Biologically Driven Assembly of Polyelectrolyte Microcapsule Patterns To Fabricate Microreactor Arrays

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.

Encapsulation of Low-Molecular-Weight Drugs into Polymer Multilayer Capsules Templated on Vaterite CaCO3 Crystals

Polyelectrolyte multilayer capsules (PEMCs) templated onto biocompatible and easily degradable vaterite CaCO₃ crystals via the layer-by-layer (LbL) polymer deposition process have served as multifunctional and tailor-made vehicles for advanced drug delivery. Since the last two decades, the PEMCs were utilized for effective encapsulation and controlled release of bioactive macromolecules (proteins, nucleic acids, etc.). However, their capacity to host low-molecular-weight (LMW) drugs (<1–2 kDa) has been demonstrated rather recently due to a limited retention ability of multilayers to small molecules. The safe and controlled delivery of LMW drugs plays a vital role for the treatment of cancers and other diseases, and, due to their tunable and inherent properties, PEMCs have shown to be good candidates for smart drug delivery. Herein, we summarize recent progress on the encapsulation of LMW drugs into PEMCs templated onto vaterite CaCO₃ crystals. The drug loading and release mechanisms, advantages and limitations of the PEMCs as LMW drug carriers, as well as bio-applications of drug-laden capsules are discussed based upon the recent literature findings.

3D Lattice Structure Control of Ordered Macroporous Material by Self-Assembly of Liquid Droplets

Microfluidic devices, which can continuously fabricate single emulsion with monodispersed droplets having a pore diameter of more than 100 μm in large numbers, can be applied to manufacture ordered macroporous films. 3D ordered macroporous films with a diameter of more than 100 μm can be fabricated using ordered arrays of the monodispersed droplets as templates of the macropores, which are self-assembled in the space between two parallel flat glass plates. As the gap between the glass plates increases, the number of the layer increases. Furthermore, in the case with two or more layers, the lattice structure of the macroporous films also changes due to the confinement effects.

Individually addressable patterned multilayer microchambers for site-specific release-on-demand

Patterned arrays of light-responsive microchambers are suggested as candidates for site-specific release of chemicals in small and precisely defined quantities on demand. A composite film is made of poly(allylammonium)-poly(styrene sulfonate) multilayers and gold nanoparticles incorporated between subsequent stacks of polyelectrolytes. The film shaped as microchambers is loaded with colloid particles or oil-soluble molecules. The microchambers are sealed onto a glass slide precoated with an adhesive poly(diallyldimethylammonium)-poly(styrene sulfonate) multilayer film. A focused laser beam is used for remote addressing the individual microchambers and site-specific release of the loaded cargo.

One-dimensional hybrid nanostructures: synthesis via layer-by-layer assembly and applications

Assembly techniques are being intensely sought for preparing nanocomposites with tunable compositions and structures. Compared to other assembly techniques, the layer-by-layer (LBL) technique, which is based on the electrostatic attraction between oppositely charged species, provides a simple, versatile and powerful method to synthesize various types of one-dimensional (1D) hybrid nanostructures. In this review, we begin with the developments in the LBL synthesis of nanocomposites, with a focus on our recent results for synthesizing 1D hybrid nanostructures via LBL assembly. Compared to previous LBL processes, we conducted the in situ reaction on the surface of 1D nanostructures via electrostatic attraction between oppositely charged 1D nanostructures and ions in the solution in an attempt to produce 1D hybrid nanostructures. Moreover, these core-shell nanostructures can be transformed into nanotubes by the removal of the templates. The as-synthesized 1D hybrid nanostructures and nanotubes with tunable composition exhibited enhanced performance for various applications such as gas sensors, lithium-ion batteries and cellular imaging.

pH-dependent control of particle motion through surface interactions with patterned polymer brush surfaces

In this Article, we show that inclined silicon surfaces patterned with poly(methacrylic acid) brushes are able to control the position and movement of 20 μm silica particles, which are propelled across the patterned surface by sedimentation forces. Three different types of behavior were observed depending on the angle between the direction in which a particle sedimented and the orientation of the polymer-brush silicon interface. At small angles, particles were found to sediment to the brush interface and then sediment following the direction of the brush interface. At larger angles, particles sedimented to the interface and then followed the direction of the brush interface, but then after a certain distance changed direction to pass over the interface. At the largest angles where the brush interface was approximately perpendicular to the motion of the particle, particles were found to travel over the interface unperturbed. This behavior was also found to be pH dependent, allowing the formation of pH responsive "gates", which allow particles to pass at low pH but not at high pH. It was also found that if patterned polymer brush surfaces were oriented in the correct way, they were able to control the number of particles present at specific locations.

Controlling the motion and placement of micrometer-sized metal particles using patterned polymer brush surfaces

In this paper, we show that silicon surfaces patterned with poly(methacrylic acid) brushes are able to control the Brownian motion of 2-3 μm iron particles, which sediment onto the surface in aqueous solution and experience differences in repulsive force depending upon their position. Differences in repulsion lead to different gravitational potential energies across the surface, which gives bias to the Brownian motion taking place. Three regimes have been identified depending upon the brush height: (i) no control of Brownian motion when the brush height is small, (ii) Brownian motion that is influenced by the polymer brush when the brush 17 height is intermediate, (iii) Brownian motion that is confined by polymer brush barriers when the brush height is greatest. The height of brush found necessary to significantly influence iron particle motion was small at 39 nm or 2% of the particle diameter.

Tuning the permeability of polymer hydrogel capsules: an investigation of cross-linking density, membrane thickness, and cross-linkers

Nanoengineered poly(methacrylic acid) hydrogel capsules (PMA HCs) are promising candidate carriers for biomedical applications, especially in the areas of drug delivery, encapsulated catalysis, and cell mimicry. The assembly, stability, and degradation of these carriers, as well as their use for the encapsulation of therapeutics, have received considerable attention. However, tailoring the permeability properties of PMA HCs to various types of cargo remains largely unexplored. Herein, we investigate fundamental parameters that govern the structural integrity and the capability of PMA HCs to encapsulate macromolecular cargo. The thiol content of the constituent polymers and the number of deposited polymer layers are shown to be key factors in controlling cargo retention within the PMA HCs. We further introduce a new strategy to achieve disulfide cross-linking for PMA HCs via a thiol-disulfide exchange in order to obtain capsules with superior cargo retention characteristics. Finally, we provide evidence for the semipermeable nature of PMA HCs based on the charge of the solutes and demonstrate that rational design of these systems can yield capsules with specific cargo retention properties. This work contributes toward the development of multilayered polymer capsules and PMA HCs and associated applications in biomedicine.

Stamps, inks and substrates: polymers in microcontact printing

It's all about polymers! Polymers play a key role in the patterning and functionalization of surfaces by microcontact printing. Polymers are versatile stamps, inks and substrates and microcontact printing can provide microstructured polymer surfaces in a single printing step.

Surface-supported multilayers decorated with bio-active material aimed at light-triggered drug delivery

In this work, we report on the functionalization of layer-by-layer films with gold nanoparticles, microcapsules, and DNA molecules by spontaneous incorporation into the film. Exponentially growing films from biopolymers, namely, hyaluronic acid (HA) and poly-L-lysine (PLL), and linearly growing films from the synthetic polymers, namely, poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), were examined for the embedding. The studied (PLL/HA)(24)/PLL and (PAH/PSS)(24)/PAH films are later named HA/PLL and PSS/PAH films, respectively. The HA/PLL film has been found to be more efficient for both particle and DNA embedding than PSS/PAH because of spontaneous PLL transport from the interior of the whole HA/PLL film to the surface in order to make additional contact with embedded particles or DNA. DNA and nanoparticles can be immobilized in HA/PLL films, reaching loading capacities of 1.5 and 100 microg/cm(2), respectively. The capacities of PSS/PAH films are 5 and 12 times lower than that for films made from biopolymers. Polyelectrolyte microcapsules adsorb irreversibly on the HA/PLL film surface as single particles whereas very poor interaction was observed for PSS/PAH. This intrinsic property of the HA/PLL film is due to the high mobility of PLL within the film whereas the structure of the PSS/PAH film is "frozen in". Gold nanoparticles and DNA form micrometer-sized aggregates or patches on the HA/PLL film surface. The diffusion of nanoparticles and DNA into the HA/PLL film is restricted at room temperature, but DNA diffusion is triggered by heating to 70 degrees C, leading to homogeneous filling of the film with DNA. The film has not only a high loading capacity but also can be activated by "biofriendly" near-infrared (IR) laser light, thanks to the gold nanoparticle aggregates on the film surface. Composite HA/PLL films with embedded gold nanoparticles and DNA can be activated by light, resulting in DNA release. We assume that the mechanism of the release is dependent on the disturbance in bonding between "doping" PLL and DNA, which is induced by local thermal decomposition of the HA/PLL network in the film when the film is exposed to IR light. Remote IR-light activation of dextran-filled microcapsules modified by gold nanoparticles and integrated into the HA/PLL film is also demonstrated, revealing an alternative release pathway using immobilized light-sensitive carriers (microcapsules).

"Giant" hollow multilayer capsules by microfluidic templating

Monodisperse microgels were synthesized by a microfluidic setup and used as a sacrificial template for the fabrication of "giant" hollow polyelectrolyte capsules with rigid walls consisting of covalently cross-linked polyelectrolytes and metal nanoparticles. First, a simple tubing-needle-based microfluidic system is utilized to produce size-monodisperse, degradable "giant" microgels consisting of dextran chains linked via carbonate esters. Second, these monodisperse microgels are subjected to a layer-by-layer coating of negatively charged platinum nanoparticles (Pt(NP)'s) and a positively charged diazoresin (DAR). Three bilayers of Pt(NP)'s and DAR are used to obtain a stable membrane on the microgels. Finally, the sacrificial dextran-based microgel cores are smoothly hydrolyzed and removed without rupture of the polyelectrolyte membrane due to the covalently linked hybrid polyelectrolyte/nanoparticle nature of the membrane. Scanning electron and confocal microscopy are used to characterize the capsules. The capability of encapsulating materials of interest is also shown by the addition of fluorescent polystyrene particles to the monomer mixture and subsequent visualization of embedded polystyrene particles in coated microgels after microfluidic polymerization and layer-by-layer coating. The obtained "giant" microcapsules are envisaged to be used as microreactors or drug-delivery systems.

Patterned microcontainers as novel functional elements for microTAS and LOC

Using nanoimprint lithography, arrays of highly ordered patterns of polyelectrolyte multilayer microcapsules consisting of alternating layers of poly(allylamine hydrochloride) and poly(sodium 4-styrene sulfonate) have been achieved. Anchoring the capsules on a pre-patterned substrate facilitates the utilization of their various capabilities in lab-on-a chip devices. In this paper we have demonstrated a very effective method to entrap soft capsules into surface cavities. Supported microcapsules were applied as the depots for loading and storage of macromolecular cargo (glucose oxidase and peroxidase) and as preserved microvessels for the cascade of enzymatic reactions. The loading of capsules was achieved under a pre-determined pH environment. This development is potentially useful for the realization of novel multianalytical systems for catalytic, bio-affinity and pH detection with protected sensing molecules.

Microreactors as tools in the hands of synthetic chemists

Recent developments in the construction of microstructured reaction devices and their wide-ranging applications in many different areas of chemistry suggest that microreactors may significantly impact the way chemists conduct experiments. Miniaturizing reactions offers many advantages for the synthetic organic chemist: high-throughput scanning of reaction conditions, precise control of reaction variables, the use of small quantities of reagents, increased safety parameters, and ready scale-up of synthetic procedures. A wide range of single and multiphase reactions has been performed in microfluidic-based devices. Certainly, microreactors cannot be applied to all chemistries yet and microfluidic systems also have disadvantages. Limited reaction time ranges, high sensitivity to precipitating products, and analytical challenges have to be overcome. An overview of microfluidic devices available for chemical synthesis is provided and some specific examples, mainly from our laboratory, are discussed to illustrate the potential of microreactors.

Hollow chitosan-alginate multilayer microcapsules as drug delivery vehicle: doxorubicin loading and in vitro and in vivo studies

We report here the loading of the antitumor drug doxorubicin (DOX) in preformed multilayer microcapsules and its application in tumor treatment assayed by in vitro cell culture and in vivo animal experiments. The microcapsules, consisting completely of polysaccharides, were fabricated by deposition of oppositely charged chitosan and alginate onto carboxylmethyl cellulose (CMC)-doped CaCO(3) colloidal particles in a layer-by-layer fashion, followed by cross-linking with glutaraldehyde and decomposition of the cores by disodium ethylenediaminetetraacetic acid. The microcapsules as prepared contain negatively charged CMC-either in a free state or very possibly coupled with the excess chitosan of the first layer. They showed a strong ability to accumulate the positively charged DOX with a factor of tens to hundreds; that is, the drug concentration within the microcapsules was hundreds of times higher than the feeding concentration. Confocal microscopy and transmission electron microscopy revealed homogeneous distribution of the drug. The encapsulated DOX could be released again, following a diffusion-controlled model at the initial stage. In vitro experiments showed that the encapsulated drug can effectively induce the apoptosis of HepG2 tumor cells, as shown by various microscopy techniques after acridine orange, Hoechst 33342, and osmium tetraoxide staining. By seeding the HepG2 hepatoma cells into BALB/c/nu mice, tumors were created for the experimental studies. The results showed that the encapsulated DOX had better efficacy than that of the free drug in terms of tumor inhibition in a 4-week in vivo culture period.

Microreactors as Tools for Synthetic Chemists—The Chemists' Round-Bottomed Flask of the 21st Century?

Will microreactors replace the round-bottomed flask to perform chemical reactions in the near future? Recent developments in the construction of microstructured reaction devices and their wide-ranging applications in many different areas of chemistry suggest that they can have a significant impact on the way chemists conduct their experiments. Miniaturizing reactions offers many advantages for the synthetic organic chemist: high-throughput scanning of reaction conditions, precise control of reaction variables, the use of small quantities of reagents, increased safety parameters, and ready scale-up of synthetic procedures. A wide range of single- and multiphase reactions have now been performed in microfluidic-based devices. Certainly, microreactors cannot be applied to all chemistries yet and microfluidic systems also have disadvantages. Limited reaction-time range, high sensitivity to precipitating products, and new physical, chemical, and analytical challenges have to be overcome. This concept article presents an overview of microfluidic devices available for chemical synthesis and evaluates the potential of microreactor technology in organic synthesis.