shockwave absorption property upon laser-induced shockwave. Microstructure analysis by X-ray scattering suggests nano-segregation of alkyl side chains and charged head groups in NILs. Further post-shock observation indicates changes in the low Q region implying that the soft alkyl domain in NIL plays an important role in absorbing shockwaves. Interestingly, we observe a shock-induced ordering in the NIL with longest hexyl side chain, indicating that both nano-segregated structure and shock-induced ordering contribute to NIL’s shockwave absorption performance. NILs possess promising energy dissipation properties, but are viscous liquid at room temperature and difficult to incorporate in a material. Here we investigate soft silicone composites containing ionic liquid droplets that dissipate energy via shock induced ordering of ionic liquid droplets and impedance mismatch between the silicon matrix and ionic liquid. Composites with different NILs and varying inclusion concentrations are subjected to laser-induced shock waves. Energy absorption and attenuation of peak pressure are extracted from interferometric measurements of surface velocity. Structural changes of the ionic liquid by shock wave impact are characterized by x-ray diffraction. Consistent with the literature, we also find that the elastic modulus of the composite increases with increasing concentration of liquid inclusions due to surface tension effects.

Various water-responsive biomaterials swell and shrink in response to changes in relative humidity (RH); potentially a work cycle able to harvest energy from evaporating water. Here, we predict the energy harvestable from naturally evaporating water. We model the power output, the effect on evaporation rates, and the intermittency of the power output due to typical weather conditions. We perform steady state analysis over a range of locations across the United States and predict average energy fluxes greater than 1 W m^-2 and net water savings greater than 1 mm H₂O day^-1 due to reductions in evaporation rates. We use a non-steady state mass and energy balance to investigate daily and yearly variations in power output due regional changes in weather and power demand. Our calculations show that this model can deliver power densities surpassing wind power and comparable to current installed solar systems. These results suggest that further research into water-responsive biomaterials and devices can provide major benefits in developing novel renewable energy platforms.

As part of our continuing research on adaptive polyimide-based systems that have been shown to be mechanically responsive to light [1], heat [2] and thermal-electrical stimuli,[3] a new diamine (Ester Sulfonyl-Diamine or ES-Diamine) containing an ester sulfonyl group was synthesized via a two-step route. It was polymerized with five common dianhydrides in NMP to afford polyamic acid (PAA) solutions, which were subsequently converted either chemically at room temperature or thermally at 175 °C to a series of amorphous polyimides (PI’s) containing ester sulfonyl side groups (PI-ES’s). The chemically imidized polyimide films are tough, creasable while the thermally imidized ones are brittle since the molecular weights of the former is much higher than the latter based on the GPC results. In addition, a series of copolymers (PI-ES:A’s) of PI-ES’s and PI-A’s (polyimides containing carboxylic side groups) were prepared by heating PI-ES’s at 250 °C for 1-16 hrs via a retro-Michael-Addition reaction. For comparison purposes, PI-As and PI-Ns (PIs without stimuli-responders) were also prepared. All the polymers were thoroughly characterized by FTIR, thermal analysis, WAXD and mechanical testing. The thin films of PI-ES’s, PI-A’s and PI-ES:A’s showed remarkably motility under gradient conditions created by humidity (or methanol vapor), while PI-N, Ultem and Nafion films were non-responsive under the same conditions. Together with their excellent thermal stability, fatigue behavior and chemical resistance, these polyimides provide material platform with outstanding mechanical integrity, demonstrating >100 oscillatory and locomotion cycles without performance degradation; thus, adding humidity-gradient responsivity to their versatile capabilities.
References:
[1] (a) D.H. Wang, K.M. Lee, Z. Yu, H. Koerner, R.A. Vaia, T.J. White, and L.-S. Tan, Macromolecules, 2011, 44(10), 3840-846; (b) K.M. Lee, H. Koerner, D.H. Wang, L.-S. Tan, T.J. White, R.A. Vaia, Macromolecules 2012, 45(18), 7527-7534; (c) J.J. Wie, D.H. Wang, K.M. Lee, L.S. Tan, T.J. White, Chem. Mater, 2014, 26(18), 5223-5230.
[2] H. Koerner, R.J. Strong, M.L. Smith, D.H. Wang, L.-S. Tan, K.M. Lee, T. J. White, and R.A. Vaia, Polymer 2013, 54(1), 391-402.
[3] A.T. Sellinger, D.H. Wang, L.-S. Tan and R.A. Vaia, Adv. Mater, 2010, 22, 3430-3435.

Transition metal-ligand interactions offer a way to control mechanical properties of materials by tuning the photoreactivity and assembly of polymers. We have designed organic-inorganic metallopolymeric hybrid materials where new material properties were introduced due to unique interactions between a metal-binding termini-group on the polymer and a metal ion. Specifically, the linkage of polymer molecules through metal-ligand interactions changed the mechanical properties of the material as well as added new features to the polymeric material. In the work presented here, telechelic hydrogenated poly(ethylene-co-butylene) polymers with ligand binding end groups coordinated to chromium(III) ions showed photoresponsive, viscoelastic response. Specifically, we showed tight control over the mechanical strength of prepared metallopolymers where 455 nm light radiation gave rise to reversible changes in the modulus of the materials. We attributed this photo-responsive behavior to the generation of an excited state of the chromium(III) metal center, which locally breaks the metal-coordination bonding in the polymer, leading to softening of the Cr(III)-based metallopolymers. The magnitude of this softening was observed up to 46 kDa in storage modulus G'. Once the light stimulus was removed, however, the initial matrix elasticity is fully recovered within a minute timescale. Current efforts are now aimed to create multifunctional supramolecular polymers with multi-modal self-assembly sites to introduce self-healing sites and maximize changes in mechanical stiffness of these materials upon light exposure.

Organic nanowires represent idealized model systems for understanding the interactions between organic semiconductor building blocks. We have synthesized nanowires consisting of pi-conjugated DNA-base surrogates covalently attached to a DNA-like backbone and studied the properties of these constructs with molecular dynamics simulations. The DNA base surrogates were first parametrized through quantum mechanics calculations to create an atomistic model. Constant-temperature molecular dynamics simulations then provided an improved understanding of the kinetics of stacking between adjacent DNA base surrogates. Replica-exchange simulations in turn yielded the atomic structures of our nanowires at equilibrium. By examining the lowest energy structures obtained from our simulations, we have gained insight into the structure and integrity of our nanowires, which would not be readily available through experimental techniques.

Traditional biosensors have been fabricated with rigid materials of Young’s modulus (MPa-GPa order) of which value is higher than biological body, leading to inflammation and damage of body’s cells, tissues, and organs [1,2]. In order to realize less invasiveness and long lasting therapeutic benefits, soft materials such as thin polymers, rubbers, and gels have been applied. Recently, we developed silver nanowires-based electrodes of high aspect ratio showing high optical transparency (-94%), high electrical conductivity (-24 ohm/sheet), and high mechanical stretchability (25% strain-) [3,4]. However, the ion migration has been a critical issue of silver nanowires for electrical reliability. Here, we report silver nanowires-based multi-channel electrodes enhanced on an electrical stability which successfully obtained electrocorticography (ECoG) with wireless recording system. Patterned 16 channels electrode of silver nanowires on parylene of 1 micron thickness was modified with biocompatible materials on the wire’s surface for the high electrical stability. After depositing encapsulation layer of total 3 micron thickness of parylene and making through-hall, the size of each electrode was ca. 150 micron meter square. To improve handling of thin polymer having total 4 micron thickness, the sheet electrodes was coated with a polymer which was harden at room temperature but soften near vital warmth. With the developed sheet electrodes, brain activity of a rat has been monitored with wireless recording system. On the other hand, in the way to practice implanting on rat’s brain with multi-channel, flexible, and gold-based electrodes, we have got neural signals after 7 weeks implantation. The improved silver nanowires electrode has a high potential of biological compatibility as a long term implantable passive sensor realizing high spatial mapping of brain functions.

Reference
[1] I. R. Minev et al., Science, 347, 159, 2015.
[2] S. Lee et al., Nature Comm., 5, 5898, 2014
[3] T. Araki et al., Nano Research, 7, 236, 2014.
[4] Y. Yang et al., Nano Research, in press.

Zinc oxide is an appealing semiconductor material for industrial purposes such as gas-sensing applications. Recently, it has attracted great attention for biosensing applications, majorly due to its biocompatible characteristics where it retains the biochemical activity and conformation of biomarkers under study. ZnO provides surface states that can be used for biomolecule immobilization using different linker chemistries. One of the useful properties of ZnO is the ability to tune its electrical characteristics and surface states based on deposition. Tailoring the terminations on the polar ZnO surface would enhance the capacity of the film for performing the desired biochemical function, improving the sensor selectivity. In this study, we analyzed two types of ZnO thin films of ~ 100 nm thicknesses deposited on flexible polyamide membrane using Pulsed laser deposition (PLD) system and RF Magnetron based sputter deposition system. Structural characterization and surface morphology of the deposited films was studied using SEM images. Hall measurements were performed to compare the electrical characteristics of the two thin films in terms of resistivity and Hall mobility. We studied the interaction of these two thin films with various buffer solutions such as PBS and synthetic sweat in small volumes (less than 10µL) to understand the effect of pH variability and ionic conductivity on electronic transduction of ZnO thin film. Electrochemical Impedance spectroscopy (EIS) and Mott-Schottky measurements were carried out on these two types of films using the mentioned buffer solutions to evaluate their influence on conduction pathways in ZnO thin films. These AC impedance measurements were used to monitor variations in interfacial layers at electrode-electrolyte interfaces. The EIS measurements were analyzed through equivalent circuit modeling using software Zview 3.0 (Scribner Associates, Inc.), where RC parallel circuit and constant phase element values were compared for grain boundary and bulk of the thin film. This analysis helped in understanding the entrapment of carriers at the grain boundaries of the thin film as various ionic buffer solutions were tested. We further obtained the impedance measurements by spiking the cortisol concentrations from 1 ng/mL to 500 ng/mL in synthetic sweat on both types of thin films. Calibration curves obtained in terms of impedance changes from both types of films for detection of stress biomarker cortisol were compared. This study helped in investigating the effectiveness of two types of ZnO thin films for the purpose of stress biomarker sensor development on flexible substrates.

In recent years, viruses have been pursued as versatile, hierarchical templates with site-specific affinity. These bio-based patchy particles present an opportunity to create hybrid nanobiomaterials composed of multi-functional building blocks which have applications in a variety of disciplines from drug delivery to photocatalysis. The soft or biological components provide precision assembly and specificity, while the inorganic components add functionality. Possessing distinctive nanoparticle arrangements, such viral-templated materials are unique, with performance often surpassing the sum of the constituents. Yet, for the most part, once assembled by the host these biomacromolecules are stagnant, with a fixed size and shape that limits utility. One exception is the M13 bacteriophage. In this work, the filamentous M13 virus was studied as a structurally adaptable bio-template for the assembly of two-faced nanostructures known as Janus particles. The filamentous bacteriophage M13 was genetically modified to present a gold-binding peptide on the p8 capsid protein, as well as a ZnS-binding peptide on the p3 protein. This bi-functional bacteriophage served as an adaptable biomolecule template. When physically manipulated using a chloroform treatment, the virus transformed from an 880 nm by 6 nm filament into a spherical particle about 60 nm in diameter. Both the filamentous and spheroidal forms were explored as bio-functional supports for hybrid nanomaterials. Using aqueous-based chemistry, gold was synthesized on the p8 proteins and ZnS was synthesized on the p3 protein to form both matchstick and dumbbell-shaped metal-semiconductor Janus particles. Transmission electron microscopy and electron diffraction were used to study the size, morphology, crystal structure, and composition of the templated nanoscale heterojunctions. Furthermore, photoluminescence and UV-Vis absorption measurements were used to investigate optical behavior of the metal and semiconductor nanocomponents individually, as well as jointly. This study represents the first steps toward a reconfigurable viral template for Janus particle synthesis.

Dynamic covalent bonds (DCBs) featuring reversible bonding/debonding properties have found various applications in dynamic materials designs such as self-healing, malleable, shape-memory or environmentally-adaptive materials. However, most DCBs developed so far require either catalyst or harsh environments to facilitate bond reversion and involve special building blocks for synthesis. Here we seek to develop a novel platform for dynamic materials design using a new type of DCB – hindered urea bond (HUB). Differing from normal urea bond, HUB bears bulky substituents on one of the nitrogen atom, which distorts the atomic co-planarity, weakens the bonding stability, and allows reversible dissociation of HUB to isocyanate and hindered amine. The dynamic properties of HUB enables the design of self-healing polymeric elastomers through bond exchange and hydrolyzable polymers through quenching of dissociative intermediate – isocyanate. HUB shows many advantages in dynamic materials designs compared with other DCB systems in terms of simplicity, dynamicity, tunability and cost.

Our group is exploring the use of biological phenols and polyphenols as building blocks in the design of advanced materials with unusual dynamic and responsive physical properties. The use of biological phenols/polyphenols is also attractive because of their diverse and interesting biological properties such as antiproliferative, antiviral and anti-inflammatory activities. Here, we aim to employ polyphenols such as tannic acid, ellagic acid, and nordihydroguaiaretic acid as linkers in dynamic polymer networks formed by complexation with boronic acid containing polymers. The dynamic covalent boronate ester (BE) bonds formed between o-dihydroxyphenyl functional groups of multivalent phenols/polyphenols and boronic acid polymers produce hydrogels with interesting physical properties. Mixing of branched boronic acid terminated poly(ethylene glycol) (PEG) mixed with polyphenol cross-linker resulted in self-healing hydrogels due to the dynamic nature of BE bonds. The chemical composition of the boronic acid strongly influenced the physical properties and physiologic stability of the gels. BE linked hydrogels are stable under physiological conditions, however they dissociate at mild acidic conditions due to the pH dependence of BE bond stability, offering an opportunity to achieve pH-dependent release of biologically active polyphenol compounds.

Polymersomes, hollow spherical nano-to-microscale polymer assemblies, have increasingly become important constructs in the development of biomimetic materials that expand the library of functional and robust analogs to lipid-based vesicles. As compared to liposomes, polymersomes possess superior physical properties and the nearly unlimited potential for synthetic fine-tuning. Herein we improve on the physical properties of these polymer vesicles by introducing platinum-based metal-ligand crosslinks into the hydrophobic core, which gave the vesicles demonstrated resistance to destabilization by surfactants over uncrosslinked polymersomes. The formation of crosslinks was capable of being selectively reversed by the addition of phosphines. In addition, the Pt(0) crosslinks retained their catalytic activity for the hydrosilylation of alkenes.

Stimuli-responsive polymer gels and their application to smart bio- or biomimetic materials have been widely studied. On the other hand, we have developed “self-oscillating” gels that undergo spontaneous cyclic swelling–deswelling changes without any on–off switching of external stimuli, as with heart muscle. The self-oscillating gels were designed by utilizing the Belousov-Zhabotinsky (BZ) reaction, an oscillating reaction, as a chemical model of the TCA cycle. We have systematically studied these polymer gels since they were first reported in 1996 (JACS). Potential applications of the self-oscillating polymers and gels include several kinds of functional material systems, such as biomimetic actuators, mass transport systems and functional fluids. For example, it was demonstrated that an object was autonomously transported in the tubular self-oscillating gel by the peristaltic pumping motion similar to an intestine. Further, it is possible to create a new dynamic interface by immobilizing the self-oscillating polymer. We prepared a self-oscillating polymer brush surface by SI-ATRP and evaluated its dynamic behavior. Besides, autonomous viscosity oscillation was realized via metallo-supramolecular terpyridine chemistry, etc. Self-oscillation between unimer/micelle or unimer/vesicle structures was also realized for a synthetic block copolymer. Our studies represent innovative research, creating new concepts of functional gels and expanding their potential, and they have attracted attention in many research fields and have inspired related studies. In this presentation, our recent progress on the self-oscillating polymer gels is summarized.

Many organisms rely on metal ion coordination of biopolymers to produce materials with specific mechanical properties. Some natural materials also have 3-D patterned architectures with soft, elastic materials interfacing with hard, stiff materials, which allow for unique strength and elasticity. Our approach is to create interfaces in hard and soft by tuning the cross-linking interactions of the polymers through changes in the metal coordination bonds. These materials also have added functionality, where the metal-coordination bond can be controlled with light due to changes in the bonding interactions in the excited state. We have created Fe(III)-polysaccharide materials that show changes in stiffness (modulus) upon light irradiation. While all the materials formed from different polysaccharides (alginate, pectate, and hyaluronic acid) show changes in mechanical properties, the efficiency of the photoreaction is different for each material. The change in modulus also varies with the % composition of the polysaccharide.
In addition, we have shown that interfaces in modulus can be patterned into the material using photolithography techniques. By using a photomask, we can create materials with specific interfaces and gradients in modulus. After light irradiation, the Fe ions can be removed from the material with EDTA, and the material retains the patterned mechanical properties. Patterned materials also show mechanical properties that are different from those of the un-patterned material. This allows for the creation of materials with very unique mechanical properties and tunable changes in the overall response of the material.

In the work here, we describe sensing motifs that utilize 2-D and 3-D arrays and monolayers of particles embedded onto a molecular recognition polymer hydrogel network. The 2-D arrays alter their visually evident diffraction color because the hydrogel network swells or shrinks in response to analyte concentration changes. We developed 2-D photonic crystals for molecular recognition and chemical sensing applications. We prepared close packed 2-D polystyrene particle arrays by solvent evaporation of an assembling monolayer on a mercury surface or by lifting off the array from a water surface. We then transfer the 2-D arrays onto a hydrogel thin film that showed a hydrogel volume phase transition in response to a specific analyte. This alters the array spacing, changing the array diffraction wavelength. These 2-D array photonic crystals exhibit ultrahigh diffraction efficiencies that enable them to be used for visual detection of analyte concentrations. The 3-D arrays undergo 3-D volume changes to shift the diffraction wavelength. We developed novel protein hydrogels that are highly selective for charged species binding. These protein hydrogels act as a Coulometer that senses binding of individual charged species.

We present the formulation of photoresponsive hydrogels with potential applications in tissue engineering. Our method comprises mixed gels prepared from widely used acrylic monomers and a uronate-containing polysaccharide. The polysaccharide is coordinated with Fe(III), yielding a responsive material that can absorb 405 nm light to undergo a photochemical decarboxylation of the uronate segments. As the irradiation proceeds, the material becomes softer, showing a systematic decrease both in elastic and storage moduli. The changes in mechanical properties can be correlated with changes in the microstructure, as estudied by SEM. This method can be used for creating smooth gradients or discrete patterns on the hydrogels, which can be relevant in areas such as tissue engineering, where mechanical cues on substrates can direct the development of cells. Our preliminary results indicate the suitability of these substrates for culturing chondrocytes, where the photochemical treatment of the material has an effect on the extracellular matrix production.

A fundamental consequence of cellular organization of living systems is that the aqueous milieu, bathing the cells, is also compartmentalized. Although water equilibrates readily across the elastic cellular boundary, passive permeation of solutes is strongly hindered. As a result, gradients of concentrations of ions, salt, and soluble biomolecules are readily established across the cellular boundary, producing osmotic activity of water. To deal with any sudden environmental changes in the amount of dissolved molecules in water, free-living cells have evolved complex molecular machinaries and mechanisms (e.g., mechanosensitive channels and compatible solute accumulation), which allows them to dissipate the osmotic stress. But how might primitive cells near the dawn of life on Earth – lacking advanced biochemical or genetic capabilities and composed essentially of simple amphiphiles– have responded to such environmental insults?

Drawing from recent experiments in our labs employing simple models for the cellular chassis (i.e., giant vesicles composed of amphiphilic lipids and polymers), this talk considers how the osmotic activity of water is transduced across cell-like compartments. It highlights how water activity and accompanying dissipation of osmotic energy couples with the compartmental boundary, mechanically remodeling the membrane shape and spatially reorganizing membrane components - both through a well-orchestrated cooperative dynamics. Comparing these processes as elemental events in the homeostatic working of a living cell, these findings support the idea that water is not a mere solvent for life – a blank canvas on which biomolecules become animated – but an active medium that guides organization and dynamics of biomolecules in complex, subtle and essential ways.

This talk will highlight work performed with Doug Gettel, Jeremy Sanborn, Sean Hong, James Ho, Kamila Oglecka, Madhavan Nallani, and Bo Liedberg.

Biomolecular self-assembly has been a source of inspiration to construct nanowires¹ from biomolecules such as DNA, porphyrins, viruses and peptides. Self-assembly is an important technique for materials design by using noncovalent interactions including hydrogen bonding, hydrophobic, electrostatic, metal-ligand, π-π and van der Waals interactions¹. Various self-organized supramolecular nanostructures (nanotube, nanofiber, nanosphere and nanosheet) have been produced by using these noncovalent interactions². In this work, we designed and synthesized supramolecular semiconducting peptide nanofibers where they demonstrated high biocompatibility and biodegradability. The self-assembly and photophysical properties of these organic semiconductor peptide nanofibers were studied by various spectroscopic methods such as UV-vis, fluorescence and circular dichroism spectroscopy.

Inspired by the unique “bricks and mortar” microstructure of nacre, we developed a gel-to-film method for preparing graphene/polymer composite films with ultrahigh strength, ultrahigh toughness and high conductivity. The best films have a tensile strength up to 614 ± 12 MPa, as strong as AISI 304 stainless steel (585 MPa). They also have a high toughness of 14.89 ± 1.02 MJ cm^-3, together with a high electrical conductivity of 802 ± 29 S cm^-1 and a thermal conductivity as high as 524 ± 36 W m^-1 K^-1. We also fabricated moisture driving actuators based on graphene films, exhibiting rapid and strong actuations. They can be used switching electrodes and generate electrical powers.

Formation of ice and frost has detrimental effects on transportation and power industries. Typically their formation is combatted by dispensing antifreeze liquids. However, use of such liquids can be expensive and pose environmental problems. Bioinspired self-cleaning superhydrophobic and lubricant impregnated surfaces have been proposed as fully passive anti-icing alternatives. However, these coatings have been shown to work only in a narrow set of conditions.
In this work, we demonstrate that the icing and frosting problems can be mitigated by mimicking active natural systems that secrete functional liquids only in response to external stimuli. For example, poison dart frogs have two types of specialized glands in their skin to produce mucus and toxins. The frogs continually secrete the mucus to stay hydrated but release toxins only to deter predators. Based on this biological example, we have developed an anti-icing coating that prevents accumulation of all forms of ice by responding to its presence with release of antifreeze liquid. The coating consists of an outer porous superhydrophobic epidermis and a wick-like underlying dermis that is infused with antifreeze liquid. We validate the functionality of this responsive coating through condensation frosting, simulated freezing fog and rain experiments. In the tested conditions, the novel anti-icing skin delays onset of frost, rime, and glaze accumulation at least ten times longer than anti-icing superhydrophobic and lubricant impregnated coatings. Furthermore, our coating delays onset of glaze formation ten times longer than surfaces covered by a film of the antifreeze. We discuss the fundamental mechanisms responsible for antifreeze release and their relation to required antifreeze replenishment rates for each of the icing scenarios.

Machine technology frequently puts magnetic or electrostatic repulsive forces to practical use, as in maglev trains, vehicle suspensions or non-contact bearings. In contrast, materials design overwhelmingly focuses on attractive interactions, such as in the many advanced polymer-based composites, where inorganic fillers interact with a polymer matrix to improve mechanical properties. However, articular cartilage strikingly illustrates how electrostatic repulsion can be harnessed to achieve unparalleled functional efficiency: it permits virtually frictionless mechanical motion within joints, even under high compression. Here we describe a composite hydrogel with anisotropic mechanical properties dominated by electrostatic repulsion between negatively charged unilamellar titanate nanosheets embedded within it. Crucial to the behaviour of this hydrogel is the serendipitous discovery of cofacial nanosheet alignment in aqueous colloidal dispersions subjected to a strong magnetic field, which maximizes electrostatic repulsion⁶ and thereby induces a quasi-crystalline structural ordering over macroscopic length scales and with uniformly large face-to-face nanosheet separation. We fix this transiently induced structural order by transforming the dispersion into a hydrogel using light-triggered in situ vinyl polymerization. The resultant hydrogel, containing charged inorganic structures that align cofacially in a magnetic flux, deforms easily under shear forces applied parallel to the embedded nanosheets yet resists compressive forces applied orthogonally. We anticipate that the concept of embedding anisotropic repulsive electrostatics within a composite material, inspired by articular cartilage, will open up new possibilities for developing soft materials with unusual functions.

References
[1] Q. Wang, J. L. Mynar, M. Yoshida, E. Lee, M. Lee, K. Okuro, K. Kinbara, and T. Aida. Nature 2010, 463, 339–343.
[2] M. Liu, Y. Ishida, Y. Ebina, T. Sasaki, and T. Aida, Nature Commun. 2013, 4, 2029
[3] M. Liu, Y. Ishida, Y. Ebina, T. Sasaki, T. Hikima, M. Takata, and T. Aida, Nature 2015, 517, 68–72.
[4] Y.-S. Kim, M. Liu, Y. Ishida, Y. Ebina, T. Sasaki, T. Hikima, M. Takata, and T. Aida, Nature Mat. 2015, 14, 1002–1007.

From the cellular level up to the body system level, to keep alive and perform various functionalities living organisms cooperatively perform selective localizing and transporting of specific biological species in the complex bio-fluids. These graceful capabilities arise from the coordination of the chemo-mechanical actions of their muscles and/or tissues with their environmentally vigilant cells, such as the molecular configuration changes and micro/macroscopic mechanical motions in response to a variety of signals. Inspired by the unique self-regulating abilities, we have applied the concept of homeostasis to the design of autonomous materials and created a synthetic homeostatic material, SMARTS (Self-regulated Mechano-chemical Adaptively Reconfigurable Tunable System), which reversibly transduce external or internal chemical inputs into user-defined outputs via the “on/off” mechanical actuation of catalyst-bearing, hydrogel-driven epoxy microstructures. On the broad-based platform, we successfully used the adaptive, integrative materials to sort biologically relevant target molecules from a mixture solution. Further investigation with cancer cells and bacteria opens up the versatility of our “smart” integrative material for applications in biomolecule purification, concentration, and isolation. The dynamic material systems would have transformative impacts in areas ranging from medical implants that help stabilize bodily functions, to a low-coat high-throughput point-of-care diagnostic tool of diseased indicators in solution, and to smart devices that regulate energy usage. Further, with the high stability and specificity for a diverse array of target molecules by functionalization with various capturing entities, such a “smart” device can readily find use as a biosensor, a simple diagnostic tool of diseased indicators in solution, or even as a tool to quantitatively determine the extent of chirality achieved in the synthesis of a complex molecule.

Biology offers many examples of autonomous, multifunctional systems that can serve as inspiration for materials scientists. For example, most biological tissues can regenerate either after being damaged or as a natural consequence of the remodeling process. And while self-healing has been achieved in synthetic materials, healing is limited to relatively small (microscopic) defects and cracks. In contrast, damage that involves significant mass loss like ballistic impact represents a technological challenge that has been unachievable to date. Regenerating large damage volumes is hindered by voids or large gaps in the material as liquid healing agents bleed out of the damage area since they cannot be retained there by surface tension alone. The use of scaffolds in tissue engineering circumvents this problem by providing a support network for the growth of tissue across large gaps and voids. Recently we introduced a bi-stage regenerative chemistry in microvascular polymers that enables regeneration of significant mass loss and restoration of structural performance. In another example, plants rely on transpiration for distributing water, transporting nutrients, and providing thermal control. In the latter case, heat is supplied to leaves by the sun and removed by transpiration of water from the leaf. Supplying the water for this process requires a transport system from the roots through the trunk and branches of the tree and finally into the leaf. We introduce a microvascular composite materials system that self-cools through transpiration of the skin layer and conductance of water from a reservoir through its internal vasculature. As in natural transpiration, water is replenished from a reservoir by capillary action and requires no external power for transport. Both of these bio-inspired systems will be discussed in detail during the talk and the future directions of research in these areas will be highlighted.

Switchable optical materials, which possess reversible light transmission in response to external stimuli are of wide interest for potential applications such as energy efficient windows, roofings, and skylights that can transmit or block light. So far, most literatures have demonstrated switching between two states, color and transparency, color and whiteness, and whiteness and transparency. Few can switch between all three states. In nature, bio-organisms switch color /opaqueness and/or transparency to suit the local environment for hiding from the predators, for signaling, or for mating purposes. For example, squids and octopus in deep sea are masters of disguise. They are normally transparent in sea. They can quickly turn into red, however, thus become invisible again to fish with bioluminescent searchlights by stretching the skin.

We first fabricate tilted pillar arrays on wrinkled elastomeric polydimethylsiloxane (PDMS), which can achieve the dramatic and reversible visual effect between structural color, white, and transparent states. However, the initial state is opaque and it cannot achieve > 90% transmittance since the surface roughness cannot be completely eliminated. In the second approach, we design and fabricate a composite film consisting of a thin layer of quasi-amorphous array of silica nanoparticles (NPs) embedded in bulk PDMS. It is completely transparent in the initial state due to refractive index match between silica NPs and PDMS. Upon mechanical stretching, the transmittance was dramatically reduced to 30% and displayed angle-independent structural color depending on the size of NPs. In each system, we elucidate color switching mechanisms and their robustness against repeated mechanical stretching/release (up to 1000 times). Further, we show these materials can be patterned for display hidden images.

It remains a huge scientific challenge to understand and mimic the utilisation of chemical energy in biological systems to achieve the highly adaptable organisation and sophisticated functions like active transport, motility, self-repair, replication, and adaptability. The development of biomimetic systems with similar energy consuming organisation and functions requires a radical departure from equilibrium self-assembly approaches, towards out-of-equilibrium systems driven by the continuous input of energy.

In our research we focus on the development of active materials driven by chemicals fuels. First, I will discuss how active materials can result from the transient self-assembly of synthetic molecules, driven by the consumption of a chemical fuel. In these materials, reaction rates and fuel levels, instead of equilibrium composition, determine properties such as lifetime, stiffness, and self- regeneration capability. Then, I will discuss our recent steps to achieve temporal and spatial over fuel-driven self-assembly by the development of a chemical reaction network that allow for feedback control. Such systems will form the basis for self-organising systems and for design and construction of energy-consuming dynamic devices and materials.

References:
[1] J. Boekhoven, A.M. Brizard, K.N. Kowlgi, G.J. Koper, R. Eelkema, J.H. van Esch, Angew. Chem. Int. Ed. 2010, 49 DOI: 10.1002/anie.201001511.
[2] J. Boekhoven, W. Hendriksen, G. Koper, R. Eelkema, J.H. van Esch, Science 2015, 349, 1075.
[3] A.G.L. Olive, N. Hakimin Abdullah, I. Ziemecka, E. Mendes, R. Eelkema, J.H. van Esch, Angew. Chem. Int. Ed. 2014, 53, DOI:10.1002/anie.201310776.

Evaporation is a ubiquitously abundant, yet untapped, energy resource in nature. Bioinspired water-responsive materials that change shape in response to moisture fluctuations hold much promise for the development of devices that harness this potential¹. To demonstrate this potential, we have developed two types of evaporation-driven engines that can power locomotion and generate electricity². To build these engines, we rely on water-responsive Bacillus spores¹ deposited periodically on alternating sides of thin polyimide tapes, which then exhibit simultaneous high actuation strains and work densities. By exchanging less than 5% moisture by weight, these hygroscopy-driven artificial muscles can lift 50 times their own weight and quadruple their length. The performance of the muscles reduces only slightly even after 1 million cycles of hydration and dehydration. Using these materials, we designed device architectures that produce sustained linear oscillatory motion (piston-like) and rotary motion (turbine-like) in the presence of evaporation. When these engines are placed at the air-water interface, they self-start and continuously harvest evaporation energy to power their mechanical motions. To demonstrate their applications in powering external systems, we coupled our oscillatory engine to an electromagnetic generator. The corresponding peak power reached 60 mW and the average power was 1.8 mW across a 100 kΩ resistor. This level of power was sufficient to light up an LED periodically. To demonstrate the applications in locomotion, we created a miniature car (weighing 0.1 kg) driven by the rotary engine. As water in the rotary engine evaporates, this miniature car moves forward.

References:
1. Chen, X., Mahadevan, L., Driks, A. & Sahin, O. Bacillus spores as building blocks for stimuli-responsive materials and nanogenerators. Nature Nanotech. 9, 137-141 (2014).
2. Chen, X. et al. Scaling up nanoscale water-driven energy conversion into evaporation-driven engines and generators. Nature Commun. 6, 7346 (2015).

Synthetic spider silk has the potential to be an effective replacement or additive to a variety of products including bio-medical implants, athletic equipment, textiles, transportation materials and aerospace components. Recent advances in the production and refinement of spider silk protein have opened a path towards economic viability and commercialization. To assess the technology, modular engineering system models validated with experimental data were generated and leveraged to perform a techno-economic analysis and life cycle assessment. Modularity in the foundational engineering model facilitates the investigation of the diverse production and processing technologies being explored including spider silk protein production through transgenic E. coli, alfalfa, and silk worms. Alternative downstream processing methods under consideration include purification by affinity chromatography, salt precipitation, heat induction and spray drying. Economic analysis includes a simulation of a large-scale production facility operating 360 days with a facility lifetime of 30 years with a total annual production capacity of 1600 tons. The simulation integrates equipment depreciation, an internal rate of return of 10%, taxes, capital expenditures, and operational costs. The baseline pathway incorporating protein production through fermentation with heat induction and a salt precipitation purification stage, at 2.8 grams of expressed protein per liter of fermented media, results in a total cost of $21.70/lb. of spider silk. The majority of the costs are associated with material consumption highlighting the need for optimization of the fermentation and purification process. Sensitivity analysis shows focused research to improve protein expression as well as validating the optimized post-processing methods will dramatically impact the final selling price. Results on the environmental impact, and alternative production pathways including a conservative and optimistic scenario for fermentation will be presented.

In self-assembling biomolecular materials, such as microtubules (MTs), the dynamic, cooperative interactions of proteins and biomolecules are responsible for many of the adaptive and responsive behaviors we seek to emulate in artificial, biomimetic systems. For example, the selective binding of α and β tubulin forms an anisotropic dimer with distinct functional components, central to tubulin polymerization and ultimate function in MTs. Moreover, the binding and hydrolysis of guanosine triphosphate (GTP) on tubulin dimer building blocks regulates controlled MT assembly and disassembly. Here, we describe our recent work on self-assembling synthetic peptides composed of structural and chemical elements designed to mimic selective aspects of this biological assembly scheme. In particular, we explore several “multiblock” peptide chemistries, in which select functional segments of small peptides influence molecular solvation, hydrogen bonding, and critically, interactions with secondary molecules to control the reversible aqueous assembly of these peptides. For example, engineered electrostatic or reversible covalent interactions with secondary molecules facilitate unique, controllably reversible self-assembly into nanostructured filaments, much the way tubulin building blocks interact with their biomolecular environment to regulate self-assembly. Alternatively, modifying the size, distribution, and conformation of attractive and repulsive elements within peptide building blocks determines the form and function of assembled peptide nanostructures, again, imitating central assembly concepts in the natural biological system. These demonstrations represent promising examples of how mimicking aspects of biomolecular assembly may lead to novel molecular behaviors in engineered synthetic systems.

Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Solid-state nanopore is an emerging tool of sensing biomolecules such as DNA, protein, DNA-protein complex, and even biological cells with a very high throughput and simplicity. This biosensor has been well known for its high efficiency, ease in preparing the device and target biomolecule without any labelling, wide biological versatility of the sensor, compatibility with semiconductor technology, and so on. Despite of novel achievements reported so far in this field, the solid-state nanopore now faces a need for great improvement in terms of sensor performance; noise properties, device reliability, resolution, etc. Among the requirements for a more reliable biomolecule sensing system, temporal and spatial resolution is critical to realize the ultimate goal of the nanopores: next-generation DNA sequencing. Two major approaches taken to improve the device performance is to control the device noise level and dynamics of biomolecule translocation through solid-state nanopores.
The strategy brought to this work is to induce changes in the DNA dynamics translocating through a solid-state nanopore by structural modification of the device, so that data with better quality could be produced at the first place. While random nature of DNA movements and fast translocation speed are pointed out as factors negatively influencing on the nanopore resolution, the goal of this work is to restrain these two effects for improvements in resolution of this platform. Our approach is to geometrically confine DNA molecules before translocation. The confinement is achieved by establishing another layer of ionic channel underneath the nanopore, providing directionality in DNA movement and allowing less room for complete random thermal motion of DNA near the nanopore entrance. The fabrication technique in this work incorporates focused ion beam milling and transfer technique inspired from CVD graphene works. DNA translocation dynamics is modified by setting the additional ‘guide’ layer, and this positive influence is represented as slowed down translocation speed and enhanced percentage of real data to total peaks when compared to nanopores with the conventional structure. With the experimental data, physical origins of the improved performance and modified dynamics are also explored for better interpretation and fundamental understanding of the observed phenomena.

The kinesin-microtubule (MT) active transport system is a promising candidate for producing complex, structured nanomaterials that exhibit a wide range of emergent and adaptive behaviors currently unattainable through conventional self-assembly methods. Kinesin transport, for example, has been used for the dynamic self-assembly of ring nanocomposites and spool consisting of MTs and semiconductor nanocrystals. Here, surface-adhered kinesin motors propel biotinylated MTs across a surface, dissipating chemical energy, that in turn self-assemble into ordered nanostructures upon introduction of streptavidin-coated quantum dots. In the present work, we describe the effects of manipulating the structure and chemistries of the MT building blocks on the morphology and behavior of the resulting ring nanocomposites. Segmented, heterostructured MTs were generated via head-to-tail self-assembly of biotinylated and non-biotinylated segments mixed at different molar ratios. The assembly and transformation of ring nanostructures generated by these MTs was characterized over time, and revealed a dependence of ring morphology on relative concentration of biotinylated segments. In contrast, varying the relative concentration of biotinylated segments did not influence the overall size of the composite rings. Precisely, the outer diameter and thickness of rings remained consistent and independent of biotinylated segments available, suggesting that the assembly process is able to manage “defects” presented by non-biotinylated segments. We, in fact, observe the adaptive nature of these composites in which defects (i.e., non-compliant segments) are either incorporated and/or actively removed from composite rings. Overall, this work provides a deeper understanding of the dynamic self-assembly and adaptability of hybrid nanostructures with the goal of developing self-regulating, multi-dimensional materials. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Current drug delivery methods include oral, intravenous, and transdermal administration, which expose the entire system to the biotherapeutic in an uncontrolled manner. This can lead to adverse side effects since only a small amount of the biotherapeutic is typically required to treat a target tissue. These short and long term side effects are the result of exposing otherwise healthy tissues to the biotherapeutic. In recent decades, hybrid drug delivery systems composed of thermoresponsive polymers and magnetic nanoparticles have been developed using chemical methods, such as emulsion polymerization, that are capable of remotely delivering controlled amounts of a biotherapeutic to a target tissue. Unfortunately, these methods can be expensive, time intensive, and produce composites with unsatisfactory polymer purity due to the necessity of using surfactants during synthesis. Here a spray-dry synthesis technique is explored in which aqueous poly(N-isopropylacrylamide) (PNIPAM) solution containing Fe₃O₄ magnetic nanoparticles is nebulized into droplets and made to pass through a drying chamber to reduce the droplet sizes by evaporation of water from the surface. The accompanying increase in the polymer density transforms the droplet into a gel capsule. The role of the magnetic particles is two-fold: (i) assist in guiding the polymeric capsules within the body, (ii) generate heat in an RF field to cause the capsule to shrink, and thereby expel the drugs. A study of the effect of drying temperature on the droplet size and the subsequent formation of the gel-state was observed by SEM and revealed a linear relation with an approximate rate of -0.3 µm/°C, which is in agreement with the a computational model. Dried polymer particles were dispersed in water and subjected to RF heating to investigate the drug delivery capability. The effect of capsule size and magnetic nanoparticle density on the required RF energy to eject contents from the capsule will be presented.

Biological iridescence from structural color has existed for over 500 million years and evolved for their maximum performance and multifunctionality. Structural colors are coloration deriving from optical phenomena including interference, diffraction and scattering of visible light in a periodic structure consisting of microscopic materials. In particular, surface-confined diffraction elements for the structural color are less abundant in nature than multilayer-based photonic structures; however, diffraction from surface periodicity is very important for bees to pollinate the flowers. Here, in order to mimic the floral iridescence produced by diffractive optics, we describe the creation of artificial photo-reconfigurable microspheres made of azobenzene molecule-containing materials (i.e., azomaterials) with the purpose of making it amenable for photofluidic shaping and texturing of colloidal particles by light irradiation. The directional photo-reconfiguration of azomaterials along with the light polarization has enabled the sophisticated engineering of micro/nanoarchitectures with exquisite control over large-area 2D/3D structural shapes and geometries (collectively referred to as directional photofluidization lithography). However, its unique feature of a path change approach to the micro/nanofabrication place severe restrictions on the range of available structural motifs and their practical applications because much of efforts have been mainly limited to a simple flat or pattern as pristine structures. To overcome this incompatible challenge and address the diversification of available azomaterial structural motifs, we exploit droplet-based microfluidic techniques for the high throughput generation of azomaterial microspheres. The microfluidic approach provides a platform for the generation of uniformly sized colloidal microspheres fully made of linearized azopolymeric chain. In particular, we achieve (i) 3D shaped colloidal microparticles and (ii) Janus microsphere with different surface textures, which can be useful for the diffractive grating-barcoded microparticles.

Self-powered micro/nanomotors have attracted attention because of their potential use as micro/nanomachines and for the transport and delivery of cargos. Many approaches have been proposed to realize self-powered micro/nano structures. Micromotors consisting of segmented gold/platinum (Au/Pt) microrods are representative examples. The Au/Pt microrods are chemically powered and can “swim” autonomously in an aqueous solution. On the other hand, there are only a few self-deformable micro/nanomotors. This presentation describes a self-deformable structure consisting of tethered multiple micromotors that shows “close-to-open” deformation for the first step to realize highly controlled motion. The tethered micromotor was fabricated by combining hard inorganic segments and soft organic tethers.

The tethered Au/Pt microrods were fabricated in a porous alumina template with an array of nanoholes of a uniform diameter (200 nm). First, the nanoholes were covered with a flexible polymer bilayer formed by layer-by-layer self-assembly. Then, Ag/Pt/Au/Ag/Au/Pt rods were electrodeposited sequentially in the nanoholes. The length of each segment can be controlled by changing the charge for electrodeposition. The bottom sacrificial Ag segment and unwanted polymer bilayers were removed. Finally, the alumina template and the middle sacrificial Ag segment were dissolved in a 3 M NaOH and 2 M HNO₃ solution, respectively, to isolate the tethered micromotors grown in the template.

First, the Au/Pt micromotors consisting of a 1 μm Au segment and a 1μm Pt segment were fabricated. The motors moved in an aqueous H₂O₂ solution (3wt%) at ~10 μm/s. As a step toward realizing self-deformable micromotors, a structure consisting of two Au/Pt micromotors tethered by a flexible polymer tube was then fabricated. The behavior of the tethered micromotors was examined in an aqueous H₂O₂ solution. The rods of this scale tend to sink at the bottom and are expected to be oriented in parallel to the substrate. This is advantageous to measure the change in the angle θ of the tethered structure under the two-dimensional video observation. Using 2D observation, the angle θ between the two Au/Pt microrods was traced. The “close-to-open” motion of the micromotors was observed. This opening motion is considered to derive from individual translational movement of the two Au/Pt microrods.

The structure can be a basis to realize sophisticated micro/nanomachines that may be used for applications such as micro-scaled surgery and drug delivery.

In living organisms, most biochemical processes are implemented and mediated by a series of enzymatic reactions in a cascade way. Such cascade reactions could occur specifically and efficiently, relying on the appropriate spatial arrangement of multi-enzyme on a scaffold (e.g., cytoskeleton or cell membrane) to avoid subsidiary reactions. Mimicking such type of reactions employed by nature is a great challenge, since a reliable scaffold capable of locating multi-enzyme with precisely controlled position and distance is hard to design and obtain in vitro. Developments of DNA nanotechnology provide a promising way to solve this problem, owing to the amazing properties of DNA structures, such as structural programmability, accurate addressability and site-specific functionalization. Herein, we use a tweezer-like DNA machine as a scaffold to locate two cascade enzymes, glucose oxidase (GOx) and horseradish peroxidase (HRP), on each end of two arms. These two enzymes were covalently modified with single strand DNA and interacted with DNA machine via base pairing hybridization. A DNA motor, in the DNA nanomachine, can switch between stem-loop and double helix structures driven by strand displacement reaction, cycling DNA machine between open and closed state, which changed the distance between the two enzymes. Since the diffusion distance of the intermediate product play an important role in the cascade reaction, the efficiency of this enzyme cascade reaction could be regulated by the DNA machine in situ. When the DNA machine was in the closed state, two cascade enzymes were hold together by two arms to keep them very close, and the efficiency of enzyme cascade reaction is approximately 88% higher than cascade reaction catalyzed by both free enzymes without any modification. After adding "fuel" strands, DNA machine opened its two arms, bringing two enzymes apart, which caused an obvious decrease of reaction efficiency. The distances between the ends of two arms are about 6 nm and 18 nm for closed and open state, respectively. By sequential addition of "fuel" and "antifuel" strands to the enzyme-functionalized DNA machine system, we can reversibly regulate the enzyme cascade reaction.

Supramolecular chemistry provide nowadays an excellent prospect to construct reversible biological interfaces that can be employed for supramolecular cell manipulation experiments.[1] Making use of supramolecular chemistry is rewarding to develop functional materials and devices. Knowing the limitations involved in ordering proteins at different length scales will surely hasten developing future applications, supramolecular bionanotechnology being the most prominent. The construction of synthetic supramolecular assemblies of proteins provides an excellent tool to fabricate organized bioactive components at surfaces. I will present new synthetic procedures for site-specific noncovalent anchoring of proteins to surfaces and polymers.[2-4] Special attention is paid to orientational and conformational aspects at the surface and will be demonstrated. Using concepts of multivalency the interactions between proteins and surfaces can be modulated by design. Many of the protein complexes were patterned on surfaces using microcontact printing or nanolithography and visualized using fluorescence microscopy.[5] Furthermore, supramolecular linkers that are sensitive to remote electrochemical and/or optical stimuli will be presented, using cucurbituril (CB)[2] and cyclodextrin (CD)-modified surfaces[4, 6]. Electrochemical switching was studied using surface embedded electrodes.[8] Cell adhesion, spreading and release was studied in detail in the case of cell-adhesive peptides and growth factors.[8] With the development of reversible bioactive platforms on surfaces serving as a reversible dynamic interfaces to cells, improved scaffolds for tissue regeneration will become in hand. First steps into this directions will be introduced as well.
[1] J. Brinkmann, et al., Chem. Soc. Rev. 2014, 43, 4449. [2] A. González-Campo, et al., Small 2012, 8, 3531. [3] L. Yang, et al., J. Am. Chem. Soc. 2012, 134, 19199. [4] D. Wasserberg, et al., J. Am. Chem. Soc. 2013, 135, 3104. [5] J. Voskuhl, et al., Curr. Opin. Chem. Biol. 2014, 18, 1. [6] S. Sankaran, et al., ACS Nano 2015, 4, 3579. [7] J. Voskuhl, Chem. Commun. 2014, 50, 15144. [8] Q. An, et al., Angew. Chem. Int. Ed. 2012, 51, 12233.

Manipulating surface wettability and liquid dynamics on patterned surfaces is of great interest for a wide range of applications, including water harvesting, digital lab-on-a-chip, thermal management, inkjet printing and spray cooling. With the advancement in surface engineering, lotus leaf inspired superhydrophobic surfaces have been extensively studied to achieve excellent liquid repellency. In addition, by fabrication of various micro/nanoscale topographic features and selective chemical patterning, the surfaces can enable directional liquid motion (either rolling or spreading).
Another conceptually different liquid repellent surface was inspired by the slippery pitcher plant, which is composed of a liquid lubricating film stably locked by a micro/nano textured substrate. This liquid-infused micro/nano structured surfaces exhibit exceptional performance in liquid repellency for enhanced condensation, anti-iciting and anti-biofouling. Also, mechanical stimulus and temperature control have produced tunable wetting dynamics. However, in all of these research attempts, the wetting behavior is isotropic.
Here, we demonstrate that by the design of asymmetric groove-like liquid-infused micro/nanostructured surfaces, the droplet preferred to slide along the direction parallel to the groove, and even got pinned in the direction orthogonal to the groove, therefore, anisotropic wetting is observed with ratio of velocities in two direction as high as ~ 10². Via experiments and modeling, we determined that the dynamics of liquid sliding depends on the size of the groove and lubricant height locked by capillary filling. The modeling, based on the energy analysis, provides good agreement with experimental data. We anticipate that the insights gained from this work should provide potential new avenue to dynamically control the wetting states on demand for optimal application performance.

System-level functions of living systems, such as homeostasis, bistability, and temporal pattern formation, are controlled by complex chemical reaction networks (CRNs) whose characteristics transcend the properties of individual molecules and reactions. Despite substantial efforts in the engineering of complex molecular systems, the construction of functional out-of-equilibrium networks that take advantage of the versatility of synthetic chemistry remains a major challenge. Recently,¹ we presented a versatile strategy for ‘synthesizing’ programmable enzymatic reaction networks in microfluidic flow reactors that exhibit sustained oscillations. The production of an oscillatory concentration of an active enzyme holds considerable potential for coupling to stimuli-responsive gels and other smart materials, which opens up applications in tissue engineering and soft robotics. Our work forms the basis of a bottom-up synthetic biology approach to the development of complex synthetic systems that operate according to the principles of life. In this lecture I will outline how such CRNs could become the controlling elements in ‘living materials’.

Polynucleotide co-polymers promise a rich micellization behavior in solution and hold promise for novel functional materials in nano- and biotechnological applications. Here I report on the synthesis of biologically-inspired polynucleotides with well-defined sequence, dispersity, and assembly function that have large potential for applications ranging from delivery vehicles of medical therapeutics, sensing applications, to scaffolds for nanowires. Specifically, we exploit the ability of a specific DNA polymerase, terminal deoxynucleotidyl transferase (TdT), to polymerize long chains of single stranded DNA (ssDNA) and to incorporate unnatural nucleotides with useful functional groups into the growing polynucleotide chain.

This talk will describe the development of an approach for optically controlling receptor mechanics. In principle, the most desirable approaches for manipulation within biological systems are optical-based. This is evidenced by the rapid proliferation of photo-stimulation techniques employing caged or photoswitchable molecules, and optogenetic constructs. Therefore, the development of methods to harness light for delivering precise physical inputs to biological systems could potentially transform the study of mechanotransduction. This is achieved using optomechanical actuator nanoparticles that are controlled with non-invasive near-infrared light. Illumination leads to particle collapse, delivering piconewton forces to specific cell surface receptors with high spatial and temporal resolution. As a proof-of-concept, we applied optomechanical actuation to trigger integrin-based focal adhesion formation, cell protrusion and migration, as well as T cell receptor activation.

Reference: Zheng et al. Nature Methods, 2015

Lipid membranes form a 2D interface which imposes fundamental constraints on the self-assembly of membrane proteins such as clathrin. To satisfy its biological function of endocytosis, clathrin’s form is governed by these constraints and provides insights as to how similar self-assembling systems might be rationally designed. We have created micron scale chemically functionalized particles with structures similar to clathrin that can self-assemble at a fluid-fluid interface, which we term “Active Clathrin Mimics” (ACMs). By tailoring the structure of these ACMs we not only control the extended self-assembly but also avoid irreversible aggregation during the transition from a 3D bulk fluid phase to a 2D fluid-fluid interface. Once the ACMs are self-assembled at the interface via surface tension, they can be manipulated to cause invagination of one phase within the other through an external magnetic field. Due to the precise and flexible nature of photolithography as a fabrication system there is great potential to create many different bio-inspired networks similar to ACM.

The efficient separation and purification of metal ions from mixture fluids are important for a variety of purposes ranging from recovery of the precious catalytic metal ions to wastewater treatment and heavy metal detection. Inspired by the biological active transfer process that efficiently and seamlessly synchronize transport and localization of metal ions from one side of the cytomembrane to another through hierarchical sense-and-response structures, we integrated chemo-mechanically responsive structures and chemistries into a robust dynamic system demonstrating concerted catch and release or local concentration self-regulation of target molecules from mixed aqueous fluids at 95% sorting efficiency (He, et al. Nature 2012, Nature Chemistry 2015). Recently we explored the capability of this versatile system on metal ion removal and recycling. This non-conventional approach uniquely couples the pH-dependency of the ligand-metal ion association and the pH-responsiveness of hydrogel to realize programmable continuous catch-transport-release of metal ions from mixture fluid. Specifically, the ligand bearing flexible epoxy microstructures repetitively swing between chemical distinct fluids, transferring the target metal ions from mixture fluid to another to collect them for recycling and downstream analysis. This modular broad-based platform can be customized to separate diverse metal ions by changing binding ligands and “smart” hydrogel, showing advantages over conventional complex, costly separation methods on energy-efficient, programmable separation and purification. This new bioinspired paradigm of resource recovery from complex waste streams holds great potential in contaminant removal for numerous industrial and environmental applications.

Ensembles of interacting colloidal particles subject to an external energy injection often develop nontrivial collective behavior and dynamic (active) self-assembled phases. Dispersions of magnetic colloids suspended at a liquid-air or liquid-liquid interface and driven away-from-equilibrium by a transversal alternating magnetic field develop nontrivial dynamic self-assembled structures not generally available through thermodynamic processes. Experiments reveal new types of nontrivially ordered phases (“asters”, “magnetic snakes”) emerging in such systems in a certain range of excitation parameters. Induced self-propulsion of robust aster-like structures in a presence of small in-plane DC field perturbations has been established. A possibility of a directed cargo transport at the interface by self-assembled structures has been demonstrated
Nontrivial active self-assembly in magnetic suspensions suspended at liquid-air interfaces and driven out of equilibrium by uniaxial alternating magnetic fields applied parallel to the interface have been observed. New types of dynamic self-assembled structures are reported, ranging from gas of rotators to dynamic wires, emerging in such systems in a certain range of excitation parameters. These remarkable magnetic non-equilibrium structures emerge as a result of the competition between magnetic and hydrodynamic forces and have complex magnetic ordering. Transitions between different self-assembled phases with parameters of external driving field are observed. Molecular dynamic simulations capturing microscopic mechanisms of the non-equilibrium self-assembly in our system have been developed [8]. The first principles model is based on the Navier-Stokes equation for liquids in shallow water approximation coupled to Newton equations for interacting magnetic particles suspended at a water-air interface.
The research was supported by the U.S. DOE, Office of Basic Energy Sciences, Division of Materials Science and Engineering.