Tunable Interfacial Properties in Silk Ionomer Microcapsules with Tailored Multilayer Interactions.

Microencapsulation techniques represent a critical step in realizing highly controlled transport of functional materials in multiphase systems. The first demonstration of microcapsules prepared from minimally grafted silk ionomers (silk fibroin modified with cationic/anionic charge groups) are presented here. These tailored biomacromolecules have shown significantly increased biocompatibility over traditional polyelectrolytes and heavily grafted silk ionomers, but the low grafting density had previously limited attempts to fabricate stable microcapsules. In addition, the first microcapsules from polyethylene-glycol-grafted silk ionomers are fabricated and the corresponding impact on microcapsule behavior is demonstrated. The materials are shown to exhibit pH-responsive properties, with the microcapsules demonstrating an approx. tenfold decrease in stiffness and an approx. threefold change in diffusion coefficient when moving from acidic to basic buffer. Finally, the effect of assembly conditions of the microcapsules are shown to play a large role in determining final properties, with microcapsules prepared in acidic buffers showing lower roughness, stiffness, and an inversion in transport behavior (i.e., permeability decreases at higher pH).

Synthesis, Assembly, and Applications of Hybrid Nanostructures for Biosensing.

The robust, sensitive, and selective detection of targeted biomolecules in their native environment by prospective nanostructures holds much promise for real-time, accurate, and high throughput biosensing. However, in order to be competitive, current biosensor nanotechnologies need significant improvements, especially in specificity, integration, throughput rate, and long-term stability in complex bioenvironments. Advancing biosensing nanotechnologies in chemically "noisy" bioenvironments require careful engineering of nanoscale components that are highly sensitive, biorecognition ligands that are capable of exquisite selective binding, and seamless integration at a level current devices have yet to achieve. This review summarizes recent advances in the synthesis, assembly, and applications of nanoengineered reporting and transducing components critical for efficient biosensing. First, major classes of nanostructured components, both inorganic reporters and organic transducers, are discussed in the context of the synthetic control of their individual compositions, shapes, and properties. Second, the design of surface functionalities and transducing path, the characterization of interfacial architectures, and the integration of multiple nanoscale components into multifunctional ordered nanostructures are extensively examined. Third, examples of current biosensing structures created from hybrid nanomaterials are reviewed, with a distinct emphasis on the need to tailor nanosensor designs to specific operating environments. Finally, we offer a perspective on the future developments of nanohybrid materials and future nanosensors, outline possible directions to be pursued that may yield breakthrough results, and envision the exciting potential of high-performance nanomaterials that will cause disruptive improvements in the field of biosensing.

Interfacial Shear Strength and Adhesive Behavior of Silk Ionomer Surfaces.

The interfacial shear strength between different layers in multilayered structures of layer-by-layer (LbL) microcapsules is a crucial mechanical property to ensure their robustness. In this work, we investigated the interfacial shear strength of modified silk fibroin ionomers utilized in LbL shells, an ionic-cationic pair with complementary ionic pairing, (SF)-poly-l-glutamic acid (Glu) and SF-poly-l-lysine (Lys), and a complementary pair with partially screened Coulombic interactions due to the presence of poly(ethylene glycol) (PEG) segments and SF-Glu/SF-Lys[PEG] pair. Shearing and adhesive behavior between these silk ionomer surfaces in the swollen state were probed at different spatial scales and pressure ranges by using functionalized atomic force microscopy (AFM) tips as well as functionalized colloidal probes. The results show that both approaches were consistent in analyzing the interfacial shear strength of LbL silk ionomers at different spatial scales from a nanoscale to a fraction of a micron. Surprisingly, the interfacial shear strength between SF-Glu and SF-Lys[PEG] pair with partially screened ionic pairing was greater than the interfacial shear strength of the SF-Glu and SF-Lys pair with a high density of complementary ionic groups. The difference in interfacial shear strength and adhesive strength is suggested to be predominantly facilitated by the interlayer hydrogen bonding of complementary amino acids and overlap of highly swollen PEG segments.

Immobilization of Recombinant E. coli Cells in a Bacterial Cellulose-Silk Composite Matrix To Preserve Biological Function.

Strategies for the encapsulation of cells for the design of cell-based sensors require efficient immobilization procedures while preserving biological activity of the reporter cells. Here, we introduce an immobilization technique that relies upon the symbiotic relationship between two bacterial strains: cellulose-producing Gluconacetobacter xylinus cells; and recombinant Escherichia coli cells harboring recombinase-based dual-color synthetic riboswitch (RS), as a model for cell-based sensor. Following sequential coculturing of recombinant cells in the cellulose matrix, final immobilization of E. coli cells was completed after reconstituted silk fibroin (SF) protein was added to a "living membrane" generating the composite bacterial cellulose-silk fibroin (BC-SF) scaffold. By controlling incubation parameters for both types of cells, as well as the conformations in SF secondary structure, a variety of robust composite scaffolds were prepared ranging from opaque to transparent. The properties of the scaffolds were compared in terms of porosity, water capacity, distribution of recombinant cells within the scaffolds matrix, onset of cells activation, and ability to protect recombinant function of cells against UV irradiation. The closer-fitted microstructure of transparent BC-SF scaffolds resulted in leakage-free encapsulation of recombinant cells with preserved RS function because of a combination of several parameters that closely matched properties of a biofilm environment. Along with proper elasticity, fine porosity, capacity to retain the water, and ability of SF to absorb UV light, the composite hydrogel material provided necessary conditions to form confined cell colonies that modified cell metabolism and enhanced cell resilience to the stresses induced by encapsulation.

Bimorph Silk Microsheets with Programmable Actuating Behavior: Experimental Analysis and Computer Simulations.

Microscaled self-rolling construct sheets from silk protein material have been fabricated, containing a silk bimorph composed of silk ionomers as an active layer and cross-linked silk ß-sheet as the passive layer. The programmable morphology was experimentally explored along with a computational simulation to understand the mechanism of shape reconfiguration. The neutron reflectivity shows that the active silk ionomers layer undergoes remarkable swelling (eight times increase in thickness) after deprotonation while the passive silk ß-sheet retains constant volume under the same conditions and supports the bimorph construct. This selective swelling within the silk-on-silk bimorph microsheets generates strong interfacial stress between layers and out-of-plane forces, which trigger autonomous self-rolling into various 3D constructs such as cylindrical and helical tubules. The experimental observations and computational modeling confirmed the role of interfacial stresses and allow programming the morphology of the 3D constructs with particular design. We demonstrated that the biaxial stress distribution over the 2D planar films depends upon the lateral dimensions, thickness and the aspect ratio of the microsheets. The results allow the fine-tuning of autonomous shape transformations for the further design of complex micro-origami constructs and the silk based rolling/unrolling structures provide a promising platform for polymer-based biomimetic devices for implant applications.

Reconfigurable and actuating structures from soft materials.

The recent interest in reconfigurable soft materials may lead to the next paradigm in the development of adaptive and actuating materials and structures. Actuating soft materials eventually can be precisely designed to show stimuli-sensing, multi-length scale actuation, tunable transport, programmed shape control and multifunctional orthogonal responses. Herein, we discuss the various advances in the emerging field of reconfigurable soft materials with a focus on the various parameters that can be modulated to control a complex system behavior. In particular, we detail approaches that use either long-range fields (i.e. electrical, magnetic) or changes in local thermodynamic parameters (e.g., solvent quality) in order to elicit a precise dimensional and controlled response. The theoretical underpinnings and practical considerations for different approaches are briefly presented alongside several illustrative examples from the recent studies. In the end, we summarize recent accomplishments, critical issues to consider, and give perspectives on the developments of this exciting research field.

A robust and facile approach to assembling mobile and highly-open unfrustrated triangular lattices from ferromagnetic nanorods.

A simple and widely applicable approach to assemble long-range two-dimensional mobile arrays of functionalized nickel nanorods with tunable and "highly open" lattice structures is presented. The magnetic assembly of uniformly oriented nanorods in triangular lattices was achieved by a phase separation of the surface confined yet mobile vertical nanorods driven by a gradient magnetic field. In contrast to known approaches, the unfrustrated lattices can be further locked in place allowing for the removal of the applied magnetic field and processing without disrupting the initial order with different symmetries precisely assembled and locked in their position on the same substrate. We suggest that the tunable assemblies of magnetic nanorods provide a versatile platform for downstream handling of open lattice arrays for eventual device integration.