Shape switching of hollow layer-by-layer hydrogel microcontainers

Surface Modification with Particles Coated or Made of Polymer Multilayers

The coating of particles or decomposable cores with polyelectrolytes via Layer-by-Layer (LbL) assembly creates free-standing LbL-coated functional particles. Due to the numerous functions that their polymers can bestow, the particles are preferentially selected for a plethora of applications, including, but not limited to coatings, cargo-carriers, drug delivery vehicles and fabric enhancements. The number of publications discussing the fabrication and usage of LbL-assembled particles has consistently increased over the last vicennial. However, past literature fails to either mention or expand upon how these LbL-assembled particles immobilize on to a solid surface. This review evaluates examples of LbL-assembled particles that have been immobilized on to solid surfaces. To aid in the formulation of a mechanism for immobilization, this review examines which forces and factors influence immobilization, and how the latter can be confirmed. The predominant forces in the immobilization of the particles studied here are the Coulombic, capillary, and adhesive forces; hydrogen bonding as well as van der Waal's and hydrophobic interactions are also considered. These are heavily dependent on the factors that influenced immobilization, such as the particle morphology and surface charge. The shape of the LbL particle is related to the particle core, whereas the charge was dependant on the outermost polyelectrolyte in the multilayer coating. The polyelectrolytes also determine the type of bonding that a particle can form with a solid surface. These can be via either physical (non-covalent) or chemical (covalent) bonds; the latter enforcing a stronger immobilization. This review proposes a fundamental theory for immobilization pathways and can be used to support future research in the field of surface patterning and for the general modification of solid surfaces with polymer-based nano- and micro-sized polymer structures.

Two-Dimensional and Three-Dimensional Ultrathin Multilayer Hydrogels through Layer-by-Layer Assembly

Stimuli-responsive multilayer hydrogels have opened new opportunities to design hierarchically organized networks with properties controlled at the nanoscale. These multilayer materials integrate structural, morphological, and compositional versatility provided by alternating layer-by-layer polymer deposition with the capability for dramatic and reversible changes in volumes upon environmental triggers, a characteristic of chemically cross-linked responsive networks. Despite their intriguing potential, there has been limited knowledge about the structure-property relationships of multilayer hydrogels, partly because of the challenges in regulating network structural organization and the limited set of the instrumental pool to resolve structure and properties at nanometer spatial resolution. This Feature Article highlights our recent studies on advancing assembly technologies, fundamentals, and applications of multilayer hydrogels. The fundamental relationships among synthetic strategies, chemical compositions, and hydrogel architectures are discussed, and their impacts on stimuli-induced volume changes, morphology, and mechanical responses are presented. We present an overview of our studies on thin multilayer hydrogel coatings, focusing on controlling and quantifying the degree of layer intermixing, which are crucial issues in the design of hydrogels with predictable properties. We also uncover the behavior of stratified "multicompartment" hydrogels in response to changes in pH and temperature. We summarize the mechanical responses of free-standing multilayer hydrogels, including planar thin coatings and films with closed geometries such as hollow microcapsules and nonhollow hydrogel microparticles with spherical and nonspherical shapes. Finally, we will showcase potential applications of pH- and temperature-sensitive multilayer hydrogels in sensing and drug delivery. The knowledge about multilayer hydrogels can advance the rational design of polymer networks with predictable and well-tunable properties, contributing to modern polymer science and broadening hydrogel applications.

Anisotropic Particles through Multilayer Assembly

The anisotropy in the shape of polymeric particles has been demonstrated to have many advantages over spherical particulates, including bio-mimetic behavior, shaped-directed flow, deformation, surface adhesion, targeting, motion, and permeability. The layer-by-layer (LbL) assembly is uniquely suited for synthesizing anisotropic particles as this method allows for simple and versatile replication of diverse colloid geometries with precise control over their chemical and physical properties. This review highlights recent progress in anisotropic particles of micrometer and nanometer sizes produced by a templated multilayer assembly of synthetic and biological macromolecules. Synthetic approaches to produce capsules and hydrogels utilizing anisotropic templates such as biological, polymeric, bulk hydrogel, inorganic colloids, and metal-organic framework crystals as sacrificial templates are overviewed. Structure-property relationships controlled by the anisotropy in particle shape and surface are discussed and compared with their spherical counterparts. Advances and challenges in controlling particle properties through varying shape anisotropy and surface asymmetry are outlined. The perspective applications of anisotropic colloids in biomedicine, including programmed behavior in the blood and tissues as artificial cells, nano-motors/sensors, and intelligent drug carriers are also discussed.

Biodegradable Polymeric Multilayer Capsules for Therapy of Lung Cancer

Formulated forms of cancer therapeutics enhance the efficacy of treatment by more precise targeting, increased bioavailability of drugs, and an aptitude of some delivery systems to overcome multiple drug resistance of tumors. Drug carriers acquire importance for anti-cancer interventions via targeting tumor-associated macrophages with active molecules capable to either eliminate them or change their polarity. Although several packaged drug forms have reached the market, there is still a high demand for novel carrier systems to hurdle limitations of existing drugs on active molecules, toxicity, bioeffect, and stability. Here, we report a facile assembly and delivery methodology for biodegradable polymeric multilayer capsules (PMC) with the purpose of further use in injectable drug formulations for lung cancer therapy via direct erosion of tumors and suppression of the tumor-promoting function of macrophages in the tumor microenvironment. We demonstrate delivery of low-molecular-weight drug molecules to lung cancer cells and macrophages and provide details on in vivo distribution, cellular uptake, and disintegration of the developed PMC. Poly-l-arginine and dextran sulfate alternately adsorb on a ∼500 nm CaCO₃ sacrificial template followed by removal of the inorganic core to obtain hollow capsules for consequent loading with drug molecules, gemcitabine or clodronate. The capsules further compacted upon loading down to ∼250 nm in diameter via heat treatment. A comparative study of the capsule internalization rate in vitro and in vivo reveals the benefits of a diminished carrier size. We show that macrophages and epithelial cells of the lungs and liver internalize capsules with efficacy higher than 75%. Using an in vivo mouse model of lung cancer, we also confirm that tumor lungs better retain smaller capsules than the healthy lung tissue. The pronounced cytotoxic effect of the encapsulated gemcitabine on lung cancer cells and the ability of the encapsulated clodronate to block the tumor-promoting function of macrophages prove the efficacy of the developed capsule loading method in vitro. Our study taken as a whole demonstrates the great potential of the developed PMC for in vivo treatment of cancer via transporting active molecules, including those that are water-soluble with low molecular weight, to both cancer cells and macrophages through the bloodstream.

Shape Recovery of Spherical Hydrogen-Bonded Multilayer Capsules after Osmotically Induced Deformation

The mechanical properties of microparticles intended for in vivo applications as drug delivery vehicles are among important parameters that influence their circulation in the blood and govern particle biodistribution. We report on the synthesis of soft but mechanically robust spherical capsules via a hydrogen-bonded multilayer assembly of (poly(N-vinylpyrrolidone), M_w = 10 000 g mol^-1) with (poly(methacrylic acid) M_w = 100 000 g mol^-1)) (PVPON/PMAA)_n in methanol using 4 μm nonporous silica microparticles as sacrificial templates, where n = 5 and 10 and represents the bilayer number. The mechanical properties of (PVPON/PMAA)_n spherical capsules were assessed using the osmotic pressure difference method and resulted in an elasticity modulus of 97 ± 8 MPa, which is in the range of Young's modulus for elastomeric networks. We also found that hydrogen-bonded (PVPON/PMAA)₁₀ capsules demonstrated almost complete recovery from a concave buckled inward shape induced by the osmotic pressure difference from the addition of polystyrene sulfonate (PSS) to the capsule solution to their initial spherical shape within 12 h after the PSS solution was rinsed off. The permeability measurements through the capsule shell using fluorescently labeled dextran molecular probes revealed that the average mesh size of the hydrogen-bonded network assembled in methanol is in the range of 3 to 9 nm and is not permeable to FITC-dextran with a molecular weight of >40 000 g mol^-1. Our study shows that physically cross-linked polyelectrolyte multilayer capsules are capable of withstanding large deformations, which is essential to the development of adaptable particles for controlled delivery.

Multilayer Capsules Inside Biological Systems: State-of-the-Art and Open Challenges

There are many reports about the interaction of multilayer capsules with biological systems in the literature. A majority of them are devoted to the in vitro study with two-dimensional cell cultures. Multilayer capsule fabrication had been under intensive investigation from 1990s and 2000s by Prof. Helmuth Möhwald, and many of his followers further developed their own research directions, focusing on capsule implementation in various fields of biology and medicine. The aim of this future article is to consistently consider the most recent advances in cell-capsule interactions for different biomedical applications, including functionalization of clinically relevant cells, nonviral gene delivery, magnetization of cells to control their movement, and in vivo drug delivery. Finally, the description and discussion of the new trends and perspectives for improved functionalities of capsules in design and functionalization of cell-assisted drug vehicles are the major topics of this work.

Unusually fast and large actuation from multilayer polyelectrolyte thin films

Polymers responsive to external stimuli (e.g., electric field, chemical vapor, light) are of great interest for smart materials such as sensors and soft robotics. A vapor-driven multilayer polymer actuator, capable of fast and large-scale actuation, is described here. This Janus-like actuator is prepared with two polyelectrolyte multilayer systems (polyethylenimine (PEI)/poly(acrylic acid) (PAA) and polyurethane (PU)/poly(acrylic acid) (PAA)) using layer-by-layer assembly (LbL). The differing hydrophilicity of these two nanocoatings results in different swelling behavior in water and organic solvents, which leads to vapor-responsive mechanical motion. The bending/curling degree of this polymeric actuator can be precisely controlled by changing the thickness ratio of the two layers. A vapor sensor was constructed to demonstrate the environmental detection ability of this unique actuator.

Encapsulation and Ultrasound-Triggered Release of G-Quadruplex DNA in Multilayer Hydrogel Microcapsules

Nucleic acid therapeutics have the potential to be the most effective disease treatment strategy due to their intrinsic precision and selectivity for coding highly specific biological processes. However, freely administered nucleic acids of any type are quickly destroyed or rendered inert by a host of defense mechanisms in the body. In this work, we address the challenge of using nucleic acids as drugs by preparing stimuli responsive poly(methacrylic acid)/poly(N-vinylpyrrolidone) (PMAA/PVPON)n multilayer hydrogel capsules loaded with ~7 kDa G-quadruplex DNA. The capsules are shown to release their DNA cargo on demand in response to both enzymatic and ultrasound (US)-triggered degradation. The unique structure adopted by the G-quadruplex is essential to its biological function and we show that the controlled release from the microcapsules preserves the basket conformation of the oligonucleotide used in our studies. We also show that the (PMAA/PVPON) multilayer hydrogel capsules can encapsulate and release ~450 kDa double stranded DNA. The encapsulation and release approaches for both oligonucleotides in multilayer hydrogel microcapsules developed here can be applied to create methodologies for new therapeutic strategies involving the controlled delivery of sensitive biomolecules. Our study provides a promising methodology for the design of effective carriers for DNA vaccines and medicines for a wide range of immunotherapies, cancer therapy and/or tissue regeneration therapies in the future.

Covalent layer-by-layer films: chemistry, design, and multidisciplinary applications

Covalent layer-by-layer (LbL) assembly is a powerful method used to construct functional ultrathin films that enables nanoscopic structural precision, componential diversity, and flexible design. Compared with conventional LbL films built using multiple noncovalent interactions, LbL films prepared using covalent crosslinking offer the following distinctive characteristics: (i) enhanced film endurance or rigidity; (ii) improved componential diversity when uncharged species or small molecules are stably built into the films by forming covalent bonds; and (iii) increased structural diversity when covalent crosslinking is employed in componential, spacial, or temporal (labile bonds) selective manners. In this review, we document the chemical methods used to build covalent LbL films as well as the film properties and applications achievable using various film design strategies. We expect to translate the achievement in the discipline of chemistry (film-building methods) into readily available techniques for materials engineers and thus provide diverse functional material design protocols to address the energy, biomedical, and environmental challenges faced by the entire scientific community.

Ultrathin Shape Change Smart Materials

With the discovery of graphene, significant research has focused on the synthesis, characterization, and applications of ultrathin materials. Graphene has also brought into focus other ultrathin materials composed of organics, polymers, inorganics, and their hybrids. Together, these ultrathin materials have unique properties of broad significance. For example, ultrathin materials have a large surface area and high flexibility which can enhance conformal contact in wearables and sensors leading to improved sensitivity. When porous, the short transverse diffusion length in these materials allows rapid mass transport. Alternatively, when impermeable, these materials behave as an ultrathin barrier. Such controlled permeability is critical in the design of encapsulation and drug delivery systems. Finally, ultrathin materials often feature defect-free and single-crystal-like two-dimensional atomic structures resulting in superior mechanical, optical, and electrical properties. A unique property of ultrathin materials is their low bending rigidity, which suggests that they could easily be bent, curved, or folded into 3D shapes. In this Account, we review the emerging field of 2D to 3D shape transformations of ultrathin materials. We broadly define ultrathin to include materials with a thickness below 100 nm and composed of a range of organic, inorganic, and hybrid compositions. This topic is important for both fundamental and applied reasons. Fundamentally, bending and curving of ultrathin films can cause atomistic and molecular strain which can alter their physical and chemical properties and lead to new 3D forms of matter which behave very differently from their planar precursors. Shape change can also lead to new 3D architectures with significantly smaller form factors. For example, 3D ultrathin materials would occupy a smaller space in on-chip devices or could permeate through tortuous media which is important for miniaturized robots and smart dust applications. Our Account highlights several differences between ultrathin and traditional shape change materials. The latter is typically associated with hydrogels, liquid crystals, or shape memory elastomers. As compared to bulk materials, ultrathin materials can much more easily bend and fold due to the significantly reduced bending modulus. Consequently, it takes much less energy to alter the shape of ultrathin materials, and even small environmental stimuli can trigger a large response. Further, the energy barriers between different configurations are small which allow a variety of conformations and enhances programmability. Finally, due to their ultrathin nature, the shape changes are typically not slowed down by sluggish mass or thermal transport, and thus, responses can be much faster than those of bulk materials. The latter point is important in the design of high-speed actuators. Consequently, ultrathin materials could enable low-power, rapid, programmable, and complex shape transformations in response to a broad range of stimuli such as pH, temperature, electromagnetic fields, or chemical environments. The Account also includes a discussion of applications, important challenges, and future directions.

Highly efficient delivery of potent anticancer iminoquinone derivative by multilayer hydrogel cubes

We report a novel delivery platform for a highly potent anticancer drug, 7-(benzylamino)-3,4-dihydro-pyrrolo[4,3,2-de]quinolin-8(1H)-one (BA-TPQ), using pH- and redox-sensitive poly(methacrylic acid) (PMAA) hydrogel cubes of micrometer size as the encapsulating matrix. The hydrogels are obtained upon cross-linking PMAA with cystamine in PMAA/poly(N-vinylpyrrolidone) multilayers assembled within mesoporous sacrificial templates. The BA-TPQ-loaded hydrogels maintain their cubical shape and pH-sensitivity after lyophilization, which is advantageous for long-term storage. Conversely, the particles degrade in vitro in the presence of glutathione (5mM) providing 80% drug release within 24h. Encapsulating BA-TPQ into hydrogels significantly increases its transport via Caco-2 cell monolayers used as a model for oral delivery where the apparent permeability of BA-TPQ-hydrogel cubes was∼2-fold higher than that of BA-TPQ. BA-TPQ-hydrogel cubes exhibit better anticancer activity against HepG2 (IC50=0.52µg/mL) and Huh7 (IC50=0.29µg/mL) hepatoma cells with a 40% decrease in the IC50 compared to the non-encapsulated drug. Remarkably, non-malignant liver cells have a lower sensitivity to BA-TPQ-hydrogel cubes with 2-fold increased IC50 values compared to those of cancer cells. In addition, encapsulating BA-TPQ in the hydrogels amplifies the potency of the drug via down-regulation of MDM2 oncogenic protein and upregulation of p53 (a tumor suppressor) and p21 (cell proliferation suppressor) expression in HepG2 liver cancer cells. Moreover, enhanced inhibition of MDM2 protein expression by BA-TPQ-hydrogel cubes is independent of p53 status in Huh7 cells. This drug delivery platform of non-spherical shape provides a facile method for encapsulation of hydrophobic drugs and can facilitate the enhanced efficacy of BA-TPQ for liver cancer therapy.

STATEMENT OF SIGNIFICANCE

Many potent anticancer drugs are hydrophobic and lack tumor selectivity, which limits their application in cancer therapy. Although cubical hydrogels of poly(methacrylic acid) exhibit excellent biocompatibility and versatility, they have not been investigated for hydrophobic drug delivery due to poor mechanical stability and incompatibility between hydrophobic drugs and a hydrophilic hydrogel network. In this study, we provide a facile method to prepare a multilayer hydrogel-based platform with controlled nanostructure, cubical shape and redox-responsiveness for delivery of highly potent anticancer therapeutics, hydrophobic BA-TPQ. The BA-TPQ-hydrogel cubes have exceptional structural stability upon lyophilization which is advantageous for a long-term storage. The greatly enhanced trans-epithelial permeability and amplified anti-tumor activity of BA-TPQ are achieved by encapsulation in these hydrogel cubes. Furthermore, the anticancer BA-TPQ-hydrogel platform retains the selective activity of BA-TPQ to hepatocellular carcinoma cells. Overall, the produced BA-TPQ-hydrogel cubes demonstrate a high potential for clinical liver cancer therapy.

Shaped stimuli-responsive hydrogel particles: syntheses, properties and biological responses

This review summarizes a pool of current experimental approaches and discusses perspectives in the development of the synergistic combination of shape and stimuli-response in particulate hydrogels.

Loading Capacity versus Enzyme Activity in Anisotropic and Spherical Calcium Carbonate Microparticles

A new method of fabrication of calcium carbonate microparticles of ellipsoidal, rhomboidal, and spherical geometries is reported by adjusting the relative concentration ratios of the initial salt solutions and/or the ethylene glycol content in the reaction medium. Morphology, porosity, crystallinity, and loading capacity of synthesized CaCO3 templates were characterized in detail. Particles harboring dextran or the enzyme guanylate kinase were obtained through encapsulation of these macromolecules using the layer-by-layer assembly technique to deposit positively and negatively charged polymers on these differently shaped CaCO3 templates and were characterized by confocal laser scanning fluorescence microscopy, fluorometric techniques, and enzyme activity measurements. The enzymatic activity, an important application of such porous particles and containers, has been analyzed in comparison with the loading capacity and geometry. Our results reveal that the particles' shape influences morphology of particles and that, as a result, affects the activity of the encapsulated enzymes, in addition to the earlier reported influence on cellular uptake. These particles are promising candidates for efficient drug delivery due to their relatively high loading capacity, biocompatibility, and easy fabrication and handling.

Transformative Two-Dimensional Array Configurations by Geometrical Shape-Shifting Protein Microstructures

Two-dimensional (2D) geometrical shape-shifting is prevalent in nature, but remains challenging in man-made "smart" materials, which are typically limited to single-direction responses. Here, we fabricate geometrical shape-shifting bovine serum albumin (BSA) microstructures to achieve circle-to-polygon and polygon-to-circle geometrical transformations. In addition, transformative two-dimensional microstructure arrays are demonstrated by the ensemble of these responsive microstructures to confer structure-to-function properties. The design strategy of our geometrical shape-shifting microstructures focuses on embedding precisely positioned rigid skeletal frames within responsive BSA matrices to direct their anisotropic swelling under pH stimulus. This is achieved using layer-by-layer two photon lithography, which is a direct laser writing technique capable of rendering spatial resolution in the sub-micrometer length scale. By controlling the shape, orientation and number of the embedded skeletal frames, we have demonstrated well-defined arc-to-corner and corner-to-arc transformations, which are essential for dynamic circle-to-polygon and polygon-to-circle shape-shifting, respectively. We further fabricate our shape-shifting microstructures in periodic arrays to experimentally demonstrate the first transformative 2D patterned arrays. Such versatile array configuration transformations give rise to structure-to-physical properties, including array porosity and pore shape, which are crucial for the development of on-demand multifunctional "smart" materials, especially in the field of photonics and microfluidics.

A facile template-free synthesis of pH-responsive polyelectrolyte/amorphous TiO2 composite hollow microcapsules for photocatalysis

This paper presents a novel, facile method for fabricating pH-responsive inorganic/organic composite microspheres with hollow structures in the absence of a step for the removal of the core.

Shape-controlled synthesis and lithium storage properties of SnO2 nonspherical hollow structures

SnO₂ hollow structures, such as peanuts, capsules and pseudocubes, have been controlled-synthesized by using SiO₂ colloids with different shapes as templates, and show enhanced lithium storage performances.

Internalization of red blood cell-mimicking hydrogel capsules with pH-triggered shape responses

We report on naturally inspired hydrogel capsules with pH-induced transitions from discoids to oblate ellipsoids and their interactions with cells. We integrate characteristics of erythrocytes such as discoidal shape, hollow structure, and elasticity with reversible pH-responsiveness of poly(methacrylic acid) (PMAA) to design a new type of drug delivery carrier to be potentially triggered by chemical stimuli in the tumor lesion. The capsules are fabricated from cross-linked PMAA multilayers using sacrificial discoid silicon templates. The degree of capsule shape transition is controlled by the pH-tuned volume change, which in turn is regulated by the capsule wall composition. The (PMAA)15 capsules undergo a dramatic 24-fold volume change, while a moderate 2.3-fold volume variation is observed for more rigid PMAA-(poly(N-vinylpyrrolidone) (PMAA-PVPON)5 capsules when solution pH is varied between 7.4 and 4. Despite that both types of capsules exhibit discoid-to-oblate ellipsoid transitions, a 3-fold greater swelling in radial dimensions is found for one-component systems due to a greater degree of the circular face bulging. We also show that (PMAA-PVPON)5 discoidal capsules interact differently with J774A.1 macrophages, HMVEC endothelial cells, and 4T1 breast cancer cells. The discoidal capsules show 60% lower internalization as compared to spherical capsules. Finally, hydrogel capsules demonstrate a 2-fold decrease in size upon internalization. These capsules represent a unique example of elastic hydrogel discoids capable of pH-induced drastic and reversible variations in aspect ratios. Considering the RBC-mimicking shape, their dimensions, and their capability to undergo pH-triggered intracellular responses, the hydrogel capsules demonstrate considerable potential as novel carriers in shape-regulated transport and cellular uptake.

Colloidal micro- and nano-particles as templates for polyelectrolyte multilayer capsules

Colloidal particles play an important role in various areas of material and pharmaceutical sciences, biotechnology, and biomedicine. In this overview we describe micro- and nano-particles used for the preparation of polyelectrolyte multilayer capsules and as drug delivery vehicles. An essential feature of polyelectrolyte multilayer capsule preparations is the ability to adsorb polymeric layers onto colloidal particles or templates followed by dissolution of these templates. The choice of the template is determined by various physico-chemical conditions: solvent needed for dissolution, porosity, aggregation tendency, as well as release of materials from capsules. Historically, the first templates were based on melamine formaldehyde, later evolving towards more elaborate materials such as silica and calcium carbonate. Their advantages and disadvantages are discussed here in comparison to non-particulate templates such as red blood cells. Further steps in this area include development of anisotropic particles, which themselves can serve as delivery carriers. We provide insights into application of particles as drug delivery carriers in comparison to microcapsules templated on them.

Compartmentalized multilayer hydrogel formation using a stimulus-responsive self-assembling polysaccharide

Polymeric systems that self-assemble through strong noncovalent bonds form structures that are highly dependent on the spatiotemporal sequence of cues that trigger self-assembly. Here, we prepared capsules with a semipermeable alginate-chitosan polyelectrolyte membrane that encapsulates a solution of the pH-responsive self-assembling aminopolysaccharide chitosan. Immersion of these capsules in a basic solution triggers gelation of the capsule contents, and the details of the gel-inducing treatment dramatically affect the final structure of the gelled compartment. Specifically, we show that the sequential transfer of the capsules between the base and water can generate multilayer hydrogel structures, with the thickness of each layer being controlled by the base concentration and immersion times. We further demonstrate that these multilayer hydrogels can serve as templates for the synthesis of iron oxide particles with a complex internal structure (i.e., with a multilayer internal structure). This work demonstrates the ability to enlist the stimulus-responsive self-assembling properties of biological polymers to create materials with complex structures.

pH-responsive hydrogel cubes for release of doxorubicin in cancer cells

Doxorubicin (DOX)-loaded poly(methacrylic acid) hydrogel cubes release the drug at pH <5. These hydrogels are developed for shape-directed cellular uptake for drug delivery.

Near-incompressible faceted polymer microcapsules from metal-organic framework templates

Faceted polymer microcapsules are prepared from metal-organic framework (MOF) templates. The MOF templates are removable under mild aqueous conditions. The obtained microcapsules are stiffer than their spherical counterparts, reflecting the near-incompressibility of the facet edges, and indicating that the faceting might be a useful strategy for controlling the mechanical properties of polymer microcapsules.

Fabrication of red-blood-cell-like polyelectrolyte microcapsules and their deformation and recovery behavior through a microcapillary

Multilayer microcapsules with a biconcave discoidal shape mimicking red blood cells (RBCs) are fabricated. The structure of the RBC-like microcapsules is verified by scanning electron and confocal laser scanning microscopies. The capsules show elastic deformation after being forced through a microcapillary with a smaller diameter, exhibiting a high recovery ratio of ≈90%. When the capsules are coated with hemoglobin (Hb),they are able to reversibly bind and release oxygen.

Shape-controlled synthesis of hollow silica colloids

In this work, hollow silica colloids with different shapes, such as pseudocubes, ellipsoids, capsules, and peanuts, have been synthesized through the following process: silica coating on the surface of hematite colloidal particles with different shapes (pseudocubes, ellipsoids, capsules, and peanuts) and the sequential acid dissolution of the hematite cores. The as-obtained hollow silica colloids with different shapes have uniform sizes, shapes, and shells.

Tailoring Architecture of Nanothin Hydrogels: Effect of Layering on pH-Triggered Swelling

We have tailored the internal architecture of ultrathin poly(methacrylic acid) (PMAA) hydrogels from well stratified to highly intermixed by controlling the internal structure in layer-by-layer templates used for hydrogel fabrication. We have found pH-triggered swelling properties of these hydrogels to be significantly affected by hydrogel architecture. Well-stratified hydrogels exhibited a dramatic 10-fold increase in thickness when transitioned between pH = 5 and 7.5, unlike the 2-fold swelling observed in less-organized hydrogels.

Red blood cells and polyelectrolyte multilayer capsules: natural carriers versus polymer-based drug delivery vehicles

INTRODUCTION

Red blood cells (RBCs) and lipid-based carriers on the one hand and polymeric capsules on the other hand represent two of the most widely used carriers in drug delivery. Each class of these carriers has its own set of properties, specificity and advantages. Thorough comparative studies of such systems are reported here for the first time.

AREAS COVERED

In this review, RBCs are described in comparison with synthetic polymeric drug delivery vehicles using polyelectrolyte multilayer capsules as an example. Lipid-based composition of the shell in the former case is particularly attractive due to their inherent biocompatibility and flexibility of the carriers. On the other hand, synthetic approaches to fabrication of polyelectrolyte multilayer capsules permit manipulation of the permeability of their shell as well as tuning their composition, mechanical properties, release methods and targeting.

EXPERT OPINION

In conclusion, properties of RBCs and polyelectrolyte multilayer capsules are reported here highlighting similarities and differences in their preparation and applications. In addition, their advantages and disadvantages are discussed.

Thermosensitive multilayer hydrogels of poly(N-vinylcaprolactam) as nanothin films and shaped capsules

We report on nanothin multilayer hydrogels of cross-linked poly(N-vinylcaprolactam) (PVCL) that exhibit distinctive and reversible thermoresponsive behavior. The single-component PVCL hydrogels were produced by selective cross-linking of PVCL in layer-by-layer films of PVCL-NH(2) copolymers assembled with poly(methacrylic acid) (PMAA) via hydrogen bonding. The degree of the PVCL hydrogel film shrinkage, defined as the ratio of wet thicknesses at 25°C to 50°C, was demonstrated to be 1.9±0.1 and 1.3±0.1 for the films made from PVCL-NH(2)-7 and PVCL-NH(2)-14 copolymers, respectively. No temperature-responsive behavior was observed for non-cross-linked two-component films due to the presence of PMAA. We also demonstrated that temperature-sensitive PVCL capsules of cubical and spherical shapes could be fabricated as hollow hydrogel replicas of inorganic templates. The cubical (PVCL)(7) capsules retained their cubical shape when temperature was elevated from 25°C to 50°C exhibiting 21±1% decrease in the capsule size. Spherical hydrogel capsules demonstrated similar shrinkage of 23±1%. The temperature-triggered capsule size changes were completely reversible. Our work opens new prospects for developing biocompatible and nanothin hydrogel-based coatings and containers for temperate-regulating drug delivery, cellular uptake, sensing, and transport behavior in microfluidic devices.

Hydrogen-bonded Multilayers of Silk Fibroin: From Coatings to Cell-mimicking Shaped Microcontainers

We present a novel type of all-aqueous non-ionic layer-by-layer films of silk fibroin with synthetic macromolecules and a natural polyphenol. We found the multilayer growth and stability to be strongly pH-dependent. Silk assembled with poly(methacrylic) and tannic acids at pH=3.5 disintegrated at pH~5; while silk/poly(N-vinylcaprolactam) interactions were stable at low and high pH values but resulting in thinner films at high pH. The results suggest that the intermolecular interactions are primary driven by hydrogen bonding with a considerable contribution of hydrophobic forces. We also demonstrated that cubical, spherical and platelet capsules with silk-containing walls can be constructed using particulate sacrificial templates. This work sets a foundation for future explorations of natural and synthetic macromolecules assemblies as biomimetic materials with tunable properties.