pH-controlled assembly and properties of LbL membranes from branched conjugated poly(alkoxythiophene sulfonate) and various polycations

3D-to-3D Microscale Shape-Morphing from Configurable Helices with Controlled Chirality

Tunable and reconfigurable materials with autonomic shape transformation in response to the environment have emerged as one of the most promising approaches for a variety of biomedical applications, such as tissue engineering, biosensing, and in vivo biomedical devices. Currently, it is still quite challenging to fabricate soft, microscaled 3D shape-reconfigurable structures due to either complicated microfabrication or limited microscale photopolymerization-based printing approaches to enable adaptive shape transformation. Here, a one-step photo-cross-linking approach has been demonstrated to obtain a 3D-to-3D morphological transformable microhelix from a self-rolled hydrogel microsheet, resulting in chirality conversion. It was enabled by a custom-designed "hard" stripe/"soft" groove topography on the microsheets for introducing, which introduced both in-planar and out-of-planar anisotropies. Both experiment and simulation confirmed that a stripe/groove geometry can effectively control the 3D transformation by activating in-planar or/and out-of-planar mismatch stress within the microsheets, resulting in switching of the rolling direction between perpendicular/parallel to the length of the stripe. Furthermore, versatile 3D microconstructs with the ability to transform between two distinct 3D configurations have been achieved based on controlled rolling of microhelices, demonstrated as "windmill"-to-"T-cross" and "cylinder"-to-"scroll" transformations and dynamic blossoming of biomimetic orchids. In contrast to conventional 2D-to-3D micro-origami, we have successfully demonstrated an approach for fabricating microscale, all-soft-material-based constructs with autonomic 3D-to-3D structural transformation, which presents an opportunity for designing more complex hydrogel-based microrobotics.

Nanoporous Silver Submicrocubes Layer by Layer Encapsulated with Polyelectrolyte Films: Nonenzymatic Catalysis for Glucose Monitoring

This article describes the synthesis of nanoporous silver submicrocubes (Np-Ag) capped with poly(allylamine hydrochloride) PAH/poly(styrenesulfonate) PSS bilayers (Np-Ag(PAH/PSS)_n, 1 ≤ n ≤ 4) via layer-by-layer (LBL) assembly for the electrochemical glucose sensing. The consecutive LBL encapsulation of Np-Ag (average size ≈530 nm) with positively charged PAH and negatively charged PSS layers was monitored by using ζ-potential analyses, which showed that the sign of the ζ-potential became positive (+10 mV) or negative (-22 mV) depending on the charge of the encapsulating species. The thickness of two PAH/PSS bilayers on the Np-Ag was estimated to be ∼4 nm (consistent with a literature value of ∼1 nm per PAH or PSS layer) on the basis of a high-resolution transmission electron microscopy image of the Np-Ag(PAH/PSS)₂. Moreover, the high quality of the polyelectrolyte capping on Np-Ag was evidenced by the elemental mapping analysis of particles (obtained by using high-angle annular dark-field scanning transmission electron microscopy), which showed a uniform spatial distribution of C, N, and S (derived from PAH and PSS layers). Among the four different Np-Ag(PAH/PSS)n (1 ≤ n ≤ 4) electrodes, Np-Ag(PAH/PSS)₂ exhibited the highest electrocatalytic activity toward glucose because of the optimal thickness and density of its polyelectrolyte films (fabricated onto Np-Ag). The (Np-Ag(PAH/PSS)₂ electrode demonstrated a detection limit of 20 μM, a sensitivity limit of 472.15 μA mM^-1 cm^-2, and a wide range of detection for glucose at concentrations as high as 23.3 mM along with good selectivity toward glucose. The findings of this study are expected to contribute to improvements in the fabrication and stability of various particle-type catalysts on an electrode surface and to efforts to optimize the device performance using the LBL encapsulation technique.

Design Molecular Topology for Wet-Dry Adhesion

Recent innovations highlight the integration of diverse materials with synthetic and biological hydrogels. Examples include brain-machine interfaces, tissue regeneration, and soft ionic devices. Existing methods of strong adhesion mostly focus on the chemistry of bonds and the mechanics of dissipation but largely overlook the molecular topology of connection. Here, we highlight the significance of molecular topology by designing a specific bond-stitch topology. The bond-stitch topology achieves strong adhesion between preformed hydrogels and various materials, where the hydrogels have no functional groups for chemical coupling, and the adhered materials have functional groups on the surface. The adhesion principle requires a species of polymer chains to form a bond with a material through complementary functional groups and form a network in situ that stitches with the polymer network of a hydrogel. We study the physics and chemistry of this topology and describe its potential applications in medicine and engineering.

Water-Soluble Polymeric Interfacial Material for Planar Perovskite Solar Cells

Interfacial materials play a critical role in photoelectric conversion properties as well as the anomalous hysteresis phenomenon of the perovskite solar cells (PSCs). In this article, a water-soluble polythiophene PTEBS was employed as a cathode interfacial material for PSCs. Efficient energy level aligning and improved film morphology were obtained due to an ultrathin coating of PTEBS. Better ohmic contact between the perovskite layer and the cathode also benefits the charge transport and extraction of the device. Moreover, less charge accumulation at the interface weakens the polarization of the perovskite resulting in a relatively quick response of the modified device. The ITO/PTEBS/CH₃NH₃PbI₃/spiro-MeOTAD/Au cells by an all low-temperature process achieved power conversion efficiencies of up to 15.4% without apparent hysteresis effect. Consequently, the utilization of this water-soluble polythiophene is a practical approach for the fabrication of highly efficient, large-area, and low-cost PSCs and compatible with low-temperature solution process, roll-to-roll manufacture, and flexible application.

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 β-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 β-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.

Cellulose Nanocrystal Microcapsules as Tunable Cages for Nano- and Microparticles

We demonstrate the fabrication of highly open spherical cages with large through pores using high aspect ratio cellulose nanocrystals with "haystack" shell morphology. In contrast to traditional ultrathin shell polymer microcapsules with random porous morphology and pore sizes below 10 nm with limited molecular permeability of individual macromolecules, the resilient cage-like microcapsules show a remarkable open network morphology that facilitates across-shell transport of large solid particles with a diameter from 30 to 100 nm. Moreover, the transport properties of solid nanoparticles through these shells can be pH-triggered without disassembly of these shells. Such behavior allows for the controlled loading and unloading of solid nanoparticles with much larger dimensions than molecular objects reported for conventional polymeric microcapsules.

Robust microcapsules with controlled permeability from silk fibroin reinforced with graphene oxide

Robust and stable microcapsules are assembled from poly-amino acid-modified silk fibroin reinforced with graphene oxide flakes using layer-by-layer (LbL) assembly, based on biocompatible natural protein and carbon nanosheets. The composite microcapsules are extremely stable in acidic (pH 2.0) and basic (pH 11.5) conditions, accompanied with pH-triggered permeability, which facilitates the controllable encapsulation and release of macromolecules. Furthermore, the graphene oxide incorporated into ultrathin LbL shells induces greatly reinforced mechanical properties, with an elastic modulus which is two orders of magnitude higher than the typical values of original silk LbL shells and shows a significant, three-fold reduction in pore size. Such strong nanocomposite microcapsules can provide solid protection of encapsulated cargo under harsh conditions, indicating a promising candidate with controllable loading/unloading for drug delivery, reinforcement, and bioengineering applications.

ZnO nanowire array-templated LbL self-assembled polyelectrolyte nanotube arrays and application for charged drug delivery

Vertically oriented and robust polyelectrolyte nanotube arrays with high density, large area and high uniformity were successfully grown on substrates by a ZnO nanowire array-templated layer-by-layer (LbL) self-assembly approach for the first time, and were further used to deliver charged drugs, showing that they not only possess pH-responsive loading property, but also significantly enhance the loading capacity and sustained release time. This work could be extended to fabricate polyelectrolyte nanotube arrays with different polyelectrolyte combinations, including weak polyelectrolyte/weak polyelectrolyte, weak polyelectrolyte/strong polyelectrolyte and strong polyelectrolyte/strong polyelectrolyte. With the great versatility to use various substrates and building blocks, the polyelectrolyte nanotube arrays may have great potential for broad applications such as biosensor arrays, bioreactor arrays and optoelectronics.

Permeability and micromechanical properties of silk ionomer microcapsules

We studied the pH-responsive behavior of layer-by-layer (LbL) microcapsules fabricated from silk fibroin chemically modified with different poly amino acid side chains: cationic (silk-poly L-lysine, SF-PL) or anionic (silk-poly-L-glutamic acid, SF-PG). We observed that stable ultrathin shell microcapsules can be assembled with a dramatic increase in swelling, thickness, and microroughness at extremely acidic (pH < 2.5) and basic (pH > 11.0) conditions without noticeable disintegration. These changes are accompanied by dramatic changes in shell permeability with a 2 orders of magnitude increase in the diffusion coefficient. Moreover, the silk ionomer shells undergo remarkable softening with a drop in Young's modulus by more than 1 order of magnitude due to the swelling, stretching, and increase in material porosity. The ability to control permeability and mechanical properties over a wide range for the silk-based microcapsules, with distinguishing stability under harsh environmental conditions, provides an important system for controlled loading and release and applications in bioengineering.

Direct-write maskless lithography of LBL nanocomposite films and its prospects for MEMS technologies

Application of nanocomposites in MEMS, flexible electronics, and biomedical devices is likely to demonstrate new performance standards and resolve a number of difficult technical problems enabled by the unique combinations of electrical, optical, and mechanical properties. This study explores the possibility of making microscale nanocomposite patterns using the fusion of two highly versatile techniques: direct-write maskless UV patterning and layer-by-layer assembly (LBL). Together they can be applied to the production of a wide variety of nanostructured coatings with complex patterns. Single-walled carbon nanotube (SWNT) and gold nanoparticle LBL nanocomposites assembled with chitosan (CH) were made into prototypical patterns such as concentric helices and bus-line-and-stimulation-pads (BLASPs) used in flexible antennas and neuroprosthetic devices. The spatial resolution of the technique was established with the standard line grids to be at least 1 μm. Gold nanoparticle films revealed better accuracy and higher resolution in direct-write patterning than SWNT composites, possibly due to the granular rather than fibrous nature of the composites. The conductivity of the patterned composites was 6.45 × 10(-5)Ω m and 3.80 × 10(-6)Ω m at 20 °C for nanotube and nanoparticle composites, respectively; in both cases it exceeds electrical parameters of similar composites. Fundamental and technological prospects of nanocomposite MEMS devices in different areas including implantable biomedical, sensing, and optical devices are discussed.

Direct probing of micromechanical properties of hydrogen-bonded layer-by-layer microcapsule shells with different chemical compositions

The mechanical properties of hydrogen-bonded layer-by-layer (LbL) microcapsule shells constructed from tannic acid (TA) and poly(vinylpyrrolidone) (PVPON) components have been studied in both the dry and swollen states. In the dry state, the value of the elastic modulus was measured to be within 0.6-0.7 GPa, which is lower than the typical elastic modulus for electrostatically assembled LbL shells. Threefold swelling of the LbL shells in water results in a significant reduction of the elastic modulus to values well below 1 MPa, which is typical value seen for highly compliant gel materials. The increase of the molecular weight of the PVPON component from 55 to 1300 kDa promotes chain entanglements and causes a stiffening of the LbL shells with a more than 2-fold increase in elastic modulus value. Moreover, adding a polyethylenimine prime layer to the LbL shell affects the growth of hydrogen-bonded multilayers which consequently results in dramatically stiffer, thicker, and rougher LbL shells with the elastic modulus increasing by more than an order of magnitude, up to 4.3 MPa. An alternation of the elastic properties of very compliant hydrogen-bonded shells by variation of molecular weight is a characteristic feature of weakly bonded LbL shells. Such an ability to alter the elastic modulus in a wide range is critically important for the design of highly compliant microcapsules with tunable mechanical stability, loading ability, and permeability.